Falling weight deflectometer

Depending on its design, a FWD may be contained within a towable trailer or it may be built into a self-propelled vehicle such as a truck or van.

Comprehensive road survey vehicles typically consist of a FWD mounted on a heavy truck together with a ground-penetrating radar and impact attenuator.

[1][2] In a single-mass system, a weight is dropped onto a single buffer[clarification needed] connected to a load plate, which in turn rests on the surface being tested.

In unlocked mode the FWD works as a double mass system giving 50 kN long pulse.

In all systems, the load pulse shape and rise time is important because it can affect the peak values of center deflection by as much as 10% to 20%.

Geophones lack built-in calibration devices and are more sensitive to disturbances immediately before the impact since the initial error is integrated[clarification needed], but are much less expensive than seismometers.

Commonly used backcalculation software are: Many analysts use simplified methods to calculate related parameters that are empirical in nature.

Historically some used the radius of curvature (D0-D200) but this is out of favour now because it is clear that the steel loading plate of 300mm diameter affects the shape of the deflection bowl between the centre (D0) and the D200 sensor at 200mm.

The equipment has no radioactive sources and can be operated by one person, allowing for on-site data analysis and report printing.

A Fast Falling Weight Deflectometer (FFWD) is a FWD with pneumatic or electric actuators rather than hydraulic, making the mechanics several times faster.