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Hydraulic gradient similitude method for geotechnical modelling tests with emphasis on laterally loaded piles

University of British Columbia

A study has been undertaken to evaluate and apply the hydraulic gradient similitude method to geotechnical model testings. This method employs a high hydraulic gradient across granular soils to effectively increase self-weight stresses in the model. Testing principle and procedures are presented, and the factors affecting test results discussed. An apparatus (UBC-HGST) using this testing principle has been developed. Three applications are presented in which the hydraulic gradient similitude method is evaluated, and the existing concepts and methods of analysis for the problems studied are examined. In the footing tests, it is found that the scaling laws implied in the hydraulic gradient modelling test are satisfied, and are similar to those of the centrifuge modelling technique. Load-settlement curves are found to be similar to those in centrifuge tests. The test results illustrate the importance of the stress level in the load-settlement responses. Terzaghi's bearing capacity formula is compared with the observed bearing capacities under different stress levels. It is found that due to the stress level effects, the bearing capacity coefficient, Nγ, decreases linearly with footing width on the log-log scale which is in accordance with other model study and analytical results. In the downhole and crosshole seismic tests, results are used to evaluate the empirical equations that relate shear wave velocity and soil stresses in terms of field stress condition. It is found that although the various equations can predict the insitu shear wave velocity profile reasonably well, only the equation which is based on the significant stresses in the wave propagation and particle motion directions can predict the variation of velocity ratio between the downhole and SH crosshole tests. It is also found that the stress ratio has some effects on the downhole (or SV crosshole) tests, but not on the SH crosshole tests. This indicates that only the stress ratio in the plane of wave propagation is important to the shear wave velocity. Comparison between the downhole and SH crosshole tests shows that the structure anisotropy was about 10% in terms of shear wave velocity. Prediction of Ko values using shear wave measurement is evaluated, and its practical difficulties are addressed. In the laterally loaded pile tests, the pile response to static and cyclic loadings at various stress levels controlled by the hydraulic gradients is examined in terms of pile head response, pile bending moment and soil-pile interaction P-y curves. For the static loading, pile head response and bending moment are found to be significantly affected by the soil-pile relative stiffness, pile diameter, loading condition and pile head fixity. However, little effects of loading eccentricity and pile head fixity are found on the P-y curves. While pile diameter is found to have effects on the P-y curves at large pile deflection, its effects are negligible at small deflecton range. The effects of relative soil-pile stiffness on the P-y curves due to stress levels can be normalized by the soil modulus and pile diameter for the curves below 1 pile diameter, as computed by the plane strain finite element analysis. Two methods of generating P-y curves are suggested, and found to give satisfactory results as compared with the test data and the prediction given by API code (1987). For cyclic loading, different pile responses are observed in "one-way" as compared to "two-way" cyclic loading. The cyclic P-y curves are derived, and found to be highly nonlinear and hysteretic, and change with number of loading cycles. From these studies, it is shown that the hydraulic gradient similitude method provides a simple and inexpensive means of model testing for many geotechnical engineering problems and adds to the data base from which methods of analysis can be evaluated.

Text

2011-03-02

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http://hdl.handle.net/2429/31899

Civil Engineering

University of British Columbia

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