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• Limerick Immersed Tube Tunnel, Ireland

John Sisk and Son Ltd selected Tetra Tech to provide geotechnical design services for temporary works for a 500-meter-long immersed tube tunnel beneath the River Shannon in Limerick, Ireland. The project, which consisted of installing prefabricated tunnel segments underwater, required €14M of temporary works to retain excavations for the cut-and-cover sections at each end of the tunnel. Tetra Tech conducted a detailed analysis of the behavior of each wall during construction. By applying our thorough understanding of the difficult ground conditions and the complex construction sequence, we were able to manage the risks of overstressing the temporary structure.

The construction of the cut-and-cover sections at each end of the tunnel required excavations 130 meters (m) long, 30m wide, and up to 15m deep in soft sensitive soils over limestone bedrock.  The excavations were supported by steel combiwalls with alternating 1.4-m-diameter tubular piles and sheet piles.

Our team worked collaboratively with the structural engineer to provide information about the behavior of each individual wall throughout every construction stage, resulting in a timely assessment of the effects of the interactions. We devised a method that involved excavating the ground outside the walls and also excluding the river. This reduced the external ground and water forces on the walls.

Construction constraints prevented using more than two levels of permanent props, with an unsupported wall height up to 18m. Temporary props were used at intermediate levels. Each wall of the excavations experienced a different loading sequence during construction, and some walls were subject to variable loading from the 6m tidal range. Achieving a design to resist these loads required tight management of the construction sequence.

The southern cut-and-cover section was excavated underwater, but excessive siltation made construction of the base slab very difficult.  The client therefore decided to excavate the northern section in the dry.

The client provided a color-coded plan to define the construction sequence so that all aspects of it could be analyzed. This gave our team the opportunity to identify where modifications were needed.  With these modifications in place, construction was successfully achieved and float out of the tunnel units took place on schedule.

Our work on the temporary works enabled modifications to be made so as to achieve the safe and timely delivery of the construction.