Calculation of inrushing stability of local pits in deep excavation

Inrushing may lead to a high construction cost or even failure of excavation, while local deep pit, including elevator shaft and collecting well, is the most dangerous place of inrushing in the deep excavation. Based on the assumption of general shear failure of soils between the pit bottom and the confined aquifer top, an equation is proposed to evaluate the inrushing stability of local deep pit, which can consider the effect of soil shear strength, plan size and reinforcement. Applicable conditions of the formula and the method to calculate the shear strength are also provided. Finally, the data of deep excavations along the Qiantang River in Hangzhou are analyzed. The following conclusions are drawn: (1) the shear strength has an increase of more than 30% in inrushing stability and an extra increase of about 20% in stability is achieved by reinforcement of pit bottom soils; (2) the possibility of inrushing decreases with the increase of the height of pit bottom soils and the ratio of pit circumference to pit area; (3) it can prevent inrushing by dividing a large excavation into lots of small ones.