Design and Construction Of a Farm Dam
HAVE endeavoured to present in Parts I and II of this book aplanned agricultural landscape as the permanent background to Australian farmingand grazing enterprises. My own efforts were not just to improve land a trifle orto improve it a great deal, but to get the best result that is possible and consistentwith our natural environment. It is a moving optimum rising higher as time goes on.
There are ways and means outside the considerations of thisbook which produce great improvements in farming and grazing lands; means and proceduresthat in some countries have operated for centuries. Yet all of these must dependon the association of land and water and the life forces that depend on them. Inthese writings I have tried to avoid the customary fault of describing any one thingor aspect as "the most important" in agriculture, because I see agricultureas composed of many factors, and all, in their respective ways, are important, sinceeach 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 doesbring the optimum of these combined agricultural things into view, and it is fromthe advantages to be gained by combining prevailing climate and land shape in planningour agriculture that we obtain our optimum.
Any farmer will admit that he does not know sufficient abouthis local climate, and all of us would believe he would be much better off if hecould 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 ableto make the most of his climate in association with his land.
Behind all our thoughts on these matters we have the generalAustralian condition of water waste and water shortage, so that water becomes a dominatingclimatic influence on our land and in our agriculture as a whole.
There is little doubt that we could use all the water we nowlet go to waste, and so the question of water use and water storage is the next problem--aproblem that remains wherever water wastage occurs. Now, if we are aiming at theoptimum in the development of agricultural land, it is quite obvious that we cannotapproach it until this dilemma of water wastage is resolved. Though this is a widespreadand national problem, it is also local and intimate to almost every farm and grazingproperty, and so the custodians of our land and water are vitally concerned. It istheir particular problem as well as the nation's.
The landman on his own property has two good ways of preventingwastage of our precious water, and each of these can individually have more influencethan the widest use of national water control structures. This is because methodsand places for water storage available to the farmer and grazier will not only holdin 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 holda large storage, and the means of accomplishing this are widely applicable, greatlyeconomic and highly productive. These conditions can be achieved generally on allpasture land by a three-year programme of soil development designed to improve thesoil climate and promote a rapid and lasting increased fertility in the soil. Fertilesoil will absorb the first two or three inches of rainfall rapidly before heavy run-offcan start, and the increased moisture induced in the soil promotes a gradually improvingunderground storage that is valuable and extremely reliable for both agricultureand 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 cultivationand soil management. Although this extra water-holding potential of the improvingand improved soil could be very profitable for the landman (and soil could hold morewater than all the huge 'big' dam storages built and projected), there would stillbe heavy water wastage.
Since both this chapter and the next two deal with farm damsand irrigation some repetition in order to keep the subject intact is necessary.
If the problem of water storage on farm and grazing lands hadbeen solved in the past and all waste water had been conserved economically and usedprofitably, then there would not now be any problem of water wastage nor could therebe any great flood problem left.
The fact that we have both flood wastage and shortage of waterillustrates the need for a solution of the problem of rain run-off. Could the failureto find a solution be in faulty design and poor construction of our present farmstorages and the wrong method of using farm-stored water? This brings us to the secondtype of storage, the farm dam or the farm irrigation dam.
Sources of Infomation.--Since present methods have failedto solve the problem of water wastage, and since information on existing methodsis widely known and freely available, there is little need for me to discuss themfurther here. I think it is better for me to seek new approaches and bring othersources of information to bear on the problem.
In my own attempts to improve land by, among other things, usingwater 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 workand equipment. While it is generally impossible to say how, why or what causes theoccasional good idea to be forthcoming or inspiration arise, such things do happenif one is very interested and there is the opportunity to do things and then do themover again in a different way when they are at first not satisfactory. I have builtdams, filled them in again and remade them. I have deliberately broken the wallsof dams to improve them and had failures and also some successes. On other occasionsand 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 forsources of inspiration.
Years ago I built a dam for mining purposes, not a big one butsomewhat larger than an adequate farm irrigation dam. It was in a creek which extendedabout five miles upwards and it had a big catchment. I did not know much about theclimate or run-off but I met an old man who remembered a kind of legendary floodthat had occurred late in the last century and he was able to point out the heightthat the flood water then reached. I built the dam with a spillway capable of carryingtwice the volume of water of this former flood, but within four months I saw thespillway, which discharged the water away from the wall of the dam around a rockynob, 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 themorning during torrential rain when the spillway reached full capacity and the floodwater 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 overflowstarted 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 bankof wet earth which we maintained. This little bank saved the dam. I said there weretwo of us. The other man was our present business manager, Mr. R. H. Barnes.
This incident illustrates two important points in our problemof water wastage. First, that a dam spillway large enough to take twice the amountof 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 disproportionatemeasure of control over water.
This same dam has a further agricultural interest. It was constructedlike the usual farm dam, without a pipe outlet. A four-inch syphon was used to deliverwater from the dam (actually our reservoir) to a working dam near the mine. The syphonconsisted 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 thata syphon will work it has to be first filled with water, so there is always a footvalveon the water end that will stay closed while the syphon is being filled and a tapor valve on the outlet end which can be closed for the filling of the syphon andopened when the water is to be used. There must also be an opening in the syphonline on the wall of the dam through which the syphon is filled, which must then beclosed and remain airtight. Our syphon worked satisfactorily, though on occasionsthe foot valve beneath the water did not close properly. Thus the syphon could notbe 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 pipelineunder the wall, and not over it, is quite an advantage. Ever since then I have notbuilt a dam of even modest capacity without an outlet pipe. One wonders to what extentthis experience influenced my development of our own farm dam designs and the Keylineflow system of irrigation?
During the early years on "Yobarnie" we were buildinga moderately large dam on a creek. A 24-inch pipeline was placed in position on arock 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 finishedheight, 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 tiedto a peg which he pushed into the earth of the wall. During the night and early morningmany inches of rain fell and the dam was washed away by daylight. The rainfall hadsoftened the earth of the wall, the peg, now loose, was pulled over by the weightof the valve door and the valve shut. At the time the dam wall had not been builtup to spillway level, so the flood went over the top of the unfinished wall. Therewas not enough earth left to rebuild the wall again.
This early happening probably influenced the design of our presentlockpipe valve which cannot close of its own accord and cannot be closed too quicklyin any circumstances. Another direct result of this dam failure was a more detailedappraisal of run-off in relation to a whole complex of dam types, while still notneglecting the creek dams. Our latest creek dam on "Kencarley", at Orange,was constructed while the flow of the creek continued, and heavy rain would havehad little effect after the first few days of wall construction. The dams and theirdrains 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 waterstorage, to be obtained in other mining. activities and not only from the miningengineer.
In circumstances where tin or gold prospectors are attemptingto 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 bestof facilities, can be very valuable. The trained mining engineer may not have hadthe opportunity of learning them or had the necessity of knowing them. The prospectormay 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 thesluicing process, sometimes make a poor wall which quickly washes away, but theycan also make a good wall. Often these walls leak badly, so the miner tries to devisea way to save the dam. He will strip off his clothes and get down under water torake the wall where he thinks the leak starts and so promote a sealing effect, andvery often these methods are effective. He may try explosives. Although there wasrecent discussion in some agricultural papers in N.S.W. on the alleged new use ofexplosives for sealing leaking dams, explosives have probably been used for thispurpose ever since there was an explosive and a fuse which could be lighted abovewater and explode under water. Certainly mining men have used explosives for thispurpose for many years, as I have done also.
On other occasions a miner may want a pipeline beneath the wallof his dam, and lacking experience may simply lay the pipes and build the wall overthe pipeline with little thought that this could cause the failure of his dam. Waterwill tend to flow along the outside of a smooth pipe and make a tunnel of increasingsize until the wall collapses and all the water is lost. However, the same man islikely to be more careful next time and so will use some kind of baffles that willprevent or retard seepage and flow. One such man will mix rotted grass, chaff orhorse manure with the earth around the pipe on the theory that if water movementshould occur the lightweight material will also move and tend to block up the porousarea and automatically seal the leak. Surely this dodge is the invention of somehard-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 theyoften do when no such care is taken. just being aware of the danger is often allthat is required for the hazard to be averted. This type of knowledge and the furtherideas suggested from it are embodied in my own methods in the lockpipe system whereseveral anti-seep techniques, all simple and inexpensive, are employed as standardpractice. The combination of the techniques now make it sure that if a dam wall doestend to leak and allow heavy seepage the one place where it is least likely to occuris in the immediate vicinity of the lockpipe equipment. There have been no failureswith 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, inmy opinion, universally successful.
Often with little money, the prospector in the worst of conditionsemploys successfully the cheapest methods of water control, and such methods arenot always known even to the mining profession generally. On one occasion I inspectedan alleged alluvial gold mine which was owned by three prospectors who had very littlemoney but they did have a large flow of water from their own water source, namelya dam made of logs. The dam site was such that most engineers would have consideredthat 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 servedboth as a jet to break down the face of the alluvial deposit, and as a jet elevatorto carry the "wash" to the gold boxes for the recovery of the gold. Thepipeline carrying the water under pressure to the mine was made up of pipes not allof the one size, and they were coupled together by every imaginable means. Therewere plugs and patches in the holes in the old pipes and water was spraying fromthe line. In spite of the responsibility that the men felt towards each other tomake a success of the undertaking, there just was not enough gold to keep it goingfor long. The whole point of this story is that wealthy farmers in the area weresuffering from an acute drought and water shortage while the prospectors had a waterrace flowing a large volume of water which as an agricultural asset was almost beyondprice.
This practice of water control and transport, almost as oldas time, has an agricultural significance on the individual farm or grazing propertywhich 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 wastageis 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 inthis book, and the very low-cost irrigation that is thus available, is so valuablethat in my own circumstances it would still be worthwhile building our present storagesif we had good rivers instead of dry creeks in our lower areas. But the farm damin Keyline is a very different matter to the usual conception of a dam, and so ageneral 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 asan 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. Thewater of the dam is held above the land adjacently below. An earth tank, on the otherhand, is an excavated water-holding structure which holds its water in the excavatedhole and below the level of all the immediately surrounding land. The tank is generallya 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 wateroutlet 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 ofthe dam is behind the wall or the side away from the water. The inside of the damis the water side. Parts of the wall are its top or crest, which is the roadway alongthe 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 particularslope of the wall is described in figures such as 1 : 2, which mean that a verticalfall of 1 occurs in a horizontal distance of 2. The wall has a foundation or foundationarea which is the bottom of the wall shape on the prepared earth of the valley belowit, but the bottom of the dam itself is the land of the site which is or will bebelow the water level of the dam when it is filled with water. In preparing a sitefor a wall, the foundation area of the wall has two trenches, both usually of thesame width, called first the cut-off trench and second the lockpipe trench. The cut-offtrench runs the full length of the wall from end to end and in the finished dam isusually 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 rightangles to the cut-off trench. The lockpipe trench then runs in the same directionas 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 theminimum height along the wall about the level of the bottom of the spillway. Thespillway is usually on the down land side of a keyline dam and overflow or floodwater leaves the dam via the spillway.
The natural catchment area of a keyline dam is usually restrictedso this natural catchment area is augmented by a water conservation drain or feederdrain which falls to the spillway of the dam from the upland direction (see ChapterVI ). The water conservation drain actually is located for the greater part ofits length in the immediate valley area near the dam, above the water level of thedam. Falling at a grade right around the dam, it reaches the top water level whenit meets the spillway. It should be particularly noted that the water conservationdrain 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 thisimportant feature of our dams. The reason for this and other design features willbe discussed later in this chapter.
The two diagrams illustrate the various features of the keylinedam. Both diagrams are in the plan, one showing a completed structure and the othershowing only the "site-and-wall-foundation-area-preparations" at the timewhen construction of the wall begins. (See Fig. 10.)
These same names and features apply generally to most valley-typefarm dams except that lower valley dams, and on occasions reservoirs, may not havewater conservation drains (see Chapter VII).
A notable difference between the dams of Keyline and the usualfarm dam is that in Keyline the whole of the dam is designed and constructed andnot just the wall. The bottom of the dam, from which the wall material is generallytaken, is finished off in such a way that a natural valley shape remains. The largerthe farm dam the more important this may be, since the area of the bottom of thedam when the dam is emptied could be used as a very valuable special crop area.
Apart from Keyline construction, there are two other types offarm dams being made these days; one is the more or less haphazard type of structureconstructed by bulldozer operators and which is lacking in design and for the mostpart poorly located, badly constructed and often unsightly. The second type of farmdam is that built by perhaps the same means but from a design supplied by water authorityengineers. The design is too often a scaled-down model of the "big" earthwall. For the most part it is not followed in construction details because it islargely 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: Thestructures necessary for the walls of farm irrigation dams should not be a scaled-downmodel of anything. As I have said earlier, farm irrigation dams are completely specialiseddams 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 thedecomposition, etc., of rocks) is nearly perfect for farm dam construction. Theseresidual earths, which are characteristic of most dam sites with their decomposedrock 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 ona large creek or river several miles or even hundreds of miles from the headwatervalleys of that river. The foundation materials below the wall may be layers andlayers of sand and clay and decomposed rock and all its advantages and disadvantageshidden and often not capable of being inspected thoroughly even during the work.The wall material available may form a continuous construction problem and a hugevolume of water often has to be brought under control before real work starts. Theremay be difficulties in procuring the large volume of materials of uniform type. Greatexpense could be incurred on site examination. The problems of river control, materialsand foundations may be such that complete design and plans cannot be produced beforethe work starts and must be finally determined only by constant study of the behaviourof 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 amountof water conserved is so large that if released suddenly it would cause enormoushavoc. Many "big" dams have failed in the past, with loss of life and greatproperty damage. On the other hand, the farm irrigation dam, by comparison, conservesmanageable amounts of water; a farm dam conserving 400 acre feet of water would beconsidered a very large farm irrigation dam, but a "big" dam more thanone thousand times that size conserving 500,000 acre feet of water is consideredof moderate size.
For these reasons the design and construction features thatare necessary in the "big" dam to secure adequate margins of safety bearlittle relationship to those involved in the farm irrigation dam. The same high factorsof safety can be achieved in the farm irrigation dam without many of the expensivefeatures of design and construction that are necessary in the "big" dam.
The farm irrigation dam is a specialised dam, and although thedesign 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 considerationgiven in the design to the type of equipment that will build the dam.
A "big" dam may require very flat batters of morethan I in 3 to satisfy safety requirements, but on occasions farm dams carrying 16feet of water may be just as permanent and safe with batters of 1 in 1-1/2. Farmdams with batters as steep as this, with walls formed of a mixture of shale and theclay of its own decomposition, have remained quite stable, but whether it be batters1 in 1-1/2 or 1 in 3 entirely depends on the foundation sites, the wall materialand 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 ofthe world, and there are bound to be many failures of farm dams before all problemsare overcome. However, the consequences of failure in the two types of dams are notcomparable. If a dam of the design under discussion does fail, its repair will usuallybe a simple matter. If the farmer is on hand to open the lockpipe valve, a threatenedfailure 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 getthe whole endangered population out of the way in time.
In this connection, then, the farm irrigation dam in a Keylinedevelopment project has a great advantage over the "big" dam.
The majority of these farm irrigation dams will be constructedin or near the primary and secondary valleys, which, unless they are eroded, flowwater over a rounded grassed bottom. Many are built in the solid land before thecountry breaks. This break of the land is seen in any watercourse, where stream waterflows 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 waterconservation drains having an even grade, and is directed into the dam from the drainover the most suitable and stable area. The only silt, as can be seen in the colourof the water, that is likely to reach the dam would enter it during the first flowsof water to the new dam. In a short time these drains and raw earth, which are alwaysunder the control of the farmer, become stabilised and grassed and will flow clearwater. After this has taken place the greatest accumulation of silt likely wouldbe 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 carryvarying amounts of silt to the dam. Immediately the fast-moving water, with its siltload, reaches the still water of the dam its velocity drops and it deposits its loadinto the dam. This is a continuing process and often an accelerating one. Then thereis the ground load made up of material that the flowing water moves along the bottomof the stream but does not actually carry. This type of sediment, while it increasesin floods, is continuously on the move, even in clear flowing water. Particles insize from sand up to large boulders are rolled along the river and creek bottomsby stream action.
Since siltation is almost negligible in a farm dam, there isno reason why a good farm irrigation dam managed by generations of good farmers ona good farm should not last longer than the average life of the "big" dams.
The control that a farmer can exercise over all the factorsthat influence the permanent usefulness and good appearance of his farm irrigationdams 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 preservingthe life and usefulness of the "big" dam would occur if all the farmersin its hundreds of square miles of catchment area were all looking after their ownproperty and their farm irrigation dams. There is no force comparable to the farmersand graziers as custodian of land and water.
The essential design features that are to be providedfor in the farm irrigation dams are listed below:
The design should also provide that the top width of the wallcrest is wide enough to allow farm equipment to travel safely so that the dam canbe always properly maintained. A good minimum workable width is 10 feet.
Site Selection: In selecting the site for a dam for irrigationpurposes, full consideration should be given to the ultimate development of the propertyfrom the point of view of complete water conservation. The water from the first damshould provide profitable irrigation against its capital cost. The ultimate planthen should envisage a co-ordinated plan of dam lay-out and siting that will providestorage capacity for every drop of water that flows on the farm, and, because ofthis, the siting of the first dam becomes a very important task indeed.
In planning the full layout of dams for complete water conservationit is not by any means necessary that the lowest or the highest or any particularsite should be used first, but it is important to consider the general location ofall the dams so that the dam constructed first will fit in perfectly with the fullplan if and when it is developed. For instance, there may be three excellent Keylinedam sites contained in a series of five primary valleys which flow into the samelarger valley. There may be also two good reservoir sites and one lower valley damsite. Any one of these sites can be used first, provided it is located in such away that when the other dams are built, it will receive their overflow water or alternatelydischarge 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 withan adjacent area that is suitably developed or can be quickly developed for irrigationfrom this first selected site.
In this, the general order on undulating country, there arefour 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 conservingwater before it finally leaves the property.
The reservoir is an "intermediate" type of dam betweenthese two. If there is not a suitable site below the highest keyline dam, then thehighest other suitable site is selected and where it will receive the overflow fromthe high dams.
The first dam can quickly prove its effectiveness and profitpotential and enable the landman to determine in the shortest possible time thathe will institute a full programme aimed at conserving all the water that now goesto waste from his property.
It is usually assumed that dam site selection is a relativelyeasy matter and can be done by eye. While this is sometimes true, it is not necessarilyalways 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 floorslope; (4) suitability of earths for construction; and (5) suitable adjacent areafor irrigation land.
It is very difficult to obtain an adequate appreciation of theshape of a farming property from the point of view of complete water conservationby simply studying it by eye. A good contour map is of outstanding value by enablinga quick study of all the land forms and suitable dam sites. Good contour maps offarming properties are unfortunately as rare as they are valuable.
An alternative and completely satisfactory means of determiningland shape for farm water conservation can be obtained by laying in the keylinesof the property. As discussed in earlier chapters, a keyline should be laid in fromthe 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 thereand so on for each valley in turn. These lines immediately disclose the relationshipin height between all the suitable high conservation sites so that they can be arrangedin such a way that no water leaves the high country until all the proposed high damsare filled. Suitable reservoir sites that would catch most of the overflow waterfrom the high series can also be determined. Lower valley sites are usually obviousto the eye.
When investigating alternative sites for a first dam, the valleyfloor slope is checked with a levelling instrument over the proposed site of thedam and then compared with the valley floor slope of alternative sites. Other thingsbeing equal, the flatter valley floor slope is preferable. For instance, the conservationof water in a valley with a valley floor slope of 1 in 12 will cost more per unitof 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 doneby choosing a contour to represent the water level of the dam and marking it on theground with such an implement as a lister attached on a chisel plow. By standingon the marked contour line on one side of the valley and looking across at the contourline on the opposite side, a mental picture of the dam when filled with water maybe visualised.
If the keylines have already been marked in (rising into countryas they do in undulating land on which water is to be conserved), a satisfactorypicture of the dam can be obtained by standing at various points along the keylineon the downland side of the valley and imagine the wall in place from that pointacross to the keyline on the opposite side of the valley. The keyline then will bejust above the actual water level of the dam.
In marking out a selected dam site the depth of water is firstdetermined. Where the contour shape of the valley is suitable, it is suggested thatgreater depths of water be considered than at present used on farm dams. A 20-footdepth of water is suitable if the shape is suitable and generally provides a worthwhilequantity of water for irrigation purposes. Where the site is large and greater storageis desired, it should be borne in mind that a depth of water of 24 feet as againsta depth of 20 feet will in many circumstances double the cost of the wall. It shouldalso be understood that in considering a dam of this size, the extra four feet ofwater may double the conservation capacity of the dam, and for this reason the largerundertaking may become well worthwhile.
The Design of the Dam: Before a good farm dam can bebuilt it needs to be designed. To assist in the construction of our own dams andto help some of our friends and clients, we have produced an information sheet fromwhich, 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 supervisethe construction of a farm dam from our plans and instructions and produce a gooddam that will be valuable and permanent. After the experience of having built a damto the designs I have prepared, the same man then can usually design as well as builda second dam. (See Fig. 11. )
The information sheet referred to sets out what is requiredand defines precisely what is meant by such things as (1) depth of dam; (2) lengthof dam; (3) width of dam; (4) under wall valley floor slope; (5) excavation areavalley floor slope; (6) total valley floor slope; (7) contour shape of dam; (8) naturalcatchment area; (9) induced catchment area; and asks for details of each and forrainfall information, earth type, etc. From this information the value of the siteis 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 supplythe information.
There is such a wide field to the study, design and use of farmdams that a large volume would be needed to cover the subject fully, and so the objecthere is to present the simple fundamentals of the design and construction methodsof 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 proficiencyin the man as well as produce a good dam. Therefore, the first diagram shows thevarious batters or slopes for a selection of cross sectional shapes through the wallof a dam. The horizontal and vertical scale are the same, and so anyone using thesheet can soon visualise just what the various batters look like. These diagramsare marked A in Fig. 12.
The second diagram, B, shows a section to scale down the valleyfloor, representing the relationships between keypoint, valley floor slope, waterlevel and cross sections of the excavation area and of the wall of the dam. The thirddiagram, 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 planand lockpipe and irrigation drain (Fig. 13).
Then D is a section across the valley looking upstream frombehind the wall, showing again the relationships of the water conservation drainto the spillway and the water level to the freeboard and settlement allowance. Eis a plan of the site preparation and markingout of the dam, illustrating similarlayout 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 layoutshould be clearly understood and work on the dam can proceed. There are many damsto be built on farms that need them and which can very profitably employ them; thereforethe diagrams are directed towards making all those who have constructed one dam proficientand capable of designing many more dams. There are also included in the design sheettwo insets. Inset (b) illustrates the precise relationship between the valley floorslope under the wall of the dam and the location of the lockpipe to these two andto the cut-off trench. Inset (d) shows the detail of the relationship between thecentre or low point of the valley and the lockpipe and outlet positions.
Marking Out: In marking out the site of a dam some particularpoint must be selected as the main control point and to which all other constructionfeatures relate.
In a keyline dam the keypoint of the valley and the water conservationdrain 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 waterlevel 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 ofthe 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, thenalong this drain line where the line is 20 feet vertically above the valley bottomopposite lies the point of the spillway overflow, and the low point in the valleycan be selected as the main control point. In practice the approximate length ofthe 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 positionof the keypoint and is done in the following manner. Assume this part of the lengthof the drain to be 200 yards (from the keypoint to the spillway) and the slope ofthe drain 0.5 feet per cent. (6 inches fall in each 100 feet), then the spillwayheight at the point of overflow will be three feet lower than the peg at the keypointof 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 valley23 feet below the keypoint. This point, as well as serving as the main control duringthe building of the dam, also represents the level at the bottom of the lockpipeoutlet behind the wall. It is, as well, on the same level as the bottom of the lockpipeon the water side of the dam, since the Iockpipe lies on a horizontal line, as isseen in the diagrams. The point in the centre of the valley 23 feet below the keypointof the valley is now the main control point for the marking out and constructionof the dam. The main control point should now be marked with a small solid peg drivenwell into the earth and a longer peg, such as a steel post, driven into the groundalongside. Next, a point the same height and 50 feet down on one side of the valleyis marked with a similar peg, and a flag or marker is placed on it. This marker isaway 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 thediagrams.
The full marking out of the dam is not completed for the siteclearing 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 (10feet clear above this line is suitable) and down valley about 30 feet minimum fromthe 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 feetplus a freeboard of three feet and plus the shrinkage allowance. This is assumedto be 8-1/2%, or one inch for each foot of height, which is 23 inches, making thetotal 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 crestwidth will be 12 feet, a width I often use for a wall of this height. In order towork out the dimensions of the wall the calculations are made against its heightas planned, after shrinkage and settlement has taken place. The dimensions are asshown in the diagram. The maximum width of the wall shape is 104 feet. However, thiswidth does not represent the width of the placed wall, but the width of the wallshape after the dam is completed, when it then includes a section of the originalvalley floor below the wall. This feature of the dam promotes a new efficiency indesign affecting aspects of control, safety, efficient use of water and economy ofconstruction.
The marking out and site preparation for a farm dam should befrom plans and specifications already prepared. However, to illustrate the detailsof the design features the marking out will proceed from considerations on the siteof this proposed dam. The dimensions of the centre cross section of the wall of thedam are known. From the YY or main control peg another peg is placed in the centreof the valley and well upstream from the control peg to clearly show the line ofthe valley itself. The centre line of the long section of the wall is to fall atright 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 indicatingthe valley centre line. From this centre wall peg and at right angles to the centrevalley line is the actual centre line of the wall. Pegs are placed on this line toindicate the two ends of the wall on the sides of the valley, and two other pegs50 feet up the side of the valley beyond the end of the wall. These last two pegson the line of P pegs outside the work area should be marked clearly and so thatthey are not disturbed or lost in the work.
From the centre line of the wall further pegs are now placedon the back toe of the wall line Y pegs, so that this wall line can be determinedand clearly marked. Points marked on the back toe of the wall for each 5-foot verticalrise 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 followingmanner: There is a vertical rise from the YY peg of five feet, therefore the settledwall height at that point is 23 less 5, which leaves 18 feet. As the batter is 1in 2, this figure is multiplied by two, and half the width of the crest (which hasbeen determined at 12 feet), six feet added, so the first Y peg on each side of theYY peg will be 42 feet from the centre line of the wall (P line) at the 5-foot verticallyhigher point.
The next 5-foot vertical rise peg worked out in a similar mannerwill be 32 feet and the next is 22 feet, and the final distance of the fourth ofthese intermediate wall toe pegs will be 12 feet. (These various wall sections areshown on the diagram by the dotted lines.) (See Fig. 15.)
For the inside toe of the wall the Y pegs' distance from thecentre wall line are not calculated in this manner. It will be seen from the diagramthat 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 centreline to the back of the wall-toe pegs. For designs in the field the simplest mannerfor determining their position is with the aid of ordinary squared paper, which inour diagram shows that the Y pegs for the centre section of the wall are eight feetcloser to the centre line than the rear Y pegs. This distance therefore in the centreof the wall is 44 feet, the various distances from the centre line to the insideY 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 becompletely uniform. It is, therefore, advisable to make the Z pegs, indicating thefront toe of the finished wall shape, the same distance from the centre line as therear toe Y pegs. Steel fence posts make very suitable markers. From the P Pegs, oneach side of the wall, the width of the cut-off trench (10 feet) is marked and theexcavation area of the trench is defined by two or three pegs, as illustrated inthe diagram.
The design and marking out of the dam will be left now for amoment while other matters are considered.
Study of Materials Available: As already mentioned, thematerials available on dam sites are usually good. Practically any residual soilon farms in undulating country will make dam walls.
The requirements of a farm dam, as far as its construction isconcerned, is simply that its works and is permanent. No amount of laboratory testingand classifications of earth for a farm irrigation dam wall will give as much practicalinformation to the farmer as he can get by visiting a number of earth dams in hisown area (even if they are small) which have been constructed with the same typeof 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 levelledoff by the bulldozer travelling along the progressive top of the wan. The materialshould be travelled over each time as the bulldozer places it, and, together withfrequent smoothing off, a uniform texture is produced throughout the whole of thewall material.
The compaction of the wall of a farm dam is often unduly stressedand is frequently misunderstood. Some wall materials for farm dams do need the maximumof compaction, and where this applies implements such as the sheepsfoot roller orthe multi-tyres pneumatic roller can be used. However, most earths used in farm damconstruction do not require this costly process of complete compaction.
Some farm dams have been constructed very badly in that theyare just mere masses of loose earth, untrimmed, unpacked, and with no uniformityof texture whatever, yet on settlement and shrinkage they develop into imperviousand quite stable structures. However, such methods are inadvisable because dams sobuilt often fail at the first rain. In areas where poorly constructed dams do holdwell, no special compaction as such need be aimed at, providing that the suggestedprecautions are taken to ensure the uniformity of texture of the material as it islaid down in the wall. There are (as well as a general suitability of most materialson farms for dam construction) some areas where the problem of dam construction hasnot 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 nowthat the value of adequate farm water supplies is being better appreciated. Whileit has been my experience that the best dam building materials on a farm or grazingproperty are to be found at the site of the keyline dams, it is advisable to checkthe materials available. If the proposed dam is to be the first on the area, thedam should be dimensioned in a design and then an examination of the materials shouldbe made to a depth of two feet below the deepest excavation area indicated in thedesign. 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 halfan inch diameter three inches long and can then be bent without breaking into a curvenearly a half circle, cohesion is satisfactory, and so generally is the water-holdingcapacity. If this earth is very fine with no sand or larger particles it may lackstability, and so wall batters would be designed flatter than in circumstances ofgood stability. Very fine and uniform particle clays and very fine silt are the lessstable earths. Silts generally also lack cohesion, so silt earth walls generallyneed 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 necessaryto fill all the interstitial spaces between the coarser particles, water-holdingcapacity will be suitable, but where this condition is not met the earth will nothold water well and often not at all. In these cases special sealer blankets areemployed. Generally they are of two types; selected clay is used to cover the wholeof the inside of the dam and inside wall batter with up to two feet thickness ofclay or, alternatively, bitumen emulsion products incorporated with the earths ofthe structure itself form the blanket. Of other methods sometimes suggested I havefound none yet that are sufficiently effective and economically priced.
Most residual granite earths and the decomposed granite rocksbelow them form very satisfactory walls for farm irrigation dams on a batter of 1in 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 andmica that forms the clay fraction of such material and require flatter batters inorder to hold water satisfactorily. The less stable materials always require battersthat are less steep and perhaps greater supervision in their construction than thegeneral material available on farms.
Shrinkage allowance in dam building simply means constructinga wall to such finished height and shape that shrinkage and settlement will reduceit to the planned or designed height and shape. The same relative shrinkage is assumedto 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 constructedon 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 waterlevel when the dam is filled to the point of overflowing. A freeboard of three feetis suitable on most farm dams.
A spillway is the channel prepared for the purpose ofdisposing of excess water from a dam when the dam is overflowing. Its size dependson the natural catchment area of the dam and on the run-off intensity. A spillwayneeds to be wide enough so that the maximum storm run-off likely when the dam isfilled 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 sideof the dam and it is constructed with the bulldozer which builds the dam. From thepoint of the spillway at maximum water level of the dam the spillway is given a dropof 0.5% and the spillway section carried far enough around the land so that overflowwater from the spillway cannot flow on to the back toe of the wall.
In Keyline layout, the section of the spillway at a suitabledistance from the dam usually changes to the smaller section of a water conservationdrain which carries the normal overflow from the dam and the normal catchment abovethe water conservation drain into another dam. The main requirements of the spillwayare that it carries all the overflow water from the dam and if it does overflow thecountry it does so at a place where water cannot reach the back toe of the wall orcause loss of soil anywhere.
It is not necessary that the whole of the water from heavy floodrains flowing out of the spillway should be transported by the water conservationdrain to the next dam. Provision is made so that it can overflow at a suitable place.
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To return to the construction of our keyline dam-the topsoilis now removed. The soil from the wall site is pushed down valley to a point 15 feetbelow the Y pegs by removing about three to four inches of soil and working at exactright angles to the centre line of the wall. (See Fig. 14. )
The topsoil from the marked excavation area is moved upstream15 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 thewall, and the soil previously pushed up valley from the excavation area will be pusheddown over the excavation area, after it has been plowed with a chisel plow, and upover 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 liningup the peg position with some other mark. He replaces the peg in its original positionwhen the bulldozer has finished for the time being.
A main control peg (YY) has been placed at the back or downstreamside on the toe of the wall and a peg 50 feet away and exactly level with this pointhas been fixed. Two pegs have been placed on each side of the valley from and inline with the P peg, so that if the P pegs are lost these offsets will allow thecentre line of the wall to be relocated. Another control peg 50 feet from the P pegsand 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 exercisedto see that these pegs are not disturbed.
With the removal of the topsoil from the wall site and excavationarea site, further earth is moved from the wall site if it still contains unsuitablematerial which can be used to form the line of the pegged toe of the wall representedby the Y pegs. Care is taken to maintain the pegs in position, and the material ispushed to the line of pegs and not beyond them.
Progressively more of this material is placed on the line fromthe sides of the valley towards the centre of the valley, but no more is placed atthis stage than is necessary to remove the unsuitable earth.
Cut-off Trench: At this point, the centre line of thewall P pegs are checked, also the Z pegs, and the bulldozer starts the cut-off trenchby pushing straight down the centre line (P pegs) from the sides of the valley. Theearth 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, thecut-off trench need only be eight inches to one foot deep on the valley sides butincreasing in depth towards the bottom of the valley, where it will be the same depthas 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 in1.
Lockpipe Trench: The lockpipe trench is located at rightangles to the cut-off trench and parallel to the valley centre line on the downlandside of the valley.
From the level of the low spot of the valley at the toe of thewall (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 thevalley. 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 fromthe Y line at the back of the wall and constructed under the wall site right throughto the inside of the wall, and must be dead level (a horizontal line). The levelof the trench throughout is the same as that of the low point of the valley on theYY peg. The lockpipe trench will then be one foot into the solid ground on the Yline at the back of the wall, but, according to the valley floor slope, may be fourto seven or eight feet deep at the inflow end of the pipe on the water side of thewall. In this dam we are describing it is five feet deep. A right-angle cross isthus formed at the junction of the lockpipe trench and cut-off trench and at thispoint both trenches are the same depth. The level along the lockpipe trench is checkedduring the excavation as the tractor works, until it reaches the correct depth throughout.
The construction of the pipeline trench is best cut by the bulldozerworking first along the length of the trench to clearly locate it and then backwardsand forwards at right angles across the trench.
Filling the Cut-off Trench: With the completion of thelockpipe trench and the cut-off trench, a chisel plow is brought in to cultivateto about four inches deep the bottom of the two trenches and the whole of the areaof the wall site, particularly from the cut-off trench to the Z pegs, working parallelto the centre line of the wall. The roughing-up of this material, which is now theprepared foundation of the wall, aids the better bonding of the wall into the site.
The bulldozer then places good uniform material in the cut-offtrench, material free of stumps, sticks, rubbish or organic matter, which is obtainedfrom the excavation area but never from within the Z pegs defining the upstream edgeof the under-wall site.
Laying Lockpipe: The length of lockpipe necessary forthe dam is decided according to the maximum width of the wall plus an extra six feetof length, so that each end of the lockpipe protrudes three feet from the wall shapeon each side. There is now a total of 110 feet. The lengths of the individual pipesmaking up the full lockpipe may not exactly equal the lockpipe length required, soon occasions a slightly greater total length may have to be used. This dam has evenbatters, i.e., the same batters on both the front and back of the wall, so the centrepoint of the lockpipe trench is selected from the centre line P pegs and checkedby measuring a distance from this centre point half the total length of the lockpipeto the dam side of the wall. The number of baffle plates to be used and their distanceapart depend on the designed depth of water for the dam. Their distance apart fromthe inside toe is determined as approximately one-third the depth of water and sevenfeet apart in this case.
The bottom half of each baffle plate is now placed in its correctposition according to the measurement from the centre point of the wall site. Thelower baffle plate of each pair is let into the ground or hammered in so that thehalf 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 fromthe reference point level already established, the YY peg, and the bottom of thelower half circle of each baffle is fixed at this level.
If both long and shorter lengths of lockpipe are used, the shorterlengths can be placed at the outlet end of the lockpipe and the long lengths at theinlet end. The pipes can be dragged to the side of the trench with a small tractoror carried by any other means and rolled into the trench. All the pipes that willform the full length of lockpipe should be lined up in their correct position besidethe standing baffle plates before any are lifted into position on the cradle formedby the half baffle plates. (See pictorial section ).
"U" section rubber is used between baffle plates andpipe as an anti-seep and these gaskets are next fitted onto the lower half of thebaffle plates and the pipes lifted carefully into place. Care should be taken tosee that the holes in the flanges of the pipe are in such a position that when thevalve is coupled, it will be sitting upright. One hole should not be positioned atthe centre top; two top holes should be at the same height. The flange end of thepipes are inspected and cleaned and one rubber pipe gasket inserted between the flangejunction and the bolts and nuts placed in and firmed up. It is better to place alllockpipe sections, gaskets and bolts and nuts in position before tightening any ofthe bolts. When they are in position, all nuts should be tightened uniformly aroundthe pipe. Each nut should be tightened a little in turn right round the pipe twoor three times. The final turn must be very tight and the pipe gasket quite clearlyshow a squeezed effect.
It is suggested that the farmer who is supervising the workshould check the tightness at each junction of the full lockpipe. One loose jointcould endanger the stability of the dam.
The top half of each baffle plate is now coupled with each U-shapedgasket around the pipe and the two sections of all the baffle plates bolted firmlytogether. The volume strainer, which fits on the inside end of the lockpipe or waterside of the wall, and the valve which fits on the outlet or downstream side of thewall, 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 ofthe lockpipe trench. The earth that goes beneath the lockpipe is to be compactedby hand work particularly from the inflow end to the centre line of the wall. Firstthe bulldozer travels parallel along each side of the lockpipe trench in turn, pushinga load of earth so that a sufficient quantity spills into the trench to be hand rammedbeneath the pipe, but not sufficient to cover the pipe to any extent.
An anti-seep material is now placed. We use an inert, lightweightmaterial ranging in size from very fine particles up to about the size of large sandgrains, which is made by heat expanding a special volcanic rock. Its weight is aboutone-tenth the weight of sand. At each flanged junction of the pipe and each baffleplate from the inflow end of the pipe to the centre line of the wall a mixture ofthis material and the wall material is placed, so that if water does move, the lightweightanti-seep material will move with it to form a seal around it and so automaticallyseal a leak. The mixture is to contain about 20% by bulk of antiseep material andthe balance, wall material, mixed and placed at the flange junctions and baffle platespoints during the filling-in of the earth around the lockpipe.
Commencing from the inlet end of the pipe, two men with crowbarsram the earth, which has been brought in by the bulldozer and spread by hand shovelsbeneath the pipe. The first ramming with a crowbar should be from an oblique angleon each side of the pipe, so that the material is firmed well beneath the pipe. Onefirm stroke with the ramming end of the crowbar every two inches along the pipe isusually 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. Anothercareful row of ramming on the same lines as the first and from the inside end tothe centre with less emphasis on the downstream half of the lockpipe will ensuresuitable compaction below the pipe. A little work with a shovel and further generalramming around the area of the pipe will complete the hand work necessary. Looseearth in the wall of a dam may settle and consolidate, but not so the earth belowthe lockpipe, since there is no weight from the earth in the wall above. Hence thenecessity for this procedure.
The bulldozer is then brought in as before to place materialin the lockpipe trench from each side and just sufficient to cover the whole pipelinebut leaving the top edge of the baffle plates showing so that they form a guide forthe next bulldozer operation. The top edge of the baffle plates is left exposed toview, and the earth on each side of the lockpipe should be of sufficient height tocarry the bulldozer, which next straddles the baffle plates and lockpipe from endto end. In this operation the bulldozer is carefully signalled forward so that eachtrack straddles the lockpipe. Care is necessary to see that at no time can the undercarriageor sump of the tractor come in contact with the top of the baffle plates, and thereforesufficient earth on each side of the lockpipe is necessary to keep the tractor clearof the plates. One run up and back along the full length of the lockpipe is all thatis required with the bulldozer straddling the lockpipe. The bulldozer next standsoff at right angles to the lockpipe and positioned in such a way that each individualbaffle plate will be straddled. The operator is now signalled forward with a loadof earth, which he drops progressively in the lockpipe trench and over the baffleplate and pipeline (covering the pipe with at least 15 inches of earth), and continuesthe tractor movement forward until the front of the two tracks of the tractor havejust 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, andso on until this operation is complete. The bulldozer can then travel up and downeach side of the lockpipe trench (but not straddling the lockpipe), to produce somecompaction and uniformity of texture in the rest of the trench. The vibration ofthe tractor aids this work.
Next, a mound with a minimum of two feet six inches higher thanthe top of the baffle plates is pushed in over the lockpipe at right angles to thelockpipe.
The fill material in the wall on either side of the lockpipeis brought up to the height of the earth on the lockpipe and the whole area is travelledby the bulldozer from now on in the general course of the construction of the wall.
Immediately the operation of placing the lockpipe and fillingthe lockpipe trench is completed, markers are placed at each end of the line, sothat it will not be lost by the bulldozer covering the ends with earth in the courseof the dam construction. A 44-gallon drum placed at each end of the lockpipe withtwo steel posts driven in the earth on each side of each drum are excellent markersfor the purpose.
Maintenance of Wall Shape: Throughout the whole of theconstruction of the wall of the dam the downstream batter of the wall should be maintainedat its finished batter and line (Y pegs). The back batter is 1 in 2 and should bekept at this batter whether the wall is only five feet high or ten feet high; thisbatter should be maintained throughout the work.
The inside batter of the wall during construction is not treatedin this manner. It starts off as a very flat batter, gradually increasing in steepnessuntil it finally reaches the correct batter on the completion of the wall.
As the cut-off trench is filled with good material, this materialis spread and levelled off by the action of the bulldozer travelling occasionallyalong the length of the cut-off trench. Once the trench is filled, material is takenfrom the excavation area just beyond the site of the Z pegs and spread across thewall and travelling towards the back line of the wall which was already marked withearth placed prior to the completion of the excavation of the two trenches.
Supervision is necessary to see that the bulldozer operatordoes not dig earth from below the site of the wall, i.e., within the boundary markedby the Z pegs. This is of particular importance in overcoming one of the generalfaults in dam construction. It is to be remembered that an irrigation dam will sometimesbe filled with water and sometimes empty. The period of greatest stability for theinside of the wall is during the time when the dam is completely filled. The waterhelps to hold the inside of the wall stable. Its period of greatest instability occurswhen the water is drawn from the dam and the dam becomes empty. Inside slumping andslipping of the earth of the wall towards the bottom of the dam is the manifestationof this instability. If earth is removed from inside the Z pegs, i.e., from belowthe inside toe of the wall (a universal fault in farm dam construction) during theearly 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 occursat the most vulnerable inside point of the wall. If, however, the shape of the landbelow 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 surfaceand the wall is improved at what is usually a point of weakness. (See Fig. 14.)
This feature of my design is of relatively greater importancein all valley dams, as the valley floor slope is steeper. Generally the orthodoxfarm dam design provides a flatter batter for the inside of the wall and a steeperback batter, and this arrangement acts as a compensation against inside slumping.However, the Keyline design adds stability and allows the construction of the insidewall batter to be as steep as the rear batter. As well as reducing yardage, it simplifiesboth design and construction. The effect is lessened as the the valley floor slopeis 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 equallygood supervision.
Methods of Bulldozer Operation: Many operators can drivea 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, goodsupervision and the techniques which follow reduce earth-moving costs by a largepercentage. In the construction of farm dams we are concerned with placing the rightmaterial in position as quickly and cheaply as possible, and every technique thataids this end is worthy of consideration. First of all, the speed of the engine ofa 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 windrowsin bulldozer operation are the parallel banks of earth left on each side of the bulldozerblade as it moves the earth forward. The action of pushing a bulldozer load forwardoccasions continuous spill of material from each side of the blade. With a largeload 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 ina much smaller load at the end of the distance; earth will have been lost in spillwhich forms windrows. But windrows can be used as an aid to shifting earth fasterand cheaper.
The bulldozer should start its run by grabbing a big load asquickly as possible, and in low gear if necessary, pushing the load in a straightline and at right angles towards the wall. As soon as the load starts to reduce byspilling in the formation of windrows, then it stops and backs up and grabs anotherfull load. This load is pushed forward towards the wall, forming a larger and longerwindrow until such time as the load is reduced below the full load capacity of theblade when it rips back and grabs another full load. (Modern bulldozers can be equippedwith "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. Theoperation of the bulldozer is now confined between these windrows pushing up fromsix to twelve full-capacity blade loads to the fill site. Grabbing the load simplymeans loading the bulldozer blade to maximum capacity as quickly as possible andusing low gear if necessary, so that the rest of the trip to transport the full loadinto the new wall can be travelled in a faster gear. If the full load is not obtainedin the first pass over, say, a 20 to 25-feet run, the operator rips back and grabsa load again until the full blade load is obtained. After six to twelve full passeshave been made in this pair of windrows the tractor is moved to form a new path andnew windrows, with one windrow of the new path partially formed by the windrow ofone side of the first pass. This windrow site is worked as before. Windrows may beapproximately three feet high.
The windrows should be formed at the centre of the excavatingsite and moving out first on one side and then the other, so that after all windrowshave been formed and used there is a series of parallel equi-distance lines of windrowslying at right angles to the wall of the dam.
The next operation involves the destruction of all the windrowsof the first series of passes. The bulldozer commences the second series by travellingwith one of the old windrows in the centre of the blade pushing a maximum load ofthe old windrow material forward at right angles to the wall. This pass will formnew windrows very rapidly and the 'dozer continues operations in this newly formedpair of windrows for a suitable number of passes, generally from six to twelve full-bladeloads. The next movement of the bulldozer pushes the next windrow of the first seriesout on either side of the new pass and continues the operation until all the oldwindrows are dozed out, new windrows have been formed and the requisite number offull loads taken between them. This type of operation is followed throughout thewhole of the construction of the dam.
Systematic working and following these procedures will oftenshift 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 andbulldozer operator, is much more costly. It is therefore very important that thefull 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 withless than its full load. A bulldozer travelling onto the wall site with half a bladeof earth is shifting earth expensively.
Supervision in the construction of a dam is not providedby a man merely watching the bulldozer work. I have seen men set to work as supervisorsstand idly by and watch while a bulldozer operator did almost everything wrong, addeda hundred or so pounds to a job of modest size, and finish up with a blot on thelandscape 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 firstwhat he wants, and as most people have not seen a good farm irrigation dam, thenhe should really have a plan. The plan should be first studied so that the farmermay be convinced of the logic and necessity of every detail of the design, of themethods of work, of the construction details, and of the final finish and the useof the dam. He then should see that the operator follows his instructions. A farmermay be somewhat reluctant to instruct a bulldozer operator of wide experience, andthink that he should not be told or should not be stopped when he is not performingthe operations according to plan. However, a bulldozer operator, in almost all othercircumstances outside farm work, does work to a plan and under a supervisor, becausesuch 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 andconstruct a good dam from the seat of his tractor as he goes along. Also, the bulldozeroperator is only on the farm for a short while, but the farmer has to live with thiswork, whether it is good or bad, for very many years. He should, therefore, get thedam that he wants and which he will only get if the manner of his supervision carriesout effectively the design of the dam. It is also a good idea for the farmer whenhis first dam is finished to again study his plan perhaps quite critically, and maybetalk to someone about it. As long as the person to whom he talks is interested, thetalking will probably line up his own ideas and plans, and he may even find thathe can greatly improve the manner of the building of his next dam.
Travelling Forward Over all Earth as Placed: A bulldozeris not a completely efficient compacting machine. A farmer contemplating the purchaseof a heavy crawler tractor for the purposes of farm work will be assured by the tractorsalesman that the track-type tractor places less weight on his soil than the smallestwheel-type tractor. He may offer to place an egg a few inches below the surface andshow that this will not be broken by the weight of the tractor on its tracks unlessa growser bar on the track plates happens to hit it. Nevertheless, the travellingof the bulldozer over the earth as placed does give a measure of consolidation andstability to the loose earth and can produce a very desirable uniformity of texturein 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 crackingof the wall will be lessened. During the construction of a dam, continuous wavesof 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 loadand as the operator raises the blade. This fill material must be continuously smoothedoff and shaped up.
Levelling-off at the End of Each Day's Work: The constructionof 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 excavationmaterial to be taken from above the level of the lockpipe, as in the keyline damof this design, care should be taken to see that the bulldozers do not dig belowthis level. Those spots above the level of the lockpipe which would trap rain waterare 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 workfor the day, the windrows left in the excavation area from the day's operation shouldbe pushed into the wall area and trimmed. The whole area, including the wall, thenwill carry the minimum of loose earth and provide good drainage from the work tothe lockpipe. Preparation made at the end of each day's work should provide againstthe possibility of damage by heavy rain falling during night time. Some inches ofrain 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. Trimmedearth will absorb the minimum amount of rainfall water and then the rest of the waterwill be shed off. The lockpipe should also be maintained in an open, free and operatingcondition at all stages of the construction, and should be checked each day beforework ceases.
Rain: If rain has fallen on the work area the whole ofthe 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 becomewell mixed with the smaller amount of wet material, so that a uniformity of moisturecontent and texture is still maintained. In many types of earth work, moisture conditionsare maintained within precise limits, but generally the only occasions that may betroublesome in the construction of a farm dam are when the earth is very dry anddoes not bond properly, or is in an overwet condition, causing clays to ball-up andleave 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 inthe early stages of the work. After the task of laying, tamping and filling the lockpipetrench is completed, the formation of the wall should continue with the bank at thehighest level over the central low area of the valley bottom. From this stage onwardright through to the time when finished levels are being considered, the centralarea of the wall should be maintained as the highest point, with a slope of about10% (1 in 10) along the length of the wall from the centre to each side. When thiswall line is maintained during construction the low portion of the wall is alwayswell away from the area where the maximum earth has been deposited. In a valley ofuneven section, i.e., where the one bank of the valley at the wall is steeper thanthe other, then the lowest point of the rising wall at any time will be the end ofthe wall on the flatter side of the valley and at the greatest distance from themain earth fill. Water would flow over the wall here if the partially built dam wereflooded. The low spot acts as a safety fuse (the name for a weak or low spot), thusprotecting the main fill area of the earth wall.
In a sudden heavy downpour the lockpipe would also be flowinga full bore of water and as soon as heavy rain ceased would quickly reduce the waterlevel to below the overflow height and then empty the dam. Damage would be of theminimum, even though no expensive safety precautions had been employed.
Dams with only small natural catchments such as this keylinedam, though, would hardly be affected by heavy rain. The lockpipe provides full safetyand water would only rise a few feet of depth for a short time. This safety factoris aided by the fact that the water conservation drain, which will later help tofill in dam, is not constructed until the wall is completed, so that run-off is restrictedto the small natural catchment. It can, however, be even further restricted, if needbe, by constructing that part of the water conservation drain only that is aroundthe dam. Provision is then made for the disposal of the small natural catchment run-offthrough 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 finishedheight, but this is bad practice, since it tends to work against uniformity of textureand proper bonding of the earths in the wall. Furthermore, areas of compacted stabilisedearth are likely to be placed adjacent to areas of very loose earth. Shrinkage laterwould form large cracks between the different textured materials. There is a tendencyalso for bulldozer operators to push all the earth of the blade-load into the walland leave it as a loose mound. This is avoided by the operators starting to liftthe 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 wallsite, 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 restof the wall, which, by increasing its weight, could cause sliding and slumping ofthe rear of the wall. The back batter of the wall should be maintained throughoutby 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 alwaysbe a straight line. A concave line encourages slumping of the high point on the edgeof the crest of the wall, and once this has started the extra weight on the materialbelow causes a movement of earth which in the worst circumstances could result ina later partial failure of the wall.
Throughout the building of the dam the marking pegs should bemaintained in their proper position by lifting them out of the bulldozer path whenit is necessary for it to travel there and by the farmer stepping three or four pacesout and lining up the position of the peg between himself and another peg, so thatafter the work in that area has for the time been completed then the old peg canbe put back in its correct position.
The spillway of a dam, like all other features of goodfarm irrigation dam construction, has to be right for the dam to be a good one. Forrelatively small dams, the wall can be constructed to the completed height, whichincludes 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 constructedby pushing through the placed earth at one end of the wall down to solid ground atthe top water level contour and maintaining the flat bottom of the spillway on theside of the valley away from the water line. By constructing the spillway throughthe finished bank a considerable quantity of extra earth is available to strengthenthe spillway at its critical point, i.e., where the bank of the spillway mergeswith the wall of the dam. On other occasions, the construction of the spillway mayproduce a surplus of earth, which is then used in the building of that end of thewall.
The batter between the bottom level of the spillway and thewall of the dam is about 1 in 4, so that in the grassing of the dam site, wall andexcavation area, convenient travel with cultivating equipment is possible. A sectionof the spillway therefore will show a batter slope of 1 in 4 falling from the endof the dam wall to a dead level spillway bottom of a given width, with a similarbatter rising from the spillway bottom to the rising land on the high side of thespillway and away from the wall of the dam.
Shrinkage allowance, freeboard height and spillway size thereforeprovide than on the completion of the dam and its subsequent settlement, there willbe three feet of wall everywhere above top water level at the point where water commencesto flow out of the dam and through the spillway. The design of the spillway is suchthat the type of flood likely to occur any time in 50 years would be by-passed, withthe spillway carrying little more than one foot of water across its full width, andwhen this happened there would still be a further freeboard of two feet to compensateas a safety measure for bigger floods and to provide for wave action erosion duringhigh flood.
Larger spillways are necessary in farm dams such as reservoirsor lower valley dams, since they have considerably more catchment area than thiskeyline dam. To secure the necessary width of spillway with a dead flat floor, considerablematerial 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, thatis, earth greatly in excess of the needs of the spillway bank. The construction ofthe wall of such a dam, then is designed so that the earth of the spillway is usedin the construction of that part of the wall adjacent to the spillway. It is advisableto construct the spillway before the earth in the centre of the wall approaches itsfinished height, and when there is still plenty of wall area unfilled and availablefor the use of the spillway material. On occasions where the spillway of a dam involvesvery considerable earth moving, the construction of the spillway may be completedby placing the excavated material into the wall site immediately the site preparationis completed.
In spillway construction earth has to be excavated down to aspecific level and supervision should ensure that the earth is not excavated toodeeply, necessitating the filling of areas of the spillway with earth that wouldnot be in as good condition as the stable undisturbed material.
Final Batters: With the earth for the wall being constantlymoved in properly designed windrows at right angles to the wall, the site is in acondition to be examined continuously in order to ensure that the cheapest diggingearths go into the wall. As the work proceeds, areas of material somewhat harderthan that of the general digging conditions may be encountered. These areas are thenstudied to determine whether cheaper earth can be obtained by going back another15 feet or 20 feet for earth or whether cheaper earth is obtained by perseveringwith the cutting and digging of the harder materials. Outcropping hard rock may beencountered, and it should not exceed 30% of the earth in any blade load. Bulldozersoperating with back rippers are capable of making fairly light work of reasonablytough materials, and once the job is well opened up, continuous consideration shouldbe given to the work to. ensure that the cheapest material is being used throughout.
The cost of moving a particular quantity of earth is relatedto the distance it has to be moved, so length of movement of earth should be consideredas the work proceeds.
Finishing Stages: With the back wall batters strictlymaintained on a straight line and at the correct slope, the front wall will graduallysteepen from very flat batters towards the final finished angle. Centre line pegslined up with the P pegs on either bank of the valley should be placed at intervalsduring the final stages of construction, and measurements should be taken and markedwith temporary pegs to show the finished width as well as the final height of thewall. A continuous check with a level and measurements ensures that the right amountof material is in the right place and excavated from the correct position in theexcavation 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 andtop width of the remainder of the wall is maintained towards the sides. Final battersshould be even and continuous throughout, with straight lines up the wall. A slightlysteeper slope in part of the wall always has a tendency to be more unstable thanthe rest of the wall. Slight movement of earth may take place, the parting on eitherside of the movement being represented by a steep crack, which tends to allow movementof the flatter battered earths on each side of the steep area and thereby assistingthe movement of the steep material and progressively worsening. Such movements tendto reach a point where they stabilise themselves, although not necessarily so.
Finishing Off: When the material has all been placedand the wall trimmed to its proper top width, height and batters , the final finishing-offcommences. The windrows left in the bottom of the cut may be flattened out,- butit 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 shapeto conform to the valley area of the whole dam. When this is finished, the raw earthis cultivated with a chisel plow and then covered with the soil, which was previouslymoved from the surface of the excavation area. Some of the soil should be used alsoto cover the batter on the inside of the wall. The soil which was stripped from thewall area and pushed behind the wall is now brought up over the wall to cover theback or downstream side of the wall and the top of the wall, where an inch or twoof soil cover is all that is required.
If two tractors were used to build the wall and a big rope orcable of the type used for the roping down of trees is available, a good finish tothe work can be obtained by the two tractors hauling the rope across the inside andthen across the outside of the wall. In this operation one tractor with a cable attachedtravels along the top of the wall and the other tractor travels along the bottomof 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 overthe top and side of the wall smoothes the work. The top of the wall should be finishedwith slightly rounded top edges. If it is finished off haphazardly with a bulldozerthere is likely to be a small windrow effect left by the blade. When rain falls thiscould cause little ponding areas, which eventually break in one particular spot andflow down the wall in a concentrated stream. Further rain falling on the wall takesthe same path, and sufficient erosion could take place with an inch or two of rainto 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-runcultivation about three inches deep. The cultivation parallels the water level contourdownwards (keyline cultivation), so that flow water later spreads as it flows intothe empty dam. Next, the wall and the whole of the site is sown with the regularpasture seed mixture and combined with a dressing of fertiliser.
Hand finishing of the top of the wall to leave a good shapeand to aid the germination and growth of the grasses is well worthwhile. Slight ridgescan be raked out and slightly rounded edges left on the top edges of the wall.
Finishing Off Lockpipe: The volume strainer is coupledup to the cleaned surface of the flange of the pipe on the inside of the wall witha rubber gasket between and with the straight edge of the volume strainer downwards.
The inclined section of the lockpipe is made to point upwardsin the upstream direction. The upward tilt acts as an additional safety to preservea free and full flow of water in case there is a slight slip of wall material throughany cause. The volume strainer should be coupled up tightly.
Our strainer has an opening nine times the lockpipe size, andscreened by heavy mesh. The rate of flow of water into the volume strainer when thelockpipe valve is open is therefore very much slower than the rate of flow throughthe lockpipe, so that any matter which can enter the strainer will flow out throughthe 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 holesare level on top of the flanges of the lockpipe, the valve will fit in an uprightposition.
Our own valves are provided with a 2-inch constant-flow outleton the water side of the valve closure, so that water is always available for suchitems of smaller supply as stock troughs, etc.
The building of the dam is not completed until the working drainsare provided. These are discussed in the following chapter.