Wei-Min Ye

Marsh gas in shallow soils and safety measures for tunnel construction

Marsh gas is a danger for tunnelling through soils in Shanghai. Predrilling can assess the hazard and help to vent the gas. Additional measures include tunneling with an airproof shield, adequate ventilation, special precautions against ignition and education of the workers.

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.

Effect of salt concentration on desiccation cracking behavior of GMZ bentonite

Bentonite has been proposed for use as an engineering barrier and buffer in nuclear waste repositories and has been used frequently in municipal waste landfills. The cracking behavior and deformation properties of this material can be influenced by the chemistry of pore water. In the present work, the influence of salt concentration on the desiccation cracking behavior of GMZ bentonite was investigated with laboratory experiments. Image processing techniques and SEM tests were performed on the specimens which had undergone the desiccation testing in order to analyze the cracking mechanisms. Results show that the water evaporation process can be identified by a steady rate stage, a falling rate stage and a residual stage. The water evaporation rate is strongly affected by the salt concentration of the pore water; higher salt concentrations result in lower evaporation rates; the final water content is strongly impacted by a high initial salinity; otherwise the water contents are very similar for the residual stage. During desiccating, most of cracks appeared at the steady evaporation stage. The cracking morphology and patterns were greatly affected by the salt concentration of the pore water; and larger crack lengths and lower crack densities were obtained as the initial salinity was increased.

Advances in experimental investigation on hydraulic fracturing behavior of bentonite-based materials used for HLW disposal

Abstract Bentonite-based materials (BBM) in the form of blocks or rings have been commonly proposed as buffer materials for high-level radioactive waste disposal. In a real repository, hydraulic fracturing is likely to be present when the ground water pressure exceeds to hydraulic resistance, and the existence of technological gaps increases the risk of hydraulic fracturing. Based on the previous studies of hydraulic fracturing of BBM and technological interfaces, the present work systematically analyzes the experimental achievements. Results show that the fracturing pressure of BBM and interfaces increases with hydration time, initial dry density, bentonite content and overburden pressure, while it decreases with initial water content and gap thickness. The temperature effect on hydraulic fracturing depends on the predominant cation of bentonite. For Na-bentonite, the higher temperature leads to the larger swelling pressure, and thus the stronger hydraulic resistance. Tensile failure, shear failure and synthesis failures are increasingly accepted for hydraulic fracturing mechanism. The hydraulic fracturing of highly compacted BBM is mainly controlled by tensile failure, while the initial shear failure can be found for BBM with a low enough density. Since hydraulic fracturing is expected to occur several times during the repository operation, the long-time monitoring of hydro-mechanical behavior is of vital importance until the technological gaps are completely sealed. Furthermore, the failure mechanism of hydraulic fracturing of BBM should be further researched, as well as the sealing properties of interfaces under thermo-hydro-mechanically coupled conditions are proposed to be studied thoroughly.

Investigation on swelling-shrinkage behavior of unsaturated compacted GMZ bentonite on wetting-drying cycles

It is generally recognized that swelling/shrinkage deformation and accumulative deformation of compacted expansive soil on wetting-drying cycles are strongly influenced by the specimen’s dry density and the net stress applied. However, investigations on the coupling effects of dry density and vertical net stress on the swelling/shrinkage deformation and accumulative deformation are limited. Cyclical wetting-drying tests were conducted on unsaturated compacted GMZ bentonite specimens with three dry densities under five vertical net stresses. The influence of vertical stress on accumulative deformation was analyzed. Then, the critical vertical stress was proposed for specimens with different dry densities. The effect of dry density on accumulative deformation was studied and the critical dry density was determined for different vertical stresses. Based on these, the concept of critical swelling-shrinkage line is proposed in the e-p space. According to the relationship between the initial state of specimen and the critical swelling-shrinkage line, a state parameter is proposed for determination of the accumulative strain on wetting-drying cycles.

Diffusion of La3+ in Compacted GMZ Bentonite Used as Buffer Material in HLW Disposal

It has been studied that the GMZ bentonite which has a chemical barrier property can play an important role in retarding the migration of radioactive nuclide. In order to understand the diffusion properties of nuclide in GMZ bentonite, a series of experiments were conducted to obtain the apparent diffusion coefficient of La3+ in GMZ bentonite under different dry density and pH value conditions using a autonomous ion diffusion instrument. The results show that the apparent diffusion coefficient of La3+ decreases significantly with the increasing dry density or pH value. The influence of dry density on the diffusion is very apparent for a low dry density, and the influence gradually slow down with the increase of dry density. At last the influence mechanism of the dry density and pH value on diffusion coefficient was also analyzed.

Adsorption of Heavy Metals onto Bentonites Used as Buffer/Backfilling Materials

Based on the former studies, experimental results of heavy metals adsorption on bentonites used as buffer/backfilling materials, such as MX-80, FEBEX and GMZ, are reviewed. It is observed that the high smectite content makes these materials an effective barrier. The adsorption of heavy metals onto bentonites is affected by contacting time, pH value, bentonite content, and ionic strength, etc. The adsorption of heavy metals on bentonites can be fitted by Langmuir and Freundlich models well.

Characteristics of Shear Strength of Unsaturated Weak Expansive Soil

The strength properties of unsaturated soil are far more complicated than those of saturated soil. As a kind of typical and widely spread unsaturated soil, expansive clay shows a very close relationship between its shear strength and water content, for its high content of hydrophile, such as montmorillonite, illite and etc. The shear strength of soil is of much significance in the design of foundation and slope in expensive soil area. The determination of shear strength for expensive soil is always an important issue in this aspect. Based on the investigation of expansive clay in a slope along Han-Shi highway, Hubei Provence, some unsaturated shear tests with suction control have been conducted, and then, unsaturated mechanical parameters have been obtained using double-variables strength theory, and characteristics of the unsaturated shear strength of the specimen have been explored. Results indicate that slight shear dilatation appeared during the unsaturated shearing course and its value decreases with the increase of confining pressure and suction. For lower suction, the cohesion of the tested soil specimen linearly increases with suction increases. The shear coefficient for testing specimen is 10.7°.

Advances on Buffer/Backfill Properties of Heavily Compacted Gaomiaozi Bentonite

Compacted bentonite is considered as buffer/ backfill materials for geological disposal of high-level radioactive wastes in many countries. Through a national screening and comparison, GMZ bentonite is selected as the first choice of Chinese buffer materials for high-level radioactive waste repository. Many studies were conducted on GMZ bentonite, however, its still in the primary stage compared with other candidate bentonites. In order to better orientate the future research work, the progress on researches of GMZ bentonite was summarized, which consist of studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior. Based on analysis of the current studies, the key issues to be explored in the future were also proposed, that is the influence of temperature on behavior of the GMZ bentonite; the migration law of nuclide in compacted bentonite and sealing properties of GMZ bentonite based materials under coupled T-H-M-C conditions..