Earth dams, a study

by Burr Bassell

The San Leandro Dam, California.

A section of the San Leandro Dam, near Oakland, Cal., is shown by [Fig. 31]. This section was supplied by Mr. W. F. Boardman, hydraulic engineer, who superintended the construction of the dam, from his own private notes and data. It differs materially from sections heretofore published, and is 5 ft. higher, thus making it rank as the highest earth dam in the world of which we have an authentic record.

The dam was commenced in 1874, and brought up to a height of 115 ft. above the bed of the creek in 1898. At the present time it is 500 ft. in length on the crest and 28 ft. wide. The original width of the ravine at the base of the dam was 66 ft. The present width of base from toe to toe of slopes is 1,700 ft. The height of embankment above the original surface is 125 ft., with a puddle trench extending 30 ft. below.

FIG. 30.–CROSS-SECTION OF UPPER PECOS RIVER DAM;
COMBINED ROCK FILL AND EARTH.

FIG. 31.–DEVELOPED SECTION OF SAN LEANDRO DAM.

All that portion of the dam within a slope of 2½ on 1 at the rear and 3 on 1 at the face is built of choice material, carefully selected and put in with great care. The portion outside of the 2½ on 1 slope line at the down-stream side of the dam, was sluice in from the adjacent hills regardless of its character, and is composed of ordinary soil containing more or less rock.

This process of sluicing was carried on during the rainy season, when there was an abundance of water, and it was intended to be continued until the canyon below the dam had been filled to an average slope of 6.7 on 1 at the rear of the dam. It was thought that the location was particularly favorable for this kind of construction, the original intention being to raise the dam from time to time, not only to increase the storage as the demand for water increased, but to meet the annual loss in capacity caused by the silting up of the reservoir basin. The latter has amounted to about 1 ft. in depth per annum.

METHOD OF CONSTRUCTION.–Under the main body of the dam, the surface was stripped of all sediment, sand, gravel and vegetable matter. Choice material, carefully selected, was then brought by carts and wagons and evenly distributed over the surface in layers about 1 ft. or less in thickness. This was sprinkled with just enough water to make it pack well, not enough to make it like mud. During construction a band of horses was led by a boy on horseback over the entire work, to compact the materials and assist in making the dam one homogeneous mass. No rollers were used on this dam.

The central trench was cut 30 ft. below the original bed of the creek. In the bottom of this trench three secondary trenches, 3 ft. wide by 3 ft. deep, were made and filled with concrete. These concrete walls were carried up 2 ft. above the general floor of the trench, to break the continuity of its surface.

The original wasteway, constructed at the north end of the dam, has been practically abandoned, having been substituted by a tunnel of larger capacity. The original wasteway was excavated in the bed rock of the natural hillside, and although lined with masonry, is not in the best condition. The author considers its location an objectionable feature, as menacing the safety of the dam, and thinks it should be permanently closed.

A wasteway tunnel, 1,487 ft. in length, was constructed in 1888, through a ridge extending north of the dam. This has a sectional area of about 10×10 ft., lined with brick masonry throughout, having a grade of 2½%.

The criticism might be made of the tunnel that it is faulty in design at the entry or reservoir end, where the water must first fall over a high spillway wall, aerating the water before entering the tunnel proper. The water even then has not easy access to the tunnel, and no adequate arrangements have been made for ventilation, so as to insure the utilization of its maximum capacity. The maximum depth of water in the reservoir is about 85 ft., and the full capacity 689,000,000 cu. ft. of water. The catchment area is 43 square miles, and the surface of the reservoir when full 436 acres. The outlet pipes are placed in two tunnels at different elevations through the ridge north of the dam. There are no culverts or pipes extending through the body of the dam itself.