Links between geochemical and engineering properties in weathered pyritic shales
Natural weathering systems and resultant changes in the engineering properties of weathered rocks cannot be analysed in detail due to their inherent complexity. However, laboratory simulation of natural reactions under controlled conditions facilitates identification of specific factors which may contribute to changes in the physical behaviour of weathered rocks. Experiments in which the Carboniferous Edale Shale was reacted with sulphuric acid of a concentration commensurate with that generated under natural conditions showed that the acid composition was markedly altered. Furthermore, the acid composition became qualitatively similar to that observed in a natural shale weathering system by Vear & Curtis (1981). Natural reactions may thus be reproduced in the laboratory. The influence of pore solution composition on the residual shear strength and plasticity properties of Edale Shale were investigated using KNO , NaN03 , acid and groundwater from a major landslip at Mam Tor, Derbyshire. These engineering properties were found to be sensitive to porewater composition and concentration. The effects of physical breakdown on rock properties are less easy to assess because standard techniques for measuring grain size distributions, particularly the < 2 ~m size fraction are unreliable for indurated rocks. A statistical correlation between the < 2 ~m fraction and the Si02/A1203 ratio was found to give a better estimate of the amount of fine material present in a sample. Using this method it was determined that rapid physical weathering does not reduce consolidated material to its fundamental grain size. In addition, residual shear strength and plasticity were found to be independent of the state of physical breakdown and are apparently influenced more strongly by mineralogy. The implications of these investigations to slope stability are considered for the case of a major rotational landslip at Mam Tor, Derbyshire. Finally, a model is proposed in which weathering is divided into distinct components, each capable of separate investigation.