Research On Longitudinal Structure Responses Of Overlying Existing Tunnels Induced By Tunnel Excavation

In recent years,with the large-scale construction of underground transportation infrastructure in major cities,new tunnels are inevitably constructed in close proximity to existing tunnels.The excavation of the new tunnel will break the initial stress equilibrium state of the surrounding soil and lead to deformations and structural damages of the existing tunnel.In order to ensure the structural safety of the existing tunnel,the research on the new tunnel-soil-existing tunnel interaction has gradually become a hot topic.On the basis of previous studies,the analytical method,probabilistic analysis and field observation are used in this dissertation to conduct in-depth research on the longitudinal response of existing tunnels to the new tunneling underneath.The existing theoretical analysis methods are improved,and the related results can provide theoretical basis and engineering-practice guidance for the deformation control of the existing tunnel.The main work and outcomes are listed as follows:(1)An improved analytical method is established for evaluating the longitudinal behaviors of existing tunnel,which considers non-uniform soil deformation due to the new tunneling.The Loganathan-Poulos solution and the integral method are combined to calculate the tunneling-induced greenfield soil displacement considering non-uniform deformation modes of the surrounding soil.The analytical solution for the longitudinal tunnel responses is then derived based on the Timoshenko beam theory and the Winkler foundation theory.The proposed method is verified with field measurement data from actual engineering cases.The results indicate that the possible asymmetry of the longitudinal tunnel responses can be reflected by the proposed method.When the effect of the horizontal movement of the new tunnel is neglected,the settlements and internal forces of the existing tunnel may be underestimated.(2)Owing to the change of the construction method,reinforcement of the tunnel lining or the existence of the settlement joint,the tunnel is likely to have discontinuities in its longitudinal stiffness.An analytical solution for the new tunneling-induced mechanical responses of the existing tunnel with stiffness discontinuities is derived.The existing tunnel is modelled as a series of Timoshenko beams connected by elastic springs at the locations of the stiffness discontinuities.The accuracy of the proposed solution is verified by the degenerated solution and the numerical model.Two hypothetical examples are used to further study the normalized response of the existing tunnel with single stiffness discontinuity or with a reinforcement section.The results show that when the location of the new tunnel gets closer to the position of the stiffness discontinuity,the internal force responses of the small-stiffness section of the existing tunnel become larger.The length and the stiffness of the reinforcement section should match each other to ensure the effectiveness of the reinforcement measures.(3)A theoretical analysis method considering the longitudinal variability of soil elastic modulus is proposed to assess the longitudinal responses of existing tunnel to new tunneling underneath.Firstly,a calculation model for new tunnel-soil-existing tunnel interaction considering longitudinal variation in the subgrade reaction coefficient is established,and its accuracy is verified by the existing analytical solution.The calculation model and the random field theory are combined to evaluate the effect of the longitudinal variability of soil elastic modulus on the longitudinal tunnel responses.The results show that the increases in the longitudinal scale of fluctuation and the coefficient of variation of the soil elastic modulus lead to higher variabilities of the normalized tunnel responses.The decreasing pillar depth and mean value of the soil elastic modulus and the increasing skew angle strengthen the effect of the longitudinal variability of the soil elastic modulus on the tunnel responses.Compared to the Timoshenko beam model,the Euler-Bernoulli beam model overestimates the variabilities of the tunnel responses under the random field of the soil elastic modulus.(4)A probabilistic analysis model for longitudinal deformation of the existing shield tunnel due to twin tunneling underneath is established.The exponential equation is introduced to describe the relationship between the volume losses caused by the first and second tunnel excavation,and a deterministic model is proposed for calculating the twin tunneling-induced deformations of the existing tunnel.The deterministic model,sparse polynomial chaos expansion and Monte Carlo simulation are then combined to form the probabilistic analysis model considering the uncertainties in the parameters of soil and tunnel structure.The parametric analysis implies that the uncertainties in the greenfield soil settlement dominate the assessment of the tunnel settlement,and the parameters associated with the soil-tunnel interaction influence the joint deformations more remarkably.An increase in the allowable volume loss at the target probability level can be seen with increasing horizontal distance between twin tunnels.The reduction in the skew angle strengthens the influence of the failure mode of the tunnel settlement on the probabilistic evaluation of the system.(5)The existing metro line 2 in the muddy clay of Hangzhou was under-crossed by the large-diameter shield tunnel of Wenyi Road underground passage.The characteristics,mechanism and countermeasures of the longitudinal tunnel deformations due to large-diameter shield tunneling underneath are studied by means of field measurements.Through long-term displacement monitoring,the long-term responses of existing tunnels after new tunnel construction and the effect of the secondary grouting treatment are analyzed.Due to the buoyancy effect of the new tunnel,apparent heaves of the existing tunnels were observed after the passing of the shield tail.Increasing the pillar depth between the new and existing tunnels and injecting more grout with greater shear strength via the upper grouting holes could reduce tunnel heave effectively.The long-term settlements of the existing tunnels gradually became stable three months after the secondary grouting,and the maximum settlement was controlled within the allowable value.