Annan Zhou

Stochastic method for predicting risk of slope failure subjected to unsaturated infiltration flow

The increase of rainfall and rise of groundwater level can cause frequent failures of unsaturated soil slopes that result in catastrophic landslides. This paper proposes a stochastic method for predicting the risk of failure of an infinite soil slope subjected to unsaturated infiltration flow. Stochastic models for shear stress and strength at an arbitrary plane of an infinite slope are developed. The accuracy of the proposed method is verified with the Monte Carlo simulation method. The merit of the proposed method lies in its analytical form, which can easily facilitate practical applications. Furthermore, a risk-based sensitivity analysis is undertaken in this study to identify the factors that are the most random and that affect the slope failure most significantly. It is found in this study that the developed stochastic models can capture the randomness of all factors that contribute to slope failure. It is also found that the slope inclination angle (), soil air entry value (b), water table rise from matric suction (yb), dry soil unit weight (d), and soil-specific constant (a phi) are important random variables in the accurate prediction of slope failure. The paper concludes that the proposed stochastic method can serve as a tool for geotechnical engineers to predict the risk of slope failure with accuracy and ease.

Undrained Behavior of Macau Marine Clay with Various Strain Rates and Different Stress Histories

Hydro-mechanical behaviors of a saturated clay with various strain-rate and different stress histories are investigated through a series of consolidated-undrained and K0 consolidated undrained (K0CU) tests which were conducted on reconstituted specimens. The samples were taken from an excavation site in Macau and reconstituted in the laboratory to provide homogenous specimens. A total of four CU tests and four K0CU tests were undertaken on 38 mm diameter samples. The samples were saturated first by use back pressure saturation method (back pressure 200 kPa), after saturation, they were loaded to a confining stress 400 kPa and then unload to 50 kPa, 100 kPa, 200 kPa, 400 kPa (OCR = 8, 4, 2, 1) and finally were sheared at constant shear rate (0.1%/h, 1%/h, 10%/h) to failure. In this research the responses of marine clay under various strain rates under different OCRs on the strain-rate effects will be discussed, parameters for accounting the strain-rate effects on undrained shear strength will be proposed.

A Constitutive Model for Unsaturated Soils Using Degree of Capillary Saturation and Effective Interparticle Stress as Constitutive Variables

This paper discusses the role of the degree of capillary saturation in modelling the coupled hydro-mechanical behaviour of unsaturated soils and proposes a new constitutive model for unsaturated soils by using the degree of capillary saturation and the effective inter-particle stress. In the model, the shear strength, yield stress and deformation behaviour of unsaturated soils are governed directly by the above two constitutive variables. The model is then validated against a variety of experimental data in the literature, and the results show that a reasonable agreement can be obtained using this new constitutive model.

Geological and hydrogeological environment with geohazards during underground construction in Hangzhou: a review

The urban area of Hangzhou is located in the northeast region of the Hangzhou Bay, beside the estuary of the Qiantang River. Three main ancient rivers are present in the case study area. The ancient rivers, together with the influence of three transgressions, influence the geological and hydrogeological formations of the urban area of Hangzhou. Soft clay is widely deposited in the study area and the Quaternary strata are rich in pore water, karst water, and bedrock fissure water. Confined aquifers located in the case study area have high pressure, high permeability, and rich water, and their levels fluctuate with tide. Karst strata, soft clay, ancient confined aquifer, and shallow gas deposits are potential geological features, which may threaten underground construction in the urban area of Hangzhou. As such, corresponding pre-treatments should be adopted to control the potential geohazards. Karst caves are proposed be infilled or reinforced before the commencement of underground construction works. Ground improvement methods can be adopted to enhance the strength of soft soil. Foundation dewatering in foundation pits with pensile curtains can be adopted to control ancient confined aquifers. Pre-exhaustion of shallow gas prior to commencement of construction works is an effective measure to control shallow gas with high pressure. Moreover, the impacts of the Qiantang River tide on the groundwater level and the deformation of underground structures should be considered.

Modelling Hydro-Mechanical Interactive Behaviour of Unsaturated Soils

This paper presents an alternative approach for interpreting unsaturated soil behaviour, which is built in the space of stress versus degree of saturation. A new volume change equation is proposed in terms of stress and degree of saturation, to give a better explanation to the non-linear change of soil compressibility under constant suctions. An alternative approach to simulate hydraulic hysteresis and hydro-mechanical interaction is then introduced, which enables the calculation of the effective degree of saturation under complex stress and suction paths. The loading-collapse yield surface is derived based on the proposed volume change equation in the plane of the effective degree of saturation and the Bishop effective stress. The proposed volume change equation and the corresponding yield surface are generalized to three-dimensional stress states by incorporating with the Modified Cam-clay model. The proposed model is validated against a variety of experimental data in literature.

Modelling Volume Change Behaviour for Unsaturated Soils in the Stress–Saturation Space

Volume change behaviour of unsaturated soils is usually modelled in terms of stress and suction. This approach is consistent with laboratory tests where suction is a controllable variable. However, it also suffers some limitations. This paper presents an alternative approach for modelling volume change behaviour of unsaturated soils. A new volume change equation is proposed in terms of stress and degree of saturation, to give a better explanation to the non-linear change of soil compressibility under constant suctions. The soil compression index is assumed to be a function of the effective degree of saturation and is interpolated from the known compressibility at the fully saturated state and that at a dry state. The proposed model for volume change is validated against a variety of experimental data in literature.

Modeling Unsaturated Soil Behaviour in Stress-Saturation Space

This paper presents an alternative approach for interpreting unsaturated soil behaviour. In this approach, all constitutive laws are built in the space of stress versus degree of saturation. A new volume change equation is proposed in terms of stress and degree of saturation, to give a better explanation to the non-linear change of soil compressibility under constant suctions. The soil compression index is assumed to be a function of the effective degree of saturation. The yield surface is established in the plane of the effective degree of saturation and the Bishop effective stress. The proposed model is then validated against a variety of experimental data in literature.