Experimental and Computational Study of Performance of Highway Embankments
Highway embankments require careful selection of the borrow material and construction practices. Most highway agencies in North America specify the soil selection criteria based on gradation or Atterberg limits. Furthermore, current construction practices also include specifications for the placement and compaction of the selected borrow materials. However, embankment material selection criteria and embankment construction specifications were discovered to be variant among the agencies. Some agencies use plasticity index requirement as the only specification regarding selection of embankment material. Some other agencies specify using dry unit weight of the compacted soil in the form of a ratio known as relative compaction (RC), as their main construction requirement. Although this traditional approach has resulted for the most part in safe embankments, there have been unsatisfactory performances in some cases. For example, longitudinal cracking and shallow slope failures have been reported for some embankments, despite the fact that material selection criteria and material placement criteria were met. Review of the literature reveals that in the process of embankment design, for the most part, no information is provided regarding embankment slope stability or, embankment allowable settlement.In this dissertation, an alternative approach for the selection and placement of borrow soils for highway embankments has been investigated. The alternative approach is a performance-based methodology where the selection and placement of the candidate borrow soil must meet performance criteria for slope stability and deformation levels. The study focused on five test soils from the Piedmont region of North Carolina. For each test soil, extensive laboratory testing was performed to allow assessment of the aforementioned performance criteria. This includes a series of analyses to assess slope stability and deformation levels both short-term and long-term. To investigate performance of embankments, soil strength parameters obtained from both total stress analysis (TSA) or undrained conditions and effective stress analysis (ESA) or drained conditions have been considered. A set of unconsolidated-undrained (UU) triaxial tests was used to obtain total stress soil strength properties, and a set of consolidated-undrained (CU) triaxial tests with pore pressure measurements was considered to achieve effective stress soil strength properties. Moreover, a set of one-dimensional creep compression tests were considered to study long-term deformation characteristics of embankment soil materials. Sixteen embankment geometric sections have been considered in total for the study. For the highway embankment deformation analysis, two-dimensional plane strain conditions were assumed. It is noted that the scope of this study considers only failures and settlements related to the highway embankment and not due to poor foundation soil conditions. The study found that the proposed performance-based criteria is generally a viable alternative to the traditional approach. No cases showed that the TSA factor of safety was lower than the minimum value of 1.3. In many of the TSA cases, FS was well above the minimum value. However, in the effective stress stability analysis many cases were found to have FS lower than 1.3. Observed modes of failure consisted of non-shallow mode, local mode, and shallow mode. For the effective stress slope stability analysis, shallow failure (infinite) must be checked as there is a high likelihood for this mode of failure. Some findings might give ground to the idea that soils with higher PI, such as Soil 2 (A-7-6 class), perform slightly better under saturated conditions. This finding may cast doubt on specifications set by some agencies to limit Atterberg limits of embankment material as a selection criterion. This fact may also reject the specifications which abandon using A-7 group soil as embankment material. Providing suitable vegetation cover as well as drainage systems (to reduce infiltration and promote runoff, respectively) for the highway embankments could be useful measures in avoiding the detrimental effects of the water presence in the body of embankment. There was no strong evidence indicating that the soils with higher PI had a higher creep rate. The creep rate of 6×10-6 %/min may be introduced as a rough number for the silty soils compacted at optimum moisture content which were studied in this research.