Client name: John Sisk & Sons Ltd

Duration: 2005 – 2008

Location: Limerick, Ireland

Effective construction sequencing and collaboration facilitating design modifications – saving time and money. 

Temporary retained excavations for tunnel supporting road across water

This 500m long immersed tube tunnel carries the road across the River Shannon and required €14M of temporary retained excavations for the cut and cover sections at each end of the tunnel.

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

Construction constraints prevented the use of more than two levels of permanent props, with an unsupported wall height up to 18m. Each wall of the excavations experienced a different loading sequence during construction, and in addition 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. It was therefore decided to excavate the northern section in the dry.

Construction sequencing and detailed structural behaviour analysis

We conducted a detailed analysis of the behaviour 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.

Our team worked collaboratively with the structural engineer and provided information about the behaviour of each individual wall in every construction stage. This made a timely assessment of the effects of the interactions possible.

We devised a method which involved excavating the ground outside the walls and also excluding the river. This reduced the external ground and water forces on the walls. Temporary props were used at intermediate levels.

A colour coded plan was used to define the construction sequence so that all aspects of it could be analysed. This gave us 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 program.

Cost-effective design modifications allowed for safe and timely construction

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

2008, British Geotechnical Association, Fleming Award.

2009, Geotechnical Prize for presentation on the Limerick Tunnel Project, Engineers Ireland Excellence Awards.