Variation of Schmidt hammer rebound values depending on bed thickness and discontinuity surfaces

In this study, Schmidt hammer rebound (HR) values of fine to medium-grained sandstone were measured to investigate the anisotropy effect on strengthaccording to thickness. The highest HR was measured from the thickest (132 cm) bed, with an average value of 43, while the lowest was measured from the thinnest (8 cm) bed, with an average value of 35. The variation of HR is determined as 23%. Similarly, the average of highest uniaxial compressive strength is 77.73 MPa, while the lowest uniaxial compressive strength (UCS) is 66.13 MPa. The variation of UCS is fixed as 17.5%. The porosity and dry unit weight of sandstone are relyingon the internal characteristics (such as orientation-size-ratio of grain). Because of similar internal characteristics, these properties show small and irregular changes. The dry unit weight values show 1.77% variation, while porosity 0.61%. So they have small importance on bed strength. Semi-continuous bedding cause additional discontinuities. Moreover, application distance of the Schmidt hammer from the discontinuity surfaces highly affects rebound values. Closeness of single discontinuity surfaces (30 cm) cause more than 10% increase in HR, while two discontinuity surfaces cause more than 20% variation when distance from the junction of surfaces is roughly 42 cm. This study clearly indicates that bedding and other anisotropic features can cause significant changes in strength. Key words: Schmidt hammer, sandstone, bed thickness, discontinuity surface.