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Design and Construction Of a Farm Dam
HAVE endeavoured to present in Parts I and II of this book a planned agricultural landscape as the permanent background to Australian farming and grazing enterprises. My own efforts were not just to improve land a trifle or to improve it a great deal, but to get the best result that is possible and consistent with our natural environment. It is a moving optimum rising higher as time goes on.
There are ways and means outside the considerations of this book which produce great improvements in farming and grazing lands; means and procedures that in some countries have operated for centuries. Yet all of these must depend on the association of land and water and the life forces that depend on them. In these writings I have tried to avoid the customary fault of describing any one thing or aspect as "the most important" in agriculture, because I see agriculture as composed of many factors, and all, in their respective ways, are important, since each is a necessary part and without one the rest mean nothing, or, at any rate, nothing that is permanent and stable. Nevertheless, there is an order of importance, an order of permanence which when taken to its logical conclusion in planning does bring the optimum of these combined agricultural things into view, and it is from the advantages to be gained by combining prevailing climate and land shape in planning our agriculture that we obtain our optimum.
Any farmer will admit that he does not know sufficient about his local climate, and all of us would believe he would be much better off if he could know the weather for a month ahead. At present he is without that aid, but, in regard to our second factor, land shape, this he can know well and is thus able to make the most of his climate in association with his land.
Behind all our thoughts on these matters we have the general Australian condition of water waste and water shortage, so that water becomes a dominating climatic influence on our land and in our agriculture as a whole.
There is little doubt that we could use all the water we now let go to waste, and so the question of water use and water storage is the next problem--a problem that remains wherever water wastage occurs. Now, if we are aiming at the optimum in the development of agricultural land, it is quite obvious that we cannot approach it until this dilemma of water wastage is resolved. Though this is a widespread and national problem, it is also local and intimate to almost every farm and grazing property, and so the custodians of our land and water are vitally concerned. It is their particular problem as well as the nation's.
The landman on his own property has two good ways of preventing wastage of our precious water, and each of these can individually have more influence than the widest use of national water control structures. This is because methods and places for water storage available to the farmer and grazier will not only hold in the aggregate more water, but the water is held where the farmer needs it most.
The first of these two ways is to make the soil itself hold a large storage, and the means of accomplishing this are widely applicable, greatly economic and highly productive. These conditions can be achieved generally on all pasture land by a three-year programme of soil development designed to improve the soil climate and promote a rapid and lasting increased fertility in the soil. Fertile soil will absorb the first two or three inches of rainfall rapidly before heavy run-off can start, and the increased moisture induced in the soil promotes a gradually improving underground storage that is valuable and extremely reliable for both agriculture and industry. The same type of valuable storage can be secured also on crop lands, as well as in the crop phases of pasture land, by improved methods of cultivation and soil management. Although this extra water-holding potential of the improving and improved soil could be very profitable for the landman (and soil could hold more water than all the huge 'big' dam storages built and projected), there would still be heavy water wastage.
Since both this chapter and the next two deal with farm dams and irrigation some repetition in order to keep the subject intact is necessary.
If the problem of water storage on farm and grazing lands had been solved in the past and all waste water had been conserved economically and used profitably, then there would not now be any problem of water wastage nor could there be any great flood problem left.
The fact that we have both flood wastage and shortage of water illustrates the need for a solution of the problem of rain run-off. Could the failure to find a solution be in faulty design and poor construction of our present farm storages and the wrong method of using farm-stored water? This brings us to the second type of storage, the farm dam or the farm irrigation dam.
Sources of Infomation.--Since present methods have failed to solve the problem of water wastage, and since information on existing methods is widely known and freely available, there is little need for me to discuss them further here. I think it is better for me to seek new approaches and bring other sources of information to bear on the problem.
In my own attempts to improve land by, among other things, using water more effectively, I had the same problems earlier that many landmen have had, and, as has been acknowledged, I also had many unusual advantages in my own work and equipment. While it is generally impossible to say how, why or what causes the occasional good idea to be forthcoming or inspiration arise, such things do happen if one is very interested and there is the opportunity to do things and then do them over again in a different way when they are at first not satisfactory. I have built dams, filled them in again and remade them. I have deliberately broken the walls of dams to improve them and had failures and also some successes. On other occasions and in completely unrelated projects there are experiences which later were helpful. It is often an advantage to look outside the confined field of our endeavours for sources of inspiration.
Years ago I built a dam for mining purposes, not a big one but somewhat larger than an adequate farm irrigation dam. It was in a creek which extended about five miles upwards and it had a big catchment. I did not know much about the climate or run-off but I met an old man who remembered a kind of legendary flood that had occurred late in the last century and he was able to point out the height that the flood water then reached. I built the dam with a spillway capable of carrying twice the volume of water of this former flood, but within four months I saw the spillway, which discharged the water away from the wall of the dam around a rocky nob, become a roaring torrent of water and racing almost to its maximum capacity. As with many another crisis, this one finally occurred in the early hours of the morning during torrential rain when the spillway reached full capacity and the flood water started to creep across the crest of the earth wall of the dam.
Two of us had been keeping watch all night, and when this overflow started we began placing earth with hand shovels on the advancing lip of the water. We worked for an hour or two holding back the peak of the flood with this small bank of wet earth which we maintained. This little bank saved the dam. I said there were two of us. The other man was our present business manager, Mr. R. H. Barnes.
This incident illustrates two important points in our problem of water wastage. First, that a dam spillway large enough to take twice the amount of water of the previous greatest known flood rains may be none too large, and, secondly, that a very little work and a small quantity of earth can give a disproportionate measure of control over water.
This same dam has a further agricultural interest. It was constructed like the usual farm dam, without a pipe outlet. A four-inch syphon was used to deliver water from the dam (actually our reservoir) to a working dam near the mine. The syphon consisted of a metal pipeline over the wall of the dam with one end, the outlet end, discharging into the creek below and the other end in the dam water. In order that a syphon will work it has to be first filled with water, so there is always a footvalve on the water end that will stay closed while the syphon is being filled and a tap or valve on the outlet end which can be closed for the filling of the syphon and opened when the water is to be used. There must also be an opening in the syphon line on the wall of the dam through which the syphon is filled, which must then be closed and remain airtight. Our syphon worked satisfactorily, though on occasions the foot valve beneath the water did not close properly. Thus the syphon could not be primed and someone had to go down the pipe under water to clear the footvalve. One has only to do this a few times in the cold of winter to decide that a pipeline under the wall, and not over it, is quite an advantage. Ever since then I have not built a dam of even modest capacity without an outlet pipe. One wonders to what extent this experience influenced my development of our own farm dam designs and the Keyline flow system of irrigation?
During the early years on "Yobarnie" we were building a moderately large dam on a creek. A 24-inch pipeline was placed in position on a rock shelf on the creekside and about 12 feet below the top water level of the dam. A 24-inch hinge-type valve was fitted and, with the wall approaching its finished height, the valve was held in the open position in case of heavy rain. Unfortunately, the man who placed the valve in position left it held open with a piece of wire tied to a peg which he pushed into the earth of the wall. During the night and early morning many inches of rain fell and the dam was washed away by daylight. The rainfall had softened the earth of the wall, the peg, now loose, was pulled over by the weight of the valve door and the valve shut. At the time the dam wall had not been built up to spillway level, so the flood went over the top of the unfinished wall. There was not enough earth left to rebuild the wall again.
This early happening probably influenced the design of our present lockpipe valve which cannot close of its own accord and cannot be closed too quickly in any circumstances. Another direct result of this dam failure was a more detailed appraisal of run-off in relation to a whole complex of dam types, while still not neglecting the creek dams. Our latest creek dam on "Kencarley", at Orange, was constructed while the flow of the creek continued, and heavy rain would have had little effect after the first few days of wall construction. The dams and their drains above it would have reduced the greatest run-off to manageable proportions. (See Fig. 18, Chapter XX.)
There is much information, useful in connection with farm water storage, to be obtained in other mining. activities and not only from the mining engineer.
In circumstances where tin or gold prospectors are attempting to develop a water supply for their workings, much may be learned. The various arts, techniques and dodges that the prospector employs, often far distant from the best of facilities, can be very valuable. The trained mining engineer may not have had the opportunity of learning them or had the necessity of knowing them. The prospector may attempt to build a dam by discharging his tailing across a valley to form a wall. The tailings themselves, classified and separated with the earth broken down in the sluicing process, sometimes make a poor wall which quickly washes away, but they can also make a good wall. Often these walls leak badly, so the miner tries to devise a way to save the dam. He will strip off his clothes and get down under water to rake the wall where he thinks the leak starts and so promote a sealing effect, and very often these methods are effective. He may try explosives. Although there was recent discussion in some agricultural papers in N.S.W. on the alleged new use of explosives for sealing leaking dams, explosives have probably been used for this purpose ever since there was an explosive and a fuse which could be lighted above water and explode under water. Certainly mining men have used explosives for this purpose for many years, as I have done also.
On other occasions a miner may want a pipeline beneath the wall of his dam, and lacking experience may simply lay the pipes and build the wall over the pipeline with little thought that this could cause the failure of his dam. Water will tend to flow along the outside of a smooth pipe and make a tunnel of increasing size until the wall collapses and all the water is lost. However, the same man is likely to be more careful next time and so will use some kind of baffles that will prevent or retard seepage and flow. One such man will mix rotted grass, chaff or horse manure with the earth around the pipe on the theory that if water movement should occur the lightweight material will also move and tend to block up the porous area and automatically seal the leak. Surely this dodge is the invention of some hard-pressed mining man, and while it is difficult to prove that the practice works, it is known that when it is employed pipelines do not usually fail, and that they often do when no such care is taken. just being aware of the danger is often all that is required for the hazard to be averted. This type of knowledge and the further ideas suggested from it are embodied in my own methods in the lockpipe system where several anti-seep techniques, all simple and inexpensive, are employed as standard practice. The combination of the techniques now make it sure that if a dam wall does tend to leak and allow heavy seepage the one place where it is least likely to occur is in the immediate vicinity of the lockpipe equipment. There have been no failures with the lockpipe system, but I have had failures when I have used brittle pipes, and so also have some of my friends. Only sufficiently heavy steel lockpipe is, in my opinion, universally successful.
Often with little money, the prospector in the worst of conditions employs successfully the cheapest methods of water control, and such methods are not always known even to the mining profession generally. On one occasion I inspected an alleged alluvial gold mine which was owned by three prospectors who had very little money but they did have a large flow of water from their own water source, namely a dam made of logs. The dam site was such that most engineers would have considered that only a concrete wall would be suitable , but the log dam, a real work of art, was successful. The water entered a pipeline on a rise above the mine and served both as a jet to break down the face of the alluvial deposit, and as a jet elevator to carry the "wash" to the gold boxes for the recovery of the gold. The pipeline carrying the water under pressure to the mine was made up of pipes not all of the one size, and they were coupled together by every imaginable means. There were plugs and patches in the holes in the old pipes and water was spraying from the line. In spite of the responsibility that the men felt towards each other to make a success of the undertaking, there just was not enough gold to keep it going for long. The whole point of this story is that wealthy farmers in the area were suffering from an acute drought and water shortage while the prospectors had a water race flowing a large volume of water which as an agricultural asset was almost beyond price.
This practice of water control and transport, almost as old as time, has an agricultural significance on the individual farm or grazing property which has been almost entirely overlooked.
There are many failures that have turned out to be successes, and many of them go unrecorded, but the solution of the problem of water wastage is important enough for a landman to pursue every avenue that may be of assistance, and worth any amount of effort on the part of the Government authorities.
The conserving of all the run-off in the manner discussed in this book, and the very low-cost irrigation that is thus available, is so valuable that in my own circumstances it would still be worthwhile building our present storages if we had good rivers instead of dry creeks in our lower areas. But the farm dam in Keyline is a very different matter to the usual conception of a dam, and so a general review of this aspect is now made before the details of the methods of design, construction and use are set out. (See Diagrams and Plates.)
Definitions: To begin with, a farm dam is defined as an artificial or man-made water conservation structure that holds or "backs-up" the stored water by means of a wall constructed of earth obtained on the site. The water of the dam is held above the land adjacently below. An earth tank, on the other hand, is an excavated water-holding structure which holds its water in the excavated hole and below the level of all the immediately surrounding land. The tank is generally a stock-water storage only and is not a part of these discussions.
There are many types of dams and shapes of dams in Keyline, but they all hold water above the level of at least some of the surrounding land; they a possess some kind of a constructed wall or bank, and they all have a water outlet or "tap" under the wall.
The word "dam" refers to and means the whole thing, including the valley or basin area below top water level, the water and the wall. This is a different meaning for the word to that generally employed for the "big" dam, where "dam" applies to the wall itself In the farm dam the back of the dam is behind the wall or the side away from the water. The inside of the dam is the water side. Parts of the wall are its top or crest, which is the roadway along the top of the wall. The wall has an inside and an outside or front and rear batter (or slope), the rear batter in behind the dam-the downstream batter. The particular slope of the wall is described in figures such as 1 : 2, which mean that a vertical fall of 1 occurs in a horizontal distance of 2. The wall has a foundation or foundation area which is the bottom of the wall shape on the prepared earth of the valley below it, but the bottom of the dam itself is the land of the site which is or will be below the water level of the dam when it is filled with water. In preparing a site for a wall, the foundation area of the wall has two trenches, both usually of the same width, called first the cut-off trench and second the lockpipe trench. The cut-off trench runs the full length of the wall from end to end and in the finished dam is usually directly beneath the crest or roadway which is along the top of the wall. The lockpipe trench crosses the wall foundation area from front to back at right angles to the cut-off trench. The lockpipe trench then runs in the same direction as the bottom of the valley but is located to one side of the valley centreline. The water line of the dam, when filled, is also referred to as the water-level contour. The dam has a spillway (overflow, or by-pass), and freeboard, which relates to the minimum height along the wall about the level of the bottom of the spillway. The spillway is usually on the down land side of a keyline dam and overflow or flood water leaves the dam via the spillway.
The natural catchment area of a keyline dam is usually restricted so this natural catchment area is augmented by a water conservation drain or feeder drain which falls to the spillway of the dam from the upland direction (see Chapter VI ). The water conservation drain actually is located for the greater part of its length in the immediate valley area near the dam, above the water level of the dam. Falling at a grade right around the dam, it reaches the top water level when it meets the spillway. It should be particularly noted that the water conservation drain does not feed water directly into the dam. Some farm dams, not on our properties, which have been constructed with a drain falling directly into the dam, missed this important feature of our dams. The reason for this and other design features will be discussed later in this chapter.
The two diagrams illustrate the various features of the keyline dam. Both diagrams are in the plan, one showing a completed structure and the other showing only the "site-and-wall-foundation-area-preparations" at the time when construction of the wall begins. (See Fig. 10.)
These same names and features apply generally to most valley-type farm dams except that lower valley dams, and on occasions reservoirs, may not have water conservation drains (see Chapter VII).
A notable difference between the dams of Keyline and the usual farm dam is that in Keyline the whole of the dam is designed and constructed and not just the wall. The bottom of the dam, from which the wall material is generally taken, is finished off in such a way that a natural valley shape remains. The larger the farm dam the more important this may be, since the area of the bottom of the dam when the dam is emptied could be used as a very valuable special crop area.
Apart from Keyline construction, there are two other types of farm dams being made these days; one is the more or less haphazard type of structure constructed by bulldozer operators and which is lacking in design and for the most part poorly located, badly constructed and often unsightly. The second type of farm dam is that built by perhaps the same means but from a design supplied by water authority engineers. The design is too often a scaled-down model of the "big" earth wall. For the most part it is not followed in construction details because it is largely impractical and uneconomical in its application to farm water supply. Furthermore, a farm dam should be a lot more than just an earth wall.
"Big" Earth Wall Dams and the Farm Dam: The structures necessary for the walls of farm irrigation dams should not be a scaled-down model of anything. As I have said earlier, farm irrigation dams are completely specialised dams and should be studied and designed as such.
The engineering problems associated with the two types of dams, the "big" earth wall and the farm irrigation dam, are totally different. A farm irrigation dam is generally sited in or near the primary and secondary valleys. Almost any type of residual earth (earth in the place where it was formed from the decomposition, etc., of rocks) is nearly perfect for farm dam construction. These residual earths, which are characteristic of most dam sites with their decomposed rock above the harder rock below, are nearly perfect foundations for dams. But the "big" earth dam rarely, if ever, has such favourable conditions of site, foundation and wall material, and it is situated well down the fall of the land on a large creek or river several miles or even hundreds of miles from the headwater valleys of that river. The foundation materials below the wall may be layers and layers of sand and clay and decomposed rock and all its advantages and disadvantages hidden and often not capable of being inspected thoroughly even during the work. The wall material available may form a continuous construction problem and a huge volume of water often has to be brought under control before real work starts. There may be difficulties in procuring the large volume of materials of uniform type. Great expense could be incurred on site examination. The problems of river control, materials and foundations may be such that complete design and plans cannot be produced before the work starts and must be finally determined only by constant study of the behaviour of the materials and site as the work proceeds.
The "big" dam must never, under any circumstances, fail, because it could cause great property damage and heavy loss of life. The amount of water conserved is so large that if released suddenly it would cause enormous havoc. Many "big" dams have failed in the past, with loss of life and great property damage. On the other hand, the farm irrigation dam, by comparison, conserves manageable amounts of water; a farm dam conserving 400 acre feet of water would be considered a very large farm irrigation dam, but a "big" dam more than one thousand times that size conserving 500,000 acre feet of water is considered of moderate size.
For these reasons the design and construction features that are necessary in the "big" dam to secure adequate margins of safety bear little relationship to those involved in the farm irrigation dam. The same high factors of safety can be achieved in the farm irrigation dam without many of the expensive features of design and construction that are necessary in the "big" dam.
The farm irrigation dam is a specialised dam, and although the design is different to the "big" dam, correct design is just as important. It must be designed in relation to the nature of the foundation, the type of material, the depth of the water, and its future purpose, and also with adequate consideration given in the design to the type of equipment that will build the dam.
A "big" dam may require very flat batters of more than I in 3 to satisfy safety requirements, but on occasions farm dams carrying 16 feet of water may be just as permanent and safe with batters of 1 in 1-1/2. Farm dams with batters as steep as this, with walls formed of a mixture of shale and the clay of its own decomposition, have remained quite stable, but whether it be batters 1 in 1-1/2 or 1 in 3 entirely depends on the foundation sites, the wall material and the depth of water.
The farm irrigation dam, as outlined in this book, will be, in my opinion, as safe from floods, collapse and destruction as the "big" earth dam. There have been many failures of "big" dams in other parts of the world, and there are bound to be many failures of farm dams before all problems are overcome. However, the consequences of failure in the two types of dams are not comparable. If a dam of the design under discussion does fail, its repair will usually be a simple matter. If the farmer is on hand to open the lockpipe valve, a threatened failure may be averted, and if it did occur it should only be partial. There is, however, not much anyone can do in the failure of a "big" dam except get the whole endangered population out of the way in time.
In this connection, then, the farm irrigation dam in a Keyline development project has a great advantage over the "big" dam.
The majority of these farm irrigation dams will be constructed in or near the primary and secondary valleys, which, unless they are eroded, flow water over a rounded grassed bottom. Many are built in the solid land before the country breaks. This break of the land is seen in any watercourse, where stream water flows over raw earth of some kind, sand, clay, gravel, etc., and not over grass, as in a rounded unbroken valley.
Water will be brought to many farm irrigation dams by water conservation drains having an even grade, and is directed into the dam from the drain over the most suitable and stable area. The only silt, as can be seen in the colour of the water, that is likely to reach the dam would enter it during the first flows of water to the new dam. In a short time these drains and raw earth, which are always under the control of the farmer, become stabilised and grassed and will flow clear water. After this has taken place the greatest accumulation of silt likely would be such dust as comes from the atmosphere.
The "big" dam is built across a stream or, more often, a large river. According to the condition of the catchment area, flood rains carry varying amounts of silt to the dam. Immediately the fast-moving water, with its silt load, reaches the still water of the dam its velocity drops and it deposits its load into the dam. This is a continuing process and often an accelerating one. Then there is the ground load made up of material that the flowing water moves along the bottom of the stream but does not actually carry. This type of sediment, while it increases in floods, is continuously on the move, even in clear flowing water. Particles in size from sand up to large boulders are rolled along the river and creek bottoms by stream action.
Since siltation is almost negligible in a farm dam, there is no reason why a good farm irrigation dam managed by generations of good farmers on a good farm should not last longer than the average life of the "big" dams.
The control that a farmer can exercise over all the factors that influence the permanent usefulness and good appearance of his farm irrigation dams is not matched by the care or control that is possible in the "big" earth dam. In fact, the greatest influence that can be brought to bear in preserving the life and usefulness of the "big" dam would occur if all the farmers in its hundreds of square miles of catchment area were all looking after their own property and their farm irrigation dams. There is no force comparable to the farmers and graziers as custodian of land and water.
The essential design features that are to be provided for in the farm irrigation dams are listed below:
The design should also provide that the top width of the wall crest is wide enough to allow farm equipment to travel safely so that the dam can be always properly maintained. A good minimum workable width is 10 feet.
Site Selection: In selecting the site for a dam for irrigation purposes, full consideration should be given to the ultimate development of the property from the point of view of complete water conservation. The water from the first dam should provide profitable irrigation against its capital cost. The ultimate plan then should envisage a co-ordinated plan of dam lay-out and siting that will provide storage capacity for every drop of water that flows on the farm, and, because of this, the siting of the first dam becomes a very important task indeed.
In planning the full layout of dams for complete water conservation it is not by any means necessary that the lowest or the highest or any particular site should be used first, but it is important to consider the general location of all the dams so that the dam constructed first will fit in perfectly with the full plan if and when it is developed. For instance, there may be three excellent Keyline dam sites contained in a series of five primary valleys which flow into the same larger valley. There may be also two good reservoir sites and one lower valley dam site. Any one of these sites can be used first, provided it is located in such a way that when the other dams are built, it will receive their overflow water or alternately discharge its overflow water into the others, and fit in with the whole development.
It is not necessary to use the keyline sites first. Generally, the site should be one that will provide low water storage cost in conjunction with an adjacent area that is suitably developed or can be quickly developed for irrigation from this first selected site.
In this, the general order on undulating country, there are four distinct types of valley dams, including the creek dam:
The keyline dam represents the first highest storage of run-off, whereas the lower valley dam (or the creek dam) represents the last chance of conserving water before it finally leaves the property.
The reservoir is an "intermediate" type of dam between these two. If there is not a suitable site below the highest keyline dam, then the highest other suitable site is selected and where it will receive the overflow from the high dams.
The first dam can quickly prove its effectiveness and profit potential and enable the landman to determine in the shortest possible time that he will institute a full programme aimed at conserving all the water that now goes to waste from his property.
It is usually assumed that dam site selection is a relatively easy matter and can be done by eye. While this is sometimes true, it is not necessarily always so. The critical factors in site selection are: (1) sufficient catchment available; (2) the contour shape of the valley at the proposed water line; (3) the valley floor slope; (4) suitability of earths for construction; and (5) suitable adjacent area for irrigation land.
It is very difficult to obtain an adequate appreciation of the shape of a farming property from the point of view of complete water conservation by simply studying it by eye. A good contour map is of outstanding value by enabling a quick study of all the land forms and suitable dam sites. Good contour maps of farming properties are unfortunately as rare as they are valuable.
An alternative and completely satisfactory means of determining land shape for farm water conservation can be obtained by laying in the keylines of the property. As discussed in earlier chapters, a keyline should be laid in from the keypoint of the first valley as a line rising with the general rise of the country, and then, after selecting the next valley keypoint, marking in a rising line there and so on for each valley in turn. These lines immediately disclose the relationship in height between all the suitable high conservation sites so that they can be arranged in such a way that no water leaves the high country until all the proposed high dams are filled. Suitable reservoir sites that would catch most of the overflow water from the high series can also be determined. Lower valley sites are usually obvious to the eye.
When investigating alternative sites for a first dam, the valley floor slope is checked with a levelling instrument over the proposed site of the dam and then compared with the valley floor slope of alternative sites. Other things being equal, the flatter valley floor slope is preferable. For instance, the conservation of water in a valley with a valley floor slope of 1 in 12 will cost more per unit of conservation area than a site in a valley with a 1 in 30 slope.
The contour shape of the valleys are then studied. This is done by choosing a contour to represent the water level of the dam and marking it on the ground with such an implement as a lister attached on a chisel plow. By standing on the marked contour line on one side of the valley and looking across at the contour line on the opposite side, a mental picture of the dam when filled with water may be visualised.
If the keylines have already been marked in (rising into country as they do in undulating land on which water is to be conserved), a satisfactory picture of the dam can be obtained by standing at various points along the keyline on the downland side of the valley and imagine the wall in place from that point across to the keyline on the opposite side of the valley. The keyline then will be just above the actual water level of the dam.
In marking out a selected dam site the depth of water is first determined. Where the contour shape of the valley is suitable, it is suggested that greater depths of water be considered than at present used on farm dams. A 20-foot depth of water is suitable if the shape is suitable and generally provides a worthwhile quantity of water for irrigation purposes. Where the site is large and greater storage is desired, it should be borne in mind that a depth of water of 24 feet as against a depth of 20 feet will in many circumstances double the cost of the wall. It should also be understood that in considering a dam of this size, the extra four feet of water may double the conservation capacity of the dam, and for this reason the larger undertaking may become well worthwhile.
The Design of the Dam: Before a good farm dam can be built it needs to be designed. To assist in the construction of our own dams and to help some of our friends and clients, we have produced an information sheet from which, when it is filled in, a complete design for a particular dam site is determined. It has been found by actual trial that an inexperienced man can control and supervise the construction of a farm dam from our plans and instructions and produce a good dam that will be valuable and permanent. After the experience of having built a dam to the designs I have prepared, the same man then can usually design as well as build a second dam. (See Fig. 11. )
The information sheet referred to sets out what is required and defines precisely what is meant by such things as (1) depth of dam; (2) length of dam; (3) width of dam; (4) under wall valley floor slope; (5) excavation area valley floor slope; (6) total valley floor slope; (7) contour shape of dam; (8) natural catchment area; (9) induced catchment area; and asks for details of each and for rainfall information, earth type, etc. From this information the value of the site is assessed and the water storage capacity, wall yardage and cost are readily determined. It is also not difficult for any farmer or grazier to follow the sheet and supply the information.
There is such a wide field to the study, design and use of farm dams that a large volume would be needed to cover the subject fully, and so the object here is to present the simple fundamentals of the design and construction methods of those of our farm dams which I believe will have the widest application in Australia. To this end we may assume that we are building a keyline dam to the design illustrated. The instruction sheet which we use in Keyline work is designed to produce proficiency in the man as well as produce a good dam. Therefore, the first diagram shows the various batters or slopes for a selection of cross sectional shapes through the wall of a dam. The horizontal and vertical scale are the same, and so anyone using the sheet can soon visualise just what the various batters look like. These diagrams are marked A in Fig. 12.
The second diagram, B, shows a section to scale down the valley floor, representing the relationships between keypoint, valley floor slope, water level and cross sections of the excavation area and of the wall of the dam. The third diagram, C, is a keyline dam in plan showing the general relationships of all features; water level to wall, conservation drain and spillway; excavation area to wall plan and lockpipe and irrigation drain (Fig. 13).
Then D is a section across the valley looking upstream from behind the wall, showing again the relationships of the water conservation drain to the spillway and the water level to the freeboard and settlement allowance. E is a plan of the site preparation and markingout of the dam, illustrating similar layout relationships.
These diagrams are produced in this particular order, A to E, so that a study of each in turn leads to the marking-out diagram when the whole layout should be clearly understood and work on the dam can proceed. There are many dams to be built on farms that need them and which can very profitably employ them; therefore the diagrams are directed towards making all those who have constructed one dam proficient and capable of designing many more dams. There are also included in the design sheet two insets. Inset (b) illustrates the precise relationship between the valley floor slope under the wall of the dam and the location of the lockpipe to these two and to the cut-off trench. Inset (d) shows the detail of the relationship between the centre or low point of the valley and the lockpipe and outlet positions.
Marking Out: In marking out the site of a dam some particular point must be selected as the main control point and to which all other construction features relate.
In a keyline dam the keypoint of the valley and the water conservation drain may serve as controls or points used to locate a main control point. For instance, the water conservation drain of a keyline dam has a fall right around the top water level contour of the dam, meeting the water level at the floor of the spillway. Therefore, a line of levels falling in the downland direction from the selected keypoint of the valley will represent the water conservation drain, and, somewhere along it, also the position of the spillway of the dam. If the dam is to be 20 feet deep, then along this drain line where the line is 20 feet vertically above the valley bottom opposite lies the point of the spillway overflow, and the low point in the valley can be selected as the main control point. In practice the approximate length of the water conservation drain from the keypoint of the valley to the spillway is obvious. Therefore, the location of this main control point is determined from the position of the keypoint and is done in the following manner. Assume this part of the length of the drain to be 200 yards (from the keypoint to the spillway) and the slope of the drain 0.5 feet per cent. (6 inches fall in each 100 feet), then the spillway height at the point of overflow will be three feet lower than the peg at the keypoint of the valley (0.5 per, cent. fall for 200 yards is equal to a fall of three feet). Therefore, the dam being 20 feet deep, it is necessary to find a point down the valley 23 feet below the keypoint. This point, as well as serving as the main control during the building of the dam, also represents the level at the bottom of the lockpipe outlet behind the wall. It is, as well, on the same level as the bottom of the lockpipe on the water side of the dam, since the Iockpipe lies on a horizontal line, as is seen in the diagrams. The point in the centre of the valley 23 feet below the keypoint of the valley is now the main control point for the marking out and construction of the dam. The main control point should now be marked with a small solid peg driven well into the earth and a longer peg, such as a steel post, driven into the ground alongside. Next, a point the same height and 50 feet down on one side of the valley is marked with a similar peg, and a flag or marker is placed on it. This marker is away from the work area, so that it will not get lost in the latter work of dam building. The main control peg is called the "double Y" peg and marked YY on the diagrams.
The full marking out of the dam is not completed for the site clearing work, but the water level contour should be pegged and marked with a furrow. The site is then cleared of timber to a little beyond the water level contour (10 feet clear above this line is suitable) and down valley about 30 feet minimum from the YY peg. The timber is pushed into heaps outside the area of the dam site.
Now the dam may be marked out. The dam is to be 20 feet deep, therefore the height of the finished wall above the main control YY peg is 20 feet plus a freeboard of three feet and plus the shrinkage allowance. This is assumed to be 8-1/2%, or one inch for each foot of height, which is 23 inches, making the total height of the wall above the YY peg a maximum of 25 feet (24 feet 11 inches). The batter will be fixed as 1 in 2 for the two batters of the wall, and the crest width will be 12 feet, a width I often use for a wall of this height. In order to work out the dimensions of the wall the calculations are made against its height as planned, after shrinkage and settlement has taken place. The dimensions are as shown in the diagram. The maximum width of the wall shape is 104 feet. However, this width does not represent the width of the placed wall, but the width of the wall shape after the dam is completed, when it then includes a section of the original valley floor below the wall. This feature of the dam promotes a new efficiency in design affecting aspects of control, safety, efficient use of water and economy of construction.
The marking out and site preparation for a farm dam should be from plans and specifications already prepared. However, to illustrate the details of the design features the marking out will proceed from considerations on the site of this proposed dam. The dimensions of the centre cross section of the wall of the dam are known. From the YY or main control peg another peg is placed in the centre of the valley and well upstream from the control peg to clearly show the line of the valley itself. The centre line of the long section of the wall is to fall at right angles to this valley line. Then the centre point of the wall is determined. The section through the wall is 104 feet, so a point half this distance, 52 feet, is marked upstream from the YY peg and is placed in line with the two pegs indicating the valley centre line. From this centre wall peg and at right angles to the centre valley line is the actual centre line of the wall. Pegs are placed on this line to indicate the two ends of the wall on the sides of the valley, and two other pegs 50 feet up the side of the valley beyond the end of the wall. These last two pegs on the line of P pegs outside the work area should be marked clearly and so that they are not disturbed or lost in the work.
From the centre line of the wall further pegs are now placed on the back toe of the wall line Y pegs, so that this wall line can be determined and clearly marked. Points marked on the back toe of the wall for each 5-foot vertical rise from the centre of the valley up the sides are sufficient for this purpose. So the next Y peg will be placed at a distance from the centre wall line (P pegs), which is calculated for the first one in the centre of the valley in the following manner: There is a vertical rise from the YY peg of five feet, therefore the settled wall height at that point is 23 less 5, which leaves 18 feet. As the batter is 1 in 2, this figure is multiplied by two, and half the width of the crest (which has been determined at 12 feet), six feet added, so the first Y peg on each side of the YY peg will be 42 feet from the centre line of the wall (P line) at the 5-foot vertically higher point.
The next 5-foot vertical rise peg worked out in a similar manner will be 32 feet and the next is 22 feet, and the final distance of the fourth of these intermediate wall toe pegs will be 12 feet. (These various wall sections are shown on the diagram by the dotted lines.) (See Fig. 15.)
For the inside toe of the wall the Y pegs' distance from the centre wall line are not calculated in this manner. It will be seen from the diagram that the inside Y peg is not the finished shape of the wall when it is completed, so its dimensions from the centre line will be less than the distance from the centre line to the back of the wall-toe pegs. For designs in the field the simplest manner for determining their position is with the aid of ordinary squared paper, which in our diagram shows that the Y pegs for the centre section of the wall are eight feet closer to the centre line than the rear Y pegs. This distance therefore in the centre of the wall is 44 feet, the various distances from the centre line to the inside Y pegs on the ground according to the diagram are, from the wall centreline: 44 feet, 35 feet, 27 feet, 18 feet, and 10 feet. These lengths will not be strictly accurate, because the slope across the wall of the dam at the various positions will not be completely uniform. It is, therefore, advisable to make the Z pegs, indicating the front toe of the finished wall shape, the same distance from the centre line as the rear toe Y pegs. Steel fence posts make very suitable markers. From the P Pegs, on each side of the wall, the width of the cut-off trench (10 feet) is marked and the excavation area of the trench is defined by two or three pegs, as illustrated in the diagram.
The design and marking out of the dam will be left now for a moment while other matters are considered.
Study of Materials Available: As already mentioned, the materials available on dam sites are usually good. Practically any residual soil on farms in undulating country will make dam walls.
The requirements of a farm dam, as far as its construction is concerned, is simply that its works and is permanent. No amount of laboratory testing and classifications of earth for a farm irrigation dam wall will give as much practical information to the farmer as he can get by visiting a number of earth dams in his own area (even if they are small) which have been constructed with the same type of materials and built by the same methods as those that he would be using himself.
During the course of construction of a wall, fill material, particularly when moved by bulldozer operation, needs to be flattened out or levelled off by the bulldozer travelling along the progressive top of the wan. The material should be travelled over each time as the bulldozer places it, and, together with frequent smoothing off, a uniform texture is produced throughout the whole of the wall material.
The compaction of the wall of a farm dam is often unduly stressed and is frequently misunderstood. Some wall materials for farm dams do need the maximum of compaction, and where this applies implements such as the sheepsfoot roller or the multi-tyres pneumatic roller can be used. However, most earths used in farm dam construction do not require this costly process of complete compaction.
Some farm dams have been constructed very badly in that they are just mere masses of loose earth, untrimmed, unpacked, and with no uniformity of texture whatever, yet on settlement and shrinkage they develop into impervious and quite stable structures. However, such methods are inadvisable because dams so built often fail at the first rain. In areas where poorly constructed dams do hold well, no special compaction as such need be aimed at, providing that the suggested precautions are taken to ensure the uniformity of texture of the material as it is laid down in the wall. There are (as well as a general suitability of most materials on farms for dam construction) some areas where the problem of dam construction has not been solved. An instance is found in the deep cracking black clay soils which, when soaked, swell greatly. While I have not had direct experience on these earths, I believe serious investigations of the problem may be worthwhile, particularly now that the value of adequate farm water supplies is being better appreciated. While it has been my experience that the best dam building materials on a farm or grazing property are to be found at the site of the keyline dams, it is advisable to check the materials available. If the proposed dam is to be the first on the area, the dam should be dimensioned in a design and then an examination of the materials should be made to a depth of two feet below the deepest excavation area indicated in the design. A few small holes made with hand tools or even a small auger may be sufficient. The material is to be examined for cohesion, stability and water-holding capacity. If the dampened material will roll into a small bar or sausage three-eighths to half an inch diameter three inches long and can then be bent without breaking into a curve nearly a half circle, cohesion is satisfactory, and so generally is the water-holding capacity. If this earth is very fine with no sand or larger particles it may lack stability, and so wall batters would be designed flatter than in circumstances of good stability. Very fine and uniform particle clays and very fine silt are the less stable earths. Silts generally also lack cohesion, so silt earth walls generally need the flattest batters. Water-holding capacity is low in sand and high in clays, so that if there is in the earth a little more of the clay fraction than is necessary to fill all the interstitial spaces between the coarser particles, water-holding capacity will be suitable, but where this condition is not met the earth will not hold water well and often not at all. In these cases special sealer blankets are employed. Generally they are of two types; selected clay is used to cover the whole of the inside of the dam and inside wall batter with up to two feet thickness of clay or, alternatively, bitumen emulsion products incorporated with the earths of the structure itself form the blanket. Of other methods sometimes suggested I have found none yet that are sufficiently effective and economically priced.
Most residual granite earths and the decomposed granite rocks below them form very satisfactory walls for farm irrigation dams on a batter of 1 in 1-3/4 for water depths of 16 feet and batters of 1 in 2 for 20 feet of water. On the other hand, the same type of earths may have lost most of the felspar and mica that forms the clay fraction of such material and require flatter batters in order to hold water satisfactorily. The less stable materials always require batters that are less steep and perhaps greater supervision in their construction than the general material available on farms.
Shrinkage allowance in dam building simply means constructing a wall to such finished height and shape that shrinkage and settlement will reduce it to the planned or designed height and shape. The same relative shrinkage is assumed to take place along the wall at its various heights and a uniform allowance of 10% is adequate for most farm earths. It has been found on many of the dams constructed on these principles that an allowance for shrinkage of 8-1/2% is satisfactory. Generally, clay material shrinks more than mixtures of clay and sand.
Freeboard is the minimum height of the bank above water level when the dam is filled to the point of overflowing. A freeboard of three feet is suitable on most farm dams.
A spillway is the channel prepared for the purpose of disposing of excess water from a dam when the dam is overflowing. Its size depends on the natural catchment area of the dam and on the run-off intensity. A spillway needs to be wide enough so that the maximum storm run-off likely when the dam is filled will flow through the spillway as a stream not much more than one foot deep.
In keyline dams, the spillway is situated on the downland side of the dam and it is constructed with the bulldozer which builds the dam. From the point of the spillway at maximum water level of the dam the spillway is given a drop of 0.5% and the spillway section carried far enough around the land so that overflow water from the spillway cannot flow on to the back toe of the wall.
In Keyline layout, the section of the spillway at a suitable distance from the dam usually changes to the smaller section of a water conservation drain which carries the normal overflow from the dam and the normal catchment above the water conservation drain into another dam. The main requirements of the spillway are that it carries all the overflow water from the dam and if it does overflow the country it does so at a place where water cannot reach the back toe of the wall or cause loss of soil anywhere.
It is not necessary that the whole of the water from heavy flood rains flowing out of the spillway should be transported by the water conservation drain to the next dam. Provision is made so that it can overflow at a suitable place.
* * *
To return to the construction of our keyline dam-the topsoil is now removed. The soil from the wall site is pushed down valley to a point 15 feet below the Y pegs by removing about three to four inches of soil and working at exact right angles to the centre line of the wall. (See Fig. 14. )
The topsoil from the marked excavation area is moved upstream 15 feet beyond the limit line of this area. (On the completion of the structure, the topsoil from behind the wall will be pushed up over the back and crest of the wall, and the soil previously pushed up valley from the excavation area will be pushed down over the excavation area, after it has been plowed with a chisel plow, and up over the inside batter of the wall.) As the soil is moved from the area at a peg, the supervisor should remove the peg, stepping three or four yards away and lining up the peg position with some other mark. He replaces the peg in its original position when the bulldozer has finished for the time being.
A main control peg (YY) has been placed at the back or downstream side on the toe of the wall and a peg 50 feet away and exactly level with this point has been fixed. Two pegs have been placed on each side of the valley from and in line with the P peg, so that if the P pegs are lost these offsets will allow the centre line of the wall to be relocated. Another control peg 50 feet from the P pegs and at the same height and on the downland side of the peg should also be established, so that the wall height cannot be lost during the work. Care should be exercised to see that these pegs are not disturbed.
With the removal of the topsoil from the wall site and excavation area site, further earth is moved from the wall site if it still contains unsuitable material which can be used to form the line of the pegged toe of the wall represented by the Y pegs. Care is taken to maintain the pegs in position, and the material is pushed to the line of pegs and not beyond them.
Progressively more of this material is placed on the line from the sides of the valley towards the centre of the valley, but no more is placed at this stage than is necessary to remove the unsuitable earth.
Cut-off Trench: At this point, the centre line of the wall P pegs are checked, also the Z pegs, and the bulldozer starts the cut-off trench by pushing straight down the centre line (P pegs) from the sides of the valley. The earth from the cut-off trench is placed to form part of the back toe of the wall.
If the material below the wall is stable, as it usually is, and contains no excess of gravel or sand or other unstable or porous material, the cut-off trench need only be eight inches to one foot deep on the valley sides but increasing in depth towards the bottom of the valley, where it will be the same depth as the lockpipe trench, which is to be constructed next. (See Fig. 14.)
The sides of the cut-off trench have a batter of about 1 in 1.
Lockpipe Trench: The lockpipe trench is located at right angles to the cut-off trench and parallel to the valley centre line on the downland side of the valley.
From the level of the low spot of the valley at the toe of the wall (the main control peg YY, previously placed), a spot is selected on the Y line, on the downland side of the valley one foot higher than the centre bottom of the valley. The lockpipe trench crosses the wall line, including the cut-off trench, at right angles from this point. The lockpipe trench is cut in one foot deep from the Y line at the back of the wall and constructed under the wall site right through to the inside of the wall, and must be dead level (a horizontal line). The level of the trench throughout is the same as that of the low point of the valley on the YY peg. The lockpipe trench will then be one foot into the solid ground on the Y line at the back of the wall, but, according to the valley floor slope, may be four to seven or eight feet deep at the inflow end of the pipe on the water side of the wall. In this dam we are describing it is five feet deep. A right-angle cross is thus formed at the junction of the lockpipe trench and cut-off trench and at this point both trenches are the same depth. The level along the lockpipe trench is checked during the excavation as the tractor works, until it reaches the correct depth throughout.
The construction of the pipeline trench is best cut by the bulldozer working first along the length of the trench to clearly locate it and then backwards and forwards at right angles across the trench.
Filling the Cut-off Trench: With the completion of the lockpipe trench and the cut-off trench, a chisel plow is brought in to cultivate to about four inches deep the bottom of the two trenches and the whole of the area of the wall site, particularly from the cut-off trench to the Z pegs, working parallel to the centre line of the wall. The roughing-up of this material, which is now the prepared foundation of the wall, aids the better bonding of the wall into the site.
The bulldozer then places good uniform material in the cut-off trench, material free of stumps, sticks, rubbish or organic matter, which is obtained from the excavation area but never from within the Z pegs defining the upstream edge of the under-wall site.
Laying Lockpipe: The length of lockpipe necessary for the dam is decided according to the maximum width of the wall plus an extra six feet of length, so that each end of the lockpipe protrudes three feet from the wall shape on each side. There is now a total of 110 feet. The lengths of the individual pipes making up the full lockpipe may not exactly equal the lockpipe length required, so on occasions a slightly greater total length may have to be used. This dam has even batters, i.e., the same batters on both the front and back of the wall, so the centre point of the lockpipe trench is selected from the centre line P pegs and checked by measuring a distance from this centre point half the total length of the lockpipe to the dam side of the wall. The number of baffle plates to be used and their distance apart depend on the designed depth of water for the dam. Their distance apart from the inside toe is determined as approximately one-third the depth of water and seven feet apart in this case.
The bottom half of each baffle plate is now placed in its correct position according to the measurement from the centre point of the wall site. The lower baffle plate of each pair is let into the ground or hammered in so that the half circle of the plate is clear of the bottom of the lockpipe trench by three inches. All the baffle plates are lined up and levelled-in with a levelling instrument from the reference point level already established, the YY peg, and the bottom of the lower half circle of each baffle is fixed at this level.
If both long and shorter lengths of lockpipe are used, the shorter lengths can be placed at the outlet end of the lockpipe and the long lengths at the inlet end. The pipes can be dragged to the side of the trench with a small tractor or carried by any other means and rolled into the trench. All the pipes that will form the full length of lockpipe should be lined up in their correct position beside the standing baffle plates before any are lifted into position on the cradle formed by the half baffle plates. (See pictorial section ).
"U" section rubber is used between baffle plates and pipe as an anti-seep and these gaskets are next fitted onto the lower half of the baffle plates and the pipes lifted carefully into place. Care should be taken to see that the holes in the flanges of the pipe are in such a position that when the valve is coupled, it will be sitting upright. One hole should not be positioned at the centre top; two top holes should be at the same height. The flange end of the pipes are inspected and cleaned and one rubber pipe gasket inserted between the flange junction and the bolts and nuts placed in and firmed up. It is better to place all lockpipe sections, gaskets and bolts and nuts in position before tightening any of the bolts. When they are in position, all nuts should be tightened uniformly around the pipe. Each nut should be tightened a little in turn right round the pipe two or three times. The final turn must be very tight and the pipe gasket quite clearly show a squeezed effect.
It is suggested that the farmer who is supervising the work should check the tightness at each junction of the full lockpipe. One loose joint could endanger the stability of the dam.
The top half of each baffle plate is now coupled with each U-shaped gasket around the pipe and the two sections of all the baffle plates bolted firmly together. The volume strainer, which fits on the inside end of the lockpipe or water side of the wall, and the valve which fits on the outlet or downstream side of the wall, are not coupled at this time, but are left until the dam construction is completed.
Filling the Lockpipe Trench: The lockpipe equipment, when laid and tightened, is held by the baffle plates a little above the bottom of the lockpipe trench. The earth that goes beneath the lockpipe is to be compacted by hand work particularly from the inflow end to the centre line of the wall. First the bulldozer travels parallel along each side of the lockpipe trench in turn, pushing a load of earth so that a sufficient quantity spills into the trench to be hand rammed beneath the pipe, but not sufficient to cover the pipe to any extent.
An anti-seep material is now placed. We use an inert, lightweight material ranging in size from very fine particles up to about the size of large sand grains, which is made by heat expanding a special volcanic rock. Its weight is about one-tenth the weight of sand. At each flanged junction of the pipe and each baffle plate from the inflow end of the pipe to the centre line of the wall a mixture of this material and the wall material is placed, so that if water does move, the lightweight anti-seep material will move with it to form a seal around it and so automatically seal a leak. The mixture is to contain about 20% by bulk of antiseep material and the balance, wall material, mixed and placed at the flange junctions and baffle plates points during the filling-in of the earth around the lockpipe.
Commencing from the inlet end of the pipe, two men with crowbars ram the earth, which has been brought in by the bulldozer and spread by hand shovels beneath the pipe. The first ramming with a crowbar should be from an oblique angle on each side of the pipe, so that the material is firmed well beneath the pipe. One firm stroke with the ramming end of the crowbar every two inches along the pipe is usually adequate for this ramming operation.
Now more earth is brought in by the 'dozer or shovelled in. The level of this earth is brought up to the centre line of the lockpipe. Another careful row of ramming on the same lines as the first and from the inside end to the centre with less emphasis on the downstream half of the lockpipe will ensure suitable compaction below the pipe. A little work with a shovel and further general ramming around the area of the pipe will complete the hand work necessary. Loose earth in the wall of a dam may settle and consolidate, but not so the earth below the lockpipe, since there is no weight from the earth in the wall above. Hence the necessity for this procedure.
The bulldozer is then brought in as before to place material in the lockpipe trench from each side and just sufficient to cover the whole pipeline but leaving the top edge of the baffle plates showing so that they form a guide for the next bulldozer operation. The top edge of the baffle plates is left exposed to view, and the earth on each side of the lockpipe should be of sufficient height to carry the bulldozer, which next straddles the baffle plates and lockpipe from end to end. In this operation the bulldozer is carefully signalled forward so that each track straddles the lockpipe. Care is necessary to see that at no time can the undercarriage or sump of the tractor come in contact with the top of the baffle plates, and therefore sufficient earth on each side of the lockpipe is necessary to keep the tractor clear of the plates. One run up and back along the full length of the lockpipe is all that is required with the bulldozer straddling the lockpipe. The bulldozer next stands off at right angles to the lockpipe and positioned in such a way that each individual baffle plate will be straddled. The operator is now signalled forward with a load of earth, which he drops progressively in the lockpipe trench and over the baffle plate and pipeline (covering the pipe with at least 15 inches of earth), and continues the tractor movement forward until the front of the two tracks of the tractor have just crossed the pipeline, which, in the process, has become covered with more earth. He is then signalled back and does the next baffle plate in the same manner, and so on until this operation is complete. The bulldozer can then travel up and down each side of the lockpipe trench (but not straddling the lockpipe), to produce some compaction and uniformity of texture in the rest of the trench. The vibration of the tractor aids this work.
Next, a mound with a minimum of two feet six inches higher than the top of the baffle plates is pushed in over the lockpipe at right angles to the lockpipe.
The fill material in the wall on either side of the lockpipe is brought up to the height of the earth on the lockpipe and the whole area is travelled by the bulldozer from now on in the general course of the construction of the wall.
Immediately the operation of placing the lockpipe and filling the lockpipe trench is completed, markers are placed at each end of the line, so that it will not be lost by the bulldozer covering the ends with earth in the course of the dam construction. A 44-gallon drum placed at each end of the lockpipe with two steel posts driven in the earth on each side of each drum are excellent markers for the purpose.
Maintenance of Wall Shape: Throughout the whole of the construction of the wall of the dam the downstream batter of the wall should be maintained at its finished batter and line (Y pegs). The back batter is 1 in 2 and should be kept at this batter whether the wall is only five feet high or ten feet high; this batter should be maintained throughout the work.
The inside batter of the wall during construction is not treated in this manner. It starts off as a very flat batter, gradually increasing in steepness until it finally reaches the correct batter on the completion of the wall.
As the cut-off trench is filled with good material, this material is spread and levelled off by the action of the bulldozer travelling occasionally along the length of the cut-off trench. Once the trench is filled, material is taken from the excavation area just beyond the site of the Z pegs and spread across the wall and travelling towards the back line of the wall which was already marked with earth placed prior to the completion of the excavation of the two trenches.
Supervision is necessary to see that the bulldozer operator does not dig earth from below the site of the wall, i.e., within the boundary marked by the Z pegs. This is of particular importance in overcoming one of the general faults in dam construction. It is to be remembered that an irrigation dam will sometimes be filled with water and sometimes empty. The period of greatest stability for the inside of the wall is during the time when the dam is completely filled. The water helps to hold the inside of the wall stable. Its period of greatest instability occurs when the water is drawn from the dam and the dam becomes empty. Inside slumping and slipping of the earth of the wall towards the bottom of the dam is the manifestation of this instability. If earth is removed from inside the Z pegs, i.e., from below the inside toe of the wall (a universal fault in farm dam construction) during the early stage of wall construction, then fill material will later have to replace it. The result is that a greater length of material that will settle and shrink occurs at the most vulnerable inside point of the wall. If, however, the shape of the land below the wall is preserved in its original form (less the stripping of topsoil), then there will be a very much smaller length and total area of shrinkage surface and the wall is improved at what is usually a point of weakness. (See Fig. 14. )
This feature of my design is of relatively greater importance in all valley dams, as the valley floor slope is steeper. Generally the orthodox farm dam design provides a flatter batter for the inside of the wall and a steeper back batter, and this arrangement acts as a compensation against inside slumping. However, the Keyline design adds stability and allows the construction of the inside wall batter to be as steep as the rear batter. As well as reducing yardage, it simplifies both design and construction. The effect is lessened as the the valley floor slope is flatter, but it is still of significant importance in design. To be fully effective, good design features should be preserved in the construction of the dam by equally good supervision.
Methods of Bulldozer Operation: Many operators can drive a bulldozer well by performing accurately all the tasks of cutting and filling required, and though working hard all the time, still move earth at double what it should cost. I have frequently made tests and conducted trials on such matters and always, good supervision and the techniques which follow reduce earth-moving costs by a large percentage. In the construction of farm dams we are concerned with placing the right material in position as quickly and cheaply as possible, and every technique that aids this end is worthy of consideration. First of all, the speed of the engine of a bulldozer is governed to a maximum speed. It cannot overwork or strain itself, so the bulldozer should be worked with the throttle fully open always. Then windrows in bulldozer operation are the parallel banks of earth left on each side of the bulldozer blade as it moves the earth forward. The action of pushing a bulldozer load forward occasions continuous spill of material from each side of the blade. With a large load in front of a bulldozer, the movement of the load forward over a distance of, say, 80 feet, without further digging as the bulldozer progresses, will result in a much smaller load at the end of the distance; earth will have been lost in spill which forms windrows. But windrows can be used as an aid to shifting earth faster and cheaper.
The bulldozer should start its run by grabbing a big load as quickly as possible, and in low gear if necessary, pushing the load in a straight line and at right angles towards the wall. As soon as the load starts to reduce by spilling in the formation of windrows, then it stops and backs up and grabs another full load. This load is pushed forward towards the wall, forming a larger and longer windrow until such time as the load is reduced below the full load capacity of the blade when it rips back and grabs another full load. (Modern bulldozers can be equipped with "back rippers", which tear up the earth as the 'dozer moves back empty.) The bulldozer proceeds in this manner until windrows sufficiently high are formed, which will enable a full load to be transported forward to its final position. The operation of the bulldozer is now confined between these windrows pushing up from six to twelve full-capacity blade loads to the fill site. Grabbing the load simply means loading the bulldozer blade to maximum capacity as quickly as possible and using low gear if necessary, so that the rest of the trip to transport the full load into the new wall can be travelled in a faster gear. If the full load is not obtained in the first pass over, say, a 20 to 25-feet run, the operator rips back and grabs a load again until the full blade load is obtained. After six to twelve full passes have been made in this pair of windrows the tractor is moved to form a new path and new windrows, with one windrow of the new path partially formed by the windrow of one side of the first pass. This windrow site is worked as before. Windrows may be approximately three feet high.
The windrows should be formed at the centre of the excavating site and moving out first on one side and then the other, so that after all windrows have been formed and used there is a series of parallel equi-distance lines of windrows lying at right angles to the wall of the dam.
The next operation involves the destruction of all the windrows of the first series of passes. The bulldozer commences the second series by travelling with one of the old windrows in the centre of the blade pushing a maximum load of the old windrow material forward at right angles to the wall. This pass will form new windrows very rapidly and the 'dozer continues operations in this newly formed pair of windrows for a suitable number of passes, generally from six to twelve full-blade loads. The next movement of the bulldozer pushes the next windrow of the first series out on either side of the new pass and continues the operation until all the old windrows are dozed out, new windrows have been formed and the requisite number of full loads taken between them. This type of operation is followed throughout the whole of the construction of the dam.
Systematic working and following these procedures will often shift earth as mentioned for less than half the cost of another type of operation, which, though it may appear quite satisfactory and economical to both farmer and bulldozer operator, is much more costly. It is therefore very important that the full use of windrows is maintained throughout the whole of the work. The bulldozer, except in the final finishing-off of a job, should not travel to the wall site with less than its full load. A bulldozer travelling onto the wall site with half a blade of earth is shifting earth expensively.
Supervision in the construction of a dam is not provided by a man merely watching the bulldozer work. I have seen men set to work as supervisors stand idly by and watch while a bulldozer operator did almost everything wrong, added a hundred or so pounds to a job of modest size, and finish up with a blot on the landscape which both operator and supervisor mistakenly thought was a good farm dam. That is not the sort of supervision that is required.
A supervisor, whether a farmer or anyone else, must know first what he wants, and as most people have not seen a good farm irrigation dam, then he should really have a plan. The plan should be first studied so that the farmer may be convinced of the logic and necessity of every detail of the design, of the methods of work, of the construction details, and of the final finish and the use of the dam. He then should see that the operator follows his instructions. A farmer may be somewhat reluctant to instruct a bulldozer operator of wide experience, and think that he should not be told or should not be stopped when he is not performing the operations according to plan. However, a bulldozer operator, in almost all other circumstances outside farm work, does work to a plan and under a supervisor, because such methods of operation have been found to produce lower costs and efficient work. Moreover, it is quite unfair to a bulldozer operator to expect him to design and construct a good dam from the seat of his tractor as he goes along. Also, the bulldozer operator is only on the farm for a short while, but the farmer has to live with this work, whether it is good or bad, for very many years. He should, therefore, get the dam that he wants and which he will only get if the manner of his supervision carries out effectively the design of the dam. It is also a good idea for the farmer when his first dam is finished to again study his plan perhaps quite critically, and maybe talk to someone about it. As long as the person to whom he talks is interested, the talking will probably line up his own ideas and plans, and he may even find that he can greatly improve the manner of the building of his next dam.
Travelling Forward Over all Earth as Placed: A bulldozer is not a completely efficient compacting machine. A farmer contemplating the purchase of a heavy crawler tractor for the purposes of farm work will be assured by the tractor salesman that the track-type tractor places less weight on his soil than the smallest wheel-type tractor. He may offer to place an egg a few inches below the surface and show that this will not be broken by the weight of the tractor on its tracks unless a growser bar on the track plates happens to hit it. Nevertheless, the travelling of the bulldozer over the earth as placed does give a measure of consolidation and stability to the loose earth and can produce a very desirable uniformity of texture in the material. Uniformity of texture is important, as it assures that shrinkage, when it does take place, is also uniform, while cross cracking and longitudinal cracking of the wall will be lessened. During the construction of a dam, continuous waves of earth in the wall site are to be avoided by the means suggested previously, e.g., the bulldozer travelling forward over the earth as it progressively drops its load and as the operator raises the blade. This fill material must be continuously smoothed off and shaped up.
Levelling-off at the End of Each Day's Work: The construction of a farm irrigation dam may occupy as little as two days, or may take several weeks, depending on its size, the implements used, and digging conditions and weather.
In dam sites with valley floor slopes that permit all the excavation material to be taken from above the level of the lockpipe, as in the keyline dam of this design, care should be taken to see that the bulldozers do not dig below this level. Those spots above the level of the lockpipe which would trap rain water are also to be avoided. The full depth is to be maintained only in the lockpipe area, so that rain water will drain to and through the lockpipe. Before finishing work for the day, the windrows left in the excavation area from the day's operation should be pushed into the wall area and trimmed. The whole area, including the wall, then will carry the minimum of loose earth and provide good drainage from the work to the lockpipe. Preparation made at the end of each day's work should provide against the possibility of damage by heavy rain falling during night time. Some inches of rain could fall on the construction then and not prevent work on the following day. However, loose earth on the walls and excavation site would, under the same conditions, absorb much water, create ponds, and possibly hold up the work for days. Trimmed earth will absorb the minimum amount of rainfall water and then the rest of the water will be shed off. The lockpipe should also be maintained in an open, free and operating condition at all stages of the construction, and should be checked each day before work ceases.
Rain: If rain has fallen on the work area the whole of the wall section should be travelled and ripped up if necessary before starting again, so that continuous bonding of the earth as it is placed in the wall occurs satisfactorily. Windrows are then formed again as usual, when the deeper, drier earth will become well mixed with the smaller amount of wet material, so that a uniformity of moisture content and texture is still maintained. In many types of earth work, moisture conditions are maintained within precise limits, but generally the only occasions that may be troublesome in the construction of a farm dam are when the earth is very dry and does not bond properly, or is in an overwet condition, causing clays to ball-up and leave air pockets in the wall.
Progress of Work: During the construction of the dam, all 'dozer paths should be at right angles to the wall and be particularly so in the early stages of the work. After the task of laying, tamping and filling the lockpipe trench is completed, the formation of the wall should continue with the bank at the highest level over the central low area of the valley bottom. From this stage onward right through to the time when finished levels are being considered, the central area of the wall should be maintained as the highest point, with a slope of about 10% (1 in 10) along the length of the wall from the centre to each side. When this wall line is maintained during construction the low portion of the wall is always well away from the area where the maximum earth has been deposited. In a valley of uneven section, i.e., where the one bank of the valley at the wall is steeper than the other, then the lowest point of the rising wall at any time will be the end of the wall on the flatter side of the valley and at the greatest distance from the main earth fill. Water would flow over the wall here if the partially built dam were flooded. The low spot acts as a safety fuse (the name for a weak or low spot), thus protecting the main fill area of the earth wall.
In a sudden heavy downpour the lockpipe would also be flowing a full bore of water and as soon as heavy rain ceased would quickly reduce the water level to below the overflow height and then empty the dam. Damage would be of the minimum, even though no expensive safety precautions had been employed.
Dams with only small natural catchments such as this keyline dam, though, would hardly be affected by heavy rain. The lockpipe provides full safety and water would only rise a few feet of depth for a short time. This safety factor is aided by the fact that the water conservation drain, which will later help to fill in dam, is not constructed until the wall is completed, so that run-off is restricted to the small natural catchment. It can, however, be even further restricted, if need be, by constructing that part of the water conservation drain only that is around the dam. Provision is then made for the disposal of the small natural catchment run-off through the spillway area of the site.
A well-planned and supervised job looks right all the time. Some bulldozer operators like to concentrate on one spot to bring it up to its finished height, but this is bad practice, since it tends to work against uniformity of texture and proper bonding of the earths in the wall. Furthermore, areas of compacted stabilised earth are likely to be placed adjacent to areas of very loose earth. Shrinkage later would form large cracks between the different textured materials. There is a tendency also for bulldozer operators to push all the earth of the blade-load into the wall and leave it as a loose mound. This is avoided by the operators starting to lift the blade at the correct position on the wall so that the earth is distributed evenly. Again, there is a likelihood of earth being carried forward too far onto the wall site, the result being that much loose material is spilt over the back of the wall. This loose material in a finished wall tends to absorb a lot more rain than the rest of the wall, which, by increasing its weight, could cause sliding and slumping of the rear of the wall. The back batter of the wall should be maintained throughout by trimming with the bulldozer when it becomes necessary.
Sometimes a bulldozer operator "rushes-for-height", which often results in a concave line up the wall. The line up the wall should always be a straight line. A concave line encourages slumping of the high point on the edge of the crest of the wall, and once this has started the extra weight on the material below causes a movement of earth which in the worst circumstances could result in a later partial failure of the wall.
Throughout the building of the dam the marking pegs should be maintained in their proper position by lifting them out of the bulldozer path when it is necessary for it to travel there and by the farmer stepping three or four paces out and lining up the position of the peg between himself and another peg, so that after the work in that area has for the time been completed then the old peg can be put back in its correct position.
The spillway of a dam, like all other features of good farm irrigation dam construction, has to be right for the dam to be a good one. For relatively small dams, the wall can be constructed to the completed height, which includes freeboard and shrinkage allowance, right through from one side to the other. Then the spillway, which outflows at the downland side of the wall, can be constructed by pushing through the placed earth at one end of the wall down to solid ground at the top water level contour and maintaining the flat bottom of the spillway on the side of the valley away from the water line. By constructing the spillway through the finished bank a considerable quantity of extra earth is available to strengthen the spillway at its critical point, i.e., where the bank of the spillway merges with the wall of the dam. On other occasions, the construction of the spillway may produce a surplus of earth, which is then used in the building of that end of the wall.
The batter between the bottom level of the spillway and the wall of the dam is about 1 in 4, so that in the grassing of the dam site, wall and excavation area, convenient travel with cultivating equipment is possible. A section of the spillway therefore will show a batter slope of 1 in 4 falling from the end of the dam wall to a dead level spillway bottom of a given width, with a similar batter rising from the spillway bottom to the rising land on the high side of the spillway and away from the wall of the dam.
Shrinkage allowance, freeboard height and spillway size therefore provide than on the completion of the dam and its subsequent settlement, there will be three feet of wall everywhere above top water level at the point where water commences to flow out of the dam and through the spillway. The design of the spillway is such that the type of flood likely to occur any time in 50 years would be by-passed, with the spillway carrying little more than one foot of water across its full width, and when this happened there would still be a further freeboard of two feet to compensate as a safety measure for bigger floods and to provide for wave action erosion during high flood.
Larger spillways are necessary in farm dams such as reservoirs or lower valley dams, since they have considerably more catchment area than this keyline dam. To secure the necessary width of spillway with a dead flat floor, considerable material may have to be excavated into the rising country near the wall of the dam. In these circumstances, an appreciable amount of earth may have to be moved, that is, earth greatly in excess of the needs of the spillway bank. The construction of the wall of such a dam, then is designed so that the earth of the spillway is used in the construction of that part of the wall adjacent to the spillway. It is advisable to construct the spillway before the earth in the centre of the wall approaches its finished height, and when there is still plenty of wall area unfilled and available for the use of the spillway material. On occasions where the spillway of a dam involves very considerable earth moving, the construction of the spillway may be completed by placing the excavated material into the wall site immediately the site preparation is completed.
In spillway construction earth has to be excavated down to a specific level and supervision should ensure that the earth is not excavated too deeply, necessitating the filling of areas of the spillway with earth that would not be in as good condition as the stable undisturbed material.
Final Batters: With the earth for the wall being constantly moved in properly designed windrows at right angles to the wall, the site is in a condition to be examined continuously in order to ensure that the cheapest digging earths go into the wall. As the work proceeds, areas of material somewhat harder than that of the general digging conditions may be encountered. These areas are then studied to determine whether cheaper earth can be obtained by going back another 15 feet or 20 feet for earth or whether cheaper earth is obtained by persevering with the cutting and digging of the harder materials. Outcropping hard rock may be encountered, and it should not exceed 30% of the earth in any blade load. Bulldozers operating with back rippers are capable of making fairly light work of reasonably tough materials, and once the job is well opened up, continuous consideration should be given to the work to. ensure that the cheapest material is being used throughout.
The cost of moving a particular quantity of earth is related to the distance it has to be moved, so length of movement of earth should be considered as the work proceeds.
Finishing Stages: With the back wall batters strictly maintained on a straight line and at the correct slope, the front wall will gradually steepen from very flat batters towards the final finished angle. Centre line pegs lined up with the P pegs on either bank of the valley should be placed at intervals during the final stages of construction, and measurements should be taken and marked with temporary pegs to show the finished width as well as the final height of the wall. A continuous check with a level and measurements ensures that the right amount of material is in the right place and excavated from the correct position in the excavation area. The centre area of the wall is first finished off to its final height, crest width and batters. The construction of the finished batters, the heights and top width of the remainder of the wall is maintained towards the sides. Final batters should be even and continuous throughout, with straight lines up the wall. A slightly steeper slope in part of the wall always has a tendency to be more unstable than the rest of the wall. Slight movement of earth may take place, the parting on either side of the movement being represented by a steep crack, which tends to allow movement of the flatter battered earths on each side of the steep area and thereby assisting the movement of the steep material and progressively worsening. Such movements tend to reach a point where they stabilise themselves, although not necessarily so.
Finishing Off: When the material has all been placed and the wall trimmed to its proper top width, height and batters , the final finishing-off commences. The windrows left in the bottom of the cut may be flattened out,- but it is not necessary or advisable to move all loose material from the excavation area. However, it is important that the excavation area be smoothed into a natural shape to conform to the valley area of the whole dam. When this is finished, the raw earth is cultivated with a chisel plow and then covered with the soil, which was previously moved from the surface of the excavation area. Some of the soil should be used also to cover the batter on the inside of the wall. The soil which was stripped from the wall area and pushed behind the wall is now brought up over the wall to cover the back or downstream side of the wall and the top of the wall, where an inch or two of soil cover is all that is required.
If two tractors were used to build the wall and a big rope or cable of the type used for the roping down of trees is available, a good finish to the work can be obtained by the two tractors hauling the rope across the inside and then across the outside of the wall. In this operation one tractor with a cable attached travels along the top of the wall and the other tractor travels along the bottom of the dam near the wall, with the other end of the cable attached and slightly
in advance of the tractor on the wall. The rope dragging over the top and side of the wall smoothes the work. The top of the wall should be finished with slightly rounded top edges. If it is finished off haphazardly with a bulldozer there is likely to be a small windrow effect left by the blade. When rain falls this could cause little ponding areas, which eventually break in one particular spot and flow down the wall in a concentrated stream. Further rain falling on the wall takes the same path, and sufficient erosion could take place with an inch or two of rain to spoil the appearance of the new wall and necessitate some repair work.
The area of the dam is cultivated with a chisel plow in a single-run cultivation about three inches deep. The cultivation parallels the water level contour downwards (keyline cultivation), so that flow water later spreads as it flows into the empty dam. Next, the wall and the whole of the site is sown with the regular pasture seed mixture and combined with a dressing of fertiliser.
Hand finishing of the top of the wall to leave a good shape and to aid the germination and growth of the grasses is well worthwhile. Slight ridges can be raked out and slightly rounded edges left on the top edges of the wall.
Finishing Off Lockpipe: The volume strainer is coupled up to the cleaned surface of the flange of the pipe on the inside of the wall with a rubber gasket between and with the straight edge of the volume strainer downwards.
The inclined section of the lockpipe is made to point upwards in the upstream direction. The upward tilt acts as an additional safety to preserve a free and full flow of water in case there is a slight slip of wall material through any cause. The volume strainer should be coupled up tightly.
Our strainer has an opening nine times the lockpipe size, and screened by heavy mesh. The rate of flow of water into the volume strainer when the lockpipe valve is open is therefore very much slower than the rate of flow through the lockpipe, so that any matter which can enter the strainer will flow out through the pipe. The outlet valve should be coupled in the same manner as the strainer, but on the downstream end of the lockpipe and tightened up and closed. All surfaces, gaskets and flanges should be clean and no earth left in the end of the lockpipe. As care has already been taken in the laying of the lockpipe to ensure that two holes are level on top of the flanges of the lockpipe, the valve will fit in an upright position.
Our own valves are provided with a 2-inch constant-flow outlet on the water side of the valve closure, so that water is always available for such items of smaller supply as stock troughs, etc.
The building of the dam is not completed until the working drains are provided. These are discussed in the following chapter.