Study On Stability Control Of Foundation Pit And Adjacent Existing Stations On Subway Hub During Construction In Marine Silt Ground Layers

The two key problems, one is how to grasp the deformation features and control the stability of silt ground layer, the other is the interaction among several adjacent engineering and the stability control, are often encountered in underground engineering of large transportation hub in coastal urban areas like Shenzhen. This paper is based on deep excavation engineering of QianHaiwan station in Shenzhen Metro Line 5, taking adjacent foundation pit engineering, the existing station of Metro Line 1 and planned station of Metro Line 11, into consideration as characteristic boundary conditions. According to the hydrogeology geological conditions, engineering geological conditions and environmental characteristics, researches were conducted mainly focused on the four key issues:firstly, the characteristics and mechanical response of marine silt ground layer; secondly, the spatio-temporal effects of deep excavation engineering in marine silt ground layer; thirdly, the interaction law among several adjacent engineerings; finally, stability control techniques for stratum and engineering structure, including information-based construction technology. The deformation law of all-around strata and engineering structure during approaching construction for several foundation pits in sea sludge layer was clarified, and the key parameters and construction procedures were optimized. Moreover, the key technique problems in both design and construction for adjacement foundation pit were solved, which ensured the safety and stability of surrounding environment and station structure during construction.Through the researches, the main results are as follows:(1) The marine silt ground layers in Qianhai region of Shenzhen has significantly mechanical properties than the normal formation. The mud formation outside the foundation pit was sensitive to the first excavation disturbance, and the plastic zone begins far earlier than the normal formation. During the foundation pit excavation of sludge layer, the pile displacement was generally greater than ordinary formation, especially more than 15 meters deep pile, which maximal displacement of the pile and displacement of pile tip are respectively 40-50 percent and 100-300 percent larger than normal formation. The surface subsidence curve around the sludge formation and ordinary formation obeys "groove" distribution, but the maximum subsidence value was 100～300 percent larger than normal formation.(2) By the analysis on time-space effect of deep foundation pit excavation in the marine silt ground layers, the optimized key parameters of excavation are provided. When the longitudinal section excavation length of foundation pit in QianHaiwan station was within 10 meters and stage excavation thickness was about 2 meters, the better control effect in the displacement of retaining pile was achieved. Excavation speed had the greatest influence in pile deformation, surface subsidence, uplift, strut axial forces of excavation step 1 and step 2, and have little impact on the excavation step 3 and 4. Pile top displacement, the maximal displacement of piles was double exponential decreased with excavation speed, while the maximum ground settlement was negatively linear correlated with excavation speed.(3) The interaction law between the new subway station and the existing adjacement station in the marine silt ground layers during the construction had been obtained in the study. The travel path of soil displacement caused by the excavation of new station foundation pit changed because of the existing station. Surface subsidence and the horizontal displacement of piles close to the existing station were lower than those on the other side of foundation pit. Besides, it was found that the new foundation pit excavation in marine sludge layer made the existing station a certain deflection, which had not affected the overall security. Specifically, the maximum horizontal displacement was 0.9 mm, and the maximum vertical displacement was 2 mm, which were both far less than the allowable values of 36 mm.(4) The law how the planned adjacement foundation pit excavation affects the new station in the marine silt ground layers was researched. The deeper the foundation pit of new station excavates, and the closer it laid to the new station, the lager the disturbance occurs. It was proved that using diaphragm wall as the enclosure structure of the planned foundation pit can effectively reduce the displacement of new station caused by excavation. Compared with the retaining piles, it can reduce the horizontal displacement of 3 mm. However, the depth of enclosure structure can hardly influence the displacement of the new station. Moreover, the uplift pile of planned foundation pit excavation played a significant role in controlling the stability of the new station, which can reduce the horizontal displacement of 2.5 mm compared without the uplift pile.(5) Reinforcement scheme, which mainly about limiting silt with cement mixing piles that distributed as gird, has been put forward and implemented, aimed at solving the potential threat problems in the excavation of foundation pit such as the large loading on nearby roads, the high compressibility and rheology of marine silt. Creep of silt layer proved to have been limited effectively. The negative effect on enclosure structure caused by the overload on surface was controlled by the load reduction in formation.The research achievements of this paper may contribute to enriching the experiences in support designs, construction methods and techniques of deep foundation pit, the complex engineering conditions of adjacement engineerings in coastal urban areas with silt ground layer, and provide a valuable references for the similar projects in the future.