Effect of the specimen length on ultrasonic P-wave velocity in some volcanic rocks and limestones

Highlights

• We investigated the influence of specimen length on ultrasonic P-wave velocity in some volcanic rocks and limestones.

• The 200 NX-sized core specimens having lengths between 50 and 150 mm were tested under dry and saturated conditions.

• In order to determine the threshold specimen length, a method was developed using regression analyses.

• The threshold specimen length is 79 mm for volcanic rocks and 109 mm for limestones.

Abstract

Ultrasonic P-wave velocity (UPV) is commonly used in different fields such as civil, mining, geotechnical, and rock engineering. One of the significant parameters which affect the UPV of rock materials is likely to be the length of test cores although it is not mentioned in the literature. In this study, in order to explore the influence of the specimen length on the UPV, rock samples were collected from eight different locations in Turkey. The NX-sized core specimens having different length of 50, 75, 100, 125, and 150 mm were prepared. Before the analyses, rocks were divided into two groups in terms of their geological origins such as volcanic and chemical sedimentary (limestone) rocks. The UPV tests were carried out under dry and saturated conditions for each 200 core specimens. By evaluating the test results, it was shown that the length of the specimens significantly affects the UPV values. Based on the regression analyses, a method was developed to determine the threshold specimen length of studied rocks. Fluctuations in UPV_dry and UPV_sat values were generally observed for cores smaller than the threshold specimen length. In this study, the threshold specimen length was determined as 79 mm for volcanic rocks and 109 mm for limestones.

Introduction

Determination of the engineering characteristics of rocks is considered to be among the most significant components in geoengineering projects. One of the ultrasonic techniques, UPV, has been widely used physical parameter of rocks which is determined both in the laboratory and on-site. It is destructiveness and simple to perform, which is why it has been used for many years in civil, mining and geotechnical engineering such as quarrying, rock slopes, underground opening, blasting and ripping. The UPV is affected by many factors such as lithology, unit weight, grain shape and size, anisotropy, porosity, degree of saturation, weathering and alteration, confining pressure, temperature, and discontinuity properties (roughness, filling material, orientation, etc.). Additionally, the shape and size of the samples may affect the UPV (Vasconcelos et al., 2008, Fener, 2011).

Most researchers have investigated the relationships between the UPV and engineering properties of rocks (uniaxial compressive and tensile strengths, Schmidt rebound hardness, Poisson's ratio, modulus of elasticity, porosity, unit weight, etc.). They obtained that UPV is closely related with properties of rock (e.g. Tugrul and Zarif, 1999, Yasar and Erdogan, 2004, Kahraman, 2007, Sharma and Singh, 2008, Yagiz, 2011, Khandelwal, 2013, Aydin, 2014). A number of authors reported that it has been used for a variety of other specific fields such as the assessment of grouting (Knill, 1970, Turk and Dearman, 1987), rock bolt reinforcement (Price et al., 1970), efficiencies of blasting in the rock mass (Young et al., 1985), the estimation of strength and deformation of rock mass (Gladwin, 1982), the weathering degree rock (Karpuz and Pasamehmetoglu, 1997), fractured rock mass characterization (Kahraman, 2002), and thermal conductivity of rocks (Ozkahraman et al., 2004). UPV is also used in seismic, seismology and in petroleum geosciences (Ikelle and Amundsen, 2005).

The effect of saturation and porosity on the sound velocities was studied by some researchers. Thill and Bur (1969) researched the effect of water saturation on sound velocity for granodiorite. The authors indicated the UPV changes considerably with porosity and degree of saturation although intact rock has only a minute amount of porosity. Ramana and Venkatanarayana (1973) reported that both weight and UPV of the samples increased with increasing time of saturation for sedimentary rocks. Lama and Vutukuri (1978) indicated the UPV increased as the degree of saturation increased.

Vasconcelos et al. (2008) correlated the UPV with pysico-mechanical (uniaxial compressive and tensile strengths, porosity and unit weight) and elastic (Young's modulus) properties of granites. They used the cylindrical specimens (diameter: 75 mm, length: 150 mm) for uniaxial compressive tests and prismatic specimens (40 × 50 × 80 mm³) for tensile tests. They considered that the size of samples utilized in the tensile tests were too small for the UPV measurements of coarse-grained granites. Thus, they also performed the UPV measurements in cubic specimens (150 × 150 × 150 mm³) used in the porosity tests.

Fener (2011) investigated the effect of the specimen diameter on the UPV in laboratory. He performed the UPV measurements on the core specimens having different diameters (the smallest core diameter is 29.68 mm and the largest one is 113.50 mm) and the core specimens have the same length. He found exponential and polynomial relationship between the specimen diameters and UPV. The author stated that a remarkable increase in the UPV was shown for the specimens having largest diameter; however some groups show a decrease in UPV based on the increase in diameter of core sample.

Since the researchers prefer to perform the UPV and other rock mechanics tests on same specimens, the length of the specimens are chosen according to the standards or suggested methods. For example, according to the ISRM (2007) suggested method to determine the uniaxial compressive strength (UCS), the test samples shall be right circular cylinders having a length to diameter ratio of 2.5–3.0 and a diameter preferentially of not less than NX-sized core, roughly 54.7 mm. Further, before testing the UCS, UPV test is generally measured on the same core specimens. Thus, the rock specimens having a non-standard length are ignored in tests based on the suggested methods. However, more reliable correlations may be obtained by adding the rock specimens having an threshold specimen length even if the non-standard lengths are used for only UPV testing. In relevant literature published around the world, no study has been carried out to investigate the effect of the core sample length on the UPV in intact rocks. In this study, it is hence aimed to investigate the effect of core sample length on the UPV in volcanic rocks and limestones, exploring the threshold specimen length.