Abstract
Fibre reinforced shotcrete (sprayed concrete) is often used to support tunnels in hard rock. The shotcrete should be designed to carry the load from a loose block, which is assumed to be transferred to the surrounding rock mass over a narrow band. It is commonly accepted that the width of this band is constant, and independent of the shotcrete thickness. In this paper, numerical simulations are used to show that the width of this band and the load capacity is strongly correlated to the mean shotcrete thickness around the perimeter of the block. Furthermore, it is also shown that a strong linear correlation exists between the mean bond strength and the rock support capacity. Based on this, a lower bound function to estimate the structural capacity for the lining is suggested. No correlation was found between the structural capacity and the minimum or maximum shotcrete thickness or bond strength. This indicates that, under certain conditions, local areas with low thickness and bond strength can exist around the block perimeter without having any significant effect on the rock support capacity.
Highlights
The main design principle of tunnels excavated in hard and jointed rock of good quality, is that the rock mass should be able to support its own weight [1]
This paper aims to investigate how local variations in shotcrete thickness and bond strength along the interface between shotcrete and hard rock affect the structural capacity when the shotcrete is subjected to the load from a loose block
Two case studies were presented to investigate how the structural capacity for the shotcrete lining subjected to the load from a loose block is affected by (i) local variations in bond strength and (ii) local variations in shotcrete thickness
Summary
The main design principle of tunnels excavated in hard and jointed rock of good quality, is that the rock mass should be able to support its own weight [1]. Depends on the bond strength between shotcrete and rock. If the bond strength is insufficient, the shotcrete debonds and the load from the block should be transferred to the rock bolts through bending of the shotcrete. The theoretical knowledge regarding the width of the load transferring band between shotcrete and hard rock is limited, with only a few papers focusing on the stress transfer between shotcrete and rock, see e.g. Banton et al [4]. Most of the knowledge and design guidelines regarding the load transfer band δb between shotcrete and rock is based on two large-scale experimental studies, performed by Fernandez-Delgado et al [5] and Holmgren [6,7]. In the Swedish guidelines [8] and design code [9] for rock tunnels, the width of this band is limited to 30 mm, as suggested by Holmgren [6]
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