The A55 North Wales Coast Road between Chester and Holyhead forms a strategic part of Euroroute E22 (Sassnitz to Dublin) and provides a major artery to the British and European motorway network for traffic arising not only from North Wales but also from Ireland via the international ferry terminal at Holyhead (Fig. 1). The highway comprises 145km of modern dual carriageway. Unfortunately rockfall activity poses a significant geohazard in the vicinity of the mountainous headland at Pen-y-Clip and especially affects the westbound carriageway approaches to the road tunnel through the headland (Fig. 2). This area has a history of high rockfall frequency and, in 1984, a rock avalanche blocked the headland road and caused a two week closure (McWilliam 1991). At Pen-y-Clip, steep slopes and adverse geological factors combine to create potentially unsafe ground above the road.

The Pen-y-Clip promontory constitutes a natural obstacle along the coastal plain and routes tend to converge towards the seaward point of the headland. The Pen-y-Clip road tunnel, opened to traffic in October 1993, was driven through the headland and a wide variety of rockfall protection measures including rockfall checkfences were incorporated into the design and construction of the project (Brady et al. 1994). The checkfences were installed in advance of construction to protect the workers on the scheme and to safeguard the headland road particularly against rockfall initiated by blasting during construction.

Geologically, the headland at Pen-y-Clip comprises a core of igneous rocks associated with the Penmaenmawr intrusion (Fig. 3). The intrusion measures

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