An experiment has been carried out to study the soil moisture increasing characteristics caused by Pot-cover Effect under unidirectional freezing-thawing cycle. After 30 freezing-thawing cycles, frost heave and thaw settlement has occurred in the soil sample. The results of temperature and water content monitoring indicate that the moisture of the soil sample increase significantly and the water content of the top soil reaches nearly liquid limit. It is also observed that the freezing front move down gradually with the number of the cycle increases. The test results show that freezing-thawing cycle can accelerate the increase of soil moisture caused by Pot-cover Effect.

Soil-bentonite (SB) cut-off wall is widely used for controlling seepage and contamination migration due to its extremely low hydraulic conductivity. Low strength and stiffness of SB can lead to high flexibility and deformability during and after construction, which is one of the merits as cutoff walls, but it limits the application of SB walls when higher strength is required. In such a case, cement addition is a possible way to improve the strength and stiffness properties of SB, but this approach can affect its barrier performance. Soluble elements in cement may decrease the swelling of montmorillonite which may increase hydraulic conductivity of SB. Also, cement addition provides alkaline benefits for SB barrier performance. In this study, the influence of adding cement on hydraulic conductivity and sorption performance of SB cut-off wall was evaluated. Hydraulic conductivity of amended SB was studied by using flexible-wall permeameters with the falling head system with distilled water. The batch test is used to discuss the sorption performance against trivalent arsenic. The results show that adding cement will increase the initial hydraulic conductivity for around 10 times, while hydraulic conductivity of groups with 100kg/m3 cement will decrease to 20% after 60 days curing. The sorption performance of SB with cement will increase with amount of cement increase.

This paper presents a 1D model for electro-osmotic consolidation of unsaturated soils. Pore-water pressure, pore-air pressure, and soil settlement are obtained in the Laplace domain. Then, Crump’s method is adopted to obtain semi-analytical solutions for the time domain. The present solutions are more general and can be degenerated into conventional solutions to 1D consolidation of fully saturated soils and unsaturated soils without electro-osmosis. The parametric studies indicate that electro-osmosis has little impact on the dissipation of excess pore pressures at the first stage, while electro-osmosis is suggested to start at the second stage after pore-air pressure dissipates completely.

One strategy currently being explored in the development of a technically simple and practical countermeasure when utilising geogenic contaminated soils as fill materials in an embankment involves the installation of a compacted soil layer with attenuation capacity underlain the contaminated geomaterials. Water retention property, which is one of the important soil hydraulic properties in the simulation of water flow, needs to be better understood for selecting a suitable composite of base material and stabilizing agent as the attenuation layer. In this study, the water retention characteristics of six composites with different sandy soils and calcium-magnesium composite sizes were investigated. It was found out that mixing the sandy soil with the mineral-based agent improves its water retention characteristics, especially when the soil is a well-graded type. The highest residual saturation (Srf) was found for the decomposed granite soil mixed with the powder agent of under 2 mm, which reached 87%. Poorly-graded sandy soils like the silica sand were found not to be suitable base material for the attenuation layer, because they drastically become impermeable, where k reaches nearly 1×10-14 m/s from very small changes in negative pressures.

This paper presents a new and simple method to investigate the unsaturated sand with a compressible fluid under undrained condition via the Distinct Element Method (DEM). In contrast to distinct element method and computational fluid dynamic approach (CFD-DEM) coupling, the pore pressure in this method is treated as the difference value between the initial confining pressure and the effective stress of the soil skeleton according to the effective stress principle. Furthermore, a compression coefficient is introduced to take into account the fluid compression, by which the changes of pore pressure and vertical strain rate of fluid can be calculated to control the movement of vertical walls. A series of three-dimensional (3D) cyclic simple shear tests were simulated to study the influence of fluid compressibility on the mechanical properties of soil. The DEM results were compared with the experimental data to verify the feasibility of the method.

Methane hydrate (MH) is regarded as a potential future clean energy source, which has attracted lots of attention in recent decades. The sediment containing MH is called as Methane Hydrates Bearing Sediment (MHBS). The evolutions of mechanical behaviors of MHBS from macro and micro scales are needed in detail for safe MH exploitation (e.g., chemical injection method). For this purpose, a novel 3D thermo-hydro-mechanical-chemical (THMC) bond contact model was proposed and implemented into a DEM commercial software to capture the mechanical behavior of MHBS under exploitation stress path. Firstly, MHBS samples were compressed to different deviator stress levels, which represents the different initial stress state. Then the chemical injection process was simulated by increasing the salinity from 3wt% to 14wt% to allow MH dissociate while the deviator stress remained constant. After which, the recovery stage was simulated by decreasing the salinity from 14wt% to 3wt%. The results show that the salinity can affect the mechanical behavior of MHBS by changing the properties of inter-particle methane hydrate. During chemical injection process, the axial strain increases with the increase of salinity. The chemical injection method will result in soil collapse when initial deviator stress is larger than the strength of pure sand at exploitation stage but slight deformation at recover stage.

This paper investigates the mechanical behavior of Methane Hydrate-Bearing Sediments (MHBS) at different back pressures via the distinct element method (DEM). A series of triaxial compression test were carried out in order to analyze the micro- and macro-mechanical behavior under different back pressures, e.g. stress–strain relationship, void ratio and mechanical coordination number. Numerical results show that: the stress-strain relationships of MHBS exhibit more obvious strain softening characteristics at a lower confining pressure and higher back pressure. The final void ratio increases as back pressure increases. The increasing rate of the total mechanical coordination number with back pressure was significantly lower than that of hydrate cementation mechanical coordination number.

Hexavalent chromium may elute from a part of cement-improved soils when cement solidifications are used for the purpose of ground improvement. Particularly, the likelihood of elution increases when the type of soil is volcanic cohesive soil. Hexavalent cohesive chromium is extremely toxic to the human body. Therefore, in the above-mentioned cement solidifications, it is recommended to consider its elution when volcanic soil is used. In this study, strength and dissolution tests of hexavalent chromium are conducted on cement-improved soils in which cement solidifications and andosols are mixed. Three different waste materials (sugar syrup, burned fish bone, and rice husk ash) are used to verify their reduction properties for hexavalent chromium. According to the results of the test, the strengths of the specimens with the three different materials do not change significantly, and the improved soils maintain the required strengths. In addition, the elution amount of hexavalent chromium from the specimen mixed with a sugar syrup is lower than that from an unmixed specimen. This fact suggests that the waste syrup can reduce the elution of hexavalent chromium.

The strength and deformation characteristics of loess under vertical unloading path is important for the study of slope instability in excavation area. From the perspective of engineering application, direct shear test is carried out. Moreover, a discrete element method model is established to study micro-mechanism of multi-stage unloading. It is found that the shear displacement-shear stress curve of the soil under the multi-stage unloading path is different from those under the loading state. The soil sample is prone to shear failure during small deformation under multi-stage unloading. The shear strength is related to the initial consolidation pressure and unloading ratio. During the multi-staged unloading process, the number of contacts between particles does not change significantly, while the contact forces between particles decrease obviously.