Time-space Effect Analysis And Safety Evaluation Of Deep Excavation In Ningbo Soft Clay Area

To ease the pressure of difficulties in uran land use and traffic jams, developing theunderground space and building uran rail transit vigorously have become inevitable tendency for acity’s healthy development. During the construction of Ningbo uran rail transit project, thefoundation pit of metro station and cut-and-cover section face to large-scale excavation engineering,and a large number of deep excavation engineering occur in soft clay area. Therefore, a practicalexample in the first phase of line2of Ningbo uran rail transit is taken for in this paper, and asystematic study on the field monitoring result, the time-space effect of deep excavation in softcaly area, back analysis of soil strata’s m-value and the safety evaluation for deep excavation arecarried. The main content of this paper includes:1. Combining with a practical deep excavation in Ningbo uran rail transit project, theconstruction monitoring scheme is presented, and the displacement of diaphragm wall, axial forceof the support, vertical displacement of diaphragm wall, the surface settlement, building settlementand the underground water are analyzed based on the in-situ measure data. Furthermore, thedeformation rule and machanic characteristics of deep excavation in Ningbo soft clay area arestudied.2. Based on the time-space effect characteristics of the deep excavation in soft soils, the finiteelement model of a practical engineering example is established by considering the time-spaceeffect. Such effect factors as the thickness and the buried depth of the diaphragm wall, horizontalspacing, different preloading and erection time of the support, and excavation width on the internalforce and deformation of supporting structure are studied in detail through the example.Furthermore, the construction methods considering the time-space effect are presented.3. Based on the monitoring data during the deep excavation construction, the m-value of soilstrata in typical excavation conditions is analyzed by using the displacement back analysis method,through comparing the m-value provided by geotechnical investigation report, the sutable m-valuefor main layers of soil involved the excavation process is suggested.4. According to the practical cases of the deep excavation, the safety evaluation level andindexes are established, and the safety evaluation model is established by using the fuzzycomprehensive evaluation method, so as to give a quantitative safety evaluation for the practicalproject based on the in-situ measure data.