Geotechnical Slope Research Articles

Gigantic seismogenic landslides of Summer Lake basin, south-central Oregon

Subsidence of Summer Lake basin, situated in the northwestern Basin and Range province in south-central Oregon, has exposed a kilometer-thick Neogene sequence of dense volcanic flow rocks overlying very weak tuffaceous sedimentary rocks in the bounding Winter Ridge–Slide Mountain escarpment. Subsidence occurs along the active, 40-km-long, Winter Ridge–Slide Mountain fault, which is capable of producing Mw ≈ 7 earthquakes with near-field, maximum horizontal acceleration approaching 1 g. Gigantic landslides cubic kilometers in volume scallop the southwestern part of the escarpment, and the deposits run out several kilometers, characteristic of rock avalanches. Geotechnical rock-mass characterization and slope- stability analyses confirm observations that these landslides were initiated within the weak tuffaceous sedimentary rocks along shallow, east-dipping, planar failure surfaces, are insensitive to groundwater fluctuations, and are stable under static conditions. Pseudostatic analyses reveal that strong shaking is required to trigger landsliding. The Bennett Flat landslide failed as a single event between 900–270 ka and 16.8 ka; the landslide has a depletion-zone volume of 1.1 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Foster Creek landslide also appears to have failed as a single event between 180 and 16.8 ka; the landslide has a depletion-zone volume of 2.9 km3, but only 0.3 km3 of debris is discernible in the accumulation zone. The Punchbowl landslide evidences multiple events, the most recent of which failed in the Holocene and had a volume of 0.4 km3; 0.5 km3 of debris is found in the accumulation zone.

A flow slide in central Italy

The paper describes a slope failure in granular colluvium, together with its geomorphological, seismic and geotechnical setting. The predictions from conventional geotechnical slope stability analyses, taking into account topography, pore pressure and seismic effects, are compared with the observed pattern of instability. It is concluded that the failure was a flow slide and resulted from a combination of factors. Geotechnical limit equilibrium stability analyses of entire slopes are rarely able to predict the smaller-scale initiation events leading to flow slides, because these are controlled by local topography, ground and groundwater conditions. Nor are they able to predict the speed and run-out of such slides.

Design and application of an automated non-circular slip surface search within a combined hydrology and stability model (CHASM)

With geotechnical engineering the form of slope stability analysis most commonly used is the Bishop circular method. By assuming a circular shape, however, the method results in an overestimation of the factor of safety. This is because failure surfaces are often non-circular, given the heterogeneity of both hydrological and geotechnical slope characteristics. This problem is addressed by adapting an automated search algorithm for non-circular failure surfaces. The numerical method is based on a random search technique for the global minima of a constrained non-linear optimization problem. This is incorporated into a combined hydrology and stability model (CHASM) that allows the simulation of changes in pore-water pressures in response to individual rainfall events, and considers their role in maintaining slope stability. Initial results suggest that the implementation of the automated slip search algorithm leads to an improvement in the evaluation of slope stability in comparison to other less rigorous approaches.

On theoretical and numerical methods in the theory of porous media based on polar and non-polar elasto-plastic solid materials

The consideration of saturated and non-saturated porous solid materials as for instance soil, concrete, sinter materials, polymeric and metallic foams, temperate ice, living tissues, etc. naturally falls into the category of multiphase materials, which can be described by use of a macroscopic continuum mechanical approach within the well-founded framework of the Theory of Porous Media (TPM) . In the present contribution, granular elasto-plastic porous solid skeletons ( frictional materials) are taken into consideration, where, regardless of whether or not the solid is fluid-saturated or empty, localization phenomena can occur as a result of local concentrations of plastic strains. As a consequence of localization phenomena, the numerical solution of the governing equations generally reveals an ill-posed problem. In particular, the shear band width strongly depends on the mesh size of the finite element discretization by the fact that each mesh refinement leads to a decrease of the shear band width until one obtains a singular surface. In the present article, it is shown that the inclusion of fluid-viscosity in the saturated case and the inclusion of micropolar grain rotations both in the saturated and in the non-saturated case leads to a regularization of the shear band problem. On the other hand, the inclusion of micropolar degrees of freedom in the sense of the Cosserat brothers additionally allows for the determination of the local average grain rotations. The numerical examples are solved by use of finite element discretization techniques, where, in particular, the computation of shear band localization phenomena is carried out by the example of the well-known geotechnical slope failure problem and two additional academic problems, which clearly demonstrate the efficiency of the proposed procedure.

A temporal model for landslide risk based on historical precipitation

Of the recognized nonsteady-state factors that influence slope stability, probably most critical in many field situations is the character of precipitation and infiltration activity. A groundwater response model used in conjunction with precipitation records can provide a historical catalog of estimated maximum groundwater levels in a particular study area. An extreme-value statistical analysis of this catalog is linked with geotechnical slope stability analyses to provide a landslide hazard model for estimating the probability of slope failure within a given time. This modeling approach can provide meaningful input to risk assessments for landslide mitigation programs and to decision analyses and cost-benefit studies important for land-use planning and resource management.