Judith River Slide Repair

S-426 (Hanover Road) near the intersection with Ross Fork Road and the Judith River Crossing had been experiencing ongoing movement related to landslide(s) for several years. The roadway had been releveled several times with asphalt millings. However, in the spring of 2011, heavy spring rains and flooding occurred along the Judith River Drainage, and significant roadway movement occurred requiring the roadway to be closed. The recent movement resulted in vertical offsets as large as 5 feet along the slide area.

SK Geotechnical was retained to evaluate several alternatives for either stabilization or realignment of the existing roadway. Based on the alternative analyses, it was decided the best alternative would be to realign the existing portion of S-426 that currently travels across the historic slide area and locate it further south of the current alignment and away from the active slide.

The new alignment will diverge off of the existing roadway near Station 48+97, and cut through an existing ridge line that will have a maximum cut depth of about 35 feet. The alignment then extends across the Little Rock Creek Drainage, where about 30 feet of fill will be required. Little Rock Creek will be realigned and routed through a 12- by 10-foot reinforced concrete box culvert that will be about 215 feet in length. The alignment then cuts through the existing terrace that overlooks the Judith River Drainage until it intersects with the current alignment near Station 77+00. This cut will have a maximum depth of about 38 feet.

As part of the S-426 alignment, it will also be necessary to realign the existing county road, Ross Fork Road, that currently intersects near Station 61+20. Ross Fork Road will be realigned on top of the existing plateau to intersect the new S-426 alignment at Station 76+50. The Ross Fork Road realignment will generally extend across a fairly level plateau with cut and fill depths generally less than 10 feet.

As part of the realignment, it will be critical to control seepage along future cut slopes to prevent future slide development. This seepage will be collected in a combination of interceptor trenches and slope armoring, and seepage will be routed to nearby ditches and/or culverts.

The S-426 alignment will be constructed as a rural paved surface, and Ross Fork Road will remain a gravel-surfaced roadway.

SK Geotechnical performed soil borings and installed one inclinometer to evaluate if movement was currently occurring. Laboratory tests consisting of moisture content, classification, corrosion, unconfined compression, specific gravity, standard Proctor, unconfined compression, consolidation, and triaxial shear tests were performed. Detailed slope stability analysis to evaluate the stability of the future embankment and cut sideslopes was also performed. Evaluating settlement potential of the box culvert and providing recommendations regarding foundation report, and developing recommendations to control seepage in future cut slopes were also performed.