Análise de estabilidade para redução do diâmetro de tirantes em mina subterrânea

Abstract

In underground mining, support systems are applied to promote the stability, enabling the safe advancement of excavations. Active reinforcements such as tensioned bolts are being used widely due to their benefit as a permanent system, essential in galleries with a considerable lifespan. The Aripuanã mine in Mato Grosso, Brazil, the resin-filled rock bolts represent the primary reinforcement for ongoing excavations. These bolts consist of ovalized steel bars with a length of 2.4 m, nominal diameter of 22 mm, tensile capacity of 25t according to the main supplier. Despite their effectiveness in promoting excavation stability, there is a significant cost associated with acquiring these bars. One approach to reduce this expense is by reducing the bolt dimensions. It is estimated that reducing the diameter from 22 mm to 19 mm would result in a cost-saving from 15 to 20 % associated with bolts purchase. In order to achieve the financial gains and maintain the required safety level, this study uses the limit equilibrium methods to assess the stability of excavations with smaller diameter bolts and reduced tensile capacity to 18.9 t. Prior to apply these techniques, crucial parameters of the rock bolting and rock mass, such as bond strength and damage zone, were determined. It was used the SEPT (Shot Encapsulation Pull Test) methodology in 36 pull-out tests and an average Grip Factor of 18.9 t/m was established. Borehole filming before and after rock blasting allowed estimating a damage zone of 1.1 m for the rock mass in its most severe condition. These parameters serve as inputs for subsequent analyses. The study utilized a minimum acceptable safety factor (SF) of 1.3. Kinematic analysis using tetrahedral wedges indicated that 99% of the considered scenarios for galleries and intersections exhibited an acceptable FS for working with smaller diameter bolts. Analyses by the Lang and Bischoff methodology (1982) also pointed to stable conditions for this change, except in cases that the bolt spacing exceeds 1.6 m. The Dead Weight technique proposed by Pakalnis (2008), used for intersection evaluation, indicated acceptable SF for spans less than 11.5 m. Additionally, the study further compares these methods and investigates the impact of operational deviations associated with bolt spacing and excavation overbreak on SF.