查看更多>>摘要：? 2022 Elsevier B.V.A spent fuel reprocessing site (SFRS) is under constructions on granitic beds in Northwest China. To safely manage the potential migration of nuclides, it is critical to accurately characterize the natural fracture system in the bedrock and accurately predict the variability in the geological environment. However, a workflow for site-scale discrete fracture network (DFN) modeling has not been fully established in China. Therefore, a new site-scale DFN modeling framework utilizing borehole core data as constraints was proposed for fractured biotite granite within several kilometers of the SFRS. This method quantified the P10 values of 227 pre-implemented engineering cores and obtained local P32 values using a conversion factor. Based on geostatistics, this method realized the fast estimation of global P32, a key parameter in DFN modeling, at minimal cost under a scenario with extremely limited outcrop data. Further, 1933 fractures obtained from extensive field mapping supplemented the fracture orientation and size information. By integrating the modeling parameters into the proposed framework, a site-scale DFN model database was successfully established. Finally, an acceptable site-scale DFN model was obtained from the database by comparing the measured and simulated P10 values, which demonstrated the effectiveness and feasibility of this method. Compared with the traditional DFN modeling, the proposed framework captured the heterogeneity of the fracture spatial intensity. The modeling results revealed the fracture distribution in the SFRS, and illustrated the broad potential applicability of this modeling framework in the field of geological engineering. This study provides a new insight for site-scale DFN modeling in China.
查看更多>>摘要：? 2022 Elsevier B.V.Catastrophic landslides with basal shear zones usually involve a number of mechanisms that are poorly understood. One mechanism issue is the transition from slow sliding to fast movement, leading to catastrophic failure. We perform laboratory experiments on landslide shear-zone materials to investigate how rate effects influence the slow-to-fast transition. Two types of ring-shear tests are carried out, namely residual state shear tests involving a wide range of shear rates and residual state creep tests with the applied shear stress close to the residual strength. Our tests show that the shear-zone soil exhibits rate-weakening behaviour at low shear rates and rate-strengthening behaviour at high shear rates. These two opposite mechanisms are distinguished by a transition rate, which also regulates whether the creep motion falls into the viscous flow regime or returns to the slow sliding regime.
查看更多>>摘要：? 2022 Elsevier B.V.Landslides are one of the most common geological disasters with the complicated spatiotemporal processes, which can be partially reproduced in slip shear failure along the discontinuity in laboratory tests. However, the assessment and evaluation of shear behavior along the discontinuity is time-consuming and high-cost. To solve this problem, a new constitutive model is proposed to predict the shear deformation behavior of the discontinuity. In this model, the discontinuity is assumed to be composed of intact units and failed units. In the shearing process, intact units and failed units jointly bear the external loads, and intact units are gradually transformed into failed units. A damage control variable is introduced to link the mechanical response of units with the macroscopic discontinuous deformation, thereby obtaining the final expression of the new model. The model parameters can be determined directly from several standard laboratory shear tests. Subsequently, the comparison analysis among the predictive results of the new model, previous models and engineering projects is conducted. Finally, the sensitivity of model parameters is systematically discussed. A hardening index HI is proposed to characterize the shear deformation characteristics in the hardening stage, and its relationship with model parameters C and r is in a power form. The results are expected to provide theoretical guidance for landslide prediction and thus reduce the occurrence frequency of geohazards.
查看更多>>摘要：? 2022 Elsevier B.V.Many engineering practices are carried out within the unsaturated soil. Soil-water retention curves (SWRCs) of many soils exhibit bimodal characteristics that significantly affect strength behavior, this issue still remains unresolved. This work employs suction stress concept to examine the uniqueness of strength parameters for coarse- and fine-grained unsaturated soils with bimodal SWRCs. Capillary degree of saturation is used to upscale pore-scale suction to macroscopic suction stress. In the suction stress-shear strength plane, both coarse- and fine-grained soils display the bi-linear strength envelope, differing from unsaturated soils with unimodal SWRC.
查看更多>>摘要：? 2022 Elsevier B.V.The current study investigates the influence of temperature on the water retention capacity of Gaomiaozi (GMZ) bentonite by conducting free swelling tests at different temperatures. The experimental results reveal that the equilibration time, as well as the water retention capacity, decreases significantly with temperature. A constitutive model for predicting the water retention characteristics was derived and the temperature effects were taken into account. More consistent with previous studies, the model divides the pore space into microstructural pore space (always saturated) and macrostructural pore space (presence of capillary water), with the macroscale quantifying temperature effects on water retention capacity by incorporating the suction variation and the microscale considering the introduction of a temperature-dependent microstructural deformation coefficient. The efficiency of the model was also verified using experimental data. For the GMZ bentonite with the initial dry density of 1.70 g/cm3, the threshold value for the transition from adsorbed water to capillary water is greater than 22 MPa. The microstructural deformation coefficient shows a linearly increasing relationship with temperature. The presented model allows an effective application to the hydraulic characteristics of bentonite artificial barriers for the long-term operation of nuclear waste repositories.
查看更多>>摘要：? 2022Microbially-induced calcium carbonate precipitation (MICP) is a promising grouting material for subsurface remediation due to its water-like viscosity and excellent penetration. Current studies of MICP-grouting for subsurface remediation of both rock fractures and highly-permeable rock matrix focus on the spatio-temporal distribution of precipitated bio-CaCO3 and the resulting reduction in permeability. Conversely, we focus on the improvement of mechanical response following MICP-grouting. We contrast the improved mechanical response of MICP-treated Berea sandstones with distinctly contrasting initial mechanical properties - contrasting associated pre- and post-treatment microstructures with various durations of MICP-grouting. Results indicate that although the precipitated CaCO3 mass with time within these two rock types is similar, significant differences exist in the evolution of mechanical properties (UCS, Young's modulus and brittleness). The evolution of mechanical properties for the low-strength sandstone (initial UCS 25.7 MPa) exhibits three contrasting phases: an initial slow increase, followed by a rapid-increase and then saturation and asympotic response. After ten cycles of MICP-grouting, UCS, elastic modulus and brittleness index for low-strength sandstone increase by 229%, 179% and 177% compared with before grouting. In contrast, the mechanical properties for the high-strength sandstone (initial UCS 65.1 MPa) are not significantly enhanced, increasing UCS by only 22%, 14% and 12%. Imaging by scanning electron microscopy (SEM) indicates that the cementing minerals fill the quartz framework for the high-strength sandstone but are sparse for the low-strength sandstone. Sandstone is a clastic sedimentary rock consisting of a framework of quartz grains bonded by cementing minerals. For the high-strength sandstone infused with a large mass of cementing minerals, the calcium carbonate crystals only precipitate in the gaps between the cementing minerals or adhere to the cementing minerals. This is only capable of relatively limited enhancement in the bio-bonding strength and volume of the quartz framework. For the low-strength sandstone with fewer cementing minerals, the precipitated calcium carbonate is evenly distributed on the surfaces of the quartz gains. The bulk strength is progressively increased with the ongoing bio-cementation between quartz gains. Cementing mineral contents not only exert a considerable control on the integral mechanical properties and penetration for the sandstone, but also have a direct influence on the microscopic distribution of bio-accumulated CaCO3, controlling the effectiveness of bio-cementation by incrementing the mechanical properties.
查看更多>>摘要：? 2021Residual soils are commonly found in unsaturated condition, and they are associated with high variability. It is necessary to develop a soil database for simplicity of geotechnical analyses and design with respect to cost optimization. This study focused on developing a framework for creating a soil database based on boreholes from different projects, locations and time in Singapore. The soil database was developed utilizing user-friendly and low-cost platforms, i.e. Microsoft Access and Visual Basic Application (VBA). The developed soil database was used to generate saturated and unsaturated soil properties envelopes in Singapore. The developed soil properties envelopes were evaluated by the results from experimental works in this study. The five-parameter logistic curve was chosen as the best-fitting function to develop envelopes with reference to the boxplot. The analyses indicated that soils located at deeper depths have lesser percentage of fine particles which results in a higher saturated permeability, effective friction angle and unsaturated ?b angle. The results from statistical analyses indicate certain upper limit, typical and lower limit for air-entry value (AEV) for residual soils from Jurong Formation, Bukit Timah Granite and Old Alluvium. Residual soils from each formation located at the ground surface produced the highest AEV while soils at deeper depths exhibit a lower AEV.
查看更多>>摘要：? 2022 Elsevier B.V.For backfill projects in seasonally frozen regions, freeze-thaw and compaction are two critical factors that affect engineering safety by changing the soil structure. This study investigated the effects of freeze-thaw cycles on the microstructure characteristics of saline soil with different compaction degrees using mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and laser particle size analysis (LPSA). The compacted soil samples show a bimodal pore size distribution (PSD). With an increase in the freeze-thaw cycles, the volume of intra-aggregate pores (<4 μm) gradually decreases, while that of inter-aggregate pores (>4 μm) varies with the density of soil samples. Furthermore, compaction has a decisive effect on the volume of inter-aggregate pores. Under the effect of freeze-thaw cycles, the void ratio of the low-density soil sample decreases, and the soil structure gradually becomes denser, showing thaw settlement deformation. However, in the high-density soil sample, the void ratio increases and the soil structure becomes loose, showing frost heaving deformation. Moreover, the freeze-thaw cycle also leads to the breakage of coarse particles and the aggregation of fine particles. Correspondingly, the soil structure type changes from a flocculent structure to a granular stacked structure and then to a cemented-aggregated structure. After 60 freeze-thaw cycles, the cracks are penetrated and the water migration channels are formed, the soil structures all reach a new equilibrium state. Meanwhile, the void ratios of different compacted soil samples are close to the same residual value. Test results suggest that there may be an optimal compaction degree between 90% and 95%, where the soil microstructure is least affected by freeze-thaw cycles. This study may provide theoretical guidance for the construction of backfill engineering projects in seasonally frozen regions from a micro-perspective.
查看更多>>摘要：? 2022 Elsevier B.V.Several nuclear waste disposal concept designs take advantage of bentonite based materials to seal underground galleries and shafts. Safety assessment and long-term predictions of the material behaviour have been the main objective of a number of experimental campaigns and of constitutive models development. All these studies have underlined that the multi-porosity bentonite structure affects undeniably the strongly coupled hydro-mechanical processes taking place during water saturation. Due to this, in recent years, many classic experimental tests on unsaturated soils have been performed in conjunction with multi-scale observation techniques (for instance MIP, i.e. mercury intrusion porosimetry analises). Despite the well-known limitations of such observation methods, they provide interesting quantitative measurements in terms of pore diameters families, which differ by several orders of magnitude, and their distribution with respect to different assemblies' types (namely pellets mixtures and compacted bentonite blocks). On the other hand, very few studies have been focusing on the role of such pore size distributions with respect to the hydro-mechanical response, both from an experimental and a numerical point of view. The aim of this paper is to present the experimental campaign and the numerical modelling strategy adopted to analyse the role of different pore size distributions characterising MX-80 bentonite in different forms (i.e. 32 mm pellets mixture, 7 mm pellets mixture and compacted sample surrounded by gap) with same overall dry density during isochoric hydration tests. Taking advantage of multisensor-equipped cells and post-mortem analyses and of the finite element code LAGAMINE, the hydro-mechanical response of these bentonite assemblies is examined. Experimental and numerical outcomes result in good agreement and provide complementary information regarding the features of each assembly type.
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