ABSTRACT

            The stiffness-suction-moisture relationship of fine-grained compacted subgrade soils was investigated to understand the stiffness behavior of unsaturated soils at the initial compaction and the in-service conditions as the moisture and suction regimes change in response to the climatic and environmental fluctuations.  Compacted subgrade specimens were prepared at varied initial compaction moisture contents ranging from dry to wet of optimum using three levels of compaction energy: enhanced, standard, and reduced Proctor efforts.  Bender elements, a non-destructive elastic wave propagation technique, was utilized to assess the shear wave velocity and the corresponding small-strain shear modulus (G_o) of the compacted subgrade specimens.

In the as-compacted state, G_o normalized with initial compaction moisture content correlated well with the matric suction for a given soil.  A mathematical model was proposed to incorporate the effect of the compaction condition including the compaction moisture content and compaction energy.  In the post-compacted state, G_o of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated.  A test apparatus was designed to control two independent stress state variables, the net confining pressure and the matric suction, and allow G_o measurements.  The test cell allows concurrent G_o measurement and SWCC determination under a constant net confining pressure.  A mathematical model was proposed to quantitatively describe the G_o behavior of the unsaturated compacted soils.  G_o-desorption SWCC relationships were developed for a wide range of fine-grained compacted soils.  The influence of compaction condition and soil type on the proposed relationship was also investigated. 

The contributions of this study are divided into two parts: during construction and in-service.  In the first part, both stiffness and compaction moisture content normalized with respect to their corresponding values obtained at optimum moisture content based on the standard Proctor effort provides a method of estimating the stiffness of compacted subgrade soils used in earthwork construction.  The second contribution involves the quantification of stiffness change as the in situ moisture and suction regimes vary in response to climate at the site during the service life.  For a given soil under a given climatic condition (e.g. rating in term of Thornthwaite climatic index), the change in stiffness (in percent) can be estimated using the developed mathematical model for stiffness of unsaturated soils.