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Copyright of Railway Construction Technology is the property of Railway Construction Technology Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Pit-in-pit foundation pit is a complex and not yet fully studied foundation pit. In order to study the internal force development law of the inner pit support piles during the excavation of the foundation pit, the field test of the internal force of the support piles during the construction process was conducted. The bending moment of the supporting pile was calculated by collecting the stresses in the longitudinal reinforcement of the pile, and the development law of the bending moment of the supported pile during the construction process was analyzed. The experimental results were fitted by finite element numerical calculation, and the trend of the bending moment of the inner pit support pile was studied when the depth of the outer pit excavation and the width of the platform were varied. The results show that there is a close relationship between the pit toe coefficient (the ratio of platform widthto excavation depth of the outer pit ) and the variation of the maximum bending moment of the inner pit support pile. The function curve of pile maximum moment growth rate of pit toe factor is given by fitting. The maximum moment growth rate of pile can be considered as the safety factor in the design of pit retaining pile in pit-in-pit foundation pit. [ABSTRACT FROM AUTHOR]

Based on the pit excavation project in a station of Lanzhou Metro, the horizontal displacement of bored piles, the ground settlement around the foundation pit, and the supporting axial force in the foundation pit were tested on site. The changing rules and influencing factors of horizontal displacement of external and internal bracing piles during foundation pit construction are analyzed. Comparing the measured results with the numerical simulation results, the results show that the finite element calculation results can meet the engineering needs, the pile-support system can effectively control the deformation of the foundation pit, and the bottom of the outer pit has an obvious uplift. The numerical simulation is used to study the influence of different working conditions on the deformation of the foundation pit. The results show that the inner support of the foundation pit can control soil deformation more effectively than the outer support of the foundation pit. The study can provide a reference for the construction of such foundation pit in Lanzhou in the future.

Engineering practice shows that the width of the external foundation pit and the pit in the pit will have a influence on the ways of formation process of the slip surface at the bottom of the foundation pit.Thus, Rankine earth pressure theory based on conventional semi-infinite space is not applicable for calculating the earth pressure acting on the retaining structure for pit-in-pit excavations.Based on the limit equilibrium theory and the fractional force equilibrium method, the theoretical formulas for calculating the passive earth pressure of wide pit and narrow pit are derived, considering the cohesive force of soil and the friction resistance between the retaining structure and sliding soil.For example, the variation trends of passive earth pressure of wide pit and narrow pit in pit condition are compared.The results show that the passive earth pressure considering friction is larger than the passive earth pressure without friction.For both wide and narrow foundation pits, the passive earth pressure with the inner pit is less than that without the inner pit.When there is no inner pit, the increase of the width of the outer foundation pit leads to the decrease of the passive earth pressure, while when there is an inner pit, the increase of the width of the outer foundation pit increases the passive earth pressure.In the narrow foundation pit, the earth pressure decreases first and then increases with the movement of the pit position, and the increase of the pit in the pitplane dimension leads to the gradual decrease of the earth pressure.Thus, the existence of the inner pit will greatly reduce the passive pressure of outer one, and attention should be paid to the existence of inner pits in engineering.

Copyright of Geology & Exploration is the property of Geology & Exploration Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The research on the influence of groundwater level fluctuation on pit properties is a new subject in actual engineering, and the research on pit-in-pit has not been deeply studied. In this article, the finite element (FE) model of pit-in-pit is established based on FE method, and DISP subroutine is programmed to realize the fluctuation of groundwater level over time. The deformation characteristics of exterior enclosure structure under the condition of with or without groundwater level fluctuation are compared considering effect of different inner pit's excavation locations, depths, sizes, and insertion ratios. The results show that the maximum lateral displacement of the pit exterior enclosure structure is inversely related to the pit toe coefficient and the insertion ratio, and is positively related to the depth ratio and the area ratio, and there is a critical value for the pit toe coefficient. Considering the fluctuation of the groundwater level, the lateral displacement of the cantilever enclosure structure is larger than that without considering the groundwater level fluctuation. [ABSTRACT FROM AUTHOR]