Responses of Shallowly Buried Pipelines to Adjacent Deep Excavations in Shanghai Soft Ground

Abstract

Analyses of well-documented field instrumentation data from two deep excavations in Shanghai soft ground indicate that displacements of shallowly buried pipelines caused by adjacent excavation are predominantly governed by the locations of the pipelines relative to the pits and are much less affected by pipeline flexural rigidity or soil bedding stiffness. On average, circular excavations produce much smaller pipeline settlement (uy<.16%He) than do rectangular excavations (uy≤.42–.46%He), where uy and He denote pipeline settlement and final excavation depth, respectively. Despite this discrepancy, uy of pipelines behind corners of rectangular pits behave similarly to those behind circular excavations in terms of both magnitude and settlement profile. For pipelines parallel to rectangular excavations, their uy exhibit obvious corner stiffening behavior, i.e., the uy near the pit middle span is as much as to 9 times the uy near the pit corner. Consistent with ground settlement, primary uy is observed at those pipelines buried within the zone approximately .5–1.5He behind the retaining wall for both circular and rectangular excavations. Compared with uy, the corresponding lateral pipeline displacement, ux, is much smaller, less than .16%He. Furthermore, the magnitudes of ux along pit sides are independent of the distance from the pit corners and more closely related to lateral displacements of adjacent retaining walls. Based upon regression analyses on amounts of field measurements, empirical formulas are developed for estimating adjacent excavation-caused ux and uy. Once the location of the pipeline relative to the pit, He, and pit geometry are known, both ux and uy corresponding to different intermediate excavation levels, H, can be reasonably predicted.