There are many case studies which show the close relationship between the degree of maintenance of the openings created in the argillacious rock mass such as mudstone and shale and their moisture conditions. This is because the physical properties of the argillacious rock are more or less dependent on the change of moisture condition.

In this paper, some important aspects of these water-sensitive properties obtained by using moisture controlled rock samples are stated. Particular attension is payed for the mechanism of strength dependency on moisture change of the argillaceous rocks in unsaturated conditions.

Sample preparation

Five rock types, two are mudstone, one is shale and the remainders are sandstone, were used in this investigation. Ten to twenty blocks were collected at different sampling sites of the advancing faces of tunnels. The blocks were usually wrapped up in polyethylene sheet to prevent rapid change of moisture and carried to the laboratory.

Then, specimens of cylindrical shape were carefully manufactured using a lathe not to damage them. Two types of mudstone, whose natural water contents were high and very fragile, were obliged to be dried under the constant relative humidity of 59.5% to give sufficient strength to be capable of preparation work.

Most of the specimens were controlled their water contents by the following procedure procedure: They were kept for about one month until their water contents were stabilized in the cell under the constant relative humidities varying from 0.0 to 95.5% which were created in various saturated vapors at temperature of 30°C. Water contents attained by this procedure were varied from 1.45 to 7.50%. Resulting graph of plots of the water contents against the relative humidities was established for each rock type. This diagram, which will be referred to as moisture characteristic curve in the present paper, is available for the evaluation of the change in free energy (see Sec.3).

Measures of water sensitivity

It is well known that the samples of argillaceous rocks, when immersed in water, swell and loose their strength to the extent that they, in the extreme case, disaggregate or even change into muddy solution (slaking phenomena). The water-sensitivities are more quantitatively expressed by the following measures; 1. Swellabilitye εV1OO is defined as the amount of expansion of rock accompanying change of the relative humidity:

  • (Equation in full paper)

where, Vi= specific volume at the 1% relative humidity. 2. Degree of slaking, devided into the ranks shown in Table 1, is determined by the following procedure;
  1. Rock sample of some 2OOg in weight are oven-dried at 100°C for 24 hr.

  2. Then, allowed to cool gradually to room temperature

  3. Immerse the samples in the distilled water for 8 hr. Thereafter determine the degree of slaking according to Table 1, based on the observation of the state of the samples. 3. A measure of strength reduction due to moisture increase, strength reduction ratio σ tW/σ tD ' is defined as;

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