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.