Guidelines for Open Pit Slope Design in Weak Rocks

✍ Scribed by Derek Martin (editor), Peter Stacey (editor)

• Publisher: CSIRO Publishing
• Year: 2018
• Tongue: English
• Leaves: 415
• Category: Library

✦ Synopsis

Weak rocks encountered in open pit mines cover a wide variety of materials, with properties ranging between soil and rock. As such, they can provide a significant challenge for the slope designer. For these materials, the mass strength can be the primary control in the design of the pit slopes, although structures can also play an important role. Because of the typically weak nature of the materials, groundwater and surface water can also have a controlling influence on stability.

Compiled and written by industry experts and containing significant case histories, Guidelines for Open Pit Slope Design in Weak Rocks follows the general cycle of the slope design process for open pits, from first steps such as field data collection and aspects of determining the strength of weak rocks, to an examination of weak rock types, and finally design implementation and operational considerations. It summarises the current state of practice for the design, implementation, and assessment of slopes in open pits, with a view to meeting the requirements of safety, as well as the recovery of anticipated ore reserves.

Guidelines for Open Pit Slope Design in Weak Rocks is a companion to Guidelines for Open Pit Slope Design (2009), which dealt primarily with strong rocks. It provides guidance to practitioners involved in the design and implementation of open pit slopes, particularly geotechnical engineers, mining engineers, geologists, and other personnel working at operating mines.

✦ Table of Contents

Cover
Contents
Preface and acknowledgements
About the editors
1 Introduction
1.1 Background
1.2 General descriptions and definitions for weak rocks
1.2.1 Cemented sediments
1.2.2 Weak sedimentary rocks
1.2.3 Saprolites: weathered rock and residual soil
1.2.4 Soft iron ores and leached rocks
1.2.5 Hydrothermally altered rocks
1.3 Slope design terminology
1.3.1 Slope configurations
1.3.2 Instability
1.4 Design implications
1.5 Design implementation
2 Field data collection and methodology
2.1 Introduction
2.2 General field descriptions
2.2.1 Field estimate of strength
2.2.2 Weathering and alteration
2.2.3 Moisture-sensitive weak rocks
2.2.4 Residual soils and weathered rocks
2.3 Field mapping
2.4 Core logging, sampling and sample preservation
2.4.1 Core boxing and photography
2.4.2 Core logging
2.4.3 Discontinuities and bedding plane shears
2.4.4 Sampling
2.5 Laboratory testing
2.5.1 Routine test methods
2.5.2 Characterisation tests
2.5.3 Strength testing
2.6 In-situ borehole characterisation
2.6.1 Borehole strength and stiffness methods
2.6.2 Geophysical methods
2.6.3 Geochemical testing
2.7 Summary
3 Weak rock strength models
3.1 Introduction
3.2 Geological and structural models
3.2.1 Lithology and alteration
3.2.2 Major structures
3.2.3 Structural fabric
3.2.4 Seismicity
3.3 Role of rock mass classifications systems
3.4 Shear strength and failure criteria of weak rocks in laboratory tests
3.4.1 Shear strength and failure envelopes
3.4.2 Stresses and stress path during open pit excavation
3.4.3 Factors affecting weak rock strength
3.4.4 Laboratory testing of weak rock
3.4.5 Weak rock behaviour
3.4.6 Anisotropic shear strength
3.4.7 Partially saturated strength
3.4.8 Coupled hydromechanical response during mining
3.5 Erodibility/degradability
4 The role of water in slope design for weak rocks
4.1 Introduction
4.2 Hydrogeological background
4.2.1 General classification
4.2.2 Discussion of basic parameters
4.2.3 Unconfined and confined groundwater
4.2.4 Pore pressure
4.2.5 Hydromechanical coupling
4.2.6 Piping
4.2.7 Fault gouge
4.3 Groundwater characterisation
4.3.1 General
4.3.2 Defining the goals
4.3.3 Data collection and compilation
4.4 Development of a conceptual hydrogeological model
4.4.1 General
4.4.2 Regional- and mine-scale model
4.4.3 Sector-scale model
4.5 Analysis and modelling of pore pressure
4.5.1 Defining the goals
4.5.2 Pore pressure input to the geotechnical analysis
4.5.3 Analysis of pore pressures
4.5.4 Planning of numerical models
4.5.5 Inclusion of hydromechanical coupling
4.5.6 Development of numerical models
4.6 Depressurisation of weak rocks
4.6.1 Importance of time
4.6.2 Importance of recharge
4.6.3 Methods for depressurisation of weak rock units
4.7 Characterisation of surface water
4.7.1 Sources of surface water
4.7.2 Estimating flow rates
5 Slope design considerations
5.1 Introduction
5.2 Modes of failure
5.2.1 Primary modes
5.2.2 Secondary modes
5.3 Instability mechanisms
5.3.1 Cohesion-loss processes
5.3.2 Weak planes in bedded deposits
5.3.3 Collapse in high porosity weak rocks
5.3.4 Summary of instability mechanisms
5.4 Stability analyses and predictions
5.4.1 Approaches
5.4.2 Seismic analysis
5.5 Limit equilibrium analyses
5.5.1 LEM methods
5.5.2 Role of Factor of Safety
5.5.3 Deformations and Factor of Safety
5.5.4 The postulated slip surface
5.5.5 Anisotropy
5.6 Numerical approaches
5.6.1 Role of numerical models
5.6.2 Continuum models
5.6.3 Discontinuum models
5.6.4 Simulating common behaviour modes in numerical models
5.6.5 Pore water pressures
5.6.6 Shear strength reduction
5.6.7 Application of numerical models
5.6.8 Summary of numerical approaches
5.7 Role of back analyses
5.8 Acceptance criteria
5.8.1 Limit-based design (pre-feasibility)
5.8.2 Performance-based design during operations
5.9 Summary
6 Cemented sediments
6.1 Introduction
6.2 General geological setting
6.2.1 Geology
6.2.2 Structural geology
6.3 Hydrogeology
6.3.1 Basin and range deposits
6.3.2 Cemented bedded sediments: Carlin Formation
6.4 General geotechnical properties
6.4.1 Cemented bedded sediments
6.4.2 Cemented gravels
6.5 Slope design considerations
6.6 Goldstrike Betze-Post open pit, Nevada: instability in the Carlin Formation
6.6.1 Introduction
6.6.2 Geology
6.6.3 East wall development history
6.6.4 Engineering geology
6.6.5 Hydrogeology
6.6.6 Laboratory testing and material and strength properties
6.6.7 North-east layback slope analysis and design
6.6.8 Numerical modelling of deep-seated slope deformation
6.6.9 Carlin Formation instability
6.6.10 Continued instability, monitoring, and remediation
6.6.11 Control of slope movement in the Carlin Formation
6.6.12 Conclusions
6.7 Nine Points slope failure at Newmont’s Gold Quarry open pit
6.7.1 Introduction
6.7.2 Geology
6.7.3 Hydrogeology
6.7.4 Midway slope instability
6.7.5 April 2009 Nine Points slope instability
6.7.6 Post-April 2009 Nine Points instability behaviour
6.7.7 December 2009 failure event
6.7.8 Geological investigation and model update
6.7.9 Initial slope modelling results
6.7.10 Geotechnical drilling results
6.7.11 Hydrogeology results
6.7.12 Material strength results
6.7.13 Slope remediation design
6.7.14 Summary of lessons learned
6.8 Overview of open pit experience in cemented (alluvium) gravels found in south-western United States
6.8.1 General
6.8.2 Geological setting for cemented gravels of the south-western United States
6.8.3 Material properties characterisation
6.8.4 Hydrogeology
6.8.5 Pit slope performance
6.8.6 Design considerations
6.8.7 Operational considerations
6.8.8 Slope performance
6.9 Ministro Hales Mine, Codelco: bench failure in massive gravels
6.9.1 Background
6.9.2 Characterisation
6.9.3 Failure description
6.9.4 Back analysis
6.9.5 Proposed change in bench geometry
6.9.6 Conclusions and recommendations
Acknowledgements
7 Weak sedimentary mudrocks
7.1 Introduction
7.2 General geological setting
7.2.1 Geology
7.2.2 Structural geology
7.3 Hydrogeology
7.3.1 Mudrocks
7.3.2 Borates
7.4 General geotechnical properties
7.4.1 Terminology
7.4.2 Micro-fabric, macro-fabric, fissures and bedding plane shears
7.4.3 Mineralogy and plasticity
7.4.4 Strength, modulus and moisture
7.4.5 Swelling, softening and time-dependent deformations
7.4.6 Classification parameters
7.5 Slope design considerations
7.5.1 Failure kinematics
7.5.2 Design process
7.6 Voorspoed Mine, South Africa: open pit diamond mining in weak mudrock
7.6.1 Introduction
7.6.2 Geological setting
7.6.3 Slope performance
7.6.4 Feasibility study and current geotechnical domains
7.6.5 Laboratory testing database and core logging information
7.6.6 Precipitation and groundwater
7.6.7 Hypothesised failure mechanisms
7.6.8 Summary of Voorspoed failure mechanisms and consequence on design
7.6.9 Risk management strategy
7.6.10 Future design and mining strategy
7.6.11 Conclusions
7.7 Rio Tinto Minerals Boron operation: design considerations for weak lakebed sedimentary rocks
7.7.1 Introduction
7.7.2 Geology
7.7.3 Hydrogeology
7.7.4 Rock mass strengths and design considerations
7.7.5 Design considerations
7.7.6 Conclusions
Acknowledgements
8 Weak sedimentary coal, chalk and limestone
8.1 Introduction
8.2 General geological setting
8.2.1 Geology
8.2.2 Structural geology
8.2.3 Material properties
8.3 Slope design considerations
8.3.1 Typical failure modes
8.3.2 Slope designs
8.4 Weak coal measures
8.4.1 Overview
8.4.2 Derivation of empirical rock mass shear strength models
8.4.3 Interpretation of groundwater conditions with coal measures rock masses
8.4.4 Large-scale open pit mining in low-strength rock masses at the PT Kaltim Prima Coal Sangatta and Bengalon Projects
8.4.5 Geotechnical conditions for mining the coal measures rocks of the Western Canada Sedimentary Basin
8.5 Chalk and weak limestones
8.5.1 Introduction
8.5.2 General geology and classification
8.5.3 Sampling
8.5.4 Material properties
8.5.5 Hydrogeology
8.5.6 Case histories
8.5.7 Slope designs
8.5.8 Design implementation
9 Saprolite: weathered rock and residual soil
9.1 Introduction
9.2 Terminology
9.2.1 Saprolith
9.2.2 Pedolith
9.3 Weathering processes and geology
9.3.1 Chemical weathering
9.3.2 Physical weathering
9.3.3 Weathering rates
9.3.4 Influence of parent rock
9.3.5 Weathering profile examples
9.4 General geotechnical properties
9.4.1 Weathering descriptions
9.4.2 Composition and structure withdepth
9.4.3 Effect of weathering on strength
9.5 Hydrogeology
9.5.1 Typical hydrogeology profiles
9.5.2 Dewatering
9.5.3 Hydrogeology observations at Rosebel Gold Mine
9.6 Slope design considerations
9.6.1 Failure kinematics
9.6.2 Design process
9.6.3 Design implementation
9.7 Cowal Gold Mine: back analysis
9.7.1 Introduction
9.7.2 Geology
9.7.3 Slope stability assessments (pre-mining)
9.7.4 Mining
9.7.5 East wall instability
9.7.6 Back analysis
9.7.7 Discussion of back analysis results
9.7.8 Conclusions
9.8 Newmont Boddington Gold: slope design optimisation in oxide/saprolite
9.8.1 Introduction
9.8.2 Site condition
9.8.3 Oxide material at NBG
9.8.4 Historical slope failures in oxide slopes at NBG
9.8.5 Laboratory testing of oxide/saprolite at NBG
9.8.6 Groundwater conditions
9.8.7 Geotechnical assessment for oxide/saprolite slope design
9.8.8 Slope design optimisation for oxide/saprolite slopes at NBG
9.8.9 Surface drainage in oxide slopes
9.8.10 Summary
Acknowledgements
10 Soft iron ores and other leached rocks
10.1 Introduction
10.2 Background
10.3 Soft iron ores: geology
10.3.1 Lithology
10.3.2 Alteration
10.3.3 Structure
10.4 Soft iron ores: geotechnical characteristics
10.4.1 Definitions
10.4.2 Field characterisation
10.4.3 Laboratory characterisation and testing
10.4.4 Conclusions
10.5 Weathered country rocks
10.5.1 Weathering and strength
10.5.2 Characterisation
10.5.3 Strength parameters
10.6 Hydrogeology of soft iron ores and associated rocks
10.6.1 Hydrogeology in the Iron Quadrangle of Brazi
10.6.2 Dewatering and slope depressurisation
10.6.3 Carajás hydrogeology
10.7 Leached quartzites and quartzitic sediments
10.8 Slope performance and case histories
10.8.1 Introduction
10.8.2 Patrimônio: back analysis
10.8.3 Carajás: failures of the south and south-east walls of the N4E pit
10.8.4 Pau Branco Mine – Iron Quadrangle, State of Minas Gerais: weathered phyllites associated with soft iron ores
10.8.5 Pico Mine: an assessment of the mechanism of flexural toppling in weak phyllite
11 Hydrothermally altered rocks
11.1 Introduction
11.2 General geological setting
11.2.1 Igneous deposits
11.2.2 Epithermal deposits
11.2.3 Structural geology
11.3 Geotechnical properties
11.3.1 Description and classification
11.3.2 Strength and deformation
11.4 Hydrogeology
11.5 Slope stability and engineering geology at the Pierina Mine
11.5.1 Introduction
11.5.2 Engineering geology
11.5.3 Slope stability experience
11.5.4 Stability analysis and disturbance factor (D)
11.5.5 Surface runoff and hydrogeology
11.5.6 Pit slope monitoring
11.5.7 Results and conclusions
11.6 Instability in weak rocks, El Tapado Pit north wall, Yanacocha Operation
11.6.1 Introduction
11.6.2 Site conditions
11.6.3 Phase 2 El Tapado Pit
11.6.4 Deep-seated instability in north wall
11.6.5 Completion of mining
Acknowledgements
11.7 Lihir Open Pit Mine in argillic materials
11.7.1 Introduction
11.7.2 Engineering properties of argillic materials
11.7.3 Gw28 slope
11.7.4 Western Stockpile
11.7.5 Conclusions
12 Design implementation and operational considerations
12.1 Introduction
12.2 Mine planning
12.2.1 Pre-mining (Levels 1 and 2)
12.2.2 Feasibility level and detailed design
12.3 Design implementation
12.3.1 Excavation and scaling
12.3.2 Blasting
12.4 Surface water control
12.4.1 Surface water diversion
12.4.2 Collection of runoff water on catch benches and haul ramps
12.4.3 Control of recharge
12.4.4 Managing large surface flow volumes
12.4.5 Maintenance of surface water management systems
12.4.6 Case history
12.5 Slope protection
12.6 Performance assessment andmonitoring
12.6.1 Geotechnical model validation
12.6.2 Slope performance
12.6.3 Slope movement monitoring
12.6.4 Mine water monitoring
12.7 Ground control management plans
12.8 Mine closure
12.8.1 Slope stability considerations
12.8.2 Hydrogeological considerations
12.8.3 Post-closure monitoring
References
Index