State Route 28
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 intent of the design was to alleviate congestion through the addition of travel lanes and new interchanges at the 40th and 31st Street Bridges.Part of the scope of work included the construction of a permanently anchored, soldier beam retaining wall with a cast-in-place architectural finish. Moretrench was selected by Trumbull to drill and install 157 drilled shafts for the soldier beams and 412 permanent tiebacks for lateral support of the wall.
RETAINING WALL DETAILS
The wall extended for a length of approximately 1,600 feet with a maximum height of 35 feet, requiring up to four levels of permanent anchors. The geology of the area consisted of a silty clay overburden followed by horizontally bedded shales, claystone, sandstone and siltstone. Due to the poor quality of the weathered shales and claystones, the design required the tiebacks to be bonded in sandstone and siltstone layers known as the Upper and Middle Saltsburg formations. Anchor lengths therefore varied from approximately 35 to 145 feet. The tiebacks varied in size from 5 to 12 strands and required the installation of pre-production test anchors to verify bond stress values.
Access to the drilled shaft and tieback work was from a work bench constructed along the length of the wall. The 42-inch diameter drilled shafts extended to depths of 60 feet and were socketed into competent bedrock. As drilled shafts were finished, soldier piles were then set and concrete was placed in each hole. Upon completion of the soldier pile installation, the tieback work could commence.
Drilling for the tiebacks involved advancing 7-inch diameter permanent casing, inclined 30 degrees from horizontal, to the top of rock then drilling of a smaller diameter hole through bedrock to develop the tieback bond length. The casing was installed utilizing duplex drilling methods while the rock socket drilling incorporated rotary duplex drilling, The rock anchors were then installed, grouted and subsequently tested as required by specification.
Following the successful completion 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.