In-situ monitoring of earth pressures upon large shield tunnel with small overburden

摘要

The earth pressure on shield tunnel is an old but important issue in tunnel design. It is wellrnknown that the earth pressure is depending on the soil type and construction conditions. There are increasingrnneeds for short but large traffic tunnel in the area with many rivers, such as theYangtze River delta of China. Thernoverburden of the short cross-river tunnel (normally less than 1 km) is a crisis issue for tunnel design, especiallyrnin silty sand ground. However, few literatures can be found on this topic. In this study, an in-situ monitoringrnwas carried out to measure the earth pressure and pore-water pressure acting upon a large cross-river shieldrntunnel with small overburden (7 m). A PAD type earth pressure gauge was used to measure the earth pressure.rnThe distribution and the time history of earth pressures and the distribution of pore-water pressures on segmentsrnare obtained. A comparison between the measured values and theoretical values calculated by the Japanesernconventional design method is carried out. It is showed that the pore-water pressure accounts for a large portionrnin the total earth pressure in silty sand ground. Tail brush pressure and backfill grouting pressure had a largerneffect on the earth pressures during shield machine passing through. Excessive grouting pressure resulted in thernmeasured earth pressures much higher than the design values. However, the measured earth pressure droppedrndown a lot after several rings, and the final values were close to the total overburden pressure.