STRESS STATE CONDITION THAT LEADS TO HYDRAULIC FRACTURING ON THE CLAY CORE OF THE ROCKFILL DAMS
Hydraulic fracturing becomes one of the major problems in rockfill dams, since it plays significant role in the initiation and extension cracks in the clay core, which may result in leakage and internal erosion. Hydraulic fracturing may occurs in the upstream face of clay core a rockfill dam in case the vertical effective stress in the upstream face of the clay 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 where the core settles relative to the rock fill. Pore water pressure in the core will also increase during impounding, and this will further reduce the effective stresses in the core. Wedging due to water pressure may crack the upstream face of clay core. This study was a part of the hydraulic fracturing research in the laboratory using hydraulic fracturing test apparatus. The specimens were hollow cylinder compacted soil with outer diameter 104 mm, and 120 mm in height, the inner borehole diameter for applying the hydraulic pressure was 18 mm. Soil used for this study was Batubulan dam clay core material which has 30.14% fine content. The soil was classified as CL based on the USCS classification system. The soil specimens were compacted using ASTM D 698-00 standard. The maximum dry density and optimum moisture content obtained from compaction test are 15.32 kN/m3 and 20% respectively, while the internal angle of friction and cohesion obtained from triaxial UU test were 18.15 degree and 78.20 kPa. An investigation to the initial stress state on the specimen that lead to the tension fracture was carried out. Five (5) different initial stress states were applied to the specimen. The evaluation to the test results were carried out using calculation of the stress equilibrium on the specimens when the crack was occurred on the specimen. The test results indicated that three (3) specimens where the initial stress state in the condition ½(y - x)< c, produced tension failure. The finding on this test was very important, since the laboratory test results may reflected the actual situation on the upstream face of the clay-core, where if the stress states on the upstream face of the clay core of a rockfill dams at the end of the construction and before impounding fall in the condition of ½(y - x )< c, the hydraulic fracturing may occurred to the clay core of those rockfill dam.