| With the continuous development of our country's transportation construction and energy development,especially,high-speed railways,highways,and water conservancy projects under construction or to be built in the western mountainous areas cannot avoid countless bad geological environments,such as large and small fault fracture zones,high in-situ stress,and large deformation.How to improve the accuracy of tunnel advance geological prediction is a key and difficult problem for underground engineering construction in high in-situ stress area.Furthermore,the stress wave attenuation caused by the reflection and transmission of stress wave on the jointed rock masses with high in-situ stress has not been well solved so far.This not only makes the law of stress wave propagation and attenuation in the rock mass unclear,but also causes the accuracy of the tunnel advanced geological prediction to be low.Therefore,this dissertation will focus on the propagation characteristics of stress wave in jointed rock masses under different in-situ stress,and apply the theoretical results to the tunnel advanced geological prediction,which has important theoretical significance and engineering application value.Based on the g-? model(0<?<1),the stress wave propagation and attenuation in single joint and multiple joints under different in-situ stress is analyzed by using methods such as theoretical analysis,numerical simulation,model test and engineering application.The magnitude and direction of in-situ stress are mainly unidirectional.The formulas for calculating the particle velocity of the transmitted stress wave at a single joint and multiple joints are derived based on the g-? model.A model test of stress wave in jointed rock masses under different in-situ stress is carried out.A method of generating stress wave by using ZDF-3(Zhiliu Dian Fire-3)spark source for shock excitation is proposed.The research results are comprehensively applied to the tunnel advanced geological prediction.Therefore,this dissertation delivers five key research results as follows.(1)Based on the g-? model and the displacement discontinuity theory,the formulas for calculating the particle velocity of the transmitted wave particle under a normal P-wave incident single joint with/without considering the in-situ stress are derived through the method of characteristics.The BB hyperbolic model(Barton-Bandis model)is a special case of the g-? model when ? approaches 1.The viewpoint of equivalent strain in damage mechanics is introduced.The ratio of the joint closure amount to the maximum allowable joint closure is set as the damage factor.The calculation formulas of the reflection coefficient and the transmission coefficient which can reflect the nonlinear change of the joint stiffness under a single joint condition are derived,and named as stiffness reflection coefficient and stiffness transmission coefficient,respectively.These researches expand the application range of the g-? model and make up for the shortcoming of larger deviation between the BB model and the experimental results at moderate stress level.(2)The influence of different parameters on the attenuation law of the transmitted wave,including the in-situ stress,the value of ?,the number and spacing of joint,the joint stiffness,the incident wave frequency and amplitude,et al,is analyzed through the finite difference method.The results show that:(a)if the value of ? increases(the degree of joint weathering is less and the two walls of joint are well matched),the stiffness transmission coefficient increases and the stiffness reflection coefficient decreases under a half-sine incident wave;(b)the energy of the transmitted stress wave decreases as the incident wave frequency increases,and increases as the incident wave amplitude increases;(c)as the in-situ stress increases,the effective wave velocity increases.(3)A combined model test consisting of multiple cement mortar blocks that can simulate joint distribution and apply different in-situ stress is carried out,and its dimension is length × width × height = 180 cm × 80 cm × 25 cm.A method of replacing the explosive-induced stress wave with the ZDF-3 spark source is proposed.54 sets of data are collected in the model test,and the test data with 2500 V spark excitation voltage is mainly analyzed.The experimental results show that:(a)with the increase of in-situ stress,both the model materials and the joints have attenuating effect on the stress wave;(b)under elastic state,the stress wave velocity increases with the increase of the in-situ stress;(c)the transmission coefficient in jointed rock mass with a single joint and multiple joints decreases with the increase of the in-situ stress.(4)In order to analyze the rationality and accuracy of the spark source,the difference between the explosive source and the ZDF-3 spark source is compared through crack simulation experiments.The stress wave generated by the 9 explosive blasts of Model 1 not only produced a crack along the loading direction on the upper surface of Model 1,but also appeared a profile of blasting crater at the bottom of Model 1,which is about 30 cm in diameter,and has not fallen off.Model 2 uses ZDF-3 spark source only to excite stress waves,and the voltages are set to 2500 V,3000 V,and 3500 V,respectively.At the end of experiment,no cracks are generated on the upper surface of the model 2,but a Livingstone Blasting Crater with a diameter of about 52 cm and a depth of 10 cm is formed at the bottom,and the blasting crater has fallen off.The experimental results show that although the energy of ZDF-3 spark source is not as strong as it of the explosive source,it can play the role of stress wave propagation.The crack simulation experiment verifies the similarity between spark source and explosive source,and explores a feasible new way of shock excitation for similar model experiments.(5)The g-? model and model test results are applied to the high-stress tunnel advanced geological prediction.The effect of high in-situ stress on identifying fault position is investigated through multi-source seismic interferometry.It is concluded that the increase in in-situ stress and fault dip angle will make the predicted fault position closer to the tunnel face than the actual position.A correction formula containing in-situ stress and fault angle is fitted based on the resulting correction value from numerical calculation by binary quadratic function regression analysis.The advanced geological prediction for the bad geological body in front of the face of Baima tunnel is done through the HSP(Horizontal Sonic Profiling)method.The maximum in-situ stress value of the tunnel prediction section increased by about 2.2 MPa is calculated.The correction to the result of the tunnel advanced geological prediction is about 7.5 m,which is basically consistent with the actual excavation result.The accuracy of the traditional advanced geological prediction for bad geological body position under high in-situ stress is improved. |
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