The soil modulus can be determined by the deflection profile and the back-calculation method.

These limitations can be overcome by conducting a test with a portable falling weight deflectometer (PFWD), a nondestructive and cost-effective tool used to determinate the soil modulus. And the PLT takes several hours so that it can slow down the construction process. The FWD is not always applicable to the project site. The falling weight deflectometer (FWD) and plate load test (PLT) can be used to determine the stiffness of pavement, but these devices have some limitations when they are used during the construction of pavement subgrade. The soil modulus is typically determined by different laboratory tests or in situ nondestructive tests. The Mechanistic-Empirical Pavement Design Guide and other existing pavement design guides use elastic modulus ( E) as the primary input parameter for subgrades. More accurate estimation of modulus can save maintenance cost in the future. Moduli of back-calculation using LEM and VEM methods were used to perform flexible pavement analysis, which showed that with an 8% reduction of modulus, the pavement service life reduced by 25%. However, for stiffer soil, the average error of back-calculation using the LEM and VEM method was 12.4% and 4.3%, respectively. In the case of low stiffness soil, the average error of back-calculation using the LEM and VEM was 53.1% and 14.8%, respectively. The back-calculated modulus using the LEM and VEM was higher than that using the PLT. In situ test results showed that a time lag existed between the peaks of deflection and load, and load-deflection curves were nonlinear, which indicated the viscoelastic nature of the soil.

The effectiveness of the proposed method was verified with the in situ plate load test (PLT) conducted on a highway embankment. The back-calculation program was compiled with MATLAB. A quasi-static dynamic analysis technique of Laplace transformation and a modified Gauss–Newton optimization algorithm were adopted in the proposed method. To improve the accuracy of back-calculation of soil modulus using the portable falling weight deflectometer (PFWD), a viscoelastic method (VEM) overcoming the limitations of the conventional linear elastic method (LEM) was proposed.

Modcomp backcalculation software