Corrib Gas Pipeline Project, Tunnel Start and Receptions Shafts

Cofferdams

AGL were the temporary works designer to the BAM Civil and Wayss & Freytag Joint Venture for construction of a 4.9 km long, 3.5m internal diameter tunnel under Sruwaddacon Bay in Co. Mayo. The project was subject to very strict planning conditions, which applied a unique set of constraints on the design and construction of the start and reception shafts.

The key features of the temporary works design carried out by AGL are outlined below:

• Start and reception shafts for the tunnel. The shafts were 12m deep and design elements included sheet pile retaining walls, secant pile wall “soft eye” and sealing blocks for TBM launch and recovery, props, ground anchors, uplift tension piles and system of temporary dewatering wells.
• The retaining walls and sealing block design was carried out by finite element methods. AGL set trigger limits for retaining wall deflection and monitoring was carried out by “real time” SAA inclinometers which provided alerts for excessive movement.
• Geotechnical interpretative reporting and specification of additional investigation for shaft design.
• Approach pipeline sections comprising trench box excavations up to 6m deep.
• Stability analysis for the recovery of the 160t TBM transport vehicle which became stuck on a minor road over peat when the wheels went into the soft verge.
• Foundation design for the temporary infrastructure required for tunnel construction including buildings, tanks, gantry crane and 500t crane for lift of TBM.
• Design reporting, drawings and risk assessments for all the design elements.