Cyclic Parameters of High Plasticity Clay in Normally Consolidated Unsaturated Condition

Study of the seismic response of a site, requires the accurate estimation of the Shear modulus (G) and damping ratio (D) of under ground layers in that area. According to the unsaturated condition of an extensive part of the earth surface, it is necessary to perform unsaturated tests to determine dynamic or cyclic parameters of these regions. On the other hand, because of inherent complications of unsaturated testing equipment, this field of experience has had less attention. But in recent years by development of advanced experimental equipment some studies have been developed based on the dynamic parameters of unsaturated soils.

A large amount of the researches related to cyclic and dynamic parameters of unsaturated soils are the studies about determination of these parameters in very small strain levels (initial shear modulus and initial damping ratio) and the effects of some factors such as suction, mean net stress, suction history, anisotropy and pre-consolidation on them, using bender element technique and resonant column torsional shear apparatus. But there is less attention in experimental studies in the strain ranges of medium to large and determination of the parameters G (shear modulus) and D (damping ratio), and also the normalized shear modulus reduction and damping ratio curves for unsaturated soils.

In this research, it is tried to determine the shear modulus and damping ratio parameters in medium to large strain levels using suction controlled cyclic triaxial apparatus and study the effect of changes in matric suction and mean net stress on these parameters in a kind of unsaturated clay with plasticity index of 24 under high loading rates. In this regard, some tests are performed on different paths including two suction levels (zero and 300 KPa), in mean net stress level of 200 KPa and three deviatoric cyclic stress ranges (18, 42 and 81 KPa) up to 60 loading cycles. Also a comparison is done between the results obtained from the current research and the results of another research which was performed in the same paths on a fine grained soil with plasticity index of 12 using the same equipment.

The results of this research show that increase in suction level results in raising shear modulus and decreasing in damping ratio values. In addition in the same strain level, by increasing the number of loading cycles, the shear modulus values are increased and the damping ratio values are decreased.

Considering the results of current research (unsaturated cyclic tests on unsaturated normally consolidated fat clay with plasticity index of 24) with the results of another experimental research in the field of unsaturated cyclic tests on unsaturated normally consolidated lean clay with plasticity index of 12, in the same sample preparation process and the same stress paths, is indicated that the changes of the shear modulus values of the high plasticity samples are in the lower level related to the values of the samples with plasticity index of 12. In the other word, the increase in plasticity index decreases the stiffness of the samples considerably. But the change in damping ratio values is shown relatively the same trend in both groups of the samples.