Water-holding capacity of rock fragments in rehabilitated opencast mine soils

Abstract

The water-holding capacity of rock fragments sampled from spoil materials underlying rehabilitated opencast coal mine soils was determined. Four rock types, ranging from grit and coarse sandstone to carbonaceous shale, and four fractions of different size were investigated by means of ceramic plate extraction. Water retention was determined at each of four matric potentials, viz. −33, −80, −500 and −1500 kPa. Between approximately 2 and 40% (average 13.8%) water, on a volume basis, was released from saturation to −1500 kPa. In respect to smaller sized fragments (2–4 mm), 60% or more of the plant available water, taken as the fraction contained at a potential of greater than −1500 kPa, was released at high potentials (0 to -33 kPa), and in respect to larger fragments (above 4 mm), 10 to 40%. There was no relationship between water content and rock type. Rock fragment size correlated reasonably well with water held between 0 and −1500 kPa (R²0.78) as well as water released between 0 and −33 kPa and between −33 and −80 kPa (R² 0.64 and 0.85 respectively). Rock fragment size correlated rather poorly with water released between −80 and −1500 kPa. Application of the results is discussed.