Underground space in dry sand and effect of dredging on diaphragm wall using helical anchors − A model study

Abstract

Deep excavation has become inevitable for creating underground spaces due to the restricted space in urban areas. The diaphragm wall is an effective solution for restricting the lateral movement of soil and providing a sequence of stable excavation. The anchors in the backfill hold the diaphragm wall at its position and restrict its lateral movement and subsidence during excavation as well as serviceable time. The present research work has developed a scaled-down diaphragm wall with helical anchors based on past case histories of the deep excavations. Parameters such as offset distances (distance of the loading from the edge of the wall), anchor inclinations, and anchor lengths are varied. The footing offsets x/H = 0.1, 0.2, 0.3, and 0.4 (x/H is the ratio of horizontal distance between the edge of footing and wall to the height of the wall), anchor inclinations from 5° to 20° (with horizontal) and anchor lengths of 0.6H and 0.9H have been tried in model tests. Comparative analyses of anchored diaphragm wall versus unanchored diaphragm wall have been made based on the responses imparted by anchors. Thereby, an optimized design of anchors has been developed. It was observed that increasing the anchor inclination up to 15° relative to the horizontal reduced the pressure on the wall, lateral displacement, and subsidence near the wall. However, inclinations beyond 15° reduced the stability of the diaphragm wall. Additionally, the unanchored diaphragm wall experienced higher pressure than the anchored wall. The excavation depth was less when the offset distance of loading was closer to the wall due to excessive lateral pressure. The pressure on the anchored wall was reduced by 37 % when the offset distance increased from 0.1H to 0.4H. An increase in the length of the anchor from 0.6H to 0.9H caused a reduction in pressure on the wall by 31 %. The model tests suggest that anchors in the backfill at 15° inclination and length of 0.9H exhibit the highest reduction of pressure on the diaphragm wall.