Studies On Effects Of Metro Tunnel Under Historical Building

The large-scale subway construction makes shield tunneling to be used widely,which promotes the development of underground engineering. The construction ofdeep foundation pit and tunnel has many risks in soft soils; different protectivemethods cause different results. It is important to analyse the influence of differentconstruction phase on buildings, the formation mechanism of grouting reinforcement,additional stress in soil caused by shield thrust, etc.Firstly, based on the analysis of field measured data to buildings, the influencesinduced by excavation, dewatering, grouting, freezing and underlying tunneling areanalysed. The grouting lift is modeled by finite element method, the results confirmwith the measured data. It is shown that the settlements of buildings can be minimizedby applying the optimized shield parameters. Leakage during the tunneling may causesignificant settlement of overlying buildings, if soft clayey soil layer exists betweenfoundation and tunnel, grouting can not effectively lift buildings with large weight,the rise of buildings can be part offset by consolidation of soft clayey soil due todissipation of the excessive pore water pressure.Furthermore, finite element parameter verfied by field measured data is used tocalculate additional stress induced by shield frontage thrust and friction. Additionalstress distribution of shield right ahead, lateral, above and beneath is analysed, theinfluence on underground pipeline and structure is also considered. The results showthat influence of x normal stress induced by shield front thrust in plane graduallybecomes greater with the increment of distance from tunnel face; however, theadditional stress reduces rapidly. Distributional law of additional stress induced byshield friction in soils is same as the stress induced by shield front thrust, but thescope of influence is much greater and damping is relatively slow. The additionalstress induced by shield thrust is in form of soil pressure acting on pipeline andstructure, excessive deformation can cause the leakage of pipeline and cracking ofstructure, it should be taken seriously.Based on the finite element model verified by field measured data, it is analyzedthe influence of grouting in different layers and elastic modulus in surrounding soils.The results show the real effect of grouting takes on70percent of actual grouting volume. However, grouting will generate excessive pore water pressure insurrounding soils, the pressure gradually dissipate after grouting, small settlement willhappen to buildings. Too large elastic modulus of upper soil for grouting body mayhave the effect of expansion constraint, which cannot effectively lift buildings.However, for the lower soils of grouting body, the larger elastic modulus is, the moreobvious grouting lifts the great self-weight buildings, but for the little self-weightbuildings is not clear.Frozen soils experiment is conducted for Tianjin area. The physical andmechanical properties of frozen soils are analyzed. Triaxial test shows that stress staincurve expresses the characteristic of strain softening when at the temperature of-5degree; however, for the temperature of-10degree and-15degree it shows to strainhardening.Finally, it is analyzed the freezing method, and it is simplified as thick wallcylinder subjected to internal pressure, using elastic-linear model to get the thicknessof the frozen wall. The field measured data shows that freezing curtain develops veryfast at early stage; the ice wall is not uniform at the stage of closure of freezing walland happens to uniform at later stage. Current design parameters are too conservative;it should be optimized combining with the test parameters.