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. [Illustration: 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.