Settlement trough parameters for tunnels in Irish glacial tills

Glacial till (or boulder clay) is the most widespread sediment on the island of Ireland. The behaviour of these tills, especially Dublin Boulder Clay, is now better understood as a result of ground investigations and associated testing for major construction projects, particularly in the east of the country over the past decade. Despite an increase in tunnelling activity in the country over the same time period, there is very little documented evidence on the settlements induced by tunnelling operations in glacial till. In this paper, transverse surface settlement data from two glacial till sites are presented. Four profiles are presented for the ‘soft ground’ TBM-bored section of the Dublin Port Tunnel. Nine profiles are reported for pipe-jacked microtunnels constructed for a sewerage scheme in Mullingar in the Midlands; one of which was measured at the top of a railway embankment under which the pipeline passes. The measured settlements have been interpreted using a standard Gaussian error function, and trough width parameters show dependence on the fraction of the till, fine or coarse, that governs its behaviour. In addition, conservative design estimates of maximum trough settlement and volume loss have been provided, and the impact of boulders is discussed. This paper provides the first empirical guidance for predicting surface settlements above tunnels in Irish soils.

► We have shown that the trough width parameter in till depends on the dominant fraction, fine or coarse. ► Trough width parameters for Irish tills are broadly in agreement with international experience. ► The presence of boulders has significant influence on trough settlements and volume losses. ► Empirical predictions of settlements and volume losses are proposed for fine and coarse tills. ► Time dependent settlements in gravel are believed to be related to the amount of bentonite used.