Case Record of a Strut-free Excavation with Buttress Walls in Soft Soil

Abstract

This paper presents a well-documented strut-free excavation case with diaphragm walls, buttress walls, and partial floor slabs as the strut-free retaining system. The final excavation level was 9.2 m, covering an area about 17340 m2. Because of its large excavation geometry, this project utilized the strut-free retaining system to reduce the construction cost and period. The soil layers above the final excavation level are dominated by the soft to medium clay layer (SPT = 2–7) and the loose silty sand layer (SPT = 2–6). According to the interpretation of field monitoring results, the following significant findings were drawn, such as (1) the measured maximum wall deflection was similar compared with strutted excavation case histories in the Taipei area. The maximum wall deflections to final excavation level ratio (dhmax/He) were between 0.27 and 0.55; (2) at the long-side of diaphragm wall, the pattern of the wall deflections is a cantilever shape with a translational movement at the wall toe and the location of maximum wall deflection was near the top of the wall; (3) at the short-side of diaphragm wall, the pattern of the wall deflections when reaching the final excavation level was a curvature shape and the location of maximum wall deflection was slightly lower than the final excavation level; (4) The maximum ground surface settlements to final excavation level ratio were below dvmax/ He = 0.3%. Although it was quite small, the ground surface settlements extend to the significant distance behind the diaphragm wall; (5) the strut-free retaining system was proven successful to retain soil during excavation.