Mark J. Farrow, P.E.¹ & Terry W Oswald, P.E.²

The design of roadway and aircraft pavement requires determinations of subgrade CBR values Laboratory CBR tests simulate the most severe subgrade deterioration that can occur due to softening caused by ponding water conditions. If routine maintenance is performed whereby cracks are sealed to prevent surface water infiltration., excessive subgrade softening can be prevented. If the subgrade soils remain stiff due to routine crack sealing, the design life of rehabilitated pavement is usually under-estimated using laboratory CBR values. As a result, field CBR tests are often desired for pavement rehabilitation programs to determine the actual in-situ subgrade modulus. If sample borings are drilled prior to pavement rehabilitation and the sub grade soils are found to be stiff, the actual sub grade modulus is much higher than determinations obtained from laboratory methods. There are correlations between soil compressive strength and sub grade modulus. Field CBR values can be estimated from a comparison o(penetrometer compressive strength determinations performed on subgrade soil samples obtained from shallow borings if penetrometer readings are also performed on CBR specimens compacted at varying densities and moisture contents. This paper presents an overview of the methods and correlations used to approximate field CBR values.