Page:Popular Science Monthly Volume 61.djvu/269

Rh a further examination of the sub-surface material at the site. Consequently the boring parties of the commission sunk a large number of bore holes at six different sections or possible sites along the river in the vicinity of the French location. These borings revealed great irregularity in the character and disposition of the material below the bed and banks of the river. In some places the upper stratum of material was almost clear clay, and in other places clear sand, while all degrees of admixture of clay and sand were also found. At the French site the bedrock at the deepest point was found 143 ft. below sea level, with large masses of pervious and semi-pervious sand, gravel, and mixtures of those materials with clay. Apparently there is a geological valley in the rock along the general course of the Chagres River in this vicinity filled with sand, gravel and clay, irregularly distributed and with all degrees of admixture, large masses in all cases being of open texture and pervious to water. The site adopted by the commission for the purpose of its plans and estimates is located nearly half a mile down the course of the river from that selected by the new Panama Canal Company. The geological valley is nearly 2,000 ft. wide at this location, but the deepest rock disclosed by the borings of the commission is but 128 ft. below sea-level. The actual channel of the river is not more than 150 ft. wide and lies on the extreme easterly side of the valley. The easterly or right bank of the river at this place is clean rock and rises abruptly to an elevation of about 40 ft. above the river surface at ordinary stages. The left or westerly bank of the river is compact clay and sand and rises equally as abruptly as the rocky bank of the other side, and to about the same elevation. From the top of the abrupt sandy clay bank a plateau of rather remarkable uniformity of elevation extends for about 1,200 ft. in a southwesterly direction to the rocky hill in which the Bohio locks would be located. The rock slope on the easterly or northerly bank of the river runs down under the sandy river bed, but at such an inclination that within the limits of the channel the deepest rock is less than 100 ft. below sea-level.

After the completion of all its examinations and after a careful study of the data disclosed by them, the commission deemed it advisable to plan such a dam as would cut off absolutely all possible sub-surface flow or seepage through the sand and gravel below the river surface. It is to be observed that such a sub-surface flow might either disturb the stability of an earth dam or endanger the water supply of the summit level of the canal or both. The plan of dam finally adopted by the commission for the purposes of its estimates is shown by the accompanying plans and sections. A heavy core wall of concrete masonry extends from bed rock across the entire geological valley to the top of the structure, or to an elevation of 100 ft. above sea-level, thus absolutely closing the entire valley against any possible flow. The thickness of this wall