Ground Heave Due to Jet Grouting Near an Existing Structure

Renovations of the MBTA Copley Station in Boston included construction of a new elevator shaft to improve disabled access to the existing Green Line station. The site is immediately adjacent to the Eastern façade of the historic Old South Church. The construction work required excavation support including a perimeter secant pile wall and a jet-grouted base plug. Significant ground and structural movements were observed during jet grouting, mainly associated with soil displacements during grout injection. A three dimensional numerical model was developed, using the Plaxis 3D Foundation™ program, in order to test the hypothesis that the observed movements of the structure could be associated with the installation of the jet grout piles. The amount of volume expansion associated with installation of jet grout piles is estimated based by calibrating the model to measured ground movements. The finite element model results give a consistent explanation for the observed pattern of movements, including the heave of the church wall and lateral displacements at inclinometers located within the vicinity of the structure, measured at the time when damage occurred. The model assumes there is a vertical line of weakness in the masonry, representative of a pre-existing structural crack, as observed by structural investigations; and hence, confirms the underlying mechanical hypothesis for the source of ground movements.