Damaging Landslides of Cut Slopes Along Major Highways in Jordan: Causes and Remedies

Abstract

Several cases of instability of cut slopes along major highways in Jordan are reviewed in this study, in some detail. Emphasis was placed on the Amman-Irbid Highway, but some cases along the Na9ur-Dead Sea Highway were also reviewed, with the aim of establishing a wider data base of case studies and examining all possible mechanisms and factors influencing stability. This study shows that major cut slope failures are caused by the presence of weak cohesive layers (mainly clayey marl) interbedded within other, stronger formations, in addition to the steep cutting angles and unfavorable dip of strata combined with a relatively high piezometric surface brought about by poor surface and subsurface drainage. Sections II and III of the Amman-Irbid Highway suffer from an alarmingly high level of instability in cut slopes where more than 50 percent of the entire route length was found to have a high level of instability. To investigate the influence of various parameters on slope stability, several analyses were performed, in addition to back analyses, to determine shear strength parameters; in some cases, parametric sensitivity studies were also performed. Data was obtained from previous investigations by local and international firms, screened and modified where needed, before being used in the stability analyses. Considerable differences between back analysis and test result values for shear strength parameters were observed in many cases. Failure surfaces and mechanisms were accurately depicted in most cases, allowing back analyses to be performed with considerable confidence. The effective residual shear strength parameters for clayey marl needed for the stability analysis were found to be: c r ′ = 5 to 18 kPa, and (ϕ r ′ )/ = 13° to 18°. A classification system was developed for factors influencing landslides based on the case studies considered, and a stability grading scheme was also introduced. This system was applied to several existing cases along the Amman-Irbid Highway and the resulting assessments were in agreement with the observed stability conditions.