LABORATORY TESTING OF SMALL-STRAIN MODULI IN NATURAL SANDY DEPOSITS

A vast majority of experimental results have been accumulated to assess the shear modulus of various soils at very small levels of strains., so called small strain modulus or maximum shear modulus, Gmax. The cyclic triaxial test procedure due to overriding popularity in recent decades has become widely used methods for measuring modulus and damping. For fine granular soil materials Cyclic Simple Shear Apparatus is also used to perform the cyclic shearing on soil samples to evaluate shear modulus in more explicit way. One of the key aspects of assessing the small strain modulus by laboratory element testing is precise monitoring of axial and shear strains. Bender elements technique has become one of the most popular to measure small strain stiffness since it is non-destructive, can be performed multiple times on the same specimen and it is user friendly technique. The local measurements of strains from triaxial experiments using on-sample transducers are generally considered very accurate with little complexity in their installation on soil specimen. This paper presents part of the long-term study of dynamic behavior of natural sandy deposits from river terraces at Skopje value, Republic of North Macedonia. Extensive laboratory and field studies have already clarified many aspects of the cyclic stress-strain characteristics of these natural sandy deposits. Skopje city and valley is located in the central Balkan region, known for its seismic destructive history. This region was struck by several earthquakes in recent history producing damage and loss of lives. This study tries to fill the gap which exists regarding the seismic response of local site conditions in Skopje valley. Results from extensive laboratory investigation are presented in this study. Both techniques of precise monitoring of soil strains, on-sample transducer and bender elements technique were used on Dynamic Triaxial System to assess the maximum shear modulus. Direct Simple Shear Apparatus with special dual-sample concept was used to investigate the shear stress-strain relationships. The soil specimens in the triaxial tests were prepared using wet-tamping method at three different initial relative densities, then confined at three levels of initial effective stress p0 = 50, 100, 200 and 400kPa before application of cyclic loading. Results obtained from both measuring techniques show good agreement with very small scattering of data on very low level of strain.