Om Prasad Dhakal

Temperature-dependent changes in residual shear strength are important for slope stability, landslide reactivation, and thermally loaded geotechnical structures. Although this response is controlled by mineral composition, there is no clear method for extending laboratory results to larger areas. This study combines temperature-controlled ring shear testing with short-wave infrared (SWIR) spectroscopy for eight mixtures of quartz-rich sand, kaolin, and bentonite. The specimens were tested at normal stresses of 50, 100, and 150 kPa and subjected to heating-cooling cycles between 20 and 80 °C. Sand and kaolin rich mixtures showed little change, whereas mixtures containing at least 50% bentonite showed clear heating-induced strengthening. For pure bentonite, the residual friction angle increased from 6.18° to 9.02°, corresponding to Δϕr=+2.84°. The SWIR spectra distinguished clay-bearing from clay-free mixtures using the absorption feature near 2209 nm and differentiated high-activity from low-activity clays using the feature near 1912 nm. Partial least-squares regression was then used to relate the spectra to residual friction angle. Finally, the spectral data were interpolated across a sand–kaolin–bentonite ternary space and combined with the laboratory results to map Δϕr. The study provides a first step toward mapping mineral-dependent thermal changes in soil strength.

The residual shear strength governs the long-term stability of slow-moving landslides and reactivated clayey shear zones. Temperature variations, driven by seasonal fluctuations, geothermal gradients, or climate change, can alter interparticle interactions and change the soil’s resistance to shearing. While the thermal response of soils has been studied with respect to various soil properties, its specific impact on residual shear strength remains poorly understood, particularly with respect to the coupled effects of mineralogy, normal stress, and shear rate. This study investigates the thermal sensitivity of residual shear strength in two fine-grained soils: (i) a natural illitic clay from the Dubičná landslide (Czechia) and (ii) a commercial Malaysian kaolin. Multi-rate ring shear tests were conducted at normal stresses of 50, 100, and 150 kPa under controlled temperatures of 20 °C and 70 °C. To capture varying structural states (apparent overconsolidated, transitional, and normally consolidated), specimens were prepared at initial water contents ranging from 55% to 100%. Shearing rates were varied from 0.018 to 1.8 mm/min to evaluate potential rate-dependence under residual conditions. Statistical analyses, including Welch’s t-test with defined significance thresholds, were employed to isolate systematic thermal effects from experimental scatter. The results reveal stress-dependent, mineralogy-controlled thermal responses. At a high normal stress of 150 kPa, the illitic Dubičná clay exhibits thermal strengthening, whereas the Malaysian kaolin demonstrates systematic thermal weakening. At a low normal stress of 50 kPa, temperature effects remain within experimental variability for both soils. These findings highlight that normal stress and clay mineralogy are primary drivers of thermal sensitivity under residual shear conditions, offering insights for the stability assessment of reactivated clay shear zones under changing environmental conditions.

Our research investigates the complex thermo-mechanical coupling within landslide shear zones, specifically focusing on how temperature variations influence the residual shear strength of clayey soils. This parameter is a critical determinant for the stability of reactivated, slow-moving landslides.In our laboratory investigations, we utilised a modified ring-shear apparatus equipped with a temperature-control system to conduct heating-cooling cycles (typically between 20 °C and 70 °C) on various soil samples. We have tested materials ranging from low-plasticity mountain soils to high-plasticity marls and bentonites. Our findings reveal that the thermal response of soil is sensitive to both mineral composition and rate of shearing.Regarding slope modelling, we developed numerical frameworks, utilising finite-element analysis, that incorporate temperature-dependent failure criteria. By simulating scenarios from the 1960s to the 2060s based on historical and projected climate data, our results suggest that gradual ground warming may increase the factor of safety in certain clay-rich slopes, potentially transitioning active landslides into long-term dormancy. We also quantified the nonlinear coupling between temperature variations and groundwater table fluctuations, demonstrating that their combined impact on stability is more significant than the sum of their individual effects.Our field monitoring and regional activities involved extensive sampling at disaster-prone sites and the calibration of thermal parameters using historical meteorological data to accurately reproduce ground temperature profiles. Furthermore, we have implemented spatial probability analyses at a national scale, utilising soil composition and topographic data to map expected changes in slope stability under global warming scenarios.In the future, we plan to incorporate the soil-vegetation-atmosphere nexus into our framework. By evaluating how vegetation and root systems modulate heat and moisture fluxes, we should be able to capture the thermo-mechanical behaviour of the shallow subsurface more accurately. Additionally, we intend to expand our experimental investigations to a broader range of soil compositions and refine testing protocols to objectively assess the thermal sensitivity of landslide-prone formations.

Global warming reshapes geohazard dynamics by influencing both precipitation and temperature. While precipitation remains the primary landslide trigger, temperature-driven changes in soil strength are often overlooked. This study integrates temperature-controlled shear testing with a physically based multi-hazard model to quantify thermal impacts on slope processes in Nepal’s Melamchi catchment, affected by the 2021 debris flood. Laboratory ring-shear experiments on landslide soils revealed a linear reduction in friction angle with a 4.9 °C temperature increase, indicating thermal weakening. Incorporating this relationship into a calibrated multihazard model intensified hazard metrics without altering rainfall input. The warmer scenario increased peak discharge by 7%, sediment discharge by 36%, while producing longer runouts and higher impact pressures. These findings demonstrate that rising temperatures alone can amplify disaster severity, underscoring the importance of integrating thermally dependent soil properties into future multi-hazard assessments under climate change.

Temperature variations, within the typical range experienced in temperate climates, have been shown to influence the shear strength of clay soils, with the effect depending on factors such as soil mineral composition, confining stress, and shear rate. Seasonal temperature fluctuations and long-term climatic trends propagate from the atmosphere into the subsurface, attenuating and lagging with depth. In the upper few meters, where landslides frequently occur, seasonal temperature variations of 2–5 °C are common.We present field monitoring data from the Dubičná landslide, a slow-moving, clay-rich (primarily illitic) roto-translational slide in the Czech Republic. The landslide exhibits displacement rates of a few millimetres per year and displays a seasonal pattern not entirely attributable to precipitation trends. Using ring-shear experiments on shear-zone samples, we investigated the influence of temperature on the residual shear strength under different conditions. A linear relationship between temperature and shear strength was identified, indicating mild strengthening at higher temperatures under low shear rates.Slope stability analyses, incorporating air and subsurface temperature data, were performed for current temperature conditions and future projections under climate change scenarios. The results indicate that temperature effects on the factor of safety are modest, with a slight stabilising influence due to thermal strengthening. However, fully understanding the role of temperature in the stability of clay slopes requires further experiments and advanced modelling to account for the complexities of thermo-hydro-mechanical coupling and atmosphere-ground interactions.

The shear strength is a fundamental mechanical parameter that controls the occurrence and propagation of landslides. In pure clays, this parameter is temperature-dependent according to the clay’s mineral composition and hydro-mechanical boundary conditions. Landslide soils are typically heterogeneous mixtures with a variable content of clay minerals. Particularly for low-plasticity soils, the impact of changes in temperature on the mechanical response remains to be determined, and little can be said about possible macroscopic alterations of slope stability or landslide dynamics.In this study, we conducted ring-shear tests using natural soils from the Melamchi catchment in central Nepal, which suffered widespread instabilities and rainfall-induced debris flows. We performed experiments under typical landslide stress levels (50-150 kPa) in water-saturated conditions and under a constant rate of shearing (0.1 mm/min). We controlled the temperature during testing and performed a heating-cooling cycle (20-50-20 °C) only after attaining the residual shear condition. We prepared multiple samples from the same soil by retaining its finest portion under different cutoff grain sizes (0.125-0.020 mm) to evaluate the fine fraction's role in the thermo-shear response.As expected, we observed a decrease in shear strength with the clay fraction increasing. Samples with a coarser cutoff (and hence a lower clay fraction) did not exhibit any change in shear strength during the heating-cooling cycle. However, as the clay fraction increased, a heating-induced weakening emerged, corresponding to up to a 1° difference in friction angle in the samples with a 0.020 mm cutoff. In the specific case study, this weakening may be minor and will not affect evaluations of slope stability in simple limit equilibrium analyses, especially in the absence of explicitly accounting for spatial heterogeneities in soil properties and boundary conditions. Nevertheless, incorporating this effect into physically-based models, either entailing advanced soil constitutive models or equations for surface flows (both of which can include additional temperature-dependent parameters), may provide useful insights into the complexity of thermo-hydro-mechanical responses and their effects on landslides at the local and regional scales.

Temperature significantly affects the hydraulic and mechanical properties of geomaterials on a slope. However, results from the laboratory show the change in material behaviour depends on the slip rates and the type of minerals present. Active clays are observed to be the most sensitive to temperature oscillations in mechanical deformations. In this research, the temperature change effects will be addressed in laboratory experiments on natural soils. We will deploy laboratory samples to obtain the frictional coefficient as a function of temperature (20-50 degrees). Further, reconstituted samples will be tested under different shear rates to understand the dependencies on temperature. Temperature-sensitive parameters (such as the internal friction angle) will be incorporated in a physically-based modelling framework analysing effects for a sloping unit. The factor of safety will be calculated based on spatial grids representative of the sampling location. In the second part of this research, the frictional coefficient obtained from the laboratory as a function of temperature will be synchronised with projected climate change (surface temperature, hydro-meteorological forcing) and will be simulated to a catchment scale multihazard modelling. This model will incorporate measured data and geostatistical interpolation of remotely sensed data to fulfil the dataset to run the physically based equations. The final output will be the comparison of hazard intensities (e.g., debris flow impact pressure, inundation heights, solid velocity and depth) for the projected future years.