Study On The Relationship Between Engineering Geological Conditions And Surface Settlement In Shallowly Buried Sections Of Tunnels In Mountainous Areas

In the shallow-buried sections of mountainous tunnels,the excavation process is prone to disturbing the surrounding rock and soil mass,leading to a redistribution of the surrounding rock stress and,consequently,causing surface settlements above the tunnel.Generally,in areas with relatively stable geological conditions,the overall surface settlement is minimal and has little impact on tunnel construction.However,in areas with complex geological conditions,such as those containing faults,underground rivers,and other weaker rock formations,even minor disturbances can lead to significant settlements.Therefore,it is necessary to study the laws of surface settlement and adopt appropriate countermeasures in advance to reduce or avoid the risk of large-scale uneven settlement and surface collapses.At present,scholars at home and abroad have conducted research on the influence of construction methods and tunnel depth on surface settlement through empirical formulas,theoretical analysis,and numerical simulations.However,there has been relatively little research on the relationship between surface settlement during construction and the engineering geological conditions.In this regard,this paper takes the shallow-buried section of the Wafangzhai Tunnel in Gujin Expressway,located in the hilly area of central Guizhou Province,as the research subject.The relationship between engineering geological conditions and ground settlement is investigated through on-site measurements,theoretical analysis,and numerical simulations.Firstly,the original ground settlement data is obtained through on-site ground settlement monitoring measurements,and the specific geological conditions of the tunnel’s shallow-buried section are further explored using geological advance prediction.Next,the influence of different geological conditions in the Wafangzhai Tunnel’s shallow-buried section on ground settlement is studied,a logarithmic linear model for cross-sectional ground settlement is established,and the model is validated.Finally,the ground settlement of different geological condition sections is numerically simulated using MIDAS/GTS NX finite element software,and the simulation results are compared with the measured data to analyze the applicability of the established model.The main research work and achievements of this thesis are as follows:(1)Surface settlement measurements were carried out in the shallowly buried section of the Wafangzhai Tunnel using the total station method.68 sections were laid out on the left and right lines,each with 7 measuremlent points,and surface settlement curves were obtained for each section of the shallowly buried section of the Wafangzhai Tunnel by this method.(2)To further determine the specific geological conditions of the shallowly buried section of the Wafangzhai Tunnel,a comprehensive geological advance prediction scheme combining the horizontal sonic profiling(HSP)and geological-radar(GPR)method was used to investigate the engineering geological conditions of the shallowly buried section of the Wafangzhai Tunnel in more detail according to the Wafangzhai Tunnel construction drawing design documents.On this basis,a more mature forecasting process under the"HSP+GPR" integrated geo-advance prediction scheme and the characteristic radar waveform images of different geological conditions in the shallowly buried section of the Wafangzhai Tunnel are summarised,and a method to improve the accuracy of geo-advance prediction image identification based on field data acquisition and data processing interpretation is proposed.Based on the above work,the geological conditions of the shallowly buried section of the Wafangzhai tunnel are classified into three main categories:intact section of surrounding rock,fractured section of surrounding rock and cavern developed section of surrounding rock.(3)Based on the original data of surface settlement in the shallowly buried section of the Wafangzhai Tunnel,a study on the relationship between engineering geology and surface settlement was conducted to investigate the influence of different geological conditions on surface settlement.It was found that,from a longitudinal point of view,the maximum surface settlement in the intact and broken sections of the surrounding rock was stable at around 17mm and 22mm respectively,while the surface settlement in the cavern-developed section varied greatly,fluctuating from 30 to 37mm,with an overall trend of increasing and then decreasing before and after the cavern.(4)By generalising and analysing the measured surface settlement data,a log-lincar model S=A(x2+1)B for surface settlement in cross-sections under different geological conditions was established,based on which the model fitting law was analysed for each geological condition,and the model was verified by the measured surface settlement data of the intact section of the surrounding rock and the subsequent unexcavated section of the fractured section of the surrounding rock.The results show that the measured settlement conforms to the distribution of the log-linear model,and the error between the calculated and measured settlement of the model can be controlled within 15%,indicating that the log-linear model can better describe the surface settlement law of the shallow buried section of the tunnel under different geological conditions.(5)Based on the MIDAS/GTS NX finite element software,a numerical model of the shallowly buried section of the Wafangzhai tunnel under different geological conditions was established to numerically analyse the surface settlement pattern of the section under different geological conditions,with emphasis on the surface settlement pattern of the cavern-developed section of the surrounding rock.The results show that the surface settlement at each point in the cross-section of the three geological conditions simulated by the numerical model varies more gently than the actual situation;the maximum surface settlement between the longitudinal continuous sections of the intact and broken sections of the surrounding rock is consistent with the actual measurement,and both show a gradual decrease and stabilisation from outside the cave to inside the cave,at about 17mm and 20mm respectively,with less error than the actual situation.The settlement of the surrounding rock in the cavern development section shows an obvious pattern of first decreasing,then increasing and then rapidly decreasing,and the difference between the simulated maximum settlement and the actual maximum settlement is about 9mm,but the influence on the upper surface is smaller.Comparing the measured settlement,the modelled settlement and the numerically simulated settlement under different geological conditions,the log-linear model and the numerical analysis model developed in this paper are consistent with the surface settlement pattern of the shallow buried section of the tunnel under different geological conditions.