Discussion on shaft wall failure mechanism in Linh

Discussion on the mechanism of shaft wall failure in Linhuan Mining Area

since the 1980s, more than 30 coal mine shafts in Xuhuai mining area have been deformed and damaged, which has seriously affected the safety production of coal mines. The causes of shaft deformation and failure are complex. In order to further explore its failure mechanism, we have carried out model tests and numerical simulation analysis of shaft deformation and failure

1 model test study 1.1 the test scheme takes the auxiliary shaft of Linhuan Coal Mine of Huaibei Mining Bureau as the test prototype. Inner diameter of shaft  φ 7.2m, outer diameter  φ 9.9m, shaft wall thickness 1.35M, shaft wall concrete strength grade C40. The stratum penetrated by the shaft: the thickness of the first aquifer ~ the third aquifer is 115m, the third partition 1 is usually 12m when the stress is lower than the yield limit and there is no obvious plastic deformation, and the fourth aquifer is 12M, and the strong and weak weathering zone is 1 and needs to be polished; 3M, and the bedrock is below it. According to the nature of the research problem, the geometric ratio of the model is determined to be 1:40, and the unit weight ratio is 1:1.4vtt says that most of them are landfilled or incinerated. 3 according to the similarity theory, the stress ratio is 1:57.14. The mechanical parameters of rock and soil around the shaft are taken from the experimental data. There are three media around the shaft, i.e. 20m for soil, 12m for bedrock weathering zone and 8m for bedrock. The similar materials used are: Sand: soil =12:1 sandy soil is used for the soil layer, and the compressive strength is 2KPa; Sand for weathered zone: Lime =11:1 mixture, with a compressive strength of 45kpa; The bedrock adopts gypsum concrete with sand: Lime: Gypsum =8:0.5:0.5, and the strength is 720kpa. The above model materials can maintain similar strength. After several tests, the shaft material is finally determined to use the gypsum binder of quartz sand: Gypsum: water =1:1:1, which mainly meets the needs of circular shaft molding. The compressive strength of this material is about 3.6mpa and the elastic modulus is 2gpa. The inner diameter of the wellbore model is 180mm and the outer diameter is 250mm, which remains geometrically similar to the prototype. Simulate two kinds of wellbore stress modes. Model I: understand the deformation and failure state of shaft under the action of horizontal in-situ stress. According to the measured data of in-situ stress in Linhuan Mining Area ②: the average maximum principal stress at -245m level in the loose layer is 3.07mpa, and the azimuth is close to NS- 27. The operation steps of the fatigue endurance testing machine are as follows: the maximum principal stress in the bedrock is 13.5mpa, the azimuth is ne98.5 °, the minimum principal stress is 9.1mpa, and the azimuth is ne8.8 °. Both are horizontal stresses. Model II: understand the distribution of negative friction caused by formation settlement caused by water loss in the bottom aquifer. The model test system is: embed micro pressure boxes in the soil and rock around the shaft to understand the external load of the shaft; In the wellbore model, the surface should be directly pasted with longitudinal and transverse strain gauges at the bottom aquifer, bedrock weathering zone, the interface between weathering zone and bottom aquifer and other key parts, and 48 strain gauges of 4 measuring lines are counted to reflect the axial compression degree and radial deformation of the wellbore. Yj-17 strain gauge and computer data acquisition and processing system are used for testing. Model frame (long × wide × Height) is 1080mm × 1 000mm × 1 000mm large test stand. The loading equipment is wy-300 type loading voltage stabilizing device, which is loaded and tested step by step. 1.2 test process in model I, in order to simulate the effect of horizontal in-situ stress along the shaft axis, the shaft model is placed horizontally, with the surrounding soil thickness of 50cm, the weathering zone thickness of 30cm, and the bedrock thickness of 20cm. The soil layer, bedrock weathering zone, and bedrock are subjected to variable pressure loading from above the model in a direction perpendicular to the shaft axis. The loading is divided into two ways: one is to load the soil layer, and the other is to load the rock layer. The compression ratio of soil and rock is 3:13, which is close to the ratio of prototype in-situ stress. The loading is divided into 10 levels, and the whole process from shaft deformation to failure is observed. In model II, the shaft is placed in the middle of the model. The surrounding soil is 60cm thick, the weathered zone is 30cm thick, and the bedrock is 10cm thick. In order to simulate the water loss compression of the bottom aquifer, a row of  is placed horizontally in the lower part of the bottom aquifer φ 16mm reinforcement, which shall be extracted after loading. In order to simulate the influence of negative friction on shaft deformation and failure, special measures are taken to treat the outer surface of shaft wall to increase the friction coefficient between shaft and rock and soil. There are two ways to load the model: one is to load the shaft, simulating the weight of the shaft and well tower; The other way is to load the soil layer. The soil layer loading is divided into 10 levels, which are loaded step by step and tested step by step. 1.3 test results and analysis (1) from the model test of horizontal in-situ stress (model I), due to the sudden change and difference of horizontal in-situ stress at the soil rock interface, according to the observation of the surrounding pressure box near the shaft, there is a stress transition section near the soil rock interface when the shaft is loaded (Fig. 1), and the horizontal force on the shaft in the soil layer is less than that on the bedrock. This external load distribution will cause the shaft wall to be subjected to longitudinal bending. The longitudinal bending action is confirmed by the deformation and failure pattern of the shaft wall. The measurement of shaft wall deformation shows that the transverse strain of shaft wall is tensile strain, which indicates that under this stress condition, the shaft becomes elliptical; The longitudinal strain has tension and pressure near the soil rock interface, indicating the existence of longitudinal bending, which leads to the bending and tension failure of the shaft wall. The observation of shaft wall failure shows that the failure section is concentrated near the soil rock interface, showing the characteristics of cross-sectional shear failure and axial tensile failure. Fig. 1 Schematic diagram of external load distribution on the shaft wall (2) from the test of bottom water loss model (model II), the bottom water loss compression will indeed cause negative friction on the shaft wall. The axial strain of the shaft wall increases in a large gradient when the bottom aquifer loses water. The strain of wellbore model is roughly linear with the medium pressure, which shows that the distribution law of friction is consistent with the medium pressure; The shaft does produce large compressive strain near the soil rock interface. This model test adopts special measures to increase friction. According to the elastic modulus of the model material and the relationship between medium pressure and strain, the friction coefficient f is estimated to be about 0.35, but it is difficult to determine the value of F after the shaft wall is separated from the soil layer. Different shaft walls may differ greatly, which is worth further study. The model has a linear relationship between the longitudinal strain of the wellbore and the depth of the wellbore, indicating that the force on the wellbore increases linearly with the depth of the wellbore. It is worth noting that the model is loaded to the maximum level pmax=465kpa, and the wellbore is not damaged. The comparison of the results shows that if there is only negative friction, it is not enough to make the simulated wellbore unstable

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