Limerick Main Drainage Contract 3.1 Dock Road Tunnel

Description of Works

The Limerick Main Drainage scheme is a major environmental infrastructure project which provides a new drainage system and treatment facility for the City and its surrounding area. At present there are no waste water treatment facilities in the City and approximately four million gallons of waste water is discharged daily to the Abbey and Shannon Rivers and the canal through some fifty separate outfalls. The new system will be linked to a state-of-the-art waste water treatment plant, there by eliminating untreated discharges to the rivers. The Dock Road tunnel is the main spine sewer of the system and feeds directly into the pumping station adjacent to the treatment works.

Scope of the Works

Contract 3.1 involved the construction of 2.55km of 2.82m ID segmentally lined tunnel from Corcanree Business Park to Harvey’s Quay in the City Centre. Thirteen segmentally-lined shafts sunk to depths of 15m provide both access and connection points for existing sewers. The connections involved the driving of approximately 130m of spur tunnels. In addition, there was in the order of 1200m of sewer laid in open cut together with alterations and connections to the existing system.

Construction

Ground conditions along the route of the tunnel were complex and difficult, ranging from very hard limestone bedrock to overlaying very soft alluvial and glacial deposits. The close proximity to the River Shannon and high water tables added to the construction problems.

The varied geology required a range of specialist construction techniques. The tunnel was constructed in two separate drives from the main drive shaft at Corcanree – a short drive of 300m to Corcanree Pumping Station, followed by the main drive to shaft 12 at Harvey’s Quay a distance of some 2250m.

A TBM was purchased from Lovat, designed to cut the limestone rock but also incorporated an earth pressure balance (EPB) facility to enable it to excavate the softer strata via a screw conveyor to maintain face stability. The machine was equipped with 22No 14” disc cutters, which could be changed for ripper teeth when required, the installed power of the head being 1200hp.

Shaft sinking also required a number of methods and techniques. Generally shafts have been sunk as caissons through the overlaying alluvium and glacial deposits and then founded into the bedrock. Excavation has then continued by drill and blast techniques, and underpinning of the segmental rings. Ground treatment and jet grouted piles were used in sinking the drive shaft due to the depth of the overlaying soft strata and high ground water pressure.

The tunnel boring machine was designed and built specifically to deal with the varying ground conditions – approximately one third of the tunnel length follows the interface of limestone bedrock and the overlaying soft deposits. This required a machine with the capability to deal with both hard rock and also soft materials, whilst being able to maintain face stability and avoid creating excessive settlement. The remaining two thirds of the tunnel were wholly in the limestone bedrock and therefore the machine required adequate power to make good progress within these strata.

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