The phenomena derived from the water action on argillaceous rocks, and especifÍcally those of swelling, have a great significance in the geotechnical field. The influence of water (humidity) variations on the time-dependent weathering, progressive deterioration and slaking behaviour of mudrocks has already been assessed by some authors (e.g, Van Eeckhout, 1976; Olivier, 1979; Harper et al., 1979),In this aspect the present paper deals essentially with the swelling performances exhibited by some mudrocks in relation to its textural characteristics and physical properties.
For this study, a series of samples representing four types of mudrocks, from the area of Asturias-Galicia (N.W. of Spain) has been selected in order to compare their geomechanical behaviour. Two samples are typically sedimentary argillaceous rocks (Ribadesella marlstone and Villabona siltstone), and the other two samples are metamorphic of low degree (Rande phyllite and Luarca slate). Field observations, microscopic examinations, and semi-quantitative X-Ray diffraction analysis have been conducted to describe the principal lithological features of the selected samples, which are summarized in Table 1,
(Table in full paper)
Some physical properties, such as dry apparent density (fd), total porosity (n), saturation moisture content or void index (is) and permeability (K), have been measured from the rocks, according to the suggestions of the ISRM (1979). The results are shown in Table 2.
As it could be expected, the values of air permeability for the saturated specimens are significantly lower than in dry conditions. The difference or contrast between these two values is particularly remarkable in sedimentary materials (the most porous), where the permeability diminishes to the order of 10 2–103 when saturated in water. On the other hand this contrast can be esteemed as an indicator of rock degradability: the higher the initial contrast is, the higher the proclivity is to breakdown with moisture fluctuations, as will be seen further on.
Swelling is one of the most conspicuous properties shown generally by mudrocks, and in a certain way can be regarded as a contributory factor to its alterability or durability.
The unconfined, "free" swelling was measured using an apparatus designed on ISRM (1979) suggestions for this test. The curves of expansion with respect to time of the tested materials, and the respective swelling coefficients (εs = ∆ L/L) are plotted in Fig, 1, As it can be observed, the four types of mudrocks showed swelling tendencies (although εs indicates low or very low intensities), S eing a noticeable difference in the magnitude of swelling with relation to lithological and genetic character. Thus, the sedimentary specimens, containing a significant quantity in clay minerals and a relatively high porosity, display higher swelling coefficients than the metamorphic specimens, with a small clay fraction and less porosity.
(Figure in full paper)
On the other hand, it can be also deduced that, within a similar mineralogical composition (especially referring to clay fraction), a higher permeability facilitates a quicker water absorption, so that the swelling generally increases in rate and intensity.
