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UBC Theses and Dissertations

On the critical state locus of the Bennett South Moraine sand Mertz Antunes, Anays


Findings of a fundamental study of the mechanical consequences of internal erosion are reported from testing of soil sampled from the borrow source of the W.A.C. Bennett Dam, a glacial deposit termed the Bennett South Moraine (BSM). The BSM sample was characterized with respect to mineralogy, grain shape and index void ratios. It is found to be mostly composed of silicate minerals with a portion (approximately 25%) of carbonate minerals. A decrease in angularity of the grain shape was observed with increasing grain size. Similarly, the maximum and minimum void ratios were found to decrease with increasing grain size. A relation between maximum void ratio and grain shape was also observed. Triaxial compression tests were performed on a uniformly graded BSM sand and two gap-graded (or binary) mixtures of coarse and fine BSM sand. The results are analyzed with reference to the critical state strength of these gradations, and compared with other sands in the literature. A compilation of the critical state locus (CSL) of uniformly graded BSM and other sands shows an effect of the grain shape on the relative position of the CSL in the e-log(pʹ) space. The CSL of uniformly graded BSM sand was found to be located near that of other subangular sands, in particular Ticino sand. Additionally, the relative position of the CSL in this space, as measured by the value of critical void ratio at low effective stress, was found to be related to maximum void ratio for uniformly graded sands. The testing of the gap-graded BSM sands revealed that the introduction of 15% fine sand content to the mixture causes a downward shift of the CSL in the e-log(pʹ) space, confirming the finding of others. Furthermore, increasing the size ratio between coarse and fine fraction (the gap ratio, D′15/d′85) in the gap-graded BSM sands also causes a downward shift of the CSL, a phenomenon for which limited evidence is reported in the literature.

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