Excavatability assessment of rock masses using the ...

Bull Eng Geol Environ (2010) 69:13–27
DOI 10.1007/s10064-009-0235-9

ORIGINAL PAPER

Excavatability assessment of rock masses using the Geological
Strength Index (GSI)

G. Tsiambaos Æ H. Saroglou

Received: 30 June 2009 / Accepted: 20 July 2009 / Published online: 14 August 2009
Ó Springer-Verlag 2009

Abstract In the present study a new classification method exist, (b) ripping, for moderate to difficult excavation
for the assessment of ease of excavation of rock masses is conditions, and (c) blasting for very difficult excavation
proposed, based on the Geological Strength Index and the conditions. The knowledge of the physical and mechanical
point load strength of the intact rock. The data originate characteristics as well as the behavior of the geo-materials
from excavation sites in Greece in sedimentary and meta- to be excavated is vital for the selection of the most
morphic rock masses. A wide variety of rock structures effective method of excavation.
were considered, ranging from blocky to disintegrated, and
different excavation methods have been used (blasting, Previous research
hydraulic breaking, ripping and digging). The proposed
method cannot be applied to heterogeneous rock masses Assessment of rock excavatability
and soft rocks/hard soils.

Keywords GSI Á Excavatability Á Rockmass Á All the methods used for the assessment of excavatability
Rippability Á Rock strength or rippability of rock take into account the uniaxial com-
pressive strength, weathering degree and spacing of dis-
Introduction continuities. Some of them also include seismic velocity, as
well as the continuity, aperture, orientation and roughness
Predicting the ease of excavation of rock and rock masses of joints. A detailed review of the principal excavation
is very significant in earthworks for highway construction methods is given in MacGregor et al. (1994) and Basarir
or other civil engineering works, in surface mines and also and Karpuz (2004).
for foundations. In order to describe the excavation of
rocks, different terms have been used, related to the prin- Duncan (1969) states that the assessments to determine
ciple of excavation and the mechanics of fracture. These the ease or difficulty with which a rock mass may be
include cuttability, rippability, excavatability, diggability excavated are based upon the consideration of:
and drillability. In the present work, the term excavatability
is used as a broad term that refers to the ease of excavation (a) the rock material forming the rock blocks within the
of rock and rock masses and includes the methods of in situ rock mass—because excavation entails frag-
(a) digging, when easy/very easy excavation conditions mentation and rupture of the rock materials when the
block volume is large,
G. Tsiambaos (&) Á H. Saroglou
Geotechnical Engineering Department, School of Civil (b) the nature, extent and orientation of the fractures, and
Engineering, National Technical University of Athens, (c) the geological structure with respect to folding and
9 Iroon Polytechniou str., 157 80 Athens, Greece
e-mail: [email protected] faulting.

Initially, Franklin et al. (1971) proposed a method to
assess the excavation of rock based on the point load
strength of intact rock, Is50, and on the fracture spacing
index, If, which is the mean spacing of joints along a

123

14 G. Tsiambaos, H. Saroglou

scanline. Atkinson (1971) suggested that the ease of parameters based on Barton et al. (1974) Q system. Fowell
excavation can be predicted using the velocity of longitu- and Johnson (1982), Smith (1986), MacGregor et al. (1994)
dinal waves in the rock mass for different rock types. and Hadjigeorgiou and Poulin (1998) have also developed

Scoble and Muftuoglu (1984) proposed a classification Fig. 1 Layered marble corresponding to the blocky rock mass type
of rock excavatability based on the rock mass weathering
degree, the intact rock strength, the joint spacing and the
spacing of bedding planes in a layered rock mass. Pettifer
and Fookes (1994) stated that the excavatability of rocks
depends on their individual properties, on the excavation
equipment and on the method of working. They also stated
that, apart from the strength of rock expressed by point
load index, the discontinuity characteristics define the
individual size of rock blocks, which constitutes one of the
most important parameters for rock rippability. They pre-
sented a detailed chart, which is similar to that proposed by
Franklin et al. (1971) but with a more detailed categori-
zation of excavation methods. McLean and Gribble (1985)
estimated relationships between uniaxial compressive
strength and Schmidt hammer hardness (rebound number)
of intact rock and the rocks’ rippability. Karpuz (1990) and
Basarir and Karpuz (2004) proposed a rippability classifi-
cation system for Coal Measures and marls for use in lig-
nite mines. This is based on the seismic P-wave velocity,
the point load index or uniaxial compressive strength, the
average discontinuity spacing and the Schmidt hammer
hardness. Singh et al. (1987) have also proposed a rippa-
bility index for Coal Measures. Ripper performance charts
have also been proposed for a wide variety of rocks based
on their P-wave seismic velocity (Church 1981; Caterpillar
2001).

Although a number of methods are available to predict
excavatability, no particular method is universally accepted
for several reasons, e.g., lack of awareness of previous case
studies or difficulties in determining input parameters and
limitations of applicability to a specific geological envi-
ronment. A successful classification system should be easy
to use (quantifiable data, easy to determine, user friendly)
and should also give information about currently available
equipment.

Rock mass classification for estimation
of excavatability

Rock mass classification systems have also been used for Fig. 2 a Sandstone and b limestone, both corresponding to the very
the assessment of excavatability. Weaver’s (1975) classi- blocky rock mass type
fication was based on the RMR system (Bieniawski 1974).
Kirsten (1982) proposed a system for the excavatability
assessment in terms of rock mass characteristics, such as
mass strength, block size, relative orientation of geological
structure and joint walls strength. His classification system
is based on engineering properties for the weakest soil to
the hardest rock. Kirsten (1982) formulated the excavat-
ability index (N), which is determined by the use of several

123

Excavatability assessment of rock masses using GSI 15

Fig. 4 Heavily fractured limestone corresponding to the disintegrated
rock mass type

Fig. 3 Folded (a) thinly bedded limestone (b) schist, both corre-
sponding to the blocky/disturbed/seamy rock masses

grading classification systems for the assessment of rock Fig. 5 Studied rocks superimposed on the Franklin chart
rippability.
In the present study the Geological Strength Index
Additionally, Abdullatif and Cruden (1983) presented an (GSI), as proposed by Marinos and Hoek (2000) was used
assessment of ease of excavation and productivity in rela- in order to describe the rock masses and correlate each rock
tion to rock mass quality using the RMR system. Recently, mass type with the applicability of the available excavation
Hoek and Karzulovic (2000) used the data from Abdullatif methods. In this approach, the intact rock strength was
and Cruden (1983) to estimate the Geological Strength taken into account and the properties of the discontinuity
Index, GSI and strength of these rock masses and suggested sets and fracture spacing (controlling the size of rock
a range of GSI for different excavation methods. They blocks) were carefully evaluated. The advantage of the
proposed that rock masses can be dug up to GSI values of proposed classification is that it is a qualitative tool for easy
about 40 and rock mass strength values of about 1 MPa, and quick assessment of excavatability.
while ripping can be used up to GSI values of about 60 and
rock mass strength values of about 10 MPa. Blasting was
the only effective excavation method for rocks exhibiting
GSI values greater than 60 and rock mass strengths of more
than 15 MPa.

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16 G. Tsiambaos, H. Saroglou

Table 1 Range of point load strength and rock mass classification for different geological formations

Rock mass GSI Rock Discontinuity Is50 (MPa) Is50 (MPa) If (cm) If (cm)
type structure surface average range average range

Gneiss 35–60 S2, S3 D2, D3, D4 2.30 1.30–4.80 65 30–150
Weathered gneiss 35 S3 D4 0.6 25
Schist 15–70 S2, S3, S4, S6 D2, D3, D4 2.20 0.80–4.60 49 23–160a
Limestone 20–65 S2, S3, S5 D2, D3, D4 2.45 0.70–4.00 45 20–80b
Sandstone 30–60 S2, S3, S4 D1, D2, D3, D4 2.30 0.70–4.80 40 20–100
Marble 65–75 S2 D1, D2 2.80 1.80–4.20 50 40–70
Siltstone 25–30 S4, S5 D3, D4 0.50

a Fracture spacing in schists is meaningful only in rock masses with blocky, very blocky and disturbed/seamy structure. Fracture spacing due to
schistosity planes (acting as discontinuity planes) in laminated/sheared rock masses is not applicable
b Fracture spacing in disintegrated limestones affected by fault activity is not applicable

Geological Strength Index therefore built on the linkage between descriptive geolog-
ical terms and measurable field parameters such as joint
The Geological Strength Index (GSI) was introduced by spacing and roughness.
Hoek et al. (1992), Hoek (1994) and Hoek et al. (1995).
This index was subsequently extended for weak rock The rock mass type is a controlling factor in the
masses in a series of papers by Hoek et al. (1998) and assessment of the excavation method, as it is closely
Marinos and Hoek (2000). Later, Marinos and Hoek (2001) related to the number of discontinuity sets and reflects the
proposed a chart of the Geological Strength Index for rock mass structure. The Geological Strength Index, in its
heterogeneous rock masses, such as flysch, which is fre- original form, was not scale dependant, thus the rock block
quently composed of tectonically disturbed alternations of size is not directly related to the rock mass type. Never-
strong and weak rocks (sandstone and siltstone, respec- theless, each rock type has a broad correlation to the rock
tively). This chart was modified by Marinos et al. (2007). block size, i.e., a blocky rock mass has larger blocks than a
very blocky rock mass or a disintegrated rock mass which
The GSI relates the properties of the intact rock ele- is made up of very small rock fragments. This correlation is
ments/blocks to those of the overall rock mass. It is based only informative, however, and is not applicable to certain
on an assessment of the lithology, structure and condition rock mass types, e.g., sheared schist rock masses, as the
of discontinuity surfaces in the rock mass and is estimated spacing of the schistosity planes equates to the disconti-
from visual examination of the rock mass exposed in out- nuity planes and hence the concept of block volume is not
crops, surface excavations such as road cuts, tunnel faces applicable. For this reason, the present classification for the
and borehole cores. It utilizes two fundamental parameters assessment of excavatability is based on the original GSI
of the geological process (blockiness of the mass and charts (2000 version), but specific reference to the block
condition of discontinuities), hence takes into account the volume is made.
main geological constraints that govern a formation. In
addition, the index is simple to assess in the field. Characteristics of investigated rock masses

Quantification of GSI classification—block volume Field investigation—methodology
of the rock mass

According to Palmstro¨m (2000), block size and disconti- The field investigation was carried out at highway con-
nuity spacing can be measured by means of the Volumetric struction sites in Greece. In general, the rocks involved
Joint Count Jv, or the mean block volume, Vb. Sonmez and were sedimentary (limestone, sandstone and siltstone) and
Ulusay (1999) quantified block size in the GSI chart by the metamorphic (gneiss, schist and marble). The most pre-
Structure Rating coefficient (SR) that is related to the Jv dominant rock types were sandstone and limestone.
coefficient. Cai et al. (2004) presented a quantified GSI
chart and suggested that the block size is quantified by the The field investigation in sixty-one (61) selected loca-
mean discontinuity spacing S or by the mean block volume tions included the determination of rock mass properties,
Vb. The structure was quantified by joint spacing in order to the excavation method and its performance in terms of
calculate the block volume, and the joint surface condition production against time. In order to describe and classify
was quantified by a joint condition factor. The GSI is the rock masses the following parameters were recorded
(following ISRM 1981):

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Excavatability assessment of rock masses using GSI 17

Fig. 6 Studied rocks
superimposed on the
Pettifer–Fookes chart

(a) rock type, Rock mass classification
(b) joint set number,
(c) joint spacing, The rock masses studied generally have a blocky (18 sites)
(d) joint orientation, and very blocky structure (29 sites). The discontinuity
(e) joint surface condition, conditions of the blocky rock masses are fair, good and
(f) degree of weathering. very good. For the very blocky rock masses, the disconti-
nuities are poor, fair and good. Some rock masses (7 sites)
Laboratory testing of the block samples from each site have a blocky/disturbed/seamy structure and good to fair
included determination of unit weight and point load discontinuity surface conditions. Finally, a few disinte-
strength in accordance with the methods suggested by grated (5 sites) and laminated/sheared rock masses (2 sites)
ISRM (1985). All the rock masses examined were rated were found with fair to poor joint surface conditions.
according to the Geological Strength Index.

123

18 G. Tsiambaos, H. Saroglou

The sandstone, limestone, gneiss, marble and schist Assessment of excavatability using existing methods
(amphibolitic and micaceous) rock masses have a blocky
structure, as shown in Fig. 1. Gneiss, limestone and sand- Franklin et al. (1971) method
stone rock masses were also found to have a very blocky
structure (Fig. 2a, b). Blocky/disturbed/seamy rock masses The oldest graphical indirect rippability assessment method
were found in folded thinly bedded limestone (Fig. 3a) and is that of Franklin et al. (1971). It considers two parame-
in folded schist environments (Fig. 3b). Finally, some ters: the fracture spacing, If, and strength values of intact
heavily fractured limestones affected by tectonic activity rock. Franklin’s method has been re-evaluated and modi-
appear totally disintegrated and broken (as shown in fied by many researchers; the most well known being
Fig. 4). The laminated/sheared structure was encountered Pettifer and Fookes (1994). Although this graph allows
only in the schists. excavatability to be assessed rapidly, the subdivisions have
become outdated as more powerful, more efficient equip-
The point load index (Is50) of the different rocks ran- ment has become available.
ges between 0.5 and 5.0 MPa. The lower values originate
from weathered rocks. The range of point load strength, The Franklin et al. (1971) chart shows that most of the
Is50, and fracture spacing, If, of discontinuities as well as rock masses encountered in the selected sites would have to
the rock classification of the different geological forma- be excavated with blasting to loosen the rock mass and
tions are given in Table 1. The fracture spacing (If) had a some (9 of the 61) with ripping. However, as shown in
relatively wide range. The average fracture spacing is Fig. 5, most of the rock masses (29) were excavated using
higher for the gneiss and marble rock masses with a rippers, indicating that the chart is quite conservative and
blocky and very blocky structure. The limestone, schist predicts more difficult excavation conditions than is actu-
and sandstone rock masses with a blocky/disturbed/seamy ally the case with modern machinery.
and disintegrated structure have lower average fracture
spacings. Pettifer–Fookes (1994) classification method

It should be emphasized that a realistic determination Pettifer and Fookes (1994) emphasized the value of a three-
of fracture spacing is often difficult. The three-dimen- dimensional discontinuity spacing index as this provides a
sional development of discontinuities should not be more realistic assessment of the average block size.
underestimated when calculating the fracture spacing.
Moreover, fracture spacing in laminated/sheared schist With Pettifer and Fookes’ chart (Fig. 6), the evaluation
rock masses expressed by the schistosity planes (acting of excavatability is simple and hence the chart is still
as the predominant discontinuity) and in disinte- commonly used (Kentli and Topal 2004; Gurocak et al.
grated limestones, which are brecciated by faults, is not 2008). However, the rock mass data from the present study
meaningful. indicate that it underestimates the difficulty of excavation.

Fig. 7 Relationship between point load strength and excavation Fig. 8 Plot of point load strength versus GSI for different excavation
method methods

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Excavatability assessment of rock masses using GSI 19

For material falling in the region of the chart where D6 and subjective, and also to the fact that in many sites other
D7 rippers are proposed, in four sites D8 rippers were construction matters may have been involved in the deci-
required and in six sites D9 rippers were used. In only three sion to use heavier equipment.
sites were the D7 rippers appropriate. In ten sites the pre-
dicted D8 equipment was used, but in six sites heavier (D9) Prediction using the RMR and Q rock mass
rippers were necessary. In eight sites where D8 or D9 classification systems
rippers were predicted, hydraulic breaking, or rippers and
hydraulic hammers were used. Abdullatif and Cruden (1983) proposed that a rock mass
can be dug up to Rock Mass Rating (RMR) values of 30
This deviation from the predicted conditions could be and ripped up to RMR values of 60 while a rock mass rated
attributed to the accuracy of measuring the fracture index as ‘‘good’’ or higher would require blasting. They also state
of the predominant joint sets, which is somewhat

Fig. 9 GSI classification for
tested rocks with intact rock
strength (Is50 \ 3 MPa)

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20 G. Tsiambaos, H. Saroglou

Table 2 Detailed rock mass data and excavation methods used on study sites (point load strength of intact rock Is50 \ 3 MPa)

Site Rock Structure/ GSI Fracture Is50 Excavation
number type discontinuity spacing If (cm) (MPa) method

B5 Schist S2D3 60 80 2.6 Blasting
65 40 1.7 Blasting
B6 Limestone Sparitic S2D2 75 40 2.5 Blasting
70 40 1.8 Blasting
B7 Marble S2D1 65 50 2.7 Blasting
60 100 2.7 Blasting
B8 Marble S2D1 70–75 36 1.8 Hammer
50–55 26 1.2 Hammer
B9 Marble S2D1 65 70 1.3 Hammer
55 72 1.3 Hammer
B10 Sandstone S2D2 55–60 30 1.4 Hammer
55 80 2.9 Hammer
H4 Amphibolitic Schist S2D2 60 150 2.2 Hammer
50–55 50 1.7 Ripper D8
H5 Amphibolitic Schist S2-3D3 50 80 – Ripper D8
50 40 2.3 Ripper D8
H6 Mica schist S2D2 45 1.3 Ripper D8
50–55 – 1.7 Ripper D8
H7 Mica schist S2D3 40 50 2.8 Ripper D8
35 20 – Ripper D8
H8 Amphibolitic Schist S3D2 40–45 30 0.9 Ripper D8
40 30 2.2 Ripper D8
H9 Limestone micritic S2D3 35 30 1.3 Ripper D8
50 30 – Ripper D8
H10 Gneiss S3D2 45 100 1.7 Ripper D8
45 100 0.7 Ripper D9
R11 Sandstone S2D2 35 30 0.6 Ripper D9
35–40 30 1.4 Ripper D9
R12 Sandstone S2D3 40–45 30 1.7 Ripper D9
55–60 30 1.9 Ripper D9
R13 Sandstone S2D2 55–60 50 0.8 Ripper D9
55 2.0 Ripper D9
R14 Sandstone S4D2 40–45 – 2.2 Ripper D10
40–45 – 0.7 Ripper D7-Digger
R15 Sandstone quartzitic S2D2 30 23 2.9 Ripper D7-Digger
30–35 20 0.9 Ripper D7-Digger
R16 Sandstone quartzitic S4D3 40–45 – 1.1 Ripper D7-Digger
30 – 0.5 Digger
R17 Sandstone quartzitic S4D3 25 30 – Digger
15 – 0.8 Digger
R18 Sandstone quartzitic S3D3 20 – 0.7 Digger
15 0.9 Digger
R19 Sandstone silty S3D3

R20 Mica Gneiss S3D4

R21 Gneiss S2D3

R22 Gneiss S2-3D3

R23 Limestone micritic S3D3

R24 Mica Gneiss S3D4

R25 Mica Gneiss S3D4

R26 Granitic Gneiss S3D3

R27 Sandstone S3D1-2

R28 Sandstone S3D2

R29 Sandstone S3D2

R30 Schist S4D2

R31 Sandstone S3D3

R32 Sandstone S3D4

R33 Sandstone–Siltstone S3D4

R34 Sandstone S3D3

D3 Siltstone S4D4

D4 Mylonitic limestone S5D4

D5 Schist S6D4

D6 Limestone S5D4

D7 Calcareous schist S6D4

that rocks with a Q value up to 0.14 can be dug but those rocks with Q values between 3.2 and 5.2 can be ripped and/
with Q values above 1.05 require ripping. However, they or require blasting.
pointed out that the use of Q as a guide to excavation
methods presents problems, as there is an overlap where The present study found Abdullatif and Cruden’s (1983)
ranges for digging, ripping and blasting are in good

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Excavatability assessment of rock masses using GSI 21

agreement with the methods actually used at the investi- (a) Rock masses that have a joint spacing, If, greater than
gated sites but the use of the Q system was less consistent 0.3–0.5 m and a point load strength of intact rock
with field practice. greater than 1 MPa have to be excavated using either
hydraulic breaking or blasting.
Guidelines concerning If and Is50
(b) Rock masses with fracture spacing of less than about
From the evaluation of the data from this study using the 100 mm (close to very close spacing according to
classification methods of Franklin et al. (1971) and Pettifer ISRM 1981) can be excavated by rippers or diggers
and Fookes (1994), the following conclusions can be drawn irrespective of the point load strength of the intact
concerning fracture spacing and point load strength of rock.
intact rock.
(c) Rock masses exhibiting a point load index for intact
rock of less than about 0.5 MPa can be excavated
easily by ripping or digging, irrespective of fracture

Fig. 10 GSI classification for
tested rocks with intact rock
strength (Is50 C 3 MPa)

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22 G. Tsiambaos, H. Saroglou

spacing (If). No data from rock masses with intact commonly used prediction methods, proved that the
rock strength lower than 0.5 MPa were available. selection of the excavation method depends on the
parameters which are taken into account. In the RMR and
A point load strength value equal to Is50 = 3.0 MPa and Q classification systems, ground water and joint orientation
fracture spacing of If = 0.3 m proved to be threshold values will influence the total ranking, while in both the Franklin
below which ripping was performed in the majority of the sites. and Pettifer–Fookes classification charts, the correct
assessment of the fracture spacing is significant.
The intact rock strengths obtained were analyzed for the
different excavation methods and the results are presented The study has shown that the GSI classification in con-
in the bar chart in Fig. 7. In summary, junction with the intact rock strength can produce a qualita-
tive categorization of excavation methods for rock masses. In
(a) Rock masses excavated with blasting had an intact this procedure, the rock structure and the joint surface con-
point load strength of between 2 and 5 MPa, with a ditions are important. For example, if the joints in a rock mass
mean value of 3 MPa. are tight or very tight (separation of discontinuity surfaces
less than 0.5 mm) it is most probable that the rock blocks
(b) Rock masses excavated using a hydraulic hammer in cannot be detached and thus the rock mass will not be rip-
conjunction with ripping are characterized by point pable, although, a joint spacing in the range of 0.1–0.5 m
load strengths between 1.2 and 3 MPa (mean strength would allow ripping in most circumstances. If the joints are
2.3 MPa). open (separation is between 2.5 and 10 mm) or very wide
(between 10 and 25 mm), either empty or filled with soft
(c) Rock masses excavated using rippers have point load material, and their spacing is between 0.5 and 1.0 m, rippers
strengths in the range of 0.5–5 MPa with a mean are commonly used as the rock blocks are separated relatively
value of 2 MPa. easily. However, the strength of the intact rock in the indi-
vidual rock blocks is also important as excavation with rip-
Proposed classification pers entails fragmentation and rupture of the rock itself.
Sedimentary rocks which are well-bedded and jointed or
General closely interbedded strong and weak rocks can be excavated
by ripping or digging.
An assessment of the excavatability of the rock masses
encountered on the selected sites, based on the most

Table 3 Detailed rock mass data and excavation methods used on study sites (point load strength of intact rock Is50 C 3 MPa)

Site Rock Structure/ GSI Fracture Is50 Excavation
number type discontinuity spacing If (cm) (MPa) method

B1 Schist S2D3 60 90 3.9 Blasting
70 160 4.2 Blasting
B2 Schist S2D2 65 4.2 Blasting
55–60 70 4.8 Blasting
B3 Marble S2D2 50 4.6 Hammer
55 35 3.1 Hammer
B4 Sandstone S3D2 55–60 50 3.1 Hammer
45 3.7 Ripper D9
H1 Schist S2D3 40–45 10 4.0 Ripper D9
35 20 3.4 Ripper D9
H2 Crystalline limestone S3D2 40 30 4.7 Ripper D9
40 54 4.8 Ripper D8
H3 Crystalline limestone S3D2 30 4.1 Ripper D8
35 20 3.9 Ripper D8
R1 Limestone S3D3 35 20 3.1 Ripper D8
50 30 4.8 Ripper D8
R2 Limestone S3D3 30 100 – Ripper D7
20 – – Digger
R3 Limestone S3D4 25 – – Digger
–
R4 Mica Gneiss S3D3

R5 Sandstone S3D4

R6 Sandstone S4D4

R7 Sandstone S4D3

R8 Mica Gneiss S3D4

R9 Gneiss S3D2

R10 Mylonitic limestone S5D3

D1 Mylonitic limestone S5D4

D2 Siltstone S5D3

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Excavatability assessment of rock masses using GSI 23

Fig. 11 Proposed GSI chart for
the assessment of excavatability
of rock masses (Is50 \ 3 MPa)

A first assessment of the excavation methods in the threshold values proposed in the literature; most research-
study sites based on a GSI classification of the excavated ers suggesting a UCS of 70 MPa, equivalent to a point load
rock mass and the point load strength of the intact rock is strength of 3 MPa (Bell 2004; McLean and Gribble 1985;
presented in Fig. 8. It is evident that three distinct regions Bieniawski 1975).
exist in the GSI-Is50 chart, which correspond to the dif-
ferent excavation methods (blasting and/or use of hydraulic Two classification charts are proposed for the assess-
hammer, ripping and digging). For a given strength of rock, ment of excavation method based on GSI:
the ease of excavation increases as the rock mass quality
decreases (lower GSI values), thus blasting can be substi- (a) For rock masses with a point load strength (Is50)
tuted by ripping or even digging. between 0.5 and 3 MPa;

The study also indicated the threshold value of strength (b) For rock masses with a point load strength (Is50) equal
of an intact rock, beyond which the rock mass requires to or above 3 MPa.
blasting, is equal to 3 MPa. This value is similar to the
In order to correlate the excavatability method with GSI
classification, categories of rock mass types were

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24 G. Tsiambaos, H. Saroglou

Table 4 Excavation method for different rock mass types (Is50 \ 3 MPa)

Intact rock Method of Rock mass type based on GSI (Structure-Discontinuity condition)
strength excavation

S1
S2D1
S2D2
S2D3
S2D4
S2D5
S3D1
S3D2
S3D3
S3D4
S3D5
S4D1
S4D2
S4D3
S4D4
S4D5
S5D1
S5D2
S5D3
S5D4
S5D5

Drill & Blast X X X X
or hammer

Is50<3 MPa Ripper (D8, XXXXXXX XX
D9) XX

Ripper (D7) XXX

Digging XXX

Underlined symbols represent areas of application that are suggested (with no records from the study sites)
Symbols in bold represent marginal conditions for application of the proposed excavation method

Table 5 Excavation method for different rock mass types (Is50 C 3 MPa)

Intact rock Method of Rock mass type based on GSI (Structure-Discontinuity condition)
strength excavation
S1
S2D1
S2D2
S2D3
S2D4
S2D5
S3D1
S3D2
S3D3
S3D4
S3D5
S4D1
S4D2
S4D3
S4D4
S4D5
S5D1
S5D2
S5D3
S5D4
S5D5

Drill & Blast XXX XX
or hammer XXXX
Is50 ≥ 3 MPa Ripper (D8, XXXX

D9) X
XXX
Ripper (D7)

Digging X

Underlined symbols represent areas of application that are suggested (with no records from the study sites)
Symbols in bold represent marginal conditions for application of the proposed excavation method

determined based on the structure of the rock mass and the good joint surface conditions (S3D2 to S3D4), while some
surface conditions of discontinuities. Each rock mass type have a blocky/disturbed/seamy structure (S4D2 to S4D3).
is given a code in the form of S (number) for rock mass To some extent ripping was also successful in blocky rock
structure and D (number) for discontinuity condition. For masses with fair joint surface conditions (S2D3). Easy rip-
example, the intact/massive structure is defined as S1 and ping conditions (D7 rippers) were encountered in very
the laminated/sheared rock mass as S6, while discontinu- blocky rock masses with poor joint conditions (S3D4) while
ities with a very good condition are defined as D1 and those rocks with a seamy, disintegrated and laminated/sheared
with a very poor condition D6. Thus, a rock mass that has a structure and poor joint (or schistosity) surface conditions
very blocky structure and good condition of discontinuities (S4D4 to S6D4) were excavated with digging equipment.
would be described with S3 and D2 (S3D2).
The GSI classification for rock masses with strengths
Excavatability assessment using GSI above 3 MPa is shown in Fig. 10 and their relevant rock
mass characteristics and the excavation method used are
Samples with a rock strength lower than 3 MPa are clas- summarized in Table 3. Blasting was used for rock masses
sified in the GSI chart shown in Fig. 9; the detailed data with a blocky structure and fair to good joint surface
concerning the rock mass characteristics and excavation conditions (S2D2 to S2D3) and for rocks with a very
method are presented in Table 2. blocky structure with good joint conditions (S3D2).
Hydraulic breaking was used in some very blocky rock
It is evident that blasting was required in blocky rock masses while heavy ripping equipment (D8, D9) was used
masses with a fair to very good discontinuity condition to excavate the very blocky and blocky/disturbed/seamy
(S2D1 to S2D3). Hydraulic breaking was used in similar rock masses with poor to fair joint surface conditions
rock conditions and in some cases in very blocky rock (S3D3 to S3D4 and S4D3 to S4D4). Diggers were only
masses. Most of the rock masses excavated with rippers used in the disintegrated limestone rock masses (S5D3 to
(D8, D9 and D10) have a very blocky structure with poor to S5D4).

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Excavatability assessment of rock masses using GSI 25

Fig. 12 Proposed GSI chart for the assessment of excavatability of rock masses (Is50 C 3 MPa)

Proposed excavatability charts using GSI blocky or very blocky rock structures. Hydraulic breaking
is required for the loosening of rock masses with GSI
Based on the GSI classification of the rock masses, the between 55 and 65 while ripping is successful in rock
following excavation charts are proposed: masses with GSI \ 55. The lower margin for ripping
depends on the rock structure, thus for very blocky rock
GSI excavation chart with Is50 \ 3 MPa masses it is around 25 but in blocky/disturbed/seamy and
disintegrated material it is 35. Rock masses with GSI up to
The proposed excavation method categories in the GSI 25 (or 35) can be dug, obviously with increasing difficulty.
chart for rock masses with intact rock strength less than
about 70 MPa (Is50 \ 3 MPa) are shown in Fig. 11. The applied excavation method in relation to rock mass
Blasting is necessary for rock masses with GSI [ 65 and type for material with Is50 \ 3 MPa is presented in
Table 4.

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26 G. Tsiambaos, H. Saroglou

Fig. 13 Overall assessment of
excavatability of rock masses

GSI excavation chart with Is50 C 3 MPa excavatability assessment is appropriate in the case of
flysch formations with thick beds of sandstones.
Figure 12 shows the proposed excavation method catego-
ries in the GSI chart, for rock masses with intact rock It is also not applicable to ‘‘hard soils/soft rocks’’,
strengths greater 70 MPa (Is50 C 3 MPa). It can be seen especially those characterized as very weak to moderately
that blasting is required when GSI [ 60 (the rock structure weak rocks (Hawkins 2000) with intact rock strengths
is blocky or very blocky). The transitional zone where between 1.25 and 10 MPa. In this case, the discontinuities
hydraulic breakers should be used to loosen the rock mass have a secondary and minor role in the behavior of the rock
is applicable to rock masses with a blocky, very blocky or mass (i.e., marly formations). Excavation in these forma-
seamy structure and GSI between 45 and 60, although in tions should always be undertaken using conventional
some cases blasting might be necessary in this zone of the methods, e.g., shovels and bulldozers.
chart.
An overall assessment of excavatability of rock masses
Although the rock material itself is not rippable due to is presented in the decision chart in Fig. 13.
its high strength, the fractured rock mass indicates a low
block volume which would allow ripping. Heavy rippers Conclusions
(D8 and heavier) can be used up to GSI of between 20 and
45 for very blocky rock masses and 30 for seamy and The Geological Strength Index (GSI) was used to assess the
disintegrated rock masses. It can be seen from the chart, ease of excavation of rock masses. The 61 sites investi-
however, that for rock masses with a disintegrated and gated included sedimentary (limestone, sandstone and
laminated/sheared structure, digging is only applicable for siltstone) and metamorphic (gneiss, schist and marble) rock
GSI \ 30. masses with a variety of rock structures and discontinuity
surface conditions. The majority of the rocks exhibited a
The applied excavation method in relation to rock mass blocky to very blocky structure with a significant number
type for intact rock strength higher than 3 MPa is presented of blocky/disturbed/seamy and disintegrated rock masses.
in Table 5.
The proposed classification method takes into account
Heterogeneous rock masses (flysch and molasses) the point load strength of the intact rock and the rock mass
and soft rocks structure. Two GSI classification charts are proposed:
(a) for rock masses with Is50 \3 MPa, and (b) for rock
The proposed classification cannot be used for the assess- masses with Is50 C 3 MPa.
ment of the excavation method/ease of excavation in het-
erogeneous rock masses, as the flysch or molasse formation It was found that blasting is required when GSI values
(alternations mainly of siltstone or clay shales and stronger are greater than 65 when Is50 C 3 MPa and 60 when
sandstone layers) and in bimrocks (blocks in matrix rocks) Is50 \ 3 MPa, hence blasting is usually required in mas-
such as ophiolitic complexes with strong blocks in weak sive, blocky and very blocky rock masses or when joints
surrounding material, as well as volcanic formations, i.e., are tight.
agglomerate tuffs. However, the proposed method of
Successful ripping is generally achieved for rock masses
with GSI values between 20 and 45. However, as the

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Excavatability assessment of rock masses using GSI 27

strength affects the ripping, the GSI range is between 20 Hoek E (1994) Strength of rock and rock masses. ISRM News J
and 45 for rock masses with point load strength of intact 2(2):4–16
rock Is50 C 3 MPa and between 25 and 55 for those with
Is50 \ 3 MPa. Hoek E, Karzulovic A (2000) Rock mass properties for surface mines.
Slope Stability in Surface Mining. In: Hustralid WA, McCarter
In the transitional zone between the ripping and blasting MK, van Zyl DJA (eds) Littleton, Colorado: Society for Mining,
areas of the GSI charts, excavation with hydraulic breakers Metallurgical and Exploration (SME), pp 59–70
is necessary.
Hoek E, Wood D, Shah S (1992) A modified Hoek—Brown criterion
It is emphasized that the proposed classification is for jointed rock masses. In: Proceedings of Rock Characteriza-
applicable only for rock masses where discontinuities tion, Symposium on International Society of Rock Mechanics:
control the excavation, thus is should not be used for the Eurock’92. Hudson JA (ed) British Geotechnical Society,
assessment of excavation in heterogeneous rock masses London, pp 209–214
(i.e., sheared flysch, bimrocks and soft rocks).
Hoek E, Kaiser PK, Bawden WF (1995) Support of underground
Acknowledgments The contribution of Athanasiou J., Makrinikas excavations in hard rock. Rotterdam, Balkema
A. and Zalachoris G., graduate students of the Geotechnical
Engineering Department, NTUA in the fieldwork is gratefully Hoek E, Marinos P, Benissi M (1998) Applicability of the Geological
acknowledged. Strength Index (GSI) classification for very weak and sheared
rock masses. The case of the Athens Schist Formation. Bull Eng
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