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

SLIDES. 59 places these limits may not be reached. On the whole, however, it is believed that these lines represent pretty closely the area of ground that is likely to be disturbed. This area may be narrowed down somewhat by the relieving excavation work which may be completed, say, before the close of the next dry season. It will be noted that these lines converge on the cut about station 1760, and are comparatively restricted for the next 1,000 feet northward. This is due to the strengthening effects of the sandy limestone and limy sandstone beds that occur toward the surface there. The western slope in the vicinity of 1740 to 1745 will probably be disturbed to a rather wide extent, but the sliding there will be somewhat superficial, and large movements of the break type should not occur. These superficial slides are due largely to the presence of the lens of greenish, sandy clay rock shown on Plate XV. The upper part of this becomes muddy, and any material that may overlie it easily slips into the cut. On both sides of the canal in the vicinity of stations 1737 and 1747 faults have dis- turbed the strata. These faults, however, are mostly at right angles to the cut, and will therefore be much smaller factors in producing slides than would be the case if they cut across with diagonal trend. The slides, then, along near the above-named stations give no evidence of assuming very large proportions- Northward of these stations in the region mapped the sliding problem grows less until the Empire breccia is reached. This breccia as a whole will not slide, though local masses of rock may fall from its steep slopes. In conclusion, it may be said that the crack that developed under and disturbed the Club House, Culebra, was due to renewed movement along an old minor fault plane. All the ground under the Club House may slide, but the northwest corner of the building is not more than, say, 100 feet from ground that will notslide. . . Summary.—In summary, maximum sliding in Culebra Cut will occur and is occur- ring on the west slope about opposite the Isthmian Canal Commission Chapel, because the already weak rocks there have been greatly weakened by faults which trend diag- onally across the cut. Relief excavation work vigorously pushed ahead in thia vicinity will greatly ameliorate conditions. PROBABLE LIMITS OF SLIDING GROUND. In spite of much subsequent deepening of Culebra Cut with some flattening of its slopes, the “probable limits of sliding ground” shown on Plate XV (made in 1911) hold good, with some exceptions, at the date of writing, August, 1915. In a memorandum to the Chairman and Chief Engineer of the Isthmian Canal Commission, dated May 10, 1913, the writer predicted a serious slide just north of Gold Hill, the outer limits of which would be at some cracks 1,500 feet back from the center of the canal. This slide will be far out- side the limiting line mentioned above. A somewhat unlooked-for slide on the east side of the cut, between stations 1746 and 1758 in February, 1913, was caused by two large fault zones which were masked by sloughing material so that their dangerous character was hidden until the slide came down. SUMMARY. In summary, then, the structural-break type of slide was due to oversteep slopes in places where the banks were high and the rocks weak. The remedy was to unload the unbalanced pressure and to reduce the slopes. The ends sought to be accomplished by the rem- edy were a reduction of the amount of excavation that otherwise would be necessary and noninterference with tracks, drainage, etc.