Subgrade and Foundation Dynamic Performance Evaluation by Means of Lightweight Deflectometer Tests

Evaluation of the in situ bearing capacity of subgrade and foundation layers is carried out by means of enhanced dynamic testing devices such as the lightweight deflectometer (LWD). These devices offer many practical advantages over traditional plate load tests and allow simulation of real conditions of loads induced by heavy vehicle traffic. Devices provided with a load cell and a geophone can acquire load-deflection time histories, sampling data every 0.25 ms, with a high level of accuracy. LWD data are used mainly to calculate the surface modulus of the tested materials with the Boussinesq equation; procedures specifically developed for estimating the material compaction level achieved on-site are coming into use. This paper focuses on the use of the LWD for investigating the performance of subgrade and foundation layer materials for stiffness (bearing capacity) and compaction achieved. Great effort has been devoted to improving deflectometric tests, correlating energy loss (energy spent to deflect the tested surface, determined as the area enclosed by the load-deflection graph) to the performance of tested materials, especially for compaction level achieved on site. The influence of load frequency is evaluated and described. The results, obtained at two trial fields, confirm the reliability of the LWD in roadwork quality control and for a staged design approach to earthworks.