Development of design criteria for prevention of slope failures.

PDFTech ReportFHWA/LA-88/210STUDY NO. LA HPR 86-2GT(B)SlopesAtterberg limitsEmbankmentsClay soilsLouisianaLouisiana Transportation Research CenterMutchler, Jack W.Smith, Schaun M.Hernandez, MikeBurns, Scott F.Siddiqui, AdnanLouisiana Tech University. Dept. of Petroleum Engineering and GeosciencesLouisiana Transportation Research CenterUnited States. Federal Highway AdministrationUS Transportation CollectionA total of 242 embankments were examined along a 122-mile transect of I-10 and I-20 highways in Louisiana. A total of 99 slope failures had occurred 8-15 years after construction (mean volume = 15,105 cu ft). Most of the failures occurred on slopes greater than 16 deg. Over 70% of the failures were found in modern alluvium parent material as compared to loess, sandy alluvium and Prairie Terrace alluvium. It is believed that the high amount of smectite in these soils created most of the failures when the slope moisture content rose. A predictive model for the first 15 years after construction was developed. A high-rise slope has an 85-90% chance of failure and is constructed of soil with: 47% clay content, a plasticity index (PI) 29%, a liquid limit (LL) 54%, and net smectite 33%. Low-risk slopes have a chance of less than 5% of failure and are constructed of soils with: 32% clay, 16% PI, 36% LL, and net smectite 18%. Determination of the risk category can be done easily in the laboratory using Atterberg limits. Control of slope stability depends upon control of expansive clays in the high- and intermediate-risk slope soils. It is recommended to lime stabilize these soils or use a new slope design. A map of the distribution of these soils in Louisiana is included. Four different design nomographs based on a stability model of the slopes is presented for soils in the high- and intermediate-risk categories. Depending upon the availability of space and economy, the designs include constant slope configuration, broken-back design, and soil-stabilized layers below the above two