modifications to the geological strength index (gsi) and their applicibility to slopes

Modi®cations to the geological strength index (GSI) and theirapplicability to stability of slopes

H. Sonmez, R. Ulusay*

Geological Engineering Department, Applied Geology Division, Faculty of Engineering, Hacettepe University, 06532 Beytepe, Ankara, Turkey

Accepted 10 May 1999

Abstract

Determination of the strength of closely jointed rock masses is di�cult since the size of representative specimens is too large

for laboratory testing. This di�culty can be overcome by using the Hoek±Brown failure criterion. Since its introduction in 1980,the criterion has been re®ned and expanded over the years, particularly due to some limitations in its application to poor qualityrock masses. In the latest version, the geological strength index (GSI) was introduced into the criterion by its originators.

However, the GSI classi®cation scheme, in its existing form, leads to rough estimates of the GSI values. Another particular issueis the use of undisturbed and disturbed rock mass categories for determining the parameters in the criterion, for which clearguidelines are lacking. Furthermore, the data supporting some of these revisions, particularly the latest one, have not been

published, making it di�cult to judge their validity. In this study, in order to provide a more quantitative basis for evaluatingGSI values, some modi®cations are suggested by introducing easily measurable parameters with their ratings and/or intervalswhich de®ne the blockiness and surface condition of discontinuities. In addition, a method is proposed to assess the in¯uence of

disturbance on rock mass constants due to the method of excavation. The modi®cations to the GSI and the suggested methodhave been applied to slope instability case histories selected from Turkey by performing back analysis, to discuss the validity ofthe criterion and the methodology of parameter estimation. It was shown that the failure conditions in each case werecon®rmed, i.e. the analysed failure surfaces satis®ed factors of safety of unity, when the suggested modi®cations and disturbed

rock mass condition are considered. On the basis of the results, a chart to assess the e�ect of disturbance in terms of method ofexcavation was also suggested. The back analysis of a spoil instability indicated that spoil pile materials consisting of blocky andangular rock pieces could be categorized as a disintegrated rock mass in the GSI classi®cation and the criterion seemed to be

applied to such materials. The method suggested herein must, however, be veri®ed by additional data from slope failures beforemore precise guidelines can be formulated. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

The standard method for assessing the strength of a

geotechnical material is to recover representative

samples and test them in the laboratory. In the case of

a closely jointed rock mass it is clearly not possible to

recover a sample that is large enough to represent the

joint system. In order to overcome the di�culties in

laboratory determination of the shear strength of

jointed rock masses, the empirical Hoek±Brown failure

criterion [1] is commonly used in conjunction with theGeomechanics Classi®cation System [2].

This failure criterion has been re®ned and expandedover the years [3±7] as summarized in Table 1. Thelimitations in Bieniawski's RMR classi®cation scheme[2] for very poor quality rock masses and for unrealis-tic rating adjustments for discontinuity orientation inslopes have necessitated some signi®cant changes inthe criterion. This is probably one of the main reasonswhy the originators of the technique continue modify-ing their criterion. Recently, Hoek and Brown [6±9]proposed the geological strength index (GSI) basedupon the visual impression on the rock mass structure.Fig. 1 shows twenty codes to identify each rock mass

International Journal of Rock Mechanics and Mining Sciences 36 (1999) 743±760

0148-9062/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0148-9062(99 )00043 -1

www.elsevier.com/locate/ijrmms

* Corresponding author. Tel.:+90-29-777-62.

E-mail address: [email protected] (R. Ulusay)

category and estimate the GSI value. It is also noted,on the basis of their recent studies on the Athensschist, Hoek et al. [9] introduced a new rock mass cat-egory into the GSI system called `foliated/laminatedrock mass structure'. This new category accommodatesthinly foliated, folded and predominantly shearedweak rocks of non-blocky structure. The equivalentGSI contours range from a new value of 5 up to 30 inthe lower right portion of the disintegrated rock masscategory.

The latest version of the GSI chart [8] (Fig. 1) is suf-®cient for ®eld observations, since it is only necessaryto note the letter code which identi®es the rock masscategory. The GSI also seems a more practical par-ameter to estimate the strength of jointed rock masses

from ®eld observations when compared to the methodemploying rock mass classi®cation. Because rock massclassi®cation requires time consuming procedures andhas some limitations as discussed by Sonmez et al. [10]in detail. However, due to lack of measurable andmore representative parameters, and related intervallimits or ratings for describing the surface conditionsof the discontinuities, value of the GSI for each rockmass category appearing in Fig. 1 represents a rangeof values. For example, for a blocky rock with verygood surface condition of discontinuity (B/VG), GSIvalues varying between 63 and 85 are obtained fromFig. 1. This consideration placed focus on the question``how can a more precise GSI value be obtained fromthe existing chart for design?''. Hoek [11] indicates that

Fig. 1. Characterization of rock masses on the basis of interlocking and surface condition of discontinuities: GSI classi®cation (rearranged from

Tables 3 and 4 given by Hoek and Brown [8]).

H. Sonmez, R. Ulusay / International Journal of Rock Mechanics and Mining Sciences 36 (1999) 743±760744

although some geologists go to extraordinary lengthsto try to determine an `exact' value of GSI or RMR,geology does not lend itself to such a precision and itis simply not realistic to assign a single value. He alsostates that for preliminary ®eld investigations or low-budget projects, it may be prudent to assume largerstandard deviations for the input parameters (uniaxialcompressive strength of the intact rock, intact rockmaterial constants and GSI) and they can be rep-resented by normal distribution. Although its origin-ators have always pointed out the criterion'sapproximate nature, it seems questionable to obtain amean value represented by a normal distribution fromthe existing form of the GSI chart. It is also con-sidered that it is possible to estimate di�erent GSI

values for the same rock mass by di�erent persons,depending on their personal experience, when the chartgiven in Fig. 1 is employed.

From the review of the criterion, it is clear that thecontinuous update of the Hoek±Brown failure cri-terion has not been complemented by equal e�orts toverify the same. Furthermore, the data supporting ofthese revisions have not been published, making it dif-®cult to judge their validity. One important issue is useof the undisturbed and disturbed rock mass categorieswhen determining the parameters in the criterion forwhich clear guidelines are lacking. Any disturbance onthe rock mass due to some local factors (e.g. blasting,the presence of discrete fault zones, etc.) should beconsidered and, therefore, an adjustment should be

Table 1

Historical development of the Hoek±Brown criterion (rearranged from Hoek and Brown [8])

Publication Coverage Equations

Hoek and Brown

[1]

original criterion for heavily jointed rock masses with no ®nes; Mohr

envelope was obtained by statistical curve ®tting to a number of (sn 't ) pairs calculated by the method published by Balmer [28]. s1 ', s3 'are major and minor e�ective principal stresses at failure,

respectively; s1 is the tensile strength of the rock mass, m and s are

material constants; sn ', t are e�ective normal and shear stresses,

respectively

s 01 � s 03 � sci

��ms 03=sci

p � s; s1 ��sci=2��mÿ

��m2p

� 4s�; t � Asci��s 0n ÿst�=sci�B;s 0n � s 03 � ��s 01 ÿ s 03 �=�1� @s 01=@s 03 ��; t � �s 0nÿ s 03 �

��@s 01@s

03

p; @s 01=@s

03 � msci=2�s 01 ÿ s 03 �

Hoek [3] original criterion for heavily jointed rock masses with no ®nes with a

discussion on anisotropic failure and solution for the Mohr envelope

by Dr. J.W. Bray

s 01 � s 03 � sci

��ms 03=sci

p � s; t � �cot f 0i ÿcos f 0i �msci=8; f 0i � arctan�1=

��4hcos2yÿ 1p

�;y � �90� arctan�1=

��h3pÿ

1�=3�; h � 1� �16�ms 0n � ssci�=�3m2sci��Hoek and Brown

[4]

as for Hoek [3] but with the addition of relationships between

constants m and s and a modi®ed form of RMR (Beniawski) [2] in

which the groundwater rating was assigned a ®xed value of 10 and

the adjustment for joint orientation was set at 0; also a distinction

between disturbed and undisturbed rock masses was introduced

together with means of estimating deformation modulus E

disturbed rock masses:

mb/mi=exp((RMRÿ100)/14);s= exp((RMRÿ100)/6); undisturbed or

interlocking rock masses:

mb/mi=exp((RMRÿ100)/28);s= exp((RMRÿ100)/9); E = 10((RMRÿ10)/40);mb, mi are for broken and intact rock, respectively

Hoek et al. [5] modi®ed criterion for account for the fact the heavily jointed rock

masses have zero tensile strength; Balmers technique for calculating

shear and normal stress pairs was utilised

s 01 � s 03 � sci�mbs 03=sci�a; s 0n � s 03 � ��s 01 ÿs 03 �=�1� @s 01=@s 03 �� ;t � �s 0n ÿ s 03 �

��@s 01=@s

03

p; @s 01 �

1� amab�s 03=sci��aÿ1�

Hoek [6] and

Hoek et al. [7]

introduction of the generalised Hoek±Brown criterion, incorporating

both the original criteiron for fair to very poor quality rock masses

and the modi®ed criterion for very poor quality rock masses with

increasing ®nes content; The geological strength index, GSI, was

introduced to overcome the de®ciencies in Bieniawski's RMR for

very poor quality rock masses; the distinction between disturbed and

undisturbed rock masses was dropped on the basis that disturbance

is generally induced by engineering activities and should be allowed

for by downgrading the value of GSI

s1 '=s 3 '+sc (ms3 '/sci+s )a;

for GSI>25;

mb/mi=exp((GSIÿ100)/28);s= exp((GSIÿ100)/9); a=0.5; for GSI < 25;

s= 0; a=0.65ÿGSI/200

Hoek and Brown

[8]

as for Hoek [6] and Hoek et al. [7], but with the addition of a chart

to estimate GSI based upon the visual impression of the rock

structure and the surface condition of the discontinuities indicated

by joint roughness and weathering

as for Hoek [6]

Hoek et al. [9] as for Hoek and Brown [8], with the addition of a new `foliated/

laminated' rock mass structure category to accommodate thinly

laminated or foliated, folded and predominantly sheared weak rocks

of non-blocky structure

as for Hoek [6]

H. Sonmez, R. Ulusay / International Journal of Rock Mechanics and Mining Sciences 36 (1999) 743±760 745

applied to RMR values [2]. Although the rock massclassi®cation schemes and the Hoek±Brown criterionhave been generally applied in tunnelling and under-ground mining, the back analyses of slope failures inheavily jointed rock masses, by assuming disturbedrock mass conditions showed good agreement betweenthe estimated and the back-calculated strength par-ameters from the observed slope failures [10]. On thecontrary, in the latest version of the criterion [7,8]average undisturbed in situ conditions are consideredto estimate the GSI without application of any adjust-ment. Hoek and Brown [8] indicated that one of thepractical problems which arose when assessing thevalue of GSI in the ®eld was related to blast damage.According to these investigators, where all the visiblefaces have been damaged by blasting, some attemptshould be made to compensate for the lower values ofGSI obtained from such faces. Also, in recently blastedfaces, new discontinuity surfaces occurring due toblasting will give a GSI value which may be as muchas 10 points lower than that for the undisturbed rockmass. Therefore, Hoek and Brown [8] suggest thatsevere blast damage can be allowed for by moving upone row in Fig. 1. This approach may be correct forthe estimation of GSI from blasted rock exposuresduring excavation. However, moving the GSI value upone row seems to be a rough approach and also resultsin an increase in uniaxial compressive strength of therock mass by more than 70%. Therefore, the reason ofthis assumption is still open to discussion. On theother hand, method of excavation, major planes ofweakness or change in stress are treated as local fea-tures which have in¯uenced the rock mass at a particu-lar location, and are not rock mass constants [12±14].Therefore, in order to compensate the in¯uence ofsuch local factors, necessary adjustments should betaken into consideration. An additional practical ques-tion arising from the latest version of the Hoek andBrown's approach is ``how can the in¯uence of themethod of excavation can be taken into account byusing the existing Hoek±Brown's equations which con-sider only undisturbed rock mass, when the GSI isestimated from borehole cores or natural exposuresbefore excavation or blasting?''. It is also noted thatthere is no any published case history on the backanalysis of slopes in heavily jointed rock masses whichcon®rms that the latest GSI classi®cation yields satis-factory results when an adjustment factor is not takeninto consideration.

This paper is an attempt to provide a more quanti-tative numerical basis for evaluating GSI by introdu-cing new parameters, and ratings, such as surfacecondition and structure rating. For meaningful in-terpretation and for providing a common basis forcommunication between engineers and designers, stan-dard interval limits and ratings for the input par-

ameters are suggested. In addition, a method isproposed to assess the in¯uence of disturbance. Themodi®cations and the suggested method have beenapplied to slope instability case histories from Turkeyto discuss the validity of the criterion and the method-ology of parameter estimation.

2. Suggested modi®cations for estimating GSI values

Once the GSI has been estimated, the parameterswhich describe the rock mass strength characteristics,are calculated as follows:

mb � mi exp

�GSIÿ 100

28

��1�

For GSI>25, i.e. rock masses of good to reasonablequality,

s � exp

�GSIÿ 100

9

��2�

and

a � 0:5 �3�For GSI < 25, i.e. rock masses of very poor quality,the criterion applies with

s � 0 �4�

a � 0:65ÿ GSI

200�5�

From the above equations it is clear that the rockmass strength parameters are sensitive to the GSIvalue. The lack of parameters to describe surface con-ditions of the discontinuities and the rock mass struc-ture prevents to obtain a more precise value of GSI.For these reasons, the authors suggest two termsnamely, `structure rating, SR' based on volumetricjoint count (Jv) and `surface condition rating, SCR',estimated from the input parameters (e.g. roughness,weathering and in®lling).

The suggested ratings by the RMR system [2] forthese parameters are selected for the purpose.According to the rating of each input parameter (Rr,Rw and Rf ) estimated from the right upper margin ofthe table given in Fig. 2, the total rating for surfaceconditions (SCR) is obtained using the following ex-pression:

SCR � Rr � Rw � Rf �6�where Rr, Rw and Rf denote the ratings for roughness,weathering and in®lling, respectively. Since the sum of


the maximum ratings of these parameters is 18, theSCR axis in Fig. 2 is divided into 18 equal divisions.

In the earlier version of the criterion (Fig. 1), Hoeket al. [7] used the terms BLOCKY/SEAMY andCRUSHED, following the terminology proposed byTerzaghi [16]. After they recognized that these termsproved to be misleading, they have been replaced, byBLOCKY/DISTURBED and DISINTEGRATED.

The authors agree with this change. On the otherhand, Hoek et al. [9] proposed a new rock mass cat-egory to accommodate thinly foliated or laminated,folded and predominantly sheared weak rocks of non-blocky structure. However, Hoek [6] emphasizes thatthe criterion is only applicable to intact rock or toheavily jointed rock masses which can be consideredhomogenous and isotropic. On the contrary, the

Fig. 2. The modi®ed GSI classi®cation suggested in this study.


strength and deformability characteristics of such rockmasses are governed by the displacements along thenumerous very thinly spaced presheared and slicken-sided foliation planes. Due to anisotropic and inhom-ogenous features of such rocks, introducing of thisnew category into the GSI scheme seems not to be rea-listic. The other questionable issue is that the validityof the GSI values assigned for this new category hasnot been con®rmed yet by case studies. Therefore, onlyfour structural categories as previously suggested byHoek and Brown [8] are considered in this study.

Block size is an extremely important indicator of arock mass. Large blocks tend to be less deformableand develop favourable arching and interlocking inunderground openings. In the case of slopes, a smallblock size may cause rotational slides instead of struc-turally controlled modes of failure. Block dimensionsare determined by three rock mass parameters, namelydiscontinuity spacing, the number of discontinuity setsand the persistence of the discontinuities delineatingpotential blocks. However, in order to decrease thenumber of inputs, the use of a single parameter whichcan take into account one or two of the above men-tioned parameters was considered to be more practical.Thus, volumetric joint count (Jv), which is de®ned asthe sum of the number of joints per meter for eachjoint set present, is suggested to be used for thedescription of structure of the rock mass. Jv is esti-mated by one of the following expressions:

Jv � N1

L1� N2

L2� . . .� Nn

Ln�7a�

Jv � 1

S1� 1

S2� . . .� 1

Sn�7b�

where S is the true spacing, N is the number of jointsalong a scanline, L is the length of the scanline and nis the number of joint sets.

On the other hand, estimation of Jv for heavilyjointed rock masses with no identi®able structural pat-tern is extraordinarily di�cult. Since the discontinuitiesin such rock masses do not introduce considerabledi�erences in their spacing in all directions, they canbe assumed as homogeneous and isotropic. Therefore,expressions given in Eqs. (7a) and (7b) are not advisedto be used for the determination of Jv. Instead ofthese, the authors suggest the following approachwhich is more practical in the estimation of the num-ber of discontinuities in a rock mass with a volume of1 m3.

Jv � Nx

Lx� Ny

Ly� Nz

Lz�7c�

where Nx, Ny and Nz are the number of discontinuities

counted along the scanlines (Lx, Ly and Lz ) perpen-dicular to each other. However, in some cases it canbe di�cult to ®nd exposures along which three scan-line surveys in perpendicular directions can be carriedout. In such circumstances, by assuming the rock massis homogeneous (i.e. the terms appearing in Eq. (7c)are equal to each other), Eq. (7c) can be rewritten inthe following form.

Jv ��N

L

�3

�7d�

The intervals of Jv and related descriptionssuggested by ISRM [15] were adopted for the blocki-ness categories to be used in the GSI classi®cation(Table 2). Based on the intervals of Jv and correspond-ing descriptions for the blockiness ratings, structuralrating (SR) was assigned to each category by the fol-lowing procedure.

1. Using a semi-logarithmic sheet, SR and Jv are puton y and x axes, respectively.

2. While the SR axis is divided into the ratings rangingfrom 0 to 100, logarithmic Jv axis is divided accord-ing to the boundaries suggested for four structuralcategories as described in Table 2. The upper limiton the Jv axis is selected as 104 to consider pebblesize.

3. Since the boundaries between the structural cat-egories in the existing GSI table are equally divided(Fig. 1), the SR limits between the codes B-VB, VB-B/D and B/D-D are selected as 75, 50 and 25, re-spectively.

4. By plotting the Jv values for each category assuggested in Table 2 against the boundary values ofSR mentioned in item (c), the curve shown in theleft margin of Fig. 2 is obtained. This curve can beused to assign a rating for SR of any rock massusing the value of Jv.

It is now possible to estimate a more precise GSIvalue from the intersection point of SCR and SR rat-ings when the modi®ed GSI chart (Fig. 2) is used.

Table 2

Descriptive terms corresponding block size and intervals of Jvsuggested by ISRM [15] and by the authors of this study

Descriptions by ISRM Jv (joint/m3) Descriptions for GSI (this study)

Very large blocks < 1 BLOCKY (B)

Large blocks 1±3 BLOCKY (B)

Medium sized blocks 3±10 VERY BLOCKY (VB)

Small blocks 10±30 BLOCKY/DISTURBED (B/D)

Very small blocks 30±60 DISINTIGRATED (D)

Crushed >60 DISINTIGRATED (D)


3. Validity of the Hoek±Brown estimates of rock massand their impact on the assessment of stability of slopes

3.1. Theoretical background of the suggestedmethodology for the assessment of disturbance e�ect

Some factors, such as method of excavation, majorplanes of weakness or change in stress, are treated aslocal features in¯uencing the rock mass at a particularlocation and are not rock mass constants. These havebeen discussed by Laubscher [12], Romana [13] andKendorski et al. [14]. The greatest in¯uence of themethod of excavation will be on the spacing of discon-tinuities. Depending on the blasting damage, blastedslopes may have closer discontinuity spacing thannatural slopes. Therefore, in order to compensate thein¯uence of such local factors, necessary adjustments[2,12±14] are taken into consideration in rock massclassi®cation. Sonmez et al. [10] showed that by assum-ing an adjustment factor based on the method of exca-vation (disturbed rock mass), a good agreement wasfound between the estimated and the back-calculatedstrengths from the observed slope failures and, there-fore, e�ect of disturbance should be taken into con-sideration.

In order to check the validity of the equations usedfor the rock mass constants and to assess the e�ect ofdisturbance, four approaches with the use of modi®edGSI chart have been suggested and applied to failedslopes from Turkey (Fig. 3). Four cases were selectedfrom the failures of the pit slopes in heavily jointedrock masses where joint spacing is a fraction of ameter and one is from the failures occurred in spoilpiles in Turkey. The authors suggest that spoil pilescomposed of rock materials possess the behaviour ofpoorly interlocked, heavily broken rock masses with amixture of angular and rounded rock pieces and,therefore, can be considered in disintegrated rock masscategory. On the basis of this assumption, a spoil pilefailure was also examined in this study.

The back analysis procedure starts with the determi-nation of the GSI value of the rock mass investigatedin each case from the modi®ed chart (Fig. 2) accordingto the suggested input parameters. Then the followingapproaches are employed in order to check the validityof the equations:

. Approach 1: the shear strength parameters of thefailed rock masses are estimated using the originalexpressions given by Eqs. (1±5) without applicationof any adjustment for the method of excavation, i.e.undisturbed rock mass condition is assumed. Theseparameters are then used in the back analyses forthe calculation of factor of safety.

. Approach 2: a disturbance (adjustment) factor of(df ) depending on the method of excavation [12,14]

is assigned in each case and multiplied by the GSIvalue appearing in the denominator of Eqs.(1), (2)and (5). The parameters determined in this way arethen employed in the back analyses.

. Approach 3: a disturbance factor (df ) is assigned ineach case and multiplied by the numbers appearingin the numerator (28 and 9) of Eqs. (1) and (2) forthe estimation of the rock mass parameters, mb ands.

. Approach 4: in the previous form of the criterionthe numbers appearing in the denominator of theequations are 28 and 14 for mb, and 6 and 9 for sfor undisturbed and disturbed rock mass conditions,respectively (Table 1). In other words, it seems to belogical to conclude that the denominator of theseequations vary between 14 and 28, and 6 and 9depending on the degree of disturbance. Based onthis fact, Eqs. (1) and (2) can be rewritten in the fol-lowing forms:

mb � mi exp

�GSIÿ 100

bm

��bm � 14ÿ 28� �8�

s � exp

�GSIÿ 100

bs

��bs � 6ÿ 9� �9�

In approach 4, ®ve di�erent values are assigned tobm (starting from 28 to 14) and to bs (starting from 9to 6) and then the value of factor of safety (FOS) cor-responding to the pairs of bm and bs for each particu-lar case history is calculated. The results of theanalyses are presented in the form of FOS±bm andFOS±bs curves. From these curves, bm and bs valueswhich lead a value of factor of safety of unity areobtained for each case as depicted in Fig. 4(a). Thevalues of bm and bs from the curves are then plottedagainst corresponding disturbance factors (df ) to estab-lish a relationship between bm, bs and df (Fig. 4b), andto check the validity of the Hoek±Brown estimates.

3.2. Software description

In this study, a computer program, HOBRSLPdeveloped and described by Sonmez et al. [10] wasemployed. The program HOBRSLP was modi®ed forthis study to include the approaches described above.It can handle slope stability analysis of circular andnon-circular slip surfaces for slopes involving manybenches with di�erent geometries, various materialsand di�erent groundwater conditions. It also incorpor-ates external loading conditions.


Fig.3.Locationmapoftheback

analysedcase

studysitesandviewsfrom

theinvestigatedslopeinstabilities:

(a)initiationoftheinstabilityin

thehighwallexternallyloaded

byaspoilpilein

Eskihisarstripcoalmine;

(b)aview

from

theheavilybroken

schistrock

mass

atBaskoyakbarite

mine;

(c)aview

from

thejointedrock

mass

inKisrakdereopen

pitcoalmine;

(d)bench

failure

inaclosely

jointedmarlyrock

mass

inHim

metoglu

ligniteopen

pitmineand(e)aview

from

theslopeinstabilityin

aspoilpileatEskihisarstripcoalmine.


3.3. Description of the rock mass conditions and theexamined slope instabilities

3.3.1. Case 1: an externally loaded highwall slopefailure

This case history involves the instability of a high-wall in Eskihisar (Yatagan-Mugla) strip coal mine insouthwestern Turkey. During a comprehensive slopestability research project by Ulusay [17], an instabilityof the southern part of the highwall of the ninth slicehas occurred due to external loads by a temporaryspoil pile (Fig. 3a).

The failed slope was excavated in marl which liesabove the coal seam with a thickness of 15±20 m.

There are highly persistent (about 8 m) three dominantjoint sets developed parallel and/or subparallel to nor-mal faults crossing the Tertiary deposits. The presenceof cross joints, faults and ¯at lying bedding planesresults in a closely jointed rock mass. The groundwaterlevel rises above the coal seam and tends to declinetoward the marl±coal seam boundary. Thus, the failedpart of the investigated slope was dry.

The structure rating (SR) and surface condition rat-ing (SCR) were estimated from the scanline surveydata obtained by Ulusay [17] (Table 3). In this pit acontrolled blasting is carried out with slight damage toloosen the marly overburden. For this condition, ablasting damage adjustment of 0.94 [14] was assigned.The average uniaxial compressive strength and mi ofthe intact marl specimens were 4.15 MPa and 9.87, re-spectively [17]. Based on available monitoring recordsby Ulusay [17] the model depicted in Fig. 5 for theslope under the in¯uence of a symmetrical vertical tri-angular spoil loading, and the failure surfaces illus-trated in Fig. 6 were used in the analyses. Averagevalues of unit weight of 13 and 16 kN/m3 were utilizedfor the spoil material (in-situ) and the marls, respect-ively. It was reported by Ulusay [17] and Sonmez et al.[10] that the back analyses, using the previous form ofthe Hoek±Brown estimates [4], which includes theRMR scheme, con®rmed these failure surfaces.

3.3.2. Case 2: slope failure in a closely jointed rockmass at a barite open pit mine

A comprehensive slope stability project was carriedout at Baskoyak barite open pit mine, in westernAnatolia between 1987 and 1988 by Ulusay and Yucel[18]. Based on the scanline surveys and a geotechnicalborehole, it was reported that the schist should beregarded as comprising two types of rock mass [18].The ®rst type consists of a heavily broken schist rockmass by closely spaced discontinuities and schistosityplanes (Fig. 3b) and the second type is a weatheredschist in di�erent degrees. Due to the heavily jointed

Fig. 5. The model with the parameters for the slope under the in¯u-

ence of a symmetrical vertical triangular spoil loading in the back

analysis of the failed slope in case 1.

Fig. 4. Basic concept of approach 4 to check the validity of the

Hoek±Brown equations.


nature of the schist, the rock mass was assumed ashomogeneous and isotropic with a joint spacing of0.04 m in all directions and, therefore, Jv of the rockmass was estimated by using Eq. (7d). The mean unitweight and uniaxial compressive strength of the heav-ily broken part of the schist are 22.2 kN/m3 and 5.2MPa, respectively. The rock mass properties are tabu-lated in Table 3. No sign of groundwater was encoun-tered through the geotechnical and previously drilledboreholes and on the pit benches. Thus, the pit slopeswas considered as dry for stability assessments. Sincethe overburden material and the ore are removed byexcavators without any blasting, an adjustment factorof 0.97 [14] was considered. One of three failureswhich occurred in closely jointed rock mass along a

Fig. 6. Slope pro®les, and the predicted and calculated failure sur-

faces employed in the back analysis of the externally loaded highwall

slope in case 1.

Table

3

Theparametersem

ployed

intheGSIclassi®cationfor®vecasesconsidered

inthisstudy

Parameters

Case

1Case

2Case

3Case

4Case

5

Spacinga(m

)S1=

0.71,S2=

0.82,

S3=

1.26,Sb=

0.65

Sx,y,z=

0.04

S1=

0.75,S2=

1.07,

S3=

0.13,Sb=

0.4

S1=0.37,S2=0.65,Sb=0.11

Sx=0.085c,Sy=0.081,Sz=0.083

Conditionof

discontinuities

andratings

smooth

surfaces(1),

slightlyweathered

(5),

softcoating<

5mm

(2)

smooth

toslickensided

surfaces(1),

highly

weathered

(1),

softcoating<

5mm

(2)

smooth

surfaces,(1),

slightlyweathered

(5),

softcoating<

5mm

(2)

slickensided

surfaces(0),

moderately

weathered

(3),

softcoating<

5mm

(2)

smooth

surfaces,(1),

slightlyto

moderately

weathered

(4),

softcoating<

5mm

(2)

Jv

6.14

15635

12.5

13.3

1773

SR

63

042

35

4

SCR

84

85

7

GSIb

43

16

37

27.5

26

dfd

0.94

0.97

0.90

0.97

0.80

aTruespacing(S

1,S2,S3forjoints,Sbforbeddingplanes).

cEstim

atedbymethodofphotoanalysisalongx,yandzaxes.

bGSIdetermined

from

themodi®ed

chart

inFig.2.

dAdjustmentfactorfordisturbance

e�ect.


circular surface was selected for this study. The resultsof the back analysis [18,10] indicated that the calcu-lated sliding surface con®rms the actual failure surfacedelineated from the site measurements (Fig. 7).

3.3.3. Case 3: a slope instability in a coal mine atwestern Turkey

A slope instability from the Kisrakdere open pitmine located at Soma lignite basin, western Turkey,was selected for the purpose of this study (Fig. 3c).The necessary data were collected by the authors fromthis pit. Fig. 8 shows the geometry of the failed slopein which a single thin coal seam with a thickness of 4.5m is overlain by a sequence consisting of compactmarl and soft clay beds about 10 m thick. The obser-vations on the slope surfaces and available recordsindicated that the groundwater was below the failedmarly rock mass, and the coal seam acted as an aqui-fer. The marly rock with a uniaxial compressive

strength of 40 MPa and mi of 9.04 has a carbonatecontent more than its clay content. The observedactual slip surface was in circular shape and passedthrough the compact marl rock mass and along theclay bed, above the coal seem. Bedding planes dip intoopposite direction of the slope. Three main joint setsmoderately and closely spaced, and bedding planes inthe marly sequence resulted in a jointed rock mass.The rock mass characteristics and the geomechanicalparameters of the clay are listed in Table 3.

3.3.4. Case 4: a bench failure in a coal mineHimmetoglu open pit coal mine, operated by the

Turkish Coal Enterprises (TKI), is located in north-west Anatolia and produces low calori®c value of coal.A local bench failure occurred in 1998 in the easternslope, excavated in heavily jointed marly rock mass, asa result of steepening of the slope (Fig. 3d). On thebasis of the scanline surveys [19], the parameters ofdiscontinuities given in Table 3 were obtained and aGSI value of 27.5 for the rock mass was determined.

Detailed instability plan and cross-section of thefailed bench are shown in Fig. 9. The visible part ofthe failure surface was in circular form. Detailedhydrogeological investigations [19] indicated that theslope was dry. Since the overburden material wasremoved by excavators without blasting, an adjustmentfactor of 0.97 was considered in this case. The backanalysis of the failure surface (surface 1 in Fig. 9)showed that circular failure did not appear as a realis-tic mechanism for this instability with a factor ofsafety considerably greater than unity. The position ofthe ¯oor strata dipping towards the excavation and thevisible upper part of the sliding surface indicated thepossibility of another mode of failure by combinationof a planar sliding surface along the weak ¯oor strataand a circular failure surface through the rock mass(Fig. 9; failure surface 2). The back analysis of themultiplanar failures along both bedding planes and thefaults in this pit indicated that the residual shearstrength parameters of the weak and slickensided bed-ding planes were cr=1.4 kPa and f=128 [19]. Byemploying these parameters, rock mass properties ofthe marls and Janbu's method of analysis [20] for thiscombined failure surface, a back analysis was per-formed. The analysis which yielded a factor of safetyof unity indicated that a combined failure surface wasthe realistic mode of failure for this instability. Afterthe removal of the failed material, the combined fail-ure surface clearly appeared and con®rmed the surfacelabeled 2 in Fig. 9 as the real failure surface.Therefore, the parameters given in Table 3 and thepredicted mode of failure were employed in this studyfor further assessments.

Fig. 7. Slope geometries before and after the failure, and critical slip

surface in closely jointed schist rock mass (case 2).

Fig. 8. Cross-section illustrating the geometry of the failed slope and

the position of the strata (case 3).


3.3.5. Case 5: spoil pile instability in a strip coal mineSpoil piles in the Eskihisar mine, su�er from numer-

ous problems. Engineering geological characterizationof the waste material and stability of the piles in thismine were investigated by Ulusay et al. [21±23], whoreported deep-seated rotational instabilities coveringagricultural areas, shallow-seated rotational failuresboth sides of the haul road and bi-planar wedge fail-ures along the operating slices. The authors of the pre-sent article consider that spoil pile materials can bede®ned as poorly interlocked and heavily broken rock

masses with a mixture of angular and rounded rockpieces, unless they do not contain high proportion of®nes as a result of hauling, dumping and subsequentdeformation. This approach indicates that the categor-ization of spoil piles mainly consisting of rock materialas an disintegrated rock mass and the use of theHoek±Brown criterion to estimate the shear strengthparameters of such materials seems to be logical.Therefore, a selected spoil instability from theEskihisar mine was also employed for this investi-gation. For this purpose, an instability occurred alongthe haul road (Fig. 3e) was back±analysed. The spoilpile consisted of marly rock. The cross-sections pre-pared from the instability plan by Ulusay et al. [21,22]revealed that the failure did not involve the foundationmaterial. The curvature of the exposed sliding surfaceand slightly curved escarpments in plan con®rmed arotational type of failure has occurred (Fig. 10). Nowater table or seepage was encountered in the pile. In-place unit weight determination indicated a meanvalue of 14 kN/m3 for the spoil material [21].

The main question in the case of spoil materials is``how can the structural rating in conjunction with Jvbe estimated?''. It was considered that determinationof the grain size distribution of the fragments in thespoil, (i.e. measurement of the distance between theboundaries of the rock fragments along the selecteddirections and calculation of the average value), can beused as a practical and economic method to estimateJv. Although sieving the excavated or blasted rockthrough screens is the most direct method to determinerock fragmentation, the cost of this method is high.Photoanalysis is one of the more recent and well estab-lished methods [24±26]. In this study the method ofphotoanalysis has been employed.

A test site near the investigated spoil pile wasselected for the fragmentation analysis. Using a shoveltruck, a small sized pile was dumped and photographsof this pile from its two sides were taken by a 35 mmcamera. Since there were no established ®gures in theliterature on the minimum number of photographsthat were needed to accurately sample a given volumeof rock fragments, analysis of two randomly selectedphotographs was considered to be su�cient for thepurpose of this study. Attention was paid to keep thecamera perpendicular to the surface of which photo-graph was taken. In each photograph a scaling object(a circular plate) and a reference area (a woodenmesh) were used (Fig. 11). Outlines of the rock frag-ments were then digitized into the computer. Alongthe x and y axis (Fig. 11), all fragments larger than ap-proximately 2 cm were automatically dimensioned. Inorder to measure the dimensions along the third axis(z ), the same process was applied on the photographtaken in perpendicular direction to the previous one.The information obtained was used in the statistical

Fig. 9. Plan of the shallow-seated bench failure in a heavily jointed

rock mass (a) and cross-section of the stability (b) deduced from sur-

veying (case 4).


Fig. 10. (a) Plan of the shallow-seated spoil instability along the haul road and (b) cross-sections of the spoil pile showing the failure surfaces

and pile geometries (case 5; after Ulusay et al. [21]).


analyses. By putting the mean fragment sizes of 0.085,0.081 and 0.083 m calculated for x, y and z axes, re-spectively, into Eq. (7c), a Jv value of 1773 which rep-resents a disintegrated rock with a very low structurerating of 4 was obtained. A surface condition rating of7 and a GSI value of 26 were estimated.

3.4. Back analysis of the selected slope instabilities

The results obtained from the back analysis of theseslope failures are evaluated by following the steps sum-marized below.

. Step 1: using the GSI value for each case (Table 3),the Hoek±Brown constants were calculated from theoriginal equations (Eqs.(1), (2) and (5)) which donot consider the disturbance e�ect or an adjustmentfactor. The factors of safety, based on these par-ameters tabulated in the ®rst column of Table 4, areconsiderably greater than unity and indicate thatfailure can not occur through these slopes. This situ-ation focuses the attention on the fact that the negli-gence of an adjustment factor (df ) yields unrealisticassessments for stability of slopes, and therefore, amodi®cation based on df value seems to be necess-ary for Hoek±Brown equations.

. Step 2: using the same model, the analyses includingan adjustment factor for each case (Table 3) wereperformed. However, in this step, the adjustmentfactor was multiplied by the GSI value to considerthe disturbance e�ect on the rock mass, as appliedin the RMR scheme. Thus, Eqs.(1), (2) and (5) wererewritten in the following forms:

mb � mi exp

�GSI� df ÿ 100

28

��10a�

s � exp

�GSI� df ÿ 100

9

��10b�

a � 0:65ÿ GSI� df

200�10c�

The results of the back analysis of the failed slopesyielded factors of safety considerably greater thanunity suggesting that the slopes were stable (Table4). This approach indicated that the above modi®-cation did not satisfy the failure condition.

. Step 3: in this step, the e�ect of disturbance wasassigned onto the numbers appearing in the numer-ators of the Eqs. (1) and (2) as follows:

mb � mi exp

�GSIÿ 100

28� df

��11a�

s � exp

�GSIÿ 100

9� df

��11b�

The results of the analysis indicated that the valuesof the factor of safety calculated for each case(Table 4) were still greater than unity.

. Step 4: in this step, considering the results obtainedfrom step 3, ®ve di�erent values were assigned to bmand bs in Eqs. (8) and (9), ranging between 14 and28, 6 and 9, respectively. Then the values of factorof safety corresponding to selected values of bm andbs for each particular case were calculated to esti-

Table 4

The results of the back analysis of the failed slopes based on di�erent approaches to assess the e�ect of disturbance. The values in parentheses in-

dicate the average values for three sections

Case No.

dfb Calculated factor of safety (FOS)

approach 1 approach 2 approach 3 approach 4: at limiting equilibrium conditiona

bm bs

Case 1: 0.94

Section 1-1 '/1 1.48 1.44 1.42 18.9 (18.55) 7.05 (6.98)

Section 1-1 '/2 1.48 1.43 1.41 18.2 (18.55) 6.90 (6.98)

Section 2-2 ' 1.45 1.40 1.39 18.55 (18.55) 6.98 (6.98)

Case 2: 0.97 1.70 1.62 1.59 20.28 7.34

Case 3: 0.90 1.41 1.34 1.28 17.15 6.68

Case 4: 0.97 1.32 1.23 1.19 20.3 7.35

Case 5: 0.80

Section 1-1 ' 2.71 1.93 2.10 14 6

Section 2-2 ' 2.64 1.80 2.03 14 6

Section 3-3 ' 2.64 1.84 2.04 14 6

Section 4-4 ' 2.69 1.84 2.07

b Adjustment factor for disturbance e�ect.a Obtained from Fig. 12.


mate the pairs of bm±bs satisfying the limit equili-brium condition. Since the disintegrated materialforming the spoil piles were the mixture of exca-vated, blasted, hauled and dumped overburden, alower bound adjustment factor of 0.8 was assignedfor this material. Besides, the values of 14 and 6,which were suggested as the lower bounds in theoriginal Hoek±Brown equations [4], were employed.Therefore, for di�erent values of bm and bs for thespoil pile instabilities mentioned in case 5 the trialand error method was not used. The back analysisof these instabilities yielded values of factor of safetyequal to unity, indicating that the suggestedapproach seemed to be satisfactory.

The results of the back analysis are presented inFOS±bm and FOS±bs forms (Fig. 12) to obtain thepairs of bm and bs satisfying limiting equilibrium con-dition. Considering that an adjustment factor (df ) of 1corresponds to bm and bs values of 28 and 9, respect-ively, for undisturbed rock masses and, similarly,values of 14 (bm)and 6 (bs) correspond to a df value of0.8 for highly disturbed rock mass and using the com-binations of df ±bm and df ±bs which lead a factor ofsafety of unity, the plots given in Fig. 13 are estab-lished. It is now possible to estimate bm and bs valuesfor closely jointed rock masses, depending on the dis-turbance e�ect, when the following expressions derivedfrom the curve shown in Fig. 13 are used.

bm � 3:14 ln

�df

df � 340�1ÿ df�� 28 �12�

bs � 0:67 ln

�df

df � 340�1ÿ df�� 9 �13�

The results of the back analysis of the slope instabil-ities in closely jointed rock masses and in the spoilpiles with high proportion of disturbed rock piecesindicated that the disturbance e�ect due to the in¯u-ence of the method of excavation could not beignored. In other words, the equations of the criterionbased on the undisturbed rock mass condition did notwork well if an adjustment factor was not considered.For this purpose, it is advised that, in the estimationof the rock mass constants, determination of the valuesof bm and bs of any particular rock mass determinedfrom the curves (Fig. 13) or from Eqs. (12) and (13)for a given df value seems to be better. The rock massconstants then should be estimated by using Eqs. (8)and (9) proposed in this study.

4. Conclusions

Due to the limitations in the RMR classi®cationscheme, particularly for very poor quality rock masses,the geological strength index (GSI) has been intro-duced into the Hoek±Brown failure criterion.However, there are no published case histories on theback analysis of slopes or underground openings inheavily jointed rock masses which con®rm that thecurrent GSI methodology yields satisfactory results.

In this study, an attempt has been made to providea more quantitative numerical basis for evaluating theGSI and to suggest quantities which make more sense

Fig. 11. A view from the spoil material taken for image processing, and the scaling factor (case 5).


Fig. 12. Variation in bm and bs values with factor of safety (FOS) for the case studies from the slope failures in closely jointed rock masses.


than that of the RMR System when used for the esti-mation of rock mass strength. For the purpose, twoterms, `structure rating, SR', and `surface conditionrating, SCR', have been introduced into the existingGSI classi®cation scheme. In order to assign the rat-ings to these terms, the use of some easily measurableinput parameters such as, roughness, weathering, in®ll-ing and volumetric joint count have been suggested.According to the selected rating intervals, the GSIchart has been modi®ed to estimate more precisevalues of GSI.

Five well documented slope instability exampleshave been given to illustrate the application of the pro-posed method in practical geotechnical engineering.The application of the suggested method and theapproaches indicated that the use of GSI value deter-mined from the suggested modi®ed chart and consider-ation of disturbance e�ect con®rmed the limitequilibrium condition for the failed slopes.

The other issues concluded in the study were thatthe spoil pile materials consisting of angular androunded rock pieces with low proportion of ®nes couldbe categorized as disintegrated rock masses in the GSIclassi®cation and it seemed possible to estimate theirshear strength parameters from the modi®ed Hoek±Brown equations presented herein.

Some engineering geologists feel that the visualdescriptions upon which the GSI system is based arepreferable to the numbers of Bieniawski's classi®-cation. On the other hand, some engineers may beunhappy with the largely descriptive nature of the GSIsystem and the comments o�ered by the authors.However, the attempt by the authors is to address thediscussion of the GSI chart and its perceived de-®ciencies to improve the GSI and to suggest a methodfor practitioners.

A better understanding of the mechanics of jointedrock mass behaviour is a problem of major signi®cancein geotechnical engineering. The authors believe thatthe Hoek±Brown failure criterion provides a good esti-mate for the shear strength of closely jointed rock

masses and even for rock spoil pile materials.However, the authors hope that the application of thesuggested modi®cations onto various failure case his-tories both from surface and underground excavationsmay lead to provide a better tool for more preciseguidelines and to check validity of the equationsemployed by the non-linear failure criterion.

Acknowledgements

The authors would like to thank Professor Dr.Hasan GERCEK from Zonguldak KaraelmasUniversity, Turkey for his valuable comments on therevised manuscript.

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modifications to the geological strength index (gsi) and their applicibility to slopes

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