In light of rapid growth in Philadelphia's northwest suburbs, Pennsylvania Turnpike officials sought a low-maintenance, cost-effective solution for increased traffic volume at its Valley Forge interchange. The answer was a $48-million project that aims to relieve traffic congestion in Chester County by building the turnpike's first four-direction, all-electronic toll interchange.

The project's scope called for building new east-bound and westbound exit and entrance ramps, three retaining walls and two electronic tolling facilities, plus widening the freeway and creating a monitoring system.

The project team had to overcome several site challenges, including the presence of three gas pipelines, a former quarry, an active commercial development, a cell tower and a wetland.

The alignment of the ramps and the bridge over the main line was greatly influenced by the three parallel gas pipelines crossing the project site. The pipeline right-of-way was inaccessible and couldn't be used for bridge foundations. The contractor was able to secure agreements to establish two temporary crossing locations to access both sides of the project's limits.

The original plan estimated steel H-pile length would be 46,462 linear ft, but the actual length driven into the ground was 72,000 linear ft. That large increase was required due to the area's geology of pinnacle limestone and solution cavities. The construction manager's team recommended using a second pile-driving rig to mitigate the potential impact to the schedule.

Due to changes in tolling technology after documents were completed for final plans, specifications and estimates, the designer and construction team were forced to add weigh-in-motion scales to the tolling area. The scales are designed to reduce the likelihood of improper toll collections.

Adding the scales required other revisions to the plan, including moving the gantry structures, revising the median island and making changes to the safety design of the ramps. The electronic tolling equipment allows vehicles to exit and enter the system at normal traffic speeds.

S.R. 29 All-Electronic Toll Interchange, Chester County, Pa.

Key Players

Owner Pennsylvania Turnpike Commission

Design Firm STV Inc.

General Contractor Allan A. Myers Inc.

Construction Manager JBC Associates Inc.