ORIGINAL PAPER An approach to predict the length-to-diameter ratio of a rock core specimen for uniaxial compression tests Ergün Tuncay 1 & Nazlı Tunar Özcan 1 & Aycan Kalender 1 Received: 24 May 2018 / Accepted: 11 February 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract This study aimed to evaluate the effect of length-to-diameter ratio (L/D) of a rock core specimen on uniaxial compressive strength (UCS) and to investigate the optimum L/D range for different rock types. For this purpose, a considerable amount of rock block was compiled and a series of laboratory tests were carried out to determine some geo-mechanical properties of the rocks and the UCS values of test specimens having different L/Ds. Some assessments were made on the limit L/Ds [(L/D) lim ] based on theoretically calculated values and those experimentally observed, and then some easily determined properties of the studied rocks were statistically evaluated to predict (L/D) lim values. By considering the results of these evaluations, some L/D classes were suggested for selection of the L/D intervals of rock specimens before uniaxial compression tests. In addition, an equation was proposed to correct the UCS values obtained from core specimens shorter than lower limits of the suggested L/D classes. Keywords Classification . Correction equation . Length-to-diameter ratio . Shape effect . Uniaxial compressive strength Introduction A number of factors such as type of pore fluid, loading rate, environmental conditions, structure and texture of a specimen and its size and shape affect the uniaxial compressive strength (UCS) of rock (Coates 1965; Hoek and Brown 1980; Hudson and Harrison 1997; Lockner 1995; Pappalardo et al. 2016 and Pappalardo and Mineo 2017). Generally, a decrease in strength of rock is experienced with increasing pore pressure, decreasing loading rate and increasing temperature (Lockner 1995). Although little change in rock strength with specimen size, which is less than the normal intragroup variation, has been reported in some studies (Obert et al. 1946; Hodgson and Cook 1970), researchers such as Mogi (1962), Bieniawski (1968, 1972), Protodyakonov and Koifman (1963), Hoskins and Horino (1969) and Pratt et al. (1972) indicated that there is a significant strength reduction with increasing specimen size. Based on the data compiled from the UCS measurements in literature on core specimens (cylindrically shaped specimen) with diameters between 10 and 200 mm, Hoek and Brown (1980) showed that UCS decreases, while specimen diameter increases, and suggested an equation (Eq. 1) to convert the value of strength obtained from any specimen with diameter other than 50 mm. UCS=UCS D50 ¼ 50=D ð Þ 0:18 ð1Þ By considering the test results on core specimens of sedi- mentary rocks having diameters between 12.5 and 150 mm, Hawkins (1998) concluded that the maximum strengths were obtained from the cores of approximately 40–60 mm in diam- eter, while lower UCS values were measured for specimens with both larger and smaller diameters. In this study, length- to-diameter ratio (L/D) of rock cores was 2. Based on International Society of Rock Mechanics (ISRM) Suggested Methods (ISRM 2007), the diameter of the core specimen used for uniaxial compression testing should not be less than 54 mm, while it is 47 mm for the American Society for Testing and Materials (ASTM; ASTM 1994; ASTM 2000). From the previous studies, in addition to standards and suggested methods, it is clear that diameters of core specimens in labo- ratory studies are specified as being approximately 50 mm. In addition to these studies, some researchers (e.g., Özkan et al. * Ergün Tuncay etuncay@hacettepe.edu.tr 1 Department of Geological Engineering, Hacettepe University, 06800 Ankara, Turkey Bulletin of Engineering Geology and the Environment https://doi.org/10.1007/s10064-019-01482-6