L. Z. Wu

Analysis of physical testing of rainfall-induced soil slope failures

Rainfall is a significant factor that triggers slope failures around the world. This paper reports a series of physical tests, which were conducted to simulate rain-induced slope failures. The experiments dealt with two scenarios including (1) rainwater infiltration into the slope and (2) slope failures induced by artificial rainfall with different initial conditions. Slope deformation and slope failures were observed and possible mechanisms were interpreted based on the experimental results. The results confirm the hypothesis that pore-water pressure and water content in a loose soil slope change rapidly and that water infiltration into cracks in the slope has a great impact on landslide development. The observed slope failures can be divided into three types: overall sliding failure, partial sliding failure and flow slide. The effect of slope gradient, rainfall intensity and distribution of initial suction on the slope deformation and failure process are also summarized for possible applications under the similar conditions.

One-Dimensional Coupled Infiltration and Deformation in Unsaturated Soils Subjected to Varying Rainfall

AbstractAssuming that the water content and hydraulic conductivity of unsaturated soil are exponential functions of the pressure head, Green’s function is used to obtain analytical solutions to one-dimensional coupled rainfall infiltration and deformation in unsaturated soils under varying surface rainfall flux. The analytical solution considers varying flux and pressure head at the top boundary and arbitrary initial conditions. In particular, nonlinearly increasing rainfall intensity at the surface boundary is considered. The result indicates that the coupling of seepage and deformation has a significant effect on the pressure-head profiles for the transient unsaturated seepage. The coupling effect is closely related to the boundary conditions. The coupling effect almost disappears if ponding at the ground surface occurs. The ponding time is different under the coupled and uncoupled conditions. There is a quick change in the pressure-head profiles near the time of ponding or peak or discontinuous rain in...

Numerical analysis of 1D coupled infiltration and deformation in layered unsaturated porous medium

Unsaturated layered soils are common in nature and engineering practices, including highway embankments and waste landfills. Based on the principle of effective stresses and considering the deformation in unsaturated porous medium and changes in permeability with the stress state during rainfall, a one-dimensional model for simulating coupled seepage and deformation in layered unsaturated porous medium is established. A finite element program was written in FlexPDE to analyze the hydro-mechanical coupled process of rainfall infiltration into a two-layer unsaturated porous medium. The factor of safety as a function of the wetting front depth is discussed based on a simple infinite slope analysis. The numerical results demonstrate that the coupling of seepage and deformation plays a significant role in the movement of wetting front, the distribution of pore-water pressure, and the slope stability. The coupling of seepage and deformation should be taken into account when analyzing the rainfall infiltration into the layered unsaturated porous medium, especially for a combination of a short-duration heavy rain and high initial suctions.

Rainfall infiltration-induced groundwater table rise in an unsaturated porous medium

Increased rainfall associated with climate change is becoming a common occurrence and can lead to a variety of geoenvironmental hazards. Infiltration during rainfall and deformations of a soil mass are coupled, and the water table can easily experience a rise during these events. A study of the coupling process is important to increase our knowledge concerning the impact of a water level rise on the environment. Based on the theory of seepage and flow in porous media, the theory of elasticity and the soil–water characteristic curve, a two-dimensional hydro-mechanical model for an unsaturated porous medium is developed and incorporated in the COMSOL Multiphysics® software. The numerical procedure is capable of considering the influences of the soil–water characteristic curve. The effects of varying the boundary conditions on the coupled unsaturated infiltration and deformation equations are investigated. The examples demonstrate that the coupling effects significantly influence the position of the groundwater levels. The rate of change in the water table is closely related to the coupled infiltration and deformation in an unsaturated porous medium.

Analytical Analysis of Partially Saturated Infiltration Coupled with Deformation in a Semi-Infinite Region

AbstractNumerical methods that incorporate hydromechanical processes in unsaturated soils meet substantial challenges when considering semi-infinite zones. Although restricted assumptions can limit...

The Role of Surface Infiltration in Hydromechanical Coupling Effects in an Unsaturated Porous Medium of Semi-Infinite Extent

Rainfall infiltration into an unsaturated region of the earth’s surface is a pervasive natural phenomenon. During the rainfall-induced seepage process, the soil skeleton can deform and the permeability can change with the water content in the unsaturated porous medium. A coupled water infiltration and deformation formulation is used to examine a problem related to the mechanics of a two-dimensional region of semi-infinite extent. The van Genuchten model is used to represent the soil-water characteristic curve. The model, incorporating coupled infiltration and deformation, was developed to resolve the coupled problem in a semi-infinite domain based on numerical methods. The numerical solution is verified by the analytical solution when the coupled effects in an unsaturated medium of semi-infinite extent are considered. The computational results show that a numerical procedure can be employed to examine the semi-infinite unsaturated seepage incorporating coupled water infiltration and deformation. The analysis indicates that the coupling effect is significantly influenced by the boundary conditions of the problem and varies with the duration of water infiltration.

The rainfall-induced Wulipo rockslide, China: a modified model for rockslide initiation

During the May 12, 2008 Wenchuan earthquake, rock masses in the earthquake region were shaken, and subsequently, a number of post-earthquake landslides and debris flows occurred, triggered by heavy rainstorms. This paper presents a study of the rainfall-induced mechanism that triggered the landslides. This study is based on pre- and post-landslide geomorphology and geological features identified from pre-landslide high-resolution satellite imagery and post-landslide aerial photographs of the Wulipo landslide. The remote sensing observations were verified by field investigations. The results indicate that the heavy storm that occurred in July 2013 was the main trigger for the landslide although existing tensions and weak rock interfaces were the main internal causes for reactivation of the landslide. The bedding in the rocks dips in the same direction as the topographic slope. A tensile trough developed at the scarp. Based on data from the Wulipo rockslide, a model is proposed for calculating the safety factor for rockslides that takes into account the hydrostatic pressure along the structural plane of the rockslide. The results show that the proposed method can effectively estimate the stability of a storm-induced rockslide in regions underlain by bedded rocks. Based on this estimate, it is concluded that the Wulipo rockslide is a reactivated old rockslide.

Neogene clay and its relation to landslides in the southwestern Loess Plateau, China

Loess occurs widely in Northwestern China, covering Neogene clay and other sedimentary units that overlie the bedrock. The Neogene clays of the Baoji Region of Shaanxi Province, north China, including the eolian Hipparion and paleo-Sanmen Lake fluvial deposits, are typical clays of the southern Chinese Loess Plateau. These clays are also sensitive key strata controlling the development of large-scale landslides along the slope of the loess tableland surrounding the Guanzhong Basin. To investigate the Neogene clay types and landslides in the region, soil samples were tested and the macro- and micro-structures of the clay strata were analyzed. Comprehensive analysis methods, including the pipette method and fine X-ray diffraction, were applied to quantitatively analyze the sample composition and determine the clay mineral types. The Neogene clays are mainly composed of illite and montmorillonite mixed-layer minerals. On the Williams’ discrimination diagram of swelling potential, both types of clay are medium to strongly expansive soil. Two case studies illustrate that rainfall reduces the shear strength of the Neogene clay and raises the groundwater level. Rainfall has gradually destroyed the structure of the Neogene clay, transforming it to sliding belt soil. The sliding belt lies mainly within the Neogene clay layers, significantly affecting the occurrence and characteristics of landslides in Baoji. The rise in groundwater level and weakening of the shear strength of the Neogene clay are important factors in the occurrence of landslides. The long-term strength is a key factor affecting the development of loess landslides along the loess tableland in Baoji.

Theoretical analysis and model test for rainfall-induced shallow landslides in the red-bed area of Sichuan

Abstract Heavy rainfall is a key cause of shallow landslides in red-bed terrains with steep topography and residual soils of degradable strength. In this study, laboratory model tests were carried out to examine the characteristics of rainfall infiltration, deformation, and failures of slopes in the red-bed area of Sichuan. The hydrological response and deformation of the slope soil during rainfall are addressed. Based on a modified Green–Ampt infiltration model, ponding along the bedrock surface is incorporated. A physically based model for shallow landslides caused by rainfall is developed. The theoretical analysis and the model test results indicate that the slope failures are related to erosion in the shallow soil layer and rainwater infiltration, particularly along preferential seepage channels. The process of rainfall-induced shallow landslides can be separated into three stages: erosion at the slope toe, tension crack formation at the slope crest, and shallow sliding. When initial underground water level is located at the bedrock surface or the preferential seepage flow quickly reaches the bedrock surface, it is easier for the soil slopes to slide along the bedrock surface than along the wetting front.