Major construction has been under way for more than two years on the downtown I-93 portion of Boston’s Central Artery/Tunnel (CA/T) project. This section includes approximately 7 km (23,000 linear feet) of Soldier Pile Tremie Concrete (SPTC) slurry wall with soldier piles used to reinforce the slurry walls spaced at 1.2 m to 1.8 m (4 feet to 6 feet). The slurry walls will form the walls of the highway tunnels, which will be excavated beneath the 6-lane I-93 viaduct as it remains in use (Figure 1). The viaduct will be underpinned by grade beams and needle beams supported by the slurry walls.

About a third of the length of the slurry walls must be constructed under low head room beneath the expressway viaduct. The vertical constraint for low head room construction can be as tight as 1.2 m (4 feet) from the bottom of the I-93 viaduct to top of grade. Walls as deep as 36.6 m (120 feet) must be built under this confined space.

Work Zones Prepared

Figure 1: Plan (top) and Elevation (bottom)

Figure 2: Elevation - Soldier Pile no Splice Zones (not to scale)

Figure 3: Method of Installation of the Soldier Piles in the Middle SPTC Wall (not to scale)

Figure 4: Stage 1 (top) and Stage 2 (bottom)

Contractors prepare work zones by pre-excavating to a depth of 1.2 m to 1.8 m (4 feet to 6 feet) below grade to increase the vertical working dimension. SPTC guide walls are then constructed at the lower elevation. The depth of pre-excavation is limited for two reasons:

• The site, adjacent to Boston Harbor, has a high water table-only 1.8 m to 2.4 m (6 feet to 8 feet) below grade.
• The guide walls must be constructed above the water table because the slurry in the trench typically requires the hydrostatic head above the in-situ water table to provide for a stable trench during excavation.

Soldier Piles Spliced

Sections of the long steel soldier piles must be spliced in order to be installed beneath the viaduct. Bolted connections were designed instead of welded connections because there was concern that welded splices could not be installed fast enough, considering the time constraint of keeping the slurry trench open. Specifications typically allow a maximum of 72 hours from completion of trench excavation to completion of reinforcement and tremie concreting. The longer the slurry trench remains open before tremie concreting, the greater the risk of debris falling into the trench or the trench sides caving in.

The splice design features full moment transfer using high strength A490 bolts. Bolts were designed in bearing, double shear, and installed to a “snug tight” condition. Soldier pile splices were positioned vertically along the pile to avoid conflicts with later structural connections to the tunnel roof and base slabs. The design called for a “no splice zone,” where the contractor was not permitted to detail splices. (Figure 2 on the following page). The layout for this was carefully calculated and checked during the design phase.

Two Splicing Methods Used Successfully

To date, the two contractors performing low head room construction have used different methods to install the splices.

One Method. One contractor designed equipment to lower two segments of soldier pile into position at a time (Figure 3). The segments are suspended from the guide walls, and their position is determined by using a custom-designed frame. The splicing process is as follows:

• Two segments are lowered into position beneath the I-93 Artery, the A490 bolts are installed, then the vise clamps on the side of the frame, which temporarily held the segments in position, are released.
• At this point the soldier pile assemblage is supported by a crane, which slowly lowers the piles into the slurry trench down to the next splice point.
• The vise clamps are resecured, the crane hook is released, and the next two segments are brought over for splicing.

The other method. The second contractor has also chosen to install SPTC soldier pile segments two at a time. Instead of suspending the installed segments from a crane and vises on the guide walls, however, the contractor has erected a frame with two winches (Figure 4). Cables from the winches are attached to the bottoms of the soldier pile segments. The winches are used to lower (and occasionally raise) segments as the spliced soldier piles are installed.

Both Methods are Successful

The splicing operation has proceeded without difficulty using both methods. The second contractor’s operation has the advantage of requiring less equipment, and it appears to be more flexible. On the other hand, in some cases the position of the winches and frame have made it more difficult to clean the bottom of trench prior to tremie concreting.