58 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

for every site and will be organized into an overall database. The data will provide the basis fora detailed quantitative assessment of current design criteria of emergency spillways and will assistin the development of any needed revisions.

I believe that Dr. Kirsten's idea of applying his erodibility index (expressed as N) to a rockmass that forms an excavated spillway channel should be explored in our (SCS) assessment. Oncethe data base is established from the spillway performance reports it would be productive todetermine an empirical relationship between J, and the relative dip angle and block shape forhydraulic erosion of spillway channel beds. The analysis should be useful in our assessment ofdesign criteria for excavated rock emergency spillways.

Nick Barton'

Rock Mass Classification and TunnelReinforcement Selection Using theQ-System

REFERENCE: Barton, N., "Rock Mass Classification and Tunnel Reinforcement SelectionUsing the Q-System," Rock Classification Systems for Engineering Purposes, ASTM STP 984,Louis Kirkaldie, Ed., American Society for Testing and Materials, Philadelphia, 1988, pp. 59-88.

ABSTRACT: This paper provides an overview of the Q-system and documents the scope of caserecords used in its development. A description of the rock mass classification method is given usingthe following six parameters: core recovery (RQD), number of joint sets, roughness and alterationof the least favorable discontinuities, water inflow, and stress-strength relationships. Examples offield mapping are given as an illustration of the practical application of the method in the tunnelingenvironment, where the rock may already be partly covered by a temporary layer of shotcrete. Themethod is briefly compared with other classification methods, and the advantages of the method areemphasized.

KEY WORDS: rock mass, classification, tunnels, rock support, shotcrete, rock bolts jointing

This paper provides an analysis of the Q-system of rockmass characterization and tunnel supportselection. The 212 case records utilized in developing the Q-system (Barton et al, 1974) arereviewed in detail, so that application to new projects can be related to the extensive range ofrock mass qualities, tunnel sizes, and tunnel depths that constitute the Q-system data base.

Ultimately, a potential user of a classification method will be persuaded of the value of aparticular system by the degree to which he can identify his site in the case records used to developthe given method. The most comprehensive data base of the seven or eight classification systemsreviewed is utilized in the Q-system. This body of engineering experience ensures that supportdesigns will be realistic rather than theoretical, and more objective than can be the case when fewprevious experiences are utilized to develop a support recommendation.

Classification Systems Currently in UseTable I is an abbreviated listing of most of the rock mass classification systems currently in

use internationally in the field of tunneling. These are:

Terzhagi (1946) Rock Load ClassificationThis has been used extensively in the United Statesfor some 40 years. It is used primarily to select steel supports for rock tunnels. However, it isunsuitable for modem tunnelling methods in which rock bolts and shotcrete are used.

Lauffer (1958) Stand- Up Time ClassificationThis introduced the concept of an unsupportedspan and its equivalent stand-up time, which was a function of rock mass quality. It appearsexcessively conservative when compared with present-day tunneling methods.

' Norwegian Geotechnical Institute, Oslo 8, Norway.

59

TABLE 1Major rock mass classification systems.

Name of

Originator CountryClassification and Date of Origin Applications

Rock Loads Terzhagi (1946) USAStand-Up-Time Lauffer (1958) AustriaRQD Deere et al (1967) and USA

Deere et al (1970)RSR Concept Wickham et al (1972)

USAGeomechanics Bieniawski (1973) S. Africa

(RMR System)Q-System Barton et al (1974) Norway

tunnels with steel supportstunnelingcore logging, tunnelling

tunnels with steel supportstunnels, mines, etc.

tunnels, large chambers

BARTON ON Q-SYSTEM 6160 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

The NATM relies on performance monitoring for prediction and classification of groundconditions. It is adapted to each new project based on previous experience. The classification isalso adapted during a single project based on performance monitoring. A particular classificationis therefore only applicable to the one case for which it was developed and modified, so use byothers on other projects may be difficult.

The NATM is essentially a design method in which the rock mass is allowed to yield onlyenough to mobilize its optimum strength, by utilizing light temporary support. With correct timingof final support, this initial yielding is arrested in time to prevent loss of strength. On occasion,the desire to allow deformation to occur by installing canals of deformable material within theshotcrete, and steel ribs with sliding joints, has resulted in loss of ground control and severedamage to final concrete linings and bolt arrays (Barton, 1982).

" Sec Bibliography for details.

Deere et al (1967) Rock Quality Designation (RQD)This is a simple description of thecondition of recovered drill core. It has been successfully adopted as part of subsequent classificationsystems. On its own, it fails to account for the condition of joint surfaces and filling materials,and may be overly sensitive to orientation effects. Deere et al (1970) utilized RQD to developsupport recommendations for 6 to 12 m span tunnels, but pointed out that the details of jointing,weathering, and groundwater should also be taken into account when selecting support. Fourteencase records were utilized in developing these recommendations.

Wickham, Tiedemann, and Skinner (1972) Rock Structure Rating (RSR)This conceptintroduced numerical ratings and weightings to relate rock mass quality, excavation dimensions,and steel support requirements. The method was an immediate forerunner to the two methods nowused most frequently on an international basis (the RMR and Q systems).

Bieniawski (1973) RMR Geomechanics ClassificationThis evolved from several earliersystems and has undergone several changes (1974, 1975, 1976, and 1979) since its first introductionin 1973. The method was eventually based on 49 case records, though details of these cases withtheir relation to support recommendations have not been published. Recent applications of theRMR system have been made in mining, which has extended the data base considerably.

Barton, Lien, and Lunde (1974) Q-SystemThis classification system was developedindependently of the Wickham et al (1972) and Bieniawski (1973) methods, but it builds extensivelyon the RQD method of Deere et al (1967), introducing five additional parameters to modify theRQD value to account for the number of joint sets, the joint roughness and alteration (filling), theamount of water, and the various adverse features associated with loosening, high stress, squeezing,and swelling. The classification method and the associated support recommendations were basedon an analysis of 212 case records. Full details of these cases are given later in this paper.

NATM

One further tunnel support concept which should be mentioned here is the New AustrianTunneling Method (NATM), which was developed by Rabcewicz, Packer, and Muller (Rabcewicz,1963 and Rabcewicz and Packer, 1975). As acknowledged by Milner (Salzburg), almost everyoneusing this method has a different conception of it, and numerous economic and technical failuresin past years demonstrate the amount of confusion that prevails in this field. In NATM's defense,it must be understood that the method is principally utilized in squeezing ground conditions, whichcould present technical and economic problems for any tunneling method.

Updating Case Records

Some of the support methods recommended by the above classification methods are quite laborintensive and will need updating as new support methods become more generally available. Forexample, the development of high strength, but highly ductile, steel fiber reinforced microsilicashotcrete is a revolutionary advance in tunnel support. It can be applied by one robot operator andone back-up person right at the tunnel face. The extra strength of this product removes the needfor mesh in shotcrete, and it has sufficient early strength to replace steel arches and cast concreteunder a large range of tunnelling conditions.

Comparison of RMR and Q SystemsThe two classification systems that appear to be in widest use in tunneling that do not rely on

performance monitoring (though they can be used in conjunction with monitoring) are the RMRand Q systems. These two systems are therefore compared in some detail here.

Bieniawski (1976) rates the following six parameters in his RMR system:

I. Uniaxial compressive strength of rock material.Drill core quality RQD.Spacing of joints.Condition of joints.Groundwater conditions.

6. Orientation of joints.

In contrast, the Q-system (Barton et al, 1974) rates the following six parameters:

I. RQD.Number of joint sets.Joint roughness.Joint alteration.Joint water.

6. Stress factor.

The common parameter used in both systems is Deere's RQD. Bieniawski also includes jointspacing and orientation, while the Q-system considers the number of joint sets. Orientation isincluded implicitly in the Q-system by classifying the joint roughness and alteration of only themost unfavorably oriented joint sets or discontinuities.

Bieniawski (1975) appears to have favored the mean rating for spacing and orientation of the

62 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON 0-SYSTEM 63

different joint sets according to the example given in his paper. He also indicates (1979) that whenonly two joint sets are present the average spacing will prove conservative, since the rating isusually based on the presence of three sets of joints.

The very detailed treatment of joint roughness and alteration, perhaps the strongest feature ofthe Q-system, is not particularly emphasized in the RMR Geomechanics Classification. In hisoriginal version Bieniawski (1973) considered the condition of joints under three descriptive terms:weathering (5 ratings), separation of joints (5 ratings, 5 mm) and continuity ofjoints (5 ratings, not continuous up to continuous with gouge). In his 1974 publication Bieniawskicondensed these three terms to "condition of joints" which again had five ratings; from very tight,separation 5 mm, continuous gouge >5 mm. In his laterpublications (1976, 1979), Bieniawski also includes joint roughness in his fourth parameter"condition of joints."

In the RMR system, rock stress is not used specifically as a parameter though it is apparentlywhen selecting support measures. In his 1975 paper Bieniawski gives support recommendation fora tunnel of 5 to 12 m span in which the vertical stress should be less than 30 MPa. In his 1976version, the same support recommendations are given specifically for 10 m span tunnels, with thevertical stress limited to 25 MPa.

In the Q-system, the ratio (o.c/o,) (unconfined compression strength/major principal stress) isevaluated when treating rock stress problems. The onset of popping, slabbing, and rock burstproblems can be quite accurately predicted in hard rocks. The Q-system also accounts for looseningcaused by shear zones and faults, and squeezing and swelling ground. However, very few caserecords could be utilized in the squeezing category, so support recommendations are tentative.

The Geomechanics Classification was based initially on Lauffer (1958), which is nowacknowledged to be excessively conservative. Bieniawski increased Lauffer's maximum unsupportedspan of 8 m (in his 1973 version) to 20 m (1975 version) and finally to 30 m (1979 version).Despite these later modifications, Bieniawski's chart of stand-up time versus unsupported span isstill seen to be very conservative compared with the Q-system.

In a detailed comparison of various classification methods, Einstein et al (1979) compared eachmethod's support estimates with Cecil's (1970) unsupported cases. They showed that Bieniawski's(1976) method predicted considerable support in all cases, the Deere et al (1969) method evenlarger amounts of support. The Q-system predicted no-support, since Cecil's cases formed thebackbone of the method. Such discrepancies reflect the important differences in support philosophiesbetween Scandinavia, South Africa, and the United States. These differences appear to be narrowinggradually.

The Einstein et al (1979) comparison of each method with Cecil's supported cases indicatedthat the Q-system support recommendations also agree well with the actual support. The Bieniawski(1976) and Deere et al (1969) RQD method were more conservative.

One of the most recent reports of comparisons between rock mass classification systems fortunneling was published by Einstein et al (1983) from work performed at the Porter Square Station,a 168 by 14 by 21 m excavation located 20 to 30 m below the surface in argillite, in Cambridge,Massachusetts. Five of the classification methods listed in Table I were compared in detail duringseveral stages of the project and by several observers.

The methods were compared while classifying drill core, mapping an inspection shaft and apilot tunnel, and when the rock was exposed in the final excavation. Only the Q-system was foundto be applicable to the worst conditions encountered in the main excavation. Category 32 supportpredicted by the Q-system, consisting of systematic bolts at I m centers, and 40 to 60 cm ofshotcrete, was "practically identical to the actual support" used. Careful monitoring using MPBX(multiple position borehole extensometers) and convergence measurements revealed maximumdeformations of only 8 mm; in other words, stable conditions were established.

Description of the Q-SystemRationale

The vast majority of the thousands of kilometers of tunnels constructed world-wide every yeardo not have the benefit of performance monitoring. Design decisions are nevertheless requiredboth before and during construction. No matter how many sophisticated rock mechanics testprograms and finite element analyses are performed, design engineers will come back to the basicquestion: "Is this bolt spacing, shotcrete thickness, or unsupported span width reasonable in thegiven rock mass?"

At present we have to rely on engineering judgment, or on classification methods, where thedesign is based on precedent, and where a good classification method will allow us to extrapolatepast designs to different rock masses and to different sizes and types of excavation. Undergroundexcavations can be supported with some confidence primarily because many others have beensupported before them and have performed satisfactorily.

Method for Estimating Rock Mass Quality (Q)The six parameters chosen to describe the rock mass quality (Q) are combined in the following

way:

Q = (RQD/J,,) (J,1J) (J,/SRF) (1)

where

RQD = rock quality designation (Deere et al, 1967),J = joint set number,J, = joint roughness number (of least favorable discontinuity or joint set),J,, = joint alteration number (of least favorable discontinuity or joint set),J. = joint water reduction factor, and

SRF = stress reduction factor.

The three pairs of ratios (RQD/J, J,/.1, and J./SRF) represent block size, minimum inter-blockshear strength, and active stress, respectively. These are fundamental geotechnical parameters.

It is important to observe that the values of J, and J relate to the joint set or discontinuity mostlikely to allow failure to initiate. The important influence of orientation relative to the tunnel axisis implicit.

Detailed descriptions of the six parameters and their numerical ratings are given in Table 2.The range of possible Q values (approximately 0.001 to 1000) encompasses the whole spectrumof rock mass qualities from heavy squeezing ground up to sound unjointed rock. The case recordsexamined included 13 igneous rock types, 26 metamorphic rock types, and 11 sedimentary rocktypes. More than 80 of the case records involved clay occurrences. However, most commonly thejoints were unfilled and the joint walls were unaltered or only slightly altered.

The Q-system is more detailed than any of the other methods as regards the factors jointroughness (or degree of planarity), joint alteration (filling), and relative orientation. The classificationof "least favorable features" (for J, and .1,,) represents one of the strongest features of the method.It also seems to be a factor that is virtually ignored in the other classification schemes. Forexample, in Bieniawski's RMR method, although data for all joint set and discontinuities arecollected, only the average data are incorporated in the numerical ratings. Furthermore, in theRMR it is impossible to separately vary the degree of joint roughness and the degree of infilling,as obviously may occur in practice.

(J.) (Or)(approx.)(a) Rock wall contact

Tightly healed,hard,non-soften-ing,impermiable filling i.e.quartz or epidote

0.75 ( - )Unaltered joint walls,surfacestaining only 1.0Slightly altered joint walls.Non-softening mineral coatings,sandy particles, clay-freedisintegrated rock etc.

2.0Silty-,or sandy-clay coatings,small clay fraction (non-soft.) 3.0

E. Softening or low friction claymineral coatings, i.e.kaoliniteor mica. Also chlorite,talc,gypsum,graphite etc., andsmall quantities of swellingclays.

(b) Rock wall contact before10 .ms shear

BARTON ON Q-SYSTEM 6564 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

TABLE 2-Ratings for the six Q-system parameters(table continues on pp. 65 and 66).

1. ROCK QUALITY DESIGNATION (RQD)

Very poor 0 - 25

Poor 25 - 50

Fair 50 - 75

Good 75 90E. Excellent

90 - 100

Note: (i) Where RQD is reported or measured as 10,

(including 0) a nominal value of 10 is usedto evaluate Q in equation (1).

(ii) RQO intervals of 5, i.e. 100,95,90, etc. aresufficiently accurate.

(J )2. JOINT SET NUMBER

Massive,no or few joints One joint set One joint set plus random

Two joint sets Two joint sets plus random Three joint sets Three joint sets plus random Four or more joint sets,random,heavily jointed,"sugar cube" etc. .... 15

J. Crushed rock,earthlike 20

Note: (1) For intersections use (3.0 x Jn)Note: (ii) For portals use 12.0 JD)

0.5 - 1.02

469

12

3. JOINT ROUGHNESS NUMBER

Rock wall contact andRock wall contact before (Jr)

10 ems shearDiscontinuous joints 4Rough or irregular,undulating 3

Smooth,undulating 2

Slickensided,undulating 1.5

Rough or irregular,planar 1.5Smooth,planar 1.0

Slickensided,planar 0.5

Note: (1) Descriptions refer to small scale featuresand intermediate scale features,in thatorder.

(c) No rock wall contact when shearedZone containing clay minerals thick enoughto prevent rock wall contact 1.0

J. Sandy,gravelly or crushed zone thick enoughto prevent rock wall contact 1.0

Note: (ii) Add 1.0 if the mean spacing of the relevIntjoint set is greeter than 3m.

(iii) J r w0.5 can be used for planar slickensidedjoints having lineations,provided the line-ations are orientated for minimum strength

TABLE 2-(continued).4. JOINT ALTERATION NUMBER

Sandy particles,clay-freedisintegrated rock etc.

4.0 (25-30')Strongly over-consolidatednon-softening clay mineralfillings (continuous,but

66 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON Q-SYSTEM 67

Method of Selecting Suitable SupportThe Q-system is essentially a weighting process, in which the positive and negative aspects of

a rock mass are assessed. A store of experience (case records), which is itself based on earlierexperience, is searched to try to find the most appropriate support measures for the given excavationsand rock mass conditions. The whole procedure is probably not dissimilar to the mental processoccurring when a very experienced tunneling consultant is asked for his support recommendations.While the assessment of most of the parameters is admittedly subjective, the process of supportselection is organized and reasonably objective. The trial-and-error adjustment and readjustmentof parameter ratings necessary during the development of the Q-system was an important factorin reducing the need for subjective judgments on the part of the developers. The large number ofcase records made it possible to generate the support recommendations quite objectively.

Figure I shows that the tunnel or excavation span width and the rock mass quality (Q) are thedecisive parameters for placing an excavation in a given support category (Boxes 1 to 38).However, there is an important user requirement for different degrees of safety. The excavationsupport ratio (ESR), which reduces the effective span in Fig. 1, reflects construction practice inthat the degree of safety and support demanded by an excavation is determined by the purpose,presence of machinery, personnel, etc. The list of ESR values in Table 3 was developed throughexhaustive trial and error, and seems to be the most workable solution to the problem of variablesafety requirements.

Increased safety can be selected at will by reducing the ESR value (e.g. by using ESR = 1.3for an important permanent mine opening in place of 1.6). Similarly, support for oil storagecaverns could be selected by using ESR = 1.6 instead of 1.3, assuming the occasional fall ofsmall stones (from the walls) were acceptable. The use of ESR = 1.0 for power station chambersand major road tunnels ensures a high factor of safety, as obviously required.

Most of the 38 numbered "support boxes" shown in Fig. 1 contain case records. The supportrecommendations for cases that plot in these same boxes are listed in Table 4. Further details ofthe use of these support recommendations are given by Barton, Lien, and Lunde (1975). Roofsupport, wall support, and temporary support can be selected as required.

Examples of Case RecordsA considerable data base for developing the Q-system was provided by Cecil (1970), who

described numerous tunneling projects in Sweden and Norway, including detailed evaluations ofthe rock, the jointing, the type of support, and the apparent stability. Figure 2 shows three examplesof Cecil's cases, and Table 5 gives the abbreviated descriptions of the key rock mass parametersand describes the support actually used. Note that the alphabetic descriptions given in brackets inTable 5 can be checked directly against the same letters given in Table 2. This convenientshorthand method can be used during tunnel mapping, when writing conditions are unfavorable.

Examples of Tunnel MappingExamples of some tunnel projects in which the Q-system has been used extensively in day-to-

day follow-up mapping are shown in Figs. 3a and 3b. It will be noticed that the treatment ofcrushed zones and major discontinuities shown in Figs. 3a and 3b is often on an individual basis.The quality of the rock mass between the zones is a decisive factor in deciding between individual"stitching" and general support.

One of the examples (Fig. 3b) is a tunnel excavated by a full-face tunnel boring machine(TBM). The minimal disturbance caused by TBM excavation makes it particularly important tomap as close to the advancing face as possible (for optimal joint definition), followed by repeated

6. STRESS REDUCTION FACTOR TABLE 2(continued).

(a) Weakness zones intersectingexcavation,which may causeloosening of rock mass whentunnel is excavated. (SRF)

Multiple occurrences of weak-ness zones containing clay orchemically disintegrated rock,very loose surrounding rock(any depth)

10 Single weakness zones cont-aining clay or chemicallydisintegrated rock(depth ofexcavation 50m) Single weakness zones cont-aining clay or chemicallydisintegrated rock (depth ofexcavation > 50m ) 2.5Multiple shear zones in compet-ent rock (clay-free),loose surr-ounding rock (any depth) 7.5

E. Single shear zones in competentrock (clay-free) (depth ofexcavation S 50m ) 5.0Single shear zones in competentrock (clay-free) (depth of excav-ation 50m I 2.5Loose open joints,heavily jointedor "sugar cube" etc. (any depth) 5.0

Note: (i) Reduce these values of SRF by25 - 50% if the relevant shearzones only influence but do notintersect the excavation.

(b) Competent rock, rock stress problems

oc ia lof/al (SRF)

Low stress, near surface >200 >13 2.5Medium stress '00-10 13-0.66 1.0High stress, very tightstructure (usually fav-ourable to stability,may be unfavourable forwall stability) 10-5 0.66-.33 0.5-2Mild rock burst(massive rock) 5-2.5 0.33-.16 5-10Heavy rock burst(massive rock) 02.5 10,reduce oc and a t to 0.60cand 0.60 t , where o cunconfinedcompression strength, and a t = tensilestrength (point load), and 0 1 and 0 3 arethe major and minor principal str?,SeS.

(iii) Few case records available where depth ofcrown below surface is less than spanwidth. Suggest SRF increase from 2.5 to 5for such cases (see H).

Squeezing rock:plastic flow of incompetentrock under the influence of high rock pressure

(OAF)M. Mild squeezing rock pressure ,

5 100. Heavy squeezing rock pressure 10 20

swelling rock:chermecal swelling activitydepending on presence of water

P. Mild swelling rock pressure 5 - 10R. Heavy swelling rock pressure 10 15

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68 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON 0-SYSTEM 6900 0

el 0 0 in .1, PITABLE 3Excavation Support Ratio (ESR)pr a variety of underground gveavations.

NumberESR of Cases

Temporary mine openings etc. ca. 3-5? 2Permanent mine openings, water tunnels for hydro power (excluding high pressurepenstocks), pilot tunnels, drifts and headings for large openings 1.6 83Storage caverns, water treatment plants, minor road and railway tunnels, surgechambers, access tunnels, etc. 1.3 25Power stations, major road and railway tunnels, civil defense chambers, portals,intersections 1.0 79

E. Underground nuclear power stations, railway stations, sports and public facilities,factories ca. 0.8? 2

mapping before permanent support is chosen. There will then be improved possibilities forobserving the character of narrow clay-bearing discontinuities. The effective RQD of the zone ofrock around a TBM excavated tunnel will generally be higher than that around a blasted tunnelowing to the relatively slight disturbance of incipient joints and tight structures.

Figure 4 illustrates ten parallel (100 m long) sewage treatment caverns constructed near Oslo.At the feasibility and planning stage, surface mapping and drill core analysis were interpreted interms of the Q-system parameters. Support requirements were predicted on this basis. Duringconstruction, support decisions were also guided by the method outlined. The general improvementin rock conditions as the parallel caverns advanced from the shale into the nodular limestone wereclearly reflected in the six parameters and support was reduced accordingly:

Shale: B 1.25 in c/c, L 3.5 m + S (mr) 12-15 cmNodular limestone: B 1.5 m c/c, L= 3.5 m+ S 5 cm

(B = bolting, c/c = spacing, L = length, S = shotcrete, mr = mesh reinforced)

The Q-system has also been used in the area of mine stability. In a recent assessment of stabilityin two limestone mines with 13 to 15 m span rooms or drifts, respectively, the quality of thelimestone varied as follows:

80 100 I 1.5 1Q = = 4 38 (fair -- good)4 9

1 2 1

In the great majority of the drifts the quality was "good" (Q = 18 38). By comparing thesequalities with the permanently unsupported cases (Fig. 1, black circles) it was possible todemonstrate satisfactory conditions for the great majority of the excavations. However, in places,stability was apparently nearer the "temporary mine openings" category (ESR = 3-5, Table 1).There were in fact limited areas in the mines where fall-out occurred from the pillars or walls. Alimited number of pillars in one of the mines were instrumented as a precaution.

Rock Mass Requirements for Permanently Unsupported ExcavationsAn important area of application for the Q-system is the recognition of rock mass characteristics

required for safe operation of permanently unsupported openings. The relationship between themaximum unsupported span and the Q-value is clearly seen in Fig. I. Detailed analysis of theavailable case records reveals the following requirements:

Type of Excavation

70 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON Q-SYSTEM 71

TABLE 4-Support recommendations for the 38 categories shown in Fig. I (see Barton et al, 1974, 1975for notes: table continues on pp. 71 and 72). TABLE 4-(continued).

Cottilatonal factorsSupport Ro J. SPANrate-gory .1 1, ESR Tvpe of .support NotesI" -

-

sblutg) -

-- -

sblutg) -- - - sblutg) -

4' - sblutg) -5 -- sblutg)

6 - - sblutg) -7' - - - sblutg) -g'

--

sblutg) -Note. The type of suppon to be used in categories I to 0 will depend on the

blasting technique. Smooth wall blasting and thorough barring-downmay remove the need for suppon. Rough-wall blasting may result in theneed for single applications of shotcrete, especially where the excavationheight is >25 m Future cast records should differentiate categories Ito 8.

9 020 - .sblutg))

cr)w

0

If)

O

-0

0.

O

O

72 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON Q-SYSTEM 73

TABLE 4(continued).

Conditional factors5"PP.'" RQD 1, SPAN!UM-

80ry 1, .1, ESR Type of support Notes

TABLE 5-Comparison of support used and support recommended for three case records described b y Cecil (1970).

CaseNo.

3.

DESCRIPTION OF ROCK MASSNature of instabilityPurpose of excavation,location, reference

SPAN Height

(m ) ( m )

Depth

( m)Support used

RQD L._1.r.J

.3n

(Code :Tables

,w

SRF

1 to 6)

Q ESR SPAN Estimate ofpermanentroof support

ESR

24 60 m length, including a

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12.5 6.5 60 shotcrete 6 6

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Orginally large waterie akageROCK

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SiONet)

TUNNEL MAPPING - SUPPORT PROJEC TNO 71610LOCALITY HYLLEN , ULLA - FORRE 5NEE T NO

ROCK MASSDiscontinuity,thickness 50cm

Crushed zone,without clay

Crushed zone,weathered withclay

80Strike/dip

Dikes and sills

0'.o- 1

o

...

PI lb. -

ritlipe0,1041.

11111.

111

4 (01.401,111.

10,., . .Are Like

-I-ntictine-r-

..,411,0,4 40

ri0 0

1090

1080-

1070-

1060

1050 -

!.!.

S I m 111111114

1441%

11111111111111

S 5 c m

10cm

S Im I10cm

.

..

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,-.

1

o -H.0

..2 m

. -

'c'l

I

a-

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-

-

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.

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a.,

e--,...o

.2: c-..1,,,,,,

-

o,, - -

. ,?, I - ' -

r-.. / t' '- ,'" \ , _r zt ' -_C-%.l,c,5 ROCK MASS ic TEMPORARY RECOMMENDED

DESCRIPTION ": SUPPORT SUPPORTiLo

z7(.T.,

,_o

z

-L' a --,N,,

I

I

L'ezL---".,

t,x-, r

,.-_-,-sr."z

i,='n2, a -'

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NOTES- 1060-1075 Fault 20 cm crushedwith 10 cm clay filling1080 Fault 3m crushed zone1090 2 m crushed zoneROCKNodular lime -

stone and shale 1100 Discontinuity with 5cm clay fitting A TI 80 s2,TUNNEL MAPPING - SUPPORT PROJECT NONO 716 28LOCALITY HOL ME N (right I 50000 NO

SUPPORT

B L0 o fi Expansion bolts

Bolts with bands

Grouted bolts

Shotcrete

Mesh reinforcedshotcrete with grouted bolts

Cast concrete arch

S (mr) , ;

76 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON Q - SYSTEM 77

FIG. 3aExample of tunnel mapping using the Q-system: 160 m 2 headrace tunnel. FIG. 3bExample of tunnel mapping using the Q-system: 3.3 m diameter TBM driven sewage tunnel.

=CY T--)-5 --CM77:nF--116m

0

BO

1.0'At

60U.0

CO

02

20

0

BARTON ON 0-SYSTEM 7978 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

SPAN OR DIAMETER (m)

1

60m

40mNODULAR LIMESTONE AND SHALE

20m

==n==r-)==r-)==n 0FIG. 4Vertical section through the VEAS sewage treatment plant, Oslofjord.

General requirements for permanently unsupported openings (i.e., it is preferable that):

J 9, J,.- 1.0, J-- 1.0, J,= 1.0, SRF 2.5 (see Table 2).Conditional requirements for permanently unsupported openings:

If RQD 40, need J 2.If J = 9, need J, 1.5 and RQD

90.If f, = 1.0, need J, < 4.If SRF > 1, need J, % 1.5.If SPAN > 10 m, need J, < 9.

7. If SPAN > 20 m, need 4 and SRF 1.

The shorthand in No. 3 gives the following recommendations:

3. If there are as many as three joint sets (J = 9), then one needs joint roughness (J,) at leastequivalent to rough-planar or smooth-undulating (J, % 1.5) and one needs RQD = 90 (i.e.,"excellent").

Existing natural and man-made openings indicate that very large unsupported spans can besafely built and utilized, if the rock mass is of sufficiently high quality. Our case records describeunsupported man-made excavations having spans from 1.2 to 100 m.

Analysis of Q -System Case RecordsUltimately, a potential user of a classification method will be persuaded of the value of a

particular system by the degree to which he can identify his site in the case records used to developthe given method. In this section, the characteristics of the 212 case records used to develop theQ- system arc analyzed by means of histograms. The potential user can evaluate to what extent hissite fits with the data base, or whether he would be relying on few, extreme value cases, whichwould inherently reduce the reliability of associated support recommendations.

Histograms of the principal parameters used to codify the 212 case records used in the Q-systemhave been developed. The following brief summary describes the extreme values and the mostcommon values of the various factors affecting tunnel stability in the case records considered.

Support Method

The large majority (180) of the 212 case records were supported excavations. Only 32 caseswere permanently unsupported. Support ranged from spot bolting (as little as 50 bolts over a roofarea of 6000 m 2) to very heavy rib-and-rock-bolt-reinforced concrete of 2 to 3 m thickness, poured

1-5 5-10 10-15 15-20 20-30 30-100

FIG. 5Histogram of tunnel spans.

in multiple arch and wall drifts. The predominant form of support in the case records was rockbolts,or combinations of rockbolts and shotcrete, often mesh reinforced. Occasionally, extreme conditionscalled for extreme varieties of support (e.g., 9.8 m long rockbolts on 0.9 m centers together with14.6 m long bolts on overlapped 0.9 m centers) and mesh-reinforced gunite.

Dimensions

The cases studied ranged from unsupported 1.2 m wide pilot tunnels to unsupported 100 mwide mine caverns (Fig. 5). The predominant tunnel dimensions (span or diameter) were 5 to 10m (78 cases) and 10 to 15 m (59 cases). Excavation heights ranged from extreme values of 1.8to 100 m. A significant body of the case records came from hydroelectric projects; consequentlythere were some 40 cases of large caverns with spans in the range of 15 to 30 m and wall heightsin the range of 30 to 60 m.

Depths

Excavation depths ranged from 5 to 2500 m, though most were commonly in the range of 50to 250 m. The Scandinavian bias caused predominantly by Cecil's (1970) case records is shownin Fig. 6. Note that the other case records contribute most of the data on the deeper-seatedexcavations, including numerous hydropower caverns.

RQDThe "rock quality designation" (RQD) ranged in a quite uniform manner from 0 up to 100%.

Forty (40) cases lay in the "very poor" category (0 to 25%) and fifty-three (53) cases in the"excellent" category (90 to 100%).

Number of Joint Sets (.1,)The number of joint sets was most commonly in the range of one set (plus random) to three

sets (plus random). Fifty-two (52) cases, the largest group, had exactly three joint sets. Extremecases consisted of massive, unjointed rock and completely crushed, disintegrated rock.

60 m

40m

20m

0

0-25 25-50 60-100 100-260 250-500 600-250

501-

0

30

20

10

0

16

10

O

80 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES BARTON ON Q-SYSTEM 81

TUNNEL DEPTH (meters)

SCANDINAVIAN CASES (CECIL. 1970)

ME OTHER CASESFIG. 6Histogram of tunnel depths.

Joint Roughness (sr)The joint roughness numbers most commonly found in the 212 case records were 1.0-1.5-2.0,

which represent smooth-planar, rough-planar, and smooth-undulating surfaces, respectively.Extreme values consisted of discontinuous joints in massive rock (16 cases) and plane slickensidedsurfaces (17 cases) typically seen in faulted rock with clay fillings.

Joint Alteration (.L)The joint alteration parameter most commonly seen in the case records was represented by the

number 1.0 (unaltered or unweathered). One hundred and three (103) cases were in this class.However, more than eighty (80) of the case records involved clay mineral joint fillings of variouskinds; these included twelve (12) swelling clay occurrences. Thirteen (13) cases consisted ofhealed joints, which are obviously very favorable for stability and for their inherently lowpermeability.

Joint Water (.1w)The joint water reduction factor describing the degree of water inflow was strongly biased in

the direction of "dry excavations or minor inflow" (

40

30

20

1

0

I BARTON ON 0-SYSTEM

TABLE 6Frequency of occurrence of rock types in examined case records.

83

I. Igneous II. Metamorphic III. Sedimentary

Basalt 1 Amphibolite 8 Chalk 1Diabase 4 Anorthosite (meta-) 1 Limestone 3Diorite 2 Arkose I Marly LimestoneGranodiorite 1 Arkose (meta-) 3 MudstoneQuartzdiorite I Claystone (meta-) 2 Calcareous MudstoneDolerite 1 Dolomite 1 Sandstone 4Gabbro 2 Gneiss 14 Shale 2Granite 46 Biotite Gneiss 1 Clay Shale 2Aplitic Granite I Granitic Gneiss 4 Siltstone 2Monzonitic Granite 1 Schistose Gneiss 2 MarlQuartz Monzonite 2 Graywacke I Opalinus ClayQuartz Porphyry 2 GrecnstoneTuff 2 Schistose meta Graywacke

Quartz HornblendeLeptite IMarbleMylonite 4Pegmatite 2Syenite 1Phyllite 1Quartzite 13Schist 17Biotite Schist 1Mica Schist 2Limestone Schist 1Sparag mite 2

82 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

ROCK MASS QUALITY (0)

.001-.01 .01-0.1 0.1-1.0 1.0-4 4-10 10-40 i400 100-1000

-

-

-

-

-

-

EXCEPT.POOR

EXTREM.POOR

VERYPOOR POOR FAIR 0000

VERY0000

EXT.EXC.

GOOD

FIG. 8Histogram of Q-value for all 212 case records.

The predominant rockmass characteristic in the cases with popping, slabbing, or rockburstingwas relatively massive rock, with few joint sets and/or wide joint spacing. The mean RQD forthese cases was 91% (range 70 to 100%), and the mean J, value was 5.0 (two joint setstwoplus random). Jointing ranged from three widely spaced sets (J = 9) to massive intact rock (J= 1.0).

Squeezing or swelling problems (groups (c) and (d)) were encountered in only nine of the caserecords, although a total of twelve cases were listed as rock containing swelling clay such asmontmorillonite.

Q-ValueThe whole spectrum of rock mass qualities exhibited by the case records is shown in Fig. 8.

As expected, the majority of cases (76%) fall in the central categories "very poor" (Q = 0.1 to1.0), "poor" (Q = I.0 to 4), "fair" (Q = 4 to 10), and "good" (Q = 10 to 40). The wholespectrum of case records utilized in the Q-system ranges from qualities of 0.001 (extreme squeezing)to 800 (essentially unjointed, massive rock).

Rock TypesThe distribution of rock types represented in the case records can be summarized as follows:

igneous rock (13 types), metamorphic rock (26 types), and sedimentary rock (II types). Table 6provides a complete breakdown on the rock types and the number of case record occurrences ofeach type. The statistics are dominated by granite (48) and gneiss (21). However, there aresignificant numbers of case records involving schist (21), quartzite (13), leptite (11), and amphibolite(8). Sedimentary rocks are relatively poorly represented with only 19 cases.

Conclusions

I. The large number of case records utilized to develop the Q-system ensures that reliablesupport recommendations are provided for a very wide range of tunnel sizes, types of excavation,depths, and rock mass qualities.

Detailed analysis of the case records has revealed the overall distribution of individual rockmass parameters such as joint roughness, alteration, and stress-strength ratios, so that extremevalue cases can be readily identified.

Squeezing ground is the only class of problems that is inadequately represented in the originaldata base. Swelling, slabbing, and rock bursting problems are represented in a sufficient numberof case records for reliable determination of support requirements. General tunneling conditionsare extremely well represented, with 160 case records in the range of Q-values from 0.1 (verypoor) to 40 (good).

Fifty individual rock types are represented in the case records. Their characteristics arequantified in such a manner that the individuality exhibited by many rock types is carried all theway through to support selection. Application of the Q-system to other rock types than thosedescribed in the case records can be performed with confidence, provided that any specialcharacteristics of the new rock type are adequately represented in the six parameters. A case inpoint would be the susceptibility to alteration by exposure to moisture. The environment expectedunder tunnel use must always be carefully considered.

The Q-system has been used for several years in conjunction with Norwegian tunnelssupported with fiber-reinforced microsilica shotcrete, a revolutionary new material that is rapidlyreplacing labor-intensive mesh-reinforced shotcrete. Very poor rock qualities previously requiring

84 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES DISCUSSION ON Q-SYSTEM 85

cast concrete arches are being successfully supported by fiber-reinforced shotcrete and rock bolts.Updating of the Q-system for use in countries with access to this new temporary and permanentsupport method is underway at NGI.

BibliographyBarton, N., Lien, R., and Lunde, J., "Engineering Classification of Rock Masses for the Design of Tunnel

Support," Rock Mechanics, Vol. 6, No. 4, 1974, pp. 189-236.Barton, N., Lien, R., and Lunde, J., "Estimation of Support Requirements for Underground Excavations,"

in Proceedings, 16th Symposium on Design Methods in Rock Mechanics, Minn., 1975, published byASCE, New York, 1977, pp. 163-177; discussion on pp. 234-241.

Barton, N., "Unsupported Underground Openings," Rock Mechanics Discussion Meeting, Befo, SwedishRock Mechanics Research Foundation, Stockholm, 1976, pp. 61-94.

Barton, N., Loset, F., Lien, R., and Lunde, J., "Application of the Q-System in Design Decisions ConcerningDimensions and Appropriate Support for Underground Installations," in Proceedings, InternationalConference on Sub-Surface Space, Rockstore, Stockholm, Sub-Surface Space, Vol. 2, 1980, pp. 553-561.

Barton, N., "Characterizing Rock Masses to Improve Excavation Design," Panel Report, Theme II, 4th IAEGCongress, India, 1982.

Bieniawski, Z. T., "Engineering Classification of Jointed Rock Masses," Transactions of the South AfricanInstitution of Civil Engineering, Vol. 15, No. 12, 1973, pp. 335-344.

Bieniawski, Z. T., "Geomechanics Classification of Rock Masses and Its Application in Tunnelling," inProceedings, 3rd International Congress on Rock Mechanics, ISRM, Denver, Colo., Vol. IIA, 1974, pp.27-32.

Bieniawski, Z. T., "Case Studies: Prediction of Rock Mass Behavior by the Geomechanics Classification,"in Proceedings, 2nd Aust.-N.Z. Conference on Geomechanics, Brisbane, Australia, 1975, pp. 36-41.

Bieniawski, Z. T., "Rock Mass Classifications in Rock Engineering," in Proceedings, Symposium Explorationfor Rock Engineering, Johannesburg, A.A. Balkema, Vol. I, 1976, pp. 97-106.

Bieniawski, Z. T., "Rock Mass Classifications in Rock Engineering Applications," in Proceedings, 4thInternational Congress on Rock Mechanics, ISRM, Montreaux, Vol. 2, 1979, pp. 51-58.

Cecil, 0. S., 111, "Correlations of Rock Bolt-Shotcrete Support and Rock Quality Parameters in ScandinavianTunnels," Ph.D. thesis, University of Illinois, Urbana, 1970, 414 pp.

Deere, D. U., Hendron, A. J., Jr., Patton, F. D., and Cording, E. J., "Design of Surface and Near-SurfaceConstruction in Rock," in Failure and Breakage of Rock, C. Fairhurst, Ed., Society of Mining Engineersof AIME, New York. 1967, pp. 237-302.

Deere, D. U., Peck, R. B., Parker, H. W., Monsees, J. E., and Schmidt, B., "Design of Tunnel SupportSystems," Highway Research Record, No. 339, 1970, pp. 26-33.

Einstein, H. H., Steiner, W., and Baecher, G. B., "Assessment of Empirical Design Methods for Tunnels inRocks," in Proceedings, 4th Rapid Excavation Tunneling Conference, AIME, New York, Vol. 1, 1979,pp. 683-706.

Einstein, H. H., Azzouz, A. S., McKown, A. F., and Thompson, D. E., "Evaluation of Design andPerformancePorter Square Transit Station Chamber Lining," in Proceedings, Rapid Excavation andTunneling Conference, Chicago, Ch. 36, Vol. I, 1983, pp. 597-620.

Lauffer, H., "Gebirgsklassifizierung far den Stollenbau," Geologie und Bauwesen, Vol. 24, 1958, pp. 46-51.

Palmstrom, A., "The Volumetric Joint CountA Useful and Simple Measure of the Degree of Rock MassJointing.'' in Proceedings, Fourth Cong. Int. Assoc. of Engineering Geology. New Delhi, India, Vol.V, Theme 2, 1982, pp. V.22I-V.228.

Rabcewicz, L., Bemessung von Hohlraurnbauten, Die "Neue Osterreichische Bauweise" und ihr Einlluss aufGebirgsdruckwirkungen und Dimensionierung, Felsmech. u. Ing. Geol., Vol. I, H. 3/4, 1963, p. 224.

Rabcewicz, L. and Pacher, F., Die Elemente der "Neuen Osterreichischen Tunnelbauweise" und ihregeschichtliche Entwicklung," Osterr. Ing. Zschr., Vol. 18, Jg., 1975, p. 315.

Terzaghi, K., "Rock Defects and Loads on Tunnel Supports," in Rock Tunneling with Steel Supports, R. V.Proctor and T. White, Eds., Commercial Shearing Co., Youngstown, Ohio, 1946, pp. 15-99.

Wickham, G. E., Tiedernan, H. R., and Skinner, E. H., "Ground Support Prediction Model (RSR Concept),"in Proceedings, 1st Rapid Excavation Tunneling Conference, AIME, New York, 1972, pp. 43-64.

DISCUSSION

H. A. D. Kirsten' (written discussion)The table of Barton et al [DI, Table 2] for the jointalteration number (.10 ) is quite complex and relatively open to interpretation. The criteria may besystematized and extended as shown in Table D-1 to overcome these problems to a large extent.

The stress reduction factor (SRF) is not a geological property that varies for different regimesof rock around an excavation. It is a parameter that characterizes the loosening of the rock aroundthe excavation as a whole.

Barton et al provide qualitative criteria for determining SRF for incompetent, non-homogeneousrockmasses [DI, Table 3a1 and competent, homogeneous rockmasses [Dl, Tables 3b, 3c, and34 The non-homogeneities provided for involve single or multiple clay filled weakness or clayfree shear zones. The determination of SRF is open to interpretation, because of the qualitativenature of the criteria and the difficulties that often arise in deciding whether the rockmass ishomogeneous or not. Further difficulties arise in the case of homogeneous rockmasses with regardto assessing the degree of stressing, bursting, squeezing, and swelling.

These problems can be overcome by observing that, in the case of non-homogeneous rock, SRFis related to the overall quality of the rock, Q, and, in the case of homogeneous rock, to the fieldstress state relative to the rockmass strength. The respective relationships are given by theexpressions

TABLE D-1Joint alteration number (1,).

Description of Gouge

Joint Alteration Numberfor Joint Separation of

Less than1.0 mm" mmb

Larger than5.0 mm

Tightly healed, hard, non-softeningimpernieable tilling 0.75

Unaltered joint walls, surface staining only 1.0Slightly altered, non-softening, non-cohesive rock mineral or

crushed rock tilling2.0 4.0 6.0

Non-softening, slightly clayey non-cohesive tilling 3.0 6.0' 10.0'Non-softening strongly over-consolidated clay mineral tilling,

with or without crushed rock3.0' 6. 10.0

Softening or low friction clay mineral coatings and small quantitiesof swelling clays

4.0 8.0' 13.0'

Softening moderately over-consolidated clay mineral tilling, withor without crushed rock

4.0' 8.0 13.0

Shattered or micro-shattered (swelling) clay gouge, with or

without crushed rock5.0' 10.0 18.0

Joint walls effectively in contact.b Joint walls come into contact before 100 mm shear.

Joint walls do not come into contact at all upon shear. Also applies when crushed rock is present in clay gouge and there is no rock wall contact.

Figures added to original data to complete sequences.

' Steffen, Robertson, and Kirsten, P. 0. Box 8856, Johannesburg 2000, South Africa.

/00

/0

5

/.0

0./

i`o

Z0

c r4

o R 2Cl...se 2E ..?:_.0 1' 0-) S ;

Contours,..,/-- for P h /

F4

.

J w z' 1,0

of SRFQ n .-1.0

Pc Pe.00e.0%

ei)11,7CIP0 / 6.4

/41.,,Z,o (ROD/JII)(JrAtIksq

/Jo)

,

,../0VO

P13Ltvo.3-.0

0,,,099,,, , .C15'

TN.,n5'*-

E

?iii

itiC>....-:

1$:60.

. 48.- 0C C elle 29rErta)0 .7,0 kE o

Contoursf n r n in

increasingloosening -.1,-4.--e-burstingpotential

r'ili)of SRF

---1 n \

increasing

potential-

1,0( H / UCS)

FIG. D-1Graphs of QhIQ, versus H/UCS for range of (RQD1l)(.1,11).

SRF = 1.809 Q -0329SRFh = 0.244 K 0346 (H/UCS)' 722 + 0.176(UCS/H)' 413

whereQ = (RQD/J)(J,JJ)(J/SRF),K maximum-to-minimum principal field stress ratio,H = head of rock corresponding to maximum principal field stress, m, and

UCS = unconfined compressive strength of rock, MPa.

The corresponding rockmass indices are given by the expressions

Qn = I ( RQD/J,,)(Jr/.1 )(4)1 .809)I"' - 329)Q, = (RQD/J)(,/,/,/)1,//0.244 e346(H/UCS)1 322 + 0.176(UCS/H) 1 4 I

The ruling value for Q may be determined by calculation as the minimum of these twoexpressions. The corresponding value for SRF may also be determined. In the event of this valuecorresponding to SRF h it may further be established which of the two terms in the expression forSRFh is the larger. These two terms represent respectively the stress and structurally controlledbehavior of competent rock.

The proposed procedure for determining SRF replaces that given by Barton et al IDI] and is

HEAVYROCK BURSTSQUEEZINGSWELLING

MILDROCK BURSTSOUSE ZINGSWELLING

MINIMALFRACTURINGSQUEEZINGSWELLING

NOFRACTURINGSQUEEZINGSWELLING

IIIIII11111.1n1111111111

III1111111illi IlaN=1 II11111I

:"." 4' Z

,...:........MMENEWMEM

roil1 IEEE".IIIIMIIIIMnleMMMINMENOIN=MO

11111111111=11011LBINIIMINIMINIIMICM III II IIIIMMOMMIl'

Illiek..III

I IIii

--4111alimmonNMI.

Mn10=113= e6,

__ mo IN mmiumnImlommmannsm211112INIMEIMIIELEINIMIoG END:g22/ TEFCAGHI(after Barton et al (1)) 111MMIIIII=11111+ BA RTON TABLE 3(al

...J.., to Kirsten)lCi CECIL(after Barton et d Mt

1lIllibrAlill 11111111111irEormdleiliiiikin_... fratOMIMMii. imam,

I II Stress Reduction WollSRF .8090 0,329(applicable to incompetent rock)

=linSELF SUPPORTED CASES(otter Barton et al (I))ii X R L m 1111POOR POOR

^

00 V RYPOOR GOODliMill XCGOOD GOOD4 40 /00 400 1000

ROCK MASS QUALITY 0

FIG. D-2Plot of stress reduction factor versus rock mass quality.

illustrated graphically in Fig. D-1 for K = 1.0 and J,. = 1.0. The significance of the proposedalternative procedure is illustrated in Fig. D-2. The straight-line graph corresponding to SRF =1.809 Q-"9

represents the squeezing behavior associated with shear or weakness zones inincompetent ground. As such it represents the lower bound values for SRF. The domain abovethe graph represents the loosening behavior of competent rock, to which the expression for SRFin terms of K, H, and UCS applies.

The various case histories shown plotted in Fig. D-2 confirm the proposed alternative determinationof SRF. The correspondence between Barton's and Terzaghi's implied relationships between SRFand Q for incompetent ground is remarkable. The fact that Terzaghi worked in tunnels which weregenerally of a squeezing nature and not self-supporting is confirmed in Fig. D-2. It is also evidentthat tunnels in ground in which the loosening behavior is governed by shear or weakness zonesor by compressible gouge on the joints are generally not self-supporting.

The behavioral characteristics of the ground surrounding a tunnel are detailed in Table D-2.The proposed direct calculation of SRF from a limited number of geological parameters is the

only quantification of the loosening potential or stress relaxational behavior of tunnelled groundavailable to date. Barton and his co-authors should be congratulated for this contribution, althoughthey were not explicitly aware of the quantitative implications of their findings.

0.00/ 0.006 05 4/00

/Op

Ca

0 /,

0/

0,0,0/

DISCUSSION ON Q-SYSTEM 8786 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

88 ROCK CLASSIFICATION SYSTEMS FOR ENGINEERING PURPOSES

TABLE D-2Behavioral characteristics of tunnelled ground in terms of SRF (J,, = K = 1.0)Stress

ReductionControlling Factor

Criterion (SRF)

ProbableRock Mass

Quality(0

Excavation-InducedGround

Condition

Excavation-Nature ofInduced Ground-Ground Supporting

Displacement Action

Larger than 10 0.001-25/ Heavy rock-burst- Indefinite creep Visco-plastic

Stress in re-spect ofcompetentrock/Shearor weakness

5-10

0.001-0.006

0.006-50/0.006-0.05

ing, squeezing,or swelling

Mild rock-bursting,squeezing, orswelling

arch

Terminating creep Partially plasticarch

zones in re-spect of in-competentground

2.5-5 0.05-100/0.05-0.4

Minimal fracturing,squeezing, orswelling

Large elastic de- Highly stressedformation elastic arch

0.4-2.5 0.4-640/ No facturing, Moderate elastic Moderately0.4-100 squeezing, or

swellingdeformation stressed elastic

arch0.4I 6.0-640 No fracturing,

squeezing, orswelling

Limited elastic

Lowly stressed

deformation elastic arch

Geologicalstructure in

1-2.5 0.4-250 No fracturing,squeezing, orswelling

Random block Lowly stressed

displacement

elastic arch

respect ofcompetent

2.5-5 0.05-10() No fracturing,squeezing, or

Random block Keystone arching

displacement by mechanicalrock swelling interlocking of

blocksLarger than 5 0.001-50 No fracturing,

squeezing, orswelling

Block fall outs or

Non-arching in

runs of ground completelyloosenedground

The two probable ranges of Q correspond to the given controlling criteria.

Discussion References[DI] Barton, N., Lien, R., and Lunde, J., "Engineering Classification of Rock Masses for the Design of

Tunnel Support," Rock Mechanics, Vol. 6, No. 4, 1974, pp. 189-236.

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