Subway subgrades typically consist of alternating deposits of soil layers with significantly different physical and mechanical properties. However, the overall dynamic characteristics and the evolution of micro-porous structures in stratified soils is often overlooked in current studies. In this study, cyclic triaxial tests were conducted on homogeneous sand, silt and stratified soils with different height ratios, and nuclear magnetic resonance (NMR) was used to investigate the changes in pore structure and moisture content. The dynamic behavior and macroscopic deformation mechanisms were systematically investigated in terms of stress amplitude, confining pressure, and layer height ratio (the ratio of sand to silt height). The results show that as the sand height ratio increases, the axial strain and pore water pressure first increase and then decrease, reaching the maximum when h(Sand): h(Silt) = 2:1. When the confining pressure is 100 kPa, the axial strain of h(Sand): h(Silt) = 2:1 is 181.08 % higher than that of silt. Under the dynamic loading, the stratified soils form a dense skeletal structure near the stratification plane, which hinders the flow and dissipation of pore water, so that the pore water agglomeration phenomenon occurs near the stratification plane, which aggravates the accumulation of residual pore pressure and reduces the deformation resistance. However, when h(Sand): h(Silt) = 4:1, the influence of the stratification planes is significantly reduced, and the deformation characteristics approach homogeneity. This study reveals the dynamic characteristics of stratified soils by comparing and analysing homogeneous samples.

In silty (fine) sand aquifer, water-soil gushing (WSG) of shield tunnel may occur, causing structural damage and even collapse. A comprehensive understanding of the mechanism behind ground displacement and tunnel deformation during WSG in stratified soil was required for guiding disaster-relief in practice. In this article, the responses of ground and shield tunnel to WSG in stratified soil were investigated using a material-point method (MPM). First, a typical case of WSG in stratified soil was studied. By comparing the results with those of WSG in a homogeneous sand, both the ground and tunnel responses to WSG in a stratified soil were clarified. It was found that in stratified soil, the tunnel lining may deform first and then became stable, while in homogeneous sand, the tunnel deformation was shown to continuously develop with time due to unremitting soil loss. Then, the effects of the WSG locations, the sand layer position to tunnel, the layers number and permeability of clay and the discharge rate on WSG were further analyzed.

Stratified soil is a type of widely distributed special soil, consisting of alternating interlayered soils with distinct properties in both terrestrial and marine sedimentation conditions. It is endowed with anisotropic physical properties and mechanical behavior by its unique laminar structure features. So far, its mechanical behavior has not been fully understood. To systematically investigate the laminar structure effects of stratified soil, artificially prepared stratified soil samples of silty clay interlayered by silty sand were studied. First, the laminar structure features of stratified soil in Yangtze River floodplain deposits at Nanjing, China, were summarized. Then, based on the laminar structure features, preparation method for stratified soil samples was proposed by stacking soil layers one by one, which was basically an integration of soil paste plus consolidation method for silty clay layer preparation and water pluviation plus freezing method for silty sand layer preparation. After verification of the sample preparation method, a series of consolidated-undrained triaxial compression tests were carried out to study the mechanical behavior of stratified soil. The effects of thickness of constituent layers, consolidation conditions (isotropic or anisotropic consolidation), and loading paths (conventional triaxial compression, constant-p compression, and lateral extension) were investigated. The results show that the mechanical behavior of stratified soil (including stress-strain curves, excessive pore pressure accumulation, sample failure modes, and strength index) generally falls in between the behavior of the two constituent layers of soil, i.e., a normally consolidated silty clay and a medium-dense silty sand. The silty clay layer thickness (with fixed silty sand layer thickness), consolidation conditions, and loading paths together determine the stratified soil behavior, either silty sand dominant or silty clay dominant. Laminar structure can improve volumetric dilation trend and thus increase undrained shear strength of stratified soil. The presence of silty clay layer would suppress shear banding development in stratified soil. The strength of stratified soil can be underestimated by experiments using disturbed or remolded samples where the laminar structure is partially or completely lost.

Permanent deformation and uplift caused by fault rupture is one of the most significant hazards posed by earthquakes on the built environment. In this paper, we use smoothed particle hydrodynamics (SPH) to explore the effects of soil layering or stratification on the trajectories and deformation patterns caused by rupturing reverse faults in bedrock, as well as in the foundations of engineered earth structures. SPH is a continuum meshfree numerical method highly adept at modeling large deformation problems in geotechnics. Through the use of constitutive models involving softening behavior as well as critical state type models, we isolate the effects of rigid body rotation from critical state behavior of soil in helping explain the frequently observed rotation of shear bands emanating from the bedrock fault. This analysis is facilitated by the fact that the SPH method allows us to track the propagation of shear bands over substantial amounts of vertical uplift (more than 50% of the total height of the soil deposit), far beyond many previous computational studies employing the finite element method (FEM). We observe and characterize various emergent features including fault bifurcations, stunted faults, and tension cracking, while providing insights into practical guidelines regarding the potential surface distortion width, and the critical amount of fault displacement required for surface rupture depending on the multilayered constitution of the soil deposit. Finally, we predict the expected amount of surface distortion and internal damage to earthen embankments depending on varying fault location and soil makeup.

Rainfall infiltration plays a crucial role in the near-surface response of soils, influencing other hydrological processes (such as surface and subsurface runoff, groundwater dynamics), and thus determining hydro-geomorphological risk assessment and the water resources management policies. In this study, we investigate the infiltration processes in pyroclastic soils of the Campania region, Southern Italy, by combining measured in situ data, physical laboratory model observations and a 3D physically based hydrological model. First, we validate the numerical model against the soil pore water pressure and soil moisture measured at several points in a small-scale flume of a layered pyroclastic deposit during an infiltration test. The objective is to (i) understand and reproduce the physical processes involved in infiltration in layered volcanoclastic slope and (ii) evaluate the ability of the model to reproduce the measured data and the observed subsurface flow patterns and saturation mechanism. Second, we setup the model on the real site where soil samples were collected and simulate the 3D hydrological response of the hillslope. The aim is to understand and model the dynamics of hydrological processes captured by the field observations and explain the redistribution of water in different layers during 2 years of precipitation. For both applications, a Monte Carlo analysis has been performed to account for the hydrological parameter uncertainty. Results show the capability of the model to reproduce the observations in both applications, with mean KGE of 0.84 and 0.68 for pressure and soil moisture data in the laboratory, and 0.83 and 0.55 in the real site. Our results are significant not only because they provide insight into understanding and simulating infiltration processes in layered pyroclastic slopes but also because they may provide the basis for improving geohazard assessment systems, which are expected to increase, especially in the context of a warming climate. Combining physical model and in situ measurements of soil water content and soil water pressure together with a 3D hydrological models, we detailed and disentangled the infiltrations processes trough layered pyroclastic soils. The finding will be relevant for accurate geo-hydro risk management in a changing climate. image