Short wave infrared (SWIR) image analysis provides an accurate procedure to measure the degree of saturation (Sr) of soils. Changes in Sr result in reflectance changes, which are recorded by a ...special digital camera optimized for a particular range of wavelengths that provides the maximum resolution of the method. This paper describes the developed methodology that relies on sample-determined calibration of soil reflectance for a few values of Sr covering the dry to fully saturated range of moistures. The accuracy of the method was checked by comparing equilibrated profiles of SWIR-determined Sr with other procedures (sensor readings and water retention information). It was found that the SWIR-based method correlates well with the degree of saturation of the soil but not with its water content, which may change for a given Sr when the soil deforms volumetrically. The transient variations of Sr are also well captured by the method. The recorded evolution of two-dimensional Sr maps of an initially unsaturated soil column compares satisfactorily with the results of a numerical model.
The re-activation of a large (40 Mm.sup.3) landslide on the valley slopes of a reservoir motivated a research initiative to estimate the risk of a fast-sliding mass moving into the reservoir. A ...previous simplified analysis had suggested that a joint consideration of strain rate effects on friction and thermal pressurization phenomena in the sliding surface could provide a rational approach to answer the question raised. The paper describes first the capability of strain rate effects on friction to reproduce long-term creeping records of two real cases. The joint and coupled phenomena of creeping motion and thermal pressurization in shearing bands was incorporated into a material point method computational technique for hydromechanical analysis of porous materials. A representative cross section of the Canelles landslide was then analysed, profiting from previous finite element investigations of the landslide. It was found that a rapid rate of landslide acceleration could be a possibility under extreme external actions. However, it was also found that a moderate strain rate effect on the basal residual friction angle could create conditions that avoid the triggering of a fast motion.
A dam construction project for the heightening of a dam is threatened by a large landslide located in the Spanish Southern Pyrenees. A marked syncline structure, folding a flysch formation overlying ...a marl substratum, is the main geological feature of the unstable valley slope. The landslide is characterized by two main and superimposed slip surfaces which follow the bedding layers. The motion observed is very slow and its acceleration can be related to rainfall regimes and changes in loading condition due to excavations.
The paper provides an insight into the complexities associated with stability and kinematic evolution of a landslide under static and dynamic actions. Simplified procedures are proposed to predict the complex landslide behaviour. A comprehensive hydrogeological model is developed to relate rainfall history and water pressure which incorporates available data about the anisotropic permeability of flysch and the natural drainage obtained from pumping test and insitu measurements. The stability analysis of the slope focuses on the discussion of safety factors and their practical implication not only in terms of static limit equilibrium, but also on creeping velocity. The latter was evaluated under the simplified assumption of planar landslide and invoking the strain-rate effects on residual strength. A derived analytical expression provides a relationship between Safety Factor and creeping velocity. Under such hypothesis, the increment of the strength required to avoid predicting unrealistic landslide acceleration should be imposed for a lower range of shear velocity than the usual value imposed in the laboratory shearing tests.
The seismic response of the landslide is also analysed. A Newmark's approach and a stress-strain finite element (FE) analysis are compared, with special emphasis on the effect of the superimposed slip surfaces on the dynamic landslide response. To do that, the Newmark's analysis has been extended to two overlaying sliding surfaces with independent strength properties and pore water pressure. It is concluded that assuming a single deeper sliding surfaces may underestimate the co-seismic permanent displacement. The comparison of two-block Newmark's analysis with two-dimensional elastoplastic FE approach highlights the effect of geometry in the landslide motion.
•Description of a large landslide characterized by two distinct sliding surfaces, which threatens two existing dams.•Development of a practical theoretical relationship between SF and creeping velocities.•A comparison between Newmark’s method, its extension to two overlying sliding surfaces and a comprehensive FE analysis.
Except for simple sliding motions, the stability of a slope does not depend only on the resistance of the basal failure surface. It is affected by the internal distortion of the moving mass, which ...plays an important role on the stability and post-failure behaviour of a landslide. The paper examines the stability conditions and the post-failure behaviour of a compound landslide whose geometry is inspired by one of the representative cross-sections of Vajont landslide. The brittleness of the mobilized rock mass was described by a strain-softening Mohr–Coulomb model, whose parameters were derived from previous contributions. The analysis was performed by means of a MPM computer code, which is capable of modelling the whole instability procedure in a unified calculation. The gravity action has been applied to initialize the stress state. This step mobilizes part of the strength along a shearing band located just above the kink of the basal surface, leading to the formation a kinematically admissible mechanism. The overall instability is triggered by an increase of water level. The increase of pore water pressures reduces the effective stresses within the slope and it leads to a progressive failure mechanism developing along an internal shearing band which controls the stability of the compound slope. The effect of the basal shearing resistance has been analysed during the post-failure stage. If no shearing strength is considered (as predicted by a thermal pressurization analysis), the model predicts a response similar to actual observations, namely a maximum sliding velocity of 25 m/s and a run-out close to 500 m.
A large landslide (40 × 10
6
m
3
) was reactivated on the left bank of Canelles reservoir, Spain. The instability was made evident after a considerable reduction of the reservoir level. The drawdown ...took place during the summer of 2006 after several years of high water levels. The drawdown velocity reached values between 0.5 and 1.2 m/day (registered at low elevations). The paper reports the geological and geotechnical investigations performed to define the movement. The geometry of the slip surface was established from the detailed analysis of the continuous cores recovered in deep borings and from limited information provided by inclinometers. Deep piezometric records provided also valuable information on the pore water pressure in the vicinity of the failure surface. These data allowed validating a flow–deformation coupled calculation model, which takes into account the changes in water level that occurred 4 years previous to the failure as well as the average rainfall. The analysis indicates that the most likely reason for the instability is the rapid drawdown that took place during the summer of 2006. The potential sudden acceleration of the slide is also analysed in the paper introducing coupled thermal hydraulic and mechanical effects that may develop at the basal shearing surface of the sliding mass. The results indicate that the slide velocity may reach values around 16 m/s when displacement reaches 250 m.
Over the last few decades, the particle image velocimetry (PIV) technique has become an interesting tool used to measure displacements in the field of experimental mechanics. This paper presents a ...procedure to interpret PIV displacements, measured following an Eulerian scheme, with the purpose of providing accumulated displacements, velocities, accelerations, and strains on points representing physical particles. Strains are computed as the gradient of displacements. When compared with other standard procedures already published, the presented methodology is especially well suited to interpret large strains. The basis of the procedure is to map displacement increments measured through PIV analysis on the subset (or patch) centres into numerical particles that are defined as portions of the moving masses whose deformation is analyzed. The implementation of the method is explained in detail, highlighting its simplicity. The procedure can be used as a post-processor of currently available PIV software packages. The methodology is first applied to synthetic cases of rectangular samples in which known displacements are imposed and also to a sandy slope failure experiment involving large displacements. The method reproduces satisfactorily the recorded images.
Vaiont slide has been represented by a model of two interacting evolutive wedges. Mass conservation during the motion implies that the upper wedge transfers mass to the lower one through an internal ...shearing plane. The model respects available in situ observations. It was formulated in dynamics terms. Outcomes of the analysis are the determination of safety factors of the valley before dam impoundment, and the calculation of run-out distance once the motion starts. Rock strength degradation as motion develops has also been included. This degradation, even if it is intense, was unable to explain the very high estimated landslide velocities.
This paper presents the influence of temperature on the residual strength by analyzing shearing tests carried out in a temperature- and velocity-controlled ring shear apparatus. Temperature was ...applied in steps of cooling and heating ranging between 6 °C and 50 °C at a constant shearing velocity of 0.442 mm/min. Additional tests include the evaluation of the strength at 0.890 mm/min before and after the imposed temperature change. Six basic soil types characterized by different mineralogy, grain size and plasticity, as well as, mixture soils having different proportions of sand and clay, are evaluated and the results are interpreted in terms of their properties. The experimental results reveal that the temperature effect becomes relevant when fine particles and plasticity increase being negligible in the case of the sandy soil. Shear strength reacts immediately to temperature changes and a new residual strength establishes soon. The temperature effect is symmetric. An increase in temperature leads to a decrease in strength, and an equal and opposite trend is observed when temperature decreases. The magnitude of the strength variation is significant in clayey soils. For the case of high plasticity soils, the residual friction angle variation observed during cooling and heating (±50 °C) is around 2°. The strain rate effect evaluated in a smaller number of tests indicated a strain rate strengthening in all the materials tested except for the case of the sandy soil, which also was not reactive to temperature changes. The experiments are calibrated with a velocity, state, and temperature-dependent friction model.
•Influence of temperature on residual shear strength have been characterized.•Heating leads to decrease in friction and cooling leads to an increase for high shearing rates.•Relevance of residual friction change with respect of temperature correlates with the soil plasticity.•No effect of temperature was measured for the clean sand tested.•The friction constitutive model developed by Chester (1994) can reproduce the tests performed.
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