Normal Stresses in a Granular Material Under Falling Weight Deflectometer loading

The vertical normal stress under a falling weight deflectometer (FWD) was measured in a sand. The material had more than 90 percent falling within the sand fraction from 60 micrometer to 2 mm. The stress was measured with three different transducers. All transducers were installed at a depth of 280 mm. The sand was uniform to a depth of 700 mm and at a distance of 600 mm to either side of the centerline. An FWD was used to exert a known vertical force on the surface of the sand, using a loading plate 300 mm in diameter. The average stress under the plate was about 300 kPa. All the loads were imposed in the centerline of the three gauges, but at different horizontal distances from FWD to transducers. An integration of the measured stress on the plane of the transducers results in a force 10 to 14 percent larger than the peak force exerted by the FWD. The measured force is thus reasonably close to the actual force. When the measured stress is compared to the the stress predicted using Boussinesq's equation for an elastic half-space, a very large difference is observed. At the centerline of the load, the measured stress is about twice the theoretical value. This difference cannot be explained by variation in the material characteristics, including nonlinearity, or by dynamic effects.