Early Thermal Mechanical Coupling Analysis And Cracking Mechanism Of Frozen Shaft Outer Wall In Deep Alluvium

At present,coal is still the main energy source in our country.With shallow resources gradually exhausted,it is necessary to develop deep coal resources and build new mine shaft.In the middle and east of China,some newly-built shafts will cross the deep alluvium.When freezing method is used to drill shafts,circumferential cracks often appear in concrete during the construction of outer wall,which not only affect the bearing capacity of outer wall,but also become water diversion channels when the shafts are built and the freezing wall is thawed,threatening the safety of shaft operation.Therefore,based on the engineering background of the second auxiliary shaft in Dingji Coal Mine,this paper carried out the early thermalmechanical coupling analysis and cracking mechanism research of the outer wall of freezing shaft in deep alluvial formation.The main research contents and achievements are as follows:Firstly,from the micro point of view,the equivalent age and minimum hydration degree of concrete are introduced,and the relationship between cement hydration degree,minimum hydration degree and concrete strength is derived.Based on the relationship between age and modulus of elasticity of existing concrete,analytical formulas for modulus of elasticity,degree of hydration and temperature of concrete at variable temperature caused by hydration heat are established.Secondly,experimental research on basic mechanical properties of frozen shaft outer wall concrete at early age is carried out.Evolution characteristics of early mechanical properties of frozen shaft outer wall concrete are analyzed according to the test results.Time-dependent models of elastic modulus composite index and splitting tensile strength-age time-dependent model of frozen shaft outer wall concrete are constructed.Thirdly,theoretical analysis of vertical restraint temperature stress and spontaneous temperature stress caused by early hydration reaction of frozen shaft outer wall concrete is carried out and corresponding calculation formulas are obtained.According to the engineering case parameters,the vertical temperature tensile stress caused by temperature drop of shaft wall and temperature difference between inner and outer edges of shaft wall is calculated and the early cracking mechanism of concrete on outer wall of freezing shaft in deep alluvium is explained from the angle of theoretical analysis.Then,the thermal-mechanical coupling numerical calculation model of outer wall of freezing shaft in deep alluvium is established.With the second auxiliary shaft of Dingji Coal Mine as the engineering background,the early temperature field,stress field and strain field of outer wall are simulated and analyzed.The results show that the outer wall concrete is under pressure at this stage due to hydration temperature rise in the early stage.Then,the temperature is reduced due to the influence of air in the freezing wall and shaft.At this time,under the combined action of the freezing wall temperature reduction and the restriction of the freezing wall and the temperature gradient in different positions inside the shaft wall,the compressive stress will gradually change into tensile stress,with the maximum tensile stress of 5.08 MPa,which is very easy to cause the cracking of the shaft wall.The calculation results are basically in agreement with the field measured data,thus revealing the cracking mechanism of concrete on the outer wall of freezing shaft in deep alluvium.Finally,the early-age splitting tensile strength test of fiber concrete is carried out.The test results show that the early-age tensile strength of fiber concrete is significantly higher than that of ordinary concrete,which can significantly improve the tensile performance of Outer-wall concrete.In addition,the anti-cracking measures using fiber concrete instead of ordinary concrete as outer-wall material are proposed.Figure 57 table 15 references 103...