Bulletin of Engineering Geology and the Environment最新文献

The fracture behavior of granite with pegmatite veins, which influences brittle spalling at the Shuangjiangkou Hydropower Station’s underground powerhouse in China, was investigated. This study explored the fracture characteristics of veined granite through three approaches: an in-situ geological survey, true triaxial compression experiments, and microscopic fracture analysis. The results revealed that pegmatite veins, due to their high brittleness, are prone to spalling and often serve as boundaries for rock mass instability. Under true triaxial compression, the stress–strain curve of granite shifted from Class I to Class II in the presence of veins, with thicker veins resulting in lower overall granite strength. In granite with gently dipping veins, deflection of the main fracture at the vein–granite interface was observed, while steeply dipping veins led to fracture propagation either within the veins or along the lithologic interface. Factors influencing the fractures included differences in elastic modulus and Poisson’s ratio between the veins and granite near the lithological interface, as well as mineral composition and structural characteristics. A greater aggregation of shear fracture signals was observed in specimens with thicker veins before reaching the damage stress threshold. Stress-relieving measures are necessary to reduce stress concentration near veined rock masses, and microseismic techniques are recommended for monitoring shear fractures to provide timely warnings of rock mass instability when excavating granite with steep and thick veins in a deep underground cavern.

Landslides in colluvial soils under rainfall have been identified as a significant problem due to their loose, heterogeneous nature and low shear strength. Evaluation of the stability of colluvial slopes under rainfall conditions is challenging. This study investigated two landslide failure case studies of colluvial soils to understand the failure patterns using finite element (FE) and limit equilibrium (LE) slope stability analysis methods under unsaturated conditions. Transient seepage conditions due to rainfall infiltration and failure were analysed using hydromechanical models. Here, a FE fully coupled hydromechanical model and a sequential coupling of a FE hydrological and LE mechanical model were used to evaluate the failure of variably saturated slopes. Results from the case studies revealed that the failure occurred due to the rise in the groundwater table in both cases. It was evident that there can be significant disparities in the pore water pressure profiles with the fully coupled and sequentially coupled analysis. The dynamic capability of the two models can also affect the interplay between the hydrological and mechanical aspects. When the thickness of the colluvium layer is large, the failure could potentially occur as a deep-seated failure along the boundary of overburden and the bedrock surface due to the large driving force. However, when the thickness is small, failure can occur along the colluvium-weathered rock surface. The outcomes from the study will contribute to mitigate the uncertainty of failure prediction of landslides in colluvial soils.

Water-softening effect has been widely recognized as one of the primary causes triggering large deformation and failure in soft-rock engineering; however, there is still a lack of a full-stage constitutive model for rock considering the water-softening effect and non-linear deformation characteristics at the compaction stage under triaxial stress conditions at present. In this paper, laboratory tests are firstly carried out to estimate the deterioration characteristics of mechanical properties with increase of saturation coefficient for shale samples. And then, a full-stage constitutive model of shale subjected to water-softening effect is proposed, which consists of the pre-yield and the post-yield constitutive relationships. The pre-yield constitutive relationships could well describe the non-linear deformation characteristics of compaction stage, which are derived based on the generalized Hooke’s law considering water-softening effect under anisotropic stress conditions. On the other hand, by introducing correction coefficients to solve the problem of numerical discontinuity at the yield point of the pre-yield and the post-yield constitutive relationships, the post-yield constitutive relationships are derived on the basis of the statistical damage mechanics theory. The comparison results with the experimental data show that the proposed model could well characterize the full-stage stress–strain relationship for shale under triaxial loading considering the water-softening effect.

Rock pore structure coupled with fluid pressure plays an important role in controlling fault slip behavior. Observation of fluid-induced seismicity in geoenergy extraction has raised fundamental questions about the physics of fault rock structure and fault frictional stability in the presence of fluid. Here, we change the pore structure of faults by thermal treatment and report on the frictional stability of granite faults with pore evolution and pore fluid pressure in velocity stepping experiments under the rate-and-state framework, where the variation of pore fluid is monitored. The experiments under constant fluid pressure show that pore structure propagation leads to an increase in friction coefficient from 0.71 to 0.78. As the degree of pore propagation increases, the drained fault exhibits a transition from velocity strengthening to weakening behavior. The decrease in frictional stability could be caused by the coupling between the pore fluid and the well-connected pores, namely “fluid oscillation”. Pore pressure overpressurization could develop and cause non-uniform stress distribution along the fault surface due to pore fluid oscillation at velocity steps. The time required to equilibrate fluid pressure could be prolonged by fluid oscillation, leading to intrinsic velocity strengthening behavior appearing as velocity weakening. The decrease in rate-and-state parameter with elevating pore fluid pressure on high-porosity fault corroborates the fluid-induced fault destabilization. The fluid oscillation at the greater pore pressure could be responsible for fault reactivation. Therefore, the coupling effect of rock pore structure with pore fluid could be a potential mechanism governing fault frictional stability.

Residual stresses are known to exist within the microstructure of crystalline materials as a result of material formation processes. Research has proven their existence and implications, and engineering applications have been derived for glass and metal materials. In the rock engineering field, limited research has been published on the topic in recent decades. Literature on residual stress in rock is presented regarding the formation mechanisms, magnitudes, and observed implications. Numerical modelling techniques, such as Grain-Based Modelling, can be used to gain insight into residual stresses in rock. Micromechanical numerical models were created using RS2’s Voronoi network to study rock simulations that include residual stress. Using a simplified modelling sequence, a residual stress field (microstresses) was created within a hypothetical rock mineral structure and three main scenarios were simulated. The first explores a potential relationship between residual stress and compression test crack closure strain. Secondly, the possibility of sample damage due to residual stress redistribution and the influence of residual stresses on the propagation of a slot cut was investigated. Finally, the anticipated displacements around a circular excavation in a rock block containing residual stresses were examined. The numerical investigations suggest that residual stress may have real and non-negligible influence on rock behaviour. This includes the effects of crack opening/closure, sample damage, and rock displacements that are not currently accounted for with implications for rock engineering projects.

The permeability of cementitious soils is substantially controlled by the permeability of the soil itself, the dosage of binder, the water-to-binder ratio, and other factors. This study employed metakaolin as the additive to enhance underground engineering construction's economic benefit since it could improve the impermeability of cement-stabilized soils and replace cement partly. A series of indoor permeability tests on cement- and metakaolin-stabilized fine sandy soils (CMSFSSs) with different cement-to-metakaolin ratios, water-to-binder (the mixture of cement and metakaolin) ratios, total binder contents, and curing times were conducted. The influences of these factors on the impermeability of CMSFSSs were investigated. Their impermeability improvement mechanism at the microscale was explored by Scanning Electron Microscopy and Mercury Injection Porosimeter tests. The empirical permeability coefficient prediction formulas about these influence factors, compressive strength, and porosity were discussed. The results showed that the best impermeability of CMSFSSs was achieved when the cement-to-metakaolin ratio was 5:1, saving 1/6 cement consumption. This ratio did not vary with the total binder content. The permeability coefficient of CMSFSSs increased nonlinearly with the water-to-binder ratio but decreased rapidly at first and then slowly with the increase of total binder content and curing time. The optional water-to-binder ratio should be less than 0.6 if both their liquidity and impermeability requirements were met together. The total binder content for fine sandy soil stabilization should be less than 15% since it was not more reliable to improve the impermeability of fine sandy soils by using excessive binder in terms of economic benefits. The hydrated gels in CMSFSSs formed rapidly at the early curing time. The calcium hydroxide formed by cement hydration disappeared over the curing time. The internal pore volume and sizes in CMSFSSs decreased over the curing time, resulting in worse and worse connectivity. All of them proved the contribution of metakaolin to cement-stabilized soil's impermeability improvement. Six empirical formulas for the permeability coefficient of binder-stabilized soils were summarized regarding the water-to-binder ratio, total binder content, curing time, unconfined compressive strength, and porosity. The results of this study provide theoretical and technical references for improving the impermeability of binder-stabilized soils.

The research on slope stability of open-pit mine has always been a key technical problem that plagues safe production, and it is also an important research topic in geotechnical engineering. Changshanhao open-pit mine is the largest gold mine in northern China. The increasing mining depth has led to various slope instability problems. Based on the background of ' 2021·10 landslide ' in the northeast stope, this paper carries out a new round of slope stability evaluation and disaster prevention countermeasures from the strategic height of the future development of the mining area. Firstly, the site engineering geological survey is carried out by using scanline method, and the main structural characteristics such as faults are summarized, and the comprehensive zoning map of engineering geology is established to reveal the failure mode of the slope in northeast stope are planar sliding and wedge sliding. Secondly, based on the principle of slope classification analysis, the study area is divided into overall slope, combined step slope and step slope, and the structural plane is matched with it. The stability of mining slope is preliminarily evaluated by stereographic projection method and Q-slope method. Approximately 13.9% of the area was classified as high-risk. Finally, the three-dimensional calculation model of the stope is established, and the FLAC3D is used to invert the landslide to reveal the influence mechanism of the fault on the slope stability. Combined with the analytical solution and the numerical solution, a suggestion is made to optimize the slope angle by raising the boundary., which lays a foundation for the sustainable and safe mining of the mine.

A shaking table model test was carried out to study the failure mechanism of an anti-dip rock slope with complex structural planes. The effect of the input seismic wave frequency, types and structural plane on the slope’s dynamic response were considered. The test results show that the input seismic wave frequency is closer to the slope’s natural frequency, the acceleration amplification factor is greater. The amplification effect of input bedrock seismic waves is higher than that of soil seismic waves for rock slopes. The existence of soft-hard rock interface and tectonic fissures inhibit the slope’s amplification effect on seismic waves, and the inhibitory effect of tectonic fissures is higher than that of soft-hard rock interface. Slope displacement increases with the increase of input wave amplitude, but the change is not obvious with the increase of frequency. The time cumulative effect is more obvious under high amplitude input seismic wave for the slope’s displacement. The slope deformation and instability mode can be called ‘bending-shear slip instability’. The results of this paper are meaningful for the further understanding the dynamic failure mode of anti-dip rock slope with complex structural planes.

Granite, as a ty pical crystalline rock in the earth's crust, is one of the ideal media in protection works for radioactive waste disposal. Grain size significantly influences the conventional triaxial compression mechanical properties of granite, thus also on the safety and stability of nuclear waste repositories. Thus, triaxial compression tests were performed by combining experiments with GBM3D based on PFC3D to study granite's conventional triaxial mechanical properties in grain size. Furthermore, the flexible boundaries were constructed in the numerical model using wall-zone coupling to apply lateral constraints in the model. The experiment outcomes demonstrate that coarse-grain granite contains more defects than the fine-grain granite. Concerning mechanical properties, fine-grained granite has much better mechanical properties than coarse-grained granite. However, the compressive of the initial defects at high confining pressures narrow this gap. The numerical simulation results show that the GBM3D model based on the flexible boundaries applied to the confining pressure can reasonably simulate the macroscopic mechanical behavior and non-uniform deformation of granite. This method better reflects the inhomogeneous deformation of granite specimens. The larger the grain size, the greater the spatial variation between mineral crystals, the more significant the effect on compressive stress atrophy and the tendency to produce macroscopic shear failure zones. As the confining pressure increases, the damage to the specimen is dominated by P_CT/P_IC leading to P_CS/P_TC transformation.

Landslide disasters have occurred at excavated soil dumpsites in China’s soft soil regions due to poor drainage and weak strength of clay soil. A potential solution is a geocomposite drainage layer (GDL), which drains and reinforces the soil. Understanding the consolidation effect on the shear behavior of clay soil-GDL interfaces is vital for using GDLs in dumpsites. Large-scale interface direct shear tests were conducted on excavated clay soil (a common type of excavated soil in China’s soft soil regions) and three geosynthetics (GDL, nonwoven geotextile, and geogrid) under different normal stresses and consolidation degrees. The results showed: 1) The shear curves of clay soil-GDL interface were strain-hardened and were significantly affected by the soil consolidation. A consolidation-dependent shear constitutive model was proposed and can accurately describe those shear curves. 2) The shear strength envelops of clay soil-GDL interface satisfied the Mohr–Coulomb failure criterion. As the soil consolidation degree increased, the cohesion c exhibited a slight linear decrease, i.e., c(U) = –1.128U + 6.487, whereas the friction angle φ showed rapid linear growth, i.e., φ(U) = 10.300U + 2.685. 3) The significant increase in shear strength of clay soil-GDL interface during soil consolidation is primarily due to the improvement in soil shear strength and the “dimple effect” occurred in the interface. 4) When the soil consolidation degree exceeded 30%, the reinforcement efficiency of the clay soil-GDL was superior to that of the clay soil-geotextile and unconsolidated clay soil-geogrid. These findings shone a light on the confidence of using GDL as an efficient tool to improve the stability of excavated clay soil dumpsites.