| The role of the metro in future urban development has become more prominent.As the level of urbanization increases and the scale of cities continues to expand,metro construction has ushered in explosive growth.During the "14th Five-Year Plan" period,China’s urban rail transit mileage is expected to increase by 5,000 km,with an average annual increase of about 1,000 km.By this time,the total operational mileage will reach13,000 km.During the construction of metros in China,soil-rock composite strata are frequently encountered,which significantly impacts the safety and efficiency of shield construction.The shield posture deviation is severe in the frequently changing soil-rock composite strata.It often exceeds the limits,leading to segment misalignment,segment breakage,and poor sealing,which may cause problems in the regular operation of metro tunnels.However,the shield posture deviation mechanism needs to be better understood due to the shield construction’s concealment.Posture control mainly depends on the experience of shield drivers,which can easily lead to terrible control effects and snakelike motion.Therefore,research on the shield posture deviation mechanism and control method in soil-rock composite strata is urgently needed to provide a reference for improving shield construction technology.Given this,this paper used a combination of model tests,numerical simulations,theoretical analysis,and example verification to aim at the shield tunnel construction in soil-rock composite strata.The mechanism of shield posture deviation in soil-rock composite strata was revealed.The driving forces prediction method of the earth pressure balance(EPB)shield in soil-rock composite strata was established,and the analytical model of the posture adjustment moment of the shield with additional response feedback was constructed.A compliance allocation method of posture adjustment thrust for the propulsion system was proposed.The main research contents and understanding are as follows:Firstly,a testing device for shield bias load was designed in response to the bias load monitoring functional deficiencies of the existing shield test platform.The shield propulsion system was optimized,and the influence of shield thrust on bias load monitoring was eliminated.Then,a model test platform for EPB shield with bias load measurement was developed to enable real-time monitoring of shield bias load.Excavation model tests under different stratum distribution conditions were carried out,and the development law of bias load in soil-rock composite strata was studied.It is found that in the soft-to-hard composite stratum,the shield is prone to produce deviant moments that tend to head up.Then,based on the shield bias-load model test in soil-rock composite strata,an analytical model for the dynamic interaction between the shield machine and soil-rock composite strata was established.The influence of tunneling angles,soil and rock modulus,and their coupling effect on the shield machine’s bias load was studied.Moreover,the shield posture deviation mechanism in soil-rock composite strata was revealed.It has been found that changes in the load acting on the shield shell due to changes in the stratum are the leading cause of shield posture deviation.The more significant the difference between the softness and hardness of the strata,the faster the shield’s deviant moment increases.Thirdly,based on the quasi-static analysis of EPB shield,a mechanical calculation model of shield driving forces in soil-rock composite strata was established.The influences of burial depth,the proportion of hard rock in the tunnel surface,shield penetration,rock strength,and the opening rate of the cutter head on the shield driving forces were revealed.The analysis shows that the friction between the shield and stratum is the main component of shield driving forces.A random forest algorithm-based method for predicting driving forces was also proposed to consider the influence of construction factors.Its suitability for shield construction in soil-rock composite strata was analyzed.Furthermore,the influence degree of geological conditions and various construction parameters on shield driving forces was evaluated.Tunnel depth,foam dosage,propulsion speed,and screw conveyor speed were found to significantly influence shield driving forces in soil-rock composite strata.Fourthly,Based on the interaction between the cutter head,shield shell,and stratum,the analytical model for posture adjustment moment was established,considering the feedback of the additional offset of the shield.The influence of the adjustment angle,stratum parameter,and stratum distribution on the shield’s response and posture adjustment moment was discussed.It was found that as the adjustment angle increases,the shield undergoes additional deflection to the soil side,and the corresponding posture adjustment moment increases significantly.The posture adjustment moment increases significantly with the increase in the foundation reaction coefficient of the soil and is not significantly influenced by the foundation reaction coefficient of the rock.When the percentage of rock in the excavation face is larger than 0.5,the posture adjustment moment in the direction of soil-rock distribution increases with the increase of the percentage of rock;when the percentage of rock in the excavation face is less than 0.5,the posture adjustment moment in the direction of soil-rock distribution is not significantly influenced by the percentage of rock.Moreover,the trajectory adjustment method of a shield in soil-rock composite stratum was proposed considering the additional offset correction to improve the efficiency of shield posture adjustment.Finally,a load compliance allocation method for a shield propulsion system based on optimizing load transfer characteristics indexes was proposed.An asymmetric equivalent mechanism model of the anti-bias propulsion system was established.The load compliance of the propulsion system was investigated for different directions and amplitudes of posture adjustment loads,and the application effect of the load compliance allocation method was evaluated based on the mechanical properties of the segment.It is shown that the proposed allocation method of posture-adjustment thrust significantly improves the bias-load compliance of the propulsion system and has no significant damage to the segments. |
