Experimental Study On Mechanical Properties Of Shaft Lining High Strength Concrete Under Hydraulic Pressure

With the increase of the thickness of the new well bore through the alluvial layer, the external load will also be increased by the borehole wall. The use of high-strength concrete to improve the bearing capacity of the shaft lining is one of the freezing wall supporting problems in the effective way. After thawing, the wellbore operation stage is in a high hydraulic pressure environment, and then leakage phenomenon often happens, which bring a large number of capillary and void the concrete. Under the action of hydraulic pressure, the water will penetrate into the crack of concrete, and the mechanical properties of concrete will be affected. Therefore, it is necessary to study the mechanical properties of high strength concrete with hydraulic pressure. In recent years, domestic and foreign scholars have studied the compressive strength, elastic modulus and deformation characteristics of ordinary concrete in high pressure water environment. Research results show that the ordinary concrete compressive strength and modulus of elasticity decreased gradually with increase of the osmotic pressure, and the loss rate increasing, the loss rate of which is increasing. And the hydraulic pressure, the aggregate and the strength of concrete have great influence on the compressive strength of concrete. However, from the existing research conclusions, the mechanical properties of high-strength concrete at home and abroad have not been reported.This thesis takes the newly established shafts of Wanfu coal mine in Juye, Shandong as engineering background. The influence of hydraulic pressure on the mechanical properties of high-strength concrete was studied by carrying out a large number of systematic experiments using the TAW-3000 type electro hydraulic servo rock three axis testing machine system. All the 267 experiments were divided into three types: the uniaxial and triaxial experiments of high-strength concrete without water, the uniaxial and triaxial experiments of high-strength concrete subjected to pore water pressure, and the triaxial experiments of high-strength concrete subjected to hydraulic water pressure. Among them, the latter two kinds of experiments have not been reported at home and abroad.The main research achievements of this thesis are presented as follows:1.The uniaxial experiment of high strength concrete and the triaxial experiment with confining pressure equal to 0Mpa, 5Mpa, 10 Mpa, 15 Mpa and 20 Mpa were completed by the TAW-3000 type electro hydraulic servo rock triaxial testing machine system, and three mixture proportion, HC-1, HC-2 and HC-3, of high strength concrete were used. The failure pattern, compressive strength, Young’s Modulus and stress-strain curve of high strength concrete were analyzed.Experimental results show that: in uniaxial test, the brittle failure of high strength concrete is mainly dominated by column crack; in three axial compression test, the plastic failure is mainly the diagonal shear failure. The failure surfaces are all through the coarse aggregate. The peak strength of high strength concrete are increased with the increase of confining pressure, and the elastic modulus increases with the increase of confining pressure by power function. Based on these, failure criterion function with two parameters are put out, and the fitting results are closed to the experimental results.Confining pressure improves the ability of deformation of high strength, the greater the confining pressure, the bigger the peak strain of high strength, and the growth of peak strain with confining pressure are fitting with power function. According to this, failure criterion function with two parameters are put out, and the fitting results are closed to the experimental results.The stress-strain curve of uniaxial experiment has conspicuous peak point, the curve drops sharply after peak point, and residual strength is near to 0. With the increase of confining pressure, the curve is gentle and the peak platform is gradually extended, the curve after peak drops gradually and the residual strength increases. Based on uniaxial and triaxial experimental results, constitutive functions of high strength concrete under uniaxial and triaxial condition presented by equation are simulated, in which, the decent stage equation is proposed the first time. The fitting constitutive functions fit the experimental results very well.2.Using hydraulic loading equipment, the pore water pressure(0Mpa、1Mpa、2Mpa、3Mpa、4Mpa、6Mpa、8Mpa) of high-strength concrete specimens lasted for 48 hours. Uniaxial experiment and conventional triaxial experiment of confining pressure with 5Mpa and 10 Mpa were carried out by using triaxial test machine to test the effect of pore water pressure. The uniaxial failure process of the specimen was recorded by digital high-speed photography recording equipment. And the failure pattern, compressive strength, young’s modulus and stress-strain curves under uniaxial and triaxial experiments were analyzed.Experiment results show that: the pore water pressure does not change the uniaxial and the conventional triaxial macroscopic failure modes, but the brittleness of the high-strength concrete is improved, and the failure of the high-strength concrete is more sudden. The peak strength of high-strength concrete decreases with the increase of pore water pressure. The pore water pressure decreases the deformability of high-strength concrete, thus the high-strength concrete reaches the peak stress point with smaller deformation. With the increase of pore water pressure, the stress-strain curve decreases gradually, and the rising phase uniaxial experiment stress-strain curve is more steeply ascent. In the case of uniaxial experiments, the young’s modulus increases with pore water pressure, which is opposite of the results in triaxial experiments. The results show that the pore water pressure increases the damage of high strength concrete, which has little effect on the mechanical properties of high strength concrete before the peak, the main effect is to reduce the mechanical properties of high strength concrete.According to the experimental data,a new failure mode function with two parameters equation is obtained by replacing the confining pressure with effective stress, the relationship among parameters and pore water pressure is analyzed by using nonlinear regression method considering confining pressure equal to 0MPa, 5MPa and 10 MPa, the failure mode of high strength concrete is established considering the interaction of confining pressure and pore water pressure.Recurrent the constitutive model parameters and transfer the constitutive equation to the new one, which include the pore water pressure influence. New relationships have wilder application scape and match the experimental data better.3.Using hydraulic loading equipment to keep the pore water pressure(0Mpa、1Mpa、2Mpa、3Mpa、4Mpa、6Mpa、8Mpa)of high strength concrete specimens for 48 hours. Triaxial-seepage coupled experiments with confining pressure equal to 5Mpa and 10 Mpa after pore water pressure tests are carried out, the effects of seepage pressure on the failure pattern, compressive strength, Young’s Modulus and stress-strain curves were analyzed.Experimental results show that: seepage pressure reduces the ability of compressive resistance and deformation of high strength concrete, which result in the decrease of peak stress and strain. With the increase of seepage pressure, the ascent stage slope of stress-strain curve increases, i.e. Young’s Modulus increases, while the range ability of curve slope is affected by pore water pressure. With the increase of seepage pressure, the peak platform of stress-strain curve is shortened and accompanied by decrease of residual strength, and the acceleration of curve drop speed after peak point, the influence of seepage pressure to stress-strain curve is greater than the curve affected by pore water pressure, because the seepage pressure causes more drastic unstable expansion of micro-cracks.Based on the experimental results, a new failure criterion equation is built considering confining pressure and seepage pressure. The parameters in the failure criterion called effective stress and seepage pressure take the place of confining stress and pore water pressure. Besides, the relationship between and is given under 5MPa, 10 MPa confining pressure through nonlinear regression analysis.The parameter in constitutive model equation is corrected to another one which has considered the impact of hydraulic water pressure.4. According to the research results of rock failure process, the rupture process of high strength concrete is divided into five stages. The fracture index of high strength concrete(crack initiation strength, damage strength and peak strength) is calculated using the above test result, and the residual strength of 1.5 times and 2 times peak strain is proposed as the index of the cracking process. The effects of water pressure and confining pressure on the cracking process of high strength concrete are studied, and the mechanism of the fracture water is analyzed. By establishing the relationship between the hydraulic pressures, the flow of the fracture water pressure and the flow in the three axial seepage coupling test, the development process of the microstructure of the high strength concrete is deduced.Experimental results show that: it is difficult to enter the high strength concrete in the crack compaction stage, the water pressure as the external load affects the strength of high-strength concrete; during the crack growth phase, the water pressure in the crack weakens the strength of the high strength concrete, and accelerates the macro damage of the concrete; however in the development stage, the water pressure increases the crack growth, which greatly reduces the residual strength after the peak.5.The development of micro-cracks in the structure can be speculated by seepage pressure and history data of flow to some extent. According to the experimental results, a new equation reflects the dynamic crack size and flow rate is builded. It is based on the seepage regulation in dynamic crack expansion. Also, the relation is a practical method to infer concrete micro-cracks’ expansion through the flow of seepage water in experiment.Experimental study on mechanical properties of shaft lining high strength concrete under hydraulic pressure” is a new research at home and abroad. Results in the paper provide a basis point when designing shaft lining as well as in predicting shaft lining disasters and guiding pre-reinforcement project. The results in this paper avoid unnecessary economic losses in case of shaft lining security problems. Also it provides a scientific basis to improve the level of structure design and to achieve more effective accident prevention. At last, the paper offer a realistic computational mechanic model for high strength concrete structure durability design, prediction of concrete duration of life, damage assessment and reinforcement in different groundwater environment.