Sequential analysis of ground movements at three deep excavation sites with mixed ground profiles

Field measurements of settlement and lateral deformation obtained from three deep excavation sites constructed in mixed ground profiles are presented and analyzed. Settlement measurements were obtained throughout the construction process, categorized in three stages as: (1) preexcavation (i.e., preliminary site work and support wall installation); (2) main excavation and bracing/anchor installation; and (3) postexcavation (i.e., removal of bracing as basement construction proceeds). Maximum preexcavation stage settlements of 0.03%H_w to 0.06%H_w (where H_w=wall or trench depth) were measured at two sites, with the maximum settlements occurring adjacent to the wall during its installation. Maximum ground surface settlements during the main excavation stage ranged from about 0.15%H_e to 0.30%H_e (where H_e=final excavation depth) and the distribution of ground settlement extended to a distance of 1.5H_e to 2.0H_e from the wall. Maximum settlements occurred at distances of about 0.3H_e to 0.5H_e from the wall at two sites where the wall consisted of concrete cast in situ (concrete diaphragm and concrete secant pile walls), creating a significant reverse curvature in the settlement distribution. The maximum postexcavation stage settlements ranged from 0.07%H_e to 0.10%H_e for the three sites, representing roughly 10 to 60% increases in settlement over the main excavation settlements, depending greatly on the specific support removal methods as well as the basement floor construction details employed at an individual site. Lateral deflections during the main excavation stage were consistent with trends reported in the literature, ranging from 0.12%H_e to 0.23%H_e, while lateral movement during postexcavation stage ranged from 0.03%H_e to 0.09%H_e. Finally, the settlements measured during the main and postexcavation stages are related to the support system stiffness.