Feng Xiong

Excavation-induced unloading alters the stress distribution and support demand in high and steep rock slopes. This study investigates a rock slope near the mainline toll station of the Xiangyang–Yichang Expressway. A FLAC3D model simulates seven-stage excavation. Displacement, plastic-zone distribution, factor of safety, and bolt axial force are presented for unsupported and sequentially supported conditions. Rock-bolt support reduces the final maximum total displacement from 2.895 to 1.442 mm, a reduction of 50.2%. It also increases the factor of safety from 1.129 to 1.316, an increase of 16.6%. The factor of safety reaches a temporary peak at Stage 3 without support and at Stage 4 with support. It then decreases. The maximum bolt force increases sharply at Stage 6. This indicates increased support demand during the later excavation stages. Comparisons with field measurements yield a mean absolute relative error of 11.6% for slope-surface displacement. The monitoring points lie near the heads of the bolts carrying the stage-wise maximum axial forces. The mean absolute relative error of the computed maximum bolt axial force is 9.7%. Five fixed monitoring locations further characterize the spatial distribution and temporal evolution of local responses. Sensitivity analyses examine bolt length, inclination, and horizontal and vertical spacing. They quantify the effects of these parameters on the factor of safety. The results clarify the relationship between excavation sequence, reinforcement mobilization, and slope stability for the investigated slope.

Reservoir water level fluctuations can reactivate landslides and cause severe losses. This study examines the Niulanjiang landslide, reactivated by the impoundment of the Xiluodu Hydropower Station in Southwest China, using field investigations, in situ displacement monitoring, and direct shear tests on soil–rock mixtures. The results show that the land-slide experienced a progressive failure process, evolving from long-term shear creep in the sliding zone to localized abrupt creep and finally to overall fracture sliding. The loose soil–rock mixture provided the structural basis for instability, whereas reservoir water level fluctuation was the dominant trigger. Rising water levels increased shear stress and promoted seepage-induced weakening, causing local failure of the sliding surface and gradual formation of a shear outlet. Laboratory tests indicate that rock block content and moisture content strongly affect mechanical behavior: higher rock block content enhances shear dilatancy and strain softening, while higher moisture content promotes shear contraction, plastic deformation, and linear reductions in cohesion and internal friction angle. The failure mechanism involves coupled strength degradation and increased seepage force. Initial instability occurred in the middle slope under hydrostatic–hydrodynamic pressure, then propagated rearward and forward, reducing front resistance and driving overall sliding toward the Niulanjiang River. These findings support early warning and mitigation of similar reservoir-induced landslides.

In this study, we investigate the bearing capacity characteristics and controlling mechanisms of coastal beach sand in Quanzhou Bay, Fujian Province, China. The results provide support for coastal engineering construction and vehicle trafficability assessment in beach areas, while field sampling, laboratory static plate load tests, and data-based modeling were conducted to examine the effects of moisture content, particle size distribution, and relative density on the bearing behavior of beach sand. In total, 52 groups of static load tests were performed, with the results showing that relative density is the dominant controllable factor affecting the bearing capacity of coastal beach sand. When the relative density increased from 40% to 65%, the ultimate load increased by 80%, and the deformation modulus increased by 139.9%. The optimal relative density range was approximately 52–65%, and the improved particle size distribution enhanced bearing performance. The ultimate load of well-graded sand was 60% higher than that of poorly graded sand, and moisture content showed a threshold effect, with the best mechanical performance occurring at a moisture content of about 7%, whereas excessive moisture content significantly reduced the bearing capacity. Under natural conditions, the proportional limit load of medium-dense coastal beach sand in Quanzhou Bay was approximately 200 kPa, the ultimate load was 250 kPa, and the characteristic value of bearing capacity was 125 kPa, while the dominant failure mode was general shear failure. A semi-empirical bearing capacity model was also developed; its average relative error was 10.35%, indicating that it has both physical meaning and engineering applicability. The findings provide a reference for foundation design evaluation in Quanzhou Bay and similar coastal beach areas.

Underground coal gasification (UCG) is an emerging energy technology that involves the in situ conversion of coal into syngas through controlled combustion within a subsurface excavation. The geomechanical processes associated with UCG can lead to significant overburden caving and surface subsidence, posing risks to surface infrastructure and groundwater systems. To accurately predict the size of overburden caving zones and associated surface subsidence, a prediction model was developed based on simulation results using discrete element method (DEM) numerical models. The main purpose of developing such a model is to establish a systematic and computationally efficient method for the rapid prediction of the height of overburden caving and its associated surface subsidence induced by underground excavation. The model is broadly applicable to different types of underground excavations, and UCG is used in this study as a representative application scenario to demonstrate the relevance and performance of the model. Sensitivity analysis indicates that excavation span, tensile strength, and burial depth are the primary controls on the height of the caving zone within the ranges of parameters investigated. Rock density is retained as a secondary background parameter to represent gravitational loading and its contribution to the in situ stress level. The derived model was validated using published numerical, experimental, and field measurement data, showing good agreement within practical ranges. To further demonstrate the application of the model developed, the predicted caving geometries were incorporated into finite element method (FEM) models to simulate surface subsidence under different geological conditions. The results highlight that the arch structure formed by overburden caving can help redistribute stresses and thereby reduce surface deformation. The proposed model provides a practical, parameter-driven tool to assist in underground excavation design, environmental risk evaluation, and ground stability management.

Understanding the evolution of micromorphology and shear behavior of joints is essential for evaluating the stability of rock engineering in an acidic environment. Limestone joint samples from the Three Gorges Reservoir area, China, were subjected to acidic erosion using sulfuric acid solutions with varying pH values and immersion times. The influence of acid erosion on joint morphological characteristics and shear mechanical response was experimentally investigated using a large-scale rock direct shear testing system. Results indicated that key joint roughness parameters showed a slightly increasing trend from 1.75% to 19.23% with increasing acid exposure. Conversely, the peak shear strength of the joints decreased nonlinearly with erosion time and acid concentration. In the initial 10–20 days of immersion, shear strength dropped from 8.19 to 6.91 MPa, with the rate of decrease subsequently slowing. Under strongly acidic conditions (pH ≤ 3), strength reduction was significantly greater than under weakly acidic conditions (pH ≥ 5), reaching a 20.6% decrease from the initial value after 80 days at pH = 1. The surface damage ratio of joints increased from 13.59% to 19.71% as pH decreased from 7 to 1. Microstructure observations revealed that localized dissolution areas and craters with a diameter of 9.0 μm appeared on joint surfaces initially, contributing to increased roughness. However, with prolonged exposure, acid infiltrated the rock matrix, triggering secondary crack expansion and pore structure degradation, thereby reducing matrix strength. Despite a minor increase in joint roughness from acidic erosion, the significant attenuation of rock matrix strength was the dominant factor controlling the degradation of joint shear strength.

This study explores the effects of confining pressure Pz, shear displacement u, and rotational shear angle θ on the nonlinear hydraulic behavior during the rotational shearing of rough rock fractures. A model for fractures under rotational shear was first established, and subsequent seepage tests were carried out at various flow rates. The data revealed a strong correlation between the hydraulic gradient and flow rate by Forchheimer's law. The coefficients of the inertial and non-inertial terms of Forchheimer's law decreased in two distinct phases with increasing u. Both Pz and θ were found to enhance these coefficients, with the effect being more pronounced at a smaller u. The critical Reynolds number Rec increased by up to 2.58 times as u ranged from 1 to 15 mm, with a higher Pz and the θ corresponding to lower Rec values. The Forchheimer coefficient decreased rapidly with increasing u due to dilatancy effects, and the rate of decrease slowed as these effects diminished. Increases in Pz and θ led to a concomitant rise in β, although the influence of u and Pz on β was mitigated by the increase in θ. By solving the Navier–Stokes equations, simulations of the seepage flow in a fracture under rotational shear were conducted. The simulation results indicate that as rotational shearing occurs, the number of streamlines within the fracture decreases, while the number of vortices increases. The findings may offer some insight for underground space development projects.

The two-phase displacement in rock fractures is a critical scientific challenge in deep energy development projects, characterized by the intricate and dynamic flow patterns resulting from the complex fracture geometry and mutual interference between the two-phase fluids. The present study involves the construction of a three-dimensional rough fracture fine model using the fast Fourier transform and Gaussian distribution method. The phase field-finite element method is employed to simulate the dynamic displacement of water-oil flow and the evolution of the water-oil interface, aiming to investigate the impact of roughness, aperture, and wettability on the viscous fingering pattern observed in rough fractures. The numerical results indicate that the immiscible two-phase displacement presents a viscous fingering pattern at a high flow rate. Moreover, the greater the roughness is, the higher the number of fingering is, and the more volatile the change in displacement velocity is. The width of the wet-phase flow channel increases proportionally with the fracture aperture increasing. Furthermore, the morphology of immiscible two-phase interface within fractures is also influenced by the wetting angle, and the fingering, bifurcation, and crushing phenomena appear in the oil-wet state with the strong displacement interface instability. Finally, a fractal model of relative permeability for waterflooding in rough fractures is proposed based on the cubic law of flow and fractal dimension of aperture distribution, which has been validated through comparison with numerical results.

Underground coal gasification (UCG) is an efficient method for the conversion of deep coal resources into energy. The scope of this work is to model the subsidence of four gasification cavities with a size of 30 m × 30 m × 15 m, separated by 15 m wide pillars. Two scenarios of gasification sequence are modelled, one with the gasification of cavities 1 and 2 followed by 3 and 4, and the other one with the sequence of cavities 1 and 3, followed by 2 and 4. The results show that the final surface subsidence after gasification of four cavities is 9.8 mm and the gasification sequence has an impact only on the subsidence at the intermediate stage but has no impact on the final subsidence after all four cavities are formed, when only the elasticity regime is considered. Additionally, the maximum surface subsidence for the studied cavities of different sizes ranges from 0.016 mm to 7.14 mm, and the relationship between the subsidence and the cavity volume is approximately linear. Finally, a prediction model of surface subsidence deformation is built up using the elastic plate theory, and the formula of surface deformation at a random point is given. The maximum difference between measured and calculated deformation is 4.6%, demonstrating that the proposed method can be used to predict the ground subsidence induced by UCG.