When Moretrench was awarded the contract to install soil anchors to stabilize a failing gabion wall, a creative approach to equipment staging, cou- pled with remote drilling operations, overcame the access and installation challenges.
The gabion wall, some 500 feet in length, retained more than 28 feet of fill material, placed to bring the site up to grade for construction of four, 3-story duplexes in Ridgefield, NJ. Soon after completion and occupancy, outward movement of portions of the wall and cracking occurred in three of the buildings. The Borough of Ridgefield Building Department declared the buildings to be unsafe and enforced evacuation. The borough then retained geotechnical engineering company Schoor DePalma, Parsippany, NJ to conduct a subsurface investigation, global slope stability analyses and a stability study of the existing gabion wall and make recommendations for remediation.
Soil Anchors Considered Best Option
Soil borings revealed approximately 22 feet of sand fill with varying amounts of silt, gravel and construction rubble. Typical SPT N values were less than ten. Beneath the fill material, dense glacial till extended to weathered rock at 27 to 30 feet below grade. Global stability analyses indicated marginal existing factors of safety (1.05 to 1.08 under static conditions) in the slope behind the three affected properties. Schoor DePalma therefore recommended soil anchors to improve the global factor of safety of the wall under longterm static and seismic conditions.
The wall stabilization design developed by Schoor DePalma consisted of 1.25-inch diameter, 30-kip capacity grouted anchors installed through pre-drilled, cased holes and terminated a minimum of 6 feet in the glacial till.
Several conditions precluded conventional drilling techniques:
- No access was available either above or in front of the gabion wall.
- Additional surcharge on the severely distressed wall was undesirable.
- Casing of drill holes cannot be accomplished with small equipment located on a scaffold system.
However, one of the greatest concerns was the effect that the drilling could have on a gabion wall in this condition. This type of operation, if not unique, is extremely rare, with no reported history.
In light of all of these considerations, Moretrench developed a remote drilling system. This consisted of a 300-ton crane, with a working radius of 170 feet, based in the parking area in front of the development. With the uncontrolled nature of the fill material inadequate to handle the surcharge of the crane, Moretrench also designed and installed minipile- supported platforms to transfer the crane load to the glacial deposits. In order for the crane-supported drill boom to cover the entire length of the wall, two crane set-ups were required. Each set up had 2 platforms, each supported by 2 minipiles.
The platform-mounted drill boom was suspended from the crane while the body of the rig remained on land. Connection between the boom and the engine was by hydraulic lines.
One hundred and ten anchors, with an average length of 29 feet, were installed on a six-foot horizontal spacing. All anchors were locked off at 20 kips and two percent were performance tested. A final shotcrete facing completed remediation of the wall, which was successfully accom- plished ahead of schedule.