Cork city, situated on the southern tip of Ireland, is witnessing one of the largest environmental clean-up campaigns ever undertaken by an Irish local authority. The river Lee, which bisects the city and flows into Cork harbour, has long been a focal point for visitors and locals alike and the discharge of up to 13 million gallons of raw sewage a day is unacceptable. The client, Cork Corporation, has undertaken the IR£260M ($260M) Cork main drainage scheme in a bid to alleviate the untreated flow and comply with the EU Directive on Wastewater Treatment 91/271/ECC. This requires secondary treatment of all wastewater discharge to estuarine waters from a population equivalent of 2000 or more.
The scheme includes construction of a 2.6km long, 3m id TBM driven interceptor sewer tunnel and a 175m long, 2.1m diameter siphon 25m under the River Lee which the contractor anticipates will be pipe-jacked. Also included are 10 shafts, an 11.5m id, 10m deep drive shaft, eight 6m diameter with 1m wall intermediate shafts and a 13.6m diameter, 25m deep reception shaft.
Ground conditions allow for the shafts to all be sunk as cast in situ concrete wet caissons. The IR£15M tunnel construction contract was won in a standard competitive bid in autumn 1999 by the JV of Ascon and Nuttall, both owned by Dutch contractor HBG. Nuttall are providing tunnelling expertise while Ascon provide knowledge of working in the Irish market.
The scheme
The main trunk sewer tunnel will run generally at a depth of 6m from the Atlantic Pond starter Shaft to the reception shaft at Kennedy Quay. From here the main tunnel will be connected to the sewer network to the north of the River Lee by the siphon tunnel.
Tunnel Agent, Ivor Thomas explains: “The scheme is designed for picking up flows on the north side of the river into the siphon to a shaft in the middle of the island (Custom House Quay) then on to the south side TBM tunnel. The bottom end of the tunnel will feed into a new pumping station under construction in the coffer dam next to Atlantic Pond Shaft.” The pumping station is being constructed by Ascon under a seperate contract valued at IR£7.3M.
Another spur, yet to be constructed, will transfer the sewage from the new Atlantic Pond pumping station to a new IR£52M treatment works at Carrigrennen, Little Island 4km to the east of Atlantic Pond. The contracts for the extension of the pumping main and the sewage treatment works are currently out to tender.
An extensive site investigation of the area carried out by local firm Geotech Specialists, under the supervision of consultant EG Petit, identified three main geological strata, ranging from silts to gravels to limestone, that the tunnel alignment passes through and through which the shafts are sunk.
From Atlantic Pond shaft (Chainage 0) to chainage 400, the ground consists mainly of silts with occasional gravel in the tunnel invert and lower face. Soft to very soft silts extend from the crown to surface. From chainage 400 to access shaft 7 (Chainage 580), the tunnel passes through gravel/sandy gravel for the full and lower face portion with the upper portion consisting of soft sandy silt with occasional gravel. A loose sandy gravel extends from the tunnel crown to surface. Between access shaft 7 (Chainage 580) and access shaft 5 (Chainage 1185), the lower portion of the face consists of gravel with the upper portion comprising silt from tunnel axis to crown. From the tunnel crown to surface, silt predominates. Within this section, boreholes show a very strong, slightly weathered limestone (100-200MPa) and the presence of a short section of bedrock in the invert has not been discounted. From access shaft five (Chainage 1185) to Kennedy Quay (Chainage 2640) the tunnel passes mainly through gravel with occasional silts. From the crown to surface there is generally 0.5-1m cover of gravel overlain by 3-4m of silt.
The Nuttal/Ascon JV is driving the 2.6km long main trunk sewer tunnel using a refurbished Lovat EPBM, previously used on Nuttals’ Portsmouth transfer tunnel (T&T Feb 2000). The machine, which started boring on 14 August 1999, has been upsized and refurbished under the supervision of UK consultant Mechanised Tunnelling Services at Nuttall’s Crayford depot. The machine’s original 3.3m diameter has been increased to 3.47m by GDC Engineering of Manchester to accommodate the larger cut diameter of the Cork tunnel. Extensive modifications have been carried out on the arrangement of the head dressing by UK contractor TBM Cutterhead and Wearparts to suit the chosen ripper mode of excavation. To improve penetration, the rippers have been reduced from 64mm wide to 50mm. The scrapers have been set between 70mm to 80mm below the rippers, They are intended to serve as loaders, but will be used as emergency cutters if a ripper tool becomes dislodged. To this end, the scrapers have been equipped with an increased carbide content. With the addition of two rippers to the nose cone, a five ripper configuration has been achieved.
“The problem is the various type of ground from very soft silts to sands and gravels to fresh limestone” explains Thomas, “Our main concern is the limestone in the invert between shaft 6 and 7. We’ve designed the head to deal with the limestone with very thin narrow rippers, a tight profile and a lot of carbide on the tool, it’s a matter of we’ll see.”
The JV has developed a system for inspecting the head at every shaft. Each shaft is filled with a very weak mix concrete with a manhole inspection chamber cast in it. The machine is driven through to the inspection chamber and stopped. The head can then be turned and the tools inspected within the chamber.
Launching technique
Because of the shallow nature of the drive and the wet ground conditions, the JV developed a somewhat unorthodox method of launching the TBM. Instead of going to face from inside the drive shaft, the TBM was dropped into a launch chamber built in a cofferdam adjacent to the shaft. A series of five rings were dropped into the drive shaft and taken forward to the TBM and built. The five rings were then pushed back to a steel shove frame in the shaft. The headwall was concreted and a hydrotyte seal created. A major advantage of this process is being able to achieve a decent seal at the shaft due to the complete visibility of the working area. After the rings were built, the cofferdam was backfilled and the machine readied to start the drive. The piles were taken out around the cofferdam and the machine shoved away against the shove frame in the pit bottom. The sidewalls of the cofferdam were left in place to prevent any settlement on the built ring.
“In this ground, to put in a stub tunnel would have been expensive and time consuming, we’d have had to work with compressed air as the ground’s so wet.” says Thomas. The main problem with the technique was predicted to be the machine start up. After backfilling, the head and the screw are not surcharged with muck. After setting off, the JV envisaged a sudden rush of muck into the system. He explains the way they overcame the potential problem: “We backfilled the area with weak concrete but left the head free. We ran sand between the cofferdam and the machine, then ran the machine up to fill the spiral with sand. It’s just like a granny’s mincer.”
Due to the varying condition and wet nature of the ground, much of the tunnel runs through reclaimed land, the contractor is treating the face prior to excavation. “A lot of the tunnel dryness is due to the polymer.” says Thomas, “We started off injecting into the screw to get a plug, but that’s expensive so we are now injecting into the head which gives the polymer more time to react with the ground.” Face conditioning has also helped the JV drive through alternating geology. “At one point, within ten rings, we went from shoving at 200t to shoving at 560t,” recalls Thomas.
Polymer and foam lubricates, waterproofs and improves the flow of the muck. Thomas continues, “It’s doing the trick, it has made a significant increase in production. We’re getting quicker shove times and less wear and tear on the machine and rings.” Muds are provided by Morrison Muds.
A narrow gauge railway takes care of spoil removal. The system comprises a Clayton 5t loco, a man rider, four muck skips, a grout car, three segment loaders and a muck skip. The muck is removed via a tipping frame system and taken to tip.
The tunnel has a six-piece tapered trapezoidal lining with segments provided by Macrete of Northern Ireland, all with hydrophilic seals from VIP Heinke Gasket. The tapered design was chosen because of the tunnel curves which have a minimum radius of 185m. The JV has opted for spearbolts on the segments instead of conventional bananabolts. “It’s giving us a more rigid build that’s more appropriate for this type of ground. We find with the bananabolt, as the ring slips off the tail skin, it tends to flex a bit” says Thomas.
Current progress
An eight-man crew is working each 12-hour shift on a 24-hour system. Each shift involves a team consisting of the TBM operator, two miners, one grout man, one belt man, one fitter, one electrician and the shift engineer.
At the time of T&TI’s visit, the machine had tunnelled approximately 580m, the construction team having successfully holed through on both shafts 8 and 7 on line and on level. The intermediate shafts are all sunk to level with the reception shaft at Kennedy key having sunk 12m of the 25m.
Related Files
Cork – River Lee
Cork City, situated on the River Lee in Southern Ireland
