Abstract
The deformation prediction of foundation pit is still an open issue in geotechnique engineering. This paper focuses on a new deformation prediction method of foundation pit in soil-rock composite stratum in Nanjing area. To do so, the field monitoring deformation data of deep foundation pit adjacent to the railway in a soil-rock combined stratum is firstly analyzed. Based on these field data, the characteristics of horizontal displacement and surface settlement of rock-socketed support pile, as well as its relationship with Earth excavation are discussed. Results show that the average value of maximum horizontal displacement of support pile in zone with shallow rock depth (soil layer >15 m) is relatively large, which is 3.5 times of that with shallow soil depth (10–12 m into moderately-weathered mudstone). The depth of maximum horizontal displacement is obviously linearly related to the thickness of soil layer (H), while the surface settlement does not change with the increase of soil/rock layer thickness ratio. Utilizing the relationship between the maximum water level displacement and excavation depth, a new Gaussian-type empirical formulation is established to predict horizontal deformation of soil-rock composite stratum with rock-socketed support pile. By comparison with two other methods, it is found that the improved Gaussian curve is in good agreement with the measured horizontal displacement curve. This indicates that the new method can provide a valid reference for the design and construction of foundation pit with rock-socketed support pile in Nanjing area.
Highlights
With the rapid development of urban rail transit (URT) in recent years, the underground zone gradually involves deeper foundation pit, more complex geology, and the gradually increasing scale and difficulty of construction works, which triggers higher requirements for rapid, safe and economical construction
The deformation sketch of foundation pit is shown in Figure 3, in which H and, h respectively refer to the excavation depth and the thickness of soil layer embedded in pile, ymax refers to the maximum horizontal displacement of pile body, xc refers to the depth where the maximum horizontal displacement is located, y1 refers to the horizontal displacement of pile top, R refers to the influence range of surface settlement, and δmax refers to the maximum surface settlement
It is found that the relationship between xc and t, ymax and t, can be described by a hyperbolictype functions, It can be seen that the maximum horizontal displacement and its depth decrease with the increase of the pile depth into the rock (t), and tend to be stable, indicating that there is a limit value for the maximum horizontal displacement and its depth, and the diaphragm increase of the depth into the rock has no inhibitory effect on the deformation of the foundation pit
Summary
With the rapid development of urban rail transit (URT) in recent years, the underground zone gradually involves deeper foundation pit, more complex geology, and the gradually increasing scale and difficulty of construction works, which triggers higher requirements for rapid, safe and economical construction. Chen et al [20] studied a deformation case of pit in pit (PIP) excavation support structure in Shenzhen soil-rock composite stratum. In terms of on-site analysis methods, Xie [26] analyzed the excavation effect of deep foundation pit in soil-rock composite stratum in Jinhua area, and two methods for predicting horizontal displacement by volume and displacement relation are proposed. The monitoring data of deep foundation pit of underground metro are analysed, deformation regularities of rock-socketed supporting piles and surface settlement is summarized, and deformation prediction is carried out by using Gauss curve, which can provide reference for the design and construction of underground foundation pit under such soil-rock composite stratum in Nanjing.
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