The formulation of credible geotechnical models is based on very accurate characterization of sites, which is achieved through diamond drill core logging. In the process, two very important indices need to be identified – Total Core Recovery (TCR) and Rock Quality Designation (RQD). TCR refers to the ratio of the length of recovered rock core compared to the total length of drilled material of a particular run. RQD is an advanced index of core recovery representing the ratio of the length of intact rock fragments larger than 10 cm to the length of a drill run. These indices are the foundation of modern rock mass classification systems (Narimani et al., 2025).
The proper method of logging TCR starts with careful treatment of core material. Before taking measurements, drilling block markers should be checked in order to provide a precise account of the depth of drilling. The logger needs to assemble core very carefully, considering any spaces created by drill water eroding soft material. The value of TCR is found by dividing the length of recovered rock by the run length expressed in percentage. Accurate logging of TCR will allow engineers to identify zones of core loss, which is evidence of geological faults.
The conformity to the basic RQD definition is necessary to create consistent domain models. Only the core sections which are solid, intact, and longer than 10 cm in the core direction are taken into account when calculating the RQD value (Pells et al., 2017). The core segments which are highly weathered, weathered to the degree of soil, or crumbling at hand pressure cannot be accounted for RQD despite being longer than 10 cm. Such approach will make RQD calculation better reflect mechanical competence.
One of the primary sources of error in core logging that negatively impacts the credibility of geotechnical models is the misidentification of fracture types. RQD must account only for natural geological discontinuities like joints, bedding planes, and shear zones. The fractures caused by drilling, manipulation or release of stresses called mechanical breaks should be ignored. The appropriate practice is the physical reconstruction of mechanically broken fragments. If the reconstructed fragment is longer than 10 cm, it is regarded as an intact section.
The accurate logging of cores is central to the validity of geotechnical domain models in 3D form. The field logging of core data forms the basis of rock mass classification using empirical tools such as the RMR and GSI models (Somodi & Vásárhelyi, 2023). When the cores are logged accurately, the engineers are able to create geotechnical domains representing zones of similar geomechanical behavior. Inaccurate logging causes false variability which makes the domain models invalid for the design of critical pit slope or underground excavations (Eggers, 2016).
In summary, the correct method of logging the RQD and core recovery depends on standardization and accurate reassembling of cores as well as differentiation between natural and mechanical fractures. Geotechnical domain models cannot be made scientifically through core logging based on subjective opinions.
References
Eggers, M. (2016). Engineering geological modelling for pit slope design in the porphyry copper-gold deposits of Southeast Asia. Proceedings of the First Asia Pacific Slope Stability in Mining Conference, 49–82. https://doi.org/10.36487/acg_rep/1604_0.4_eggers
Narimani, S., Davarpanah, S. M., Bar, N., & Vásárhelyi, B. (2025). Analyzing Drill Core Logging Using Rock Quality Designation–60 Years’ Experience from Modifications to Applications. Applied Sciences, 15(3), 1309. https://doi.org/10.3390/app15031309
Pells, P. J., Bieniawski, Z. T., Hencher, S. R., & Pells, S. E. (2017). Rock quality designation (RQD): time to rest in peace. Canadian Geotechnical Journal, 54(6), 825–834. https://doi.org/10.1139/cgj-2016-0012
Somodi, G., & Vásárhelyi, B. (2023). Borehole Analysis with the Modification of RQD Value. Geotechnics, 3(4), 1017–1032. https://doi.org/10.3390/geotechnics3040055
