Installation of rock anchors for a 50-foot high, exposed retaining wall constructed in steeply sloping terrain was the challenge facing Moretrench during reconstruction of a section of State Route 38 in northwestern Pennsylvania. A value-engineered design and anchor installation using Moretrench’s specially adapted severe- access drill rig were instrumental in the success of the project.
As part of the reconstruction of Route 38, a roadway fill was constructed to accommodate realignment. The eastern portion of the project included six structural retaining walls designed to support the realigned roadway along steeply sloped hillside. The tallest of these walls, Wall 4, is a 50-foot high, tied-back soldier pile retaining wall.
Value-Engineered Alternate Design
The original Wall 4 design required 116 tiebacks installed through double beams for the 360-foot long wall. Drilling and anchor installation would be accomplished using a platform-mounted drill rig supported by a crane located on the existing roadway behind the wall.
A value-engineered alternate to the original design, reducing the number of tiebacks to 81, was developed by consulting engineer SAI, Inc. The VE design utilized single soldier piles in lieu of the double beams, with pre-fabricated steel wales installed in every other soldier pile bay to support the anchors.
Subsequently, Moretrench consulted with general contractor Mekis Construction Corporation and SAI, and offered constructability input that would allow the use of its flexible, specially designed drill rig in lieu of the crane-supported drill platform. This approach would improve the overall safety of the work by allowing drilling and anchor installation of the bottom rows of anchors to be accomplished from a work bench constructed in front of the retaining wall. The drill rig would access the upper rows by reaching over the wall from the work bench behind. Moretrench also offered an experience-based revised construction sequence that would reduce equipment downtime and streamline operations between the various crews.
Both duplex and overburden drilling methods were used to install the 5- to 10-strand, 135 to 278 kip anchors through a soil profile consisting of the sequentially placed sand/gravel embankment fill behind the wall and highly weathered and fractured sandstone and siltstone bedrock. Using “bottom-up” construction techniques, the lowest row of anchors was installed first, with subsequent levels installed as the wall itself was constructed upwards between the soldier piles. Overall anchor lengths ranged from 45 to 90 feet, with a minimum 20 foot bond length in the rock. Due to the variable rock conditions present at the site, some rock anchors experienced significant grout takes while others required consolidation grouting and re-drilling to meet the design requirements. With anchor installa- tion accomplished, a continuous cast-in-place concrete facing was then installed to complete the wall.