Engineers Australia

Geotechnical History And Rehabilitation of the Rhyndaston Rail Tunnel, Tasmania

Peter B Hills

Abstract

Rhyndaston Tunnel is located 40 km (73 km by rail) north of Hobart in southern Tasmania. It was the first and remains the longest rail tunnel excavated in Tasmania and in 2021 was the only one still in operation. It was excavated by traditional drill and blast in sandstone between 1873-1876. The 955 m tunnel was recognised as having a small, if not limiting, profile soon after operations began. The advent of containerised freight in the late 1950’s and its rapid adoption across the globe in the 1960’s meant that the tunnel would soon become redundant and a viable and cost-effective alternative was sought.

At about the same time, the Hydro Electric Commission of Tasmania chose emerging tunnel boring technology for the excavation of the Headrace and Tailrace Tunnels for the Poatina Power Station 80 km to the northwest. This was to be the first use of tunnel boring technology in Australia, and it also provided a practical option to enlarge the Rhyndaston Tunnel while maintaining the majority of services on the Main Line between Hobart and Launceston. Consequently, after the completion of tunnel boring activities at Poatina in 1963, the tunnel boring machine (TBM) was transported to Rhyndaston and reamed the Rhyndaston Tunnel in 1964-1965.

The original Rhyndaston Tunnel was largely unlined and unsupported and when, after almost ninety years of operation, it was enlarged, it remained largely unlined and unsupported. A further 50 years on, and an extreme rainfall and extensive flooding across Tasmania saw the Rhyndaston Tunnel on the list of the States many civil assets to sustain damage. The extent of that damage was small, but in the course of remediation, the opportunity was taken to repair and upgrade a number of identified defects with a view to ensuring the Tunnel’s long-term security.

The paper describes the geotechnical history, enlargement and recent remediation and upgrading of the Rhyndaston Tunnel. All photographs are from the period 2016 – 2020 unless stated otherwise.