| In recent years,great development has been taken place in tunneling and undergournd space technology in China.At the same time,more than ten thousands of landfills have been built in our country.Some tunnels might be inevitabley designed to cross underneath the landfill because of financial or technology reasons.At present,researches on the environmental geotechnical engineering hazards induced by tunnel construction have rarely been reported.In this paper,the project of the Jigongshan Mountain Tunnel excavated under the Xiaping landfill in Shenzhen was taken as an example to study the interaction mechanism between the landfill and the underlying tunnel.Multiple methods including intelligent back analysis,field tests and numerical simulation were employed to identify the characteristics of the stress and seepage field in the study area.The research results,including the integrated intelligent back analysis method of the stress field under the landfill,the influencing mechanism and evaluation criteria of the underlying constructing tunnel on the landfill,the integrated detection method of the leachate in and under the landfill,and the influencing mechanism of the landfill leakage and the underlying tunnel on the seepage field,could be valuable either for the project of the Jigongshan Mountain Tunnel,or for similar researches on the environmental geotechnical engineering hazards induced by tunnel construction.The main content and innovation were as follows:(1)An integrated strategy comprising multisource information analysis,multiple linear regression and artificial intelligence algorithm was developed to predict the initial ground stress for tunneling under the Xiaping landfill.The distribution of the initial ground stress was studied and the influence of the landfill was considered by geological analysis,rock sample analysis,field measurements and intelligent back analysis.This new strategy could consider the effects of tectonic movement,rock and fault parameters and landfill load on ground stress simultaneously.It was revealed that the horizontal tectonic stress,with a range of 2.5-5.5 MPa,was significant in the tunnel region under the landfill.The state of the three principal stresses is σH>σh>σV,and the range of the lateral pressure coefficient is 1.0-2.5 at present.The landfill load would chiefly influence the vertical stress under the landfill,but would not change the original 3D state of stress.(2)The influence of tunnel excavation on the landfill was studied from two aspects of environmental vibration and rock damage.As for the study of environmental vibration induced by tunnel excavation,field measurements and numerical simulation were combined to identify the vibration propagation law and influence factors.The failure criterion of landfill bottom liner system was established,and the peak shear strength of the geomembrane/geotextile interface,230 kPa for the current landfill height,was selected as the critical index.The relevant critical PPV of the landfill liner was calculated as 12 cm/s.The results showed that the landfill liner system was safe under the blast induced by tunnel excavation.As for the study of rock damage induced by tunnel excavation,the modified Drucker-Prager model coupled with Yang-Liu damage model was established to consider the characteristics of tensile damage for surrounding rock under blasting load.This elastoplastic damage model of rock was employed in the dynamic numerical simulation of tunnel construction.It was found that the largest damage depth was 6.1 m when considering the effects of weak intercalated layer.Compared to the vertical distance between the landfill and the tunnel,the effects of rock damage induced by blast on the landfill could be neglected.(3)With no intrusion into the landfill liner system,an integrated geological and geophysical investigation,including electrical resistivity tomography(ERT),tunnel geological prediction(TGP),transient electromagnetic method(TEM),geological drilling,and sample analysis,was carried out on the ground and in the tunnel under the Xiaping Landfill.The geological conditions under the landfill were ascertained,and the leachate distribution and potential leakage paths were identified.It was proved that the proposed integrated detecting system was of high precision,and would not affect the operating safety of the landfill.The results showed that the leachate level was 15-20 m below the landfill surface.Two possible leachage pathways from the landfill to the tunnel were found,and the width was 20-40 m.The leachate was well confined by the landfill liner system,and the probability of remarkable leachate leakage was low.(4)The unsaturated hydrologic-mechanical coupled numerical simulation was performed to study the influence of landfill leakage and tunnel construction on the seepage field.The anisotropy and evolution of the landfill and the rock were also taken into consideration.It was identified that the pore pressure would reach the peak in 45 d after the landfill leakage.Factors affecting the evaluation of the seepage field included dip angle and width of the leakage path and the size of the leakage point,among which the effect of the path width was the largest.After the excavation of the tunnel under the landfill,the pore pressure in the tunnel region would rise,and then fall in three steps.It would re-rise with the completion of the lining construction.The effect of rock damage induced by blast on the seepage field was significant,and time order of landfill leakage and tunnel construction would not change the final pore pressure.The safety of the tunnel waterproof and drainage system could not be ensured when large-scale leakage with the diameter of over 40 m occurred in the landfill.Therefore,the original scheme of tunnel waterproof and drainage was analyzed and evaluated from four aspects as grouting,lining impermeability,drainage measures and partition waterproofing system.Finally,suggestions of tunnel waterproof and drainage optimization were given,and emergency measures of leachate and landfill gas leakage were proposed. |
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