In-situ tests for subgrade resilient modulus characterization

Subgrade soil characterization, expressed in terms of resilient modulus (M_R) has become crucial for pavement design. For a new design, M_R values are generally obtained by conducting repeated triaxial tests on reconstituted/undisturbed cylindrical specimens. Because of the complexities encountered with the test, in-situ tests would be desirable if reliable correlation can be established. In pavement evaluation to arrive at the type of rehabilitation required, laboratory tests for subgrade M_R are even more complex and costly since coring through pavement structure is required. The main focus of this study is to determine subgrade M_R employing in-situ techniques, namely Dynamic Cone Penetrometer (DCP) and Falling Weight Deflectometer (FWD). To accomplish this objective M_R test results on field cores are compared with both DCP-based and FWD-based resilient modulus values. Concurrently, another objective is to analytically study the effect of soil parameters on Dynamic Cone Penetration resistance.; Twelve as-built test sections reflecting typical subgrade soil materials of Mississippi are selected and tested for DCP and FWD before and after pavement construction. Undisturbed samples are extracted using a Shelby tube, and tested in a repeated triaxial machine for M_R. Other routine laboratory tests are conducted to determine physical properties of the soil. In analyzing the data, the soils tested are categorized into two groups, fine-grain and coarse-grain soils.; DCP results (DCP index, penetration/blow) froth tests conducted directly in the prepared subgrade are employed to develop regression models for laboratory M_R predictions. A unique feature of the modulus is that physical soil properties are included in the models. The predictability of the models is substantiated by repeating DCP tests at an independent site.; The laboratory M_R values are also compared with backcalculated moduli determined from deflections measured directly on prepared subgrade as well as atop the pavement surface. The backcalculated moduli from FWD deflection measurements on the prepared subgrade are in agreement with laboratory M _R. In contrast, the backcalculated modulus derived from deflection measurements atop pavement surface is consistently larger than the laboratory M_R with two distinct ratios, one for fine- and another for coarse-grain soil.; Employing pore collapse and cavity expansion analyses, the effect of different soil parameters on Dynamic Cone Penetration resistance is investigated. Among the various parameters, the friction angle in conjunction with initial porosity, significantly, affect the dynamic penetration resistance.