Narrows Marine Crossing – Gladstone

Project Type: Design & Construct

Owner: Santos GLNG

Construction Cost: AUD 134.4 Million

Contractor: Saipem Australia (principle contractor) / Thiess (sub consultant)

Client: Thiess

Design Period: 2012 – 2013

Construction Period: 2012 – 2014

 

The Project

The tunnel package of the multi-billion-dollar Santos GLNG Gas Transmission Pipeline Project includes the construction of a 4.35km tunnel under the Galdstone harbour, a section of water known as The Narrows (environmentally sensitive marine area with mangroves). The tunnel provides a vital link in the 420-kilometre underground pipeline route transporting coal seam natural gas from the Surat Basin connecting the mainland to the LNG Plant being constructed on Curtis Island.

The 3.4m internal diameter segmentally lined tunnel serves as a conduit for the gas transmission pipeline. For construction the bored tunnel a 70,000m² temporary launch site pad and a launch box structure on the mainland at Gladstone and a reception shaft on Curtis Island was built. The majority of the TBM alignment is located within the Tertiary Narrows Group sedimentary units, which comprises hard clays with some sand and gravels.

The inclined temporary launch structure has a length of approx. 250m and a depth of 11.8m to enable launching of the 100m long, 277t heavy TBM. Whereas, the retrieval shaft is 10.3m in depth and has a rectangular shape of 18m × 9.8m.

After completion of the tunnel works a bulkhead was constructed, at both ends of the tunnel, and filled with sea water to buoy the approx. 1m diameter pipe as it was pushed through.

Key Aspects

• Single bore utility tunnel
• Total tunnel length: approximately 4.35km
• Pre-cast concrete lining segments: 5 segments, 1 key, inner diameter 3.4m, 1.2m segment width, 200mm segment thickness
• Excavation by earth pressure balanced (EPB) shield TBM
• Geological profile comprises of marine alluvium, quaternary alluvium, tertiary Narrows group and Devonian Wandilla formation
• Launch box using sheet pile walls with struts and walers as well as anchors for the shallower sections
• Retrieval shaft using contiguous bored pile wall

Scope of Works

NOMA Consulting was engaged by Thiess to carry out the detailed design of the following design aspects:

• Design certification of segmental lining design
• Detailed design of launch frame, TBM cradles and launch seal
• Specification and detailing of instrumentation & monitoring requirements
• Detailed design of muck bin
• Detailed design of end plug for permanent closure of service tunnel
• Detailed design of backfill requirements
• Construction support to address construction related issues and design modifications