Use of In Situ Testing to Optimize Slope Design in Highly Weathered Shale

Slope stability of embankments bearing over highly weathered shale encountered at shallow depths is a major concern in roadway design. This condition was encountered at a bridge location in southeastern Iowa, where the proposed 120.5-meter x 12.0-meter creek crossing dual bridges are part of a four-lane state highway. The maximum height of the embankment at the north abutment location is approximately 12 meters and 8 meters at the south abutment location, with 3H:1V slopes in the longitudinal and transverse directions. Conventional soil borings with standard penetration testing indicated the presence of weathered shale at very shallow depths, in some cases as shallow as 0.9 meters. The subsurface profiles indicated the presence of sloping shale at both the north and south abutments overlain by sandy lean clay. The average slope ratio of the top of the shale is approximately 10H:1V at the north abutment location and 3H:1V at the south abutment location. As per Iowa DOT guidelines, a 0.3-meter-thick layer of highly weathered shale was assumed at the clay/weathered shale interface and assigned undrained shear strength of 10kPa. Slope stability analyses indicated global slope instability with failure surfaces propagating through the sloping and highly weathered shale layer. As a result, various ground improvement, retaining wall, and increased bridge length alternatives were evaluated. These alternatives were estimated to cost between $3,000,000 and $5,000,000. In view of these high costs, an additional comprehensive subsurface exploration and testing program was developed and executed at the site to verify that the shear strength parameters, especially with respect to the highly weathered shale, were reasonable. The supplemental program consisted of conducting 35 in situ Iowa borehole shear tests at various depths in the 10 boreholes, particularly at the shale-clay interface. Slope stability analyses were performed using SLIDE software, including probabilistic analyses to evaluate the probability of potential global slope instability. The analyses indicated the slopes to be stable under both short-term (end of construction) and long-term (drained) conditions. The judicious use of in situ testing in addition to conventional laboratory testing resulted in the elimination of potential high-cost alternatives and kept the project within budget limits.


  • English

Media Info

  • Media Type: Print
  • Features: Figures; References; Tables;
  • Pagination: 17p
  • Monograph Title: Proceedings of the 2005 Mid-Continent Transportation Research Symposium

Subject/Index Terms

Filing Info

  • Accession Number: 01004302
  • Record Type: Publication
  • ISBN: 9780965231084
  • Files: TRIS
  • Created Date: Sep 23 2005 9:59AM