Selection of Soils as Clay Core Embankment Materials for Rock Fill Dams to Resist Hydraulic Fracturing

Soil is widely used as clay core materials in rock fill dam engineering. As a clay core material, soil should be impermeable, for it has sufficient shear strength and tensile strength to resist against hydraulic fracturing. Hydraulic fracturing may occur in the upstream face of clay core of a rock fill dam in the case of vertical effective stress in the core is reduced to levels that are small enough to allow tension fracture to occur. This situation may arise if the total stress in the core is reduced by arching effect, and by increasing pore water pressure in the core during impounding that will further reduce the effective stresses in the core. Wedging due to water pressure may crack the upstream face of clay core. This paper presents the research on hydraulic fracturing of clay core of rock fill dams on their variation of fine contents in the laboratory. Soil specimens were obtained from six major rock fill dams in Indonesia, and were modelled into six (6) different fine contents (ϕ < 0.075 mm), there are; 30%, 40%, 50%, 60%, 70% and 80%. The soil specimens were of hollow cylinders which were compacted to follow ASTM D 698-00a standard. The hydraulic fracturing tests were carried out on three (3) different initial stress states. The test results consistently indicate that soils which have greater cohesion will also have better resistant towards hydraulic fracturing.