apêndice a-análiseclassificaçõesgeomecânicas

8/13/2019 Apndice A-AnliseClassificaesGeomecnicas

1/17

A

GAP 416

APPENDIX A

APPENDIX A

ANALYSIS OF CURRENT ROCK MASSCLASSIFICATION SYSTEMS


2/17


3/17

GAP 416

APPENDIX A

A1

A.1 Introduction

The design of an underground excavation at the early stage in a project, when little informationis known about the strength properties of the rock mass, is normally based on practicalexperiences encountered in similar geological environments and on engineering judgement.However, in cases where construction works are to be undertaken in formations where suchprior information is lacking, some form of classification system, which enables ones own set of

conditions to be related to conditions encountered by others, is essential. The ultimate aim ofsuch a classification system is to provide a good estimate of average strength of the rock mass.The aims of rock mass classification for rock engineering application were identified byBieniawski (1984) as follows:

a- to divide a particular rock mass into groups of similar behaviour;

b- to provide a basis for understanding the characteristics of each group;

c- to yield quantitative data for engineering design, and

d- to provide a common basis for communication.

All relevant information gathered from observations, experience and engineering judgementwould facilitate a quantitative assessment of rock mass conditions and recommendation ofappropriate support systems.

Bieniawski (1984) stated that these aims can be fulfilled by ensuring that a classification systemhas the following attributes:

a- it is simple, easily remembered, and understandable;

b- each term is clear and the terminology used is widely accepted by engineers and geologists;

c- the most significant properties of the rock mass are included;

d- it is based on measurable parameters which can be determined by relevant tests, quicklyand cheaply in the field;

e- it is based on a rating system that can weigh the relative importance of the classificationparameters; it is functional by providing quantitative data for the design of rock support.

A.2 Common rock mass classification systems

A number of rock mass classification systems, beginning with Terzaghis (1946) rock loadfactor, have been proposed over the years. The commonly used systems which have foundvarious applications in mining, such as the Rock Structure Rating (RSR) concept, BieniawskisRock Mass Rating System (RMR), the NGI Q system, the Mining Rock Mass Rating (MRMR),the Modified Basic RMR (MBR) and the Rock Mass Index (RMi), are reviewed.


4/17

GAP 416

APPENDIX A

A2

A.2.1 The Rock Structure Rating (RSR) system

The RSR concept was the first complete rock mass classification system proposed since thatwas introduced by Terzaghi in 1946. The RSR concept, a ground support prediction modelpresenting a quantitative method for describing the quality of a rock mass and for selecting theappropriate ground support, was developed in the United States in 1972 by Wickham,Tiedemann, and Skinner.

Bieniawski (1984) described the RSR concept as a step forward in a number of respects: firstly,

it was a quantitative classification, unlike Terzaghis qualitative one; secondly, it incorporatedmany parameters unlike the RQD index that is limited to core quality; and thirdly, it was acomplete classification, having an input and an output that relies on practical experience todecide on a rock mass class.

The introduction of a rating system for rock masses is the main contribution of the RSR concept,which was developed using case histories as well as reviews of various books and technicalpapers dealing with different aspects of ground support in tunnelling. Two general categories offactors influencing rock mass behaviour in tunnelling, namely geological parameters andconstruction parameters, are considered in the RSR concept as follows;

- Geological parameters (rock type, joint pattern, joint orientations, type of discontinuities,

major faults, shears and folds, rock material properties, and weathering or alteration).

- Construction parameters (size of tunnel, direction of drive, method of excavation)

All the above factors are grouped into three basic parameters, A,B, and C, which in themselvesare evaluations as to the relative effect of various geological factors on the supportrequirements.

An adjustment to the RSR value is suggested as a result of a significant decrease in the supportrequired for machine-bored tunnels compared with those excavated by drill and blast methods.Wickham et al. (1974) have prepared support requirement charts providing a means ofdetermining typical ground control systems. These systems are based on RSR prediction as to

the quality of the rock mass through which the tunnel is to be driven.

However, it is concluded that this system is not suitable for predicting support requirements formine excavations. The major limitations for this purpose are presented below:

1- It does not provide for the influence of stress changes on the rock masses.

2- The ratings given to typical rock masses found in deep mines are close to the end of theoperating range of their support chart.

3- It does not consider the types of support which are effective in deep mines.

4- It is not recommended for the selection of rockbolt and shotcrete support because of a lackof sufficient and reliable data used in the development of the system.

5- Lack of clear definitions may lead to some confusion.


5/17

GAP 416

APPENDIX A

A3

A.2.2 The Rock Mass Rating (RMR) system

The RMR System or Geomechanics Classification, which was developed by Bieniawski (1973),enables determination of the RMR, tunnel span, stand-up time, support requirements, in siturock mass modulus, and the cohesion and friction of a rock mass.

Bieniawski (1973) indicated that the first step in the application of the geomechanicsclassification is to divide the tunnel route into a number of structural regions, i.e. zones in whichcertain geological features are more or less uniform within each region. Six of the most

significant parameters that are measurable in the field and also obtained from borehole data,and which are utilised for each area to describe the behaviour of jointed rock masses, are:

1- Uniaxial compressive strength (UCS) of rock material,

2- Rock quality designation (RQD),

3- Spacing of joints,

4- Conditions of joints,

5- Ground water inflow,

6- Orientation of joints.

In detail;

1- UCS of rock material

Bieniawski believes that the engineering classification of intact rock proposed by Deere isparticularly realistic and convenient for use in the field of rock mechanics. However, thedetermination of the uniaxial compressive strength of rock materials is a simple and cheap

process for which standard techniques are available. Tests may be conducted on preparedrock specimens in the laboratory. Alternatively, as a measure of intact rock material strength,for all but very low strength rocks the point load index may also be used. This involves thetesting on site of unprepared rock cores or hand specimens using simple portable equipment.In this test the specimen is compressed between two points. The point-load strength index iscalculated as follows:

I P

Ds = 2

where: P, the load required to break thespecimen,D, the diameter of the core.

The length of the tested core should be at least 1,5 times the specimen diameter. If thediameter D of the core is expressed in millimetres, an approximate relationship between the

point load index Isand the uniaxial compressive strength cis given by

= +( . )14 0175D Is


6/17

GAP 416

APPENDIX A

A4

This formula was determined empirically by comparing many sets of point load and UCS results.The correlation is reasonably reliable but the point load tests are closer to a Brazilian test than astandard compression test. The failure is thus by induced tension or an indirect method ofobtaining compressive strength.

2- Rock Quality Designation (RQD)

The RQD is a quantitative index based on a core recovery procedure in which the core recoveryis determined, incorporating only those pieces of core which are 100 mm or greater in length.

Deere (1964) defines the RQD as the percentage of core recovered in intact pieces of 100 mmor more in length compared to the total length of the borehole. The main limitations of RQD arethat it ignores the influence of joint orientation, continuity and gouge material, and it is notnormally possible to assess the spacing of joints from a single set of borehole cores.Consequently, while the RQD seems to be a commonly used parameter because it is easilydetermined, it is limited to a semi accurate and quantitative comparison of discontinuity spacingwhich can be much better determined by direct measurements in mining excavations.

3- Spacing of joints

The term joint is used to mean all discontinuities, which may be joints, faults, bedding planes

and other surfaces of weakness. Spacing and orientation of joints are of paramount importancefor the stability of structures in rock masses. The presence of joints reduces the strength of arock mass. The degree of reduction is governed by the joint spacing, as well as their dip andstrike.

4- Condition of joints

This parameter accounts for the separation or aperture of joints, their continuity, the surfaceroughness, the wall condition (hard or soft), and the presence of infilling materials in the joints.These aspects are related to the strength of joints since tight joints with rough surfaces and nogauge will have a relatively high shear strength and cohesion. The joint shear strength andstiffness increases with normal stress.

5- Ground water conditions

An attempt is made to account for the influence of ground water flow on the stability ofunderground excavations in terms of the observed rate of flow into the excavation. This isaccounted for by estimating the ratio of joint water pressure to major principal stress or by somegeneral qualitative observation of groundwater conditions.

Bieniawski (1976) emphasised that the importance of the five parameters discussed above isnot equal for the overall classification of a rock mass and importance ratings are assigned to thedifferent ranges of values of the parameters. Bieniawski, therefore, applied a series ofimportance ratings to his parameters following the concept used by Wickham et al. (1972).These ratings were determined from 49 case histories, and, in the assignment of the importance

ratings, the typical rather than the worst conditions are evaluated and it is assumed that the rockmass has three sets of discontinuities (Bieniawski, 1976). Thus, when only two sets ofdiscontinuities are present, a conservative assessment is obtained.

The influence of the strike and dip of discontinuities, which depends upon the engineeringapplication, is described as qualitative descriptions, such as favourable, fair, unfavourable,etc., rather than in quantitative terms. The studies by Wickham et al.(1972) provide the basisfor deciding whether the strike and dip orientations are favourable or not.


7/17

GAP 416

APPENDIX A

A5

For mining applications, other adjustments may be called for, such as the stress at depth or achange in stress (Kendorski et al.,1983).

The final rock mass ratings falling into one of five different rock mass classes will be determinedafter adjustment for discontinuity orientations.

The main disadvantages of the RMR system are that, although it takes into account guidelinesdepending on such factors as the depth below surface, tunnel size and shape, and the methodof excavation, for the selection of roof support to ensure long-term stability of various rock mass

classes, it was not specifically designed for deep mine tunnels nor for wide span stopes. Piper(1985) concluded that the RMR system does not consider the nature of the fractured rock massto the same extent as the Q system does. In addition, the joint spacing is included twice byconsidering both the RQD and the spacing between discontinuities. A further problem is thatthe ratings are particularly insensitive to the range of rock strengths typically found in deepmines.

A.2.3 NGI, Q system

The Q-system of rock mass classification, developed in Norway in 1974 by Barton, Lien andLunde, enables the design of rock support for tunnels and large underground chambers. Thesystem was proposed on the basis of an analysis of some 200 tunnel case histories fromScandinavia and is regularly updated - the current database includes over 1200 case histories.It is a quantitative classification system and an engineering system enabling the design oftunnel support.

The Q-system is based on a numerical assessment of the rock mass quality using six differentparameters, each of which has a rating of importance, which can be updated during subsequentexcavation. The six parameters are as follows,

1- RQD index,

2- number of joint sets,

3- roughness of the weakest joints,

4- degree of alteration or filling along the weakest joints,

5- water inflow,

6- stress conditions.

The above six parameters are grouped into three quotients to give the overall rock mass qualityQ as follows;


8/17

GAP 416

APPENDIX A

A6

Q RQD

J

J

J

J

SRFn

r

a

w=

where:

RQD = rock quality designation,

Jn = joint set number,

Jr = joint roughness number,

Ja = joint alteration number,

Jw = joint water reduction factor,

SRF = stress reduction factor.

A general description of RQD is given on page 4. In addition to this, it is normally accepted thatthe RQD should be determined on a core of at least 50 mm diameter, which should have beendrilled with double barrel diamond drilling equipment. However, when a bore core isunavailable, RQD can be estimated from the number of joints per unit volume, in which thenumber of joints per metre for each joint set are added. A simple relation can be used toconvert this number to RQD for the case of clay-free rock masses (Palmstrom, 1974):

RQD J approxv= 115 3 3. ( .)

where:

Jv = total number of joints per m3

RQD= 100 for Jv< 4,5

Because of its limitations in disregarding joint orientation, tightness and gouge (infilling)material, the RQD is not sufficient on its own to provide a proper description of a rock mass.

Jn represents the number of joint sets, which will often be affected by foliation, schistocity,slately cleavage or bedding, etc. If strongly developed, these parallel joints should obviously becounted as a complete joint set. However, if there are few joints visible, or any occasionalbreaks in the core due to these features, then it will be more appropriate to count them asrandom joints when evaluating Jn. The first two parameters, RQD and Jn, represent the overallstructure of the rock mass, and their quotient is a relative measure of a block of a particulatesize, with the two extreme values of 200 and 0,5 being fairly realistic approximations.

The quotient of the Jr and Ja parameters is used as an indicator for the evaluation of theinterblock shear strength of a joint from the weakest significant joint set or clay filleddiscontinuity in the given zone. In other words, the value of this quotient should in fact relate tothe surface most likely to allow failure to initiate. It is weighed in favour of rough, altered joints

in direct contact. Barton et al.(1973) quoted that, when rock joints have clay mineral coatingsand fillings, the strength is reduced significantly. Nevertheless, rock wall contact after smalldisplacements have occurred may be a very important factor for preserving the excavation fromultimate failure. Where no rock wall contact exists, the conditions are extremely unfavourable totunnel stability. Bieniaswki (1984) noted that, if the joint set or discontinuity with the minimumvalue of (Jr/Ja) is favourably oriented for stability, then a second, less favourably oriented jointset or discontinuity may sometimes be of more significance, and its higher value of (J r/Ja) shouldbe used when evaluating Q.


9/17

GAP 416

APPENDIX A

A7

The third quotient (Jw/SRF) consists of two stress parameters. The parameter of Jw is ameasure of water pressure, while the stress reduction factor (SRF), which is regarded as totalstress, is a measure of:

a) loosening load in the case of shear zones and clay bearing rock,

b) rock stress in competent rock, and

c) squeezing and swelling loads in plastic incompetent rock.

The quotient of Jwand SRF describes the measure of active stress. The purpose of the factorof SRF is to reduce the quality of the rock mass to take into account the influence of intact rockstrength and applied stresses. It approximates to loosening loads due to weakness zonesintersecting excavations, rock stress problems or plastic flow of incompetent rock under theinfluence of high rock pressure. In such cases the strength of the intact rock is of little interest.However, when jointing is minimal and clay is completely absent, the strength of the intact rockmay become the weakest link, and the stability will then depend on the ratio of rock stress torock strength.

Bartonet al.,

(1974) consider the parameters Jn, Jr, and Jaas playing a more important role thanjoint orientation, and, if joint orientation had been included, the classification would have beenless general. Orientation is implicit in parameters Jr, and Ja, because they apply to the mostunfavourable joint sets which are obviously oriented in space. However, the orientation withrespect to the geometry of the excavation is not considered.

The main disadvantages of the Q system for mining, as Piper (1985) emphasised, are that typesof supports listed in the tables of recommended support do not include those commonly used indeep mines. Also, the system is insensitive to subtle changes in rock mass characteristics,which have a strong influence on the stability of deep mine excavations. He also indicated thatthe Q-System was developed on the basis of examining shallow excavations in Scandinavia,and therefore it cannot be expected to provide suitable support recommendations for miningexcavations in high stress environments. Furthermore, the technique of multiplying thequotients makes the system very sensitive to changes in each parameter.

Undoubtedly, there are several other parameters, which could be added to improve theaccuracy of the classification system. One of these would be joint orientation. The parametersJn, Jr and Ja appear to play a more important general role than does joint orientation, becausethe number of joint sets determines the degree of freedom for block movement (if any), and thefrictional and dilatational characteristics can vary more than the down-dip gravitationalcomponent of unfavourably orientated joints. However, it is recognised that orientation is animportant parameter in cases involving major clay-bearing weaknesses and fault zones. Itseems very likely that the first four parameters (RQD, Jn, Jr, Ja) can form the basis for manyrock mass classification systems. However, the ratings may need to be modified, and otherparameters added.

The SRF factor ranges from 2,5 to 10 with a reduction possibility of these values by 25 % to50 % if relevant shear zone only influences but does not intersect the excavation, can have alarge influence on the final rating. The correlation between SRF and the requirements ofsupport in deep mines is not well established nor is the influence of stress changes. Also aparticular Q rating value may have been derived either with or without the SRF having asignificant influence. It is believed that the support requirement in this case could besignificantly different.


10/17

GAP 416

APPENDIX A

A8

A.2.4 The Mining Rock Mass Rating (MRMR) system

The geomechanics classification (RMR) has been extended for different mining environments.Initially, Laubscher and Taylor (1976) applied this technique in asbestos mines in Africa whileFerguson (1979) used this classification for mining tunnels and haulages.

Laubscher and Taylor (1976) made some essential additional adjustments to the RMR systemto cater for diverse mining situations. The fundamental difference was the recognition that insitu rock mass ratings (RMR) had to be adjusted according to the mining environment so that

the final ratings (MRMR) could be used for mine design. The adjustment parameters are givenas weathering, mining induced stresses, joint orientation and blasting effects.

Adjusted RMR parameters are as follows:

1- Blasting damage adjustment (AB), (0,8-1,0),

2- In situ stress and change in stress adjustment (AS), (0,6-1,2), and,

3- Major faults and fractures (S), (0,7-1,0).

Adjusted RMR=RMR*AB*AS*S where: maximum value of AB*AS*S is 0,5.

In recent years, there have been some modifications and improvements to the system.Laubscher (1984) has emphasised a comprehensive system based on a comparison betweenthe in situ rock mass strength and the mining induced stresses. The important key point here isthat the application of this system should be applied to an intact rock mass rather than to abroken rock mass.

Laubscher (1990) revealed that it is possible to use the ratings to determine an empirical rockmass strength (RMS) which is adjusted as above to give a design rock mass strength (DRMS).Obviously, this figure is extremely useful when related to the stress environment and has been

used for numerical modelling. Also, these ratings provide good guidelines for mine designpurposes.

In addition, Laubscher (1990) indicated in a separate study that the average numbers can bemisleading and the weakest zones may determine the response of the whole rock mass. It is,therefore, necessary to identify narrow and weak geological features that are continuous withinand beyond the stope or pillar, and rate them separately.

The intact rock strength (IRS), joint/fracture spacing, and joint condition/water must be includedinto the assessment of geological parameters. The analysis of these is as follows:

Intact rock strength (IRS)

The definition of IRS can be the unconfined uniaxial compressive strength of the rock betweenfractures and joints. As mentioned before, the results of laboratory testing carried out is usuallynot representative of the average values because the samples are invariably the strongestpieces. Undoubtedly, the presence of weak and strong intact rock and deposits of varyingmineralisation affects the value of IRS for a defined zone. An average value is assigned to thezone with the knowledge that the weaker rock will have a greater influence on the averagevalue. In such cases, the IRS for the weak and strong zones is determined separately, andexpressed as a ratio. Knowing the percentage of weak rock and selecting the appropriate curvewhich defines the relationship of weak rock IRS expressed as a percentage of strong rock IRS,


11/17

GAP 416

APPENDIX A

A9

the average IRS can be estimated as a percentage of strong rock IRS. A detailed empiricalchart to determine an average IRS, where the rock mass contains weak and strong zones, waspresented by Laubscher (1990).

Spacing of fractures and joints

Spacing is the measurement of all the discontinuities and partings, and does not includecemented features. Two techniques have been developed for the assessment of this parameter(Laubscher, 1990).

a) measuring the rock quality designation (RQD) and joint spacing (JS) separately.

The detailed analysis of RQD is presented in the previous sections. In the assessment of jointspace rating developed by Taylor (1980), the three closest-spaced joints are used to read offthe rating. The equations for all lines defined in the chart of assessment of joint-space rating(R), for the different number of joints, are as follows:

one joint set;

R=25*((26,4*log10*x)+45)/100

two joint sets;

R=25*((25,9*log10*xmin)+38)/100*((30*log10*xmax)+28)/100

three joint sets;

R=25*((25,9*log10*xmin)+30)/100*((29,6*log10*xint)+20)/100*((33,3*log10*xmax)+10)/100

where x=spacing*100

b) measuring all the discontinuities and recording these as the fracture frequency per metre.

It is possible to have a rating value for a rock mass either from a measurement of all thediscontinuities that are intersected by the sampling line or from a borehole log sheet.

In comparing these two techniques, it is concluded that the fracture frequency per metretechnique is more sensitive than the RQD for a wide range of joint spacings.

Joint condition and water

Joint condition is an assessment of the frictional properties of the joint (not fractures) and isbased on expression, surface properties, alteration zones, filling, and water. Originally, theeffect of water was catered for in a separate section; however, it was decided that the

assessment of joint condition allowing for water inflows would have greater sensitivity.

Adjustments

In order to estimate the value of MRMR, the rock mass value derived by the RMR system ismultiplied by an adjustment percentage. Laubscher (1990) emphasised that the adjustmentpercentages are empirical, having been based on numerous observations in the field, and theadjustment procedure requires that the engineer assess the proposed mining activity in terms of


12/17

GAP 416

APPENDIX A

A10

its effect on the rock mass. Laubscher (1990) suggested that, in order to arrive at the MRMR,the RMR should be adjusted by considering the influence of the following factors:

a) weathering,

b) joint-orientation for pillars and sidewalls,

c) mining-induced stresses, and

d) blasting effects.

Weathering

Weathering must be taken into consideration in decisions on the size of an opening and thesupport design. Its effect is time dependent, and influences the timing of support installationand the rate of mining. The basic three parameters, IRS, RQD or fracture frequency per metre,and joint condition, are affected by weathering. The relation between these parameters couldbe summarised as below:

- An increasing number of fractures will result in a decreased value of RQD.

- Chemical composition changes taking place have a significant effect on the IRS.

- Alteration of the wall rock and the joint filling will affect the joint condition.

Laubscher (1990) published a table delineating adjustment percentages related to degree ofweathering, after a period of exposure of various years.

Joint orientation

Size, shape and orientation of an excavation play a significant role in rock mass behaviour. Theattitude of the joints, and whether or not the bases of blocks are exposed, has a significantbearing on the stability of the excavation, and the ratings must be adjusted accordingly. In hisadjustment procedure, the attitude of the joints with respect to the vertical axis of the block hasthe most important role. As gravity is the most significant force to be considered (in shallowmines), the instability of the block depends on the number of joints that dip away from thevertical axis. A modified orientation adjustment applies to the design of pillars or stopesidewalls. The applicability of this rating has been described in detail by Laubscher (1990).

Mining-induced stresses

Laubscher (1990) indicated that the major influences on mining induced stresses, arising as aresult of the redistribution of field (regional) stresses, are the geometry and orientation of theexcavations. The redistributed stresses that are of interest are maximum, minimum anddifferences; hence it is essential that the magnitude and ratio of these stresses are known.


13/17

GAP 416

APPENDIX A

A11

a) Maximum stress

The maximum principal stress can cause spalling of the wall parallel to its orientation, crushingof pillars, and the deformation and plastic flow of soft zones. The deformation of softintercalates leads to failure of hard zones at relatively low stress levels. A compressive stressclose to perpendicular joints increases the stability of the rock mass and inhibits caving.

b) Minimum stress

The minimum principal stress plays a significant role in the stability of the sides and back oflarge excavations, the sides of stopes, and the major and minor apexes that protect extractionhorizons. The removal of a high horizontal stress on a large stope sidewall will result inrelaxation of the ground towards the opening.

c) Stress differences

A large difference between maximum and minimum stresses has a significant effect on jointedrock masses resulting in shearing along the joints. The effect increases as the joint densityincreases (since more joints will be unfavourably orientated) and also as the joint conditionratings decrease.

The factors, which should be considered in the assessment of mining-induced stresses, havebeen listed by Laubscher (1990).

Blasting effects

Some adjustment would be required since the blasting operation forms new fractures andloosens the rock mass resulting in some movement on joints. These factors vary between 80 %and 100 % depending on the technique of opening, i.e. boring, conventional blasting, etc.

Strength of the rock mass

Laubscher (1990) emphasised that the strength of the rock mass cannot be higher than thecorrected average IRS of a zone and large specimens, i.e. the rock mass, will be equal to80 % of the value obtained from laboratory tests on small specimens, if there is no joint.

The following empirical formula is adopted to calculate the RMS.

RMS A B C= ( )80

80100

where:

A= total rating of rock mass,

B= IRS rating,

C= IRS.


14/17

GAP 416

APPENDIX A

A12

Design strength of the rock mass

The definition of the design rock mass strength (DRMS) is given as the strength of theunconfined rock mass in a specific mining environment. Laubscher (1990) explained that thesize of the excavation will affect the zone surrounding the excavation in terms of instabilityconditions. Adjustments, which relate to that mining environment, are applied to the RMS togive the DRMS. As the DRMS is in MPa, it can be related to the mining-induced stresses.Therefore, the adjustments are those for weathering, orientation, and blasting.

It is concluded that the value of DRMS, which can be related to the total stresses, is theunconfined compressive strength of the rock mass.

A.2.5 The Modified Basic RMR (MBR) system

The MBR system was developed by the US Bureau of Mines to examine how a groundclassification approach could be used fruitfully in planning support for drifts in caving mines. Itfollows closely the RMR system and incorporates some ideas of Laubscher. Bieniawski (1984)explained that key differences lie in the arrangement of the initial rating terms and theadjustment rating. It is still possible to use very preliminary geotechnical information from drillholes. The MBR is also a multi-stage adjustment and its rating is the result of the initial stageand is the simple sum of the raw ratings.

Bieniawski (1984) pointed that the MBR is an indicator of rock mass competence without regardto the type of opening constructed in it. There are three stages to the determination of the MBRvalue:

The first step in using the MBR system is the collection of representative data on intact rockstrength, discontinuity density and conditions, and groundwater conditions as defined in the

RMR. The only difference in the application of RMR is the importance ratio of each parameterto be considered in the evaluation. Tables and figures for ratings and adjustments arepresented by Kendorski et al.,(1983).

The second stage is to consider the development adjustments. The objective in thedevelopment adjustment is to initially stabilise the opening during development so thatpermanent support may use its full capacity to resist the abutment loading increment. The thirdstage deals with the additional deformations due to abutment loadings. After the extraction ratiois computed, the blasting damage, termed as moderate, slight, severe or none, is assessed,

and the induced stress adjustment is determined. The horizontal (h) and vertical (v)components of the stress field must be computed or estimated and the adjustment can then bedone for the appropriate effective extraction ratio, depth and stress field. The next adjustment is

for fracture orientations. The third stages are development and production adjustments. Themultiplication of these three adjustments, having a value between 0,45 and 1,0, with the MBRwill give the AMBR value.

The final stage is to consider the role of structural geology and mining geometry as defined asproduction adjustments. The basic parameters to be considered in the developmentadjustments are as follows:


15/17

GAP 416

APPENDIX A

A13

Development adjustments

- blasting (AB, 0,8-1,0),

- induced stresses (AS, 0,8-1,2), and

- fracture orientation (AO, 0,7-1,0).

Adjusted MBR (AMBR)=MBR*AB*AS*AO

Production adjustments

- major structures (S, 0,7-1,1),

- distance to cave line (DC, 0,8-1,2), and

- block/panel size (PS, 1,0-1,3).

Final mining MBR;

(FMBR)=AMBR*DC*PS*S

Disadvantages

The MBR is an adapted version of prior work of Bieniawski and Laubscher with a modificationfor caving mine, which is radically different from driving a tunnel. In developing themodifications and adaptations for the MBR system, all data were collected for horizontal drifts inmines. Thus, the MBR system is not necessarily valid for non-horizontal workings (inclines,raises, and shafts) or for other mining methods.

A.2.6 The Rock Mass Index (RMi) system

The RMi system proposed by Palmstrom (1996) is based on defined inherent parameters of therock mass and is obtained by combining the compressive strength of intact rock and a jointingparameter. The jointing parameter represents the main jointing features, namely block volume(or density of joints), joint roughness, joint alteration, and joint size. Quantitatively, the RMi canbe expressed as:

RMi JPC= .

where:

c= the uniaxial compressive strength ofintact rock measured on 50 mm samples;

JP= the jointing parameter, which is areduction factor representing the block sizeand the condition of its faces as

represented by their friction properties andthe size of the joints.

The influence of JP has been found by using calibrations from test results. Because ofproblems in obtaining compression test results on rock masses at a scale similar to that oftypical rock works, it was possible to find appropriate data from only eight large-scale tests andone back-analysis. These have been used to arrive at the following mathematical expressionas:


16/17

GAP 416

APPENDIX A

A14

JP jC VbD=0 2. .where:

Vb is given in m3, and D jC= 0 37 0 2. .

The joint condition factor is expressed as:

( )jC jL jR jA= /

where:

jL = factors for joint length,

jR = joint wall roughness and,

jA = continuity and joint surface alteration

The factors jR and jA are similar to the joint roughness number (Jr) and the joint alterationnumber (Ja) respectively in the Q-system. The joint size and continuity factor (jL) has beenintroduced in the RMi system to represent the scale effect of the joints.

The RMi is numerical and therefore differs from earlier general classifications of rock masses,which are mainly descriptive or qualitative. Palmstrom (1996) discusses three applications ofthe RMi. These include;

a) determination of the constants in the Hoek-Brown failure criterion for rock masses,

b) assessment of stability and rock support in underground excavations, and

c) quantification of the classification applied in the New Austrian Tunnelling Method (NATM).

Some of the benefits and limitations of the RMi system are explained by Palmstrom (1996) asfollows:

1) The RMi will significantly improve the use of geological input data, mainly through its

systematic use of well-defined parameters in which the three-dimensional character of rockmasses is represented by the block volume.

2) The RMi can easily be used for rough estimates when only limited information on the groundconditions is available, for example, in the early stages of a project, where rough estimatesare sufficient.

3) The RMi is well suited for comparisons and exchange of knowledge between differentlocations. In this way it can improve communication between those involved in rockengineering and design.

4) The RMi offers a platform suitable for engineering judgement. RMi is a general parameter

that characterises the inherent strength of rock masses, and may be applied in engineeringas the quality for this construction material. Because the RMi is composed of real blockvolumes and common joint parameters for rock masses, it is easy to relate it to fieldconditions. This is important in applying engineering judgement.

5) The RMi system covers a wide spectrum of rock mass variation, and therefore haspossibilities for wider applications than other rock mass classification and characterisationsystems used today.


17/17

GAP 416A15

Any attempt to mathematically express the variable structures and properties of jointed rockmasses in a general failure criterion may result in complex expressions. By restricting the RMito uniaxial compressive strength alone, it has been possible to arrive at the relatively simpleexpressions in the above equations. Because simplicity has been preferred in the structure aswell as in the selection of parameters in RMi, such an index may result in inaccuracy andlimitations. The main limitations relate to:

a) The range and types of rock masses covered by the RMi. Both the intact rock material andthe joints exhibit great directional variations in composition and structure, resulting in a large

range in compositions and properties of rock masses. It is not possible to characterise all ofthese combinations in one single number. Nevertheless, the RMi probably characterises awider range of materials than most classification systems.

b) The accuracy in the expression of RMi. The value of the jointing parameter (JP) iscalibrated from a few large-scale compression tests. Both the evaluation of the variousfactors (jR, jA and Vb) used in obtaining JP and the size of the samples tested, which insome of the cases had a small number of blocks, may be sources of error in the expressionfor JP. Therefore, the value of RMi found may be approximate. In some cases, however,errors in the various parameters may partly neutralise each other.

c) The effect of combining parameters that vary in range. The parameters used to calculate

the RMi generally will express a certain range of values. As with any classification system,combining such variables may cause errors. In some cases, the result is that the RMi maybe inaccurate in its characterisation of the strength of the complex and varied assemblage ofmaterials and defects that constitute a particular rock mass. For these reasons, the RMimay best be considered as a relative index in its characterisation of the rock mass strength.

apêndice a-análiseclassificaçõesgeomecânicas

Documents