the dam corresponding to the water in the well and the water in the lake corresponding to the normal groundwater level, and the slope curve passing through the dam.

To prevent loss of water, the materials of which the dam is built must be selected from the available local deposits and placed in such a way as to retard, to the greatest possible extent, the flow of water. In very fine silts, the rate of flow is so minute that they are generally classed as impervious. Capillary attraction is a force which must be considered. It is this which keeps the surface of ordinary ground moist. The evaporation from the surface removes the moisture, but it is promptly replaced by capillary attraction from the ground-water reservoir below. With no rain, the ground water is thus gradually lowered until the capillary forces are no longer sufficient to raise the water from the increased depth to the surface which then becomes dry. This force must also be considered, although to a minor extent, in the design of the dam.

Unfortunately, the ordinary materials which are classed as impervious have a faculty for absorbing water, which softens them and reduces their capacity for self-support. With the height of water furnished by Gatun Lake, there is ample opportunity for the contents of the dam to become saturated, and materials subject to disintegration, or with a tendency to absorb, would settle and not maintain the side slopes. Clay or fine silt is particularly treacherous in its nature; yet it is upon these materials that the imperviousness of Gatun Dam must depend. The solution of this problem is to build the center of the dam of impervious

material and the outer portion on both sides of a material capable of maintaining the predetermined slopes wet or dry, but necessarily allowing water to pass. On the lake side, it must be faced with a lining to resist wave action.

Yet this is not all. The weight of the dam might produce so great a pressure on the original surface of the earth that it would sink and cause the earth to rise just beyond the toe of the dam. This actually happened only a short distance away with embankments for the Panama Railroad. The remedy was to counterweight the rising area of soft material at the toe of the embankment with fill material and thus prevent any further rise. With a structure like the Gatun Dam, settlement of this character would have dislodged the parts of the dam already built, would have created possible fissures and avenues for future flow, and would have aroused the greatest doubt in the minds of the public as to its strength and safety; therefore, the question must be investigated and settled in advance. The rising of the material is prevented by first removing any soft material, and further by making the dam very wide, with a thin extended toe, thus making the counterweight a part of the dam itself. Even with the greatest precautions, a slip in the rock fill due to the giving way of soft material near the old French canal occurred and caused great popular alarm, and led the President to order a board of eminent engineers to Panama. Their report was most reassuring and confirmed, in the main, the judgment of the commission.

Not only the dam itself must be impervious, but also the material upon which it is built must prevent the water from flowing underneath it. To increase imperviousness, the commission drove a line of sheet piling twenty feet into the earth; but on the advice of the special board of engineers this was omitted and, instead, a trench was dug along the middle, which was filled by the core of the dam.

The generous dimensions of the dam, however, principally contribute the imperviousness and stability. As finally built, the crest is 100 feet wide and 20 feet above normal water level; the thickness of the dam at the water surface is 400 feet, and it increases to a thickness of nearly one-half mile at its deepest part.

The dam, after clearing the 573 acres of site, was constructed by first building long mounds at the outer lines of the dam with the proper exterior slope. The material was spoil from the Culebra Cut, the locks and the spillway, and was dumped from trestles. When the mounds were carried to sufficient height, the interior space was filled with silty material from nearby deposits by the hydraulic dredging process. Where the course of the Chagres crosses the dam, two lines of sheet piling were driven, and the material between them, which was not of a suitable character, was excavated and replaced.

The design of the Gatun Dam was not decided on until elaborate tests had been made of the actual seepage through the material to be used in the construction. These seepage tests were made by drilling holes into the deposits that were later to form the core of

FIG. 10.-Gatun Spillway Dam, under construction, showing three permanent crest gates in position, January, 1913. The water is issuing from temporary culverts, which were permanently closed at a later date.