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

SLIDES. 61 For nearly a year, in 1912, the Cucu- racha slide showed little movement, owing to the fact that it was held back by large dikes of basalt (see Pl. XV and fig. 3). These seemed quite large enough to limit further sliding to a few hundred thousand cubic yards. However, when the cut in front of these sustaining d ikes had been deepened to the final level the earth pressures behind them became so enormous as to shear and crush. the dikes and start almost 2,000,000 cubic yards of material moving. The pressure devel- oped by this slide and the flatness of the slope on which the material moved (see figs. 3 and 9 and Pl. XXII) wore most astonishing. However, in spite of this the dikes would almost certainly have held but for the excessive jointing that had rendered them much weaker than their size indicated. Now that the cut is down to final depth in front of this slide the latter has entered its last stage of activity, and a year or two of dredging should entirely eliminate the menace of this slide. FAULT-ZONE SLIDES. The third type of slide was that occa- sioned primarily by sheared and weakened zones in the rocks, due to fault displace- ments. As the rock masses moved past each other, in the adjustment of earth blocks in late geologic time, the frictional pressure or drag was often great enough to crush, and shear the material foi sev- eral feet on each side of the piano of movement (Pls. XV, XIX, XX, and XXV). Where faults of this kind cut diagonally across the canal largo masses of rock in the acute angle between the fault plane and the plane of the canal Figure 9.—Cross section of slide shown in Plate XXII. slope generally fell out into the excava- tion. Where the fault plane had a considerable dip it leit a mass of rock with a narrow base and wide top, resting insecurely agauis