Cracking in Walls of a Building Adjacent to a Deep Excavation

A major concern for projects involving deep excavations in urban areas is the response of adjacent buildings and utilities to excavation-related ground movements. Unfortunately, a purely theoretical approach to estimating building response to excavation-related deformations is not possible due to the variability of the many factors that contribute to the response. Consequently, building response must be estimated and evaluated primarily based on empirical observations and various structural approximations. The goal of estimating and evaluating building response is to provide limiting criteria that will safeguard the structure against unacceptable damage. Thus, estimating the extent of the building response and consequently the severity of excavation-related building damage is critical to establishing rational limiting criteria for excavation support system designs. The most common measure of damage severity is the onset and growth of cracks in interior walls of adjacent structures. Although several procedures have been suggested for estimating excavation-related crack growth, all of the procedures have a common aspect in that they require the input of a critical strain, or the strain at the onset of cracking, as a critical input parameter. This paper presents the results of three-dimensional finite-element analyses of a building adjacent to an excavation. The analyses were used to evaluate the magnitude of strain that developed in the interior walls in response to the excavation-related ground movements. This paper describes the procedures used to model and analyze the building. The paper also presents computed building responses at dates corresponding to observations of cracking and presents discussions of strain levels in infill panel walls where cracking was observed and in panels where cracking was not observed. The analyses showed that the initial cracking observed in selected infill wall panels could not have occurred solely in response to excavation-related deformations. Consequently, it was found that the wall panels cracked as a result of a combination of strains induced in the structure from self-weight settlement and excavation-induced displacements at the supports. These analyses allowed the writers to suggest critical strain criteria.