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.