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UBC Theses and Dissertations

The outer banks of the Fraser River delta : engineering properties and stability considerations Scotton, Steven


Roberts Bank and Sturgeon Bank are the leading edges of the Fraser River Delta. In the past half century various aspects of the delta, and the banks, have been studied by geologists, geomorphologists and engineers. Published papers and reports of these studies form the primary data base for this thesis. The geology and geomorphology reports complemented the engineering data. Logs of 6 8 boreholes were found in the engineering reports made available for this thesis. These boreholes were located such as to give a reasonable coverage of both Roberts Bank and Sturgeon Bank. The reported holes were sampled extensively with Standard Penetration Test split spoon samplers and a few thin wall Shelby tube samples were taken. The engineering reports presented the results of numerous tests performed on the samples, including shear tests, triaxial tests and consolidation tests. The upper 80 feet of sediments, which is the zone of concern for strength and stability analyses, are primarily granular in nature. These sediments exist at a medium to loose density with a relative density as low as 40 percent suggested for large areas of the banks. Some of the deeper sediments are moderately compressible in nature and are presently normally consolidated. The nature of the surficial sediments is such that, in the existing seismic environment in which they are located, there is the possibility of earthquake induced liquefaction. Methods of assessing the probability of liquefaction are discussed and the results of one such assessment are presented. The subaqueous slopes of Roberts Bank and Sturgeon Bank, which average 1.5 degrees but exceed 23 degrees in a few spots, are shown to be at least nominally stable with respect to mass wasting. There are some indications that these slopes could be subject to some erosional instability. The physical environment of the banks (wind, wave, temperature) is not particularly harsh, and presents no problems to engineering development. Certain aspects of the ecology of the banks, however, are of sufficient importance to warrant consideration as part of the design process for any project on the banks.

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