Some Engineering Problems Of The Panama Canal In Their Relation To Geology And Topography

44 ENGINEERING PROBLEMS OF PANAMA CANAL. tling at 37,128 pounds was due to the pressing out of small inequalities on the pre- pared surface and that no measurable compression of the rock mass took place. The load applied to the square foot was equal to 537 pounds to the square inch, more than double the greatest pressure likely to be brought to bear upon the sandstones by the lock walls. Further tests were made upon small pieces of rock taken from different test pits. None of these pieces was larger than one-eighth of a cubic foot. They were embedded in sand and the pressure applied to smooth surfaces one-half square inch in area. Seven tests were made upon pieces of conglomerate taken from the bottom of test pit No. 1. Some of this material had been exposed on the dump for 20 days. The samples failed at pressures of from 250 to 1,625 pounds per square inch; the mean of all the tests was 867 pounds to the square inch. Nine tests were made of the argillaceous sandstone taken from different test pits at the lock site and the site for the tempo- rary spillway. Some of this material was taken from the walls of the test pits; other pieces had been lying on the dumps for a number of days. The samples failed at from 830 to 2,800 pounds per square inch. Two samples, both of which had been exposed on the dump for several days, showed no signs of failing when under pres- sures of 6,000 pounds to the square inch. Considering these as having failed at 6,000 pounds, the mean crushing load for the nine tests was 2,572 pounds per square inch. INFLUENCE OF THE OLD CHAGRES RIVER CHANNEL, As already explained, the Isthmian land during, say, middle Pleis- tocene time emerged until it reached an elevation of at least 375 feet above present sea level. As the land went up the stream cur- rents were increased so that the streams cut deep narrow gorges for themselves. In this way the Chagres River intrenched itself some 325 feet below present sea level. After tho emergence reached com- pletion a slow sinking began, and the streams, because of diminished velocity, began to fill their channels with silt. In this way the old Pleistocene channel of the Chagres was filled so that its presence was revealed only by borings made across the Chagres Valley at Gatun and at Bohio, where dam and lock sites -were being explored. As to the character of the material which now fills the old channel or Pleis- tocene valley of the Chagres, Howo a says: When the Chagres had succeeded in cutting something more than 300 feet below this level, the land began to sink slowly and the gorges were gradually filled with gravels, sand, silt, and clay. At different horizons in these alluvial deposits are shells and trunks and branches of trees that indicate a gradual rather than a sudden subsidence. * * * * * * If subsidence takes place before the period of canyon cutting has ceased, as in the case of the Chagres, the velocity of streams is checked and they are capable of carrying fine material only, which, is deposited far back from the mouths. In other words, one should expect to find at the bottom of such, a gorge as the Chagres at Gatun a comparatively thin deposit of coarse gravel, sand, and a few bowlders, while the greater part of the alluvium should be of the finest clay and silt with a certain amount of fine gravel and sand mixed with it. Such, in fact, is the character of the alluvium filling the gorge of the ('hagres, as shown by numerous borings made at Gatun. The boring records indicate considerable sand and gravel at points com- paratively near the present surface; it is unlikely, however, that any sand or gravel, a Howe, Ernest, op. cit., p. 127.