Retaining Structure And Soil Interaction In Deep Excavation

The north anchor foundation of the south cable bridge across Changjiang River which connected Yangzhou to Zhenjiang (named Runyang) had deep excavation with 69m long, 50m wide and about 50m deep. The retaining and protection structure for this deep excavation was concrete diaphragm wall supported by plane concrete frame inside. In order to study retaining structure and soil interaction in deep excavation, monitoring data (especially deformation of the wall, settlement of ground surface and earth pressure etc.) in the process of excavating and supporting was analyzed. And three-dimensional numerical method was used simulating construction with four factor (wall thickness, support space, support section size, soil strength) changing in five lever. Some conclusions come as follows.(1)Lateral movements of the diaphragm wall increased with excavating, but had different behavior in every stage. Before excavating at depth of 22.3m, the wall deformation increased at a accelerate state. After that point it would be slowdown. The wall deformation at the midpoint of rectangular side was great than that of corner in the same horizontal plane. In vertical direction, the deformation curve of the wall was entasis, that is great in middle, and mini upside or at the bottom. The max deformation position descended with excavating. Before excavating to depth of 22.3m~26.3m, the depth of maximum deformation position was correlativity to deformation value. After that, both the decline of the maximum deformation position and the deformation increase were relatively small. Settlement of the ground surface was small before excavating to depth of 22.3m. After that, it augmented remarkably because of dewatering outside the pit.(2) A formula was gained to estimate the maximum deformation of the wall under the condition of multi-bracing system, great system stiffness, factor of safety against basal heave was about 1.4. (3) Envelope line was drawn out for estimating the maximum settlement of ground surface.(4) At the beginning of excavation, Earth pressure on the wall was linear increased with depth add, and generally still earth pressure distribution. As excavating went on deeper and deeper, the curve of earth pressure distributing was more flexure at the primary turning point. When excavating to the bottom of pit, earth pressure curve was not only flexure strongly but earth pressure value minified than that of beginning.(5) Four earth pressure patterns in the process of foundation pit excavation were sum up. those were liner increase mode, undulance increase mode, step-shape increase mode and extra load action mode.(6) Under normal construction (no dewatering outside the pit), the deformation of the wall and the earth pressure on the wall both very small before 6m excavating depth. At the following shortly period, the deformation increase a little, but earth pressure gained a sharp increase. Latterly, in the main excavating process, earth pressure presented a relatively stable state with the wall deformation increase (the slope of the curve could be equal or less than 0) except a little fluctuant. The slope of the curve could be hardly influenced by excavating depth, but changing with the depth of the wall.(7) After dewatering, above 18m of the pit, earth pressure reduced and wall deformation reversed. Below that point, earth pressure reduced and wall deformation increased. The curve of the earth pressure and wall deformation could be changed from pattern of earth pressure reducing with wall deformation reversed (close to y axis), earth pressure quickly down but deformation unchanging (parallel to y axis), earth pressure decreasing with deformation developing (away from y axis) with depth increase.(8) A formula expressing the relationship between coefficient of earth pressure & wall deformation and wall depth was proposed to calculate earth pressure induced by wall deformation.(9) Internal force of diaphragm under above four earth pressure mode was calculated and gained some points, such as: Be careful of subsection setting steel in design, steel percentage setting in the wall should augmented because of complicated wall deformation under undulance increase mode earth pressure. At the author's point it should be increased 20% for coping with the abnormity. (10) Numerical method was used to simulate interaction between retaining structure and soil considering four factors and five levels in the process of excavating.The facts were found as follow:â‘ The safety of supporting system relied on the corresponding of retaining structure to soil strength. The more strength and stiffness of the soil, the less stiffness of retaining structure could be. But if the soil was soft, there should be great stiffness of the retaining structure for soil mass stabilization in deep excavation.â‘¡Wall deformation tended to small with the thickness of wall increase. Wall deformation could decreased rapidly when wall thickness less than 1.2m. But Wall deformation decreased indistinctively when wall thickness great or equal 1.2m. Their might be a critical wall thickness value. When wall thickness large than that value, the increae of the wall thickness did little effect to wall deformation.â‘¢Wall deformation decreased with the strengthening of soil.â‘£Support space, support section size influenced the wall deformation not very clearly.The conclusions of this thesis were gained from specifically deep excavation condition of Runyang great bridge foundation and would have important guidance for similar project, and have academic value in retaining structure and soil interaction study.