USE OF ULTRASONIC TECHNIQUES FOR THE DETERMINATION OF PHYSICAL AND MECHANICAL PROPERTIES OF ROCKS AND CLAYS

Abstract

The relations between the physical and mechanical properties of rocks and clays and the propagation velocities of ultrasonic waves were investigated to assess the applicability of ultrasonic techniques as an indirect laboratory testing tool. Measurements of compressional wave velocity, physical, mechanical and geological properties of more than 100 rock and clay samples were the basis for establishing correlation curves relating the various properties to the velocity. Rock samples were obtained from 5 different geological formations, all of them are formed from sedimentary rocks of ages varying from the upper Cretaceous to the Pleistocene. Most of clay samples were retrieved from recent Nile sediments of different consistencies. Propagation velocities were measured for two waves of different frequencies (54 and 150 kHz), the two velocities were then compared for verification. Almost all rock samples indicated equal velocities, whereas many of the clay samples showed considerable differences. This is believed to be a result of clay macro-structure (fissures, laminations, lenses , crumbles, .. etc.), anisotropy, disturbance and/or variation in the distribution of moisture content due to sampling and storing. Hence, only clay samples indicating equal velocities were taken into account. Pulse velocity in rocks is found to be directly proportional to the unit weight and uniaxial compressive strength, while it is inversely proportional to the water absorption. Correlation curves were developed, thus allowing estimating these properties from pulse velocity. Because of variations in composition and texture, each rock formation is found to have characteristic relations that differ from other formations as well as the global relations gathering all rock types. It is shown also, that the geological age has a direct influence on the slope of the linear unit weight/velocity relationship. The older the formation the steeper becomes the slope. Furthermore, rock ability to absorb water can be predicted from pulse velocity. High velocities. in the order of 5000 m/sec, indicate minimal absorption thus revealing high stability against softening due to wetting. The use of ultrasonic techniques for testing clays is shown to be less applicable than for rocks. Wave propagation through many of the tested clay samples had undergone considerable attenuation that lead to slower velocities not representative of material properties.