Influence of Stope Design on Stability of Hanging Wall Decline in Cibaliung Underground Gold Mine ()
1. Introduction
Cut and fill mining is a selective open stope mining method and is applied for steeply dipping high grade deposit in weak host rock. This method has several advantages including suitable to follow irregular orebodies and easier to control the stability, especially in weak rock mass. Meanwhile, this method has several disadvantages; one of the disadvantage is low on cycle time production.
Cut and fill method recently has been widely used for underground mining in the world. Several gold mining in Indonesia applied this method including Nusa Halmahera Mineral (NHM), Aneka Tambang (Antam) Pongkor, and Cibaliung underground gold mine. Cibaliung underground gold mine located in Banten province is one of the gold mines in western part of Java Island in Indonesia. Map of study area is provided in Figure 1. This mine is operated under Cibaliung Sumberdaya Company (CSD). The resources of gold deposit are estimated amount 1.5 million of ore with grade 9.8 ppm.
In order to develop stope excavation, it is essential to understand the stability of multiple excavations. The development of stope excavation also should consider the influence of adjacent opening excavation and influence stope geometry to decline stability because shotcrete failure has often occurred in hanging wall decline.
To support these studies, some parameters are required. The intact properties of rock, rock mass condition, and in situ stress are naturally geological parameters to analyze underground design and stability.
Excavation geometry, distance and position between openings, and support design are the responsibilities of engineer judgments based on their experience and analysis. Stope design has an impact on stope stability [1,2]. Although there have been many research works that focused on the interaction between underground openings [3-5], very few researches and publications are found for the influence of stope design on decline stability.
The Mathews method [6] was introduced in 1980 to predict open stope stability. This method is based on a stability graph; that is related to Mathews stability number (N) and the hydraulic radius (HR) of stope geometry. Mathews stability number (N) is influenced by rock mass and three other parameters including rock stress factor,
Figure 1. Location of Cibaliung underground gold mine (source: CSD).
joint orientation and design surface orientation, and size of an excavation surface (HR). However, this method is applied only for analysis of single stope stability. To consider the stability of multiple excavation, this paper highlighted results on simulation study of stope excavation influence on underground stability in cut and fill mine method.
Laboratory tests and field investigations are needed for a proper investigation of the properties of intact rock and rock mass. In this study, the properties of intact rock tests consist of UCS test, Triaxial test, and Brazilian test. This study combined field investigation and analysis of secondary data from underground company to classify rock mass.
2. Geological Condition of Study Area
The host rock of Cibaliung deposit is andesite rock type that consists of andesite, andesite breccia, polymictic, and monomictic breccia. Those host rocks are altered by chlorite-adularia and smectite-illite. The type of ore is a vein with low sulphidation epithermal deposit dominated by quartz vein. This vein follows the Cibaliung fault within NNW-striking/ENE-dipping, with two shoot target Cikoneng and Cibitung ore shoot. The Cikoneng ore geometry is 250 m length, 2 - 10 m width and 200 m depth, whereas Cibitung shoot geometry is 150 m length, 1 - 15 m width and 300 m depth with dip of ore is 80˚. The geology of Cibaliung can be seen in Figure 2.
The mine has been developed up to 100 meter depth. Overhand cut and fill method is applied in this mine. Two decline main ways are developed to support the production of mine. Those positions are in hanging wall
Figure 2. Regional geology of Cibaliung (source: CSD).
and foot wall. They are connected by cross cut to the ore stope. The decline of hanging wall consists of smectiteillite andesite breccia, while the foot wall consists of chlorite-adularia andesite breccia. The orebody consists of quartzite with some clay alteration.
3. Numerical Model
This research utilized Phase2®, a finite element method (FEM) software. Phase2® software is a two dimensional elasto-plastic finite element program to solve a wide range of mining engineering problems and to predict induced stress and displacements around underground openings or surface excavations. The program is convenient for analyses because of its easy for creating models, automatic mesh generation and many information results including displacement, stress, strength factor, and yield zone.
The basic model used in the analysis is shown in Figure 3. This model represents the typical geometry and geological conditions of cut-and-fill mine method especially in Cibaliung underground gold mine. Three underground excavations are designed including footwall decline, hanging wall decline, and stope excavation. The declines geometry both on foot wall and hanging wall are 4.2 m width and 4.8 m height. Meanwhile, standard stope geometry is 5 m width and 5 m height. The distance between stope to footwall decline (d1) set fixed as 20 m. The distance between stope to hanging wall decline (d2)
Figure 3. Initial condition for distance influence modeling.
was simulated. Three rock types are consisted in the location include breccia andesite with chlorite altered at footwall, breccia andesite with smectite altered at hanging wall, and quartzite as the ore body. The boundary conditions were set as follows: the sides of the model were restrained perpendicular to each side, whereas the bottom of the model was restricted in the vertical direction, and the top surface was free.
Dip of ore body is 80˚ and the thickness are varies between 5 m to 15 m. This is a reason to analyze the stope geometry impact on hanging wall decline failure.
Uniaxial compressive strength, triaxial compressive strength, Brazilian and density tests were conducted in accordance with the ISRM standard as parameters on numerical analysis. Based on the laboratory tests, some parameters are obtained including compressive strength (σc), young modulus (E), poisson’s ratio (n), cohesion (C), internal friction angle (ϕ), tension stress (σt) and rock density (ρ). The results of rock properties tests are summarized in Table 1. The values of minimum and maximum UCS varies, with the lower mean of compressive strength is in hanging wall rock. Hoek and Brown criterion was chosen due to the condition of field study consists of variations of rock masses. Table 2 shows the rock properties based on Geological Strength Index (GSI) system.
4. Result and Discussion
The analysis results include influence of rock mass, influence of distance between stope and hanging wall decline, and influence of stope size to decline stability will be explained following.
4.1. Influence of Rock Mass on Decline Stability
Two types of rock mass were analyzed in this study, poor rock mass (GSI 33) and good rock mass (GSI 53). Figure 4 shows the same size of geometry and distance between stope and hanging wall decline. It clearly indicates that rock mass gives high influence on decline stability. The yield zone occurs when the distance is 10 m for both of rock mass and overlap for both excavation in stope and hanging wall decline.
Figure 4 shows the percentage of yield element. The red color is 100% yield zone which means failure area. The yield element area on GSI 33 (Figure 4(a)) is wider than GSI 53 (Figure 4(b)). These results explain that poor rock mass has more failure potential than good rock. In addition, when the distance between stope and hanging wall decline increases, the yield zone decreases. The stability on the hanging wall decline occurs when the distance is more than 30 m for rock mass GSI 33. Otherwise, for good rock mass GSI 53, the stability has occurred when the distance is 20 m. According to the above analysis, the rock mass has influences on underground stability. For the poor rock mass, underground condition was weak and need more consideration for supporting planning.