Cutoff Mechanism Of Retaining Structure On Groudnwater Seepage During Dewatering Of Excavation In Soft Deposit

When deep excavation is conducted in soft deposit of coastal areas, in order to keep safety, groundwater is pumped both from phreatic aquifer and confined aquifer to lower the groundwater head. During excavation, retaining structure can be used not only to retain the soft soil but also to cut off the groundwater seepage from surroundings. Therefore, transient seepage of groundwater into and around retaining structure will be formed. To investigate the cutoff mechanism of seepage into and around retaining structure during dewatering in excavating pit, laboratory and numerical simulation are conducted. Countermeasures are proposed to reduce the impact on surroundings due to groundwater pumping during excavation. Based on the research work of this thesis, following findings are obtained:(1) Experimental investigation on the effect of insertion depth and width of retaining wall on seepage behavior: Experimental investigation on cutoff behavior of retaining structure was conducted in a rectangular sand tank in laboratory. In the tests, the insertion depth (D) and width (B) of cutoff wall were changed. For each case, water level and flow rate were measured. Test results show that drawdown of groundwater head decreases and flow rate increases with the decrease of the insertion depth of cutoff wall, whereas drawdown of groundwater head increases and flow rate decreases with the increase of the width of cutoff wall. The difference of groundwater level between upstream and downstream of cutoff wall (Δh9-10) increases dramatically when D/H (H=thickness of sand layer) is over 70% and B/W (W=width of sand layer) is over 90%.(2) Numerical investigation on effects of insertion depth of retaining wall on behavior of groundwater seepage and surface settlement: Finite element method (FEM) is employed to analyze the cutoff mechanism of retaining wall on characteristics of seepage and settlement induced by dewatering. Numerical simulation shows that groundwater drawdown and settlement outside of the pit decrease with increase of the insertion depth of the cutoff wall in the confined aquifer. FEM results show that both drawdown of groundwater head and settlement of surrounding ground decrease with increase of the insertion depth of the cutoff wall outside of the foundation pit. The difference of groundwater level between outside and inside of the foundation pit increases dramatically when cutoff wall blocks 70% of the depth of the confined aquifer. Further studies are carried out with variation of hydraulic conductivities and specific storage of the aquifers.(3) Countermeasures to mitigate impact on surroundings during dewatering: A new dewatering technology is proposed, which is called Unified Dewatering Technology for Pumping Groundwater both from Phreatic Aquifer and Confined Aquifer from one well (UDTPPCA). In UDTPPCA, a valve is assembled in the well pipe at the depth of aquitard to shut off water from the confined aquifer during pumping of phreatic water. Field test was carried out to confirm the effectiveness of the proposed UDTPPCA technology. The test result shows that the influential region of the groundwater drawdown is negligible.(4) Verification from engineering practice: The behavior of three field cases from different geological formations and hydro-geological properties was collected to verify the aforementioned research result. Field data and numerical analysis confirmed that the groundwater head outside of the pit is much smaller than groundwater level inside of the pit while the relative insertion depth of cutoff wall is more than 70% in confined aquifer. When pumping wells are allocated in the foundation pit, in case of low hydraulic conductivity of the aquifer, cutoff wall entering into more than 70% of the depth of the aquifer is acceptable. However, in case of high hydraulic conductivity of the aquifer, e.g. k>1 m/d, it is suggested that cutoff wall penetrates into the aquitard below the confined aquifer.