Settlement Caused by Basement Excavation of Adjacent Development

For very soft clay, the zone of influence of the excavation is known to extend to four times the maximum depth of excavation. However, when excavation is carried out in unfavorable soil conditions such as when the soft clay is underlain by sandy soil, the zone of influence of the excavation can extend to 11 times the maximum depth of excavation as demonstrated by a case history of basement excavation in very soft marine clay underlain by very dense silty sand in Singapore. The temporary earth retaining support system for the basement excavation along the perimeters of the basement was a row of long sheet piles with three levels of struts. The maximum excavation depth was 11.5 m. Settlements as large as 2.6% of the excavation depth occurred at 11 times the excavation depth from the edge of excavation. The cause of such excessive settlements was investigated. Many practicing engineers may not be aware of such risks. Lessons learned from this case history are discussed particularly with respect to precautionary measures that can be taken to eliminate or minimize the settlements of existing adjacent developments that are located further than four times the maximum excavation depth.