Degao Zou

Stress-Dilatancy Relationship of Zipingpu Gravel under Cyclic Loading in Triaxial Stress States

AbstractIn this study, cyclic triaxial tests were performed on Zipingpu gravel, and they were followed by a discrete element study that was designed to investigate the stress-dilatancy relationship of gravelly soils under cyclic loading in triaxial stress states. Several conclusions emerged from the results. (1) A nearly linear relationship was found between the stress ratio η=q/p and the dilatancy ratio Dp=devp/desp under both conventional compression and extension monotonic loading. The slope parameter α, which relates η and Dp, was smaller during triaxial compression; (2) The stress-dilatancy relationship was different during the virgin and cyclic loading. The dilatancy line under cyclic loading was located inside the virgin/monotonic loading dilatancy lines, and the dilatancy relationship was related to the location of the most recent load reversal point; and (3) A nearly parallel linear relationship was found between η and Dp in the dη>0 and dη<0 paths under cyclic loading, with α smaller under cycli...

Experimental Study on the Behaviors of Sand-Gravel Composites Liquefaction

By use of medium scale dynamic triaxial apparatus(φ 200×500mm) the development of axial strain and pore water pressure of sand-gravel composites are studied in cyclic loading. Adopting same relative density, a series of substituted material specimens gained by eliminating the oversized (>5mm) gravel particles are studied. The results show that with isotropic consolidation, the development of excess pore water pressure and axial strain in sand-gravel composites differs from that in substituted material. A series of undrained cyclic triaxial tests were performed on sand-gravel composites specimens with relative density of 50%, 55%, and 60%. Test results showed that the increase of relative density may delay the development of pore pressure of sand-gravel composites.

Three-Dimensional Scaled Memory Model for Gravelly Soils Subject to Cyclic Loading

AbstractA comprehensive constitutive model has been formulated to capture the complex behaviors of gravelly soils under irregular cyclic loading. The application of this model aims to analyze the n...

Dynamic responses of concrete-faced rockfill dam due to different seismic motion input methods:

The concrete-faced rockfill dam valley foundation was considered as an open energy system and a reasonable non-uniform seismic motion input method was applied to the dynamic analysis of a concrete-faced rockfill dam based on the generalized plastic model. First, the corresponding program was validated by means of the scattering question of an idealized semicircle valley. Subsequently, the seismic elasto-plastic finite element analyses were performed to compare and investigate the performances of a concrete-faced rockfill dam under different seismic motion input methods. The results show that the dynamic responses of the concrete-faced rockfill dam are decreased by 10%–30% approximately with the use of non-uniform seismic motion input method. As a result, the traditional uniform seismic motion input method would overestimate the responses of the dam. From the perspective of seismic safety evaluation, the overestimations would disturb the reasonable assessment of the aseismic capacity of the dam. Moreover, the slope stability analysis results might be conservative and unreasonable due to overestimating the accelerations during the earthquake.

A Simple Measurement of Membrane Penetration in Gravel Triaxial Tests Based on Eliminating Soil Skeleton Plastic Deformation with Cyclic Confining Pressure Loading

Previous investigations of membrane penetration mainly focused on uniform sandy soils, and few studied gravelly soils. A common problem in most previous investigations was that unverified assumptions were made to evaluate or mitigate the membrane penetration of a cylindrical specimen in triaxial test. In this study, a simple measurement of the membrane penetration of gravelly soils was proposed, based on small-amplitude cyclic confining pressure Δσ3 loading, in which the cyclic membrane penetration amplitude ΔVm is recoverable, and the cyclic soil skeleton volume deformation amplitude ΔVk gradually decreases with increasing number of cycles and becomes elastic. The accuracy in the proposed method is related to the accuracy in measuring ΔVk. Because of the natural limitations in measurements of the radial strain of a cylindrical specimen, ΔVk is calculated based on the isotropic assumption. The proposed method provides sufficiently accurate ΔVm values for gravelly soils because of the small portion of ΔVk in the total cyclic volume change amplitude ΔVc (=ΔVm+ΔVk), and the accuracy increases as the coarse fraction increases. Furthermore, the method can be easily implemented for individual specimens to remove inconsistences in specimen preparation. The test results indicate that ΔVm values corresponding to the same Δσ3 exponentially decrease with the confining pressure σ3. Membrane penetration increases with increases in the void ratio and coarse fraction, but the trend is becoming weak with decreasing coarse fraction and void ratio, respectively. It also demonstrated that there exists a clear difference between the results of Nicholson’s empirical formula and the test results.

Numerical Simulation of Seimic Behavior of Pipeline in Liquefiable Soil

This study focused on the behavior of a burial pipe with special reference to its stability against floatation subject to soil liquefaction. The excess pore water pressure response behaviors of soil foundations, and the effectiveness of different types of drainage or reinforcement measures were investigated using Finite Element Method (FEM). FEM numerical model is a coupled stress-flow finite element procedure, based on u-p formulation of dynamic Biot’s equations (Zienkiewicz, 1982). The hyperbolic stress and strain relationship was used in the numerical model, which takes into account the stiffness and strength degradation. Pore pressure generation due to earthquake loading was calculated via the pore pressure model (Seed et al, 1979). Performance of the numerical models was studied by simulating a series of shake table tests. Excess pore pressures predicted by numerical models were compared with the pore pressure transducer records during experiments. Also, the effectiveness of different drainage measures against uplifting of pipelines was compared. It was demonstrated that the models were able to provide results in agreement with experiments.