The purpose of this work is mainly to contribute towards the characterization of fine-grained sedimentary rocks (mudrocks). The influence of the mineralogical and textural aspects of these rocks on their geotechnical properties is given particular emphasis, so that the performance of these materials, when involved in engineering works, i.e. in cut - slopes and embankments as a construction material, can be predicted. A wide ranging programme of laboratory characterization of Jurassic mudrocks collected in the Arruda and Obidos areas (north Lisbon area) was carried out.
The geological and geotechnical classifications usually used to describe and classify mudrocks were reviewed as part of the studies made. It is considered that the distinction between the various lithotypes, that comprise mudrocks, should be based on the grain size and stratification of these materials. A field and laboratory classification is presented, based on the following parameters: strength, colour, stratification thickness, Rock Name, discontinuity spacing, other characteristics (when applicable) and weathering state.
The rocks studied are basically composed of quartz, feldspars (potassium feldspar and plagioclase), carbonates (calcite and dolomite), clay minerals (kaolinite, illite, chlorite, smectite and mixed-layer clays), pyrite and organic material. The microtextural characterization of the rocks was undertaken mainly by using scanning electron microscopy (SEM). The samples are described according to the following features: particle orientation, grain to grain relations, clay fabric and carbonate occurrence mode.
The geotechnical characterization of the mudrocks studied comprised mainly the following properties: swelling, strength and slaking (durability). The results obtained confirm that the performance of these materials is mainly affected by the importance of the total void space (pores and fissures) and by the expandable clay mineral content. It is accordingly found that increased porosity and swelling correspond, in the materials studied, to lower strength and durability characteristics. The compaction and strength characteristics of rock mixtures were also determined. The results reflect the durability and lithological composition of the materials used to make up these mixtures. It was noted that mixtures formed by rocks with coarser lithological composition (siltstones and sandstones) and with a high slaking durability have higher dry density and strength values.
It was also found, by natural exposure and simulated ageing tests, in addition to the influence of the above mentioned factors on the behaviour of mudrocks, that the presence of carbonate cements has a short term effect on the durability of these materials. Accordingly, rocks with a carbonate content of more than 10% (classified as calcareous) have greater disintegration at the end of a 4 to 6 month test period, while materials with a very low carbonate content have a rapid disintegration rate, so that the weathering of the samples basically occurs during the first two months of the test period. The dissolution of carbonate cements allows greater water access into the materials and can lead to physical disintegration of rocks caused by air compression in voids and micro-discontinuities and clay mineral expansion.
The detailed characterization of mudrocks in order to evaluate their durability in engineering works, involves the use of complex laboratory resources and high costs, which may not be justified in feasibility studies of materials, the aim of that is merely to distinguish between potentially problematic and non-problematic materials. It was therefore sought to evaluate the durability of the materials using index tests that can be determined rapidly and cheaply. The durability index - DI - was proposed for this purpose. Dry density, methylene blue adsorption and the percentage of material retained in the second cycle of the slake durability test were adopted as the parameters of this index, because they evaluate distinct characteristics of the material, i.e. the pore space, the content and importance of clay minerals (particularly expandable clay minerals) and slaking susceptibility. The index is calculated from the sum of the ranking coefficients attributed to the classes defined for each of the parameters selected. The results obtained using DI index for the rocks studied, show that it has potential for use in soft mudrocks in order to distinguish between non-durable materials and materials with medium and high durability.
Finally some comments are made on the information obtained in the experimental work carried out on the behaviour of mudrocks in natural and cut slopes and in fills as construction material.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)