Chapter 1

INTRODUCTION AND CLASSIFICATION OF ROCKS

Dr. R.K.DuttaAPCEDNIT Hamirpur

Textbook Rock Mechanics in Engineering Practice: K.G. St

agg Under ground excavation in rock: Hoek and Bro

wn Rock Mechanics in Engineering Practice: Cambr

idge University Introduction to Rock Mechanics: R.E. Goodman

Prerequisites CE 242; CE-471

Class Website http://www.nitham.ac.in/~rkd/?Teaching_Assignment

Course ContentIntroduction:

Rock Mechanics and its relationship with soil mechanics and engineering geology, application of rock mechanics to civil engineering problems.

Classification of RocksLithological classification, engineering classific

ation of rocks, classification based on wave velocity ratio, R.Q.D. Classification of rock masses i.e. RMR and Q systems.

Rock PropertiesLaboratory test, compression, tensile, void index

, permeability and shear, effects of size of specimen, rate of testing, confining pressure etc. Stress strain curves of typical rocks, strength of intact and fissured rocks, effects of anisotropy, saturation and temperature effects, shear strength of jointed rock mass.

Field TestsUniaxial tests in tunnels and open excavations, s

hear test, pressures tunnel tests etc.

Stability of Rock SlopesMode of failure of rock slopes, plane wedge analysis,

3D‐wedge analysis circular mode of failure, back analysis of slopes, stability charts, types and design of rock bolts.

Determination of Insitu StressesStresses in rock, methods of determining in situ stresses i.

e hydraulic fracturing, flat jack test and over coring.Design of TunnelRock pressure theories, ground reaction curve, rock suppo

rt interaction analysis empirical and semi empirical methods of analysis, simple method of tunnel, design, types and design of tunnel lining.

Foundation on Rocks Stress distribution in foundation, methods of determinatio

n of bearing capacity of rocks, improvement of rock properties, pressure grouting for tunnels and dams, dental concreting, shear zone treatment.

Grading Policy Two 90-min Exams ……. 30% Homework ……………... 20% Final Exam ………. 50%

TOTAL 100%

Homeworks Counts 20% towards the final grade Approx. 10 Homework Assignments Due at the beginning of class period No late homeworks accepted If you are not attending the class,

have your homework delivered to my room or mail box.

Exams 2 Exams each 90-mins. Exam dates are shown on Academic

Calendar. Each exam counts 15% towards your final

grade. Final exam counts 50% towards your final

grade. Formula given in the UG manual will be

used for calculating grades.

What is Soil Mechanics?Soil mechanics is a discipline that applies principles of engineering mechanics,

e.g. kinematics, dynamics, fluid mechanics, and mechanics of material, to predict the mechanical behavior of soils. It is the basis for solving many engineering problems in civil engineering.

Some of the basic theories of soil mechanics are the

1. Basic description and classification of soil2. Effective stress3. Shear strength4. Consolidation5. Lateral earth pressure6. Bearing capacity7. Slope stability8. Permeability

Foundations, embankments, retaining walls, earth works and underground openings are all designed in part with theories from soil mechanics.

What is Engineering Geology?Engineering Geology is the application of the geologic

sciences to engineering practice for the purpose of assuring that the geologic factors affecting the location, design, construction, operation and maintenance of engineering works are recognized and adequately provided for.

Engineering geologists investigate and provide geologic and geotechnical recommendations, analysis, and design associated with human development. The realm of the engineering geologist is essentially in the area of earth-structure interactions, or investigation of how the earth or earth processes impact human made structures and human activities.

Engineering geologic studies may be performed during the planning, environmental impact analysis, civil or structural engineering design, value engineering and construction phases of public and private works projects, and during post-construction and forensic phases of projects.

Works completed by engineering geologists include

1. Geologic hazards2. Geotechnical3. Material properties4. Landslide and slope stability5. Erosion6. Flooding7. Dewatering8. Seismic investigations

The principal objective of the engineering geologist is the protection of life and property against damage caused by geologic conditions.

Engineering geologic practice is also closely related to the practice of geological engineering, geotechnical engineering, soil engineering, environmental geology and economic geology. If there is a difference in the content of the disciplines described, it mainly lies in the training or experience of the practitioner.

What is Rock Mechanics?Rock mechanics is the theoretical and

applied science of the mechanical behaviour of rock and rock masses; it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment.

Rock mechanics itself forms part of the broader subject of geomechanics which is concerned with the mechanical responses of all geological materials, including soils. Rock mechanics, as applied in

1. Mining2. Petroleum3. Civil engineering practice

is concerned with the application of the principles of engineering mechanics to the design of the rock structures generated by mining, drilling, reservoir production, or civil construction activities such as

4. 1.Tunnels5. 2. Mining shafts6. 3. Underground excavations7. 4. Open pit mines8. 5. Oil and Gas Wells9. 6. Road cuts10. 7. Waste repositories, and other structures built in or of rock.

It also includes the design of reinforcement systems such as rock bolting patterns.

Need of Rock MechanicsEvery developmental activity particularly of water

resources call for construction of a large number of capital intensive massive structures like

1. Dams (Impose additional stresses on these materials)2. Power houses and under ground openings (Under

ground openings cause stress changes)3. Other construction activities (modify the insitu

conditions significantly)Safe and economic design and construction of such

structures cannot be conceived without a close knowledge of the behaviour of the geological materials like rock and its products which support and closely interact with such structures for their safety.

Presence of 1. Active faults2. Joints3. Cracks

make the situation more complex particularly for hydraulic structures.

Under such circumstances it is very much essential to have full understanding of the

4. Natural forces5. Characterisation of the rockmass6. Behaviour of rockmass

in the natural environment under the influence of stresses and deformation.

In this context rock mechanics subject deals more rationally with the problems on the behaviour of rock under the force field of its own environment using theoretical and experimental approaches.

Application of Rock MechanicsLarge Dams: The most challenging surface

structures with respect to rock mechanics is large dam that impose high stresses on rock foundations. In such a case the rock supporting the dam should have no fault zone and should be able to take stresses due to construction of the dam. At the same time, the rock strata should be such that upstream water may not seep through the foundation bed. For these details and proper design, a knowledge of rock mechanics is essential.

Blasting: For rock clean up work, some times blasting has to be engineered to preserve the integrity of the remaining rock and to limit the vibrations of neighbouring structures to acceptable levels. An aspect of engineering for tall buildings that involve rock mechanics is to control of blasting so that the vibrations do not damage neighbouring structures or irritate local residents.

Design of cut slopes: Cut slopes for highways, railways, canals may involve testing and analysis of the system of discontinuities. Considerable cost savings are possible if the orientation of the right of way can be adjusted based on the rock mechanics studies. The decision to place portions of such routes underground is partly determined by judgements about the rock conditions and relative costs of open cuts and tunnels.

Under ground excavations: Underground excavations call upon the discipline of rock mechanics in many ways. The design of cutters and drills can be tailored to the rock conditions which are determined by suitable laboratory tests. The rock condition and state of stress is fundamentally important in the design of the tunnels.

Power Houses: More and more hydro electric power houses are now being constructed under ground. The stability of such large under ground chambers depends upon the state of stress and the pattern and properties of discontinuities. The lay out and orientation of these openings depends almost entirely upon rock mechanics and geological conditions.

Inherent Complexities in Rocks1. Rock fracture

─ under compressive stresses2. Size effects

─ response of rock to loading affected by the size of the loaded volume” (joints & fractures)

3. Tensile strength ─ is low (similar to concrete); HOWEVER a rock

mass can have even less tensile strength

4.Groundwater effects─ water in joints: if under pressure, reduces

normal stress (less resistance along joints)─ water in permeable rocks (e.g. sandstone)

→ soil like response

─ softening of clay seams & argillaceous rocks (e.g. shales)

5. Weathering ─ chemical/physical alteration, reduction of

engineering properties─ limestone caverns, sinkholes: ”Karst”─ basic rocks with olivine (e.g. basalt) and

pyroxene minerals are reduced to montmorillonite by hydrolysis

Discontinuities 1) Bedding planes2) Folds

tension joints at the crest of a fold (strike, dip & shear joints)

folding may cause shear failure along bedding planes (axial plane or fracture cleavage)

Folding

3) Faults shear displacement zones - sliding

Faults may contain Fault gouge (clay) – weak Fault breccia (re-cemented rock) – weak Rock flour – weak Angular fragments – may be strong

Defects

Defects

4) Shear zones bands of materials - local shear failure

5) Dykes igneous intrusions (near vertical) weathered dykes, e.g. dolerite weathers to

montmorillonite unweathered dykes attract high stresses

6) Joints breaks with no visible displacement

Joint Patterns

sedimentary rocks usually contain 2 sets of joints, orthogonal to each other and the bedding plane

JOINTS1) Open

Filled Healed (or closed)

2) SteppedUndulating Planar

2B) each of the above can be RoughSmooth

Slickensided

JOINT CLASSES

IIIII

Stepped RoughSmoothSlickensided

IVVVI

Undulating RoughSmoothSlickensided

VIIVIIIIX

Planar RoughSmoothSlickensided

Order of Description of Rocks

ROCK MATERIAL COMPOSITION

rock namegrain size

texture and fabriccolour

e.g. Basalt, fine, massive, vesicular, dark grey to black

ROCK MATERIAL CONDITION

strength

weathering

e.g. VL strength, XW

ROCK MASS PROPERTIES

structuredefects (much information required)

weathering of joints

Structure: sedimentary rocks – bedded, laminated

metamorphic – foliated, banded, cleaved

igneous rocks – massive, flow banded

DEFECTS – information needed tightness cementation or infill smoothness or irregularity of surfaces

class of joint water in joints joint orientation joint spacing

Rock Classification

Classification is the arrangement of things in classes according to the characteristics they have in common. The need for an appropriate classification of rocks has long been recognised as it serves as an effective communication between the engineer and the geologist or between the engineer and the contractor. By nature, rocks are heterogeneous due to the presence of discontinuities such as macro and micro fissures, bedding plane, joints and faults.

These discontinuities introduce the concept of rock mass. An understanding of the behaviour of rock masses is of paramount importance as many developmental works such as tunnels, dams and other under ground storage tanks and excavation in mines are being constructed in and on rockmasses. Several classification system to describe rockmasses have been proposed but still prediction of rockmass behaviour, support pressure and tunnel closure has remained one of the most difficult problems in rock mechanics despite the fact that a lot of technological advancements have been made in the recent years.

It is therefore necessary to evolve an easy and yet dependable classification system which is applicable to underground openings and tunnels particularly suited to highly complex geological conditions as prevailing in Himalayan Region.

Aims of Rock Classification1. To classify a particular rock mass into

groups of similar behaviour2. To provide a basis for understanding

the characteristics of each group3. To yield quantitative data for

engineering designs4. To provide a common basis for

communication

Requirements of Good Classification System

1. It should be simple, easily remembered and understandable

2. Each term should be clear and the terminology used should be widely accepted by engineers and geologists

3. As many as significant properties of the rock masses should be taken into considerations

4. It should be based on measurable parameters which can be obtained by relevant quick tests on samples and cheaply in the field

5. It should be based on rating system that can weigh the relative importance of the classification parameters

6. It should be functional by providing quantitative data for design of rock support

Rock Classification SystemsA list of major classification systems currently in use are

as follows1. Lithological classification2. Engineering classification3. Terzaghi rock load classification4. Lauffer-Pacher Classification5. Deere’s Rock Quality Designation6. Rock Structure Rating7. Geomechanics Classification System8. NGI Classification System9. Geological Strength Index

Lithological ClassificationLithology of rock is the study of its physical

character. It includes the study of1. Mineralogical composition2. Texture3. Colour4. Physical AppearanceThe above parameters help in the selection of

a particular rock for engineering purpose.

Generally engineers are concerned with the strength properties of rock material. Hence if an engineer is conversant with the lithological classification of rocks, he can select the rock for his purpose. To ascertain the engineering properties of rocks it is necessary to know the following rock properties which can be ascertained by visual examination to make a preliminary inference about the suitability of a particular rock for a particular purpose. In order to describe the rock fully for a particular engineering purpose, it is necessary to describe the following properties.

1. Texture2. Structure3. Composition4. Colour5. Grain size

TextureRock materials may be of any of the following

textural group.Crystalline: Crystalline rock materials are

composed of visible interlocking crystal grains. When scratched by the blade of a pen knife, particles do not come out of the rock mass. If particles come out due to scratching, the rock will not be taken in crystalline group.

Indurated: Indurated rock materials are those in which interlocking crystals and crystal grains are not visible by naked eye. Grains are fine but the rock is strong as particles do not come out of the rock mass when scratched by the edge of a knife.

Crystalline-Indurated: These rock materials fall between crystalline and indurated rock materials. Its individual crystal grains or crystal aggregates are finer than crystalline structure but coarser than indurated. Rocks of this type are hard because the grains do not come out when scratched by the edge of a knife.

Compact: In compact rock materials, the particles are held together purely by tightness for grain packing. Grains are finer. Particles or powder come out of the rock mass when scratched by the edge of a knife.

StructureIt refers to placing of various textures

within the rock material. The various types of structures are as follow.

Homogeneous: If the grains and crystals are having random orientation the structure will be called homogenous. By visual examinations only the homogenous structures in a rock mass can be ascertained.

Lineated: If the material particles are having a proffered orientation in a particular linear direction/directions the structure will be known as lineated.

Intact-foliated: When the minerals in the rock mass are having a proffered orientation of a planer nature.

Fracture-foliated: When the planer struture is having closed or incipient fracture such as bedding planes or cleavage planes.

Generally lineated structure pose problems because properties of the rock mass is not the same in all directions in such cases.

Presence of calcite is of prime importance when considering mechanical and physical characteristics of rock mass. The important sub-divisions are

Noncalcareous: Rock materials are those in which calcium carbonate is absent

Composition

Part-calcareous: The rock contains mainly non-calcareous materials. The calcareous material is present as a band between the grains.

Calcareous: Rock materials which are mainly composed of calcite.

If the rock is of basic nature, it will be of dark colour where as acidic rocks are of light colour. Light coloured rocks are generally feldspathic where as dark coloured rocks are generally contain ferromagnesium minerals. Calcareous rocks which contain impure materials are dark in colour where as pure calcareous rocks are light.

Colour

Sometimes classification of rocks is done on the basis of their grain sizes. In such cases origin or type of rock is not so important.

Coarse grained: When the particles are larger than 2 mm in diameter

Medium grained: When particles size lies between 2 mm and 0.1 mm.

Fine grained: Particles of less than 0.1 mm size and invisible to the naked eye.

Grain size

Engineering Classification

The basis for engineering classification of rocks is UCS and modulus of elasticity. Based on UCS the rock is classified as class A, B, C, D and E. This classification system is valid for intact rocks only.Class Description UCS (kg/cm2)A Very high strength >2250B High strength 1125-2250C Medium strength 562.5-1125D Low strength 281.25-562.5E Very low strength <281.25

The UCS value is based on the results of the specimen having L/d ratio of 2.

Engineering classification of intact rocks on the basis of modulus ratio

MR = Et50/sult

Et50 = Tangent modulus at 50 % ultimate compressive strength of rock

sult = UCS

Class Description Modulus ratioH High >500M Average 200-500L Low <200

Engineering classification of intact rocks on the basis of modulus ratio

On the basis of the above two tables, engineering classification is done like AM, BH, CM (CM means medium strength and average modulus ratio)

Main Features of Engineering Rock Mass Classification Schemes

• Developed for estimation of tunnel support • Used at project feasibility and preliminary design stages• Simple check lists or detailed schemes• Used to develop a picture of the rock mass and its

variability• Used to provide initial empirical estimates of tunnel

support requirements• Are practical engineering tools which force the user to

examine the properties of the rock mass• Do not replace detailed design methods• Project specific

Terzaghi’s Rock Mass Classification (1946)

Terzaghi (1946) rock load classification is the first known rational system of rock classification for design of tunnel supports. The system for assessing the rock loads under different types of rock conditions had been widely used till mid seventies for design of supporting system in tunnels and is relevant even today to a limited extent, for design of cavities using steel supports. Terzaghi’s classification which is quantitative indirect method of assessing the support requirement relates rock loads to rock conditions. The Terzaghi rock load classification has proved very successful for tunnelling with steel supports but is not appropriate for tunnels built according to the modern tunnelling philosophy where displacements are controlled and where the rock is activated to self support the field stresses.

During construction of a tunnel, some

relaxation of the rockmass will occur above and on the sides of the tunnel. The loosened rock with in the area acdb will tend to move in towards the tunnel. This movement will be resisted by friction forces along the lateral boundaries ac and bd and these friction forces transfer the major portion of the over burden weight W onto the material on either side of the tunnel. The roof and sides of the tunnel are required only to support the balance which is equivalent to a height HP. The width B, of the zone of rock in which movement occurs will depend upon the characteristics of the rockmass and upon the tunnel dimensions Ht and B.

Rock Mass DescriptionsTerzaghi (1946)

– Intact– Stratified– Moderately jointed– Blocky and Seamy– Crushed– Squeezing– Swelling

Intact rock contains neither joints nor hair cracks. Hence, if it breaks, it breaks across sound rock. On account of the injury to the rock due to blasting, spalls may drop off the roof several hours or days after blasting. This is known as a spalling condition. Hard, intact rock may also be encountered in the popping condition involving the spontaneous and violent detachment of rock slabs from the sides or roof.

Stratified rock consists of individual strata with little or no resistance against separation along the boundaries between the strata. The strata may or may not be weakened by transverse joints. In such rock the spalling condition is quite common.

Moderately jointed rock contains joints and hair cracks, but the blocks between joints are locally grown together or so intimately interlocked that vertical walls do not require lateral support. In rocks of this type, both spalling and popping conditions may be encountered.

Blocky and seamy rock consists of chemically intact or almost intact rock fragments which are entirely separated from each other and imperfectly interlocked. In such rock, vertical walls may require lateral support.

Crushed but chemically intact rock has the character of crusher run. If most or all of the fragments are as small as fine sand grains and no recementation has taken place, crushed rock below the water table exhibits the properties of a water-bearing sand.

Squeezing rock slowly advances into the tunnel without perceptible volume increase. A prerequisite for squeeze is a high percentage of microscopic and sub-microscopic particles of micaceous minerals or clay minerals with a low swelling capacity.

Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to swell seems to be limited to those rocks that contain clay minerals such as montmorillonite, with a high swelling capacity.

Rock Load in Tunnel within Various Rock Classes

Modified Terzaghi Theory for Tunnel and Cavern

Terzaghi classification Singh 1995 classification Remarks

Lauffer-Pacher ClassificationTunnel Span: It is the width of the tunnel or

the distance from the face to the support if this is less than the tunnel width.

Standup time: It is the period of time that a tunnel will stand unsupported after excavation and is affected by factors like orientation of tunnel axis, shape of cross section, excavation method and support method.

The main significance of the Lauffer-Pacher classification is that an increase in tunnel span leads to a major reduction in the stand up time. This means that while a tunnel having a small span may be successfully constructed full face in fair rock conditions, a large span opening in the same rock conditions may prove highly problematic to support in terms of stand up time. According to this classification, the rocks are classified as moderately jointed rock, Blocky and seamy rock, crushed but chemically intact rock, squeezing rock, swelling rock. But the problem with this system is that only large cross section tunnel can be constructed in such rock conditions. As in this system, the rockmass classes are developed on the basis of experience, the out put will be in terms of stand up time and span. In short this classification system introduced the concept of standup time and span as relevant parameters in determining the type and amount of tunnel support and has also influenced the development of more recent rockmass classification systems.

Rock Quality Designation Index (RQD)(Deere et al. 1967)

1. Aim : to provide a quantitative estimate of rock mass quality from drill logs

2. Equal to the percentage of intact core pieces longer than 100mm in the total length of core

3. Directionally dependant parameter4. Intended to indicate rock mass quality in-situ5. Used as a component in the RMR and Q systems

Direct Method of Calculation of RQD

Indirect Method of Calculation of RQD

• Palmstrom (1982) • Priesta i Hudsona (1976) l - number of joints per unit length

Jv = Volumetric Joint Count

vJRQD 3.3115 ll 1.01.01100 eRQD

/g and

Vs/ Edynamic where

2)/(66.566.6/(1

VtVsEdynamic

Estatic

Multi parameter Rock Mass Classification Schemes

• Rock Mass Structure Rating (RSR)• Rock Mass Rating (RMR)• Rock Tunnelling Quality Index (Q)• Geological Strength Index (GSI)

Rock Mass Structure Rating (RSR) (1972)

• Introduced the concept of rating components to arrive at a numerical value

• Demonstrates the logic in a quasi-quantitative rock mass classification

• Has limitations as based on small tunnels supported by steel sets only

• RSR = A + B + C

Rock Structure RatingParameter A: General area geology

Considers (a) rock type origin(b) rock ‘hardness’(c) geotechnical structure

Considers (a) joint spacing(b) joint orientation (strike and dip)(c) direction of tunnel drive

Rock Structure RatingParameter B: Geometry : Effect of discontinuity pattern

Considers (a) overall rock mass quality (on the basis of A + B)(b) joint condition(c) water inflow

Rock Structure RatingParameter C: Groundwater, joint condition

RSR support estimates for a 7.3m diametercircular tunnel

(After Wickham et al. 1972)

ExamplesRSR = 622” shotcrete1” rockbolts @ 5ft centres

RSR = 305” shotcrete1” rockbolts @2.5ft centresOR 8WF31 steelsets @ 3ft centres

Based on the study of 53 projects, the following empirical relationship has been developed between RSR and the predicted rock load.

Metresin is H and B where

80)30(

8800)..(26.0LoadRock

RSRHB

Geomechanics Classification orRock Mass Rating System (RMR) (Bieniawski 1976)

Based upon• uniaxial compressive strength of rock material• rock quality designation (RQD)• spacing of discontinuities• condition of discontinuities• groundwater conditions• orientation of discontinuities

Rock Mass Rating System

• Rock mass divided into structural regions• Each region is classified separately• Boundaries can be rock type or structural, eg: fault• Can be sub divided based on significant changes, eg:

discontinuity spacing



BUT: 1976 to 1989 Bieniawski• System refined by greater data• Ratings for parameters changed• Adapted by other workers for different situations• PROJECT SPECIFIC SYSTEMS

Development of Rock Mass Rating System


(After Bieniawski 1989)


Rating Class Description Bearing Capacity (t/m2)

81-100 I Very Good Rock 600-440

61-80 II Good Rock 440-250

41-60 III Fair Rock 250-145

21-40 IV Poor Rock 145-55

Less than 21 V Very Poor Rock 55-45


Guidelines for excavation and support of 10mspan rock tunnels in accordance with the RMR system

(After Bieniawski 1989)

Prediction of in-situ deformation modulus Em

from rock mass classifications


• Nicholson & Bieniawski (1990)

• Bieniawski (1978) and Serafim & Pereira (1983)

• Hoek and Brown (1997)

• Verman (1993

• H – depth, a = 0.16-0.3 (decreases with rock strength)

)82.22/(2 9.00028.0 RMR

s

rm eRMREE

)(501002 GPaRMRforRMREm )(5010 40/)10( GPaRMRforE RMR

m

40/)10(1010

RMRcm

RE

)(103.0 38/)20( GPaHE RMRm

a

Prediction of in-situ deformation modulus Em from rock mass classifications

Estimates of support capacity for tunnelsof different sizes


Support pressure - Unal (1983) s - tunnel width

sRMRpv

100

100

Hoek (1994): m m eiRMR

100

28 s eRMR

100

9

mi - constant – from 4 (weak shales) to 32 (granite).

R sRcrm c RR

m m srrmc

242

Aydan & Kawamoto (2000) 5.20016.0 RMRRcrm

Kalamaras & Bieniawski (1995) 8515

2 RMRRR c

crm


Aydan & Kawamoto (2000) RMRRMRRMRRR ccrm

1006

Let’s assume: 60RMR MPaRc 80

Hoek:

Aydan:

Kalamaras & Bieniawski:

MPaRc 67.8

MPaRc 62.44

MPaRc 18.21

Aydan & Kawamoto (2000) RMRrm 05.022

rm

rmcrmrm

Rc

cossin1

2

Rock Tunnelling Quality Index Q – Barton, Lien, Lunde

• Based on case histories in Scandinavia• Numerical values on a log scale• Range 0.001 to 1000

‘Q’ Classification System

(After Barton et al. 1974)



• represents the structure of the rockmass• crude measure of block or particle size



• represents roughness and frictional characteristics of joint walls or infill material



• consists of two stress parameters• SRF can be regarded as a total stress parameter measure of

– loosening load as excavated through shear zones– rock stress in competent rock– squeezing loads in plastic incompetent rock

• JW is a measure of water pressure

Classification of individual parameters used in the Tunnelling Quality Index Q

Classification of individual parameters used in the Tunnelling Quality Index Q (cont’d)

Classification of individual parameters used in the Tunnelling Quality Index Q (cont’d)

‘Q’ Classification System – SRF update

Q Classification Scheme

Resolves to three parameters• Block size ( RQD / Jn )

• Interblock shear strength ( Jr / Ja )

• Active stress ( Jw / SRF )

• Does NOT include joint orientation

Equivalent Dimension De

Estimated support categories based on the tunnelling quality index Q



Roof pressure: 31

Q

JJ

pr

nroof

Length of the bolts: (roof) (walls)ESR

sL 15.02

31

32.0

QJ

Jp

r

nroof

Bhasin & Grimstad (1996): 3140

Q

Jsp

rroof

Young’s modulus:

Seismic wave velocity: ]/[100

log5.3 skmRQV cp

LH

ESR

2 0 15.

MPaRQE c33

310

RMR – Q - Correlations

• RMR and Q system or variants are the most widely used• both incorporate geological, geometric and

design/engineering parameters to obtain a “value” of rock mass quality

• empirical and require subjective assessment

Approach:• accurately characterise the rockmass ie: full and

complete description of the rockmass• assign parameters for classification later• always use two systems for comparison

Geological Strength Index (GSI)

• Method to link the constants m and s of Hoek-Brown failure criterion to observations in the field ie: a possible solution to the problem of estimating strength of jointed rockmass

• A system for estimating the reduction in rockmass strength for different geological conditions

• Overcomes deficiencies of RMR for poor quality rock

Estimate of Geological Strength Index GSI

based on geological descriptions

Estimation of constants based upon rockmass structure and discontinuity surface

conditions



Estimate of Geological Strength Index GSI based on geological descriptions.

Plots of cohesive strength and friction angles for different GSI and mi values

Evaluation of Tunnels based on RMR

Example: 10 m spanRMR = 80Stand up time > 4 yearsRMR = 50Stand up time 2 days

RMR

DE

Q

Shotcrete thickness

Areas within the chart area 1 area 2 area 3 area 4 area 5 area 6 area 7 area 8 area 9

unsupported spot bolting systematic bolting (SB) SB + 40-50 mm shotcrete SB + 50-90 mm FRS SB + 90-120 mm FRS SB + 120-150 mm FRS SB + 150-120 mm FRS, ribbed Cast concrete lining

FRS = fibre reinforced shotcrete

Tunnels and the Q rating Example: 10 m span ESR = 2Q= 40

10 m span ESR = 1Q= 40

Evaluation of Tunnels based on Q ratingExample: 10 m span & ESR = 2 Q = 40Area 1: UNSUPPORTED

10 m span & ESR = 1 Q = 40Area (2): SPOT BOLTING Requires rockbolts at 3 m spacing, 3 m long (max)

Tunnels and the Q rating Example: 10 m span ESR = 1Q = 1.0

KEY POINTS? Rock mass rating systems are a useful way of

forming an evaluation of rock masses

The Q or NGI system was based on tunnelling

The RMR (CSIR) system is more commonly used for slope stability

The strength of rock masses can be judged from these systems

Documents

Chapter 1