ABSTRACT
When assessing the feasibility of underground construction works impact assessment is usually undertaken in the first instance employing empirical means for the determination of excavation-induced ground movements as part of the widely-accepted 3-stage procedure for potential impact assessment. Ground movements generated by the excavation of boxes and shafts are predicted using an empirical approach validated by data from case studies of similar excavations. Vertical and horizontal ground movements are estimated on the basis of conservative greenfield assumptions. For box excavations, ground movement depends on the depth of the excavation and support system stiffness. The values for settlement at the wall and the extent of the settlement trough are expressed as functions of the support system stiffness and the excavation depth. One of the key limitations of the current approaches is the estimation of ground movements around the corners of box excavations. The increased stiffness evident at the corners of box structures should result in reduced excavation-induced ground movement behind the embedded retaining walls in these zones; field measurements support this assertion, the observations of wall deformations and ground movements at the corners of box structures being lower than elsewhere within the footprint of the box. However, this beneficial effect is largely ignored in the initial phases of the impact assessment process; the assumption that the ground movements are constant around corners is commonly made. A methodology for incorporating the reduction in ground movement evident behind the corners of box structures is presented in this paper. The use of this methodology in conjunction with other complementary empirical methods allows for more informed decision-making and early discussion with affected Third Parties whose approval will be sought as part of project implementation.
