Evaluation of PFWD Factors on Stiffness Modulus Determination of Unbound Materials

Worldwide use of portable falling weight deflectometer (PFWD) for construction quality control of earthworks and unbound pavement layers and their stiffness determination is increasing rapidly. However there are still many unknown factors related to application of PFWD, precision of the test results that may affect the PFWD data quality and it is necessary to identify them.
In this research, field testing was carried out on different unbound base and sub-base layers on several highway construction sites in Tehran. In addition, a laboratory testing box was built in which several layers of unbound materials were compacted using a portable vibrating compactor. The stiffness moduli of the compacted layers were determined using PFWD (TML model Japan). Then, factors affecting PFWD measurements such as drop weight, drop height, plate diameter, plate-surface contact, and distances of extra geophones were investigated. The results showed that the PFWD moduli increased with increasing drop weight for the considered values and a model was presented for prediction of PFWD stiffness modulus through increasing of the drop weight. But the effect of drop height on PFWD moduli was not significant. It was found that the E0 moduli from the 100 mm loading plate were about 1.85 times higher than those from the 300 mm loading plate. Also, it was shown that the plate-surface contact was important in PFWD results. Finally, distances of extra geophones were discussed and it was found that they don’t have any important effect on upper layer moduli.