ABSTRACT:

The major findings of the field investigation component of a research programme investigating the dynamic response of rock anchorages are presented. The field work was conducted at the construction sites of three U.K. tunnels which employed rock anchorages as the primary mechanism of support. The effect of blast characteristics and prestress load on anchorage response in the time and frequency domain is considered, as is the dynamic stress distribution within the anchorages.

RESUME:

Les resultats de pratiques sur Ie terrain, d'un programme de recherche qui examine la reponse des ancrages de roc, sont presentes. Les travaux etaient entrepris à I'emplacement de trois tunnels en Grande Bretagne où on a utilise des ancrages de roc comme Ie moyen Ie plus important de soutien. L'effet des charaeteristiques du souftlage et de la force precontrainte sur la reponse de I'ancrage est èvalue, aussi bien que la repartition des contraintes.

ZUSAMMENFASSUNG:

Es werden die wichtigsten Ergebnisse des prakatischen Tells einer Untersuchung ueber das dynamische Verhalten von Felsankern vorgestellt. Die Feldarbeit wurde an drei britischen Tunnelbauvorhaben durchgefuehrt, bei denen Ferlsanker als primare Stuetzmechanismen verwendet wurden. Der Einfluβ von Explosions-Charakteristiken und Spannungsbeladung auf das Verhalten der Felsanker, sowie die dynamische Spannungsverteilung in den Ankern werden im Zeit- und Frequenzbereich betrachtet.

1
INTRODUCTION

In 1986 a joint research programme was devised by the Universities of Aberdeen and Bradford to examine the influence of transient dynamic loading on the performance of rock anchorages. The stimulus to embarking on this project was a recognised need by tunnel designers for the establishment of a rational design method for assessing the ‘safe distance’ that rock anchorages, in the form of rock bolts, could be placed to a blast face. In many hard rock tunnels, constructed by drill and blast methods, rock bolts are the primary form of support. Due to the scarcity of published research in this area, methods for assessing safe distances are currently very conservative. Design practice uses safe distances derived from precedent practice, together with trial blasting and a limiting dynamic parameter, normally peak particle velocity. If the rock bolts require to be placed closer to the blast face than the specified safe distance, the bolts are deemed to be temporary and are replaced after blasting. This leads to costly duplication of bolts which may be unnecessary. The research programme involved a combination of finite element simulations of the response of rock bolts to transient dynamic loading, large scale laboratory model tests to examine the stress distribution within dynamically loaded bolts, and full scale field investigations. The field investigations were centred around studies of rock bolt performance during the construction of two major road tunnels in North Wales, and anchorage performance close to blasting at an Edinburgh city centre site. The field research programme has been conducted to date in two phases. Phase I was carried out over the period 1986–89 and has been comprehensively reported elsewhere (Holland 1989, Littlejohn et al 1987,1989, Rodger et al 1989, Mothersille 1989), and involved a full scale field investigation of the dynamic response of resin bonded rock bolt systems during the construction of the Penmaenbach Tunnel, in North Wales. At this site, axial load and acceleration were measured at the head of resin bonded rock bolts, positioned at distances from 20m down to 0.7m from the blast face. Analysis of the data obtained from the Penmaenbach field trials revealed that resin bonded rock bolts were far more resilient to blasting than originally anticipated, with no significant load loss or resin/bolt debonding registered for any rock bolt, even when positioned only 0.7m from the blast face.

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