Humidity and temperature changes are considered to be two of the most important factors influencing the slaking and disintegration behaviour of mudrocks. An experiment in which these conditions were varied was carried out on mudrock prisms sawn dry from 12 sample blocks which had been stored in sealed plastic bags since excavation. The samples were obtained from various geological formations and parts of the country. The prisms measured 30 × 30 × 50 mm with the long axes perpendicular to bedding. The experiment was conducted in a climate test box using dial gauges to monitor the swells and shrinkages and a balance to monitor moisture changes. Small temperature changes were made at nearly constant humidity and these were followed by small humidity changes at constant temperature. Both the temperature and humidity changes were then increased. The prisms were also subjected to severe "day" and "night" cycles, i.e. high temperature and low humidity followed by low temperature and high humidity. At various stages during the experiment conditions were returned to the original (starting) conditions to determine whether irreversible changes had taken place. All the samples, except for one with a high initial moisture content, adsorbed moisture when they were subjected to a temperature of 20 °C and 88 per cent R.H. (original conditions). Temperature changes on their own did not affect the samples much. The mudrocks were very sensitive to humidity changes, with regard to both volume and moisture changes. Severe "day" and "night" cycles did not affect the samples more than severe humidity changes on their own. The state of expansion and the moisture conditions returned to relatively constant levels whenever the original conditions were applied during the experiment. It seems therefore that few irreversible changes took place during the experiment and that, for these mudrock samples, free water was needed to cause the visible slaking and disintegration which occurred in some of the samples when they were immersed in water.
It is highly probable that humidity and temperature changes are two of the main agents responsible for mudrock disintegration and slaking. Although various workers have investigated the influence of humidity changes on various properties of mudrocks, studies of the type reported here have not been described previously.
During a rock quality assessment of shale, in which samples were cycled between 100 and 0 per cent relative humidity (R.H.), Grice (1968) found that 75 per cent of the total moisture adsorption took place within the first 24 hours of the cycles. In another experiment he cycled samples in the ranges of 20 to 30°C and 60 to 90 per cent R.H.; most shales remained crack-free during nine months of these cycles.
Chenevert (1970) subjected mudrocks ground to 2,0–2,38 mm to different humidities after drying in an oven., He found that the adsorption potential depended on the R.H. levels and that for most of the samples the increments in moisture adsorbed were larger in the higher ranges (90 to 98 per cent R.H.) than at lower humidities.