Investigation on the resistant of tropical residual soils as clay core of rock fill dams against hydraulic fracturing

Abstract

Tropical residual soils derived from in-situ weathering and decomposition of parent rocks which has not been transported from its original location are mostly found in Indonesia. They have been used widely as clay core and homogenous embankment dams in Indonesiafrom long time ago, usage example as in Cipanunjang dam which built in 1927. Hydraulic fracturing may occur in the upstream face of clay core of rockfill dam in case the vertical effective stress in the core is reduced to levels that are small enough to allow tension fracture to occur. Pore water pressure in the core will also increase during impounding, and this will further reduce the effective stress in the core. Wedging due to water pressure may crack the upstream face of the clay core. This phenomenon was known as hydraulic fracturing in the rock fill dams. This paper present the investigation on the resistant of the tropical residual soils as clay core from 5 major dams in Indonesia against hydraulic fracturing. Soil samples are taken from the borrow pits and the clay mineral composition was obtained by x-ray diffraction test. The soil samples then modeled into six different fine content (q<0.075mm); there are 30%, 40%, 50%, 60%, 70%, and 80%. Hydraulic fracturing in the clay core of rock fill dam can be modeled in the laboratory using hydraulic fracturing test apparatus. The soil specimens are compacted hollow cylinder with outer diameter of 104 mm, the height was 120 mm, while the inner diameter was 18 mm. Hydraulic fracturing tests were carried out in the variation of three (3) initial stress states and also three (3) compaction moisture contents. The test results indicated that the clay core which have greater halloysite clay mineral may have greater resistant against hydraulic fracturing, while the clay core which compacted at wet sides may have greater resistant against hydraulic fracturing.