Introduction to Drilling Technology for Surface Mining(PART-1)

Introduction to Drilling Technology for Surface Mining

1               Introduction

Drilling is the process of making a hole into a hard surface where the length of the hole is very large compared to the diameter. In the context of mining engineering drilling refers to making holes into a rock mass. Surface mining requires drilling for different purposes that include:

1.    Production drilling i.e. for making holes for placement of explosives for blasting. The objective of drilling and blasting is to prepare well-fragmented loose rock amenable to excavation with better productivity from the excavation machinery. The holes drilled for this purpose are defined as blast hole.

2.    Exploration drilling for sample collections to estimate the quality and quantity of a mineral reserve. The samples are collected as core and the drilling for such purposes are referred as Core drilling. As diamond bits are used for such drilling, core drilling is often called diamond drilling.

3.    Technical drilling during development of a mine for drainage, slope stability and foundation testing purposes.

Opencast mining involves removal of waste rock and subsequent winning of the mineral. In case of deposits underlying hard and compact waste rock called overburden loosening of the rock mass is essential prior to excavation. Thus drilling and blasting is the important ground preparation job. Unless the rock mass is highly weathered and very much unconsolidated drilling of holes for placement of explosives and detonating them for blasting is required for any mining operation. Modern machines like continuous surface miner can however eliminate the need of drilling and blasting in certain surface mining operations.

Successful drilling under specific site conditions requires blending many technologies and services into a coherent efficient team, particularly if it is for deep exploration drilling. Blast hole drilling is comparatively simpler. However, to minimize costs and optimize the performance and post drilling operations technical managers and decision managers must understand the language and technology of drilling operations.

This lecture covers the basic elements of drilling to give you a good understanding of the drilling process and how it integrates into the 'overall mining success'. You will learn:

                                                                  i.          An overview of drilling technology to develop an awareness of the equipment terminology and operations associated with the drilling process in surface mines.

                                                                 ii.          Classification of drilling methods

                                                                iii.          Principle of rock tool interaction in drilling

                                                                iv.          Definition and terminology

                                                                 v.          Application of different drilling methods

                                                                vi.          Selection of drills

                                                               vii.          Method of measuring drilling performances

                                                             viii.          Bailing and hole clearance

                                                                ix.          Measure while and log while drilling

                                                                 x.          Current safety practices

                                                                xi.          Drilling trends and new technology

                                                               xii.          Basic communications and supervisory skills to ensure a safe, efficient operation 

FUNDAMENTALS OF ROCK MECHANICS(PART 2)

Rock mass classification systems

Rock mass classification constitutes an integral part of empirical mine design. They are traditionally used to group areas of similar geo-mechanical characteristics, to provide guidelines for stability performance and to select appropriate support.

• The first step of the application of a classification system is to characterize the rock mass and in the second step use the advance forms of the classification systems to estimate the rock mass properties, such as modulus of elasticity, rock strength, m and s for Hoek and Brown failure criterion, etc., which are more appropriate inputs for strength parameters for any numerical analysis. Consequently, the importance of rock mass classification systems has increased over time (Milne et al., 1998)

In the recent years, Rock mass classification systems have been successfully used in tandem with analytical and numerical tools for the design of underground openings.

The most widely known systems, including Deere’s RQD, Bieniawski’s RMR, and Barton’s Q, have been used extensively throughout the world.

Rock mass classifications have been successful (Bieniawski, 1988) because they:
 

• Provide a methodology for characterizing rock mass strength using simple measurements.
• Allow geologic information to be converted into quantitative engineering data.
• Enable better communication between geologists and engineers and
• Make it possible to compare ground control experiences between sites, even when the geologic conditions are very different.

 

FUNDAMENTALS OF ROCK MECHANICS (1)

FUNDAMENTALS OF ROCK MECHANICS

Rock Material Classification

Compressive Strength (MPa)

 

Point Load Strength Index

 

Angle of Internal Friction (Degrees)

 

Rock as an Engineering material

Rock by nature is a heterogeneous, anisotropic and inelastic material and it exists in a very wide range with many geological structures built in its greater volume.

Rock Mass is an assemblage of intact rock materials separated by geological discontinuities.

 

ROCK MECHANICS (PART 20)

Horizontal stress - Longwall

•  Horizontal stress can not pass through gob area or broken or collapsed roof; therefore zones of stress relief and stress concentration are created.
• Their location depends on panel orientation, direction of retreat and sequence of extraction.

 

ROCK MECHANICS (PART 19)

Horizontal stress

• Lithostatic stress occurs when the stress components at a point are equal in all directions and their magnitude is due to the weight of overburden.

• The  other assumption is that rock behaves elastically but is constrained from deforming horizontally.
• This applies to sedimentary rocks in geologically undisturbed regions where the strata behave linearly elastically and are built up in horizontal layers such that the horizontal dimensions are unchanged. For this case, the lateral stresses σx and σy are equal and are given by:

• During the past 20 years, methods for measuring in situ stresses have been developed and a database established. Based on a survey of published results, Hoek and Brown (1980) have complied a survey of published data that is given in Fig. The data confirm that the vertical stresses measured in the filed reasonably agree with simple predictions using the overlying weight of rock.

 

 

 

 

 

Horizontal stress - Measurement

• Insitu Tests

              - Hydro Fracturing Method

- Over Coring Method

                     Magnitude and Direction

                         - Field Observations

                         - Stress Mapping – only Direction

 

 

 

 

 

 

 

 

 

 

ROCK MECHANICS (PART 18)

MECHANISM OF STRATA FAILURE

• Failure through intact material due to overstressing.
• Failure along bedding surface due to overstressing.
• Localized failure of discrete joint bounded blocks.
• Localized failure of thinly bedded roof sections.
• In coal measure strata

           - Bedded, low to moderate strength rock types
                 - Subjected to varying stress levels.
           - Expected behavior of strata
                 - Function of roadway shape, lithology & stresses acting on the roadway.

 

ROCK MECHANICS (PART 17)

1. Magnitude and orientation of Insitu stresses vary considerably within geological systems.
2. The pre-existing stress state changes dramatically due to excavation/construction therefore load must be redistributed.
3. Stress is not familiar – it is a tensor quantity and tensors are not encountered in everyday life.
4. It is a means to analyze mechanical behaviors of rock.
5. It serves as boundary conditions in rock engineering problems as a stress state is applied for analysis and design.
6. It helps in understanding groundwater fluid flow.
7. At large scale shed some light on the mechanism causing tectonic plates to move or fault to rupture with the added uncertainty in that there is no constraint on the total force, as is the case with gravity loads.

 

In situ stresses

Stress conditions often may change significantly across structures such as faults, dyke contacts and major joints. Stiffer geological materials tend to attract stress, so that stresses say in a dyke may be higher than in a rock such as quartzite in close proximity. These effects may influence the vertical stress to some extent. The effect of topography on vertical stresses depends on the height of the hill or valley in relation to its width.

 

In situ stress – World Stress Map

World Stress Map Project has now been working for more than 20 years on its data base.
Types of stress indicators. To determine the tectonic stress orientation different types of stress indicators are used in the World Stress Map.

They are grouped into four categories:

• Earthquake Focal Mechanisms                                        (69%)
• Well Bore Breakouts and Drilling Induced Fractures             (19%)
• In-situ Stress Measurements – Over coring                                                     Hydraulic Fracturing, Borehole Slotter                                (8%)
• Young Geologic Data (From Fault Slip Analysis and                                        Volcanic Vent Alignments                                                (4%)