Geotechnical properties of the Ptolemais lignite

Summary The paper describes an investigation of the geotechnical properties of the Ptolemais lignite (brown coal) and shows that the soil mechanics principles used in the study of natural and cemented soils are applicable. The material has a high and variable void ratio incompatible with its preconsolidation pressure, which may be attributed to the existence of structure (chemical bonding). Bond strength and degradation have been studied and are found to be isotropic, while the stress-strain response in the prevield domain is intensely cross-anisotropic. The shear strength determined from triaxial CU and CD tests is variable but strongly dependent on the in situ moisture content. Tests on high quality intact specimens having a wide range of moisture contents (100–150%) verify a hypothesis that the magnitude of bond strength (expressed by the peak shear strength) is inversely proportional to the moisture content. A Hvorslev-type normalization process shows that the peak shear strength of the material can be estimated from the in situ moisture content with reasonable accuracy, thus facilitating preliminary stability calculations of production cuts in lignite mines. The properties of the Ptolemais lignite are compared with the properties of a lignite from another location in Greece (Megalopolis) and an Australian lignite (Morwell). It is shown that the shear strength characteristics of the lignite types studied are similar and strongly dependent on the in situ moisture content, which seems to be a measure of the magnitude of inter-particle bonding.