The Determination of Soft Subgrade Modulus for Airport Pavement Rehabilitation via Backcalculation of Falling Weight Deflectometer Data

The integrity of existing highway and airfield pavement is increasingly being evaluated based on the analysis of falling weight deflectometer (FWD) deflection basin data. Measured deflection bowls are matched to deflection bowls computed from multi-layered elastic analysis; the resulting "backcalculated" layer moduli are sometimes blindly used to forge pavement rehabilitation design decisions. However, as has been clearly documented in the literature, the static analysis of dynamic phenomena and the existence or lack of a near surface rigid layer dramatically affects the backcalculated moduli, particularly the subgrade moduli. This paper presents a detailed investigation of soft subgrade moduli via backcalculation from FWD data, triaxial testing, and pressuremeter testing. FWD testing was conducted and data was collected at more than 80 locations along a 1200 m general aviation flexible runway pavement. Deflections were significant, reaching 1.9 mm for an applied stress of 700 kPa. Layer moduli were determined using the widely accepted and Strategic Highway Research Program-recommended MODULUS backcalculation program. For comparison, the subgrade modulus was determined using the pavement pressuremeter and laboratory triaxial testing. Both the pressuremeter and the triaxial test results revealed very soft subgrade soil with moduli ranging from 5 to 47 MPa. Conversely, moduli backcalculated from FWD tests varied considerably depending on the depth to bedrock. Using the shallow depth to bedrock estimated by MODULUS, backcalculated subgrade moduli ranged from 25 to 60 MPa, however, using the actual depth to bedrock measured through site investigation, backcalculated subgrade moduli ranged from 65 to 180 MPa.