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Seismic wave propagation in surficial stratified soil and deep rock is studied in many engineering fields like Geotechnical earthquake engineering, Geophysics and seismology. Seismic waves might be generated by a significant seismic event, volume collapse in earth’s mantle, chemical or nuclear explosions and surface impact sources. Although the seismic waves’ path in soil layers may be shorter than their path in bedrock, they are influenced significantly by the mechanical properties of surficial soil layers. Soil layers may be saturated or not fully-saturated by a single fluid, which is known as unsaturated soil. Seismic waves generated at the source are known to be body waves of two categories (a) compressional wave (P-wave), (b) shear wave (S-wave).In spite of the abundance and deepness of theoretical analyses, experimental results on measuring the compressional waves in unsaturated soils and rocks are inadequate and mainly have focused on the relation between first compressional wave velocity and degree of saturation instead of suction. Furthermore, the experiments focus on the specimens of sandy soils and rocks with a series of repeated experiments in various degree of saturation conditions. This paper presents the results of three series of ultrasonic tests carried out on fine grained soils.The soils chosen for experimental study are three commercial kaolin named ZK1, ZK2, and ZK3, from Zenoz mine in northwest Iran. These materials have plasticity index (IP) of 9%, 15%, and 19%, and classified as lean clay (CL), silt (ML), and elastic silt (MH) respectively according to Unified Soil Classification System. 15 specimens were compacted at different initial water contents and void ratios and subsequently allowed to dry gradually until air-dry. cylindrical samples, 50 mm in diameter and 100 mm high, were prepared in a mold by compacting a soil – distilled water mixture at proctor optimum dry density and another four points of standard proctor compaction curves; two at 0.5 kN/m3 less than optimum dry density in both dry and wet side of optimum water content point and two at 1 kN/m3 less than optimum dry density in dry and wet side of optimum water content point. All samples were compacted in seven layers using the under-compaction technique to ensure specimen homogeneity along the height. Measurements of compressional wave velocity (Vp) (using ultrasonic) and matrix suction (using the filter paper technique), together with water content, were made at various stages during the drying process (4 times for each specimens; at the time of making the sample and after 4, 8, and 16 hours). The results of the tests suggest that, as a soil dries, its compressional wave velocity increases with increasing in suction. The results imply that in prediction of compressional wave velocity the effectiveness of void ratio must be considered as well as the suction effects. Both compressional wave velocity (Vp) and the corresponding suction (s), have been shown to vary in consist and predictable manner as a function of the initial void ratio at compaction state (ecomp), the suction and the soil’s plasticity index (PI). Thus, an empirical expression was developed which permits estimation of the value of compressional wave velocity, Vp of compacted fine grained soils subject to drying at the suction and material properties expected in prototype conditions.

Keywords: Ultrasonic, Filter paper, Compressional wave velocity, Unsaturated fine grained soils, Suction

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