South Bay Interchange is one of the most geotechnically challenging parts of the Boston Central Artery. It involves linking a new elevated section of the Interstate 93, as it approaches downtown, to the Interstate 90 and its new extension east to Logan Airport.
The extension will be built in cut and cover, jacked tunnels and immersed tube tunnels under the Fort Point Channel. From here the I-90 will run through cut and cover to the South Boston Interchange and into the Ted Williams Tunnel.
At the junction of the I-93 and the I-90, work is gearing up for the first of three huge tunnel jacking operations that will take the I-90 from an exit ramp in the immersed tube tunnel, under railway lines and on towards the I93 in the downtown area.
The site is compact, just 200m by 300m, bounded by existing roads, a number of buildings, including the project headquarters, and the Fort Point Channel. Ground conditions are, as in most of Boston, a mixed bag. Repeated reclamation, rebuilding of the harbour area and railway construction has left variable sand, silt and gravel fill with a variety of construction material including concrete, steel and timber and granite blocks. Running across the site is a buried railway line that was built on timber piles. Beneath the fill are layers of organic sands and silts as well as peat up to 6m thick overlying the Boston Blue clay, which softens with depth. Groundwater is typically 2m to 3m below ground level.
The jacked tunnels must pass under the eight railway tracks that carry nearly 41,000 commuters and up to 3,000 long distance Amtrak passengers daily in and out of Boston's South Station. Clearance is a mere 2.5m to 9m below the tracks.
Hatch Mott MacDonald tunnel project engineer Stephen Taylor explains that the original plan was to construct nine tunnel sections beneath the tracks by a combination of cut and cover and mining, with extensive relocation of tracks, signalling and associated services to ensure that trains would keep running.
Another option was suggested by section design consultant for the South Bay Interchange, a joint venture of the Maguire Group and Frederick R Harris, along with subconsultant Hatch Mott MacDonald (brought in for development of the jacking design) aided by Kvaerner Technology. They proposed full size tunnel jacking for building the tunnels beneath the tracks.
A development of pipe jacking, tunnel jacking has been used in Europe and other parts of the world but this is the first time that it has been used on this scale in the US. It is typically used to build large rectangular sectioned tunnels in soft ground over short distances as an alternative to cut and cover.
As on the Fort Point Crossing, European expertise was brought in to assist the American engineers. Taylor, heavily involved in the development of large tunnel jacking operations in the UK, is one of a number of British expats who has moved to Boston. He will head HMM's office for the duration of the contract.
Jacking was suggested for two of the original nine tunnels nearest the station that would have had to be mined. This was given the go-ahead, but major changes to the interchange layout reduced the number of tunnels to three, and it was decided to construct them all by jacking.
Taylor explains that there are a number of advantages in using jacking over cut and cover. The original plan called for extensive relocation of the railway tracks, with work sometimes carried out between live lines. Jacking meant that no relocation was needed, removed the risk of differential settlement through excavation, construction and backfilling of the cut and cover tunnels, and allowed deep excavations to be moved further away from the lines by simply extending the tunnel segments.
Because of the tight layout of the railway lines, tunnel elements would have been skewed, with complicated construction joint detailing, reinforcement and waterproofing. Using the jacking method, these were simplified. There was a substantial reduction in spoil, which was believed to be contaminated, giving further environmental benefits.
Work started on site in January 1997 with construction of the jacking pits for the I-90 eastbound, I-90 westbound and ramp D tunnels. Main contractor for the $397M contract is a joint venture of Slattery Skanska, Interbeton, JF White and Perini.
The tunnel sections are being built in three jacking pits. Each was formed using 1,200mm thick, 30m deep, typically post tensioned, diaphragm walls, with a 1m thick base slab. This slab is 1.5m thick towards the rear of the pit where the jacks are placed.
Because of the very soft ground conditions at the base of the excavations, jet grouting was used below the base slabs to provide a low level thrust to the pit walls. Geotechnical contractor Trevi/ Icos installed the 6m to 7.5m thick zone from ground level using a triple jet system to form 1.8m diameter columns at 1.4m centres.
The tunnels are all about 24m wide and 12m high, with 1.5m to 2m thick walls and 2m thick bases. They vary in length - the I-90 eastbound is 107m, the I-90 westbound 76m and ramp D 48m. Ramp D is being built in two sections with an intermediate jacking point; the other two are being built in three sections with two intermediate jacking points.
The ground had to be treated in advance of the jacking. This was originally to be carried out using dewatering, grouting and soil nailing. Instead, the contractor proposed ground freezing (see box) to stabilise the ground below the tracks. This meant the entire ground mass could be treated in one go before jacking started, face stability would be better, and the chances of face loss were reduced because there was less chance of 'windows' in the ground treatment.
Taylor adds that the ground will thaw extremely slowly after construction. A little settlement will occur, 'but not much, as the most sensitive soils [the organic layers and Boston Blue Clay] will be removed during jacking'.
Because the casting pits were excavated before the ground freezing was carried out, the head walls either have to be able to counteract the massive loads as the ground expands or have the ability to move, controlling the increase in loading. The head wall was reinforced by large soldier piles at 1.2m to 1.5m centres.
The wall has to take the increased soil load, but if it was static, when lateral heave occurs, the forces would go to very high levels. 'The idea is to limit the amount of force by allowing flexibility in the wall but in a controlled way,' Taylor says.
This is done by fitting hydraulic jacks behind a massive concrete beam near the top. The pressure in these is measured and they are systematically released to allow the wall to move in, while maintaining the pressure. Pressure relief holes have also been drilled to allow the ground to collapse.
Excavation is open face using two hydraulic excavators working on two levels at the front of the tunnel. The tunnel is fitted with steel cutting edges, cast into temporary concrete sections, 3m long at the top and sides and 6m long at the base. These are pushed a few centimetres into the ground before excavation of the frozen ground starts. Once material is removed, hydraulic jacks at the back of the pit and in between segments push the tunnel forward. About 30 hydraulic jacks are fitted at each jacking point, with a 500t working load capacity. The front section moves forward, then the section(s) behind follow on. In this way, the tunnel shuffles forward at up to 2m a day. Each tunnel should take between four and six weeks to push through. Laser monitoring is used to keep the tunnels on line, with 150mm tolerance. Taylor says the tunnels are expected to have about 50mm settlement in their working life.
The tunnels are fitted with anti-drag cables that run along the roof and under the base: 800 of these 20mm diameter steel cables lie side by side on the base and 800 on the roof. They are lubricated with grease and create a uniform distribution of load which should help combat any severe asymmetric frictional resistance that would tend to knock the tunnels off line. The cables will greatly reduce the chance of lateral movement of the ground above the tunnels and hence protect the railway tracks. Using cables as well as increasing the length of the jacked units meant that the guide tunnels could be removed from the design.
The cables on the base are attached to the front end of the jacking base slab and on the roof to the beam cast on the head wall. Reels of cables are placed inside the tunnel and are fed out as the tunnel moves forward. They are there to make sure the ground above is not dragged or sheared, causing lateral movement of the tracks. Bentonite slurry provided lubrication between the walls and the adjacent ground.
Jacking of ramp D, the smaller and arguably simpler of the three, was due to start this month, followed by the I-90 eastbound. Jacking is expected to be complete by the end of 2000.
Because of its size, the I-90 eastbound will be jacked in two parts. The tunnel is 107m long but the biggest pit possible was 76m long, so the first 76m will be cast, pushed and left under the tracks. Work will then shift to the I-90 westbound and return when the last part of the eastbound tunnel is ready.
One concern, says Taylor, is that the ground will tend to grab the first section while it is sitting under the tracks for two to three months. 'The jacking system is designed to give the tunnel an extra push to start it moving,' he says, although he adds that the section will also be fitted with heating pipes to ensure that it does not freeze in.