Diagenesis of mudrocks, illite 'crystallinity' and the effects on engineering properties.

This study deals with the changes in mineralogy and fabric of a suite of
mudrocks due to increased induration caused by diagenetic history, and the subsequent
controls on geotechnical properties. Burial diagenesis of mudrocks has important
implications on their engineering behaviour, in particular the liberation of Si and Ca
during illitisation of smectite and mixed layer clay minerals leading to cementation. The
objective of the research has been to provide a means of predicting engineering
performance of mudrocks from mineralogical composition. A suite of 41 mudrock
samples ranging from Cambrian slates to Carboniferous Coal Measures in age and
variable diagenetic rank has been sampled and tested.
A detailed mineralogical evaluation of the samples was conducted and it was
found that the proportion of mixed layer clays and kaolinite decreased with an increase
in diagenetic rank of the material, but not necessarily with increasing ages of the
samples.
The diagenetic histories of the samples was assessed using illite 'crystalinity'
and vitrinite reflectance measurement, which indicated that the samples ranged low
diagenetic to epizonal in rank.
The textures of the samples was studied using back-scattered scanning electron
microscope imagery. The novel application of this technique to textural analysis in the
field of engineering geology resulted in the diagenetic rank parameter classification
scheme being devised. The classification parameter consists of a systematic approach to
the evaluation of pore and microfracture distribution, clay mineral orientation and
degree of recrystallization, degree of contact of clastic mineral constituent and the
degree of cementation.
The physical properties of the mudrocks were assessed in terms of bulk index
properties, swelling, slaking and strength.
In evaluating the test data, it was found that that the principal controls on the
durability of indurated mudrocks was the distribution of micro fractures and the mixed layer clay content. With increasing diagenetic rank measured by illite 'crystallinity'
determination, the durability of the samples increased due to the development of a more
mature rock fabric as seen by the use of back-scatter scanning electron microscope
imagery. With increasing diagenetic rank there is a decrease in microfracture
distribution due to recrystallization of clay mineral species and subsequent
recementation of the sample also due to release of Ca and Si from the conversion of clay
mineral species. This process reduces void space, eliminates micro fractures and creates
a non expansive clay mineral suite resulting in a durable mudrock less susceptable to
swelling and slaking effects.
An additional controlling factor on mudrock durability was found to be the
presence of calcareous and organic carbon cements. These controls were found to be
short term, as seen in natural weathering experiments, where samples of high
calcareous and carbonate contents were found to start slaking after a period of 6 to 8
months of exposure, and samples of low carbonate and calcareous content began to
degenerate almost instantaneously. The controls of cementation on the durability of
mudrocks are eventually broken down due to the effects of air breakage in voids and
swelling stress resulting from the expansive clay mineral species present.
( A range of simple index tests were used to physically characterise the mudrock
samples and their potential in determining mudrock durability was evaluated. A
mudrock rank durability classification approach is presented which is based on the
modified jar slake test, methylene blue adsorption and moisture absorption
determination. These index tests were found to correlate strongly with void and
microfracture distributions and proportions of mixed layer clays within the samples.
This classification approach was tested on the samples in the study and found to be
effective in distinguishing between non-durable, moderately durable, and durable
mud rocks.