DEEP ROTATIONAL STABILITY OF TIEDBACK EXCAVATIONS IN CLAY

This paper addresses the base stability of excavations in deep, relati vely weak deposits of clay that are supported by earth-anchored tiebac ks. Under these conditions there may be the potential for a circular s liding surface to propagate outside the anchor bond zone and daylight in the basal portion of the cut. In this paper the analysis of deep ro tational stability is described, whereby limiting equilibrium computat ions 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 as sessed by means of a case history involving an unstable excavation in Boston marine clay. The soil conditions and ground movement measuremen ts at the site are presented, followed by the results of limiting equi librium and finite-element analyses. Deep rotational stability is show n to be one of the most important failure mechanisms for tiedback exca vations in relatively weak clay, and the dimensionless charts presente d in this work are shown to provide a good estimate of the stability d ifficulties experienced at the case history site.