Jiangang Liu

Four representative TS-RCR (three-soft rock-coal-rock) structural bodies were selected according to the lithofacies analysis of coal measures in the Xiangshui Coal Mine, Guizhou, China, to analyze the mechanical properties, internal structure damage characteristics, and crack evolution law of the TS-RCR structural body composed of a soft roof, a soft floor, and soft coal seams. The mechanical model of the TS-RCR structural body was proposed. (1) The stress-strain curves of TS-RCR structures with different lithologies under cyclic loading and unloading show a change rule of sparsity, density, and sparsity with the increased cycles. The deformation of TS-RCR structures under cyclic loading and unloading can be divided into four stages. When the height ratio of the TS-RCR structural body was the same, the higher the strength of the upper and lower rocks, the more cycles the TS-RCR structural body had, and the denser the cyclic loading and unloading curve. On the contrary, the fewer cycles the TS-RCR structural body had, the sparser the cyclic loading and unloading curve. (2) The T₂ (transverse relaxation time) spectrum curves of the structural bodies were monitored at about 1 and 10ms, and two obvious peaks were detected, indicating micropores and mesopores with a large volume in the structural body. Signal intensity increased at the relaxation time of 1ms, decreased at 10100ms, and increased slightly at 1001000ms. The pore structure of the structural body with different lithologies changed significantly with the increased cycle number. (3) The damage and failure process of the structural body was roughly divided into three periods according to the time-history analysis of acoustic-emission AF (the ratio of acoustic-emission ringing count to duration) of the structural body: the active period with a relatively low AF value (AFst-2^nd cycles; the rising period with a relatively stable increase of the AF value in the 3^rd-4^th cycles; and the explosive period with a relatively high AF value (AF60kHz) in the 5^th cycles. According to the relationship between the RA (the ratio of the rise time to the amplitude of the acoustic-emission-detection wave) value and time, the evolution law of the RA value and AF value of structures with different lithologies is similar in different periods under cyclic loading and unloading.

Support technology faces challenges in view of the large deformation of surrounding rock in three-soft coal roadways under high horizontal stress in Zijin Coal Mine, China. Geostress near the tested working face of the mine was measured and its distribution law was analyzed through theoretical analysis, numerical simulation analysis, and field measurement. The original supporting scheme of the three-soft coal roadway on the tested working face was analyzed to discover the deformation and failure mechanism of the surrounding rock of the original supporting roadway and the control measures. An optimized support scheme of H-G (hollow grouting) anchor cables, high strength bolts, W-shaped steel belts, metal meshes, and sprayed concretes was proposed for field applications. Based on the roadway in the tested 3201 working face at Zijin Coal Mine, the technical parameters for optimizing the combined support of the roadway were determined. The following results were be obtained through field measurement. The roadway was kept intact after excavation and the optimized support scheme was adopted in the three-soft coal roadway. No obvious deformation in appearance existed in the roof, floor, and roadway coal sides. Compared with the original support scheme, the stability of the roadways was improved visibly. The displacement of the roadway roof decreased from 100 to 30 mm, and that of the roadway coal walls decreased from more than 100 mm to less than 50 mm. This work verifies the effectiveness of a combined support scheme of H-G anchor cables, high strength bolts, W-shaped steel belts, metal meshes, and sprayed concretes to control deformations of surrounding rock in three-soft coal roadways. The new support scheme has good social and economic benefits and can be used as a reference for other roadway supports under similar conditions.

The repair rate of deep permanent roadways is about 90%, and most of which are projects treating floor heave. The deformation behaviors of crosscut were analyzed in the work according to a trackage crosscut at the shaft station of Panyidong Coal Mine in Huainan, China. Crosscut has complex characteristics such as globality, difference, and rheology under deep stress because that crosscut passes through multiple strata. The mechanism of crosscut floor heave was studied based on on-site in situ stress tests and surrounding rock composition tests. The floor heave of trackage crosscut is water swelling in the mudstone and sandy-mudstone areas where the mineral components are mainly kaolinite and illite mixed layers. In the areas of fine and medium-fine sandstone, trackage crosscut is in shear dislocation under high horizontal stress. The slip-line field theory was used to study the ultimate load and maximum failure depth of crosscut floor heave. According to the deformation characteristics of crosscut floor heave, a collaborative control technology enhancing the bearing structure of all-sided surrounding rocks was proposed, including filling of the U-shaped steel supports, shallow grouting in the all-sided surrounding rocks and deep grouting in the floor and inverted arches. A support scheme for repairing the floor was designed based on the specific engineering geology of trackage crosscut floor heave at the shaft station of the Panyidong Coal Mine. After repairing, the crosscut floor heave was monitored for 70 d. The results showed the following. (1) After repairing, the maximum cumulative floor heave was 45.3 mm, which was only 8.1% of that before repairing. (2) Crosscut floor heave changed greatly within one week after repair, with a maximum floor heave speed of 4.7 mm/d. The floor heave speed was maintained below 1 mm/d after 40 d, and the floor heave tended to be stable after 60 d. The collaborative control technology enhancing the bearing structure of all-sided surrounding rocks could control the crosscut floor heave in soft rocks under deep high horizontal stress.

There are more and more tunnel projects in the karst-developed areas in Southwest China. Affected by karst caves and water, karst tunnels often experience geological disasters such as local collapses and water inrush. A simplified rock stress hardening-softening model was established based on the triaxial compression test results of two kinds of carbonatite to accurately analyze the deformation and water inrush characteristics of the surrounding rocks after karst tunnel excavation. The total stress–strain curve of rocks was simplified into four linear stages: the linear elastic stage, strain hardening stage, strain-softening stage, and residual stage. The volumetric strain–axial strain curve was simplified into four corresponding linear stages: the elastic expansion stage, slow expansion stage, rapid expansion stage, and stable expansion stage. The stress hardening–softening model was used to deduce the relationship between the rocks’ mechanical parameters such as cohesion, internal friction angle, dilatancy angle, and plastic strain, as well as the relationship between seepage characteristic parameters such as permeability coefficient, porosity, and volumetric strain. A karst tunnel in Chongqing, China was taken as the engineering background. The stress hardening–softening constitutive model and seepage characteristic parameters were applied to the FLAC^3D numerical simulation by the programming language FISH to analyze the stability and water inrush characteristics of karst tunnels in overlying confining caves. The results showed that rock masses between the cave and tunnel were prone to overall sliding instability. Confined water in the karst cave intruded into the tunnel through the shear-slip rupture zone on both sides instead of the shortest path. Two water inrush points existed on the tunnel surface. The variation law of the permeability coefficients of the surrounding rocks could more truly reflect whether there was a seepage channel between the tunnel and karst cave, as well as the permeable area and water inrush speed of the seepage channel. The work provides a new idea for the stability control of karst tunnels.