McMahon & Mann Consulting Engineering and Geology, P.C.

Mt. Barron Rock Reinforcement Condition Assessment

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Mt. Barron Rock Reinforcement Condition Assessment
Project Details
Location: Woodstock, NH I-93, Mt. Barron Rock Cut
Client: New Hampshire Department of Transportation Bureau of Materials and Research 11 Stickney Ave Concord, NH 03302-0483
Client Contact: Mr. Glenn Roberts, P.E. NHDOT 603-271-3151

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The New Hampshire Department of Transportation retained McMahon & Mann to evaluate the condition of 30-year old rock reinforcements supporting the Mt. Barron Rock cut along I-93 near Woodstock, NH. McMahon & Mann applied a recently developed recommended practice for evaluation of metal tensioned systems in geotechnical applications (NCHRP Project 24-13) that employs nondestructive testing (NDT), supplemented with results from invasive testing, to evaluate the elements and perform a condition assessment. We performed the condition assessment in two phases during the summer and fall of 2003 and 2004.

Two types of rock reinforcements are installed at Barron Mountain including: (1) partially bonded, polyester resin grouted, prestressed rock bolts, and  (2) fully bonded, Portland cement grouted, rock nails. McMahon & Mann's condition assessment revealed that rock nails protected by Portland cement grout are in very good condition compared to the resin grouted rock bolts. Approximately 30% of the rock bolts have suffered significant loss of prestress, and locations of increased corrosion activity were documented including a zone near the backside of the bearing plates. Observed section loss of 20% is consistent with expectations based on site conditions, installation details, date of installation, and existing mathematical models of service life. These results provide a sound technical basis for planning future maintenance and rehabilitation activities at the site, ultimately resulting in a cost savings to the NHDOT.

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