1,720,992 research outputs found
Wetting test and X-ray computed tomography of volcanic unsaturated sands
No full textWetting can induce the so-called 'capillary collapse' in loose unsaturated soils. Collapse mechanisms have been studied at macro-scale for many years, while few observations at micro-scale are available. In this work, wetting tests were conducted by combining X-ray computed tomography (CT) and image treatment to investigate the soil collapse of volcanic sands of southern Italy at both macro- and micro-scale. The specimens were tested under self-weight and the matric suction was gradually reduced until collapse. Standard techniques and image analysis were used to measure the porosity and degree of saturation for the whole specimen, while image analysis of sub-volumes was done at micro-scale. The X-ray CT-aided wetting tests allowed: (a) the analysis of soil microstructure and its variations, (b) the accurate measurements of porosity and degree of saturation for differently sized representative element volumes (REVs). The local modifications of soil internal structure were measured and compared with the overall behaviour of the whole specimen. The porosity changed both in REV-averaged values and spatial distribution. The degree of saturation increased while keeping similar spatial distribution until collapse. The insights at both micro- and macro-scale provided a rational overview of the complex mechanisms regulating the pre- and post-collapse configuration of soil
Hydro-mechanical properties of unsaturated residual soil from a flysch rock mass
No full textRainfall is a major triggering factor of landslides in flysch deposits along Europe. Physical and mechanical changes of flysch rock masses caused by the weathering process, result in a complex soil profile with residual soil typically present at the slope surface. The research encompasses residual soils from the Rječina River Valley in Croatia. Built in flysch deposits, the valley is known for several deep-seated and shallow historical landslides. This manuscript presents the hydro-mechanical properties of the soil existing in unsaturated zone of the slope that could play an important role in landslide activation on flysch slopes. Presented unsaturated soil property functions are essential for modelling of the transient rainfall infiltration process and how it affects the stability of flysch slopes in time. For the first time, shear strength and hydraulic conductivity determination were performed in saturated and unsaturated soil conditions, using both the intact as well as remolded samples of the residual soil form flysch rock mass. The results indicate that, in order to correctly define hydraulic and mechanical properties of the in-situ soil, measurements have to be performed on intact samples. Also, the presented results highlight the importance of hysteresis effects and hydraulic paths that fine-grained residual soil has been subjected to in the past
Modelling of erosion processes associated with rainfall-triggered lahars following the 2011 Cordon Caulle eruption (Chile)
No full textIn this work we investigate the sheet and rill erosion process as a possible lahar initiation mechanism in two catchments located in Villa la Angostura, Argentina (Southern Andes), in association with the 2011 eruption of Cordón Caulle (Chile). A few days after the climactic phase of the eruption, several lahar events occurred in Villa la Angostura where 15 cm of tephra mantled the slopes. The physically based model LISEM (openLISEM, 2013a, b) was applied to two small catchments where sheet and rill erosion of tephra deposits was observed. Model input parameters related to vegetation, soil surface and infiltration were obtained from field measurements, geotechnical tests and the literature. A total of 20 scenarios were simulated using a range of values of rainfall intensity and duration and hydraulic conductivity; only 16 of these scenarios resulted in erosion. Results show that hydraulic conductivity (Ks) and rainfall intensity significantly affect the runoff volume, with the smallest Ks (i.e. the tephra deposits associated with the largest fraction of fine ash) being associated with the largest eroded volumes and the largest discharges of water and sediment. The scenarios with the highest hydraulic conductivity did not show erosion for any modelled rainfall intensity. Nonetheless, we conclude that sheet and rill erosion were not the principal mechanism for rain-triggered lahars associated with the Cordón Caulle eruption, but mostly shallow landslide and similar erosion mechanisms contributed to lahars in Villa La Angostura. In fact, the eroded volumes calculated with LISEM are significantly smaller than the volume calculated with shallow landslide models for the same catchments (TRIGRS)
Possible remediation of impact-loading debris avalanches via fine long rooted grass: an experimental and material point method (MPM) analysis
No full textDebris avalanches often originate along steep unsaturated slopes and have catastrophic consequences. However, their forecast and mitigation still pose relevant scientific challenges. This is also due to the variety of mechanisms observed near high sub-vertical bedrock outcrops, such as the impact loading of soil failed upslope the outcrop, the build-up of pore water pressures in the inception zone, and the bed entrainment along the landslide propagation path. At the University of Salerno, an experimental and numerical investigation campaign started some years ago to explore the feasibility of using long-root grass to mitigate or even inhibit the inception of debris avalanches. Previous laboratory results were achieved through two twin 2-m-long columns (one bare, one vegetated), where the change in soil retention curve and soil mechanical response was assessed. As follow-up, an experimental field setup was installed in 2020 first, and in an improved configuration in 2021. Here, three different species of long-root grass were grown. In situ soil suction and water content measurements were periodically collected in the vegetated and in the original soils. In both cases, soil specimens were also collected, and laboratory geotechnical tests were performed to individuate the changes in both the water retention and strength response. Increased values of soil suction and shear strength were outlined, despite some differences, for all the grown species compared to the original soil. Using these novel experimental data, advanced large-deformation stress–strain hydro-mechanically coupled analyses were recently performed through a material point method (MPM) approach. The original slope conditions were compared to various slope configurations engineered via long-root grass. The benefits and the open issues related to this novel green technology for landslide mitigation are discussed. Some insights are outlined for the possible reduction of the soil volumes mobilized inside the inception zone of debris avalanches
Calibration of a constitutive model for volcanic sands under simple shear conditions
No full textThe simple shear response of air-fall volcanic (pyroclastic) soils under both saturated and unsaturated conditions is interpreted through an elastoplastic constitutive model with hydraulic-hardening and porosity-dependent critical state. Extensive experimental data collected under various testing protocols (i.e., simple shear, direct shear, and triaxial tests) enable the theoretical identification of the variability of the fundamental physical properties (i.e., frictional resistance, dilatancy, and water retention behavior) and the identification of a band of admissible values for prescribed confidence levels. In this paper, a constitutive model specifically developed to account for the simple shear loading is adopted and the calibration of the model parameters is performed accounting for such data scatter. The calibration procedure led to a single set of constitutive parameters through 3 main steps. The identification of range of variation for each parameter is carried out using the data derived from laboratory tests. Then, sensitivity analyses are performed on the main physical and constitutive parameters. Mathematical indicators quantifying the difference between the measurements and the predictions are proposed to investigate the role of the material properties and evaluate the model performance. Furthermore, an optimization algorithm is adopted to identify the optimal set of model parameters which best fits all the considered tests (i.e., simple shear tests under saturated and unsaturated conditions and wetting tests). The results imply that the variability of the hydro-mechanical properties must be considered to satisfactorily simulate the constitutive behaviors of the volcanic soils under a variety of simple shear testing regimes
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