Abstract

This paper describes the hydrogeological processes that caused unexpected instability and quick conditions during the excavation of a 25 m deep cutting through a drumlin in County Down, Northern Ireland. A conceptual hydrogeological model of the cutting, based on pore pressures monitored during and after the excavation, demonstrates how quick conditions at the toe of the cutting caused liquefaction of the till. Stability of the cutting was re-established by draining the highly permeable, weathered greywacke that underlies the drumlin, through the use of a deep toe drain. In spite of this drainage, the cutting was only marginally stable, owing to the presence of a low-permeability zone in the till above the bedrock, which limits the reduction of elevated pore pressures within the upper to mid-depths of the drumlin. The factor of safety has been further improved by the addition of vertical relief drains at the crest and berm of the cutting to relieve the pore pressures within the upper till by intercepting the weathered bedrock. The paper also highlights the importance of carrying out an adequate site investigation compliant with Eurocode 7 and additional monitoring in excavations in stiff, low-permeability till.

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