Rock Mechanics

INTRODUCTION AND CLASSIFICATION OF ROCKSDr. R.K.DuttaAPCEDNIT Hamirpur

Contact InformationPhone [email protected] Hours: 4:45pm-5:30pm

TextbookRockMechanicsinEngineeringPractice:K.G.StaggUndergroundexcavationinrock:HoekandBrownRockMechanicsinEngineeringPractice:CambridgeUniversityIntroductiontoRockMechanics:R.E.Goodman

PrerequisitesCE 242; CE-471Class Website http://www.nitham.ac.in/~rkd/?Teaching_Assignment

Course ContentIntroduction: RockMechanicsanditsrelationshipwithsoilmechanicsandengineeringgeology,applicationofrockmechanicstocivilengineeringproblems.

Classification of RocksLithologicalclassification,engineeringclassificationofrocks,classificationbasedonwavevelocityratio,R.Q.D.Classificationofrockmassesi.e.RMRandQsystems.

Rock PropertiesLaboratorytest,compression,tensile,voidindex,permeabilityandshear,effectsofsizeofspecimen,rateoftesting,confiningpressureetc.Stressstraincurvesoftypicalrocks,strengthofintactandfissuredrocks,effectsofanisotropy,saturationandtemperatureeffects,shearstrengthofjointedrockmass. Field TestsUniaxialtestsintunnelsandopenexcavations,sheartest, pressurestunneltestsetc.

Stability of Rock SlopesModeoffailureofrockslopes,planewedgeanalysis,3Dwedgeanalysiscircularmodeoffailure,backanalysisofslopes,stabilitycharts,typesanddesignofrockbolts.

Determination of Insitu StressesStressesinrock,methodsofdetermininginsitustressesi.ehydraulicfracturing,flatjacktestandovercoring.Design of TunnelRockpressuretheories,groundreactioncurve,rocksupportinteractionanalysisempiricalandsemiempiricalmethodsofanalysis,simplemethodoftunnel,design,typesanddesign oftunnellining.

Foundation on Rocks Stressdistributioninfoundation,methodsofdeterminationofbearingcapacityofrocks,improvementofrockproperties,pressuregroutingfortunnelsanddams,dentalconcreting, shearzonetreatment.

Grading PolicyTwo 90-min Exams . 30%Homework ... 20%Final Exam .50%TOTAL 100%

HomeworksCounts 20% towards the final gradeApprox. 10 Homework AssignmentsDue at the beginning of class periodNo late homeworks acceptedIf you are not attending the class, have your homework delivered to my room or mail box.

Exams2 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

Basic description and classification of soilEffective stressShear strengthConsolidationLateral earth pressureBearing capacitySlope stabilityPermeability

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

Geologic hazardsGeotechnicalMaterial propertiesLandslide and slope stabilityErosionFloodingDewateringSeismic 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

MiningPetroleumCivil 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

1.Tunnels2. Mining shafts3. Underground excavations4. Open pit mines5. Oil and Gas Wells6. Road cuts7. 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 Dams (Impose additional stresses on these materials)Power houses and under ground openings (Under ground openings cause stress changes)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 Active faultsJointsCracks make the situation more complex particularly for hydraulic structures. Under such circumstances it is very much essential to have full understanding of the Natural forcesCharacterisation of the rockmassBehaviour 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 RocksRock fracture under compressive stressesSize effects response of rock to loading affected by the size of the loaded volume (joints & fractures)Tensile strength is low (similar to concrete); HOWEVER a rock mass can have even less tensile strength

Groundwater effectswater in joints: if under pressure, reduces normal stress (less resistance along joints)water in permeable rocks (e.g. sandstone) soil like responsesoftening of clay seams & argillaceous rocks (e.g. shales)

Weathering chemical/physical alteration, reduction of engineering propertieslimestone caverns, sinkholes: Karstbasic rocks with olivine (e.g. basalt) and pyroxene minerals are reduced to montmorillonite by hydrolysis

Discontinuities Bedding planesFolds 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

Faults shear displacement zones - sliding

Faults may contain Fault gouge (clay) weakFault breccia (re-cemented rock) weakRock flour weakAngular fragments may be strong

Defects

Shear zones bands of materials - local shear failureDykes igneous intrusions (near vertical)weathered dykes, e.g. dolerite weathers to montmorilloniteunweathered dykes attract high stressesJoints breaks with no visible displacement

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

JOINTS1) Open Filled Healed (or closed)2) SteppedUndulating Planar2B) each of the above can be RoughSmoothSlickensided

JOINT CLASSES

IIIIISteppedRoughSmoothSlickensidedIVVVIUndulatingRoughSmoothSlickensidedVIIVIIIIXPlanarRoughSmoothSlickensided

Order of Description of Rockse.g. Basalt, fine, massive, vesicular, dark grey to black

e.g. VL strength, XW

Structure: sedimentary rocks bedded, laminatedmetamorphic foliated, banded, cleavedigneous rocks massive, flow banded

DEFECTS information neededtightnesscementation or infillsmoothness or irregularity of surfacesclass of jointwater in jointsjoint orientationjoint spacing

Rock ClassificationClassification 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 ClassificationTo classify a particular rock mass into groups of similar behaviourTo provide a basis for understanding the characteristics of each groupTo yield quantitative data for engineering designsTo provide a common basis for communication

Requirements of Good Classification SystemIt should be simple, easily remembered and understandableEach term should be clear and the terminology used should be widely accepted by engineers and geologistsAs many as significant properties of the rock masses should be taken into considerationsIt should be based on measurable parameters which can be obtained by relevant quick tests on samples and cheaply in the fieldIt should be based on rating system that can weigh the relative importance of the classification parametersIt 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 followsLithological classificationEngineering classificationTerzaghi rock load classificationLauffer-Pacher ClassificationDeeres Rock Quality DesignationRock Structure RatingGeomechanics Classification SystemNGI Classification SystemGeological Strength Index

Lithological ClassificationLithology of rock is the study of its physical character. It includes the study ofMineralogical compositionTextureColourPhysical 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.TextureStructureCompositionColourGrain 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.

CompositionPresence of calcite is of prime importance when considering mechanical and physical characteristics of rock mass. The important sub-divisions areNoncalcareous: Rock materials are those in which calcium carbonate is absent

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.

ColourIf 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.

Grain sizeSometimes 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 diameterMedium 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.

Engineering ClassificationThe 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.

ClassDescriptionUCS (kg/cm2)AVery high strength>2250BHigh strength1125-2250CMedium strength562.5-1125DLow strength281.25-562.5EVery low strength

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 ratioMR = Et50/sultEt50 = Tangent modulus at 50 % ultimate compressive strength of rocksult = UCS

Engineering classification of intact rocks on the basis of modulus ratioOn 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

Terzaghis 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. Terzaghis 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

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)Aim : to provide a quantitative estimate of rock mass quality from drill logsEqual to the percentage of intact core pieces longer than 100mm in the total length of core Directionally dependant parameter Intended to indicate rock mass quality in-situ Used as a component in the RMR and Q systems

Direct Method of Calculation of RQD

Indirect Method of Calculation of RQDPalmstrom (1982) Priesta i Hudsona (1976) l - number of joints per unit length Jv = Volumetric Joint Count

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 geologyConsiders (a) rock type origin(b) rock hardness(c) geotechnical structure

Considers (a) joint spacing(b) joint orientation (strike and dip)(c) direction of tunnel driveRock 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 inflowRock 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.

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

Rock Mass Rating System

Rock Mass Rating SystemBUT: 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

Rock Mass Rating System(After Bieniawski 1989)


RatingClassDescriptionBearing Capacity (t/m2)81-100IVery Good Rock600-44061-80IIGood Rock440-25041-60IIIFair Rock250-14512-40IVPoor Rock145-55Less than 20 VVery Poor Rock55-45

Guidelines for excavation and support of 10mspan rock tunnels in accordance with the RMR system(After Bieniawski 1989)

Prediction of in-situ deformation modulus Emfrom rock mass classifications

Rock Mass Rating System 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)

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

Estimates of support capacity for tunnelsof different sizes

Rock Mass Rating SystemSupport pressure - Unal (1983) s - tunnel widthHoek (1994): mi - constant from 4 (weak shales) to 32 (granite).Aydan & Kawamoto (2000) Kalamaras & Bieniawski (1995)

Rock Mass Rating SystemAydan & Kawamoto (2000) Lets assume: Hoek: Aydan:Kalamaras & Bieniawski: Aydan & Kawamoto (2000)

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)

Q Classification System(After Barton et al. 1974) represents the structure of the rockmass crude measure of block or particle size

Q Classification System(After Barton et al. 1974) represents roughness and frictional characteristics of joint walls or infill material

Q Classification System(After Barton et al. 1974) 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 (contd)

Q Classification System SRF update

Q Classification SchemeResolves 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

Q Classification Scheme

Q Classification SchemeRoof pressure:Length of the bolts: (roof) (walls)Bhasin & Grimstad (1996):Youngs modulus: Seismic wave velocity:

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 GSIbased on geological descriptionsEstimation of constants based upon rockmass structure and discontinuity surface conditions

Geological Strength Index (GSI)

Geological Strength Index (GSI)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 RMRExample: 10 m spanRMR = 80Stand up time > 4 yearsRMR = 50Stand up time 2 days RMR

Areas within the chartarea 1area 2area 3area 4area 5area 6area 7area 8area 9unsupportedspot boltingsystematic bolting (SB)SB + 40-50 mm shotcreteSB + 50-90 mm FRSSB + 90-120 mm FRSSB + 120-150 mm FRSSB + 150-120 mm FRS, ribbedCast concrete liningFRS = 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 = 2Q = 40Area 1: UNSUPPORTED

10 m span & ESR = 1Q = 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 massesThe Q or NGI system was based on tunnellingThe RMR (CSIR) system is more commonly used for slope stabilityThe strength of rock masses can be judged from these systems

Rock Mechanics

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