It has widely been accepted that the stress-strain relationship of soils is highly nonlinear from small strains of about $10^{-3}$%. Construction of retaining walls, foundations, and tunnels in stiff soils often results in a range of strains less than 0.1~1% over which soil stiffness varies significantly.
In Korea, the weathered granite is abundant and widely distributed around one-third of the country. Undisturbed sampling of this soil has been extremely difficult because of the sensitive particle structure, so the variations of shear stiffness and damping ratio of undisturbed weathered granite with strain amplitude have not been fully investigated at small to intermediate strains ($10^{-4}$% to 1%) where soils experience in the working load conditions of geotechnical structures.
The overall purpose of this research is to determine the deformational characteristics, expressed in terms of stress-strain relationship and damping ratio, of the undisturbed and compacted weathered granites of different degree of weathering, by small-strain triaxial compression (TX), resonant column (RC), and torsinal shear (TS) tests. Triaxial testing equipment was modified to measure the lateral strain which enables to obtain the Poisson’s Ratio and to measure shear wave velocity using bender elements which allows to obtain the maximum shear modulus.
Undisturbed samples of HW(highly weathered) weathering grade and disturbed samples of RS(residual soil) and CW(completely weathered) weathering grade are obtained at Kori site. The disturbed samples were reconstituted at the density similar to field condition by using under-compaction method. The maximum shear modulus$(G_{max})$ of undisturbed HW specimen was significantly larger than Gmax of reconstituted specimen and less affected by the confining pressure. However, $G_{max}$ value of undisturbed specimen was less than $G_{max}$ obtained by field seismic methods due to the disturbance effect and the field seismic method may be needed to evaluate the $G_{max}$ in the field. The modulus reduction curves obtained by undisturbed HW specimens were similar to those from reconstituted specimens, showing the possibility of using reconstituted specimen in the evaluation. The modulus reduction curve of weathered granites was not affected by the degree of weathering and matches nicely with Seed-Idriss curve for cohesionless soil. Small-strain damping ratio was also not affected by the degree of weathering and the damping value of undisturbed HW specimens was a little bigger than damping of reconstituted specimens at intermediate strains.
