Analysis of Consolidation Grouting Effect on The Stability of Portal Slope of The Spillway Tunnel in The Bulango Ulu Dam, Gorontalo, Indonesia

The problem of portal slope instability often occurs caused by the poor lithological conditions composing the slope. During the portal excavation of the outlet spillway tunnel in the Bulango Ulu Dam, the slope failed. The slope was then reinforced by consolidation grouting. The objectives of this research were to characterize the soil and rock masses composing the slope, evaluate the grouting effectiveness by Limit Equilibrium Method (LEM), and recommend an alternative slope reinforcement method. Surface engineering geological mapping, borehole data evaluation, and laboratory testing were conducted to obtain slope stability analysis parameters. The results showed that the slope consisted of diorite having poor to very poor rock masses. After 7 days of grouting, the triaxial test showed that the soil samples had similar cohesion and increased internal friction angle insignificantly, as compared to the soil samples prior to grouting. The LEM analysis indicated that the portal slope was unstable and consolidation grouting did not increase the slope safety factor (Fs) significantly. As the grouting effect may take significantly long, an alternative slope reinforcement method by soil nailing was proposed. The LEM analysis showed that the soil nailing increased the slope Fs from 0.63 to 1.49.