| In recent years,China’s urban underground space is exploited in an increasingly multidimensional and intensive manner,nearby construction of different projects has become more and more common.For example,excavations nearby an existing tunnel are frequently encountered in underground construction.Soil mass excavation usually causes appreciable stress relief,which might lead to significant ground stress redistribution and large deformation of nearby soils,and then indirectly cause additional deformations and internal forces of the existing tunnel.Sometimes,it will even bring about the tunnel cracking,shear displacement,water leakages or other damages.In addition,the long-term exposure of the excavation base will result in continuous soil deformation and tunnel response,owing to the evolution of earth pressure and pore water pressure inside and outside the excavation area.Up to now,many researchers have studied the interaction between excavation and an existing tunnel,but most of them focus on the analysis of influence of tunnel segment.The response mechanism of stratum stress state and tunnel deformation are not yet fully understood,notably the long-term behavior.Through centrifuge modeling test and three-dimensional numerical simulation analysis,this thesis carres out a comprehensive and in-depth study on the topic of effects of construction and post contruction induced by excavation in soft clayey strata.The findings can provide references for construction close to the underground engineering in soft clay.The main works and findings of this thesis are drawn in the following aspects:(1)Centrifuge modeling with an acceleration level of 1:120 was carried out to investigate the impacts of an excavation on a nearby tunnel in soft clayey strata.Responses of undrained shear strength,pore water pressures,horizontal earth pressures around the existing tunnel,ground settlements,tunnel settlements,and bending moments are obtained.The test results show that:exposure of the excavation base leads to continuous evolution of excess pore water pressures of soils below the excavation base and around the tunnel,together with both magnitude and distribution changes of horizontal earth pressures around the tunnel.After excavation,ground surface settlements,tunnel settlements and bending moments develop continuously.It implying that the exposure time of the excavation base should be minimized in real projects.(2)In view of the disturbance of surrounding strata caused by excavation,soils nearby the existing tunnel and excavation were sampled,and the compression behavior of the specimens was examined by oedometer tests.The test results show that the initial void ratio on the side of the existing tunnel is lower than that on the other side.The existing tunnel plays the role that can restrict soil disturbance,which lead to compressibility of the soil on the side of existing tunnel is less than that on the other side of the excacation.Compared with the soil between the excavation and existing tunnel,soil compressibility on the other side of tunnel will change more than that on the symmetrical side of the excavation.(3)Based on the parameters of HSS model obtained from oedometer tests,a three-dimensional numerical simulation was carried out to study the long term response mechanism of excavation on a nearby tunnel.The results show that the horizontal continuous displacement of the tunnel is about 40%of excavation period up to 300 days after excavation.Compared with this,the vertical continuous displacement reaches about 56%of excavation period.Owing to the large rigidity of retaining wall,its continuous displacement development is only 20%of excavation period.With the continuous exposure of excavation base,normal stress around the side of tunnel spring line near the excavation continues to decrease.Due to the inhibition of the tunnel on the side of soil,the normal stress of soil in other direction gradually increases.(4)This paper analyzes responses of maximum surface settlement,maximum horizontal displacement of retaining wall,maximum vertical displacement of the tunnel springline and maximum horizontal displacement of the tunnel crown in different cases.On one hand,when the side length of excavation parallel to the tunnel direction increases,the deformation response incerases heavily.On the other hand,when the side length of excavation perpendicular to the tunnel direction increases,the deformation response decreases gradually.As the existing tunnel can inhibit the soil disturbance caused by excavation,when the existing tunnel is closer to the excavation area,ground settlement and horizontal displacement of the retaining wall gradually decrease.(5)Considering influence of the shape of excavation and the relative position of the existing tunnel,this paper conduct numerical simulation parameter analysis.The results show that with the increasing distance between tunnel and excavation base,the vertical convergence deformation of tunnel section decreases gradually.When the tunnel is located on the long side of excavation,the tunnel transverse section will have a large deformation towards excavation area.In particular,when the tunnel is located at the short side of excavation,the deformation of the tunnel section tends to decrease,even if the excavation area increases gradually.This is caused by the size effect of excavation. |
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