Abstract A new technique has been developed for the evaluation of shear wave velocity based soil liquefaction resistance criteria by centrifuge tests. It involves the application of bender elements in centrifuge tests to measure shear wave velocity while investigating the liquefaction features of a saturated model. A total of six models were tested. Shear wave velocity and shear modulus profiles of the models were obtained. Currently used case history based soil liquefaction criteria were examined. The results show that there is an obvious difference in soil liquefaction resistance before and after the application of an earthquake. It is concluded that currently used case history based soil liquefaction resistance criteria developed from the mixed parameters measured before and after earthquakes need to be modified to take into account this difference.
This paper evaluates the lateral performance of a monopile reinforced by a gravel wheel for offshore structures via centrifuge tests and three-dimensional finite-element (FE) modelings. The gravel wheel comprises a ring frame placed on the pile head and filled with large particles to potentially utilize gravel or crushed stone in offshore areas. The results of centrifuge tests and FE analyses demonstrate that the lateral loading capacity of the monopile increases when combined with a gravel wheel, and the improvement depends on the diameter and thickness of the wheel. By means of FE methods, the interaction between the pile and surrounding soils and gravel fill are illustrated to explain how the gravel wheel contributes to the lateral resistance of the hybrid system. Furthermore, an equivalent layer method adopting the conventional p-y curves is suggested to predict the lateral response of the hybrid foundation. This method is validated by comparisons with the centrifuge tests results. Finally, a case study of the monopile-gravel wheel foundation indicates that the gravel wheel is less efficient in configurations where the ultimate capacity of the hybrid system is dictated by the bending capacity of structures rather than the strengths of soils.
Abstract The time scale for dynamic events differs from that for consolidation events if the same soil and pore fluid as in the prototype are used in a model test. In order to satisfy the scaling relationship for dynamic centrifuge tests, viscous fluids such as silicone oil or glycerin-water mixtures have been used as the pore fluid. The use of a pore fluid with higher viscosity will significantly reduce the permeability of a soil, making it possible to achieve the same time scale. However, the use of viscous fluids may also affect the mechanical properties of soils such as strength and stress-strain relationship. This paper presents results of permeability tests and triaxial tests on two types of sands over a range of void ratios. It is found that using a glycerin-water mixture as the pore fluid has little effect on the strength and stress-strain relationship of Ottawa sand No. 40. For tests with both silicon oil and glycerin-water mixture as permeants, coefficients of permeability are inversely proportional to the viscosity. However, at small hydraulic gradients, it was observed that the highly viscous fluids can cause clogging of flow in sand, especially for silicone oil in dense sand. It is recommended that when a viscous fluid is used in a centrifuge test, it is desirable to conduct a sequence of laboratory tests to make sure there is no unexpected influence on the properties of a soil.
Failure of retaining walls during earthquakes has occurred many times in the past. Although significant progress has been made in analysing the seismic response of rigid gravity type retaining walls, considerable difficulties still exist in the seismic-resistant design of the flexible cantilever type of retaining walls because of the complex nature of the dynamic soil–structure interaction. In this paper the seismic response of cantilever retaining walls with dry backfill is simulated using centrifuge modelling and numerical modelling. It is found that bending moments on the wall increased significantly during an earthquake. After the end of base shaking, the residual moment on the wall was significantly higher than the moment under static loading. The numerical simulation is able to model quite accurately the main characteristics of acceleration, bending moment, and displacement recorded in the centrifuge test.
Anisotropy in elastic shear modulus Gma x exists in most soils as the result of either anisotropic soil fabric or anisotropic stress conditions. This paper presents a theoretical and experimental study on the anisotropy in a Gma x of sand due to a K0 stress condition. Elastic shear moduli of two types of sand in multiple stress planes under a K0 condition were measured using bender elements. Stress-induced anisotropy in Gma x of the sands during loading and unloading processes and the important influential factors were investigated. An empirical relationship for the estimation of K0 was proposed based on the experimental data. Shear moduli in nonprincipal stress planes were measured and compared with the results from the theory. The influence of stress cycles on Gma x in multiple stress planes was studied.
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