High Earthen Dams for Storage Reservoirs.
At the request of our worthy secretary, Peter Milne, I have gathered together some of my fragmentary notes bearing upon this subject and submit the same, with the view of contributing something to our budget of experience in this particular line oi hydraulic engineering.
The facts hereon stated refer more particularly to dams built at and near the city of San Francisco, Cal. These dams are interesting in that they are built much higher than is generally regarded safe construction. Some of them have their crests no feet above the bed of the valley, and their puddle walls often extend down to ninety-eight feet below the bed of the valley. The depth of water in the reservoirs next the dams is very frequently 90 to 100 feet. If we consider the material used in their construction it is quite remarkable that no serious accidents have so far occurred which would tend to shake one’s confidence in their stability and usefulness.
The following is a list of the principal high earthen dams, together with their chief dimensions :
In looking over the table you will notice the great height given to more modern designs for these storage dams. A notable exception is dam No. 3, fifty feet high only. This dam was intended to be eighty-two feet high, but the puddle trench was continued down to a depth of ninety-eight feet below the bed of the valley and then failed to reach the bed rock sought for. In going down with this trench the excavation passed through no less than twenty-three separate and distinct old creek beds full of gravel and water. This gave rise to alarm as to the safety of the foundation and the proposed height was reduced to fifty feet and it was so built. Some little seepage water has appeared below the dam, but nothing to cause apprehension.
PLAN OF CONSTRUCTION.
All these dams were built alike, practically, and the following is a full and sufficient statement of the operation. A trench should be dug along the centre line of dam down to and into bed-rock. The trench not less that twenty feet, and for big dams, fifty feet wide ; and at the same time the bottom of the valley and slopes on each side to be covered by the dam. should be cleared of any roots, stumps, loose rocks, loose material and loam, and cut down to a solid ledge or good foundation stratum, and the dirt and material so excavated from the trench should, as a rule, be wasted and not used in the construction of the dam.
Next, the wide slopes of the valley should be benched so as to make a good footing for the flanks of the dam to rest on. This work being done we are now ready for work of filling in the puddle trench along the centre line of dam. and across entire valley and into bed-rock, with the best of still clay (blue) material. The filling should go on slowly and successively in thin layers not to exceed three inches thick ; the clay should be put on carefully, slightly wetted, spaded up and rammed down, so as to form a homogeneous mass, so that the connection between clay and bed-rock is complete and the trench is thus entirely filled with this impervious mass, say, fifty feet thick. The bottom of the puddle trench where it rests on bed-rock should have, moreover, longitudinal grooves, so as to let the clay form a tongue and groove joint on the bedrock. Make one or more grooves. Some imes a concrete joggle is substituted for this tongue and groove work. Now when this puddle has reached up to or a little above the level of the natural bed ot the valley, then the filling of the main embankment can go on. Particular care should be given to the selection of the material which is filled on the water side of the puddle pit; but yet the portion of the dam on the down stream side of the puddle pit should also be put in with great care. Where good clay is handy we can build almost the entire dam of clay.
Before the filling of this main embankment is commenced, all the loose material and gravel in the creek bed has to be removed, to whatever depth it may go, from the site of the dam, so that there is no possibility of any water seeping along underneath the tilling and thus reach the face of the puddle pit.
The main embankment is made as follows : The clay is put into the embankment in layers, not toexceed eight inches beginning at the upstream and down stream toe of the slopes, and pitching slightly toward the centre of the dam, say, with a five per cent slope, so that when the embankment has been filled above the puddle pit, a* well an built below on the downstream side, the top ol embankment, at this stage of operations, represents a large flat dish. This 1* done to invite settlement toward the centre of the dam and tends to solidify. The clay is carted on the embankment, dumped in the proper place, spread out by shovel* to about a layer eight inches thick, the surface of the previous layer is watered, and the surface of the new dump is sprinkled also. In this manner the material is filled successively and spread over the entire layer. We take particular care that the carts (loaded) travel over the newly finished layer so as to still more consolidate it. Finally a roller of 6000 pounds i* drawn over the new layer, *0 that by the combined mean* of sprinkling the layer, wetting it, having the loaded carts run over the new layer, and finally by rolling it down hard we reduce the thickness to five or six inches, making a compact layer with the lower one. In this manner the layers are put on successively, both on up and down stream sides of dam. until the two inside toes of the filling come nearer and nearer the puddle trench. At this point, when they come within ten to fifteen feet of the edge of this puddle trench, the most extreme care is used.by selecting the choicest of the clay. The layers are now reduced from eight inches down to two, three and four inches only, very seldom exceeding three inches in thickness. This choice clay is spread over and across the puddle trench. The layers that have meanwhile been raised up to the surface or bed of the valley and to the top of the puddle pit, are wetted, rammed thoroughly, beaten down and spaded together with the previous layer, so that it is one homogeneous mass of impervious clay. In this manner the layers are put on ending on each side with a tapered down layer over the puddle pit, and so on. until finally the top of the dam is reached and the work is completed.
The water slopes of these dams are generally protected against wave action by a heavy laye»of broken stone 2.5 feet thick, clear up from the bottom to the top. so as to prevent any wash.
A large waste way is always built through the natural ground on one flank of the dam and in no way connected with it. Its capacity is made three times the maximum flow on record from the watershed.
* Paper read by L. J. Le Conte, C. E., before the Milwaukee convention of the American Water-works Association.
The supply pipe generally passes through a tunnel in the best natural ground at either flank of the dam. The tunnel is lined with brickwork. At the inlet a gate house is located, having the customary iron and brass regulating gates, etc.
Experience teaches us that high earthen dams should be built to full height at once, because it is not safe to build a low dam and afterward conclude to raise it and have to splice on the foundation. We can’t splice on to water side, after the lake is filled, hence we have to make our enlargements on the dry side, and we will have to add material on the down stream side of the puddle pit, simply to add weight to it. For example, suppose we build a ninety-foot dam, and in the course of two or three years or more we conclude to raise it to 130 or 140 feet; we would add 400 feet to the base of the dam on the down stream side, which would allow us to raise the dam forty to fifty feet higher. Experience tells us that the following results would take place; The old dam having completed its settlement would be solid, meanwhile the new addition has been put on and settling begins and continues more or less for, say, two, three or four years; the new fill would separate from the old work, forming a crack along the junction, likewise the capping at the crest would also crack in two for similar reasons. The raising of the crest of dams is, therefore, a dangerous proceeding, and is always attended with risks which a cautious engineer should try to avoid.
The great success of these dams, sustaining the pressure due to 90 to 100 feet of water for so many years, I ascribe to the care exercised in the selection of the materials and the close attention given to every detail of construction. This is, moreover, the true secret of success in nearly every engineering undertaking.
By reference to the table you will see that the designs for the more recent dams estimate on heights of 170 and 128 feet above the bed of the valley respectively. Work was begun on the foundations of the former by sinking shafts and running tunnels, when it was discovered that the entire base of the proposed high earthen dam was underlaid by a good quality of solid stone, and at moderate depths below the surface. After further prospecting ami testing the results obtained were so assuring that the plans of construction were changed, and a massive concrete dam was built instead at little additional first cost.
The contract for the construction of the San Pablo dam has been let, and the contractors have begun preliminary work.
COST OF HIGH EARTHEN DAMS.
The cost of building dams as above described is quite a different figure from that incurred in making an ordinary embankment. The cost of main embankment averages sixty-five per cent per cubic yard, while the clay filling in the puddle pit averages ninety per cent per cubic yard.
The cost of grubbing, excavating and removing the top soil and excavating the puddle trench, are items which vary much, depending upon the character of the ground.
I append an itemized estimate of the cost of the proposed 170 foot dam, which 1 think will be of interest ; also a sketch showing the characteristic section of a California high earthen dam:
ONE HUNDRED SEVENTY-ROOT EARTHEN DAM.
Making a grand average cost for tne embankment complete $0.72 per cubic yard.