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

 46 ENGINEERING PROBLEMS OF PANAMA CANAL. ^<T,„ f horizons lenticular masses of concretionary limestone are common. The physical properties of the rocks differ but little from those of the rocka at Gatun. Being more argillaceous the shales are naturally less permeable to water, and experience in sink- ing test pits showed that they were very tough and required more blasting than did the Gatun rocks under similar conditions. This greater toughness is to be attributed to the argillaceous nature of the rock; for the same reason their compressive strength is probably slightly less than that of the argillaceous sandstone of Gatun. In the vicinity of Pedro Miguel the Culebra beds have a prevailing dip of 5° to 10° to the southeast, varied by sharp local folding; moderate fracturing has accompanied the folding, and slickensides are shown at many points where exposures have been made by recent excavation. At the point where it is proposed to construct the lower gates a dike of augite-andesite or “trap” crosses the lock site in a direction slightly east of north. It has a width of about 30 feet where exposed on the southwest side of the cut, and its contacts with the shales are clean and sharp. On the northeast side of the cut the dike turns to the north and the contact is more irregular, small apophyses having been thrown off into the shales. The Culebra beds, which are here sharply flexed and considerably fractured, owe their deformation to some extent to the intrusion of the dike - the dike itself, however, is also fractured, and slickensided surfaces within its mass as well as in the shales indicate that movement has taken place subsequent to intrusion, accom- panied, it is believed, by minor faulting along the west side of the dike. These structural features have a direct bearing on the character of the foundations. The dike, on account of its fractured condition, which borings have shown to exist below the lowest points to be reached in excavating for foundations, will be permeable to water. I he shales also have been made more or less pervious as a result of their deformation and consequent fracturing. On account of thia fracturing local seepage of surface water is to be anticipated, but the character of the Culebra beds northwest- ward is such that no flow of water is likely to occur through them even when the canal in the summit region is filled with water. Although the general dip of the beds is to the south, borings have shown the rocks to be the clay shales in large part, and as such not water bearing. Furthermore, several intrusive bodies of andesite or basalt cross the canal line between Culebra summit and Pedro Miguel, which will effectually check any flow of underground waters. The difference in hardness between the trap rock of the dike and the much softer shales of the Culebra beds must be taken into account in planning the lock walls The possibility of even very slight differential movement in the zone of fracturing and dislocation in the dike and shales adjacent to it should also be considered. By constructing the lock walls in sections, damage which might be done to monolithic walls as a result of such movements or from the unequal settling of foundations of different hardness, will doubtless be obviated. FOUNDATION OK TUE COROZAL LOCKS AND DAM. The Corozal locks and dam arc built in part on the Caimito for- mation, described on page 25, and in part on what appears to be the upper layers of the Culebra formation. It is somewhat permeable but gives a relatively firm foundation. Of this lock and dam site Howea says: The rocks that underlie the alluvium at depths of from 30 to CO feet below mean tide are sandy shales * * * Rhyolite tuffs are exposed in the hills at the north- ern end of the dam site. ° Howe, Ernest, op. eit., p. 129.