Experimental investigation on dynamic characteristics of gravel soil filler for railway subgrade affected by varying initial moisture content

Gravel soil has been extensively applied to railway subgrade engineering, which would potentially be affected by rainfall-earthquake coupling effect during the process of long-term service, and its dynamic characteristics have a notable influence on the safety and stability of railway engineering projects. Test soil was derived from on-site railway subgrade filler, which is classified as poorly-graded gravel soil. To investigate gravel soil coarse-grained filler mechanical properties, this research conducted dynamic properties test, dynamic strength test, and static triaxial shear test for each soil specimen of different unsaturated states in proper sequence. During soil specimen consolidation process, the effective consolidation stress ratio K ′_c was controlled to remain constant for representing practical stress state of layered filling. The vibration loading applied in test included 12 level axial dynamic cyclic loading with gradually increasing amplitude and 1000 cycles of axial dynamic loading in turn. These research results demonstrate that (i) with vibration loading amplitude increasing, gravel soil specimens experienced this transition from small-strain elastic deformation state to intermediate-strain elastoplastic deformation state during whole experimental process, (ii) gravel soil filler possessed well dynamic characteristics under long-term vibration loading effect, the variation rule of dynamic shear stress amplitude τ _max − dynamic shear strain amplitude γ _max curve and dynamic axial strain amplitude ε¯_a,d − dynamic axial loading amplitude σ¯_a,d curve emerged approximate linear, however, damping ratio λ decreased with vibration loading amplitude increasing, (iii) moreover, the maximum values of both dynamic elastic modulus and dynamic shear modulus exhibit a polynomial fitting correlation with effective confining pressure σ ′₃ and pore water pressure coefficient B, (iv) gravel soil still maintained paradigmatic deformation characteristics of loose sand after the action of long-term vibration loading. Overall, this research has put forward further insights into dynamic response characteristics of gravel soil filler, which would provide basic design parameters and theoretical reference for seismic response analysis of railway subgrade.