查看更多>>摘要：? 2021 Elsevier B.V.Time-domain reflectometry (TDR) can help observe the initiation and evolution of localized shear planes in both rock and soil slopes, and has temporal and spatial resolutions suitable for landslide monitoring. An improved and standardized technical guide for TDR cable installation and data interpretation is needed to utilize it better. TDR technique involves sending an electromagnetic pulse into a coaxial cable grouted in a pre-drilled borehole, and capturing the reflected signal from cable deformity, which is triggered by the localized shear deformation in the underground. This study developed a large direct shear box for testing TDR responses in various cable-grout-ground assemblies, in order to reliably model the cable-grout-ground interaction at the localized shear. Different combinations of cable type, grout condition, soil type, and shear bandwidth were tested using the new physical model to re-examine TDR's response to ground deformation in sliding mode. The results revealed some misconceptions in previous studies that used cable-grout assemblies without considering the entire interaction with the ground. The implications of the experimental results are carefully considered and new technical recommendations are provided for cable installation and data interpretation to foster the improved use of TDR. A new data reduction method that involves data filtering, differential waveform, and three-sigma rule is proposed to support the recommended installation setup for robust early detection of a localized shear plane.
查看更多>>摘要：? 2021This comparative study on a very wide group of samples shows that the crystal size and crystalline defects of smectites influence microporosity of bentonite considerably more than previously considered. The smectite crystals and surface properties, including microporosity and micropore volume, were studied using high resolution transmission electron microscopy and N2 adsorption, respectively. The specific surface area obtained varied between 25 and 278 m2g?1. The micropore area ranged between 6 and 76 m2g?1, and the external area ranged from 18 to 208 m2g?1. The external surface area was related to the size of the crystals in [001] direction because of the smaller particles with few stacked 2:1 layers have more basal surfaces accessible to the N2 molecules. However, the microporosity can be related to 1) the size of the crystals, owing to the partial ability of N2 to penetrate into the interlayer space, 2) the abundance of crystalline defects affecting the stacking of the 2:1 layers, and 3) the arrangement of the crystals forming particles, in which sub-parallel aggregates generate micro and mesopores. The study shows that these bentonites have pores in the full range, from smaller micropores (related to the crystalline structure in the interior of the interlayer) to micrometric macropores. The amounts of micro, meso, and macropores varied between samples but were similar and characteristic for samples from the same geological area.
查看更多>>摘要：? 2021 Elsevier B.V.A set of materials covering a wide class comprised between soils and fractured rocks displays a variable and complex hydraulic/mechanical response to external inputs that make both the measurement of properties and the modeling of material behaviour quite complicated. In fact, weak interparticle bonding and a network of irregular and often interconnected discontinuities can strongly and unevenly affect permeability, stiffness and shear strength, making difficult the interpretation of natural events, such as landslides, and uncertain the analysis and design of engineered slopes and man-made works. This paper reports some examples taken from the Authors' experience and from the literature, which illustrate the difficulty to correctly understand or analyze the behaviour of such complex materials through the classical approaches of Soil or of Rock Mechanics, highlighting at the same time the complex role that discontinuities play in groundwater flow and pore water pressure regime.
查看更多>>摘要：? 2021 Elsevier B.V.Contact force chains (CFCs) in the heterogeneous granular materials are often considered to be structured physical systems that play a key role in their mechanical properties such as stiffness, strength, stability and flowability. In this context, quantitatively estimating the evolution of CFCs in a quasi-statically sheared granular system is essential for advancing our understanding of the mechanics of granular materials. In this paper, based on discrete element method (DEM) simulation data, an artificial neural network (ANN) is developed and applied to predict the anisotropy of the CFCs in two types of idealized granular materials with different initial relative densities undergoing triaxial shearing. Five features including particle size, coordination number and particle displacement (i.e., x-, y- and z-components of the particle displacement) at the particle-scale and the meso-scale each are used to train and test the established ANN model. The results of model prediction show that the 3D orientational distributions of the CFCs from the ANN predictions match very well the DEM simulation results during the whole shearing progress. It is found that for both dense and loose samples, the combined set of particle-scale and meso-scale features have a dominating influence on the CFC evolutions but the ANN model performs better in the CFCs estimation at the strain increment of 0–7% than at the strain increment of 7%–14%. The outcome of this study shows that machine learning is a promising tool for studying the complex mechanical behavior of granular materials.
查看更多>>摘要：? 2021 Elsevier B.V.Compacted bentonite is often utilized as a buffer material between the canister containing radioactive waste and the host rock in deep geological repositories. Extremely low permeability and a potential to swell and develop large swelling pressures upon hydration are desirable characteristics for such material. This study focused on the B75 bentonite, a Ca-Mg-montmorillonite-rich clay selected as a potential buffer material for the planned repository in the Czech Republic. The effects of the initial dry density, vertical load, and pore fluid salinity on the swelling behavior of the clay were investigated. One-dimensional swelling tests were performed on compacted samples prepared at various initial dry densities (1.25–1.95 g/cm3). Conventional oedometer tests with various hydration and stress paths were also performed. An empirically corrected diffuse double layer model proved successful in predicting the swelling pressure during the tests. Swelling under constant load, swelling pressure under constant volume, swelling upon unloading, and unconfined swelling upon hydration were compared, and a unique relationship was found between the effective stress and the dry density at saturation.
查看更多>>摘要：? 2021 Elsevier B.V.Understanding the behavior of sand-fines mixtures and assessing their shear strength or permeability require knowledge of the influence of fine particles content on the minimum void ratio. The latter is often evaluated using empirical methods or models using parameters without physical significance. Validated for both ordered and disordered packings and for concrete mixtures, the Theoretical Packing Density Model (TPDM) is tested for predicting the minimum void ratio on binary sand-fines mixtures, whatever the fines content. It requires only two parameters: the critical cavity size ratio x0 from which the loosening effect occurs and the compaction index K describing the packing efficiency. Previously collected data from 30 binary soil mixtures including river, lake, mountain and crushed sands representing 285 individual specimens for which minimum void ratios are available, were used. The coefficient of determination provided by the TPDM is equal to 0.968 on average, and greater than 0.99 for 30% of the mixtures. Thanks to the compaction index K, the mathematical model is therefore capable to capture the non-linear behavior of the void ratio variation with respect to the solid volume fraction of fine particles. Finally, a method is proposed to approximate the value of the critical cavity size ratio x0 to be used as a function of the combination of the shapes of coarse and fine particles.
查看更多>>摘要：? 2021 Elsevier B.V.The presence of active faults or potentially active faults in urban settings is of great concern to city planners and developers. The high value of property within cities means that it is not always possible to avoid construction in such areas. Thus, building codes exist in order to regulate where and how to build in the vicinity of such faults. The Israeli National Building Code defines a zone of active faulting as a 200-m wide area on each side of an active or potentially active fault trace. In this area, there is a high potential for repeated activity of undetected branches or secondary faults and construction should be avoided or built with extreme safety measures in place. The current study examines an area located between two fault strands – an active and a potentially active one - in the city of Tiberias, northern Israel. The area lies outside the active fault zones defined for each strand (i.e., at a distance of more than 200 m from each fault). Eight high-resolution ground penetrating radar (GPR) profiles were collected along the streets that crisscross this area. Results show a dense series of potential fault strands that reach the base of the artificial fill that was laid down for the construction of the roads, indicating potential seismic hazard in this seemingly “safe” zone, thus raising a “red flag” for construction plans in the area. A geological study should be conducted to validate the geophysical results. This study shows the importance in conducting a geophysical site survey in tectonically active settings, even in areas that lie outside well defined zones of active faulting.
查看更多>>摘要：? 2021 Elsevier B.V.Debris flow prediction based on rainfall monitoring is important for early warning and disaster risk reduction. Taking a typical debris flow catchment (with debris flows occurring on average 6 times per year and with a watershed area of 19.47 km2) in central China as an example, we used a machine learning approach (17 machine learning algorithms were tested) and a time series data processing algorithm (141 features were extracted from rainfall data) to predict debris flow events in advance, based on continuous rainfall records from five rain gauges in the catchment. Using 367 rainfall events (46 rainfall events triggered debris flows, and 321 rainfall events did not) from December 2012 to April 2015, rainfall prediction models for debris flows were established, which can automatically assess in terms of probability whether any given rainfall event will trigger a debris flow. After model optimization, the Extra Trees (ETs) model showed the best performance. Under testing with 16,968 rolling rainfall series which simulated actual rainfall monitoring records, the model had no false alarms and missing alarms. Taking the event on May 11, 2012 as an example, the model correctly predicted the debris flow event 35 min in advance, i.e., before the debris flow reached the catchment outlet, which provided valuable time for early warning.
查看更多>>摘要：? 2021 Elsevier B.V.High discharge debris flows in mountainous and volcanic areas are major threats to populations and infrastructures. Modeling such events is challenging because the associated processes are complex, and because we often lack data to constrain rheological parameters. In this work, we show how extensive field data can help model a rock avalanche, and the subsequent remobilization of the deposits as a high discharge debris flow, with a single one-phase thin-layer numerical code, SHALTOP, and up to two rheological parameters. With the Prêcheur river catchment (Martinique, Lesser Antilles) as a case study, we use geological and geomorphological data, topographic surveys, seismic recordings and granulometric analyses to define realistic simulation scenarios and determine the main characteristics of documented events for model calibration. Then, we model a possible 1.9 × 106 m3 rock avalanche. The resulting deposits are remobilized instantaneously as a high discharge debris flow. We show that, for a given unstable volume, successive collapses allow to better reproduce the dynamics of the rock avalanche, but do not change the geometry of the final deposits, and thus the initial conditions of the subsequent debris flow simulation. The location of the debris flow initiation has also little influence on simulation results. However, progressive remobilization of materials slows down the debris flow and limits overflows, in comparison to an instantaneous release. Nevertheless, high discharge debris flows are well reproduced with an instantaneous initiation. Besides, the range of travel times measured for other significant debris flows in the Prêcheur river is consistent with our simulation results.
查看更多>>摘要：? 2021 Elsevier B.V.Desiccation-induced curling of mud layers on the earth surface is a ubiquitous natural phenomenon, as a result of mud-atmosphere interaction. Field observations reveal that the curling behaviors of muds were closely related to the overlying layer thickness and the basal layer condition. In order to interpret field observations, laboratory experiments were conducted on ten remolded mud samples with varying overlying layer thicknesses and basal conditions. Experimental results are consistent with field observations and provide insights into the dynamic mud curling process. During drying, the mud curling is driven by the shrinkage strain gradient along the mud profile. For a thin overlying mud layer, the mud curling is unidirectional, either upward (∪) or downward (∩), depending on the basal condition. For a thick overlying layer, the mud begins with upward curling and then recovers. Increasing the layer thickness not only weakens the mud curling but also improves the curling recovery capacity. Furthermore, under a stratified structure, the overlying mud curling is significantly influenced by the interplay of the capillary and friction forces developed at the mud undersurface. Both factors are strongly associated with the basal particle size. The decreasing basal layer particle size smoothens the interface, probably causing the unidirectional downward curling of the overlying layer. However, a rougher basal layer may induce that the overlying mud curls initially at a higher water content and undergoes a more noticeable upward curling. Through field observations and experimental tests, this study is expected to advance the current understanding of various mud curling processes.
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