The effect of overconsolidation on monotonic and cyclic behaviours of frozen subgrade soil

This paper presents an experimental investigation on the monotonic and cyclic behaviours of the overconsolidated subgrade soil used in the Lanzhou-Urumchi high-speed railway at the temperature of -3 degrees C. The important mechanical parameters of the overconsolidated soil, i.e., failure strength, initial elastic modulus, accumulative permanent strain and resilient modulus were experimentally studied by using triaxial tests. Testing results indicate that the failure strength, initial elastic modulus, dilatancy and stress-strain relationship of frozen sample are all dependent to the overconsolidation ratio (OCR). The failure strength is slightly more susceptible to the overconsolidation ratio than the initial elastic modulus. The anisotropically consolidated path (k(0) = 0.5 -1) has a negligible impact on the stress-strain-strength behaviors. The development characteristics of accumulative permanent strain and resilient modulus are collectively dominated by the overconsolidation ratio and shear stress amplitude. Axial permanent strain acceleratedly accumulates with the decrease of the overconsolidation ratio at the same shear stress amplitude. According to the shakedown theory, allowable cyclic stress ratio was introduced to analyze accumulative deformation characteristics of frozen sample. The overconsolidation ratio and loading cycle number have negligible influences on the allowable cyclic stress ratio of frozen subgrade soil. Four cyclic stress-paths with the same shear stress amplitude were adopted in triaxial cyclic tests with aim to investigate the path-dependent deformation behaviors of frozen subgrade soil. The relationships of axial permanent strain under different cyclic stress-paths are roughly proportional, regardless of the cyclic stress ratio (CRS) and loading cycle number. Those quantitative relationships can be completely determined by the overconsolidation ratio and cyclic stress-path parameter. The effect of cyclic stress-path on resilient modulus is more than the effect of overconsolidation ratio and less than the effect of shear stress amplitude. Several empirical models were proposed for the overconsolidated subgrade soil at the negative temperature to characterize the evolution laws of the above-mentioned engineering properties. Those empirical models are capable of giving well descriptions of the development of those engineering parameters for frozen subgrade soil over a wide range of the oveconsolidation ratio.