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UBC Theses and Dissertations

Mechanical properties of granular deposits from self-boring pressuremeter tests Roy, Debasis


The self-boring pressuremeter test has the potential of providing the information necessary for calculating the deformation behavior of granular deposits. However, the stress path and loading orientation with respect to the depositional direction in the test is such that the stress-strain relationship obtained directly from the data following the existing procedures can be very conservative in many field problems of practical significance. A reasonably simple stress-strain model is proposed in this research accounting for inherent anisotropy for granular deposits and stress-path dependent soil behavior. The model is used to back-analyze self-boring pressuremeter tests and determine an optimum set of model parameters representing the behavior of the deposit. The exercise of back-analysis essentially involves fitting the response of the stress-strain model to the self-boring pressuremeter data by altering the model parameter by manual trial and error. To compute the model response in cylindrical cavity expansion, a commercially available explicit finite difference computer code (FLAC) is used. An implicit finite element code developed in this research can also be used for the purpose. To minimize the impact of non-uniqueness in the result, a-priori information about the bounds of values of some of the model parameters is used. Supplementary information about the state of packing of the deposit and small strain stiffness is obtained from a seismic piezocone penetration test carried out adjacent to the self-boring pressuremeter bore hole. The deformation behavior of an axisymmetric element for the model parameters predicted from back analysis of self-boring pressuremeter test is found to agree with laboratory triaxial tests on undisturbed (frozen) samples in both compression and extension at several sites. The explicit finite difference computer code FLAC is again used to predict the response of an axisymmetric element. Further validation for the procedure proposed for estimation of deformation behavior of granular deposits from self-boring pressuremeter comes from the negligible value of the undrained strength obtained for data from a mine site where there was a static liquefaction failure. The only reliable method available at present to estimate deformation behavior of granular deposits is extraction of frozen samples and laboratory testing. The cost of adopting this approach is often too great for routine use. Even in important projects economical options of estimating deformation behavior from index tests such as SPT and piezocone penetration are thus adopted. These empirical procedures are usually very imprecise. The procedure based on back analysis of self-boring pressuremeter tests proposed in this research appears to provide a more reliable alternative than the approach based on index tests. At the same time, the approach is quite economical in comparison with the option of undisturbed sampling and laboratory testing.

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