Abstract
As an important part of subway transfer station construction, pit-in-pit (PIP) excavation has always been a key object of monitoring and control. In this study, taking the PIP excavation project of Hefei metro line 4 and line 7 transfer station as the background, combined with on-site monitoring and numerical simulation, the variation law of lateral wall displacement, ground surface settlement, and strut force during the PIP excavation were analyzed. The results showed that the maximum lateral deformation of the pile caused by the excavation of the external pit accounted for 80%–90% of the total deformation and the surface settlement accounted for 70% of the total settlement. The excavation of the inner pit only made the maximum lateral wall displacement of the outer pit and the surface settlement behind the wall increase slightly, the growth rate tended to zero, and the maximum lateral deformation depth was all above the excavation surface, which indicated that, for this project, the inner excavation had little effect on the outer pit retaining structure. In addition, the increased exposure time of the soil at the bottom of the pit and the presence of the corner effect will cause the further development of the structure displacement and the surface settlement. Based on strut force measurements, using the tributary area load distribution procedure, it was obtained that 0.3γHe was used as the upper limit index of strut force in the region, where γ is the weight of the soil and He is the excavation depth of the foundation pit. The research conclusions were helpful for the construction and sustainable development of the PIP project.
Highlights
With the continuous development of the social economy and increasing urban traffic pressure, many large and medium-sized cities have to build underground transportation networks to alleviate surface traffic pressure. e subway transfer station serves as a node of the underground transportation network, and its special supporting structure in the PIP will increase the construction risk. e study of the degree of soil disturbance and the deformation law of the supporting structure in the PIP excavation is of great significance for the protection of the surrounding environment.e apparent pressure diagram (APD) proposed by Terzaghi [1] and Peck [2] is widely used in sand, hardcracked clay, and soft-to-medium clay for braced excavations to estimate the magnitude and distribution of strut loads
Due to the excavation sequence and site conditions, the struts needed to be installed layer by layer. e excavation process began from the cantilever state of the wall, and the support state transitions began from a single support to multiple supports. e first strut of the site was installed with a preexcavation groove. is was done, so there was no cantilever rotation of the wall during the initial stage of the excavation, which would have been in the form of a bulge
Measured Results of the Strut Forces. e west side data points SF17 and SF18 and the south side data points SF27, SF28, and SF29 were selected in the two orthogonal directions of the intersection. e monitoring results of the strut axial force during the PIP excavation are shown in Figure 9. e last digit of the monitoring point indicates the vertical brace position; for example, SF17-1 indicates the first brace of the SF17 measurement point
Summary
With the continuous development of the social economy and increasing urban traffic pressure, many large and medium-sized cities have to build underground transportation networks to alleviate surface traffic pressure. e subway transfer station serves as a node of the underground transportation network, and its special supporting structure in the PIP will increase the construction risk. e study of the degree of soil disturbance and the deformation law of the supporting structure in the PIP excavation is of great significance for the protection of the surrounding environment. E subway transfer station serves as a node of the underground transportation network, and its special supporting structure in the PIP will increase the construction risk. E above studies on PIP excavation are based on specific assumptions using theoretical and numerical analysis methods. E research of the field measured data is often more representative than the theoretical and numerical analysis, and the research results are more instructive to the construction of the PIP project. In this study, taking a PIP excavation in a subway transfer station in Hefei as the background, the lateral wall displacement, ground settlement, and strut force during the whole construction process were monitored in detail. Rough the analysis of monitoring data and comparison with numerical simulation results, the spatiotemporal evolution of the support structure deformation and ground settlement during PIP excavation is discussed. As the capital city of Anhui Province in southeastern China, Hefei has multiple subway lines under simultaneous construction. e impact assessment of subway construction on the surrounding environment is important [24]. ere are few studies on the PIP excavation in this area, and it is hoped that the results of this study will provide guidance for the follow-up subway construction
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