Longitudinal tendons are 300 feet long and staggered so that stressing takes place at every 100-foot section. Four tendons stress, while eight tendons run through every construction joint

State Route 29 snakes its way through Trenton, New Jersey, along the Delaware River. Just south of downtown, it veers away from the river and proceeds through a residential area. Over the years, increased vehicular traffic on this major artery has contributed to mounting traffic congestion, causing problems for local residents as well as headaches for commuters.

In the mid-1990s, the New Jersey Department of Transportation developed a plan for a bypass to alleviate congestion and divert traffic away from the residential areas. The route of the bypass, hovering along the Delaware River, called for the construction of a large 700-meter tunnel, standing six meters tall, with a width of 39 meters at its maximum point.

Proposals for this major design/build project undertaking were solicited in 1997. The winning bid was submitted by PKF/NCI, a joint venture of PKF-Mark III, Inc. and Neshaminy Constructors. DMJM/Harris served as the consulting engineer for the winning team.

Key to winning the contract was the proposed cut-and-cover tunnel design. Among its highlights was the ability to implement larger concrete pours that are double the conventional length - thereby significantly improving construction time on that portion of the project.

The contractor's original intent was to construct the tunnel using conventionally reinforced concrete and 15-meter sequential pours (approximately 50-foot pours) for the floor slab, roof slab, center and east wall. However, post-tensioning allowed for 100-foot pours, which would allow for a shorter schedule, fewer concrete pour set-ups, and more efficient pours.

The west wall of the tunnel, along the Delaware River, is designed with 760-millimeter square columns, spaced six meters on center. The base slab is designed with a maximum thickness of 915 millimeters at the center wall haunch, with the roof slab having a maximum thickness of 760 millimeters at that point. The maximum wall thickness is 610 millimeters.

VSL, a unit of Structural Group, a national concrete repair and strengthening specialty contractor, was also part of the original PKF/NCI project team. Based on an analysis conducted by VSL engineers, a post-tensioned concrete design was proposed, which was approved by the project team. Instead of taking the conventional construction approach, the floor slab, center and east walls of the tunnel are being constructed using longitudinal tendons. The roof slab uses a combination of longitudinal and transverse tendons. In addition, the use of a VSL multi-strand coupler design allows for continuous linking of longitudinal tendons across the construction joints, thus creating greater continuity between concrete pours.

Once the decision was made to utilize post-tensioning, a tendon layout was developed that utilized tendons with few strands, evenly spaced throughout the tunnel slab and walls. The design allowed for using larger tendons spaced further apart, thereby reducing both time and cost.

Figure 1:
Post-tensioning tendon data

Location	# of Tendons	Strand Size
East Wall	8	1/2" Strands
Center Wall	3	1/2" Strands
Base slab longitudinal	17	1/2" Strands
Roof slab longitudinal	17	1/2" Strands
Roof slab transverse	19 (1st tunnel section) 31 (second tunnel section because of wider roof slabs	1/2" Strands

Applying these design innovations, the use of prestressed concrete has enabled the concrete pour length to be increased to 30 meters - nearly 100 feet - by 26 meters wide, thereby lowering the number of required concrete pours by half. As a result, PKF/NCI is completing the tunnel project at a very competitive cost, coupled with a faster turnaround time.

As an added factor, the ability to shorten the construction time was considered a plus from an environmental standpoint. With the construction taking place in a residential section of Trenton, any steps that could be taken to minimize the impact of noise and congestion on the neighborhood would certainly be welcomed.

Tunnel construction began in 2000. Several construction delays were encountered, which necessitated a halt in construction between October and March 2001. The cause of the delays pertained to contaminated soil conditions, not the cut-and-cover construction operation. According to John Crigler, VSL Senior Vice President and a member of the Route 29 project team, the time frame for construction of a post-tensioned tunnel of this size would normally be about 12 months - or about one-half the time required using conventional tunnel construction techniques. This 12-month time frame also minimizes winter weather interference.

Tunnel construction was completed in three stages: the base slab; the east, center and west walls; and the roof slab

In addition to assisting with the development of the engineering plans for the cut-and-cover tunnel design, VSL is responsible for supplying all post-tensioning materials and performing the installation. According to Crigler, the novelty of the project meant that VSL was called upon to provide extensive support to the rest of the project team. "We really worked closely on the specific post-tensioning issues, which always tend to be more specialized. The developmental time was longer than normal, but it enabled us to really add value in the form of engineering and design support, in addition to the VSL systems and installation services," he noted.

The Route 29 tunnel construction is scheduled for completion in the summer of 2002. The local residents and commuters can hardly wait.