Acta Geotechnica最新文献

Fluid injection into subsurface reservoirs may cause existing faults/fractures to slip seismically. To study the effect of temperature on injection-induced fault slip, at a constant confining pressure of 10 MPa, we performed a series of injection-induced shear slip experiments on critically stressed sandstone samples containing saw-cut fractures (laboratory-simulated faults) under varying fluid pressurization rates (0.1 and 0.5 MPa/min, respectively) and temperatures (25, 80, and 140 °C, respectively). At 25 °C, slow fault slip events with a peak slip velocity of about 0.13 μm/s were observed on a tested sample in response to a low fluid pressurization rate of 0.1 MPa/min. In contrast, fluid injection with a high pressurization rate of 0.5 MPa/min caused fault slip events with a peak slip rate up to about 0.38 μm/s. In response to a given fluid pressurization rate, several episodes of slip events with a higher slip velocity were induced at an elevated temperature of 140 °C, indicating an appreciable weakening effect at elevated temperatures. We also experimentally constrained the rate-and-state frictional (RSF) parameters at varying effective normal stresses and temperatures by performing velocity-stepping tests. The obtained RSF parameters demonstrate that for a relatively high normal stress, increasing temperature tends to destabilize fault slip. Post-mortem microstructural observations reveal that elevated temperatures promote the generation of abundant fine-grained gouge particles associated with injection-induced shear slip. Our experiments highlight that injection-induced fault slip is affected by temperature-related wear production over the fault surface.

Municipal solid wastes (MSWs) disposed in landfills are generally exposed to drying and wetting cycles because of the variation in environmental conditions, decomposition of organics and leachate recirculation. This paper studies the water retention curves (WRCs) of fresh and degraded MSWs under various numbers of drying and wetting cycles with water and leachate exposure. The result indicates that the water retention capacities of MSWs decrease with drying and wetting cycles. The maximum hysteresis between the drying and wetting cycles is observed in the first cycles for all MSW samples. The WRCs of medium to highly decomposed MSWs under drying and wetting cycles are similar to those of soils. The WRCs of fresh MSWs can undergo substantial changes due to the discharge of intra-particle moisture caused by decomposition and compression. For both fresh and decomposed MSWs, the WRCs stabilize after 3 drying and wetting cycles. However, only the MSWs of one initial composition with similar void ratios were investigated. Further research should be conducted to investigate the water retention behavior of MSWs with diverse initial compositions (e.g., food contents) and void ratios.

The stability of gentle slopes is rarely accessed in existing studies, which are at risk of below-toe failure in soils with low shear strength. The inherent spatial variability of soil shear strength poses a huge complication to the probabilistic stability evaluation of large-scale three-dimensional gentle slopes, which usually forces a trade-off between precision and efficiency. In view of this, a semi-analytical method is developed in the framework of discretized limit analysis, which gives a unified mathematical representation of toe failure and below-toe failure of slopes. The proposed method inherits the high efficiency of analytical methods and has the ability to integrate spatially variable shear strengths into the slope mechanical model. The model validation is conducted by comparisons with a widely recognized analytical method developed for uniform soils. The random fields are introduced to achieve a relatively accurate characterization of soil shear strength, and the Monte Carlo simulation is employed to obtain a sufficient number of factors of safety of slopes for the subsequent statistical analyses. In the parametric study, spatial variability-related parameters, including the coefficient of variation of soil cohesion cov_c or internal friction angle cov_φ, the autocorrelation lengths along vertical and horizontal directions ξ and k, the cross-correlation coefficient ρ_cφ, are varied systematically to reveal their influences on the slope stability from a statistical perspective. It is found that the ranking of the impact on the probabilistic stability of a gentle slope is given as: cov_c or cov_φ > ξ > k > ρ_cφ. Finally, the failure probabilities of the gentle slope are computed considering the variations of key parameters, which may have implications for practical slope designs.

The objective of this study is to assess the impact of spatial variability in the subgrade layer on the critical response of pavements and the effectiveness of geogrid reinforcement, employing the random field finite difference analysis (RFFDA). A comprehensive parametric study was conducted to examine the influence of two crucial factors: the coefficient of variation (({{text{COV}}}_{E})) and scale of fluctuation (SOF) of the subgrade modulus. Further investigation was conducted to uncover the statistical and mechanical mechanisms underlying the impact of subgrade spatial variability with emphasis on the critical strain distributions and their correlation with both the overall modulus and the local spatial variability of the key influence zone. Furthermore, this study explored the influence of subgrade spatial variability on the effectiveness of geogrid in reducing critical strains, considering various placement positions and geogrid moduli. The following main conclusions are drawn: (a) subgrade spatial variability has a substantial amplifying effect on critical pavement strains due to low modulus dominating effect, (b) there exists a worst value of SOF that results in the most unfavorable statistics of critical subgrade strain, (c) the effect of subgrade spatial variability on critical subgrade strain is more pronounced compared to its effect on critical asphalt strain, (d) the mean value of critical subgrade strain in RFFDA can be significantly underestimated when assuming fixed location for the strain, and (e) the effectiveness of geogrid in reducing critical strains is impacted by subgrade spatial variability, with the impact varying with the type of critical strain and geogrid location. Specifically, when placed at the base course–subgrade interface, the ability of geogrid to reduce critical subgrade strain is significantly compromised due to the subgrade spatial variability.

Long integral bridges experience an enhanced cyclic soil structure interaction with their granular backfills, especially due to seasonal thermal loading. For numerical modelling of this interaction behaviour under cyclic loading, it is important to employ a suitable constitutive model and calibrate it thoroughly. However, up to the present, experimental data and calibrated soil models for this purpose with focus on typical well-graded coarse-grained bridge backfill materials are rarely available in the literature. Therefore, one aim of this paper is to present results of a comprehensive cyclic laboratory testing programme on highly compacted gravel backfill material. Based on this, a hypoplastic constitutive model with intergranular strain extension for small strain and cyclic behaviour is calibrated and evaluated against the experimental test data. The soil model’s abilities and limitations are discussed at element test level. In addition, cyclic FE analyses of an integral bridge are conducted with several hypoplastic parameter sets from the literature and compared to the calibrated gravel backfill material. The investigation highlights that poorly-graded sands show significantly smaller cyclic earth pressures compared to well-graded gravels intended for the backfilling of a bridge. The soil structure interaction behaviour is clearly governed by the general soil model stiffness, including the small strain stiffness.

Flow of suspensions through the complex porous network is typically characterized by the initial spurt and then the formation of internal/ external filter cake which impedes the flow velocity. The transient mechanisms involved during the particle migration phenomenon need to be studied carefully as it is crucial for effectively managing the flow characteristics of drilling fluids and their impact on subsurface reservoirs. In this study, constant pressure permeation experiments are carried out using a specially designed apparatus to quantify the formation and evolution of particle migration zones using advanced image processing algorithms. Additionally, a comprehensive pre-test and post-test characterization of drilling fluids/ filtrates and the porous medium revealed intricate insights into the dynamics of particle migration. Four distinct particle migration/ filtration zones such as internal filter cake, primary filtration, secondary filtration and fluid loss are identified based on the change in the concentration gradient. The influence of additives on the growth of these zones is quantified during the filtration process. The concentration of barite/ micronized calcium carbonate and xanthan gum predominantly controls the filtration process by enhancing the particle plugging and retention time, respectively. In addition to the in-depth understanding of the particle migration zones, the transition from kinetic to capillary flow is identified by performing the fractal analysis. The analysis revealed that drilling fluid containing more barite exhibits a dominant capillary flow. Finally, an analytical model has been modified by considering the influence of different additives to predict the depth of penetration, which is comparable with the experimental results.

This paper presents the first effort to unravel and quantify the strengthening of a shale induced by nanoparticle injection from the perspectives of cross-scale and multi-constituent mechanical properties. After being subject to injection of pure water and an aqueous suspension of carbon black nanoparticle of ~ 50 nm in diameter under a differential pressure of 850 kPa, the shale specimens were characterized by big data nanoindentation (BDNi) to probe the mechanical properties of both individual constituents at the microscale and the bulk rock at the macroscale, leading to comparatively assessing the effects of injecting pure water and aqueous nanoparticle suspension on the mechanical properties. Microstructural characterization by electron microscopy and X-ray computed tomography validates the successful injection of nanoparticles into the microcracks and micropores of the rock. While the nanoparticles can infiltrate to depths of up to 100 s μm in zones with densely populated microcracks, the maximum depths of injection in crack-free zones are only 2–5 μm. Moreover, the injected nanoparticles mostly act as inert fillers in the interconnected micropores and microcracks but can seldom enter the isolated micropores. Comparison of the BDNi results from pure water versus nanoparticle-injected specimens shows that the Young’s modulus of the clay matrix experiences the highest increase by 23.1%, while the counterpart of non-porous quartz the lowest by 12.8%. Overall, the bulk shale’s Young’s modulus increases by 21.5%. Such data are consistent with the microcharacterization results that the injected nanoparticles mainly remain in the micropores and microcracks within the clay matrix. Owing to their hydrophobic nature, the carbon black nanoparticles have little effect on the rock’s hardness. The findings can shed light on the practical applications of nanoparticle injection for improved wellbore stability in shale formations.

Understanding accurately the influence of non-plastic fines on stress-dilatancy of coral sand mixture-packing is crucial for marine engineering in various geotechnical applications. This work experimentally examined the effects of non-plastic fines and initial test conditions on stress-dilatancy behavior of mixture. Based on test results, equivalent void ratio (({e}^{*})) was determined to quantify the global effect of fines on shear behavior across different shear stages. Test results show that ({e}^{*}) exhibits a reduction as the mean effective stress (({p}{^prime})) increases, following a power function relationship. Besides, ({e}^{*}) variation under phase transformation, peak state, and critical state can be described by a normalized curve. Reduced fines content and increased relative density can contribute to the enhancement of both peak strength and internal friction angle within the mixture. However, the smooth shape and lubrication function facilitated by fines actively contribute to initiation of shear contraction. Furthermore, the stress paths observed in the CD shear tests manifest as a sequence of parallel straight lines within the (q)-({p}{^prime}) plane. The length of these lines progressively extends as the stress level escalates. Moreover, deviator stress in (q)-({p}{^prime}) curves under character state presents lower and upper limits which are 0.334 and 0.639 corresponding to tested samples determined by fines content and relative density. Elevated fines content combined with reduced relative density can lead to a reduction in both peak-state friction angle and maximum angle of dilation.