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

62 ENGINEERING PROBLEMS OF PANAMA CANAL. the slippery crushed rock of the fault zone and quite separated by this crushed material from the solid rock on the other side of the break. This development, of course, throw an extra strain on the base of the faulted-off mass, and if that failed, the whole block crushed down. The most notable slides of the fault-zone type were the La Pita slide (Pl. XXVII) and the large slide (Pl. XXVI) that occurred about 600 feet north of La Pita between. Empire and Las Cascadas on the east side of the canal. In both cases a brownish, breccia locally covers a dark-gray volcanic agglomerate to a depth of 60 feet, the whole forming a steep bank of 90 feet at La Pita and of over 125 feet in the case of the other slide. In each a fault (Pl. XXV) crosses the cut, making a diagonal angle with the trend of the canal. The differential movement of the rocks along this fault plane had sheared and crushed them. This plane of shearing dips northerly at an angle of about 65°, thus leaving a weakened overhanging mass which, rested inse- curely against the rocks from which, it had been faulted off. A diver- sion ditch, on top of the bank came within 100 feet of the slope here, and the water from this ditch leaked out in places along the breccia- agglomerate contact, but chiefly along the sheared fault zone. In the case of the northmost slide the basal agglomerate contained volcanic mud-lava flows which on cooling assumed columnar jointing (Pl. XXVIII), and these joints afforded channels for the invasion of seep- age and ground water. The water softened the agglomerate, espe- cially down near the foot of the slope, where it was already weakened by the fault zone and under great strain from the overhanging mass. The result was that the basal agglomerate failed and crushed down under the weight of a huge block of the breccia that contained about 20,200 cubic yards in the case of the La Pita slide, and 300,000 cubic yards in the case of the slide 600 feet north of La Pita. Among slides of this type may be classed that which occurred near the division office, Empire. It was due to a fairly large fault which trends across the canal and has considerably sheared the rocks on both sides of the cut. Because of the higher slope more sliding occurred on the west than on the other side (Pl. XXIX). Here the crushed basaltic rock which forms the ridge on which the office stands (build- ing near left side of Pl. XXIX) gradually slid on a steep, slickensided slope of clay and sheared rock, letting about 278,000 cubic yards of material into the cut. The fault-zone type of slide, unlike the others, occurs in rocks strong enough to stand at a steep slope except that large blocks of them are weakened by diagonal canalward sloping faults, which leave over- hanging parts of large rock masses resting insecurely against slippery fault-zone material. Slides of this character were not common. The only remedy for them was to lessen the slopes in the vicinity of the