Bin Ye

Strength and Dilatancy of Overconsolidated Clays in Drained True Triaxial Tests

AbstractThe long-term mechanical behavior of overconsolidated (OC) clays in three-dimensional (3D) stress states plays an important role in many geotechnical problems, such as slope failures. However, previous experimental studies of OC clay were carried out under undrained conditions. This paper presents the results of a series of drained true triaxial tests on normally consolidated (NC) and OC clays. The stress paths were maintained in the same π-plane with Lode angles of θ=0–60°. The test results show that OC clays exhibit positive dilatancy tendencies and greater shear strengths and smaller strains to failure than NC clays. The shear strength and dilatancy are largely influenced by the Lode angle. As the Lode angle increases, the shear strength and dilatancy decrease. The decreasing gradients of shear strength and dilatancy versus Lode angle are almost the same, indicating that the constitutive models for OC and NC clays can share the same rules for strength and dilatancy. The formation of shear bands...

Automated True Triaxial Apparatus and its Application to Over-consolidated Clay

A better understanding of long-term mechanical behavior of over-consolidated clay (OC) is important to numerous geotechnical problems. Most previous experimental studies, however, have used sand or normally consolidated clay (NC); only limited reports of the drained true triaxial test of OC are available. In this paper, an electro-pneumatic (E/P) regulator based automatic control true triaxial apparatus is developed. A mixed boundary loading device designed by Nakai Research Group is used in the apparatus. Unlike other loading devices, the membrane covers not only the vertical but also the horizontal rigid platens, which can apply arbitrary three different principal stresses to the block specimen. The loading pattern that can reduce the corner effect and the interference problem is also analyzed. The method to increase the pressure control precision of the E/P regulator as well as the automatic control system is presented. An extra volume change due to water-absorption of membrane during long time drained test is discussed and a simple solution is proposed. The apparatus is used to study the mechanical properties of NC and OC clays under drained condition. The test results show that the initial stiffness, shear strength and dilatancy are largely influenced by the Lode angle. Furthermore, both the NC and OC clays obey the Matsuoka-Nakai criteria well.

A pneumatic shaking table and its application to a liquefaction test on saturated sand

The shaking tables that are used in earthquake engineering are normally driven by hydraulic actuators, which require high maintenance and operation costs. In some studies, when it is only desirable to conduct small-scaled model tests, a pneumatic shaking table can be considered as an alternative to a hydraulic shaking table. This paper describes the design, development, calibration, and performance of a pneumatic shaking table system. It was proved that the pneumatic shaking table can offer satisfactory performances. Using the pneumatic shaking table, a liquefaction test on saturated sand was conducted. During liquefaction, both the stiffness and acceleration response of the ground greatly decreased. In comparison to the sharp increase in excess pore water pressure (EPWP) when liquefaction started, the dissipation process lasted for a much longer time period.

A New Device for Measuring the Supercritical CO2Permeability in Porous Rocks Under Reservoir Conditions

This paper describes a newly developed device for measuring the CO2 permeability coefficient in porous rocks under reservoir conditions, in which the pressure and temperature of injected CO2 are usually beyond the critical point of 7.38 MPa and 31.8°C. The device consists of a pressure cell, a syringe pump for CO2, a pressure amplifier, a heating unit, and a measurement system. Two buffer tanks are set at the inlet and outlet of the seepage chamber to maintain stable CO2 injection and back pressures during testing. The supercritical CO2 flowing through the rock sample is depressurized to the gaseous phase to measure its flow rate at low pressure. An average flow rate over time is used to calculate the permeability coefficient of CO2 in the rock sample. A performance test was conducted to demonstrate the detailed characteristics of the device. A sample application using silt rock for testing proved that the device can effectively measure the permeability coefficient of supercritical CO2 in porous rock.

Explanation of liquefaction in after shock of the 2011 great east Japan earthquake using numerical analysis

During the 2011 Great East Japan Earthquake, severe liquefaction occurred in reclaimed ground in Urayasu city, Chiba prefecture. This liquefaction provided important lessons for us to re-recognize the liquefaction mechanism. A distinct feature of the liquefaction in this earthquake is that severe liquefaction happened not only in the main shock but also in an aftershock with a maximum acceleration of 25 gal. In some areas, liquefaction happened in the aftershock is even more serious than that happened in the main shock. In this paper, focus is placed on the characteristic features in the occurrence of liquefaction and consequent ground settlement. Based on the observed data, a series of dynamic–static analyses, considering not only the earthquake loading but also static loading during the consolidation after the earthquake shocks, are conducted in a sequential way just the same as the scenario in the earthquake. The calculation is conducted with 3D soil–water coupling finite element–finite difference analyses based on a cyclic elasto-plastic constitutive model. From the results of analyses, it is recognized that small sequential earthquakes, which cannot cause liquefaction of a ground in an independent earthquake vibration, cannot be neglected when the ground has already experienced liquefaction after a major vibration. In addition, the aftershock has great influence on the long-term settlement of low permeability soil layer. The observed and predicted liquefaction and settlements are compared and discussed carefully. It is confirmed that the numerical method used in this study can describe the ground behavior correctly under repeated earthquake shocks.

Co-seismic and post-seismic behavior of an existed shallow foundation and super structure system on a natural sand/silt layered ground

Purpose – The purpose of this paper is to evaluate the co-seismic and post-seismic behaviors of an existed soil-foundation system in an actual alternately layered sand/silt ground including pore water pressure, acceleration response, and displacement et al. during and after earthquake. Design/methodology/approach – The evaluation is performed by finite element method and the simulation is performed using an effective stress-based 2D/3D soil-water coupling program DBLEAVES. The calculation is carried out through static-dynamic-static three steps. The soil behavior is described by a new rotational kinematic hardening elasto-plastic cyclic mobility constitutive model, while the footing and foundation are modeled as elastic rigid elements. Findings – The shallow (short-pile type) foundation has a better capacity of resisting ground liquefaction but large differential settlement occurred. Moreover, most part of the differential settlement occurred during earthquake motion. Attention should be paid not only to ...

Assessment of Regional Shallow Landslide Stability Based on Airborne Laser Scanning Data in the Yingxiu Area of Sichuan Province (China)

Abstract This study focuses on analyzing the slope stabilities in a landslide-prone area of Yingxiu Town, Sichuan Province (China). Airborne Laser Scanning (ALS) data were acquired to derive a Digital Elevation Model (DEM) with sufficient accuracy and resolution, as an input for the regional landslide stability analysis. The one-dimensional hydrological model—Stability Index Map (SINMAP), functioning with topographic data, geological settings, and rainfall conditions, was used as a simplified model for slope stability mapping. In this study, the investigated region was classified into six stability levels, and data reliability was subsequently checked with reference to recent landslide inventories. Several experiments have shown that the quality of ALS data played a key-role in the slope stability inside the SINMAP model regarding the point cloud density and the random error. Higher point cloud density may construct higher precision of DEM, however, it may also produce more noises. Although with these uncertainties, using ALS data and its derived high precision DEM, the physically-based SINMAP model is expected to provide a solid basis for further landslide susceptibility mapping at regional scale.

A Unified Description of Toyoura Sand

In this paper, the mechanical behavior of sand, was systematically described and modeled. Without losing the generality of the sand, a specific sand called as Toyoura sand, a typical clean sand found in Japan, has been discussed in detail. In the model, the results of conventional triaxial tests of the sand under different loading and drainage conditions were simulated with a fixed set of material parameters. The model only employs eight parameters among which five parameters are the same as those used in Cam-clay model. Once the parameters are determined with the conventional drained triaxial compression tests and undrained triaxial cyclic loading tests, and then they are fixed to uniquely describe the overall mechanical behaviors of the Toyoura sand, without changing the values of the eight parameters irrespective of what kind of the loadings or the drainage conditions may be. The capability of the model is discussed in a theoretical way.

Investigation on Stiffness Recovery of Liquefied Sandy Ground after Liquefaction Using Shaking-Table Tests

In this paper, shaking-table tests on saturated sandy model ground were carried out. During strong motions, responses in different position inside model ground, including acceleration and pore-pressure, were measured and investigated in detail. As soon as the motion stopped, hammer strikes were applied to the bottom plate of shaking-table with some time space interval. Through calculating the time difference when shear-wave pass through different accelerators, time history of shear-wave velocity was acquired and the process of stiffness recovery was analyzed.

Supercritical CO 2 Permeability in Rock: An Experiment Study

Carbon capture and storage projects have traditionally targeted deep sedimentary basins where CO2 is maintained as supercritical state (scCO2). In order to study the permeability of scCO2, a series of tests were conducted by independent research and development of the instrument with related environment variables, including temperature, confining pressure, injection pressure and different types of rocks. Through normalization of the experiment data, the results showed that the permeability of scCO2 increased with the increment of temperature and injection pressure, but with the reduction of confining pressure and effective stress. Besides, silt rock was more sensitive for temperature, while was less sensitive for confining pressure than sandstone. The conclusions summarized above possess instructional significance and referenced value for the siting selection of geological storage.