The concept of estimating vehicle loads by weigh-in-motion systems tha
t monitor deflections in strain-gaged bridges and culverts is well est
ablished. In this project, the possibility of estimating loads by moni
toring subgrade deflections in flexible pavements was investigated. A
line of vertical strain transducers was installed at a depth of 500 mm
in a dense sand subgrade across one wheel path of the outside lane of
a busy highway (this research was conducted in Australia, where the o
utside lane is on the left). The installation was calibrated with a tr
uck of known weight, enabling the weights of other vehicles to be calc
ulated and compared with weights determined by other means. Four test
runs were conducted at six month intervals. Repeatability, sources of
error, and accuracy of weight prediction are discussed. Successful res
ults were obtained with the system, but further work would be required
before it could be used as an operational system.