Crossing the Lehigh and Pohopoco Rivers: It's Not Just About the Bridges

摘要

The existing Pennsylvania Turnpike bridges over the Lehigh River and Pohopoco Creek near their confluence are scheduled for replacement with new structures that will offer increased traffic capacity. Alignment studies were performed to provide increased capacity for the approach roadways as well. As a result of the studies and identified impacts, the alignment will shift to the west from its present configuration. The roadway was originally designed and constructed in the mid 1950's, with side-hill cut and fill conditions on both the north and south approaches to the rivers. The north approach is dominated by a 130-foot high rock embankment, with only a minor cut slope. The south approach is dominated by a 115-foot high rock cut slope, with a 60-foot high embankment. All of these features show signs of only marginal stability, and the new alignment adversely impacted them. The roadway widening for the north approach originally was to have been a 47-foot high retaining wall constructed on the rock fill. When alternatives were analyzed, the projected cost of this wall was on the order of $2.5 million and measures to ensure global stability could double the cost. Subsequently, it was decided not to build the entire template width north of the bridges, and shift the alignment of the wall to minimize its height and impact on global stability. The roadway widening for the south approach includes construction of a new rock fill with a 1.5:1 slope and a tiered retaining wall through the rock cut slope. The existing fill slope has incidental walls, slope paving, and horizontal drain outlets that will have to be dealt with during construction. The rock cut slope has structural discontinuities that need to be considered in design. It is proposed to construct the cut slope with rock anchors and soil nailing. The final product is to include a BoulderScape decorative finish. While the three major roadway approach issues were all for one project, each merited individual attention which resulted in a different solution for each.