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

50 ENGINEERING PROBLEMS OF PANAMA CANAL. 3. Amount and character of the ground water. 4. Earth tremors sot up by earthquakes, blasts, passage of railroad trains, or other causes. 5. Other factors, as (a) heavy structures contiguous to excava- tions, (6) water freezing in rock openings and wedging off rock masses, (c) variation of barometric pressure, and (<Z) earth strains from the kneading of tidal pull. INFLUENCE OF CRUSHING AND TENSILE STRENGTH OF THE ROCKS INVOLVED. If a rock has high crushing strength, with few joint or other part- ing planes, the exposed face will stand almost perpendicular, without shearing or slipping, at any height with which, we need to have con- cern. If it has high crushing strength in small solid fragments, but is much cut by joints, faults, or bedding planes it will not deform or flow but will slough off masses from steep places until a certain angle, “I i i t i 8 I I I -X ....... —------1 ..-r----1----1----<----1----1----1----r- 0 too ZOO 300 400 SOO 600 700 BOO 900 Figure 7.—Ideal cross section to illustrate excavation deformations, a, Width of zone of deformation, measured from toe of slope; &, original surface; c, surface after deformation; d, bottom of excavation before deformation caused it to bulge upward; e, toe of slope, intersection of slope and bottom planes; f, crack, or break, caused by deformation of basal part of the block shown with perpendicular broken lines. The curved lines from f, with arrow, show approximately the chief zone of deformation and the direction of the motion. which might bo called the angle of repose, is attained. Fissures, faults, or bedding planes that incline toward the excavation and intersect its slopes are planes of weakness and promoters of slides, especially if the bedding planes have shale, lignite, or other greasy rock partings, or if the fault planes arc filled with clay or slippery talcose material. Even if such partings are horizontal, relatively light back pressure may push the material that rests on them out into the excavation. Beds of dense, relatively impermeable, greasy clay which slope toward an excavation are especially likely to allow any material that may rest on them to slip off their moist inclined surface. If a rock has low crushing strength, movement may manifest itself as a deformation or sinking near the excavation, with a slight ad- vance of the lower slope toward the excavation, and a bulging of the bottom (see dotted line, fig. 7). Similar movements have occurred in the Culebra Cut. In most instances, however, such a movement