LT Personnel Observing Rock Bolt Installation

The Montana Department of Transportation (MDT) had experienced several rockfall events at a site along NH-5 (US 93) located 5 miles south of Lakeside, Montana.  In February 1995, an approximate 15-foot diameter rock reached the southbound fogline.  Subsequently, MDT performed a site investigation and developed rockfall mitigation designs for a 250-foot section of US 93.

Landslide Technology (LT) was retained to provide full-time construction technical services for the rockfall mitigation elements including rock scaling and trim blasting; guiding and documenting rock bolt installation and acceptance testing; determining additional rock bolt and rock dowel locations, bar lengths and orientations; and assisting with the installation of instrumentation monitoring points.

Forty-one tensioned rock bolts with a total length of 995 feet and ten untensioned rock dowels representing a total length of 95 feet, were installed using a crane-mounted drill and rope work techniques. The general layout of the rock bolts and dowels coincided with the layout outlined by the MDT design, but final field adjustments were made by LT.  Nine bolts were painted with orange reflective paint provided by MDT for purposes of survey monitoring.  The instrumentation consisted of installing eight anchor points for monitoring with a tape extensometer and seven tiltmeter bases.