A Case Study of Protection for a Building Adjacent to Deep Excavation in Saturated Sandy Deposits

Deep excavations in water-rich sandy strata are prone to waterproof failures or through-wall leaking accidents. The ground volume losses incurred by these accidents will severely threaten the nearby sensitive buildings if proper protective measures are not taken in advance. Hence, protecting the pre-existing buildings close to excavations is of paramount significance and needs to be carefully designed and implemented. Based on a project of deep excavation in Nantong, China, protective and contingency measures for a high-rise building (i.e., Nantong TV tower) adjacent to a 10.9–26.5 m-deep subway station comprised of 4 sub-excavations were investigated and discussed. Bored-cast-in-place (BCIP) piles along with high-pressure-jet-grouting (HPJG) piles were conducted to safeguard the tower as the original protective measures. However, when the excavation reached a depth of 13–16 m at pit No. 2, the tower experienced significant differential settlements, and its maximum settlement magnitude exceeded the alert level. To safeguard the tower, remedial countermeasures consisting of metro-jet-system (MJS) piles and double-row bi-slurry columns were carried out. Field monitoring data shows that the movements of the tower were controlled effectively and the related settlement rates also lessened significantly. The lessons yielded from this case are: (1) the isolation piles with great waterproof performance should be completed prior to construction of the project, (2) the scope of protective measures for nearby sensitive buildings should be delineated carefully, and (3) the initial design of protective schemes should be adjusted timely according to the feedback of in-situ monitoring data.