Since its establishment in 1970, SMEC has provided engineering services both within Australia and overseas, including ventilation and subsidence studies, for some 370km of large-diameter tunnels as well as a number of large underground chambers and approximately 4km of vertical and sloping shafts, adding to the extensive tunnelling experience of its parent organisation, the Snowy Mountains Hydro-electric Authority (SMHEA). The services provided have included prefeasibility and feasibility studies, detailed investigations, design, documentation and construction supervision.

Experience has been gained in both conventionally excavated and machine-bored tunnels in sandstone and other sedimentary rocks in railway tunnels for the Eastern Suburbs Railway in Sydney, Australia, and pressure tunnels for the Shoalhaven Scheme, a dual purpose water supply and pumped-storage project near Sydney, and from the many tunnels which have been designed for projects in overseas countries. Shield tunnelling combined with mechanical excavation in mixed ground and with precast segmental concrete lining and pipe jacking in sewer tunnels in Perth, Western Australia has further added to the diversification of SMEC's experience.

SMEC has also carried out the engineering investigations for the Perth Central City Railway feasibility study in Western Australia. Alternatives considered in this study included 6.1m diameter tunnels in sand and cut-and-cover structures constructed with various techniques.

The New Austrian Tunnelling Method has been used in the construction of the Bylong Tunnel which SMEC designed and supervised. This particularly economic and safe method of construction has been specified for tunnels such as Avon, Kaugel and the Perisher Skitube for which SMEC provided consultancy services.

Physical details of the more significant tunnels and underground structures designed by SMEC and SMHEA in Australia, and the many tunnels SMEC has designed (forming part of hydropower and multi-purpose schemes) in overseas countries, are presented in Appendix D.

VENTILATION

SMEC provides ventilation design services for all its tunnels and underground power station projects. SMEC staff have had considerable experience in the determination and procurement of ventilation plant for underground works. For instance, the ventilation studies for the Avon Tunnel, where diesel locomotives were involved, consideration was given to noxious gas dispersion and heat output (due to braking). SMEC uses the US Department of Transport Subway Environmental computer program for these studies.

SUBSIDENCE

In the design work undertaken by SMEC for the SRA on the proposed Cordeaux Tunnel the effects of both pre-construction and post-construction mining on the tunnel were investigated. Excavation difficulty and the necessity for extra support measures were assessed, as was damage to the tunnel structure and the effects on drainage and on track and train operation. In the case of the Cordeaux tunnel, SMEC made theoretical calculations of the curvatures and subsidence of the ground settlement curve due to extraction of coal, recommended restrictions on mining near the tunnel and made an estimate of the amount of coal which could not be extracted (sterilised) in order to protect the railway.

Tunnel alignment and profile were adjusted to minimise the amount of coal which would have to be sterilised. Vertical curvature of the Avon tunnel in NSW was also investigated by mathematical modelling of mining subsidence or extraction.

Other, non-tunnelling, experience in subsidence problems includes:

• Prediction of surface settlement for multi-storey unit development above mine workings, Newcastle, New South Wales.
• Assessment of stability of excavations in mine subsidence areas near Newcastle, New South Wales.

Further information regarding SMEC's tunnelling and underground engineering experience can be obtained from hywtr@ozemail.com.au