Research On Earth Pressure In Pit-in-Pit Type Deep Excavation

In recent years,with the continuous improvement of the country’s modernization level,engineering construction has continued to develop underground.In order to meet the needs of the construction of infrastructure,elevator shafts,subway stations and large-scale underground shopping malls,secondary or multiple excavations are often done inside large foundation pits,forming“Pit-in-pit”type deep excavation.In this paper,the development and transmission of earth pressure between the pits are studied by combining theoretical analysis and numerical calculation.The main work is as follows.（1）In-depth analysis and description of the instability and failure process of the“pit-in-pit”type deep excavation.Two parameters——the degree of disturbance（the degree to which the soil approaches the limit stress state）and the critical value of the maximum width-to-depth ratio of the inner pit,are defined to study the development of earth pressure and the instability process of this type of excavation.When the width of the inter-pit area is greater than or equal to the critical value of the maximum width-to-depth ratio of the inner pit,the interaction between the inner pit and the outer pit supports can be ignored.Assuming that the active zone and the passive zone exist in the soil between pits at the same time,and they influence each other,the expressions of the effective displacement and disturbance degree of the active and passive zone are respectively established,and the critical value of the maximum width-to-depth ratio of the inner pit g=2,which is obtained through analysis.（2）Considering the interaction between the active and passive areas in the inter-pit area,the active and passive earth pressure expressions of the inner pit support and the passive earth pressure expressions of the outer pit support are respectively deduced,and the theoretical value of the earth pressure in this paper is compared with the measured value of the model test and other earth pressure theories.It is found that the calculation theory of earth pressure proposed in this paper is the most consistent with the measured value,which indicates that the earth pressure before outer pit support and after inner pit support be calculated using the effective displacement expression and influence coefficient given in this paper is reasonable.（3）The values of active and passive earth pressure influence coefficients under different geometric conditions are analyzed.Based on this,the expressions of the active and passive earth pressure influence coefficients are established,and the factors that affect their values and the size of the influence weight are analyzed,and it is considered that（?）_pav has the greatest influence on dis_p,（?）_aav has a significant influence on dis_p,and the influence of B/H₂on dis_p smaller.It is believed that（?）_{pa v} has the greatest impact on dis_a,and the impact of（?）_aavand B/H₂ on dis_a is significant,and the difference is not large.（4）Based on previous model tests,by changing the length and depth of inner and outer pit support piles and other geometric parameters,the failure process of“pit-in-pit”type excavations under different conditions was simulated using FLAC^3D.The method of judging that the soil has reached the ultimate stress state in FLAC^3D is given.Based on the existing method for judging the failure mode of“pit-in-pit”type deep excavations,the support stable failure mode results of numerical tests of“pit-in-pit”type deep excavations under different geometric conditions are analyzed.By comparing the failure mode results of support instability under different geometric conditions,the factors affecting the failure mode of the“pit-in-pit”type deep excavation,their influence law and influence weight are analyzed.It is considered that increasing the width of the inter-pit area and H₂/H₃——the ratio of the inner and outer pit support lengths will lead to the development of“pit-in-pit”type excavation damage in the direction of separation,and increasing the depth difference（35）h between the inner and outer pit supports will make the damage of the pit-in-pit type excavation develops towards the overall direction.