RockSol Consulting Group, Inc.
  • Libby Creek and Grave Creek Roadway

    Libby Creek and Grave Creek Roadway

Libby Creek & Grave Creek Roadway Embankment Slope Movement Evaluations


Client:

US Forest Service

Services:

Geotechnical Engineering

Location:

Lincoln County, Montana

Project Description:

RockSol evaluated slope movements, characterized subsurface stratigraphy, and developed roadway design and reconstruction recommendations for two low-volume, forest highways on behalf of the US Forest Service. The two sites where the roadway embankment slope movement was observed were Libby Creek and Grave Creek near Libby and Eureka, Montana. The observed distress at the Libby Creek site included longitudinal cracks extending around 180 feet along the outer fill section of the roadway alignment; while the distress observed at the Grave Creek site included longitudinal cracks extending around 75 feet and a failed zone along the outer fill section of the roadway alignment. RockSol conducted a geotechnical evaluation to characterize the subsurface stratigraphy of the roadway embankment and existing slopes at both project sites, performed slope stability analysis, and provided recommendations and geotechnical considerations for slope stabilization and roadway reconstruction. The subsurface investigation conducted by RockSol consisted of drilling three test borings within the affected roadway at each site location. One boring was located on the upslope (cut slope) side of the roadway and two on the downslope (fill slope) side of the roadway. The borings extended through the fill and overburden soils to terminal depths ranging from 31 feet to 42 feet below existing grades. Temporary piezometers were installed at each site in the cut slope boring location to allow US Forest Service to measure groundwater levels. Samples obtained from the field investigation were shipped to and tested in RockSol’s AASHTO certified Colorado lab, and characterized by our lead geotechnical and pavement design engineer. 


RockSol determined that cracking and subsidence observed on the fill slope portion of Libby Creek Road and Grave Creek Road were caused by localized instability, likely caused by improper fill placement, poor compaction, unsuitable fill material, and poor drainage. RockSol recommended deep patch road embankment repair or embankment repair using geofoam fill to stabilize these sections of roadway. Both solutions provided low-cost options that met the US Forest Service’s budget constraints while following the required FHWA guidelines for slope stabilization methods. RockSol’s roadway improvement recommendations took into account the project’s surrounding environment. Unique soil conditions at unstable slope sites, which were largely a mixture of sand, gravel, cobbles and boulders, were considered in both recommendations. In addition, both recommendations would create minimal disturbance to embankment and the contiguous woodland environment.