This research intends to use the ultrasonic properties of the rocks to classify weathering grade of sandstones and to correlate their physical and mechanical properties. Sandstone specimens were collected from Nam Phong (NP) and Phra Wihan (PW) Formations, Khorat Group, Northeast Thailand. Three weathering grades were categorized as fresh (I), slightly weathered (II) and moderately weathered (III). Petrofabric of both NP and PW sandstones at each degree of weathering were observed. Physical properties and uniaxial compressive strength tests were also conducted in the laboratory. Both ultrasonic compressional (Vp) and shear (Vs) wave velocities of the rocks were measured. The results of the research indicated that the ultrasonic wave velocities decrease as increasing of degree of weathering and porosity. On the contrary, the wave velocities increase as increasing percentage of quartz, density and uniaxial compressive strength.
The strength and other engineering properties of rock materials were usually reduced when the rocks are weathered. In generally, weathering is the process which changes the properties of rock mass due to changing the environment conditions such as physical, chemical and biological. Khorat Group, the non-marine red-beds sandstone, forms mostly in the Khorat Plateau, Northeast Thailand which is in tropical area (Figure 1). Sandstone is a sedimentary rock that composed of sand-sized grains of mineral, rock fragments, or organic material. It also contains the most common cementing materials that are silica and calcium carbonate; accordingly, it is very moisture-sensitive and easily weathered due to reaction with water. Thus, the authors interested in weathered sandstone of the Khorat Group. This Group was named by Ward & Bunnag (1964) who subdivided the group into 9 Formations, namely from older to younger sequences (Triassic to Cretaceous) as, Haui Hin Lat, Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, Khok Kruat, Mahasarakarm and Phu Thok Formations. The rock samples in this research were taken from Nam Phong and Phra Wihan Formations that Nam Phong Formation deposits in Triassic and Phra Wihan Formation deposits in Cretaceous. The Nam Phong Formation has a type section along the Nam Phong stream and distributes along with the Loei-Phetchabun Fold Belt and the western regime of the Khorat Plateau. The formation consists of resistant red-brown micaceous sandstone, siltstones, mudstones, and conglomerates of fluvi-atile origin (Chonglakmani 2011). The Phra Wihan Formation is widely distributed, occurring in the Phu Phan Rang and around the western and particularly the southern rims of the Khorat Plateau. Generally, the rock consists of light buff to gray, fine to coarsegrained quartzitic sandstones and rare siltstones and mudstones with occasional conglomerates (Meesook 2011).