Abstract:

It is of common occurrence that the flood scours the riverbed and the back-silting takes place immediately by turn, which inevitably influences the safety of shield tunnel through the Qiantang River. Hence, how to determine the thickness of covering layers reasonably is an urgent problem to be solved to ensure the safety of tunnel and the construction cost under control. According to the completed study that the riverbed thickness decreases extremely by nearly 16 m after the heaviest flood in 300 years, two embedding schemes of subway line are selected initially. That is, the minimum overburden layer thickness of tunnel is 3.5 m or 4.0 m. The results show that the two schemes are both safe for the tunnel considering the shearing effect of the soil around the tunnel and the one of the bolts between rings. However, in construction, the possibility of floating upward for shield tunnel increases greatly under the buoyant force action of back-filled grouting. More attention should be paid to construction management to minimize the disturbance of soil around the tunnel.

Key words: shield tunnel, minimum thickness of overburden layers, back-filled grouting, checking of resistance to floatation