Shaft sinking start for Narragansett Bay CSO Sep 2003

Work on the first Phase of the extensive Narragansett Bay CSO (combined sewer overflow) control program has started with freeze-controlled shaft sinking and surface assembly of the 30ft (9m) diameter shielded TBM ready for a TBM start by contractor ML Shank/Balfour Beatty JV in November 2003. Along with some 16,215ft (5km) of rock TBM tunneling through siltstone, sandstone, shale, and conglomerate of up to 25,000psi (175MPa) compressive strength, the JV's US$163.5 million 49-month contract in Providence, Rhode Island, includes six shafts of up to 290ft (88m) deep, a number of de-aeration chambers and adits, and excavation of a large 130ft long x 65ft wide x 65ft high (40m x 20m x 20m) pumping station cavern.

The contract's main site is on open space belonging to client, the Narragansett Bay Commission, near its Fields Point Treatment Plant alongside Providence River. The site sits directly above the location of the pumping station and incorporates three of the job's six permanent shafts - three into the pumping station (a 32ft (9.7m) i.d. utility shaft for man access and an 11ft (3.3m) i.d. access shaft for installation of machinery). The third is the main 35ft x 35ft (10.5m x 10.5m) access operating shaft from which the TBM will progress the full 5km of tunnel on a slightly uphill gravity flow gradient. The three other shafts further along the alignment, comprise a 12ft (3.6m) o.d. drop shaft, a 5ft (1.5m) o.d. vent shaft, and a more substantial 34ft (10m) o.d. ventilation structure shaft at the Foundry Site.

For the four largest shafts the JV selected ground freezing over slurry diaphragm walling to support the upper 170ft (52m) or so through soft waterbearing soil and engaged Moretrench as the ground freezing subcontractor. Once excavated, the frozen sections are slip-form lined bottom up with D12 x D12, 4 x 4 WWF reinforced in-situ concrete before taking the shafts to full depth using drill+blast. The freeze is suspended once laboratory tests confirm a minimum 4,000psi strength in the concrete lining.

When TunnelTalk was on site in June, excavation of the frozen section of the utility shaft on the pumping station was complete and excavation in the smaller access shaft was 150ft (46m) through the frozen section and close to top of rock. Work had also started on the Foundry Site vent structure shaft, but all concentration was on completing the main operating shaft.

“At the moment we are about five months behind schedule on this shaft," explained JV Project Manager Steve Minassian. "For reasons that are still under investigation, the freeze in the shaft didn't take properly and an extra row of freeze pipes had to be installed. That set us back by five months."

Since then, and after completing the frozen section, crews working three 8hr production shifts/day, 5 days/week, had taken the shaft 80ft (25m) into the rock by mid-June 2003 and had broken out the top heading of a 332ft long x 31ft diameter (100m x 9.4rn) drill+blast starter tunnel which will allow the TBM and its full backup to be installed before starting.

TBM and bored tunnel design

The 30ft diameter TBM is an electro-hydraulically driven machine of Shank design and manufactured largely by Hitachi Zosen in Japan with smaller elements and the backup components manufactured in the USA. The 9m-diameter cutterhead is dressed with 49 x 17in (431mm) diameter Herrenknecht face cutters and 16 gauge cutters and rotates at variable speed on a 16ft (5m) diameter Rotek bearing. With a total installed power of 3,400hp (2,535kW), the machine can generate a maximum cutterhead torque of 5,200,000lbft (7,050,000Nm).

As an alternative to the primary support of rockbolts and shotcrete - designed by tunnel subconsultant Jacobs/Sverdrup JV for Program Managers Louis Berger Group - Shank has had accepted its own system of primary precast concrete segmental support. In the shield skin and within 14ft (4.25m) of the face, support rings of four 4ft (1.2m) wide x 10in (250mm) thick concrete segments are erected. The non-bolted, non-gasketed rings are erected using twin hydraulically, rotating and extendable arm erectors and the segment rings are secured using steel key gap strut jacks in the crown. There is no caulking or grouting of the primary lining and the tunnel is finished with a final 12in (30cm) thick unreinforced in-situ concrete Iining.

The 16,150 segments needed for the 5km long tunnel, reinforced with 60kg of steel rebar/m³ for handling purposes, are being produced at a factory in Providence by one of the minority subs engaged by the JV. The four large segments in each 28ft (6.7m) i.d. ring are 22ft (6.7m) long on the inside arc and are being cast in moulds supplied by Everest of Canada. "This is the largest diameter on which we have applied this system," said Minassian. "Our largest previous applications were for a 24ft diameter tunnel in Phoenix and for 22ft diameter subway tunnels in Los Angeles." The TBM applies forward thrust off this primary segmental lining using 20 thrust rams to exert a total thrust of up to 6,362,000lb.

The main components of the machine arrived from Japan back in February for an expected start in May. As it is, TBM launch is delayed until November by the shaft freezing problems. In June, when TunnelTalk was on site, the machine was being assembled on the surface before being lowered into the starter tunnel.

Once launched, Shank's program average for the TBM and primary support system is based on an advance of about 82ft (25m) or about 21 rings/day working 24h (3 x 8h shifts)/day 5 days/week. At that rate, the 16,215ft (5km) long tunnel is expected to take 13 months to complete – including time taken for forward grouting.

Water make from the upper waterbearing deposits through cracks and discontinuities into the tunnel and rock excavations is anticipated at up to 3,300gal/min (15,000 litre/min). A minimum pumping capacity of 4,000gal/min is required in the 290ft (88m) deep TBM working shaft and there is a grouting program to control zones of excessive flow. "We are currently making about 10-50gal/min in the starter tunnel excavation and access shaft," explained Minassian. “Systematic probing ahead of the TBM is required and drills mounted on the first trailing gantry will extend probe and grout holes out through the buckets of the cutterhead."

Drill+blast lining and support

Immediate support in the rock sections of the shafts comprises pattern rockbolting using Swellex bolts and up to 3in (7.5cm) of wet mix shotcrete reinforced with synthetic fibres. "This drill+blast work will eventually be lined with reinforced in-situ concrete," explained John Kaplin, Resident Engineer for the Gilbane/Jacobs Associates JV engaged as Construction Managers by the Narragansett Bay Commission. "Synthetic fibres were accepted as tests prove they provide similar strength properties to steel fibres."

Steel fibre and different types of poly-fibre were permitted in the design and, of the options, the JV selected Barchip MHT 48mm poly-fibres. "There wasn't much difference in price," said Minassian, "but we wanted to reduce the aggressive wear of steel fibre on the equipment. We hadn't used poly-fibre before and were interested to see how these would perform." The long flat ribbon-like Barchip fibres are made of high tensile strength modified polyolefin with an embossed surface for effective matrix bonding.

Rather than batching on site, the JV is buying in ready-mixed shotcrete and the fibres are added at 17lb/yd³ (10kg/m³) to the truck-mixer before the concrete is fed down the slick line to a diesel concrete pump in the shaft bottom and applied using a Shotcrete Technologies robot and boom. “We are pleased with the overall result,” said Minassian, "but there are a few particular quirks. Where heavier steel fibre will stay put in any rebound, for example, the lightweight poly-fibres separate from the rebound easily and float in any still or flowing water. It then collects in clumps, like hair, and can cause problems with the pumps if not properly managed. At the moment we are getting about 15-20% rebound but we haven't yet investigated whether this is influenced by the long fibres or not. There is a large volume of shotcrete to be applied in the pumping cavern and we might investigate this further at that time."

Drill+blast excavation of the pumping station is supported with systematic mechanical-shell rockbolting and lined with 8in (20cm) of synthetic-fibre reinforced permanent shotcrete. The excavated cavern is finished with an in-situ concrete base slab floor and a suspended corrugated steel ceiling.

CSO program buildout

The Shank/Balfour Beatty US$163.5 million civil works contract in Providence is part of the total US$314 million first phase of a larger project designed to respond to a consent agreement with the Rhode lsland Department of Environmental Management to control continuing CSO pollution of the Providence River (Fig 1). "This first phase of a proposed three-phased program is ordered to be in operation by 2007 and will intercept and divert 40% of all CSOs in our service area for treatment through the Field's Point Treatment Plant," explained Jamie Samons, Public Affairs Manager, for the Narragansett Bay Commission. The end date of the Shank/Balfour Beatty civil contract is March 2006 and the completed pumping station cavern, from where flow will be lifted into the treatment plant, is programmed to be handed over in December 2004 for installation under separate contract of the pumping equipment.

"Following this, there is an agreed two-year moratorium on new works to review the performance of current facilities and estimate what more is needed in Phases II and III," continued Samons. "Currently Phase ll involves construction of some 30,000ft (9km) of near surface interceptors and collectors that feed into this Phase I main tunnel. Phase III identifies another large deep level interceptor tunnel at the upper reaches of the Seekonk River on the other side of Providence to convey surcharge into the Bucklin Point Treatment Plant. Construction of an agreed Phase III facility is set to start in 2012 to meet current consent mandates. Before that, Phase l, with its $314 million cost funded primarily by local Narragansett Bay Commission ratepayers, is attracting acute public interest. Start of the TBM in November will be a major progress marker."