Guidelines On Settlement Criteria For Design Of Highway Projects

This paper examines the implication of post-construction settlement and differential settlement on highway pavements constructed on soft soils by considering a number of a factors including: type of pavement, rate of settlement, ride quality, and likelihood of pavement distress. The main purpose of this paper is to provide some guidance and clarity on how differential settlement criteria should be specified and allowance made in the design, and in the selection of pavement types. Experience is drawn from monitoring and maintenance records gathered on various sections of the Pacific Highway Upgrade and other NSW roads. Extensive research has been carried out by the Roads and Maritime Services of NSW (RMS) and we have quoted extensively from their draft RMS (2009) “Guide for design and performance of concrete pavements in areas of settlement”. Structural performance of the pavement is considered in addition to ride quality functionality, and guidance is provided for designers to select the appropriate settlement and differential settlement criteria for highway projects. The RMS document uses radius of curvature as a basis for design, which is strictly speaking correct but difficult to predict when designing embankments on soft soils due to small allowable post-construction settlements. This paper provides some guidance on the correlation between the more commonly used limits on “change in grade” specified on recent road projects and “radius of curvature” and provides warning on the potential misuse of these values in assessing the length of transition zone required behind bridge abutments.