ABSTRACT:

The paper describes the performance of low to medium capacity rock anchorages when subjected to close proximity blasting during the construction of the Penmaenbach Tunnel in North Wales. The anchorages, installed in massive Rhyolite, proved resilient to routine construction blasting. Face blasts, carried out to within 2m of the 4m long fixed anchors, generated maximum residual load increases of 5% working load, which was below the ± 10% limit stipulated in UK practice. The behaviour of the anchorages can tentatively be described, within upper and lower bound limits, by a power decay law on a linear-linear scale.

INTRODUCTION

The Penmaenbach Tunnel was commissioned by the Welsh Office (UK) to provide a new carriageway for westbound traffic on the A55 North Wales coast road. It was driven by drilling and blasting through the Rhyolite volcanic rock which forms the Penmaenbach Headland, two miles to the west of Conway. The tunnel, 640m long, was constructed using a top heading and bench extraction method although further rock extraction to form service trenches, created multiple blast sources within the tunnel on occasions. The three vertical rock faces, which form the west portal of the Penmaenbach Tunnel, were stabilised by the installation of 42 low to medium capacity rock anchorages of working loads 200–600kN. To assess the long-term performance of the anchorages, the Welsh Office commissioned the joint consulting team (Travers Morgan & Partners and Howard Humphreys & Partners) to monitor the service behaviour of selected production anchorages throughout the duration of the Penmaenbach Tunnel contract. To improve understanding of the performance of rock anchorages in close proximity to blasting, a monitoring programme was developed to take advantage of the fact that routine production blasting was carried out to within 2m of the fixed anchors.

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