Embankment Widening Design Guidelines and Construction Procedures

In recent years failures have occurred in Indiana highway embankments where the embankments were widened and steepened in order to facilitate construction of longer, safer acceleration and deceleration lanes and to increase the traffic capacity and efficiency of existing thoroughfares. The objectives of this study were to investigate the cause of failure and to make recommendations for modifying the existing INDOT Standard Specifications and construction guidelines in an attempt to avoid similar failures in the future. A literature review and survey of state and federal transportation agencies was performed to collect current, available design guidelines and construction procedures for sideslope steepening projects. Several projects were investigated where widening and steepening of existing embankments was performed. Both failed and successful projects were investigated to discern the differences in approach used that may have led to failure in some cases. For each site, available design documents and construction records were reviewed to identify key aspects of individual projects that may have contributed to the projects being categorized as successful or unsuccessful. In general, very little information was available in terms of engineering design documents, field observations during construction, or as-built drawings for these projects. For the most part quality control tests were limited to the right-of-way with few tests conducted in the widened slopes. Investigations of the failed embankments involved both field investigations and laboratory tests, including SPT and CPT tests, test pits, in situ density tests, index tests, compaction tests, and strength tests. Investigations of successful projects were also performed and included site reconnaissance and hand auger borings. The results of the investigations indicate that failure of the widened embankments resulted from sub-standard compaction of fill and inadequate benching into the original embankment. Surface water infiltration from the roadway run-off contributed to the problem, possibly saturating and softening the soils. It is concluded that the slope failures investigated would not have occurred if INDOT Standard Specifications were followed during construction. Therefore, it appears that the primary cause of failure is the lack of appreciation of the potential risk by the parties involved. Several recommendations are provided to help prevent these types of failure in the future, including modifications to the existing specifications and the transfer of information among INDOT personne