Study On Constitutive Relationship Of Soft Clay Ground And Its Application In Power Pipe Jacking Project

Soft clay is widely distributed in China’s coastal areas.China’s coastal areas have developed economies and more and more power and communication pipelines.Under the situation of severe congestion on the ground,cable access to the ground has become the main trend.In soft clay areas,due to the physical characteristics of the soft clay such as high water content,high compressibility,high sensitivity,low strength,and low bearing capacity,the use of buried or open-cut tunnels has certain risks in the application process.,Shield construction and pipe jacking have become the main construction methods for underground engineering in soft clay areas.Pipe jacking tunnel segments are prefabricated segments with fewer joints than shield tunnels,which is more advantageous than shield tunnels..However,although the pipe-jacking tunnel is not slotted,it will inevitably affect the surrounding stratum during the construction process.At the same time,the external construction will also affect the existing pipe-jacking tunnel..In addition,many constitutive models of soil have been developed.How to choose a constitutive model for different projects and how to determine the constitutive model parameters are the main issues for soft clay research.The main work of this paper is as follows:（1）By consulting relevant literature,consulting a large number of actual engineering and numerical simulation cases,investigating the application of the constitutive model in soft soil areas,and the three most widely used constitutive models in soft soil areas are the main parameters of the Mohr-Coulomb model.: Compression modulus,internal friction angle,cohesion,density,water content,etc.The main parameters of the modified Cambridge model are: log-hardening modulus λ,various constant coefficients of expansion κ,critical state effective stress ratio M,small strain model Several major moduli: reference secant modulus E₅₀^ref,reference loading and unloading modulus E_ur^ref,reference tangent modulus E_oed^ref,and reference shear modulus G₀^ref.The correlation between the parameters was fitted.（2）Sampling of typical soft soil on-site and conducting indoor geotechnical tests,including triaxial consolidation drainage shear test,triaxial consolidation drainage loading-unloading-reloading test,standard consolidation test,unidirectional compression-rebound-recycling Compression test.Combined with the results of statistical analysis of parameters,the parameters of different constitutive models were input using the built-in geotechnical laboratory program in PLAXIS3 D,and the indoor geotechnical tests of soft soil sampled on site were inverted.（3）Through numerical simulation of typical engineering examples of pipe-jacking tunnel construction in soft soil areas,the ground deformation results calculated by different constitutive models are obtained,and the following conclusions are obtained by combining the field monitoring data: the modified Cambridge model and Mohr-Coulomb model can more accurately simulate ground deformation Law,but the modified Cambridge model simulation results are too large.The small strain model is not suitable for simulating the construction of pipe jacking tunnels in soft soil areas.The small strain calculation results show that the displacement around the tunnel decreases as the distance from the tunnel decreases.The longer the horizontal distance of the tunnel,the ground displacement does not tend to zero,which is also inconsistent with reality.（4）Through numerical simulation of the construction engineering examples of the foundation pit subdivision near the existing electric pipe jacking tunnel in soft soil area,the existing pipeline deformation,surface displacement around the pit,ground wall deformation,and foundation deformation calculated by different constitutive models are obtained.Monitoring the data,we found that the small strain model can better simulate the surface displacement around the pit and the deformation of the ground wall.The basement uplift calculated by the Mohr-Coulomb model is too large,which is quite different from reality.（5）The Mohr Coulomb model is still the most widely used constitutive model because of its simple and easy to obtain parameters.However,when the Mohr Coulomb model is applied to soft soil engineering,it has the problem of excessive rebound and unloading.This section combines top In the case of a tube tunnel project,the effect of such methods on the rebound of the stratum is studied by reducing the thickness of the soil below the tunnel during the modeling process and introducing a similar unloading modulus of the small strain model to increase the elastic modulus of the soil below the tunnel.The application of the Mohr-Coulomb model in the calculation of pipe jacking tunnels in soft soil areas is more reasonable.（6）Based on the Mohr Coulomb model,the main factors for soil settlement of pipe jacking projects in soft soil areas are the in-situ stress release rate（corresponding to the degree of simultaneous grouting）,the pressure of the earth bunker,the buried depth of the pipe jack,the elastic modulus of the soil,and the pipe diameter.And so on,to analyze the construction effect of pipe jacking in soft clay ground.The perturbation laws of pipe jacking on soil at different locations on the surface and stratum are analyzed.