Investigation of Pavement and Subgrade Distress at Alabama Highway 5

Abstract

The primary objectives of this study were to determine the root causes for subgrade failure and associated pavement distress along Alabama Highway 5 in Perry County, Alabama. In order to accomplish these goals photographic, video, laboratory and field data were collected regarding the preconstruction conditions of the Alabama Highway 5 research site. These tasks were performed in coordination with larger research objectives to evaluate various remediation strategies for existing shrink-swell clays in western Alabama. It is hypothesized that the seasonal variation in moisture demands of large trees growing within a zone of influence of the roadway has a detrimental effect on the subgrade through shrinking and swelling of expansive clays and consequently causes localized pavement distress. To verify this, laboratory tests – including one dimensional swell, soil water characteristic curves, and standard AASHTO classification tests – have been performed. A survey of the trees greater than 6 inches in diameter and within 60 feet of the roadway, ±10 feet based on the diameter of trees present, was conducted to draw correlations between observed pavement distress and the moisture demand of trees. Additional observations included an estimation of shear strength through published correlations. International Roughness Index surveys were performed to determine the roadway roughness. It has been shown that soils at Alabama Highway 5 have considerable swell potential generating pressures between 500 and 1500 psf. The shrink/swell cycle is predominately driven by suction pressures either from the soils themselves or nearby vegetation. The soils suction potential has wide ranging impacts including variable shear strength and volumetric changes, which have led to pavement and slope distress.