This summer saw the completion of Limerick’s Dock Road sewer tunnel, in Ireland, to the relief and satisfaction of the client, Limerick City Council.
Bored by J Murphy & Sons using a new 3.3m diameter Lovat EPBM, the 2.5km long, 12m deep sewer tunnel was constructed in two drives; one 300m, the other 2.2km.
Tunnelling started in the summer 2001, and the scheduled contract completion date was October 2002. Following a number of delays, breakthrough was then expected at the end of March 2003. This was further delayed when problems hampered a cutter change in running ground at the very end of the drive. Murphy stabilised the ground by face and surface grouting. Breakthrough was finally achieved on 14 July 2003.
“It was always going to be a very difficult job,” resident engineer for the client, Russell Naylor, told T&TI. “The limestone bedrock is 150MPa-250MPa, with boulders and very soft silts and gravels above it, and the tunnel horizon moving in and out of this interface. Although behind programme, the contractor dealt with all the challenges and delivered a well-built tunnel. In fact, vertical alignment at the end of the 2.2km drive was within 12mm of target,” Naylor said.
T&TI reported some of the problems experienced on the US$17M project in October 2002. “We went through another difficult area with hard rock in the bottom and contaminated granular fill above which was next to the old quay wall, since we last spoke,” Naylor said. “Here, the settlement monitoring above the line of the tunnel identified some abnormal ground movements. The affected section of the quay road was closed off and an explorative investigation carried out. This revealed a cavity below the road surface that was backfilled allowing the road to be quickly reopened and tunnelling to recommence,” he said.
The Dock Road Tunnel did not delay the commissioning of the whole of the Limerick Main Drainage scheme (LMD). The construction of a major new pumping station is one of the last contracts (out of 20) due soon for completion. This will allow the existing sewers that currently discharge directly into the River Shannon to be diverted into the new Dock Road sewer. The new sewer will deliver the flows to the pumping station and then on to a new state-of-the-art treatment works.
“The client is happy and appreciates all the hard work Murphy has put in,” Naylor said. He conceded that the majority of the tunnelling projects on the LMD have faced problems as contractors struggle with the combination of the hard rock, the soft silts and the many boulders
The variability of the geology and rockhead level limited any benefit of constructing a deeper tunnel. Also, the deeper the tunnel goes, the more money would have to be spent on pumping. Running the tunnel shallower was also not an option because a number of the tunnel connections pass under the River Shannon.
Naylor expects to stay with the US$240M (approx.) Limerick Main Drainage scheme as another tunnelling contract is put out to tender. Bids are currently being assessed for contract 4.3, which involves the construction of a 2km long, 1.5m diameter tunnel.
