To prevent serious shaft deflection disasters under asymmetric mining conditions, it is urgent to solve the problem of designing shaft protection rock pillar (SPRP) sizes in thick soil and thin rock strata. In this paper, based on the parallel mining model and the perpendicular mining model, a dynamic prediction model that can describe the horizontal movement of the shaft was established by the probability integration method and the Knothe time function. Next, according to the measured data of the shaft deflection in the Guotun Coal Mine, a back analysis was used to calculate the prediction parameters that were suitable for the deep soil strata. Based on the mining model, the variation law of the horizontal deflection displacement of the shaft and SPRP size was obtained. The results showed that the final displacements of the shaft under the two ideal mining models were equal, while the parallel mining model was superior to the perpendicular mining model at the initial stage of mining. The horizontal displacement of the shaft head had a nonlinear negative correlation with the SPRP, and the SPRP size in thick soil and thin rock strata calculated by the parallel mining model was more reasonable. For the Guotun Coal Mine, when the soil movement angle was 57.8% of the actual value, the horizontal displacement of the main shaft head was reduced by 87%. The results have important theoretical and practical value in preventing shaft deflection in thick soil and thin rock strata.

In the water-bearing soft rock strata of West China, a field measurement is carried out on the shaft lining in bedrock section. The shaft lining is built with freezing method. From the measurement, some changes of regularity in construction, thawing and post thawing period are obtained, including the temperature, freezing pressure and the vertical and hoop strain of the shaft lining. Measurement studies have shown that: during the plastic stage of shaft construction period, with the release of hydration heat, the thermal expansion occurs in the concrete and the strain gauge, leading to tensile strain. In the earlier stage, the elastic modulus and strength of concrete increases rapidly. When the shaft lining temperature reaches its maximum, the cooling and growth in the freezing pressure make the hoop strain increase sharply. However, the hoop strain is at lower tensile or compressive value. In the early stage and mature stage, the strain change is mainly affected by temperature. In the thawing and post thawing period, the concrete strain is also primarily influenced by temperature change. During the measurement, when the maximum compressive strain is less than ultimate value, the shaft lining is in a safe state. To ensure the security of the shaft lining, the concrete strength growth and sidewall temperature should be controlled in the similar shaft condition. In addition, the reinforcement ratio should not be too small for the shaft lining in frozen bedrock. And further, the fiber reinforced concrete, shrinkage-compensating concrete and low hydration heat concrete technology can also be adopted to reduce the cracks as much as possible and to avoid penetrating cracks. For the soft aquifers in West China, the further field measurement is still needed to obtain the stress and deformation regularities of the shaft lining, which can not only provide a basis for security assessment of the shaft, but also guide its design and optimization to improve the design theory

Artificial ground freezing is an effective method for underground constructions in deep alluvium. To study the compressive strength of frozen soil under high ground pressure and high hydraulic pressure, it is necessary to understand the mechanical behaviour of ice that is formed under triaxial compressive stress. A low-temperature triaxial test system was developed and used to study both formation and deformation of columnar ice under hydrostatic pressure. Cylindrical ice specimens 125 mm in height and 61.8 mm in diameter were prepared and tested under constant strain rates. At a strain rate of 5 × 10⁻⁵ s⁻¹, the peak axial stress showed a linear increase as the confining pressure increased from 2 to 30 MPa, while the peak deviatoric stress exhibited a slight decrease. At a confining pressure of 30 MPa, the peak deviatoric stress showed a logarithmic increase with the strain rate increasing from 5 × 10⁻⁶ to 5 × 10⁻⁴ s⁻¹, and the failure strain nearly doubled. A power law relationship between the time to failure and the strain rate was also observed. In this study, each test consistently demonstrated a ductile failure mode, with a noticeable reduction in cracking as the confining pressure increased. Due to the effect of the high confining pressure, crack propagation was suppressed, and an apparent recrystallization after peak stress was observed.

Heat treatment on shale reservoirs can promote the development of secondary fractures in a matrix on the basis of hydraulic fracturing, forming multi-scale gas–water seepage channels and strengthening the gas desorption. Experimental evidence shows that heat treatment can enhance gas recovery in the same mining life. Heat treatment on a shale gas reservoir is a multi-physical and multi-phase coupling process. However, how the thermal stimulation interacts with nonlinear two-phase flow in heterogeneous shale volume fracturing has not been clear. In this paper, a fully coupled THGM model for heating-enhanced shale-gas recovery in heterogeneous shale reservoirs is proposed. First, the governing equations are formulated for the shale-reservoir deformation involving both gas adsorption and thermal expansion, the permeability evolution model for the cracking process of fractured shale, the gas–water two-phase continuity equation considering the effects of gas solubility and the heat transfer equation for heat conduction and convection. The interactions among stress, temperature and seepage in a heterogeneous shale reservoir were studied. Secondly, a test on shale permeability after 50 °C temperature treatment was conducted. The evolution of temperature, capillary pressure, water and gas saturation and the permeability of shale during the heat treatment of the reservoir were numerically analyzed. Finally, the gas production from a shale gas reservoir was numerically simulated with this THGM model. The numerical results indicated that the thermal-induced fracturing, gas desorption and separation from water make predominant contributions to the evolution of permeability. The heat treatment can enhance cumulative gas production by 58.7% after 27.4 years of heat injection through promoting gas desorption and matrix diffusion.

Frozen wall design theory is a key technique of the freezing method. However, previous design theories for a deep artificial frozen wall have neglected the influence of shaft flank displacement, that is, the displacement of the inner boundary of a frozen wall. Thus, the associated designs tend to be unsafe and earthwork excavations tend to be underestimated. This study builds a new design theory for frozen wall thickness which considers the influence of a large strain and obtains new solution formulas for the thickness and excavation radius before deformation occurs. The analytical results are compared with numerical calculation results by analyzing the influences of various parameters, such as crustal stress, cohesion and internal friction angle of frozen soil, and cohesion and internal friction angle of unfrozen soil as well as the elastic modulus of the ground, on the frozen wall thickness and the shaft flank displacement. The results indicate that the new formula is applicable for large deformation calculation with a strain of up to 0.2. The new formula can accurately calculate the amount of excavation earthwork and serves as a safer and more reasonable theoretical support for the design of frozen walls in ultradeep soil layers.