A Note On Rockbolt Design For Load Supporting Ledges In Hawkesbury Sandstone

The objective of much of the work done by the author and his co-workers over the past decade has been to take the guesswork out of assessing rockbolting for tunnel support (Pells, Best & Poulos, 1994). A particular issue in this regard arose in connection with the design of the ledges along the sides of the piggy-back tunnels of the Eastern Distributor in Sydney. These narrow ledges carry prestressed concrete bridge planks which span across the tunnel and support the upper (northbound) carriageway (see Photograph 1). Near vertical joints of the dominant NNE oriented joint set intersect the approximately N-S tunnel at oblique angles. These joints create one sided wedges (see Figure 1). Failure of such wedges must include fracture or shearing of intact sandstone. The joints are quite widely spaced meaning that there are considerable lengths of ledge comprising effectively intact sandstone. However, at joint locations rockbolts have to be installed to provide an appropriate factor of safety against bearing capacity failure.

This paper describes an approximate method which was developed to quantify rockbolt requirements. It is not an exact solution to the problem but because it is simply an extension of planar 2D sliding it is easy to be aware of the sensitivity of designs to the assumed input parameters.

The method could be used in similar design problems which arise in association with basement excavations in the Sydney area.