When a mechanically stabilized earth (MSE) retaining wall was determined to be in danger of failing, the Pennsylvania Department of Transportation (PENNDOT) elected to construct a soldier beam and precast lagging wall immediately in front of the existing structure. Moretrench was retained to install two levels of permanent rock anchors to provide additional lateral restraint.
The 290-foot long wall is located along SR 28 in downtown Pittsburgh, adjacent to the Sarah Heinz House youth community center. Anchor installation presented Moretrench with three specific challenges:
- The 15-foot wide shoulder lane of the heavily traveled highway served as the staging area and work zone for all constructino disciplines, including general contractor Joseph B. Fay Company who installed the new wall with multiple sub-contractors,
- The upper level of tiebacks was to be installed at 18 feet above road level, and
- The installed tiebacks had to avoid existing MSE backstraps and augercast piling.
Close cooperation between all parties was therefore key to timely completion of the project.
Subsurface conditions consisted of 25 feet of gravelly sand with clay overlaying a 10 foot thick stratum of moderately weathered claystone. Beneath this, hard grey sandstone was encountered.
The 133-kip, double-corrosion protected, 5-strand anchors, ranging in length from 105 to 120 feet, were designed with a minimum bond zone of 15 feet in competent rock.
Access to the target soils was provided by angled sleeves pre-placed between welded double H-beams. At each location, Moretrench used duplex drilling techniques with a down-hole hammer to advance permanent, 80 ksi steel casing through the MSE wall and overburden soils to top of rock and cut a 6-inch diameter socket to full anchor design length. Anchors were installed and tremie-grouted with 4000-psi, Type 2 cement grout.
Following a 3-day grout curing period, the anchors heads were installed and specified testing performed to 150% of design load before lock-off. Three creep tests and five performance tests were conducted for the project, with the remaining 64 anchors proof-tested. Once anchor installation was completed, Joseph B. Fay placed aggregate to fill the annulus between the walls.