There are growing evidence indicating that earthquakes occur more frequently in the Chinese Loess Plateau, which can possibly cause saturated intact loess liquefaction, leading to geological disasters. However, the cyclic behaviour of saturated intact loess is still much less investigated. In the presented work, typical intact loess was collected to carry out a comprehensive experimental investigation, including both undrained monotonic and cyclic triaxial shear tests with different confining stresses and cyclic stress. The results indicate that the intact loess shows an overall clay-like cyclic behaviour. It shows a strain softening behaviour under undrained monotonic shearing, while under undrained cyclic shearing, a well-defined failure pattern of plastic strain accumulation is observed, and an axial strain of 3 % is determined as the failure criterion. Although different excessive pore water pressure generation patterns and failure criteria are found under monotonic and cyclic conditions, a quantitative relationship is established between the monotonic and cyclic stress-strain behaviour based on the unique relationship found between axial strain and the defined allowable stress. Additional microstructure observations and analyses further reveal the differences in loess structure destruction caused by different loading conditions. This study helps to further understand the cyclic behaviour and failure mechanism of intact loess, and to provide a useful reference for the evaluation and prevention of possible soil liquefaction failure in loess areas.

It is frequently observed that the stress-strain behaviour of soft clayey soils is affected by temperature changes. Development and verification of a reliable constitutive model with consideration of variable temperature conditions are necessary. Due to the significant rheological and other nonlinear properties of clayey soils, the coupled effects of temperature, time dependency, structuration, nonlinear creep, and anisotropy should be considered in the constitutive model. In this study, a new threedimensional (3D) thermal elastic visco-plastic model is established and verified for the time-dependent stress-strain behaviour of clayey soils considering temperature changes. The model is developed based on the existing elastic visco-plastic models with the equivalent time concept, the overstress theory, and the critical state model. The thermal elastic line and virgin heating line are introduced and generalized to construct constitutive equations for both thermal elastic and thermal visco-plastic behaviour of clayey soils in general stress conditions. After establishing the 3D basic model, further refinement is introduced to consider the nonlinear creep behaviour and structuration for natural and reconstituted clayey soils. Finally, the model is successfully validated by a series of laboratory test data on different clayey soils under variable temperature paths with reasonably good accuracy.