Rock Mass Rock Mass ClassificationClassificationGeomechanics (EBS 417/3)Geomechanics (EBS 417/3)

Rock Mass Rock Mass ClassificationsClassifications

It is not always convenient to make a It is not always convenient to make a definitive test in support of engineering definitive test in support of engineering decision involving rockdecision involving rock

Most of the problem in rock mechanics in Most of the problem in rock mechanics in mining is being solved from previous mining is being solved from previous experiences in a mine or experiences in experiences in a mine or experiences in other mineother mine

An attempt was made to relate these An attempt was made to relate these experiences to be used in any mine, so experiences to be used in any mine, so various classification schemes were derived various classification schemes were derived for the rock massesfor the rock masses

What are rock mass What are rock mass cassification schemes? cassification schemes?

• Very little detailed information obtained Very little detailed information obtained during preliminary designduring preliminary design

• As a check-list to ensure all relevant As a check-list to ensure all relevant information has been consideredinformation has been considered

• Consider them as aids to design. THEY Consider them as aids to design. THEY ARE NOT DESIGN METHODSARE NOT DESIGN METHODS

• Geomechanics classificationGeomechanics classification•Rock Mass Rating (RMR) systemRock Mass Rating (RMR) system•Q-systemQ-system

Their objectives are:Their objectives are:

1)1) Idenify the most significant Idenify the most significant parameters influencing the parameters influencing the behaviour of a rock massbehaviour of a rock mass

2)2) Partition a rock mass into cones of Partition a rock mass into cones of similar quality, and hence similar quality, and hence behaviourbehaviour

3)3) Provide a basis for understanding Provide a basis for understanding the characteristics of each rock the characteristics of each rock mass classmass class

3)3) Relate experience at one site to Relate experience at one site to othersothers

4)4) Derive quantitative data and Derive quantitative data and guideline for engineering designguideline for engineering design

5)5) Provide a common basis for Provide a common basis for communication between engineers communication between engineers and geologistsand geologists

Benifits of using them Benifits of using them include:include:

1)1)Improved quality of site Improved quality of site investigation.investigation.

2)2)Provide quantitative data for Provide quantitative data for design.design.

3)3)Enable better engineering Enable better engineering judgement and improved judgement and improved communication.communication.

Bieniawski’s geomechanics Bieniawski’s geomechanics classification systemclassification system

Developed in South Africa by Developed in South Africa by Bieniawski (1976)Bieniawski (1976)

The system provides a general The system provides a general rock mass rating (RMR) rock mass rating (RMR) increasing with quality from 0-increasing with quality from 0-100100

Rock Mass Rating (RMR) Rock Mass Rating (RMR) systemsystem

Bieniawski’s scheme is Bieniawski’s scheme is based on five parameters:based on five parameters:

1.1.Strength of the intack rock Strength of the intack rock materialmaterial

Compressive strength can be Compressive strength can be obtained from the rock core obtained from the rock core samples. For weak rocks, the point samples. For weak rocks, the point load index can be usedload index can be used

2.2.Drill core quality/RQDDrill core quality/RQDIt is rated according to Rock Quality It is rated according to Rock Quality Desigation (RQD)Desigation (RQD)

It is determined by the percentage It is determined by the percentage recovery of core in lengthsrecovery of core in lengths

Xi = length of every rock core that is Xi = length of every rock core that is equal to or exceed 100 mm length (4 equal to or exceed 100 mm length (4

in.)in.)

L = total length of the drillL = total length of the drill

100L

XRQD i

3.3. Spacing of jointsSpacing of joints

Evaluated from drill core. Spacing of Evaluated from drill core. Spacing of the joints that include all types of the joints that include all types of discontinuitiesdiscontinuities

4.4. Conditions of jointsConditions of joints

This parameter are the distance of This parameter are the distance of the opening of the discontinuity, the opening of the discontinuity, continuity or persistence, surface continuity or persistence, surface roughness, condition of the wall roughness, condition of the wall rock (soft or hard) and the rock (soft or hard) and the conditions of any infillingconditions of any infilling

Geometrical properties of Geometrical properties of discontinuitiesdiscontinuities

5.5.Groundwater conditions Groundwater conditions

Groundwater can greatly influence Groundwater can greatly influence rock mass behaviour. It is recorded rock mass behaviour. It is recorded as the flow rate that enters the as the flow rate that enters the excavation orexcavation or

The ratio of the water pressure that The ratio of the water pressure that emits from the joint to the major emits from the joint to the major principal stress of the rock orprincipal stress of the rock or

The qualitative of general The qualitative of general observation on the condition of the observation on the condition of the groundwatergroundwater

All the parameters are entered - get All the parameters are entered - get the rating. -Section (a) in Table 1the rating. -Section (a) in Table 1

The provision for the ratings shows The provision for the ratings shows that every parameter does not give that every parameter does not give the same effect on the behaviour of the same effect on the behaviour of the rock massthe rock mass

The total of RMRThe total of RMR is obtained by is obtained by adding all the ratings from all the adding all the ratings from all the parametersparameters

Since the orientation of the joints Since the orientation of the joints relative to the work can have an relative to the work can have an influence on the beheviour of the influence on the beheviour of the rock, rock, Bieniawski recommended Bieniawski recommended adjusting the SUM of RMR according adjusting the SUM of RMR according to the effect on the orientation of the to the effect on the orientation of the discontinuity. discontinuity. -Section (b) of Table 1 -Section (b) of Table 1

The range of orientation is specified in The range of orientation is specified in Table 2.Table 2.

The rock classes according to the The rock classes according to the rating and the description of the rock rating and the description of the rock mass is given in – Section (c) Table 1mass is given in – Section (c) Table 1

The meaning of the total rating is The meaning of the total rating is described in accordance to the tunnel described in accordance to the tunnel or the underground excavation stand-or the underground excavation stand-up time as given in – Section (d) Table up time as given in – Section (d) Table 1. 1.

Example problemExample problem

A tunnel is to be driven through slightly A tunnel is to be driven through slightly weathered granite with a dominant joint weathered granite with a dominant joint set dipping at 60set dipping at 60° against the direction of ° against the direction of the drive. Index testing and logging of the drive. Index testing and logging of diamond drilled core give typical Point-load diamond drilled core give typical Point-load strength index values of 8 Mpa and strength index values of 8 Mpa and average RQD values of 70%. average RQD values of 70%.

The slightly rough and slightly weathered The slightly rough and slightly weathered joints with a separation of <1 mm, joints with a separation of <1 mm, discontinuity length is 2m, are spaced at discontinuity length is 2m, are spaced at 300 mm. Tunnelling conditions are 300 mm. Tunnelling conditions are anticipated to be wet.anticipated to be wet.

Parameter Value or descriptionRating

A.1. Point Load index

8 MPa 12

A.2. RQD 70% 13

A.3. Spacing of discontinuities

300 mm 10

E.4. Conditions of discontinuities

Discontinuity length (1-3m) = 4

Separation (0.1 – 1.0mm) = 4

Slightly rough = 3No infilling = 6

Slightly weathered = 5

22

A.5. Groundwater

Wet 7

B. Adjustment for joint orientation

Driven against the dip, dipping 60º = fair conditions

-5

Total RMR 59

Guideline properties of rock Guideline properties of rock mass classesmass classes

RMR RMR = 59 = 59 (Fair rock (Fair rock quality)quality)

Initial constructionInitial construction = advisable to utilize = advisable to utilize the support system suggested, as the good the support system suggested, as the good

construction & support progressing ; construction & support progressing ; reduce the support requirementreduce the support requirement

RMR system & properties of RMR system & properties of rock massrock mass

Geomechanics system of rock Geomechanics system of rock mass ratingmass rating

The stand up time of an unsupported openings in granite with a span of 3 m would last The stand up time of an unsupported openings in granite with a span of 3 m would last for about 1 month.for about 1 month.

The relationship between the stand-up time of unsupported The relationship between the stand-up time of unsupported underground excavation and RMR (Bieniawski).underground excavation and RMR (Bieniawski).

No support required

ImmediateCollapse

Support required

Excersice 1Excersice 1

A 3 metre wide tunnel is to be excavated towards the east. A 3 metre wide tunnel is to be excavated towards the east. Information about the rock mass:Information about the rock mass:

Uniaxial compressive strength Uniaxial compressive strength -180 Mpa-180 MpaRock Quality DesignationRock Quality Designation - 95%- 95%Average joints spacingAverage joints spacing - 2.2 m- 2.2 mCondition of jointsCondition of joints - hard joint wall - hard joint wall

rock, rock, separation < 1 separation < 1 mmmm

Condition of ground waterCondition of ground water - slightly dry- slightly dry

There are 3 sets of joint within the tunnel as the following.There are 3 sets of joint within the tunnel as the following.Joint setJoint set dipdip dip directiondip direction 11 303000 70 7000

22 808000 170 17000

33 707000 80 8000

Q : Obtain the rating using the Bieniawski Geomechanics Q : Obtain the rating using the Bieniawski Geomechanics Classification.Classification.Describe the condition of this rock mass. Describe the condition of this rock mass.

Answer: Answer:

Set 1Set 1 Set 2Set 2 Set 3Set 3

Strength 180 MpaStrength 180 Mpa 12 12 12 12 12 12

RQDRQD 20 20 20 20 20 20

Joint spacing 0.9 mJoint spacing 0.9 m 20 20 20 20 20 20

Condition of jointsCondition of joints 25 25 25 25 25 25

Conditions of g/wConditions of g/w 10 10 10 10 10 10

Total Total 87 87 87 87 87 87Rating adjustment for Favour. V. unfavou.Rating adjustment for Favour. V. unfavou. V. Favour.V. Favour.joint orientationjoint orientation

-2-2 -12 -12 0 0

Total overall ratingTotal overall rating 80 80 70 70 82 82

From the three rating, take the From the three rating, take the lowest value which is in this case lowest value which is in this case is 70.is 70.

Rating = 70 = good rock = Class Rating = 70 = good rock = Class IIII

With rock type of class II, a With rock type of class II, a tunnel of 3 m span can stand-up tunnel of 3 m span can stand-up for 6 months.for 6 months.

Further, Bieniawski(1989) published a Further, Bieniawski(1989) published a set of guidelines for the selection of set of guidelines for the selection of support in tunnels in rock accordings support in tunnels in rock accordings to RMR value. to RMR value.

Note: this guidelines is for a 10 m span Note: this guidelines is for a 10 m span horseshoe shaped tunnel, constructed horseshoe shaped tunnel, constructed using drill and blast methods, in a rock using drill and blast methods, in a rock mass subjected to a vertical stress< mass subjected to a vertical stress< 25 Mpa = a depth below surface of 25 Mpa = a depth below surface of <900 m.<900 m.

Guidelines for excavation & support of 10 m span rock tunnels Guidelines for excavation & support of 10 m span rock tunnels in accordance with RMR system (Bieniawski, 1989)in accordance with RMR system (Bieniawski, 1989)

RMR = 59RMR = 59

Bieniawski suggests that a tunnel Bieniawski suggests that a tunnel

1.1.Could be excavated by top heading Could be excavated by top heading and bench with 1.5 to 3 m advance and bench with 1.5 to 3 m advance in the top heading.in the top heading.

2.2.Support should be installed after Support should be installed after each blast and the complete support each blast and the complete support should be placed at a maximum should be placed at a maximum distance of 10 m from the face.distance of 10 m from the face.

3.3. Systematic rock bolting, using 4 m Systematic rock bolting, using 4 m long 20 mm diameter fully grouted long 20 mm diameter fully grouted bolts spaced at 1.5 to 2 m in the bolts spaced at 1.5 to 2 m in the crown and walls, is recommended. crown and walls, is recommended.

4.4. Wire mesh, with 50-100 mm of Wire mesh, with 50-100 mm of shotcrete for the crown and 30 mm shotcrete for the crown and 30 mm of shotcrete for the walls, is of shotcrete for the walls, is recommended. recommended.

Developed by Barton, Lien and Lunde Developed by Barton, Lien and Lunde (1974) of the Norwegian Geotechnical (1974) of the Norwegian Geotechnical InstituteInstitute

Q for the rock mass characteristics and Q for the rock mass characteristics and tunnel support requirementstunnel support requirements

Q varies on logarithmic scale from Q varies on logarithmic scale from 0.001- 10000.001- 1000

TUNNELLING QUALITY TUNNELLING QUALITY INDEX (Q-system)INDEX (Q-system)

Defined by:Defined by:

Where Where RQDRQD = Rock Quality Designation= Rock Quality Designation

JJnn = joint set number= joint set number

JJrr = joint roughness number= joint roughness number

JJaa = joint alteration number= joint alteration number

JJww = joint water reduction factor= joint water reduction factor

SRFSRF = Stress Reduction Factor= Stress Reduction Factor

SRF

J

J

J

J

RQDQ w

a

r

n

Defined by:Defined by:

RQDRQD = Rock Quality Designation = Rock Quality Designation

JJn n = joint set number= joint set number (no. of discontinuity sets) (no. of discontinuity sets)

JJr r = joint roughness number= joint roughness number (roughness of the (roughness of the discontinuity surfaces)discontinuity surfaces)

JJaa = joint alteration number= joint alteration number (degree of (degree of alteration or weathering of the discontinuity alteration or weathering of the discontinuity surfaces)surfaces)

JJww = joint water reduction factor = joint water reduction factor (pressures (pressures and inflow rates of water within the discontinuities)and inflow rates of water within the discontinuities)

SRF SRF = Stress Reduction Factor= Stress Reduction Factor (presence of (presence of shear zones, stress concentrations, squeezing & shear zones, stress concentrations, squeezing & swelling rocks)swelling rocks)

RQD/JRQD/Jnn – – representing the structure of representing the structure of rock mass; Q increases with high RQD rock mass; Q increases with high RQD & decreases with high & decreases with high JJn n ((higher higher value = better mechanical quality)value = better mechanical quality)

JJr r /J/Jaa – represents the roughness and – represents the roughness and frictional characteristics of the joint frictional characteristics of the joint walls or filling materials; in favour of walls or filling materials; in favour of rough, unaltered joints in direct rough, unaltered joints in direct contactcontact

Thin clay mineral coatings & fillings – Thin clay mineral coatings & fillings – significant reduced in strengthsignificant reduced in strength

JJww/SRF/SRF – 2 stress parameters; – 2 stress parameters;

JJww - a measure of water pressure - a measure of water pressure

SRFSRF

1- loosening load in the case of an 1- loosening load in the case of an excavation through shear zones and excavation through shear zones and clay bearing rockclay bearing rock

2- rock stress in competent rock2- rock stress in competent rock

3- squeezing loads in plastic 3- squeezing loads in plastic incompetent rocksincompetent rocks

Example:Example:

A 15 m span crusher chamber for an underground A 15 m span crusher chamber for an underground mine is to be excavated in a norite at a depth of mine is to be excavated in a norite at a depth of 2,100 m below surface. The rock mass contains 2,100 m below surface. The rock mass contains two sets of joints controlling stability. These joints two sets of joints controlling stability. These joints are undulating, rough and unweathered with very are undulating, rough and unweathered with very minor surface staining. minor surface staining.

RQD values range from 85% to 95% and laboratory RQD values range from 85% to 95% and laboratory tests on core samples of intact rock give an tests on core samples of intact rock give an average uniaxial compressive strength of 170 MPa. average uniaxial compressive strength of 170 MPa. The principal stress directions are approximately The principal stress directions are approximately horizontal with magnitude 1.5 times that of the horizontal with magnitude 1.5 times that of the vertical principal stress. The rock mass is locally vertical principal stress. The rock mass is locally damp but there is no evidence of flowing water.damp but there is no evidence of flowing water.

Answer:Answer:

RQDRQD = Average = 90= Average = 90JJnn = two joint sets = 4= two joint sets = 4

JJrr = rough and undulating = 3= rough and undulating = 3

JJaa = unaltered joint walls with = unaltered joint walls with surface staining only surface staining only

= 1.0= 1.0JJww = minor inflow = 1.0= minor inflow = 1.0

SRF

J

J

J

J

RQDQ w

a

r

n

Stress,Stress,

Generally, the vertical stress for the lying rock Generally, the vertical stress for the lying rock

= 0.027 MPa/m= 0.027 MPa/m

So for a depth below surface of 2100 m, the So for a depth below surface of 2100 m, the overburden stress = 57 Mpa = overburden stress = 57 Mpa = σvv

The major principal stress = 57 MPa x 1.5= 85 The major principal stress = 57 MPa x 1.5= 85 MPa = MPa = σhh = = σ11

Given the compressive strength of norite, UCSGiven the compressive strength of norite, UCS

= 170 MPa = = 170 MPa = σcc

SRF = 170/85= SRF = 170/85= σcc// σ11 = 2 = 2

Expected to produce heavy rock burst Expected to produce heavy rock burst conditions , conditions ,

SRF = 15 (Between 10-20)SRF = 15 (Between 10-20)

QQ = = 9090 x x 33 x x 11 = 4.5 = 4.5 4 1 154 1 15

In relating the Q value to the In relating the Q value to the stability and stability and support requirements of underground support requirements of underground excavationsexcavations, Barton defined an additional , Barton defined an additional parameter called Equivalent Dimension, Dparameter called Equivalent Dimension, Dee..

DDee = = Excavation span, diameter of hights Excavation span, diameter of hights (m(m )) Excavation Support Ratio, Excavation Support Ratio, ESRESR

Excavation Support Ratio, ESR – Barton (1974)

The crusher station – permanent The crusher station – permanent mine openings, mine openings, ESR = 1.6ESR = 1.6

DDee = 15/ 1.6 = = 15/ 1.6 = 9.49.4

From the graph, the crusher falls into From the graph, the crusher falls into category 4category 4 which requires a pattern which requires a pattern of of rockbolts (spaced at 2.3 m) rockbolts (spaced at 2.3 m) and 40-50 mm of unreinforced and 40-50 mm of unreinforced shotcrete shotcrete