Post–Soil Interactions on Guardrails near the Crown of a Slope

Abstract

Most highway departments are very concerned with the adequacy of highway guardrails. A common problem is a narrow road shoulder of 1.5 m to 0.5 m, or even less, combined with steep side slopes and a high embankment beside the road. Although numerous studies have measured the strength of different types of guardrail posts in various soils, a theoretical model does not exist that could be used to predict soil resistance to post movement. Such a theoretical model was constructed for level ground as well as close to an embankment slope. A mechanical model of a narrow vertical object moving soil with a level surface was modified to account for a ground slope that lies within the area of influence of soil failure. The model predicts that at any distance from a slope, the resistance of a post can be increased by installing it to a greater depth. Also, when soil adjacent to a post is compacted heavily, it results in approximately a fourfold increase in horizontal resistance compared with a lightly compacted soil. A rough surface post, such as concrete, gives almost double the horizontal resistance of a smooth surface post of the same dimensions. Using a post of 400 mm width rather than 100 mm results in a horizontal soil resistance force of only approximately 20% to 30%, for a post far away from or very close to the crown of a slope of 1 vertical/2 horizontal.