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

SLIDES. 53 to the porous mass by filling its interstices. It will also soften and weaken the porous material and greatly decrease its adhesive grip on the sloping surface of the relatively impervious rock. The water will descend through the porous material, but will be deflected by the relatively impervious rock; and on this impervious surface will be deposited the mud particles and other lubricating material gathered by it in descent. The contact thus becomes a slippery zone that greatly promotes slides. Evon capillary water in a weak rock is a source of danger, especially in fine-grained rocks, for with deforma- tion much of the capillary water may bo forced into shear planes, thus giving them increased lubrication. In estimating the sliding or de- forming tendencies of a rock careful determinations of its water con- tent should be made, both fresh and air-dried samples being used. The most troublesome slides of Culebra Cut were in fine-grained basic volcanic shales and argillites of fairly massive character. These, after having been air-dried, show 6 to 17 per cent of water and con- tain considerable finely divided chlorite, a hydrous mineral. Two samples from the Cucuraclia formation—the formation in which maximum sliding has developed—were analyzed in the United States Geological Survey laboratory. They showed 12.26 and 9.47 per cent of water when heated below 100° C., and 5.41 and 6.71 per cent when heated above 100° C. Any considerable percentage of chlorite particles is an important weakening factor in rocks, and one that renders thorn slippery and unstable. EFFECT OF EARTH TREMORS. Earth tremors cause deforming movomonts in rock, massøs. Many earthquakes cause landslides and rock deformations. Heavy blasts also generate earth waves that loosen and weaken excavation walls and greatly increase their tendency to slide. A blast generates two sets of vibrations, one transmitted through, tlie earth, the other through the atmosphere. The atmospheric vibrations travel seem- ingly with tho same velocity as sound, the earth vibrations much faster. Surface blasts expend a greater proportion of their energy in atmospheric vibrations than do deep blasts. Hence, surface blasts make much more noise and jar houses more than deep blasts, but they have little influence in causing slides. On tho other hand, the tremors created by a deep blast help to bring down, even at a con- siderable distance from the explosion, rock masses already danger- ously strained. Two large blasts in Culebra Cut gave the following approximate vibration records: A blast of 2,250 pounds of dynamite, exploded in 14 holes 24 to 28 foot deep, gave a maximum amplitude of 20 millimeters. Another blast of 5,370 pounds of dynamite, exploded in 48 24-foot holes at about the same distance from the instrument, gave an amplitude of 28 millimeters. These records were made on