Earth dams, a study

by Burr Bassell

Percolation and Infiltration.

The earlier authorities on the subject of percolation and infiltration of water are somewhat conflicting in their statements, if not confused in their ideas. We are again impressed with the importance of a clearly defined and definite use of terms. The temptation and tendency to use language synonymously is very great, but it is unscientific and must result in confusion of thought. Let it be observed that filtration is the process of mechanically separating and removing the undissolved particles floating in a liquid. That infiltration is the process by which water (or other liquid) enters the interstices of porous material. That percolation is the action of a liquid passing through small interstices; and, finally, that seepage is the amount of fluid which has percolated through porous materials.

Many recent authorities are guilty of confusion in thought or expression, as will appear from the following:

One says, for instance, that a

rock is water-tight when non-absorbent of water, but that a soil is not water-tight unless it will absorb an enormous quantity of water.

This would seem to indicate that super-saturation and not pressure is necessary to increase the water-tightness of earth materials.

Again, in a recent discussion regarding the saturation and percolation of water through the lower half of a reservoir embankment, it was remarked, that

the more compact the material of which the bank is built, the steeper will be the slope of saturation.

Exception was taken to this, and the statement made, that

with compact material, the sectional area of flow is larger below a given level with porous material, and as the bank slope is one determining factor of the line of saturation, this line tends to approach the slope line; while with porous material in a down-stream bank, the slope of saturation is steeper and the area of the flow less.

In reply to this, it was said,

that it is obvious that if the embankment below the core wall is built of material so compact as to be impervious to water, no water passing through the wall will enter it, and the slope of saturation will be vertical. If it be less compact, water will enter more or less according to the head or pressure, and according to its compactness or porosity, producing a slope of saturation whose inclination is dependent on the frictional resistance encountered by the water. And the bank will be tight whenever the slope of saturation remains within the figure of the embankment.

Further,

that it was necessary to distinguish between the slope assumed by water retained in an embankment and that taken by water passing through an embankment made of material too porous to retain it; where the rule is clearly reversed and where the more porous the material the steeper the slope at which water will run through it at a given rate.

These citations are sufficient to emphasize the importance of exact definition of terms and clear statement of principles.

The latest experiments relating to the percolation of water through earth materials and tests determining the stability of soils are those made during the investigations at the New Croton Dam and Jerome Park Reservoir, New York, and those relating to the North Dike of the Wachusett Reservoir, Boston. These are very interesting and instructive, and it is here proposed to discuss the results and conclusions reached in these cases, after some introductory remarks reciting the order of events.

NEW CROTON DAM.–In June, 1901, the Board of Croton Aqueduct Commissioners of New York requested a board of expert engineers, consisting of Messrs. J. J. R. Croes, E. F. Smith and E. Sweet, to examine the plans for the construction of the earth portion of the New Croton Dam, and also the core wall and embankment of the Jerome Park reservoir.

This report was published in full in Engineering News for Nov. 28, 1901. It was followed in subsequent issues of the said journal by supplemental and individual reports from each member of the board of experts, and by articles from Messrs. A. Fteley, who originally designed the works, A. Craven, formerly division engineer on this work, and W. R. Hill, at that time chief engineer of the Croton Aqueduct Commission.

After describing the New Croton Dam, the board of experts preface their remarks on the earth embankment by saying that

it has been abundantly proven that up to a height of 60 or 70 ft. an embankment founded on solid material and constructed of well-selected earth, properly put in place, is fully as durable and safe as a masonry wall and far less costly.

There are, in fact, no less than 22 earth dams in use to-day exceeding 90 ft. in height, and twice that number over 70 ft. in height. Five of the former are in California, and several of these have been in use over 25 years. The writer fails to appreciate the reason for limiting the safe height of earth dams to 60 or 70 ft.

The New Croton Dam was designed as a composite structure of masonry and earth, crossing the Croton Valley at a point three miles from the Hudson River. The earth portion was to join the masonry portion at a point where the latter was 195 ft. high from the bed rock. The Board thought there was no precedent for such a design and no necessity for this form of construction. The point to be considered here was whether a dam like this can be made sufficiently impermeable to water to prevent the outer slope from becoming saturated and thus liable to slide and be washed out.

The design of the embankment portion was similar to all the earth dams of the Croton Valley. In the center is built a wall of rubble masonry, generally founded upon solid rock, and “intended to prevent the free seepage of water, but not heavy enough to act alone as a retaining wall for either water or earth.”

[Fig. 17] shows a section which is typical of most New England earth dams; and [Fig. 18], the sections of two of the Croton Valley dams, New York water supply. These dams all have masonry core walls, illustrating the third type of dams given on [page 33].

FIG. 17.–CROSS-SECTION OF A TYPICAL NEW ENGLAND DAM.

The board of experts made numerous tests by means of borings into the Croton Valley dams to determine the slope of saturation. The hydraulic laboratory of Cornell University also made tests of the permeability of several samples of materials taken from pits. All the materials examined were found to be permeable and when exposed to water to disintegrate and assume a flat slope, the surface of which was described as “slimy.”

Pipe wells were driven at different places into the dams and the line of saturation was determined by noting the elevations at which the water stood in them. In all the dams the entire bank on the water side of the core wall appeared to be completely saturated. Water was also found to be standing in the embankment on the down-stream side of the core wall. The extent of saturation of the outer bank varied greatly, due to the difference in materials, the care taken in building them, and their ages. [Fig. 19] gives the average slopes of saturation as determined by these borings.

The experts stated

that the slope of the surface of the saturation in the bank is determined by the solidity of the embankment: The more compact the material of which the bank is built, the steeper will be the slope of saturation.

As a result of their investigations, the experts were of the opinion that the slope of saturation in the best embankments made of the material found in the Croton Valley is about 35 ft. per 100 ft., and that with materials less carefully selected and placed the slope may be 20 ft. per 100 ft.

Further, that taking the loss of head in passing through the core wall, and the slope assumed by the plane of saturation, the maximum safe height of an earth dam with its top 20 ft. above water level in the reservoir and its outside slope 2 on 1, is 63 to 102.5 ft. This is a remarkable finding in view of the fact that the Titicus Dam, one of the Croton Valley dams examined, has a maximum height above bed rock of 110 ft. and has been in use seven years. This dam is not a fair example to cite in proof of their conclusion, because its effective head is only about 46 ft.[2]

BOG BROOK DAM.

MIDDLE BRANCH DAM.

FIG. 18.–CROSS-SECTION OF TWO CROTON VALLEY DAMS,
SHOWING SATURATION.

Mr. Fteley gave as a reason for the elevation of the water slope found in the outer bank of the Croton dams the fact of their being constructed of fine materials and stated that with comparatively porous materials they would have shown steeper slopes of saturation.

Mr. Craven argued that all dams will absorb more or less water, and that porosity is merely a degree of compactness; that slope implies motion in water, and that there is no absolute retention of water in the outer bank of a dam having its base below the plane indicated by the loss of head in passing through the inner bank and then through a further obstruction of either masonry or puddle; that there is simply a partial retention, with motion through the bank governed by the degree of porosity of the material.

[Fig. 19] is a graphical interpretation of the conclusion reached by the board of experts, as already given on [page 41]. “A” is an ideal profile of a homogeneous dam with the inner slope 3 on 1 and the outer slope 2 on 1. The top width is made 25 ft. for a dam having 90 ft. effective head, the high-water surface in the reservoir being 10 ft. below the crest of the dam. This ideal profile is a fair average of all the earth dams of the world. Not having a core wall to augment the loss of head, it fairly represents what might be expected of such a dam built of Croton Valley material, compacted in the usual way. It should be noted that the intersection of the plane of saturation with the rear slope of the dam at such high elevation as shown indicates an excessive seepage and a dangerously unstable condition.