OPTIMISING SLOPE STABILITY BY OPEN PIT SLOPE DESIGN AT IKPOBIA MARBLE QUARRY, OKPELLA, SOUTHWESTERN NIGERIA

Abstract

The adoption of Open Pit Slope Design (OPSD) used in exploitation of minerals is known to impact mining outputs and safety if slope stability is properly determined. However, slope stability is dependent on the understanding of the rock mass properties and identification of critical features such as overburden materials, structural features, and groundwater system of the mines, all of which are usually not well defined. Therefore, this study was aimed at determining the rock mass properties and associated critical features of Ikpobia Marble Quarry, Okpella, Southwestern Nigeria towards optimising slope stability. The quarry was divided into seven zones (1-7), each with approximately constant bench orientation. Geologic mapping of the quarry was conducted. Vertical Electrical Sounding (VES) using the Schlumberger array was done to determine the subsurface lithology. Quarry photographs were taken and used in photogrammetric analysis to create models of each of the seven zones. Rock mass classification (Geologic Strength Index, GSI) was carried out on the models. Core rock samples from the quarry were tested to determine Uniaxial Compressive Strength (UCS), indirect tensile strength, and density. Petrographic study and X-Ray Diffractometry (XRD) of the rocks were also undertaken. Joint sets orientations were determined from the photogrammetric models and used for kinematic analysis. Data from the analyses were used to determine rock mass properties and as input for analysis of slope stability. The heights (m) and angles (°) of slopes of the seven zones analysed were 25 and 85; 25 and 80; 20 and 80; 20 and 75; 30 and 80; 20 and 80; and 25 and 85, respectively. Rocks in the quarry were marble interbedded with calc-silicate gneiss, banded gneiss and quartz schist. The VES showed H-type curves to be dominant (62%) indicating three geoelectric layers made up of clayey sand topsoil, weathered bedrock, fractured and fresh bedrock. Joint sets observed in the zones ranged from 4 – 8 with the dominant orientation posing stability risk to the Northwest. The GSI ranged from 30 to 40. The UCS ranged from 45.7 to 52.3 MPa, tensile strength ranged from 1.5 to 4.6 MPa, and density ranged from 2710 to 2745 Kg/m3 which are in the low to medium range for intact rock. Minerals identified in the rocks were Calcite, Quartz, Alkali Feldspar, Chlorite, Apatite, Biotite, Muscovite, Epidote, Diopside, Phlogopite, Microcline and Albite. Kinematic analysis indicated low risk of slope failure across the zones but a toppling failure with 60% probability in zone 1. The current safety factors for each zone were 1.5, 1.43, 1.34, 1.69, 1.35, 1.34, and 1.5, respectively for zones 1 to 7. The optimum slope angles determined from finite element sensitivity analysis are 55°, 40°, 20°, 40°, 45°, 20° and 55° for zones 1 to 7, respectively. The optimum slope angles for optimising slope stability were determined. The quarry rock mass properties, critical features assessment and measurements met the requisites standards for acceptable limits of safety.