Influence of rock cradle block geometry on rebound

Authors: N Gunes Yilmaz R M Goktan A B Yavuz Z Karaca

Publish Date: 2015/04/19

Volume: 75, Issue: 1, Pages: 325-339

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Abstract

The Schmidt hammer SH testing of cylindrical rock cores requires the use of a cradle block core holder to hold the cores firmly during the test For this purpose the related International Society for Rock Mechanics ISRMsuggested method and American Society for Testing and Materials ASTM standard suggest the use of an ‘archshaped’ or a ‘Vshaped’ cradle block However there is little or no detailed quantitative information in the literature regarding the potential influence of cradle block type on the measured rebound values R values With this in view an investigation was carried out regarding the influence of cradle block geometry on R values by employing the two cradle blocks specified in the preceding Standards Using an Ltype DigiSchmidt hammer SH tests were performed on NXsize cylindrical cores of 20 different rock varieties both in ‘ovendry’ and ‘wet’ state Although the observed differences were usually small results showed that the magnitudes of the mean rebound values determined in the archshaped block R A were higher than those determined in the Vshaped block R V except in one case For the ovendried samples lowest and highest differences between the means of R A and R V values were 037 and 592 respectively In the case of wet samples the differences ranged from a low of 033 to a high of 470 in Runits Statistically significant positive linear correlations were observed between R A and R V values although the degree of correspondence was relatively lower for rocks having uniaxial compressive strength 50 MPa The results of additional SH tests carried out on two different engineering materials also indicated that higher mean rebound values could be attained in the archshaped block The results of the present study did not confirm the findings of a previous study where it was found that the Vshaped cradle block gave consistently higher mean rebound values than the archshaped one The possible reasons for this discrepancy were discussedThe authors would like to thank Prof Yaşar Kibici Geological Engineering Department of Dumlupınar University for his assistance in examining the thin sections and hand specimens of the test samples Sincere thanks are also to Mr Mehmet Ardıç for providing and machining the two engineering materials used in the experimental campaign

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