Bulletin of Engineering Geology and the Environment最新文献

In deep geological repositories for disposing of high-level radioactive waste, various types of bentonite interfaces exist in the engineered barrier system. The hydromechanical resistances of the bentonite buffer rely on the sealing and healing of these interfaces, especially the assembled bentonite-bentonite interfaces that can heal spontaneously upon bentonite hydration (i.e., self-healing). This study explored the interfacial self-healing behavior of densely compacted Gaomiaozi bentonite via laboratory tests and evaluated the interfacial self-healing capacity in terms of the physical and hydraulic properties. In detail, the interfacial healing test results indicate that self-healing of the assembled bentonite interface initiated at low suction values (total suction s < 20 MPa) under confined conditions (when the assembled interface is compressed by the swelling pressure). The infiltration test results suggest an increased interfacial hydraulic resistance after long-time water infiltration and a slightly higher permeability than the saturated intact bentonite specimen with the same final dry density. The swelling pressure test results show that the bentonite assembly developed much lower swelling pressure than the intact bentonite specimen (for a given total suction) and achieved a rotated and slightly enhanced swelling pressure anisotropy compared with the saturated intact bentonite specimen with the same final dry density. The MIP test results reveal that the healed interfacial zone has similar pore structure as its adjacent intact zone. Thus, it is concluded that densely compacted Gaomiaozi bentonite has strong interfacial self-healing capacity in physical and hydraulic aspects.

Tunnel boring machines (TBMs) are one of the most widely used means of excavating tunnels in rock masses in a more economical, faster, and safer manner. Despite the recent technological advances in tunneling, a reliable estimation of the TBM rate of penetration (ROP) still remains a challenging task. This study aims to develop empirical models for predicting the ROP, which can be applied to a variety of rock mass conditions. A large database with 557 instances of TBM performance parameters including the rock mass properties such as rock strength and frequency of discontinuity, machine specifications and ROP, was established. To account for the effect of the discontinuity on the ROP a rock mass fracture index is introduced via a weighting method and used in the model formulation. Numerous linear and nonlinear multiple regression models were developed; the five most accurate models were selected, and their performances were compared using the values of performance indices and the total ranking method. The results indicate that the best model is the non-linear multiple regression models that has fewer input variables and the best estimation result (total score ranking of 42, Eq. 9) in comparison with others. It was concluded that the proposed models relating the intact rock strength, the weighted fracture index in rock mass, and TBM specifications including cutter force, rotation speed and cutterhead diameter could be confidently used for TBM tunneling.

Hydrological changes affect clays used as landfill cover, liner and embankments due to wetting-drying (W-D) cycles, adversely affecting their engineering behavior. This study assessed impact of plasticity characteristics on W-D response of unreinforced and fiber-reinforced clays. Four different mixtures of kaolinite and bentonite have been used to assess a wide array of plasticity. Compositions have been mixed with 0.1, 0.2, 0.3, 0.4, 0.6 and 0.9% polypropylene (PP) fibers 6, 12 and 18 mm in length and subjected to six W-D cycles. To analyze pictures of cracks for geometric dimension determination, Image J software together with Scanning Electron Microscopy (SEM) for visual examinations have been used. Results show that unfavorable effects of W-D cycles intensify with increase in clay plasticity and inclusion of fibers significantly reduced the detrimental effects through reduction of the area, length, width and number of cracks and helped maintaining integrity of samples even after six cycles. Additions of up to 0.3% fibers proved very effective with 12 mm being the optimum length. Fiber inclusion proved more effective in controlling cracking in high compared to low plasticity clays. Longer fibers proved more effective in low plasticity and shorter fibers with larger numbers in high plasticity clays. In high and low plasticity clays cracks formed after the 1st and the 3rd W-D cycles respectively and number of cracks grew with increase in W-D cycles. SEM showed fibers create a 3-dimensional network in clays that bind particles, resist tensile stresses and prevent rise in number and size of cracks.

To overcome the limitation of the Grasselli's morphology parameter only represent the local features of the fracture roughness, this paper defines an average slope angle to reflect the ignored roughness information. A modified parameter ({theta }_{C}) is proposed by incorporating the average slope angle into the Grasselli's morphology parameter, and its ability to capture the anisotropic characteristics of joint morphology is validated. Direct shear tests are performed on joint replicas with different morphology to investigate the relationship between of the modified Grasselli's morphology parameter and shear strength. The results show the contribution of ({theta }_{C}) to peak dilation angle depends on the ({sigma }_{text{n}}/{sigma }_{text{c}}) ratio. Follow the physical constraints, a peak dilation angle model is constructed. The initial dilation angle, as determined from tilt tests, could be expressed as twice the ({theta }_{C}). Finally, a new shear strength criterion for rock joints is proposed. Compared to existing criteria, this criterion has a simpler form and provides a more comprehensive understanding of dilation behavior. The predicted results indicate that it reliably estimates the joint shear strength.

Significant amounts of waste glass are deposited each year, which poses a significant environmental problem globally. Utilizing waste glass for soil improvement has emerged as an environmentally friendly solution, attracting increased research interest in recent years. This study investigated the effects of waste glass gradation and content ratios on the compaction, strength, and swelling characteristics of medium expansive clayey soil and pulverized glass mixtures. Waste glass was crushed into three different size gradations and mixed with the soil at eight different weight ratios (0%, 4%, 8%, 12%, 16%, 24%, 32%, and 40%). Standard Proctor compaction and unsoaked and soaked California Bearing Ratio (CBR) tests, along with swelling tests, were conducted on each sample. Microstructural analysis was performed through scanning electron microscopy (SEM). The results indicated that adding up to 40% pulverized waste glass reduced the Optimum Moisture Content (OMC), whereas the Maximum Dry Density (MDD) increased. Moreover, the unsoaked and soaked CBR values increased notably with higher pulverized glass ratios. Increases in unsoaked CBR values reached up to 143.5%, 129.8%, and 122.1% for coarse, medium, and fine pulverized glass grades respectively, while those for soaked CBR were 209.2%, 197.2%, and 189.4% respectively. Additionally, the swelling percentage decreased as the pulverized glass content increased. The different grades of pulverized glass showed no significant effect on the swelling percentage at glass contents up to 8%. Statistical analysis revealed strong correlations between the tested parameters. Relationships between the soaked and unsoaked CBR values derived from OMC, MDD, and waste glass ratio were also discussed.

In the Central Sichuan region, oil-gas fields are widely distributed, and due to the toxicity and explosiveness of natural gas, tunnel construction poses safety hazards. Existing prediction methods for oil-gas tunnels have deficiencies in accuracy and applicability. Therefore, developing a new method for the classification prediction of harmful gas is crucial for the design and construction of projects in the Central Sichuan region. This research proposed a prediction method based on the Support Vector Machine (SVM) optimized using the Energy Valley Optimizer (EVO) algorithm. Initially, 114 sets of harmful gas tunnel cases were selected based on existing engineering data. Parameters such as tunnel depth, length, oil-gas field location score, structural score, and lithology score were used as inputs, while the actual gas classification served as the output for model validation. The results show that the method achieved a high accuracy of 96.67%, along with higher convergence speed and higher predictive accuracy. The method was applied to the Sichuan Basin region to predict the hazard levels of 89 tunnels within the area, obtaining the hazard levels and distribution map of harmful gas in the area, providing valuable guidance for tunnel construction projects in the area.

Shear-induced rock bridge fractures greatly threaten the stability of rock slopes and deep rock masses, owing to their connection with pre-existing discontinuities. In this research, direct shear tests on sandstone rock bridges were performed under constant normal stiffness (CNS) conditions. The effects of rock bridge length, initial normal stress and normal stiffness on the shear behavior of rock bridges were carefully investigated, encompassing both the pre-failure (cracking phase) and post-failure (sliding phase) stages. Test results revealed that these three factors variably impact the shear strength, dilation characteristics, failure pattern and acoustic emission response of the rock bridges. In particular, normal stiffness was found to greatly affect the post-peak slip behavior. It was observed that shear-induced rock bridge fractures exhibit distinctive shear contraction characteristics, which contrast with tension-induced splitting fractures that are typically marked by shear dilation. The shear contraction mechanism of rock bridge fractures was elucidated using a conceptual cracking model, termed the TST model. This research contributes fresh insights to the comprehension of dynamic slip hazards prompted by the rupture of rock bridges in deep rock engineering.

Using self-absorption tests combined with the digital speckle correlation method (DSCM), we investigated the evolution of the deformation field, expansion anisotropy, full-field strain, crack development, and deformation mechanisms in mudstone. During the early moisture-absorbing stage, the displacement field was uneven, accompanied by crack initiation and propagation. By the end of moisture absorption, the horizontal displacement exhibited an approximately 45° zonal pattern along the x-axis, while vertical displacement showed a horizontal zonal distribution. The anisotropy degree of the expansion rate over time mirrored the water absorption curve of mudstone. A three-piecewise linear model was developed to fit the relationship between expansion rate anisotropy and water absorption. Throughout moisture absorption, the average linear strain variation followed the water absorption trend, with the final mean linear strain significantly decreasing as sample height increased. The maximum ε_xx occurred at the crack tip, and the strain localization band distribution indicated crack development. The maximum ε_yy aligned with mid-span theory, showing zero vertical displacement. Deformation of mudstone during moisture absorption involved both pore water adsorption and the expansion of clay minerals within the matrix. This study introduces a novel method for examining the hygroscopic expansion and cracking behavior of red-bed mudstone, providing crucial insights into disaster mechanisms and engineering stability in red-bed regions.

Analysis of fracture propagation, evolution mechanisms, and failure characteristics is crucial for investigating the stability of rock slopes under the combined effects of rainfall and excavation. The novelty of this study lies in the comprehensive investigation of crack propagation and final failure modes of high steep rock slopes at different angles (ranging from 45° to 65°) under the coupling effect of rainfall and excavation. These investigations were based on conventional rock mechanics tests, large-scale laboratory similar model experiments, and theoretical analysis of fractal fractures. Herein the experimental results revealed that: (1) During the open-pit excavation stage, the rock masses gradually moves towards the mining area, resulting in vertical cracks during the mine-room stage. These cracks exhibit a semi-elliptical shape as they continuously develop through combination. Moreover, separation cracks appear in the overlying rock area after pillar mining, which is followed by the large-scale collapse of the overlying rock due to continuous pillar extraction. (2) The fractal dimension of the fracture connected zone is found to be the highest among the “three zones” within different slope angles, followed by the collapse zone, while the fracture extensive zone exhibits the lowest fractal dimension. (3) The fractal dimension and percolation rate initial decrease, followed by an increase, as the slope angle increases from 45° to 65°. Notably, among these angles, the highest value is demonstrated by 65° when compared to 45°, 50°, 55°, and 60°. Hence, this study can provide theoretical guidance for safe and efficient mining in the coupled rainfall-mining environment.

Dam-break flood has the characteristics of fast flow speed and large kinetic energy, which can increase the pore water pressure in slope and cause the slope deformation or even destruction. To determine the pore water pressure in slope under impact of dam-break flood, firstly, a novel approach for estimating the impact pressure of dam-break flood under condition of high Froude number is proposed based on equivalent hydrostatic pressure, and the accuracy of this approach is verified by wave flume test; then a simplified calculation method of pore water pressure is put forward based on microwave theory and Darcy's law. Finally, instability mechanism of a soil-rock mixed slope under dam-break flood is investigated, and the instability failure process of this slope is analyzed through field investigation. By constructing its geomechanical model, the slope stability is analyzed under impact of dam-break flood, and the results show that influence of dam break distance on the slope stability is significant in a certain critical range, and the maximum flood pressure distribution pattern changes from triangle to trapezoid with the increase of Froude number. The safety factor diminishes as the flood flow velocity increases, and the safety factors increase with the impact angle and internal friction angle increasing. These results can provide technical reference for landslide hazard risk assessment and emergency decision-making under dam-break flood action.