Research On Deformation And Stability Of Foundation Pits And Analysis Method Of Earth Pressure On Retaining Structures

It is of great significance to reasonably evaluate the deformation behavior of foundation pits and accurately evaluate the earth pressures on the retaining structures for improving the economy and ensuring the safety level of foundation pits.Using the centrifuge model tests and field observations,this paper investigated the deformation characteristics of the foundation pit and the interaction mechanism between the retaining structures and the soil considering excavation and seepage conditions.An improved method for earth pressures on the retaining structure were proposed on the basis of the mechanism analysis.New achievements and main conclusions are as follows:1.A new device was developed with new principles based on the understanding of the stress field in the soil base before and after excavation for simulating excavation of a foundation pit in flight in centrefuge model tests.Therefore,the coupling effect of excavation and seepage was reasonably simulated in centrifuge model tests.This device has been authorized the National Invention Patent.2.A series of centrifuge model tests,along with field observations of two foundation pits,were conducted with considering several factors such as properties of the soil and embedded depths of retaining structures.The deformation characteristics of foundation pits and the earth pressure behavior on the retaining structures were analyzed according to the observations.The tests showed that the true paths of the stress and deformation of the foundation pit could be reasonably simulated in the centrifuge model tests,which cannot be captured by the traditional method,in which the excavation was conducted previously at the 1 g level.The traditional method underestimates the earth pressures and the deformation of the soil due to the excavation of the foundation pit according to the comparison tests.Centrifuge model tests in this thesis exhibited the same rules with the field observation on the deformation behavior of the foundation pits and earth pressures on the retaining structures,indicating the effectiveness of the centrifuge model tests.3.The centrifuge model tests revealed that the excavation and seepage cause significant deformation of a foundation pit,which develops according with dependency on the movement of the retaining structure in a limited area.Significant deformation localization occurs as the deformation increases to a certain extent and develops with an invariable width.The development of the deformation localization causes the foundation pit to an understable state.An understable state index was proposed to quantitatively describe the relationship between the typical displacements and the deformation localization.This index can be used as a characteristic parameter to determine the safety level of a foundation pit.4.A potential slip surface appears and exhibits a synchronous change with the earth pressure on a retaining structure as the deformation localzaiton of the foundation pit develops.The potential slip surface approximates a plane,but only extends from the surface behind the retaining structure to the rotation center.This demonstrates that the deep soil behind the retaining structure and the soil in front of it are difficult to reach the limit state.A modified method was proposed on the basis of the division to four zones according to the retaining structure and its rotation center.The method was verified by comparing the predictions with the results of centrifuge model tests.This method considers the characteristics of the deformation localization and the potential slip surface as well as the effects of the movements of the retaining structure.The parameters of the method are easily determined and can be used in practice.