Anchorage Performance And Reinforcement Effect of Fully Grouted Rockbolts On Rock Excavations

Le mechanism de l'ancrage des boulons est elucide par une etudie experimentale et une representation numerique est propose dans les cas les boulons scelles. Ce modele est applique en vue de simuler les boulons lors des analyses de tests de traction et de 1'effet de renfort sur piliers de roche discontinue et un tunnel creuse dans une formation faible. Les resultats des analyses sont compares avec les donnaes experimentales. Les analyses ont demontre que le modele propose est capable de simuler les boulons et permettent l'evaluation quantitative leur effet de renfort.

Ankernmechanismen von Felsankern werden anhand eines experimentellen Programmes geklart und eine numerische Reprasentation von eingeklenten Felsankern in vorgeschlagen. Das Modell wird dann angewendet auf die Simulation von Felsankern in Zugversuchen, auf die Simulation der Verstarkungseffekte von Felsankern in diskontinuierlichen Felssaul en und in einem Tunnel in einer weichen Felsformation. Die Resultate der Analyse werden mit tatsachlichen Messungen verglichen. Die Analyse hat gezeigt, dass das vorgeschlagene Modell in der Lage ist, die Felsankern zu simulieren und einen in die Lage versetzt, die Verstarkung durch Felsankern quantitativ auszuwerten.

Rockbolts have become one of the widely used supporting members in geotechnical engineering practices in recent years. This probably results from the easiness of their transportation, storage and installation of and quickly developing reinforcement effects as compared with other conventional supporting members such as steel ribs with wooden lagging. This has initiated numerous theoretical, numerical and experimental researches all over the world in order to enlight and evaluate the qualitatively well known superior reinforcement of bolts in engineering practices for the optimum design of support systems for geotechnical engineering structures in quantitative terms. These research works on rockbolts can be broadly classified into two main groups: 1. works on their reinforcement effects, and 2. works on their pull-out and shear resistances. According to these works, the reinforcement effects of bolts are grouped into the followings:

1. Suspension effect,

2. Beam building effect,

3. Arch action,

4. Radial confinement, and

5. Utilising the inherent structural properties of the surrounding rock depending upon the location and ground conditions in which they are installed.

When the reinforcement effects of bolts are considered, there is a tendency to associate that with the tensile and/or shear strength of steel bar with the condition, that is, bolts have a sufficiently long anchorage. On the other hand, the works on pull-out resistance of bolts mainly involve the effect of factors such as the kind of host rock, bolt-borehole diameter ratio, surface roughness of steel bar and/or hole walls, and curing time of resin or cement based mortar, and others. These works have clarified some of several important aspects of bolts qualitatively and quantitatively up to some extent. Nevertheless, the uncertainty still exists on the role of rock bolts as a permanent supporting member for rock excavations as most of these works involved with the resultant effects of the bolts rather than the fundamental ones. It is the opinion of the authors of this paper that if any mathematical model incorporates the axial and shear behaviour of steel bars with due considerations of behaviour of grout annulus and interfaces between grout and steel bar, and grout and rock, it will be possible to evaluate the reinforcement effect of bolts in quantitative terms and remove the uncertainty on the permanency of bolts as a supporting member in securing the stability of rock excavations. Though it may be possible to obtain some closed form solutions for anchorage capacity and reinforcement effects of bolts (Aydan et al 1985, 1986), a general modelling of rockbolts for rock engineering structures with complex geometry and material behaviour will require a numerical model such as a finite element representation. Among all the finite element representations, there is only one representation proposed by John and Van Dillen (1983) which deserves mentioning herein. This representation has attempted to incorporate the behaviour of grout annulus as well as the axial and shear behaviour of steel bar. However, it fell short of considering the most fundamental behaviour of interfaces which is felt to be the most important element on overall anchorage behaviour of the bolts. In this paper, experimental and theoretical studies on the anchorage performance of grouted rock bolts are described and a numerical representation has been proposed.