Earth dams, a study

by Burr Bassell

Jerome Park Reservoir Embankments.

The Jerome Park reservoir is an artificial basin involving the excavation and removal of large quantities of soil, and the erection of long embankments with masonry core walls, partly founded on rock and partly on sand. The plan and specifications call for an embankment 20 ft. wide on top, with both slopes 2 on 1, and provide for lining the inner slope with brick or stone laid in concrete, and for covering the bottom with concrete laid on good earth compacted by rolling.

Section at Sta. 99.

Section at Sta. 76+20.

FIG. 20.–GRAPHICAL EXHIBIT OF STUDIES OF
JEROME PARK RESERVOIR EMBANKMENT.

Wherever bed rock was not considered too deep below the surface the core walls were built upon it. In other places the foundation was placed 8 to 10 ft. below the bottom of the reservoir and rested upon the sand.

It appears that the plans of the Jerome Park embankment were changed from their original design, prior to the report of the board of experts, on account of two alleged defects, namely, “cracks in the core wall” and “foundation of quicksand,” and incidentally on account of the supposed instability of the inner bank.

In describing the materials on which these embankments rest the experts remarked

that all these fine sands are unstable when mechanically agitated in an excess of water, and that they all settle in a firm and compact mass under the water when the agitation ceases. That they are quite unlike the true quicksands whose particles are of impalpable fineness and which are “quick” or unstable under water.

[Fig. 20] is a graphic exhibit of the results of tests made at “Station 76 + 20,” and at “Station 99,” to determine the flow line of water in the sand strata underlying the embankment and bottom of the Jerome Park reservoir.

The experts reported that there was no possible danger of sliding or sloughing of the bank; that the utmost that could be expected would be the percolation of a small amount of water through the embankment and the earth; and that this would be carried off by the sewers in the adjacent avenues; that a large expenditure to prevent such seepage would not be warranted nor advisable.

In concluding their report, however, they recommended changing the inner slope of 2 on 1 to 2½ on 1, and doubling the thickness of the concrete lining at the foot of the slope to preclude all possibility of the sliding or the slipping of the inner bank in case of the water being lowered rapidly in the reservoir.

Mr. W. R. Hill, then chief engineer of the Croton Aqueduct Commission, favored extending the core walls to solid rock. He took exception to the manner of obtaining samples of sand by means of pipe and force-jet of water, claiming that only the coarsest sand was obtained for examination. He did not consider fine sand through which three men could run a ¾-in. rod 19 and 20 ft. to rock without use of a hammer, very stable material upon which to build a wall.

North Dike of the Wachusett Reservoir,
Boston.

The North Dike of the Wachusett Reservoir is another large public work in progress at the present time. It is of somewhat unusual design and the preliminary investigations and experiments which led to its adoption are interesting in the extreme.[3]

The area to be explored in determining the best location for the dike was great, and the preliminary investigations conducted by means of wash drill borings, very extensive. A total of 1,131 borings were made to an average depth of 83 ft., the maximum depth being 286 ft. The materials were classified largely by the appearance of the samples, though chemical and filtration tests were also made. The plane of the ground water was from 35 to 50 ft. below the surface, and the action of the water-jet indicated in a measure the degree of permeability of the strata.

In addition to these tests experimental dikes of different materials, and deposited in different ways, were made in a wooden tank 6 ft. wide, 8 ft. high and 60 ft. long. The stability of soils when in contact with water was experimented with, as shown in [Fig. 21], in the following manner:

An embankment ([Fig. 21]) was constructed in the tank of the material to be experimented with, 2 ft. wide on top, 6 ft. high, with slopes 2 on 1, and water admitted on both sides to a depth of 5 ft. The top was covered with 4-in. planks 2 ft. long and pressure applied by means of two jack screws resting upon a cross beam on top of the planks.

With a pressure of three tons per square foot, the 4-in. planks were forced down into the embankment a little more than 6 ins., resulting in a very slight bulging of the slopes a little below the water level. Immediately under the planks the soil became hard and compact. A man’s weight pushed a sharp steel rod, ¾-in. in diameter, only 6 to 8 ins. into the embankment where the pressure was applied, while outside of this area the rod was easily pushed to the bottom of the tank.

These results corroborate in a general way the practical experience of the author, both in compressed embankments, where he found it necessary to use a pick vigorously to loosen the material of which they were composed, and in embankments made by merely dumping the material from a track, in which case the earth is so slightly compressed that an excavation is easily made with a shovel.

Fig. 21.

Fig. 22.

Fig. 23.–CAN FOR DETERMINING
FRICTIONAL RESISTANCE

Fig. 24.

Fig. 25.

FIGS. 21 TO 24.–EXPERIMENTAL DIKES AND CYLINDER EMPLOYED IN STUDIES FOR THE NORTH DIKE OF THE WACHUSETT RESERVOIR; AND (FIG. 25) CROSS-SECTION OF THE DIKE.

The difference in the coefficient of friction of the same material when dry and when wet greatly modifies the form of slope. The harder and looser the particles, the straighter will be the slope line in excavation and slips. The greater the cohesion of the earth, the more curved will be the slope, assuming a parabolic curve near the top–the true form of equilibrium.

RATE OF FILTRATION.–The rate of filtration through different soils was experimented with by forming a dike in the tank previously mentioned, as shown in [Fig. 22].

The dike was made full 8 ft. high, 7 ft. wide on top, with a slope on the up-stream side of 2 on 1, and on the down-stream side 4 on 1. This gave a base width of 55 ft. Immediately over the top of the dike there was placed 3 ft. of soil to slightly consolidate the top of the bank and permit the filling of the tank to the top without overflowing the dike. The water pressure in different parts of the dike was determined by placing horizontal pipes through the soil crosswise of the tank. These pipes were perforated and covered with wire gauze, being connected to vertical glass tubes at their ends. The end of the slope on the down-stream side terminated in a box having perforated sides and filled with gravel, thus enabling the water to percolate and filter out of the bank without carrying the soil with it.

When the soil was shoveled loosely into the tank, without consolidation of any kind, it settled on becoming saturated and became quite compact. It took five days for the water to appear in the sixth gauge pipe near the lower end of the tank. After the pressure, which was maintained constant, had been on for several weeks, the seepage amounted to one gallon in 22 minutes. When the soil was deposited by shoveling into the water, the seepage amounted to one gallon in 34 minutes.

The relative filtering capacities of soils and sands were thought to be better determined by the use of galvanized iron cylinders of known areas.

[Fig. 23] shows one of the cylinders. These latter experiments confirmed those previously made at Lawrence, by Mr. Allen Hazen, for the Massachusetts State Board of Health. They showed that the loss of head was directly proportioned to the quantity of water filtered and that the quantity filtered will vary as the square of the diameter of the effective size of the grains of the filtering material.[4]

The material classed as “permeable” at the North Dike of the Wachusett Reservoir has an effective diameter of about 0.20 mm. A few results are given in the following table:

Amount of Filtration in Gallons per Day, Through an Area of 10,000 Sq. Ft., With a Loss of Head or Slope of 1 ft. in 10 ft.

	Material.	Unit ratios.	U. S. gallons.
(1)	Soil	1	510
(2)	Very fine sand	14	7,200
(3)	Fine sand	176	90,000
(4)	Medium sand	784	400,000
(5)	Coarse sand	4,353	2,200,000

To be sure that the accumulation of air in the small interstices of the soil was not the cause of the greatly reduced filtration through it, another series of experiments was conducted in the wooden tank, as shown in [Fig. 24].

A pair of screens was placed near each end of the tank, filled with porous material, sand and gravel, and the 50-ft. space between filled with soil. The soil was rammed in 3-in. layers, and special care taken to prevent water from following along the sides and bottom of the tank. One end was filled with water to near the top, while the other end gave a free outlet.

After this experiment had been continued for more than a month, the amount of seepage averaged 1.7 gallons per 24 hours, or about 32 drops per minute.

Filtration tests were also made through soil under 150 ft. head, or 5 lbs. per sq. in., with results not materially different, it is stated, from those already given. The soil used in all these tests contained from 4 to 8% by weight of organic matter. This was burned and similar tests made with the incinerated soil, resulting in an increase of about 20% more seepage water.

PERMANENCE OF SOILS.–This last material experimented with suggests the subject of permanence of soils. This was reported upon separately and independently by Mr. Allen Hazen and Prof. W. O. Crosby. These experts agreed in their conclusion, stating

that the process of oxidation below the line of saturation would be extremely slow, requiring many thousands of years for the complete removal of all the organic matter, and that the tightness of the bank would not be materially affected by any changes which are likely to occur.

It has been remarked,

that of all the materials used in the construction of dams, earth is physically the least destructible of any. The other materials are all subject to more or less disintegration, or change in one form or another, and in earth they reach their ultimate and most lasting form.

In speaking of the North Dike of the Wachusett Reservoir, Mr. Stearns remarked that,

it was evident by the application of Mr. Hazen’s formula for the flow of water through sands and gravels, that the very fine sands found at a considerable depth below the surface would not permit enough water to pass through them if a dike of great width were constructed, to cause a serious loss of water, and it was also found that the soil, which contained not only the fine particles or organic matter, but also a very considerable amount of fine comminuted particles, which the geologist has termed “rock flour,” would be sufficiently impermeable to be used as a substitute for clay puddle.

[Fig. 25] shows the maximum section of the North Dike with its cut-off trench. The quantities and estimated cost of the completed structure are given in the table herewith:

			|––––– Cost –––––|
Work.	Quantities. (cu. yds.)	Unit Price.	Actual.	Per cent. total.
Soil	5,250,000	$0.05	$262,500	34.7
Cut-off trench	542,000	.20	108,400	19.3
Borrowed earth and gravel	200,000	.20	40,000
Slope paving	50,000	2.20	110,000	14.6
Sheet-piling, pumping, etc.			117,000	15.5
Engineering and preliminary investigations			120,000	15.9
Total cost			$757,900	100.0