DETERMINATION OF SHEAR STRENGTH FOR DESIGN OF CUT SLOPES IN PARTLY WEATHERED ROCK AND SAPROLITE. FINAL REPORT

Three active landslides located in western North Carolina were investigated. They are: Balsam Gap II landslide located along US 23/74 in Haywood County; Boone landslide located along US 321 in Watagua County; and Waterville landslide located along I-40 in Haywood County. At the Balsam Gap II and Boone landslides, borings were made to obtain soil samples, install slope indicators and piezometers, and to perform pressurementer tests. Surveys were made to determine the surface topography, and test pits were excavated to inspect the shear surface which was located in the saprolite just below the interface with the overlying colluvium. The Waterville landslide had already been investigated by the NCDOT and LAW Engineering so that only a boring was performed to obtain undisturbed samples for laboratory testing. At NCSU, the undisturbed samples were tested in direct shear, ring shear, and consolidation tests. At the NCDOT laboratory, the undisturbed samples were tested in consolidated undrained triaxial tests. The direct shear tests measured peak friction angles of 32 to 38 degrees and cohesion values of 0.25 to 0.48 ksf. The ring shear tests measured residual friction angles of 21 to 27 degrees and cohesion values of 0.04 to 0.15 ksf. The triaxial results fell between these bounds depending upon the orientation of the weakness surfaces. Back analyses show that at failure the pore pressure ratios vary from 0.2 to 0.49 while piezometers measured zero pore pressure. Rainfall causes pore pressures to rise quickly to the failure values. Laboratory measured permeabilities and observations made in the test pits indicate rainfall infiltration caused a perched water table in the colluvium at the Boone landslide while it caused an artesian condition in the saprolite at the Balsam Gap II landslide.

• Corporate Authors:

  North Carolina State University, Raleigh

  Department of Civil Engineering
  Raleigh, NC  United States  27695-7908

  North Carolina Department of Transportation

  Division of Highways, P.O. Box 25201
  Raleigh, NC  United States  27611

  Federal Highway Administration

  1200 New Jersey Avenue, SE
  Washington, DC  United States  20590
• Authors:
  • Lambe, P C
  • Riad, A H
• Publication Date: 1990-6

Media Info

• Features: Appendices; Figures; References; Tables;
• Pagination: 453 p.

Subject/Index Terms

• TRT Terms: Backcalculation; Boring; Colluvium; Design; Direct shear tests; Embankments; Groundwater; Laboratory tests; Landslides; Permeability; Pore pressure; Rainfall; Ring stress; Shear strength; Slope stability; Soil consolidation test; Triaxial shear tests; Water table; Weathered rock
• Uncontrolled Terms: Ring shear; Saprolite
• Old TRIS Terms: Artesian conditions; Perched water table
• Subject Areas: Data and Information Technology; Design; Geotechnology; Highways; Hydraulics and Hydrology; I42: Soil Mechanics; I43: Rock Mechanics;

Filing Info

• Accession Number: 00616577
• Record Type: Publication
• Report/Paper Numbers: FHWA/NC/91-001, Rept PN 23241-88-3
• Files: TRIS, USDOT, STATEDOT
• Created Date: Nov 30 1991 12:00AM