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

co ENGINEERING PROBLEMS OF PANAMA CANAL. FISSURING AND JOINTING. As mentioned previously, within the Canal Zone there are many dikes, cores, flows, and small masses of igneous rocks. These, while still in a molten condition, were projected upward through the bedded rocks. On cooling they were much cut by the developing of joints and shrinkage cracks, and later many of them were somewhat sheared by faulting. The jointing and fissuring (Pl. XI) of these rock masses has had a direct bearing on engineering results. The great amount of jointing and fissuring in the Ancon Hill rhyolite cheapened considerably the cost of quarrying and crushing that rock for use in the concrete of the Pacific locks. On the other hand, one of the reasons why the Porto Bello rock was more expensive to use was because it had few joints and fissures, and hence broke in large masses that were expensive to load and to break into smaller pieces. The difference in jointing between these two rocks was due to the fact that the Ancon Hill rock is a relatively small, tabular-shaped mass which cooled quickly and de- veloped many joints and shrinkage cracks. It was then not strong enough to resist the crushing effects of faulting, and subsequent move- ments of this kind still further sheared and crushed it. On the other hand, the Porto Bello rock mass was much larger and cooled much more slowly; consequently it developed relatively few shrinkage cracks and was little affected by faulting. Certain dikes and lava flows that for a time held back the Cucuracha slide would probably have been strong enough to have prevented fur- ther sliding there but for the fact that they were cut by many cooling and shrinkage joints and fissures. These dikes were large and for a year or more they held back the sliding material. However, when the cut had been brought to final depth in front of them, the accumu- lated stresses of the material behind caused them to shear off at points where they had been greatly weakened by joint planes (PI. X). This gave a renewed activity to the Cucuracha slide that brought down into the canal over 1,000,000 cubic yards of material. It has already been pointed out how the excessive jointing in some large outcrops of basalt a few miles south of New Frijolcs prevented the selection of this material as armor for the east breakwater. Several other basalt outcrops were, for a similar reason, also found unsuitable for such use. Because of minimum jointing, Porto Bello rock when blasted yielded a large proportion of fragments that weighed several tons; hence coarse “ armor” rock for the west breakwater was taken from Porto Bello. It was thought that the coarser product of the jointed basalt south of New Frijolcs would bo used for the first covering over the softer heart material in the east breakwater. The coarse “armor” rock for this will, however, prob- ably have to be taken from Porto Bello, or from Sosa Hill, Balboa.