The role of the rock mechanics engineer in the mining industry:-

The responsibilities of the rock mechanics engineer in the mining industry, and the available computer programs and their applications.

Over the past decade (1963 – 1973), there has been a change of emphasis in the field of rock mechanics from a purely theoretical approach to the practical solution of problems which occur in the mining industry. However, without the theoretical and research work in the period before 1967, the rock mechanics engineer would not be in a position where he can plan the mining operations safely, particularly in deep-level, hard rock mines.

Numerous papers have been published in recent years, describing in detail the computer programs and tools available to the rock mechanics engineer and their application to particular problems in the mining industry. It is not the purpose of this article to examine rock mechanics in detail, but to describe the organisation of rock mechanics in the mining industry. In so doing, it is hoped that the mechanical and electrical engineers in the mining industry will be made aware of what rock mechanics is and, as a result, where it can be of assistance to them

Rock mechanics is usually associated with the gold mining section of the industry. This is to be expected since the most urgent problems which had to be solved occurred as a result of gold mine workings extending to greater depths and the consequent increase in rock burst incidence. This was followed by the application of rock mechanics to the design of bord and pillar and longwall workings in the coal mines.
The discovery and exploitation of base metal ore bodies in recent years has expanded the field of rock mechanics to include the planning of new base metal mines.

Organisation and responsibilities:-

The rock mechanics engineer is usually responsible to the consulting engineers of the various mines of his group. He provides a rock mechanics service for the mining policy and long-term planning of the mines, and the solution of major problems which may arise. Rock mechanics officers are employed on these mines to assist him in his duties, particularly on the gold mines which are operating at greater depth. The rock mechanics officers are an essential part of the management team on the mine and, as such, provide a rock mechanics service to the manager for short, medium and long-term planning.

The following list gives an idea of some of the problems in which rock mechanics is an important and major consideration:-

• Planning of mine layouts and stoping sequences.
• Control of the incidence of rock bursts.
• Stope support.
• Haulage support.
• Control of blasting in stopes for improved hanging wall control.
• Control of blasting in haulages for improved hanging wall and sidewall control, and to ensure long-term stability.
• Overstoping of haulages, cross-cuts and inclined shafts.
• Shaft pillar design.
• Siting of main excavations within the shaft pillar.
• Support for main excavations – pump chambers, water dams, settlers, hoist chambers and refrigeration plant chambers.
• Design of pillars to act as water barriers.
• Organisation of introductory rock mechanics courses for mine production officials.

Siting main excavations and supporting them are possibly of more concern to the mechanical engineer since one of the major problems of the rock mechanics engineer is the support of large excavations such as hoist and pump chambers, particularly at the deeper levels. The cost of supporting these chambers increases rapidly with an increase in cross-sectional area of the excavation. Where possible, the dimensions should be reduced to a minimum, taking cognisance of the need to exchange and service equipment. This point should be kept in mind when planning new excavations.

Analogue and computer programs:-

Development of the techniques:-

The work of Ryder et al (1964) and Ortlepp et al (1964) shows that elastic theory could be used to predict the stresses and displacements induced by mining in hard rock mines at great depth.
The mathematical relationship between elastic theory and the equations governing the steady flow of elasticity was used by Salamon et al (1964) to develop the electrolytic tank analogue in which the elastic convergence in the excavations in the plane of the reef could be ascertained.

Once the convergence distribution of an irregular stoping configuration could be determined using the electrolytic tank analogue, it was possible, using an integration technique developed by Salamon (1964), to calculate the induced stress and displacements at a point above or below the reef plane.

The electrical resistance analogue:-

Due to the difficulties associated with the use of the electrolytic tank analogue, Cook et al (1965) designed an electrical resistance analogue based on the same principles. This had the advantage that stresses normal to the reef plane in the unmined areas could be measured and changes in the mining layouts could be modelled relatively quickly.
The manual integration process for the calculation of stresses and displacements above and below the reef plane was subsequently replaced with a digital computer program.
The operation of the electrical resistance analogue application to the solution of rock mechanics problems is described by Wilson, et al (1970). During the last three years, the analogue has been superseded by the Mining Simulator program, or MINSIM for short.

The mining simulator program:-

The electrical resistance analogue is time consuming with having to prepare the input data for the integrator program. The MINSIM program is a completely digital solution of the problems previously solved in analogue. The input data is simple and quick to prepare. The other advantages are the scaling facility which enables problems to be examined in detail, and the storage of the solution on disc for updating at a future date. Like analogue, the MINSIM program can handle moderately inclined tabular reefs at depth. However the program can be used, under certain conditions, for the solutions of problems at relatively shallow depth with very little error.

The finite element program:-

The finite element solution of problems is used extensively in the civil engineering field. This method has been adapted for rock mechanics applications and the computer program developed has been described by Deist et al (1968). The finite element program can solve problems containing any shape of excavation but, because of the limitations on the speed of available computers, is restricted to the solution of two-dimensional idealisations. Although this restriction limits the use of the program, many mining problems arise which can be represented by two-dimensional idealisations without serious error. Another advantage of the program is that surface effects can be considered under shallow depth conditions.

The major disadvantage with the initial finite element program was the preparation of the input data, namely the drawing up of the network of triangles, nodal point lists and subsequent error correction. This disadvantage has been overcome with the development of a mesh generator program described by Deist et al (1971). All that is required is to divide the network into areas of similar sized triangles ranging from large triangles for the boundary regions, to small triangles for the regions which are of particular interest, the program generating the necessary triangles and nodal point lists. The time required for the input data preparation for the mesh generator is two or three hours compared with two or three weeks for the manual preparation of the data.

A further advantage is that the network of triangles can be plotted automatically, using a different scale for the various areas.

Three-dimensional elastic problems:-

Because of the limitations on the application of the analogue, MINSIM and Finite Element Programs in rock mechanics, the Research Organisation of the Chamber of Mines of South Africa is engaged actively in the development of a program for the analysis of three-dimensional configurations. This program will be of considerable assistance for the planning, not only of gold mines, but perhaps more important, open pit operations and base metal mines with irregular shaped ore bodies. This program is eagerly awaited by the rock mechanics engineers.

Applying analogue computer programs to mechanics:-

Interaction between vertical ore body, adjacent shaft:-

A vertical elliptical shaped ore body approximately 90 x 30 m extends for a depth of 550 to 730 m below the surface. Situated approximately 45 to
50 m perpendicular to the long axis of the ore body is the main vertical hoisting shaft. The mined-out ore body has not been filled with waste material. To determine the interaction between the ore body and shaft and possible damage to the shaft system, the problem was analysed by the Finite Element Program using a horizontal section through the ore body. Considering that the vertical height of the ore body was 180 m compared with the maximum width of 90 m, the configuration could be examined with a two-dimensional approximation. The results showed that the shaft system would not be subjected to serious ground movement and damage while the shaft is used for mining the deeper levels.

Multi-reef pillar mining at depth:-

The mining of a multi-reef deposit at a depth of 2000 to 3000 m is being planned. Three reefs with a total stoping width of approximately 7 m will be extracted within a total reef series thickness of 10 to 40 m. A central line of pillars, in the dip direction of the reef series, will be left in situ to reduce the rate of energy release and alleviate the incidence of rock bursts. The MINSIM program was used for the planning of the pillars and a combination of the MINSIM and Finite Element Program was used for the design of and to examine the stability of the pillars.

Stability of a tall chimney stack:-

A 180 m tall reinforced concrete chimney stack was constructed on ground where mining was currently taking place at a depth of 210 m below the chimney. The tabular reef has a shallow dip and the rock type from the reef horizon to surface was essentially homogeneous and fault free. To determine the ground movements (tilt and strain) at the base of the chimney as mining progressed to a greater depth, a vertical section in the dip direction of the reef was examined using the Finite Element Program. The mining configuration could, within limits, be represented by a two-dimensional idealisation. However, this idealisation would represent the worst case condition and the results showed that the ground movements were within the design limits of the chimney.

Location of water inflow into a mine:-

A large inflow of water into a mine resulted as a consequence of mining which had taken place several years previously. The area in question as simulated on the MINSIM program and the movements and stresses at various elevations above the reef plane were determined. It was possible from this analysis and consider the geological features to determine the approximate location of the water bearing fissure or fissures so that the necessary action to stop the inflow of water could be planned.

Water barriers:-

When it is necessary to isolate mines or sections of mines from possible large uncontrollable inflow of water, reef pillars are left in situ to act as water barriers. With multi-reef mining where the reef thickness extracted is of the order of 7 m over a total height of 10 to 40 m, the width of each pillar acting as a water barrier must be determined separately. Most of the barriers can be represented by two-dimensional idealisations since they are usually associated with dykes, faults or mine boundaries.
The following procedure is adopted to design the barriers. The stress levels acting on the barrier are obtained from the MINSIM program which models the overall mining configuration. The various widths of the water barriers are then examined in detail using the Finite Element program, the boundary conditions on the perimeter of the network having been obtained from the MINSIM model. The design of the barrier must satisfy the following criteria:

• The width-height ratio of the pillar must be sufficiently large to ensure pillar stability.
• The zone of failed rock above and below the pillar must not bridge across the pillar.
• The minimum principal stress in the central portion of the pillar and above and below the pillar must be compressive and exceed the water pressure.

These examples illustrate the use of the computer programs to rock mechanics problems which are not associated with deep level, tabular and narrow reef deposits. Without the development of these computer techniques the rock mechanics engineer would not be in a position to solve the numerous problems which arise in the mining industry.