As part of the ongoing improvements of State Route 28, Pennsylvania Department of Transportation (PennDOT) awarded the prime contract for road widening and reconstruction of Section A10, which runs along East Ohio Street, to general contractor Trumbull Corporation of Pittsburgh. The overall design, intended to alleviate congestion and ease traffic flow, included the creation of additional travel lanes and new interchanges at the 40th and 31st Street Bridges. Trumbull’s scope of work included the construction of a permanently anchored, soldier pile and architectural, cast-in-place retaining wall near the 40th Street Bridge, in Section A10, to allow road widening and on-ramp construction. Moretrench was contracted to Trumbull to drill the 157 shafts for the soldier piles and to install 412, high-capacity permanent strand anchors for lateral support of the wall.
Anchor System Design
The wall extended for a length of approximately 1,600 feet with a maximum height of 35 feet, requiring up to 4 levels of permanent anchors. Borings revealed 15 feet of silty clay and weathered shale, followed by strata of claystone, sandstone and siltstone to depths of 80 feet. In order for the anchors to bond in what borings had indicated was high quality sandstone (Upper and Middle Saltsburg formations), anchor lengths varied from approximately 35 to 145 feet. The double-corrosion- protected anchors themselves varied from 5 to 12 strands, with design loads ranging from 220 to 527 kips. Because of constructability issues with the as-designed anchors, Moretrench modified the anchor details and provided sealed design calculations prior to verifying its assumptions with full-scale, pre-production test anchors in various locations along the wall length.
Working from a bench constructed in the existing hillside by Trumbull, Moretrench drilled the 42-inch diameter shafts to a maximum depth of 60 feet. Installation of the soldier piles by Trumbull was completed before drilling and installation of the anchors was begun.
Anchors were installed at 30o from horizontal. At each location, 7-inch O.D. permanent casing was advanced to top of rock using duplex drilling methods. The drill hole was then advanced to design depth, providing a minimum 15-foot bond length in rock. Following anchor placement, the borehole annulus was tremie-filled with 4000 psi grout. All anchors were proof-tested to 133 % of design lock-off load, with 5% performance-tested and 5% creep-tested.
Unanticipated Condition Overcome
The anchor installation contract with PennDOT did not provide for water pressure testing of the borehole, pre- grouting or re-drilling of anchors. During installation of the top row of anchors, significant grout takes signaled that the rock within the hillside was potentially highly fractured. When additional failures occurred in certain lengths of the wall in successive rows, it became apparent that there were isolated pockets of deficiency in the bond zone material. Moretrench removed, re-drilled, reinstalled, and successfully tested all previously failed anchors.
Based on the knowledge gained and successful outcome of this section of the upgrade, Trumbull awarded Moretrench the subcontract for a similar program of caisson drilling and anchor installation, together with soil nailing and temporary shoring, for Section A9, which includes the 31st Street Bridge.