Investigation on long-term dynamic properties of decomposed mudstone as a substitutive fill material in railway subgrade

In southwest China, red mudstone fill material (RMF) is widely used in constructing railway subgrades to substitute the conventional unbound granular materials (UGMs). Besides the strain-level dependent dynamic properties, RMF significantly depends on loading cycles. However, such an effect has yet to be incorporated into the current design method, which would lead to a considerable misprediction in dynamic responses of the RMF subgrade during the operation period. This paper presents a comprehensive study of long-term dynamic properties of RMF (a silty clay) over a range of water contents and cyclic stresses. The objective is to establish a normalization framework of dynamic properties that considers the effect of large numbers of cyclic loading. With this emphasis, 40 cyclic triaxial tests with 50000 loading cycles were conducted on RMF specimens compacted at various water contents. Two-stage behavior has been identified in equivalent Young’s modulus and damping ratio evolutions. An exponential model is thus proposed to capture the two-stage pattern. The proposed normalization procedure showed a competent availability for the characterization of equivalent Young’s modulus and damping ratio at different loading cycles. Soil fabric also played a decisive role in evaluating RMF’s dynamic responses. Evidence of microfabric effect on the dynamic responses of RMF was strengthened by the Mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) analysis.