Back Issues 11-20

MAJOR CAUSE OF SUBSIDENCE OF ADJACENT BUILDINGS DURING FOUNDATION EXCAVATION

Tse-Shan Hsu

President, Institute of Mitigation for Earthquake Shear Banding Disasters

Professor, Feng-Chia University, Taiwan, R.O.C., This email address is being protected from spambots. You need JavaScript enabled to view it.

Yan-Ming Wang

Director, Institute of Mitigation for Earthquake Shear Banding Disasters, Taiwan, R.O.C.

Yu-Chien Wu

Ph.D. Student, Ph.D. Program for Infrastructure Planning and Engineering, 

Feng-Chia University, Taiwan, R.O.C.

Zong-Lin Wu

Assistant Professor, National Chin-Yi University of Technology, Taiwan, R.O.C.

Abstract

In the past, technicians have used Terzaghi's safety factor formula to calculate resistance to piping failure in the design of retaining walls for adjacent buildings within excavation areas and to evaluate the collapse potential due to piping failure. However, during foundation excavation, retaining walls that are assessed as not susceptible to piping failure do experience such failure. In view of this, this paper presents a literature review and a case study involving two distinct piping failure mechanisms to identify the reason for the collapse of adjacent buildings due to piping failure in retaining walls during foundation excavation. The findings reveal that: (1) during foundation excavation, piping failure exclusively occurs within shear bands; (2) piping is inherently a non-steady-state mass-non-conserving pipe flow, rather than a steady-state mass-conserving seepage; (3) when piping occurs, groundwater sequentially transports soils including silt, sand, and gravel, spurting out towards the excavation surface through outlets formed by the pore spaces in shear bands; (4) after the failure of the retaining wall due to piping, a piping hole appears in the back of the wall, leading to notable subsidence of buildings above the location of the piping hole. Based on the findings, the authors suggest that the design of retaining walls for foundation excavation should incorporate an unsteady-state mass-non-conserving piping mechanism. This approach will enable technicians to accurately assess the piping failure potential of retaining walls, thereby ensuring the safety of both the retaining walls and adjacent buildings during foundation excavation.

Keywords: retaining wall, piping failure, shear banding, foundation excavation.