Influence of Displacement Rate on Residual Shear Strength of Clays

Abstract This paper reports on the results of direct shear tests carried out under controlled displacement rate in the range of 10 -4 -10 2 mm/min, under different normal stresses, with different shear devices. The tests were carried out on a kaolin, a bentonite, their mixtures with sand at various percentages, and the clayey soil of the Costa della Gaveta earthflow. The tests were performed on specimens reconstituted with distilled water as well as with NaCl solutions at various concentrations. Positive rate effects were exhibited by mixtures with c.f. higher than 50% and, consistently, by the natural clayey soil the c.f. of which is about 50%. The residual shear strength increases significantly for shear displacement rate higher than about 1 mm/min. The rate effect increases with the pore solution concentration. The residual shear strength independence of displacement rate has been confirmed in the range 10 -6 - 10 -1 mm/min by the results of shear tests performed under controlled shear stress, with varying chemical conditions of the pore fluid

Temperature and slope stability in temperate climate

peer reviewe

Modelling the role of material depletion, grain coarsening and revegetation in debris flow occurrences after the 2008 Wenchuan earthquake

A large amount of debris was generated by the co-seismic mass wasting associated with the 2008 Mw 7.9 Wenchuan earthquake. The abundance of this loose material along the slopes caused more frequent debris flows, triggered by less intense and/or shorter rainfalls. However, both the triggering rainfall and the debris flow frequency seem to have normalised progressively during the past decade. Although changes of rainfall thresholds for post-seismic debris flows were recorded after several major earthquakes, the factors controlling these changes remain poorly constrained. With the aid of a virtual experiment, we investigate the roles of material depletion, grain coarsening and revegetation of the co-seismic debris on the propagation and deposition of debris flows initiated by runoff, as well as their influence on the triggering rainfall thresholds. We employ a Geographic Information System (GIS)-based simulation of debris flow initiation by runoff erosion, which we first calibrate on the 14th August 2010 Hongchun gully event that occurred near the Wenchuan earthquake epicentre. We obtain, by investigating each of the aforementioned processes, changing critical rainfall intensity-duration thresholds for given debris flow runout distances. Grain coarsening appears to play a major role, which is consistent with published laboratory experiments, while material depletion and revegetation do not seem able to account alone for the actual quick decay of debris flow frequency. While the virtual experiment has proven useful in identifying the first-order controls on this decay, model improvements and verification over multiple catchments are needed to make the results useful in hazard assessments

The impact of earthquakes on orogen-scale exhumation

Individual, large thrusting earthquakes can cause hundreds to thousands of years of exhumation in a geologically instantaneous moment through landslide generation. The bedrock landslides generated are important weathering agents through the conversion of bedrock into mobile regolith. Despite this, orogen-scale records of surface uplift and exhumation, whether sedimentary or geochemical, contain little to no evidence of individual large earthquakes.We examine how earthquakes and landslides influence exhumation and surface uplift rates with a zero-dimensional numerical model, supported by observations from the 2008 Mw 7:9 Wenchuan earthquake. We also simulate the concentration of cosmogenic radionuclides within the model domain, so we can examine the timescales over which earthquake-driven changes in exhumation can be measured. Our model uses empirically constrained relationships between seismic energy release, weathering, and landsliding volumes to show that large earthquakes generate the most surface uplift, despite causing lowering of the bedrock surface. Our model suggests that when earthquakes are the dominant rock uplift process in an orogen, rapid surface uplift can occur when regolith, which limits bedrock weathering, is preserved on the mountain range. After a large earthquake, there is a lowering in concentrations of 10Be in regolith leaving the orogen, but the concentrations return to the long-term average within 103 years. The timescale of the seismically induced cosmogenic nuclide concentration signal is shorter than the averaging time of most thermochronometers (> 103 years). However, our model suggests that the short-term stochastic feedbacks between weathering and exhumation produce measurable increases in cosmogenically measured exhumation rates which can be linked to earthquakes

State Of the Art Report in the fields of numerical analysis and scientific computing. Final version as of 16/02/2020 deliverable D4.1 of the HORIZON 2020 project EURAD.: European Joint Programme on Radioactive Waste Management

Document information Project Acronym EURAD Project Title European Joint Programme on Radioactive Waste Management Project Type European Joint Programme (EJP) EC grant agreement No. 847593 Project starting / end date 1 st June 2019-30 May 2024 Work Package No. 4 Work Package Title Development and Improvement Of NUmerical methods and Tools for modelling coupled processes Work Package Acronym DONUT Deliverable No. 4.

Temperature and shear-rate effects in two pure clays: Possible implications for clay landslides

Temperature fluctuations in landslide shear zones can originate from heat exchanges with deeper layers as well as with the atmosphere through the landslide body. Shallow landslides (depth <10 m) are especially subject to seasonal temperature oscillations and rapid climatic changes. Various hydro-mechanical properties of clayey soils are sensitive to changes in temperature. Few studies suggested that the residual shear strength may vary significantly even in temperature ranges typical of shallow layers in temperate and warm regions. Here, we verified the response of two pure clays (Ca-bentonite, kaolin) to shearing at temperatures up to ∼55 °C under various normal stresses (50–150 kPa) and shear rates (0.018–44.5 mm/min) by equipping a ring-shear device with a temperature-control system. Then, we performed experiments on an ideal slope to quantify the extent to which ground temperature can condition the stability of clay slopes, across the seasons and under prolonged warming. Considering the largest effects evaluated experimentally (change in residual shear strength by ±1.5 %/°C), we determined changes in global factor of safety by ∼20 % for rotational slides ∼6 m deep, solely attributable to seasonal heating-cooling. Warming of 5 °C over decades would change the stability condition by an additional ±7 %. Although these results were obtained under simplified geometry and boundary conditions, without considering changes in triggers, preconditions, and effects of other thermo-hydro-mechanical couplings, they provide an upper bound to the role of the temperature-dependence of the residual shear strength on the factor of safety. We argue that this role should not be neglected in slope stability and landslide hazard assessments in clay-rich soils, thus warranting in-depth experimental analyses and advanced modelling