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

40 ENGINEERING PROBLEMS OF PANAMA CANAL. evidence could be obtained on this point. At any rate no part of Gatun Dam and no part of the locks is built across this probable fault zone, so that the locks would not be harmed in any way should fresh differential movement or faulting occur along it. A glance at the map comprising Plate IV will show how the whole drainage of the Gatun Basin, including the Chagres, the Trinidad, and the Gatun Rivers, joined forces and emptied out into the Caribbean through a gap in the Quebrancha Hills little more than a mile wide. In order to form Gatun Lake, therefore, it was necessary only to close this gap; but to do this was a huge task; it meant the build- ing of the Gatun Dam. THREE GREAT QUESTIONS INVOLVED IN DAM. With tho project of this great dam, three questions in geologic engineering presented themselves. They were: (1) As Gatun Lake would have an area of about 165 square miles and would be 85 feet above sea level and separated from the Caribbean by only a few miles, would the seaward rim of this lake be found to be porous, or composed of limestone and thus result in leakage? (2) Would the bedrock be adequate in strength and in water-tightness to meet the foundational requirements of the locks and dams? (3) What would bo the best typo of dam to build under the engineering and geologic conditions involved ? Regarding the first of these questions the following was found to be true. The oceanward rim of Gatun Basin was found to bo in general a fine-grained argillaceous rock. It has already been de- scribed as tho Gatun formation (p. 25). This rock is so fine grainßd as to be relatively impervious. It contains a few local lenses of gravel and conglomerate, but these seem to be discontinuous and therefore relatively harmless in thé matter of allowing leakage. Fortunately, thc.limestone locally forming the upper part of the rim is not known to extend below the 85-foot, or lake, level in any important place. If it did, a troublesome leakage that might rob the lake of considerable water might develop in it. I ho second question was also answered in the affirmative, for it was found that tho Gatun rocks are relatively solid and compact and suitable for foundations of heavy structures. However, some writers, even recently, have affirmed that tho Gatun rocks would sol ton. into mud when Gatun Lake filled, and thus the locks and dam would sink their foundations and bo damaged or destroyed,. Opinions of this sort spring from a vivid imagination and not from facts. In the first place, the Canal Zone rocks for centuries have boon saturated with all tho water tlioy can. hold; nine months in the your tlio rains are so heavy that tho percentage of absorption as compared with the