Influence of Imbalance of Earth-pressure on Wall Deflections on a Deep Excavation

Abstract

An undersea vehicular tunnel was constructed by using the cut-and-cover method. The cofferdam supporting the excavation of 35 m maximum in depth and 50 m in width comprised intermittent pipe pile walls of 50 m maximum in depth. The struts were preloaded to the magnitudes as large as 16.4 MN. Due to the difference in the depths to the rockhead levels on both sides of the cofferdam, there is the earth-pressure imbalance effect causing differential lateral wall deflections. The wall on the shallower rockhead side was pushed back by the wall on the opposite side with deeper rockhead levels through the struts. The wall deflections were as large as 194 mm on one side and 22 mm on the opposite side. As a result, the central axis of the excavation trench shifted by as large as 156 mm. The magnitude of shifting of the central axis is proportional to the ratio of the depths to the rockhead on the two sides of the excavation.