Effects of calcium-rich additives on the small-strain modulus of representative subgrade soils in Missouri
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Calcium rich additives, such as fly ash and lime kiln dust, can improve the mechanical properties of subgrade soils, resulting in better performance and more economical pavement design. Use of calcium-rich additives in Missouri pavement subgrades has been uncommon, due in part to a lack of quantitative data on the benefits derived from stabilization, and the parameters influencing the effectiveness of the stabilization efforts. A need also exists to identify and assess non-destructive testing methods to evaluate the quality of stabilized soils soon after they are placed. Laboratory measurements of small-strain moduli were performed in this study to: (1) quantify the effectiveness of calcium-rich additive stabilization of representative subgrade soils in Missouri, and (2) assess the viability of using stress wave-based, non-destructive testing (NDT) methods for quality assessment of stabilized subgrades. Two representative Missouri subgrade soils, a low plasticity soil (PI=15) and a high plasticity soil (PI=33), were used in this study. These soils were mixed with fly ash (10 percent, 15 percent, and 20 percent by weight) or lime kiln dust (4 percent and 8 percent by weight) and compacted over a range of water contents. Changes in small-strain modulus with time were measured over a period of three to seven days using the free-free resonant column (FFRC) testing method. The results from this study showed large and rapid increases in modulus for most soil-additive combinations. The three-day modulus of the low-plasticity soil more than doubled with the addition of fly ash and more than tripled with the addition of lime kiln dust. However, the results also demonstrated large variability in the effectiveness of additive stabilization. In particular, modulus values of the high plasticity soil increased with the addition of lime kiln dust but showed essentially no effect from fly ash stabilization. Possible reasons for this behavior were developed from a supplemental study of the physical and chemical properties of the soil using particle size analysis, X-ray diffraction, scanning electron microscopy, pH, and cation exchange capacity measurements. Small-strain modulus measurements were also used to evaluate the viability of using stress wave-based velocity measurements in the field for quality assessment. The results of the laboratory measurements showed that the magnitude of very short term (within one hour after compaction) velocity changes of stabilized soil would be detectable using small-strain field measurements (such as seismic surface wave methods). Therefore, wave-based velocity measurements appear to be a viable method for assessing the quality of subgrade stabilization shortly after placement.