Deep Rotational Stability of Tiedback Excavations in Clay

Abstract

This paper addresses the base stability of excavations in deep, relatively weak deposits of clay that are supported by earth-anchored tiebacks. Under these conditions there may be the potential for a circular sliding surface to propagate outside the anchor bond zone and daylight in the basal portion of the cut. In this paper the analysis of deep rotational stability is described, whereby limiting equilibrium computations are performed for circular sliding surfaces. Dimensionless charts are presented to estimate safety factors and identify the centers of rotation of critical slip circles. The suitability of the method is assessed by means of a case history involving an unstable excavation in Boston marine clay. The soil conditions and ground movement measurements at the site are presented, followed by the results of limiting equilibrium and finite-element analyses. Deep rotational stability is shown to be one of the most important failure mechanisms for tiedback excavations in relatively weak clay, and the dimensionless charts presented in this work are shown to provide a good estimate of the stability difficulties experienced at the case history site.