The inclinodeformometer (IDM) is a novel device for measuring changes in earth pressure in a sliding layer of a creeping landslide. The device makes use of existing and widely used inclinometer measurement technology. The change of earth pressure in the sliding layer leads to changes in the shape and dimensions of the incinometer pipe. Careful measurements of these changes make it possible to backcalculate the pressure increment from the solution of a boundary value problem with properly described constitutive behaviors of the pipe and surrounding soil. An advantage of the inclinodeformometer is that it does not require any additional infrastructure than standard inclinometer pipes, even long after they are sheared and became unsuitable for inclinometer measurements. Full scale laboratory tests performed in a 2 m high calibration chamber demonstrated that simple constitutive models can be used for backcalculation as a first approximation. Initial field measurements performed on the St. Moritz landslide confirmed the existence of a compression zone in this constrained creeping landslide, and gave an order of magnitude for the pressure increment. This confirms the shear strength on the sliding surface decreased over time, implying that Brattas landslide in St. Moritz has to be analyzed considering a “potential failure scenario”.

View full abstract

Soil nails have been widely used to retain excavations and stabilize steep cutslopes. A series of dynamic centrifuge model tests were conducted on nail-reinforced and unreinforced slopes during an earthquake, with several influence factors, including the nail length, nail spacing, and the inclination of slope, taken into consideration. The unreinforced slope exhibited a progressive failure in the middle and lower parts though the global slip surface did not appear due to the earthquake, which was arrested by using the nail reinforcement. The nails changed the dynamic acceleration response of the slope during the earthquake. The deformation of the slope was significantly decreased by the nails within a nail-influence zone. This zone involved the slip surface of the unreinforced slope, and was almost completely independent on the layout of the nail-reinforcement when the nails had sufficient length. A point couple analysis, a strain analysis, and a uniformity analysis were carried out in an attempt to determine why nails can increase the stability of a slope. It was discovered that the nails forced the deformation of the slope to be more uniform and thus arrested possible strain localization under earthquake conditions. As such, it is suggested that increasing nail length or decreasing nail spacing can both improve the nail-reinforcement effect, and increase the stability level of a slope.

View full abstract

About 50% of the world's dam failures are triggered by piping, which is a primary cause of embankment breaks. The phenomenon of piping results from the erosion of soil particles and their transport within a soil mass. In this paper, a numerical method is proposed to analyze the erosion within soils and the transport of eroded soil particles by adopting the concept of the erosion rate of soils. In such an analysis, the saturated-unsaturated seepage flow of the pore liquid, the detachment of the soil particles from the soil fabric, and the migration of the eroded particles are taken into consideration, and the equations related to the conservation of the pore liquid and the eroded soil particles are numerically solved. This numerical simulation allows for the procurement of the temporal alteration and the spatial distribution of the porosity, the particle size distribution, and the concentration of the detached soil particles in the pore liquid, as well as the distribution of pore liquid pressure. The results have revealed that the method can reproduce the experimental data from previous studies on the internal erosion of soils and that it qualitatively predicts, from the numerical experiments, the typical development of piping within soils, such as soil blocks and embankments, as the solutions to the initial and the boundary value problems of the governing equations.

View full abstract

A multiphase deformation analysis of a river embankment was carried out using an air-soil-water coupled finite element method capable of considering unsaturated seepage flow. A numerical model for unsaturated soil was constructed based on the mixture theory and an elasto-viscoplastic constitutive model. The theory used in the analysis is a generalization of Biot's two-phase mixture theory for saturated soil. An air-soil-water coupled finite element method was developed using the governing equations for three-phase soil based on the nonlinear finite deformation theory, i.e., the updated Lagrangian method. Two-dimensional numerical analyses of the river embankment under seepage conditions were conducted, and the deformation associated with the seepage flow was studied. We have found that the occurrence of large deformations corresponds to the large values of the hydraulic gradients at the toe of the embankment, and that the overflow of river water makes the embankment more unstable. It has been confirmed that seepage-deformation coupled three-phase behavior can be simulated well with the proposed method.

View full abstract

The paper describes an extension of the computational code LIQSEDFLOW proposed by the authors. The salient features of the code lie in its capability to describe the multi-phased physics of subaqueous sediment gravity flows. Specifically, it combines Navier-Stokes/continuity equations and equations for the advection and hindered settling of grains for a liquefied soil domain, with a consolidation equation for the underlying, progressively solidifying soil domain via a transition layer characterized by zero effective stress and a small yet discernible stiffness. Evolutions of the flow and solidification surfaces are traced as part of the solution using a volume-of-fluid (VOF) technique. The predicted features of the gravity flows of initially fluidized sediments with different concentrations conform to the observed performances in two-dimensional flume tests. The present results demonstrate the crucial role of two-phase physics, particularly solidification, in reproducing the concurrent processes of flow stratification, deceleration, and redeposition in subaqueous sediment gravity flows.

View full abstract

This paper investigates a numerical modeling of progressive failure in rock mass using the distinct element analysis. The numerical modeling consists of two processes. The first is to get the mechanical properties of synthetic specimens composed of circular rigid elements with the bonded effect between elements. The second is to analyze deformation of a reduced-scale rock slope under the gravity increased condition. With the calibrated properties of the synthetic specimen unchanged, an approach to initialize stress state in the slope model is studied. Evolution of displacements and the resulting initiation of failure surface in the slope model are displayed.

View full abstract

The Material Point Method (MPM), as proposed by Sulsky et al. (1994), has been developed to simulate large deformations and failure evolution involving different material phases in a single computational domain. A continuum body is divided into a finite number of subregions represented by Lagrangian material points, while the governing equations are formulated and solved with the Eulerian grid. Since this grid can be chosen arbitrarily, mesh tangling does not appear in the MPM. To design a simple but robust spatial discretization procedure, the MPM is coupled with the finite difference method (FDM) in the present study for simulating fully and partially saturated elasto-plastic soil responses based on the simplified three-phase method. Governing equations for the soil skeleton and the pore fluid are discretized by the MPM and FDM, respectively. Soil-water coupled analyses for fully saturated soils and seepage-deformation coupled analyses for unsaturated soils are performed, and the potential of the proposed method is demonstrated via numerical examples.

View full abstract

The damage caused by an earthquake-induced landslide can generally be classified as either a limited deformation or a catastrophic failure. From an engineering point of view, the latter can be much more dangerous because the sliding mass may continue moving until it collides with another object. If a catastrophic failure occurs near a river, the debris may block the river, causing serious damage to the adjacent area. Therefore, examination of the mechanism of such catastrophic slope failures is important with respect to the mitigation of earthquake disasters in mountainous districts, although numerical modeling of such phenomena is rather difficult.
In the present study, a new numerical model is developed to simulate an earthquake-induced catastrophic landslide that occured at a typical dip slope, namely, the Yokowatashi Landslide in Japan. In this case, the upper part of the bedrock on the planer tectonic dip surface slid more than 70 m. Only shear-strength degradation at the bedding plane could cause such a long-distance traveling failure. To investigate the strain-softening characteristics of the materials that filled the bedding plane, a series of laboratory tests involving undisturbed block samples was performed. The measured stress-displacement relationships under cyclic loading were numerically modeled as a newly proposed elasto-plastic constitutive model to be used in numerical simulations of landslide, based on the dynamic finite element method. The observed phenomena were appropriately simulated by the proposed method.
The mechanism of catastrophic failure is discussed in detail in this paper in order to clarify the relationships between the strain-softening characteristics and the global slope stability. Our newly proposed method to evaluate the possibility of a catastrophic failure was applied to the landslide, and the moment when the slope becomes unstable was able to be predicted. The results confirm that the proposed method can predict the catastrophic failure of a slope.

View full abstract

Particle breakage occurs in granular materials with various engineering applications, such as when driving piles (especially where the strength of the particles is low) and in debris flows (where the energy levels are high), and the influence of this breakage on the mechanical behaviour of soils should be given proper consideration in a constitutive model for soils. Particle breakage results in an increase in the number of fine particles and broadens the grading of particle sizes, and the primary effect of broadening the grading is to lower the critical state line and other characteristics of the volumetric response in the compression plane. In our study, an existing constitutive model, the Severn-Trent sand model, in which the critical state line plays a central role as the locus of asymptotic states, has been extended to include the effects of particle breakage. Severn-Trent sand is a frictional hardening Mohr-Coulomb model described within a kinematic hardening, bounding surface framework. The central assumption is that strength is seen as a variable quantity, dependent on the current value of the state parameter (volumetric distance from the critical state line) which varies with changes in density and stress levels. If the critical state line falls as a result of broadening grading, the state parameter tends to increase and the soil feels looser.

View full abstract