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Technical paper: Tunnel interface crossing solved by specialised consolidation - use of sophisticated chemical grout system

By M G Brain, Fondedile Foundations

This paper was first published in GE’s September 1972 issue

Introduction

Extensive grouting was required during the driving of the cooling water tunnel for the outlet system for the Hunterston nuclear power station in Scotland. This work, which proved more extensive than originally planned, was undertaken by Fondedile Foundations of London. The contract to build the Hunterston power station was let to the Thermo Nuclear Power Group which subsequently entrusted the civil work to one of the partners of this group, Sir Robert McAlpine and Sons.

The outlet tunnel, sea leg, comprised the main offshore works for the power station cooling water system, this tunnel being driven through rock into clay terminating beneath the Firth of Clyde, as shown in fig 1, and on the site plan and aerial view opposite. A strata of sandy clay, gravel and small boulders ran diagonally across the axis of the tunnel for a length of approximately 12.2m (40 ft).

This band became known as the “interface”. The clay matrix content suggested that this type of ground cou&.be regarded as impervious from the practical aspect of tunnelling operations, but during the tunnel excavation two roof collapses had occurred followed by ingress of sand and water. Small diameter drillings through the ex-isting rock cover revealed fine sand and water to be present and overlying the rock surface which freely relieved itself into the tunnel when pierced.

Further sedimentation tests showed the clay content to be small, so from these observations it became clear that the overlayer would readily accept pressurised low viscosity chemical grouting. 

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