Soils that exhibit nonassociated flow may, according to stability postulates by Drucker and by Hill, become unstable when exposed to certain stress paths inside the failure surface. Series of conventional triaxial tests on fully saturated and on partly saturated specimens were performed under drained and undrained conditions to study the regions of stable and unstable behavior. For specimens that compress and have degrees of saturation higher than critical, undrained conditions lead to effective stress paths directed within the region of potential instability, and instability was observed provided the yield surface opens up in the outward direction of the hydrostatic axis. Thus, instability occurs inside the failure surface. Instability is not synonymous with failure, although both may lead to catastrophic events. The location of the instability line is discussed. Examples of a shallow submarine slope and a nearly fully saturated steeper slope representing a tailings dam, which both should remain stable a...

Closure of "Static Instability and Liquefaction of Loose Fine Sandy Slopes"

Experimental study on face instability of shield tunnel in sand

Face stability analysis of shallow circular tunnels in cohesive–frictional soils

Three-dimensional face stability analysis of pressurized tunnels driven in a multilayered purely frictional medium

Tunnel face stability in cohesion-frictional soils considering the soil arching effect by improved failure models

This paper is an investigation of face stability on a small-scale tunnel model in a geotechnical centrifuge. By making use of symmetry, half of the tunnel cross section was considered. The support at excavation face was provided by a piston, which was adjusted during flight. Some aspects on the collapse at tunnel face are investigated for different overburden pressures: failure mechanism, surface settlement, stress acting at tunnel face, and the required face support counteracting the earth pressure. Ground deformation was observed through a transparent wall and measured by digital image correlation. The results from centrifuge model tests were compared with theoretical models.

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Centrifuge model test on the face stability of shallow tunnel

This paper presents the results of centrifuge tests on rainfall-induced instabilities in variably saturated slopes. The roles of rainfall intensity and initial conditions, such as slope angle, porosity and degree of saturation of the soil, in the failure initiation and postfailure kinematics are considered. The failure patterns, infiltration profile and deformation at prefailure and postfailure stages are characterized. The results indicate that rainfall-induced slope failures usually follow one of the following two failure modes, i.e. slide-to-flow and flowslide failure modes. The former pattern is characterized by soil mass flow after initial failure along a continuous shear surface, while the latter is more relevant to the rapid increase in the saturation at the slope surface, resulting in surface erosion channels followed by the acceleration of the soil mass. The flowslide failure pattern usually gives rise to several superficial shear surfaces and longer run-out distances. The rainfall intensity and profiles of the degree of saturation play the key roles in initiating the slope failure at the prefailure stage and subsequently in mobilizing the soil mass at the postfailure stage. Our test data, together with the data from the literature, are presented in two threshold curves to define the critical condition of slope failure under rainfall infiltration.

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Centrifuge modelling of rainfall-induced slope failure in variably saturated soil

We investigate the impact of spatially varying saturation on the localization properties of unsaturated soils. The approach is based on mesoscale modeling in which continuum state variables including the suction stress are allowed to vary both spatially and temporally within a soil specimen. Continuum hydromechanical simulations are conducted on a rectangular specimen of unsaturated soil using stabilized low-order mixed finite elements in plane strain condition. The results reveal important patterns of shear band development triggered by the spatially varying saturation.

Finite Element Simulation of Strain Localization in Unsaturated Soils

Rainfall-induced slope instabilities are ubiquitous in nature, but simulation of this type of hazards with centrifuge modelling still poses difficulties. In this paper, we introduce a rainfall device for initiating slope failure in a medium-sized centrifuge. This rainfall system is simple, robust and affordable. An array of perforated hoses is placed close above the model slope surface to generate the raindrops. The rainfall intensity depends on the centrifuge acceleration and the flow rate of the water supply, which is controlled by the size and number of the tiny pinholes in the hose walls. The rainfall intensities that are tested range from 2.5–30 mm/h, covering the intensity range of moderate, heavy and torrential rainfall events. Our model test with rainfall-induced slope failure shows that this system is capable of generating relatively uniform rainfall of wide intensities and leads to various patterns of slope failure.

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A device for rainfall simulation in geotechnical centrifuges

A set of centrifuge model tests is performed to investigate the stability and failure initiation of unsaturated soil slopes. Homogeneous model slopes of reduced size are tested in the enhanced acceleration field of the geotechnical centrifuge. The body stress increases due to increasing self-weight until failure conditions are reached. The pre-failure soil displacement and the propagation of the shear zone is visualized by the digital image correlation technique. The experimental parametric study with granular soil investigates the role of moisture content, soil density, and slope inclination. Observed failure modes can be classified as shallow translational slides of coherent soil blocks of low mobility. The shear zone emerges at regions of local weakness, and is observed to initiate at the toe of the slope or around mid-height of the slope.

Gregor Idinger

Papers

Centrifuge model test on the face stability of shallow tunnel

Centrifuge modelling of rainfall-induced slope failure in variably saturated soil

Finite Element Simulation of Strain Localization in Unsaturated Soils

A device for rainfall simulation in geotechnical centrifuges

Failure of Unsaturated Soil Slopes Initiated by Self-weight Loading