Most existing pavement subgrade criteria are based on the AASHORoad Test, where only one material was tested and for only oneclimatic condition. To study the validity of this criteria andto refine the criteria a co-operative research program entitledthe "International Pavement Subgrade Performance Study' wassponsored by the FHWA with American, Finnish and Danish partners.Thispaper describes the first test series which was carried out inthe Danish Road Testing Machine (RTM).

The first step in this program is a full scale test on an instrumentedpavement in the Danish Road Testing Machine. Pressure gauges andstrain cells were installed in the upper part of the subgrade,for measuring stresses and strains in all three directions. Duringand after construction FWD testing was carried out to evaluatethe elastic parameters of the materials. These parameters werethen used with the theory of elasticity to calculate the stressesand strains at the position of the gauges and to compare thesevalues to the stresses and strains recorded under the rollingwheel load.

Plastic strains resulting from 50,000 applications of eachof two load levels (20 kN and 40 kN) were recorded, as well asthe permanent deformation of the pavement surface. A simple modeldescribing the plastic strain bas been developed.

The test showed that currently used subgrade strain criteriaare conservative if used with the measured strains in the subgrade.If used with strains calculated from FWD tests using linear elastictheory, however, the strain criteria seem to give a reasonablygood prediction of the bearing capacity of the pavement. The differencesbetween measured and theoretical values appear to be due to inhomogeneitiesin the pavement, to the non-linear elastic subgrade modulus, andto a modulus gradient in the subgrade.