EFFECTS OF UPSTREAM SLOPE OF CLAY CORE AND HEIGHT OF THE ROCK FILL DAMS AGAINST HYDRAULIC FRACTURING

Abstract

Hydraulic fracturing in rock fill dams may occur in case where the upstream face of the clay core experiencing tension cracks due to the arching effects and water pressure from the reservoir. One of the causes of arching effects was the steep slope of the upstream clay core. A statistical research on the dam experiencing with hydraulic fracturing indicated that rock fill dams with un-compacted rock fill embankment zone with narrow and steep slope of core, where the ratio between height against base width of core > 2 were considered much more likely for hydraulic fracturing to occur, while if the ratio was between 1 to 2 were considered likely to occur. This paper investigates the height limitation of the rock fill dams on the ratio of the height against the base width of core of 2.00 and 2.50, which represent the conditions of more likely and much more likely hydraulic fracturing to occur. The clay cores were obtained from five (5) major dams in Indonesia; Batubulan, Batutegi, Pelaparado, Sermo and Wonorejo dams, where their heights vary from 37 m to 125 m. The variation of the clay core embankment materials was made in six (6) various fine contents, and compacted at their optimum moisture contents. Analysis was made on the modeled rock fill dam using finite element analysis with coupling of the stress, the deformation and the seepage analyses. The hydraulic fracturing may occur in case the vertical effective stresses in the upstream face of clay core were less than the water pressure from the reservoir. The results indicated that the maximum dam height with no hydraulic fracturing was governed by the percentage of fines in the clay core and the ratio of the height to the base width of the clay core. The clay core that consists of more fine contents, and smaller ratio of height against the base width of the clay core of the dam has greater resistance against hydraulic fracturing.