Engineering Characteristics of the Gulf Coast Continental Shelf

Abstract

Abstract The Louisiana Continental Shelf is a submarine area extending offshore asmuch as one hundred miles. The Gulf bottom in this region varies considerablyin extent, profile and composition and consists largely of sedimentarydeposits, predominantly those of the Mississippi River. It is very young in ageological sense. Up to the present time few major structures have been erected offshore inthe Gulf and no comprehensive preliminary tests have been carried out. Resultsof pile-driving tests are of little value as actual bearing capacities farexceed the dynamic resistance to driving as measured by conventional formulas.Soils encountered are, with few exceptions, cohesive and piling consequentlydevelop their supporting power through surface friction. Because of the time required and relatively high costs, no complete loadtests have been made. The results of those tests which have been made, andinformation obtained from pulling piling in one structure, indicate that designloadings could be considerably increased, and it is contended thatcomprehensive preliminary investigations including an actual load test, combined with borings and examination of materials encountered by competentsoil experts, would effect economies in design far in excess of their cost. The Louisiana Continental Shelf - General The Louisiana continental shelf is a submarine area which extends seawardfor as great a distance as one hundred miles off the present shoreline. TheGulf bottom in this region is characterized by low slopes and its outer marginis outlined by the fifty fathom (300'Ft.) contour. South of the margin of theshelf the slopes steepen abruptly in the continental slope zone and plunge intothe deeps of the Gulf of Mexico. The engineer who is called upon to design a structure on this shelf isconcerned only with the uppermost portion to a depth generally not to exceed100 to 15? ft. This layer is very young in a geological sense, and it has thecharacteristics usually associated with infancy. A few of these are lack ofstrength, lack of uniformity, and high water content. It consists of thesedimentary deposits of the various rivers and streams entering the Gulf. Thesestreams are all heavily burdened in quantities sufficient to fill the areas attheir mouths, were it not for the fact that continual subsidence, equal orgreater in amount and acting directly opposite, prevail over the sedimentaryworkings of the streams. This subsidence has been estimated by geologists to beat the rate of at least one foot per century. Influence of Mississippi River The Mississippi River, which has been the predominant factor in theformation of practically all the Louisiana continental shelf, is one of tworivers flowing into the Gulf that have been able to build protruding deltas.The Mississippi has done so because of its enormous size and load. It depositssediments in amounts sufficient to replace and actually gain on the amount ofsubsidence. T.P. 2323