ge 6477 discontinuous rock rock mass classification …web.mst.edu/~norbert/ge6477/class...

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GE 6477 DISCONTINUOUS ROCK5. Rock Mass Classification

and Empirical Design

Dr. Norbert H. Maerz

Missouri University of Science and Technology

(573) 341-6714

[email protected]

Instructional Objectives

1. Explain the rational for using rock mass classifications.

2. List the advantages and disadvantages of empirical design based on rock mass classification.

3. Select and justify which classification system you would use for a) tunneling, b) mining, c) block caving mining, mechanical rock excavation.

4. Justify the approach to rock support that is proposed by the new Austrian tunneling method.

5. Barton’s Q system is semi-analytical. Explain.

What is rock mass classification?

• CLASSIFICATION: The formal arrangement of attributes in a hierarchy

Why use mass classification?

• Can’t measure what we want to, instead measure while we can and classify

• Empirical Design

Empirical design

• Uses engineering judgement

• Uses experience

3 Pillars of empirical design and rock mass classification

• 1) Description of ground quality by a quantitative classification system, based on parameters that are easily and universally measured.

• 2) Description of ground performance by a formal set of parameters (unsupported stand time, support requirements, bearing capacity etc.).

• 3) Correlation of the above 2 based on a broad spectrum of case histories, based on local or global experience.

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Empirical design and rock mass classification method of design

• ADVANTAGES

• Incorporates everyone’s experience

• Uses parameters that are easy to measure

• Provides simple, specified, tested solutions

• DISADVANTAGES

• No predictive ability

• Difficult to extrapolate, safe only in the context for which they were formulated

Approach

• Description of the classification system

• Dependencies/ Emphasis

• How used in design

• How relates to discontinuous rock

Classification systems

• 1. Sonic velocity (1 parameter system)

• 2. Deere’s RQD system

• 3. Franklin’s size-strength classification

• 4. Franklin’s Shale rating system

• 5. Bieniawski’s RMR system

• 6. Laubshers geomechanics rock mass classification

• 7. Barton’s Q system

1. Sonic velocity classification

2. Deere’s RQD system

• Single parameter system

• Incorporates (somewhat both size and strength)

boreholeoflength

mmpiecesincoreoflengthRQD

__

100____100(%)

Picture(s) from Gonzales de Vallejo and Ferrer

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Deere’s system Deere: design chart

3. Franklin’s size-strength classification

Picture(s) From the collection of Dr. J. A. Franklin Picture(s) From the collection of Dr. J. A. Franklin


New Austrian Tunneling Method

Picture(s) From the collection of Dr. J. A. Franklin

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• New Austrian Tunneling method, design principles


Principle of stress relaxation


4. Franklin’s Shale rating system

• Strength / Durability Index

• Not consider discontinuities

• Durability in the form a slake durability test

• Strength from point load test for hard shales

• Strength from plasticity index for soft shales


Slake durability apparatus


5. Bieniawski’s RMR system

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What must a rock mass classification do:

• 1) Divide the rock mass into groups of similar behavior

• 2) Provide a good basis for understanding the characteristics of the rock mass

• 3) Facilitate the planning and the design of structures in the rock by yielding quantitative data required for the solution of real engineering problems

• 4) Provide a common basis for effective communication among all persons concerned with a geomechanics problem

The classification should be:

• 1) Simple and meaningful in terms

• 2) Based on measurable parameters which can be determined quickly and easily in the field.

What Bieniawaski though important

• 1) RQD

• 2) State of weathering

• 3) UCS (unconfined compressive strength)

• 4) Discontinuity spacing

• 5) Strike and dip

• 6) Joint separation

• 7) Joint persistence

• 8) Ground water

Final Parameters

• 1) UCS

• 2) RQD

• 3) Discontinuity spacing rating

• 4) Joint condition rating

• 5) Groundwater rating

RMR: Rating

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RMR: relationship to discontinuous rock?

• RQD, spacing, joint condition are all directly related to joints

• Ground water flow, which in most hard rock is a function of joints

• Strength is the only intact parameter

6. Laubshers geomechanics rock mass classification

• 1) RQD

• 2) IRS (Intact rock strength)

• 3) Joint spacing rating

• 4) Condition of joint rating

• 5) Groundwater rating

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Laubscher: Adjustments

• Weathering - up to 75%

• Field stresses - 120% for increase in compression, 90% for decrease

• Stresses in problem area, crown pillars, adjacent to caving, abutments, 60-100%


• Orientation Adjustment

• Blasting Adjustment


Laubscher: tunnel design chartLaubscher: tunnel design chart

explanation• a) no support

• b) grouted bolts on 1 m pattern

• c) grouted bolts on 0.75 m pattern

• d) grouted bolts on 1 m pattern, 50 mm shotcrete

• e) grouted bolts on 1 m pattern, 300 mm concrete

• f) grouted bolts on 0.75 m pattern, 100 mm shotcrete

• g) grouted bolts on 1 m pattern, 100 mm mesh reinforced shotcrete

• h) grouted bolts on 1 m pattern, 450 mm concrete

• i) grouted bolts on 0.75 m pattern, 100 mm shotcrete, yielding steel arches

• j) 450 mm concrete

• k) shotcrete, yielding arches

• l) AVOID

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Laubscher: cavability chart

• Caveability • Angles of cave

Laubscher: pit wall angle design chart

Laubsher: relationship to discontinuous rock?

• RQD, spacing, joint condition are all directly related to joints

• Ground water flow, which in most hard rock is a function of joints

• strength is the only intact parameter

7. Barton’s Q system

Barton’s Q-System

• 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 SRF

J

J

J

J

RQDQ w

a

r

n

**

RQD

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Joint set number Block size component

• RQD - rock quality designation, 10 to 100

• Jn - joint set number, 0.5 to 20

• RQD/Jn, 0.5 to 200

nJ

RQD

Joint roughness number

Shear strength component

• Jr - joint roughness number, 0.5 to 4

• Ja - joint alteration number, 0.75 to 20

• Jr/Ja, 0.05 to 5

a

r

J

J

Mathew’s Modified Barton’s Q-System

• RQD - rock quality designation

• Jn - joint set number

• Jr - joint roughness number

• Ja - joint alteration number

a

r

n J

J

J

RQDQ *'

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Joint water reduction factor

Active stress component

• Jw - joint water reduction factor, 0.05 to 1

• SRF - stress reduction factor, 0.5 to 20

• Jw/SRF, 0.0025 to 2SRF

Jw

Barton’s Q-System

• 0.5-200

• * 0.05 - 5

• * 0.0025-2

SRF

J

J

J

J

RQDQ w

a

r

n

**

Q: Design table Excavation support ratio

• Equivalent dimension of opening

• Based on Excavation support ratio (ESR)

• Sort of inverse “FACTOR OF SAFETY”

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Q: Design table Q: Case histories

Q: Design table


Design criteria Q: relationship to discontinuous rock?

• RQD, Jv, Jr, Ja are all directly related to joints

• Jw is a measure of water flow, which in most hard rock is a function of joints

• SRF is under some conditions related to zones of weakness like joints

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Q: relationship to true block size?r-squared=0.88

0

1

2

3

4

5

6

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

RQD/Jn

Blo

ck

Siz

e (m

)

Hoek Brown-Strength Criterion

GSI

Summary

• Rock mass classification characterizes rock masses based on easily measurable features

• Empirical data, or case studies can be used to predict performance base on the classification

• Rock mass classification can be though of as a model

• Model is far from perfect, however can result in useful designs, and it is often the best design approach.

ge 6477 discontinuous rock rock mass classification …web.mst.edu/~norbert/ge6477/class...

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