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

ROCK MATERIAL USED IN CANAL CONSTRUCTION. 43 by suction dredges, which cut thoir way in from the old French, canal. When the excavation had been completed the dredges were taken out, the channel they had made was dammed, the water pumped out of the excavation, and the foundations laid. Although the foundation rock of the Gatun formation is relatively impervious to tho movements of ground water, still it does not abso- lutely prevent seepage. Therefore, in order to prevent excessive hydrostatic pressure from Gatun Lake on the bottom floor of the lock chambers, the flooring of the upper lock was made strong enough to resist full lake pressure. Behind the remaining lengths of lock walls drainage was provided. The details of how experimental dams built of the material to be used in the completed dam were made and tested, also of the manner of building the Gatun Dam, may be found in the annual reports of the Isthmian Canal Commission, particularly the report for 1908. DESCRIPTION OF GATUN-LOCK FOUNDATIONS. Concerning the character of the lock foundations at Gatun, Howe a writes as follows: Excavation for the locks will be almost entirely in the fine-grained argillaceous sandstone except at the lowest point to be reached at the extreme southern end, where conglomerate occurs beneath the sandstones. The rocks as a whole are well consolidated and make excellent cores with the diamond drill. When taken out and exposed on the surface they remain firm and hard; in rare instances they have been found to crumble and break down into a sandy clay. The rocks are well com- pacted and capable of supporting heavy loads when confined, but being poorly cemented are unable to withstand erosion and should be fully protected where such action is anticipated. On account of the very considerable amount of clay present in all of the rocks it is believed that they will prove to be almost entirely impervious to water. An exception will be found in the conglomerate referred to. Although this rock con- tains considerable clay, it has been found where exposed at about sea level in test pit No. 1 to be water bearing. On April 1, 1907, a pump discharging about 12 gal- lons per minute was just able to hold the water at an approximately constant level in the pit. The surface of the conglomerate exposed in the sides and bottom of the pit was about 240 square feet. Wherever this conglomerate is encountered, it may be expected to be water bearing at all times. In the test pit this rock stands in vertical walls without timbering and as long as confined will support heavy loads. In order to obtain some idea of the compression that might take place under loads equal to or greater than those due to the weight of the lock walls, a freshly exposed surface of the argillaceous sandstone near teat pit No. 1 was leveled and smoothed by means of a file. Upon a square foot of this surface 100 steel rails, representing a weight of 77,350 pounds, were balanced upon the side of a short I beam, to the under- side of which a casting, having a smooth surface 1 square foot in area, was firmly bolted. After a load of 37,128 pounds has been applied a settling of 0.011 foot waa recorded, measurement being made by means of a Y level with reference to a pre- viously determined bench. From this point on, until 48 hours after the full load had been applied, no further compression was noted. It is probable that the set- a Howe, Ernest, Ann. Rept. Isthmian Canal Commission, 1908, pp. 124-129.