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Red-bed mudstone experiences periodic swelling and shrinkage behaviors due to alternating wetting and drying cycles, leading to a series of engineering disasters such as foundation settlement and excessive deformation of high-speed railway roadbeds and reservoir bank slopes. This paper examines the effect of cyclic wetting and drying on the swell–shrink deformation and cracking to simulate the time-dependent behavior of such rocks for high-speed railway subgrades. Red-bed mudstone specimens from the Upper Cretaceous Guankou Formation (K2g) of central Sichuan Basin, Southwest China, were subjected to laboratory tests under oedometric conditions. The swelling and shrinkage deformations in each wetting and drying cycle under different cycles were measured over time. A theoretical model with elastic and viscous elements connected parallelly was put forward, and the time-varying swelling strain was derived. The results showed that the swell–shrink strain and fractures mainly occurred in the first dry–wet cycle. With increasing number of cycles, the accumulated swelling deformation increased gradually due to the progressive evolution of the cracks, and the maximum swelling stress σm and swelling deformation modulus k increased, indicating an underestimation of the swelling potential in conventional swelling tests. The time correlation parameters of B and η also increased, verifying the significant time-dependent deformation of red-bed mudstone. The time-dependent swell–shrink behavior is an important factor in evaluating the long-term stability of rocks in deformation-sensitive engineering. [ABSTRACT FROM AUTHOR]

The catastrophic mechanism of rock masses with discontinuities under excavations and river incisions is very complex. Based on this engineering background, a standard direct shear test under unloading normal stress is performed on sandstone specimens containing nonpersistent joints to investigate their shear behaviors. The results obtained are compared and discussed with those of conventional direct shear tests. An overall instantaneous coalescence under unloading conditions is presented on the rock bridge without a precursor, while the failure under loading conditions is a segmented progressive coalescence. This overall cracking behavior, driven by tensile damage, triggers a constant dilation rate. The cohesion is smaller and the internal friction angle is larger at a larger continuity factor under unloading conditions, while the relationship under loading conditions is the opposite. The evolution mechanism of the shear strength parameter affected by the continuity factor is revealed by analyzing the damage degree of the rock bridge per unit length. Interestingly, there is a polished and scratched area on the failure plane under unloading conditions, while the failure plane is smooth with some polishing without striated scratches under loading conditions. These failure plane characteristics reflect the basement sliding behavior of a rockslide. The striated scratch-like gullies are caused by tensile damage from the analysis of the microscopic three-dimensional topography. This investigation reveals the failure mechanism of rock masses under tensile-shear stress conditions. [ABSTRACT FROM AUTHOR]

The weakening effects of increasing the water content in rocks have been studied extensively in a wide variety of rock types over the past few decades. High energy release is typically characteristic of rock burst hazards. However, few studies have investigated the mechanism of water involved in rock bursts from the perspective of energy distribution based on the water content. Therefore, in this study we investigated the evolution of microcracks in hard sandstone determined from triaxial tests combined with an acoustic emission analysis for different water contents and confining pressures. Then, the weakening effects of water on the brittleness of the rock were studied from an energy viewpoint. The study shows that the capacity of the storage energy of hard rock decreases due to the dissipation of internal elastic energy in the compression stage in the presence of water. These results revealed the mechanical and energetic mechanisms that caused the weakening effects induced by water on rock burst and can contribute to design and construction improvements in tunnel engineering. [ABSTRACT FROM AUTHOR]

Rich groundwater content can produce a complex geological environment for underground tunnels. Therefore, the brittle mechanical characteristics of hard rock exposed by tunnelling in a moist environment are of great significance. Uniaxial tests and acoustic emission (AE) analysis of sandstone with respect to different moisture conditions are described in this paper. Moisture-controlled trends of the stress-strain relationship, mechanical parameter variation, maximum instantaneous AE energy, cumulative AE energy and macro crack behaviour were analysed in detail. In addition, X-ray diffraction and scanning electron microscopy were performed to analyse the fracture surfaces of rock at the microscale. Sandstone particles were found to be more likely to slip past each other at high moisture contents, resulting in increased plastic deformation and dissipation of internal energy. Increasing moisture reduces the brittleness of hard rock and the probability of rock burst. The results reveal the mechanical mechanisms cause moisture to affect brittle hard rock and can contribute to the improvement of the design and construction of deep tunnels. [ABSTRACT FROM AUTHOR]

Many tunnels along the Dujiangyan to Wenchuan highway, located near the epicenter of the 2008 Wenchuan earthquake in China, were damaged severely. The characteristics of the tunnel failures were analyzed and categorized as avalanches and landsliding near the tunnels, cracking of the tunnel portals, collapse of the liner and surrounding rock, cracking and dislocation of the liner, uplift and cracking of the ground, deformation and cracking of the preliminary bracing. The main geological factors influencing the tunnel damage are secondary fractures of earthquake faults, sudden change in soil and rock type, weak rocks and the variable geo-stresses in the host material. The tunnel portals and their slopes, unless fully integrated into the tunnel structures and sufficiently reinforced, are likely to suffer significant distress as a consequence of seismic events. The main mitigation measures proposed are the use of reinforced concrete in the secondary lining in the area of fault zones and injection grouting to reduce the differences where there are sudden changes in the character of the host material. [ABSTRACT FROM AUTHOR]