Baltimore Metro’s Shot Tower Station was constructed under a major roadway intersection near the heart of Baltimore. It supports the triple-cell Jones Falls conduit, which carries a river that was formerly a surface water feature, some large, storm drain conduits and four 115 kV electrical lines on its roof structure (Figures 1 and 2).

Excavation for Shot Tower Station extended 21 meters (70 feet) underground. The station is 186 m (610 feet) long, 20 m (65 feet) wide and 13.7 m (45 feet) high. The two tunnels that enter the station are 5.8 m (19 feet) in diameter, spaced 11.3 m (37 feet) apart horizontally to provide a 8-meter- (26-foot-) wide column-free space for a center platform.

Selecting a Construction Method

The Shot Tower Station site is characterized by a high groundwater table and a limited excavation area. PB performed design services as part of a joint venture, called DKP, with Daniel, Mann, Johnson and Mendenhall (DMJM) and Kaiser Engineers. We investigated many subway stations across the country with similar environmental and soil conditions before deciding on the most suitable type of structure and excavation support wall. The options we considered and our decisions are as follows:

• Precast concrete panel slurry walls. This excavation support wall type was rejected because of the difficulty in transporting large panels and in making the joints between the panels watertight.
• Cast-in-situ reinforced concrete slurry walls. Although reasonably watertight, this method was rejected also because it requires a larger than usual wall thickness, typically 1.2 m to 1.5 m (4 feet to 5 feet), to transmit high bending moments and shear forces imposed by the soil and full hydrostatic pressures. Also, moment connections using reinforcing bar bends, field bending and field welding presented major structural problems.
• Cast-in-situ concrete walls with soldier piles. We decided that the walls of the station would be constructed using Soldier Pile Tremie Concrete (SPTC) with reinforcing between the soldier piles. This type of wall is relatively watertight and it provides significant strength in the vertical direction, greater flexibility in moment connections and relatively easy connections for temporary cross lot bracing.

Figure 1: Shot Tower Station Plan

Figure 2: Shot Tower Station Profile

SPTC Wall Construction

The soldier piles were designed for each construction stage as well as for the final design condition. Pipe struts and wide-flange walers were used to brace and frame each stage of the excavation. The pipe struts were preloaded to about 35 percent of their anticipated load to assure that each one would engage the bracing system.

Several of the pipe struts and walers were monitored during construction with strain gauges to observe the stress levels.

In one instance, one of the monitored pipe struts near invert slab level reached 150 percent of its design load. This problem was quickly remedied by placing a second strut adjacent to the first strut to relieve some of the stress.

The exact placement of a soldier pile is not yet a precise science, especially when the pile has to go down about 21 m (70 feet). In some instances, the soldier piles for the Shot Tower Station were off from the theoretical location by as much as 0.3 m (1 foot). The specified tolerance was 0.15 m (6 inches), but very little could be done after the SPTC had been constructed except to accommodate it by reworking structural and architectural details.

The SPTC wall was structurally connected to the base slab and used as part of the final structure. The weight of the SPTC wall helped resist high buoyancy forces at this site adjacent to the Baltimore Harbor. The bulkheads and invert slabs at the ends of the station had to be designed to accommodate the openings for the tunnels and for any construction loads such as the bracing frame for the tunnel shield and the waler and strut loads from the braced excavation.

Approximately 12 m (40 feet) of the main station was constructed using a jet grouted cutoff wall in order to accommodate, but not disturb, four existing 115 kV lines that would eventually rest on the roof slab.

Supporting the Utilities

Some major utility structures had to span the width of the station (between the clear distance of the SPTC walls) using the roof slab for support:

• A 5.3-meter- (17.25-foot-) wide elliptical, double-box storm drain
• The 20.7-meter- (68-foot-) wide triple-cell Jones Falls Conduit, which provides drainage for areas within and north of Baltimore
• Existing conduits for four 115 kV power lines
• Existing and relocated 0.7-meter- (24-inch-) wide sanitary lines
• A relocated 0.9-meter- (30-inch-) wide sanitary line.

Embedded steel plate girders in deep concrete beams had to be designed for some of the larger utilities in order to minimize the thickness and provide adequate clearance between the mezzanine and the roof slabs.

Reconstruction of the triple-cell Jones Falls Conduit across the top of Shot Tower Station required removing one cell at a time by diverting the flow into the two other cells. The empty cell was then removed so that SPTC wall panels on the north and south sides of the station box could be constructed. The permanent station roof was constructed and supported on the SPTC wall panels. The removed cell was reconstructed upon completion of the station roof structure.

Staging the SPTC wall construction beneath the conduit required careful coordination with the demolition and reconstruction work. Brick and mastic waterproofing was used between the base of the conduit cells and the station roof slab. High density polyethylene (HDPE) membrane sheets were installed at the outside faces of conduit and at the junction of the SPTC wall with the conduit cells. (See also “Groundwater Considerations for SPTC Walls at Shot Tower Station” by Ray Castelli.)

Resolving Connection Problems

We used steel beams to support the mezzanine slab and steel plate girders was to support the roof. The beams and girders sat on wide flanged steel stubs that were welded to each soldier pile underneath the mezzanine and roof levels. There was major concern about the weld between the bracket and the soldier pile and how it would be constructed and inspected. Welding experts were consulted about the feasibility, constructibility and inspectibilty of the welds. All welds were eventually tested and found to be acceptable.

Due to the location, slope and rotation of each soldier pile, each mezzanine beam and roof girder would have a slightly different length and, therefore, be custom made. A complex spreadsheet was developed to calculate each mezzanine beam, roof girder length and associated miscellaneous stationing, offsets, and eccentricities needed to check shop drawings.

The brackets were welded perpendicular to the soldier pile flange and beveled shim plates were used in the connection between the beams and stub brackets. The brackets had to be designed not only for the vertical dead and live loads, but also for any eccentricities due to the location and orientation of the soldier piles.

Communication was Key to Success

As with any major engineering project, communication played an important role in executing the design and
construction. On the Shot Tower Station project:

• Technical committees met on a regular basis to monitor and advise on the progress of the design and construction.
• Quality assurance and value engineering evaluations were done prior to every major submittal.
• Weekly construction progress meetings were held to resolve any major issues that were encountered.

Overall, continuous contact was maintained with the client and the contractors in order to discuss, accommodate and execute decisions