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On November 7, 1972, a rockslide of approximately 13,000 m3 fell into the uncompleted northbound lane of a section of Interstate 93 in the town of Woodstock, New Hampshire. The rockslide forced almost total curtailment of construction operations at the site of the failure and necessitated a total redesign of the highway section through the rock cut.

The redesign of the 44 x 213 m rock cut began with a detailed geologic mapping of the structure of the rock in the vicinity of the failure, because such a study had not been undertaken during the preliminary design phases for the project. Several weak structural trends were established for the cut area, with the most disadvantageous system striking parallel to the roadway and dipping into the open cut. This system was also troublesome because many of the joint surfaces associated with it were filled with a mylonitic material of little structural competence.

A 2:1 (vertical:horizontal) slope was specified for the rock cut following this study, rather than the 8:1 slope as had been originally designed. The neatline for the presplit along this new slope was carefully placed on the mountainside to take best advantage of the existing structure of the rock mass.

As excavations proceeded along this new slope, a second set of mylonitic joints was encountered on the new presplit face. Construction was again halted while this set was structurally analyzed and a second redesign for the section formulated.

The second redesign was based upon the gravity-mass concept and involved the following engineering and construction: (1) excavation of a rock bench at the top of the anticipated sliding failure parallel to the newly exposed mylonite; (2) placement of a system of high-strength steel tendons into the toe of the bench to further resist the anticipated failure; (3) installation of spot rock bolts along the crest of the bench to actively secure the intact mass of the bench itself; (4) drilling of an extensive system of rock drains along the toe of the cut; and (5) installation of an instrumentation system to monitor the success of the redesign.

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