FWD Determination of the In-Situ Subgrade Resilient Modulus and Effective Structural Number of Virginia's Interstate Network

Abstract

The Virginia Department of Transportation (VDOT) manages approximately 25,000 lane-miles of interstate and primary roadway; of which approximately 2000 lane-miles is comprised of interstate pavements. These pavements consist of flexible, rigid, and composite pavements. Virginia’s pavements are managed utilizing a pavement management system (PMS) which aides VDOT in determining the amount of funding required for various levels of maintenance (i.e., preventative maintenance, rehabilitation, or reconstruction). As part of VDOT’s PMS system, a portion of the interstate pavements are visually rated on an annual basis to determine a condition index based on load related and non-load related distresses. Recently, VDOT began utilizing an automated distress collection procedure for this task will incorporate the measurement of pavement condition data such as the International Roughness Index (IRI), rutting on both wheel paths, and a cracking index. However, there is no current method to asses the structural capacity of the pavement and thus determine the remaining load carrying capability of a pavement structure. Many state departments of transportation use the Falling Weight Deflectometer (FWD) equipment to collect pavement deflection data at the project or the network levels. The analysis of this data provides the effective roadway resilient modulus, the effective in-situ structural number, the pavement layer moduli, the effective in-situ layer coefficient or all of these parameters. This process is accomplished through the backcalculation procedure utilizing numerous developed in-house or commercial backcalculation routines that utilize the FWD deflection data, know as the deflection basin, the FWD load history, and using the pavement layer thicknesses as an input to this procedure. VDOT currently uses the 1993 AASHTO Guide for the Design of Pavement Structures for the design of its new or rehabilitated pavement structures. The AASHTO guide is based on empirical relationships that were developed during the 1960s as a result of the AASHTO road test. As VDOT moves to implement the Mechanistic-Empirical Pavement Design Guide (MEPDG), characterizing existing pavement conditions, including the resilient modulus of the subgrade, is an enhancement of the input level compared to using the MEPDG’s default values. Therefore, there is an urgent need to collect the in-situ structural number subgrade resilient modulus and deflection along VDOT’s interstate system and to build the associated database for future implementation of the MEPDG. In addition, this information will allow for better management of VDOT resources on the network level for the interstate system. This paper describes the results of testing Interstate 77 and also describes the details and experiences during the process.