A comparative study of slope stability methods and mitigative design of a highway embankment landslide with a potential for deep seated sliding

During the very exceptional 1991/1992 winter of heavy rainfall in Jordan, slope stability problems were encountered at an embankment at Station 47 + 300 along Irbid-Amman Highway. Geological and geotechnical investigations were conducted at the landslide area. Parametric slope stability analysis and back analysis were carried out for the embankment at the time of instability, using both Modified Bishop and Modified Janbu limiting state equilibrium methods (incorporated in the computer program PCSTABL5M (Achilleos, E., 1988. User Guide for PCSTABL5M. Joint Highway Research Project, Report JHRP-88/19, School of Civil Engineering, Purdue University, West Lafayette, IN, USA)), in order to determine with confidence the most representative values of regional shear strength parameters, explain the failure mechanism, assess the condition at the time of failure, and investigate all possible failure mechanisms through different foundation layers. Furthermore, slope stability analysis was carried out for mitigative design options. In the analysis, different conditions of piezometric water level were considered. Moreover, circular and sliding “Block” failure surfaces at shallow and deep seated levels were investigated. is showed that the main causes of instability were: excessive load of the embankment, constructing the embankment on colluvial deposits, and the ingress of water into the embankment, and the underlying colluvial material, resulting in its softening. The landslide movement occurred mainly within the colluvial deposits. The most likely location of the slip surface was at the embankment-colluvium contact. is also showed a potential for a deep seated sliding along a marl layer that governed the remediation procedures which should be implemented to stabilize the area. st estimates of effective shear strength parameters of the materials encountered at the landslide area were as follows: for colluvium, cohesion (c′)=0.0 and angle of friction (gftʹ)=25.0°, and for Marl, c′=25 kPa, and gftʹ=13.0°. st appropriate remedial measure for road design and stabilization of the sliding area was found to be a combination of vertical reduction in embankment height and horizontal realignment. This remediation was implemented in the field successfully.