Supporting Characteristics And Thickness Optimization Of Tunnels Through Expansive Paleosol Layer

The terrain of the Loess Plateau in western China is very undulating.Due to the limitation of the longitudinal slope of the line,railway lines often pass through the Loess Plateau in the form of tunnels.The Yinchuan-Xi’an high-speed railway(referred to as the Yinxi High-speed Railway)encountered a thick palaeo-soil layer with weak swelling when crossing the highland area.Existing tunnel excavation and support methods through expansive strata refer to the construction experience of loess layer without considering swelling force,which makes the mechanical properties of tunnel lining structures through expansive ancient soil strata have greater risks.Based on the Yinchaung Railway Zaosheng Tunnel,the thesis studies the stress characteristics of the lining of the tunnel with large cross section through the expansive ancient soil layer through a combination of indoor tests,numerical simulations and on-site monitoring,and reveals the lining under the effect of swelling load Based on the mechanical response and internal force distribution characteristics,the design parameters of lining thickness suitable for ancient soil layers are proposed.The research of the thesis has achieved the following results:(1)Through laboratory experiments,the engineering characteristics of ancient soils were systematically studied,and the ancient soils with water content as independent variables were obtained according to the relationship between water content and original condition,strength of remodeling ancient soil,swelling force,and swelling rate.Expressions of strength and expansion indicators.According to the damage variables of paleo-soil under dry-wet cycle and external load coupling,the damage equation of paleo-soil considering the change of water content and load is established.(2)The stratum structure model is established according to the ancient soil tunnel project in Zaosheng.According to the consistency of the expression of the seepage equation and the temperature conduction equation and the correlation between the temperature field and the moisture field,the temperature field is used to simulate the swelling force of the ancient soil.The stress-strain field of the initial support under the influence.According to the release rule of swelling potential under different water content and the deformation and internal force distribution characteristics of initial support and surrounding rock under different support thickness,the optimal thickness of initial support is obtained.(3)Establish the load structure model of the secondary lining,and obtain the design scheme of the secondary lining according to the orthogonal test.Considering factors such as safety and economy,an optimized model of secondary lining thickness was established,and the optimal thickness of the secondary lining of ancient soil tunnels in the study area was obtained.It is considered that when the thickness of the secondary lining is 50 cm and the circumferential reinforcement is 20@200,the phase is optimal.Compared with the original lining thickness(60cm),the cost decreased by 10.1%.(4)Carry out on-site monitoring tests on the lining structure at the construction site of the No.3 tunnel in Zaosheng,monitor the stress state of the lining by burying earth pressure boxes,concrete strain gauges,steel bar stress gauges,surface strain gauges and other equipment,and further analyze and summarize with numerical calculations Under this thickness parameter,the proportion of initial support and secondary lining to the surrounding rock pressure and its own stress characteristics.