Extension of AASHTO Load-Spreading Method to Include the Full Benefits of Pavements for Reliable Load Rating of Buried Culverts

Abstract

This paper develops a two-step procedure to extend the AASHTO Ad Hoc Method (AAM) for live-load spreading through soil to include the full benefits of pavement overlays. The two-step procedure is called AAMP-θ* (“P” for pavements), which combines the Fox two-layer elasticity solution for peak stress at the pavement–soil interface with the improved AAM-θ* theory for load spreading through soil. The AAMP-θ* procedure is applicable to all culvert analysis tools commonly used for Load and Resistance Factor Rating (LRFR) analysis ranging from simple frame models to sophisticated two-dimensional finite element model (2D-FEM) soil structure models. The AAMP-θ* procedure is rigorously and completely developed wherein all approximations are shown to be conservative so that the predicted benefits of pavements are trustworthy and not overstated. This is why the more accurate AAM-θ* load-spreading method is used instead of the simpler but unconservative AAM-30° method. Application of AAMP-θ* for LRFR load rating of buried culverts is described for simple frame models as well as 2D-FEM pavement–soil structure models. Lastly, the AAMP-θ* procedure is validated by comparing structural response predictions from 2D-FEM solutions with strain-gage measurements in a buried corrugated steel culvert loaded by a three-axle truck. One set of strain-gage measurements was made before, and another set of measurements was made after, asphalt pavement was installed. The correlation between field data and 2D-FEM/AAMP-θ* predictions is remarkable and underscores the bottom-line message that AAMP-θ* should be used when performing LRFR analysis on buried culverts to determine realistic rating factor values and avoid predicting false unsafe values.