Mechanical Properties Of Gravelly Sand Under Different Stress Paths

The Qinghai-Tibet Highway(QTH)covers 550 km across the Plateau Permafrost Region(PPR)with harsh ecological environment and complex geological conditions.Frequent highway damages such as thaw subsidence,wave terrain,and longitudinal cracks develop along the highway.It seriously affects the safety of Qinghai-Tibet Highway traffic.Owing to the insufficient support system for expressway construction technology in PPR,the construction of the expressway in the Tibetan Plateau does not start.The subgrade subsidence is a hidden danger in the communication and transportation.It is also the hard problem for quality control of projects in cold regions and the hot spot of academic research.The influence of roadbed filling and frozen soil layer on the subgrade settlement is significant.The engineering properties of soil are complex and is greatly affected by the region.The research on the mechanical properties of Qinghai-Tibet gravelly sand has important guiding significance for the engineering construction in the plateau area.In view of such circumstances,this paper takes the Qinghai-Tibet highway and other road constructions as the background.The initial relative density,stress path,confining pressure and drainage condition on the mechanical properties of gravelly sand were investigated based on triaxial shear tests.This paper establish constitutive model for gravelly sand in the Qinghai-Tibet Plateau and the results can accumulate basic data for the study of sand mechanics.The main research contents and conclusions are as follows:(1)Based on the triaxial shear test under different stress paths,initial relative densities and confining pressures,the mechanical properties of the gravelly sand in Qinghai-Tibet plateau were studied.The results show that in the conventional triaxial shear test under undrained condition(CTC),the stress-strain curves of the gravelly sand all show strain softening,and the curves of pore water pressure all show hyperbola.In the conventional triaxial drainage shear test under drained condition(CTC),the stress-strain curves were all strain harding,and the volume deformation all show shear shrinkage.In the triaxial shear test(TC),the stress-strain curves showed strain softening when the initial relative densities were 0.75 and the confining pressures were 50 and 100 k Pa,and the volume deformation show shear dilatation.Under other conditions,the stress-strain curves of the gravelly sand all show the strain hardening and the volume deformation shows the volume shrinkage.In the triaxial shear test(RTC),when the initial relative density of the gravelly sand is 0.75,the stress-strain curves shows strain softening.Under other test conditions,the stress-strain curves of gravelly sand is strain hardening and the volume deformation is shrinkage.(2)To describe these triaxial shearing mechanical properties of gravelly sand,a higher-order dilatancy equation was proposed based on the concept of a super yield surface.A constitutive model which can describe the mechanical properties of gravelly sand was established when the associated flow laws was applied to compare with the results of the triaxial shearing test under the consolidated drained condition.The comparison results showed that the proposed model can reflect the strain hardening and shear contraction characteristics of gravelly sands from low to high confining pressures under different initial relative densities.(3)In view of the advantages of UH model in reflecting the dilatancy and strain softening of soil,a modified UH model suitable for gravelly sand was established based on the UH model.In order to better describe the stress-strain behavior and volume deformation of gravelly sand,the new parameter ξ that reflects current state of gravelly sand is introduced into hardening parameter H,thus developing a new hardening parameter H therefrom.Finally,a modified UH model was established to describe the strain hardening,strain softening,volume shrinkage and shear dilatation of gravelly sand.The comparison between the calculated results and the test results show that the modified UH model is applicable to the gravelly sand.