Stabilization System Stops Slope Failure

By Lisa Doyle | September 28, 2010

Geotechnical Contractors Hayward Baker, Inc., North America geotechnical contractor, recently completed an ambitious landslide repair project for the Mississippi Department of Transportation (MDOT). The work was conducted along a stretch of U.S. Highway 61, to stabilize a failing slope that had caused the closure of both southbound lanes at Signal Hill on the outskirts of Vicksburg, Mississippi. As general contractor for the project, Hayward Baker installed more than 250 buried anchor blocks to stabilize the slope.

The project is noteworthy because of the size of the slide (some 1,300 feet) and the depth of the slide plane below the surface, which necessitated the use of the sophisticated anchoring technology and drilled tiebacks as long as 265 linear feet.

Evidence of landslide activity had been observed at Signal Hill as early as 1977, shortly after completion of the roadway. An earthen berm was constructed to reduce the rate of movement. A trench drain-type system was also installed in an attempt to remove water that had collected under the roadbed in the fill area, but, over the course of the next 25 years the roadbed would sink as much as 4 feet on occasion, requiring extensive repairs.

MDOT contracted with Burns Cooley Dennis, Inc. (BCD), geotechnical and materials engineering consultants, to investigate and evaluate the movement of the slope, and recommend a new stabilization system in 2004. BCD's solution was a permanent ground anchor/buried anchor block system — a retention system to resist the landslide forces and add safety. Over 250 anchor blocks were used on the project. Anchors had to be bonded in material below the failure plane, which meant anchor lengths were between 250 feet and 265 feet apiece. Each anchor also needed a load capacity of around 400,000 pounds to resist the slope's failure.

Hayward Baker was awarded the bid and served as prime contractor for the 10-month, $6-million project.

"It's an unusual length as far as anchors go," said Steve Buckner, senior projects manager for Hayward Baker. "It's quite a bit longer than many anchors."

Anchor blocks were precast on the site at the highway level and then carried down to position. Anchors are 14 strand and bear against reaction block, a large concrete block that is about 13 square feet and 30 inches thick. Each block has a hole at the center to facilitate drilling.

Buckner says Casagrande C8 and Klemm 807 drill rigs were used to drill 7-inch-diameter holes and advance a 7-inch-diameter drill casing to the target depth of the anchors. Anchor tendons were then fed through the hollow casings and a cement grout was pumped inside. Once the grout reached design strength, anchor tendons were post-tensioned to exert full force against the slope.

The design engineer team for the project also included ABMB Engineers, Inc., Advanced Engineering Resources, Inc., and the MDOT geotechnical branch. BCD received the 2007 Grand Conceptor Award from the American Council of Engineering Companies — Mississippi (ACEC-MS).