1179 1 INTRODUCTION Stiffness and damping are some the most important dynamic properties affecting wave propagation and small-strain response of soils. Proper assessment of these properties is a major requirement in many ge- otechnical analyses, although this might be particu- larly critical for analyses relying on a rigorous un- derstanding of the mechanical behavior of cyclically loaded soils such as those affected by waves, wind and/or earthquake loading. A more fundamental un- derstanding of such properties for offshore calcare- ous sediments still lacks since rigorous and system- atic studies on the characterization of such properties for these soils (e.g. Catano and Pando 2010) are ei- ther limited or not widely available. A simple illustration of stiffness changes and damping ratio is schematically shown in Figure 1 for a closed-loop shearing cycle of a soil element. Upon shearing, the secant shear stiffness (G) decreases from its initial maximum value (G max ) at small strains to the value shown at point A, for example. If the element is then unloaded from A to C and then reloaded back from C to A, a hysteretic closed-loop cycle with area ΔW can be defined, where ΔW rep- resents the energy dissipated in the unloading- loading cycle. At point A, the maximum elastic stored energy can be defined as the shaded area W (= area of OAB triangle). The damping ratio (D) of the material is proportional to the ratio of energy dissipated to the maximum stored energy. Figure 1. Basic representation of stiffness degradation and damping ratio of a soil subjected to cyclic shearing. Shear strain, ! Shear stress, # O A B $W W D " #W W G = " # G max C Frontiers in Offshore Geotechnics III – Meyer (Ed.) © 2015 Taylor & Francis Group, London, ISBN: 978-1-138-02848-7 Stiffness degradation and damping of carbonate and silica sands J.A.H. Carraro Centre for Offshore Foundation Systems, The University of Western Australia, Perth, Australia M.S. Bortolotto CNPq Science Without Borders Program, The University of Western Australia, Perth, Australia ABSTRACT: Stiffness and damping are some of the most important dynamic properties affecting wave prop- agation and small-strain response of soils. While these properties are useful in many geotechnical analyses, they are particularly relevant in analyses relying on the rigorous understanding of the mechanical behaviour of cyclically loaded soils, such as those affected by waves, wind loads and earthquakes. A fundamental under- standing of such properties for offshore calcareous sediments still lacks as rigorous characterization of such properties is less established for these materials. In this study, a state-of-the-art resonant column apparatus is used to characterise the stiffness degradation and damping of a carbonate sand from the North West shelf of Australia. A silica sand with particle size distribution identical to that of the carbonate sand tested was also used to allow for the effect of soil mineralogy to be quantified. State variables such as density and mean ef- fective stress were varied systematically to assess their effect on the stiffness degradation and damping ratio of the soils tested. Solid cylindrical specimens were subjected to mean effective stresses up to about 2 MPa during the tests. Changes in particle size distribution were monitored to quantify particle breakage, which was shown to be higher for the carbonate sand than for the silica sand tested. At similar initial states of density and stress, increasing shear strains leads to a decrease in stiffness and corresponding increase in damping of the soils tested, as expected. However, the rate of stiffness degradation was always higher for the carbonate sand compared to its silica counterpart. At similar levels of stiffness degradation, the carbonate sand system- atically shows higher damping ratio than the silica sand.