NEW HAMPSHIRE ROCK SLOPE STABILIZATION

Rock slope stabilization methods utilized by the New Hampshire Department of Transportation (NHDOT) fall into three general categories: 1. Elimination of the driving forces by reduction of water pressure and/or excavation of rock. 2. Increase the resisting force by installation of rock bolts. 3. Protect the highway from rock falls by wire mesh rock fall netting, barriers, rock fall ditches and energy absorbing stone. Remedial measures are tailored to the conditions and geology at each site. Several remedial rock slope stabilization techniques are reviewed in four case histories, each with unique challenges.

A project in Manchester involved the installation of 70,000 square feet of rock fall netting to restrain, channel and prevent rock fall from reaching the roadway. A barrier and energy absorbing material were used to catch and retain falling rock in the ditch. Twenty years later, the NHDOT installed eight 65-foot long anchors in the median section to keep the rock mass intact, to prevent a plane failure and to secure toppling slabs.

The NHDOT stabilized a 100-foot high rock slope located on I-89 in New London. The remediation consisted of removing 100,000 cubic yards of potentially unstable rock, which was dangerously close to the northbound lane. The granite rock at the site contained naturally occurring anomalous concentrations of uranium and thorium. The challenge was to presplit the entire height of the rock slope in a single lift and to safely dispose of the uranium rich rock while protecting the workers and the public.

A slide occurred in Woodstock during the construction of Interstate 93. The slide, consisting of 17,000 cubic yards of rock, buried the highway. Stabilization of the slope included extensive rock reinforcement, instrumentation and horizontal drains. Recent longevity concerns, resulted in the NHDOT undertaking a two-phase research study to assess the condition and to determine the remaining service life of the rock reinforcement. Non-destructive techniques and invasive test methods were conducted to evaluate the reinforcement.

Removal of unstable rock and the installation of prestressed rock bolts stabilized a toppling failure involving large slabs with tension cracks along the crest of a rock slope in Hinsdale. A total of 2,796 linear feet of rock bolts were installed.