History - 1980

Multidepth Deflectometer

Multi-Depth Deflectometers are used to measure in situ elastic deformation and/or permanent deformations in the various pavements layers of a test section. The basis of the patented MDD system is a series of Linear Variable Differential Transformer (LVDT) modules which are mounted on a rod in a 39 mm diameter hole in the test section. Up to six (LVDT) modules can be mounted at various depths in the hole. The modules are anchored to the soil by way of small steel balls which are forced out against the walls of the hole. The reference rod is anchored into the subgrade approximately 3 m below the pavement surface. The LVDT module measure the displacement of the soil relative to the rod to an accuracy of 10 microns. 256 points are measured on the deflection basin at each depth.

The top of the hole is sealed with a cap which contains the connector to the data acquisition system. The reference rod is connected to the anchor using a snap head connector so that the MDD modules can be removed for re-use.