The writers present a systematic experimental study delivering valuable information on a topic of soil dynamics that lacks information due to the inherent variety and non-homogeneity of organic soils. In order to supplement this study and given that the amount of data in the literature is scarce, the author presents herein results of resonant column tests organic soils tested in the frame of the PrevezaAktion immersed tunnel project in the western coast of Greece. The three samples tested (V1-V3) have been extracted from two boreholes 60 m apart in the vicinity of the tunnel portals, and show similar characteristics. They are classified as medium to high organic fibrous sandy silts with sea weed and shell fragments. An accurate description of the material is difficult. In the USCS system they correspond to MHOH soil. Grain size distribution in terms of composition and diameter for 50% passing D-50, initial density rho(0), initial water content w0, as well as organic matter content OC by the ignition-loss method are summarized in Table 6. The determination of Atterberg limits has not been considered appropriate for this type of soil. The tests have been conducted on a Drnevich fixed-free apparatus according to the specifications of the ASTM D4015 Standard. The solid specimens had a diameter of 50 mm and a height of 100 mm. After re-saturation each specimen was isotropically consolidated to the value of effective stress sigma'(0)-which corresponds to the estimated in situ mean effective stress plus the expected pressure due to the superstructure-as quoted in Table 7. Damping ratio DT was determined from the free vibration decay curve as well as from the steady-state vibration of the specimen according to the ASTM specification. Both methods yielded approximately the same values. Due to the limited amount of material available, tests have been performed as two-stage tests: The samples were first tested under a confining pressure sigma'(01) at low strain levels (less than the linear threshold strain). A re-testing at the lowest strain level was then carried out to prove that small-strain dynamic properties were not affected by the testing procedure. Subsequently, the confining pressure was increased to sigma'(02) and the sample was tested up to the highest possible shear strain. A re-testing at the lowest strain was done to assess the degradation of the small strain shear modulus and possible increase of the damping ratio. Isotropic confining pressure sigma'(0) in all tests was higher than the in situ mean effective stress. Numerical values of small strain modulus G(max) are summarized in Table 7. The variation of shear modulus and damping ratio with shear strain amplitude for the three samples is depicted in Fig. 22. Degradation of shear modulus is observed only for the samples V1 and V3 that have been tested up to gamma = 0.7%.
