Water retention curves (WRC) of loess were investigated through conducting two series of tests on intact and re-compacted specimens. Modified pressure plate extractors were used to control net stress and matric suction. Compared to the re-compacted loess, higher rates of adsorption and desorption as well as larger area of hysteresis loop were observed for intact loess. The effect of net stress on WRC seems to be more pronounced for re-compacted loess than intact one. The higher the net stress, the lower the rate of adsorption, and desorption, and the area of hysteresis loop for re-compacted loess. This is due to the reduction in void ratio as a result of net stress. Drying-induced shrinkage is significant for the re-compacted loess, but not for the intact one. Furthermore, shrinkage increases with net stress.

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The balk transfer method is one of the evaporation estimation methods. The amount of evaporated water from bare ground surfaces can be estimated from the general meteorological data using the method. A moisture exchange speed, ga, and evaporation efficiency, β are important parameters of the method, and they are influenced by volumetric water content in unsaturated soil. The authors have proposed the estimation method of the exchange speed considering the soil thermal capacity, ga,soil. In this paper, the evaporation efficiencies are obtained from our proposed procedure, the relationship between volumetric water content and evaporation efficiency of unsaturated sandy soil is discussed based on the experimental results. As the results, the properties of evaporation efficiency depend on volumetric water content are clearly explained.

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The shear strength of unsaturated completely decomposed granite (CDG) soil was studied by performing a series of unsaturated direct shear tests under different combinations of net normal stress and matric suction conditions. The axis-translation technique is applied to control matric suction. A series of typical soil water characteristic curves (SWCC) of CDG soil under different net normal stresses were obtained considering consolidation process by monitoring the vertical displacement throughout the tests. Experimental results show that both unsaturated shear strength and SWCC are affected by the soil stress states. Shear dilation behavior is observed significantly obvious at higher suction and lower net normal stress state, and the shear strength of unsaturated CDG soil increases with matric suction and net normal stress.

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Vegetation is increasingly recognised as an eco-friendly method for slope stabilisation, through mechanical root reinforcement and transpiration-induced soil suction. Several centrifuge tests have been carried out to investigate failure mechanism of slopes mechanically reinforced by roots. However, the effects of transpiration-induced soil suction were ignored, and hence any of its contribution to stability enhancement is not well-understood. This paper aims to develop and verify a new artificial root that is capable of simulating both the effects of transpiration-induced suction and mechanical reinforcement for physical modelling, including centrifuge at high-g. The artificial root was made of a high air-entry value (AEV) porous filter and supplied with vacuum pressures. The function of the porous filter is to maintain hydraulic gradient between soil and the system. Any reduction of soil moisture due to applied vacuum hence induces suction. In order to verify this newly developed system, mechanical properties and suction distribution influenced by the new artificial root and selected living tree were tested and compared in an atmospheric controlled room. The comparisons of test results showed that the suction distributions induced by the artificial root and the living tree were reasonably close, during both transpiration and a rainfall event.

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The unsaturated small strain shear modulus, G_max, is a key reference value in predicting relationships between dynamic shear modulus and shear strain amplitude and is thus a key quantity to properly model the behavior of dynamically-loaded geotechnical systems such as pavements, rail beds, and machine foundations. From the interpretation of the experimental G_max results for unsaturated soils, different definitions of trends between G_max and the stress state of the unsaturated soils and material properties are proposed. However, in most of trends, the relationship between the stress state and void ratio is considered and the effect of void ratio on the unsaturated small strain shear modulus is not fully investigated. In the study presented herein, G_max data published in the technical literature for two different types of unsaturated soils are critically reviewed with the goal of identifying trends with path-dependent stress state and void ratio. The literature data is also used to evaluate the reliability of an existing approach in predicting the small strains shear modulus of unsaturated soils under different loading conditions.

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The conventional fill compaction procedure controlling the dry density ρd and water content w has several inherent limitations due mainly to significant effects of compaction energy level CEL on the ρd vs. w relation of compacted soil and a difficulty of estimating in-situ CEL. The followings are shown. The optimum degree of saturation (Sr)opt defined as Sr when (ρd)max is obtained and the ρd/(ρd)max vs. Sr - (Sr)opt relation of compacted soil are independent of CEL and insensitive to soil type variations. The strength and stiffness of unsoaked and soaked compacted soil and the hydraulic conductivity k of saturated compacted soil are controlled by ρd and “Sr during compaction”. Empirical equations of CBR and k expressing these trends of behaviour, not including CEL, were derived by analyzing existing data. A new compaction control method examining whether Sr of compacted soil is close to (Sr)opt and ρd is large enough to achieve required soil properties, together with pre-compaction control of water content, is proposed.

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Hydro collapsible soils are of kinds of problematic soils which show high shear strength at low degrees of saturation but due to wetting, their Meta stable structure collapses and will be subjected to large deformation. In this study by applying isotropic triaxial loadings to undisturbed samples, mechanical behavior of a highly collapsible loessial soil has been assessed. During tests, matric suction of the samples was controlled as well as the mean net stress, and the variation of the degree of saturation was monitored continuously. Two types of stress paths were conducted on the samples namely “Isotropic induced collapse” under applying constant matric suctions and “wetting induced collapse” by decreasing matric suction while applying constant mean net stress. Results show increasing yielding stress of the samples by increasing the mean stress under constant matric suctions. By considering effective stress approach, a hardening model is introduced in this paper to interpret the behavior of the tested soil. Agreement obtained between the predicted and measured values of the yielding stresses during tests is promising and shows the efficiency of the presented model.

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