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Abstract Rock quality designation(RQD)，which plays an significant role in the rock mass analysis，is an important parameter in geotechnical and geological engineering. However，the size effect and spatial effect have not been comprehensively considered. To investigate the spatial effect of RQD，three-dimensional(3D) fracture network modelling is used to simulate the actual rock mass，and numerous scan-lines are set to obtain RQD values. The result shows that the RQD values vary in different regions and have obvious spatial effect. To obtain the characteristics of RQD that could represent the whole rock mass quality，the analysis based on plenty of RQD samples should be conducted. In addition，to better reflect the inhomogeneity feature of rock mass，RQD ranges of different thresholds are studied；and the optimal threshold of 4 m that can comprehensively reflect the inhomogeneity feature of the investigated rock mass is determined. The size effect is an important feature of RQD. To investigate the feature，the RQD variation is researched by changing the scan-line length. The models and equations(i.e. A-A model，T-T model，A-A-S model，Priest-Hudson equation，and Senz-Kazi equation) are proposed and summarized. The size effects of different models are studied. The results show that：the error of size effect can be decreased using A-A-S model；when the threshold is relative large，the results by Priest-Hudson equation and Senz-Kazi equation have errors；and the RQD value based on 3D fracture network is more correct.
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2012, Vol. 31 
