| With the development of Chinese social urban transportation system,a boom of subway construction has been set up in order to alleviate the increasingly crowded surface traffic.In the construction of subway,shield method is widely used because of its relatively mature technology,safe and reliable construction,free from external environment interference and other advantages.In the construction process of shield tunnel,due to the unreasonable pressure setting of the face,it is easy to cause the instability of the excavation face,resulting in the collapse of the surface,and the stability of the face has attracted the attention of experts and scholars at home and abroad.Especially in the upper soft and lower hard(gravel sand-argillous siltstone)stratum,there are few relevant studies by domestic and foreign scholars.Due to the numerous factors affecting the stability of the palm face,there is a lack of mature and reliable analysis theory of the palm face stability,resulting in frequent safety accidents caused by the instability of the palm face in the process of shield construction.Therefore,it is of great theoretical value and engineering significance to carry out the research on the stability analysis of the palm surface in soft upper strata and hard lower strata.In this paper,the stability of the palm surface of shield tunnel in upper soft and lower hard strata in Nanchang area is the main research object.Relying on the practical project of shield tunnel section between Renmin Park Station and Shangshagou Station of Nanchang Metro Line 4,the stability of the palm surface of shield tunnel in upper soft and lower hard strata is studied by theoretical calculation,numerical simulation,mathematical statistics and linear regression.The main research contents and conclusions are as follows:(1)The failure mode and displacement of soil mass caused by shield excavation in full section gravel sand layer and upper soft and lower hard strata are compared.The results show that the instability failure of gravel sand layer is mainly in the lower part of the center of the palm surface,while the instability failure of upper soft and lower hard strata is all in gravel sand layer.The prediction formula of soil displacement in front of the palm face is presented.The soil displacement and stress support ratio of the palm surface were studied under different load distribution forms.It was found that the maximum soil displacement under rectangular distribution load was larger than trapezoidal distribution load when the pressure of the palm surface was too large or too small,and the tunnel location of the maximum soil displacement was different.However,in the upper soft and lower hard stratum,when the soil is passively unstable,the soil displacement caused by the rectangular uniform load is much larger than that caused by the trapezoidal load.When the soil is actively unstable due to insufficient supporting force,the maximum displacement of the soil under the two load distribution forms is not much different,only 1.02 mm.(2)The instability failure mode of the shield tunnel in the upper soft stratum and the lower hard stratum(gravel sand-argillous siltstone)is studied in detail.By establishing a three-dimensional numerical model for the stability analysis of the face of the shield tunnel,the sand layer accounts for 100%,87.5%,75%,62.5%,50%,37.5%,25% and 12.5%,respectively.The failure law of the metacarpal face in the sand-argillous siltstone composite upper soft and lower hard strata is obtained,and on this basis,the theoretical calculation method of the ultimate supporting force of the metacarpal face under the condition of upper soft and lower hard strata is derived.The results show that the tunnel deformation and failure is mainly controlled by the upper sand layer,and the failure form of the sand layer is local instability failure,and mudstone does not participate in the failure.When the proportion of gravel sand is higher than 75%,the increase of ultimate supporting pressure is relatively gentle,and when the proportion of gravel sand is lower than 75%,the increase of ultimate supporting pressure is faster.In the process of instability failure,the tunnel face will go through three stages,that is,the slow and small deformation stage to the sensitive and large deformation stage and finally to the unstable failure stage.When the support force of the tunnel face is reduced to a certain extent,the tunnel instability failure develops rapidly and finally reaches the limit state.The calculation formula of the palm surface is similar to the numerical simulation results,and the error is less than 5%.It can better predict the ultimate supporting force of the face in practical engineering.(3)Through numerical analysis of different working conditions,the influence of the stability of the palm surface on the soil disturbance around the tunnel is analyzed,and the rule of soil displacement caused by tunnel excavation under different burial depths is compared.The results show that: From the surface down,the soil settlement always increases gently.The settlement starts to increase sharply about 1.5D above the surface(-9m,D is the tunnel diameter),and the soil displacement reaches its maximum at the top of the tunnel.In the process of shield tunneling,the surface soil settlement is in the shape of inverted Gaussian,and the maximum settlement is above the central axis of the tunnel,and the settlement of the soil in the vault is much larger than the surface settlement.The settlement value of the soil in the tunnel vault is about 2-3 times of the surface settlement value.A calculation formula for controlling the palm face pressure to predict the soil loss rate is proposed.According to the engineering practice,when the palm face pressure is greater than 1.5 times the limit supporting force of the palm face,it can be considered that the palm face pressure set at this time meets the engineering safety requirements and will not cause significant impact on the surrounding environment. |
