DETERMINATION OF PILLAR STRENGTH FROM KAMOTO ROOM DETERMINATION OF PILLAR STRENGTH FROM KAMOTO ROOM

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  • DETERMINATION OF PILLAR STRENGTH

    FROM KAMOTO ROOM AND PILLAR COLLAPSE

    BY NUMERICAL APPROACH

    Nzenga Kongolo

    A project repoit submitted to the Faculty o f Engineering, University o f the

    Witvvatersrand, Johannesburg, in partial fulfilment o f the requirements for the

    degree o f M aster o f Science in Engineering.

    Johannesburg, 1998.

  • i

    D E C L A R A T IO N

    I declare that this project report is my own, unaided work. It is being submitted for the D egree o f M aster o f Science in Engineering in the U niversity o f the W itwatersrand, Johannesburg. It has not been submitted before fo r any degree or exam ination in any other university.

    ^ — day — i 998

  • i

    D E C LA R A TIO N

    I declare that this project report Is my own, unaided work. It is being submitted for the Degree o f M aster o f Science in Engineering in the University o f the W itwatersrand, Johannesburg. It has not been subm itted before for any degree or exam ination in any o ther university.

    day of- 1998

  • ii

    ABSTRACT

    In Septem ber 1990, a section o f Kam oto mine w here two thick and superimposed

    ore bodies w ere being mined, using the room and pillar m ining method operating

    in isolated blocks, experienced a large scale collapse, Pillars 15 meters long, 10

    m eters w ide and 12 m eters high in the upj.er ore body and 14 meters high in the

    low er ore body w ere left during mining o f a block w ith 15 meter-wide-rooms.

    D ue to the unstable nature o f som e pillars within the first mined blocks, a decision

    w as taken to fill, partially or totally, the mined area with uncemented backfill.

    Unfortunately, this decision did not incorporate a re-evaluation o f pillar strength

    in term s o f rock m ass classification encountered during the developm ent stage o f

    ‘hat m ining method, hence the need to determine the pillar strength from the

    Kam oto room and p illar collapse.

    This back-analysis o f K am oto pillar strength assessment involved a combination

    o f several approaches in rock engineering such as the rock mass classification,

    rock mass strength and pillar stresses, and strength. The Bieniawski’s

    geotechnical rock m ass classification (1976) has been used to determine the

    Geological Strength Index (GSI) for each layer o f the ore bodies. This concept,

    combined w ith the strength o f intact rock and the Hoek-Brown failure criterion

    have been exploited to assess the rock mass properties, such as the strength,

    elastic modulus, P o isson’s ratio, cohesion, friction angle and Hoek-Brown

    constants. These estim ates are used as input data in the Kam oto room and pillar

    model,

    The strength values o f pillar have been determined by analytical method and

    numerical modelling. In the first approach, the strength o f a cubical specimen o f

    Kam oto rocks has been separately assessed by the procedure proposed by Ryder

  • Ill

    and Ozbay (1990) and by the Rock M ass Index (RM I) developed by Palmstom

    (1997), The m odified Salam on’s formula and Obert-Duvall form ula for pillar

    design in the hard rock have been employed to assess the strength o f pillar in the

    study area. In the second approach, an elastic boundary code nam ed M insimW has

    been perform ed to generate the elastic model o f K am oto room and pillar area from

    the real mine plans edited before the collapse, and to com pute the mining induced

    stresses acting on the pillars. Under conditions assum ed during this work, the

    numerical analysis reveals that the ultim ate average stresses acting on the pillar

    edge at the beginning o f the large scale failure mechanism are estimated at 35

    M Pa and 43 M Pa in the upper ore body and the lower ore body respectively. It has

    been noticed that these values are abcut 2.7 times greater than those calculated by

    Salam on’s formula and about 1.3 tim es greater than the strength values from

    Obert-Duvall formula.

  • To my family

    WMWWWWlWWWJOMTlTllllii

  • A C K N O W L E D G E M E N T S

    This project achievem ent m ight not be possible if the following assist ance w as not available:

    - I w ish to thank the management o f the company “ La Generate des M ines et Carrieres” (GECAMTNES) for sp o n so rirt my tertiary education.

    - I thank Prof, M .U Ozbay, my supervisor for the orientation o f this project and his endless encouragement,

    - I am grateful to M r Roger Johnson from CSIR / M iningtek for his advice in running the MinsimW program.

  • CONTENTS PAGE

    D ECLA RA TIO N ...................................................................................................................... i

    A B STR A C T................................................................ ii

    D ED IC A TIO N .........................................................................................................................iv

    A CK N OW LEDG EM EN TS..................................................................................................v

    C O N TEN TS.............................................................................................................................vi

    L IST OF FIG U R ES.............................................................................................................. x

    LIST OF TABLES................... xii

    LIST OF A BB REV IA TIO N S.......................................................................................... xiii

    1. IN T R O D U C T IO N ......................................... 1

    1.1 B ackground................................................................................................................... 1

    1.2 Purpose o f the w ork ................................................................................................... 3

    1.3 M ethodology.................................................................................................................3

    2. L IT E R A T U R E S U R V E Y ...................................................................................... 5

    2.1 In troduction ............................................................................. 5

    2.2 Rock mass classification........................................................................................... 5

    2.2.1 The NGI Tunnelling Index or Q-Ciassification..................................................6

    2.2.2 B ieniawski’s G eom echanics Classification .............................................8

    2.2.3 Laubscher’s G eom echanics Classification.......................................................... 9

    2.2.4 Discussion on rock classification sy s te m ..........................................................10

    2.2.5 Influence o f geology on pillar stability............................................................... 11

    2.3 Pillar strength theories in shallow m ines...................................................... ..12

    2.3.1 Categories o f mine pillar in shallow m ines........................................................ 12

    2.3.2 • P illar strength and D esign...................................................................................... 14

    2.3.3 Pillar loading system ............................................................................................... 20

    »■ -

  • vii

    PAGE

    2.4 Failure modes o f failure........................................................................................... 25

    2.4.1 Failure criteria...........................................................................................................26

    2.4.2 Pillar safety fac to r......................................................................................................29

    2.5 Basic concepts o f numerical modelling............................................................... 30

    2.6 Conclusion.................................................................................................................33

    3 K A M O T O M IN IN G G E O L O G Y ....................................................................35

    3.1 Introduction................................................................................................................35

    3.2 Form and structure o f the main ore body......................................................... 35

    3.3 Lithostratigraphy o f the Katangan system ........................................................36

    3.4 Stratigraphy o f the Roan G roup...........................................................................38

    3.5 M ineralisation.......................................................................................................... 39

    3.6 G eobgical m ining reserves...................................................................................39

    4 K A M O T O R O C K M ASS C L A S SIFIC A T IO N AND STR EN G TH ... 40

    4.1 Introduction........................................................................................................... ...40

    4.2 M echanical properties o f Kam oto intact rocks................................................41

    4.3 R ock mass classifi

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