Finite element analyses of steep man-made cuts in Dublin boulder clay

Abstract

This paper describes finite element analyses undertaken to gain an insight into the short-term behaviour of temporary steep slopes in Dublin boulder clay for cut and cover sections of the recently completed Dublin Port Tunnel. The analyses of two steep slopes are reported: (i) an unsupported slope for a full-scale trial excavation and (ii) a partly nailed slope for a prototype cut and cover structure. It has been found that a major contribution to the stability of these cuts is an increase in the effective stress (and thus strength) due to both (i) depressed pore-water pressures (suctions) and (ii) a tendency of the Dublin boulder clay to dilate during undrained excavation in this low permeability and very dense material. However, the presence of even thin, discontinuous but recharged granular lenses of a higher permeability significantly reduces the time to failure.