Experimental study on critical dynamic stress and dynamic resilient modulus of frozen silty clay under long-term cyclic loading
Ensuring traffic safety necessitates a comprehensive understanding of both the transient and long-term responses of subgrade materials under traffic loads.Currently,the critical dynamic stress is commonly utilized as a benchmark for regulating long-term responses,while the dynamic resilience modulus is associated with the transient response.In cold regions,the subgrade's near-surface soil layers frequently remain frozen during win-ter,making the dynamic behavior of these frozen soils under prolonged traffic loading a crucial factor for the in-tegrity of railway and highway infrastructure.However,current research predominantly concentrates on the dy-namic properties of unfrozen subgrade soils,with a significant gap in knowledge regarding the behavior of fro-zen soils.This study conducts an extensive series of cyclic triaxial tests on frozen silty clay to meticulously inves-tigate the two aforementioned indices:critical dynamic stress and dynamic resilience modulus.Initially,a de-tailed analysis of the accumulated axial strain is presented,demonstrating that higher confining pressures or low-er cyclic stress levels result in reduced axial strain accumulation over an equivalent number of loading cycles.Subsequently,this study introduces a novel approach for determining the critical dynamic stress.Building on the findings of Chen et al.(2019),the accumulation curve for frozen soils is categorized into two distinct stages:post-compaction compression and secondary cyclic compression.Post-compaction compression concludes after a limited number of loading cycles,suggesting that long-term subgrade settlement is predominantly influenced by the axial strain accumulation during secondary cyclic compression.The accumulation curve for this stage is mathematically represented by Eq.(2),where the parameter as-1 denotes the slope of the curves against,lg((Ns+N0)/N0)as illustrated in Fig.5.The control parameter as-1 for each curve in the secondary cyclic compres-sion phase is determined through least-square fitting,which quantifies the long-term accumulated deformation following a specific number of loading cycles.Additionally,three hypothetical strain accumulation demands are proposed,and the corresponding as-1 values are calculated.The critical dynamic stress for each demand is then ascertained using linear interpolation.Furthermore,the study examines the effect of confining pressure on the critical dynamic stress,revealing that higher confining pressures correspond to elevated critical dynamic stress-es,indicating a significant enhancement in the long-term bearing strength of frozen soil with increasing confin-ing pressure,particularly at high cyclic stress amplitudes.Finally,this study presents the variations in the dy-namic resilience modulus of frozen silty clay with increasing loading cycles.The results indicate an initial de-crease in the dynamic resilience modulus to a minimum value,followed by a trend towards stabilization.The im-pact of confining pressure and dynamic stress amplitude on the dynamic resilience modulus is also investigated.The findings suggest that the dynamic resilience modulus of frozen soil increases with confining pressure,with-out exhibiting a peak value,implying that the pressure-thaw weakening effect is outweighed by the strengthening influence of confining pressure under the tested conditions.Moreover,within the scope of this study's loading parameters,the effect of dynamic stress amplitude on the dynamic resilience modulus is found to be negligible.
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