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 Tsiambaos H. Saroglou
Received: Received: 30 June 2009 / Accepted Accepted:: 20 July 2009/ Publishe Published d online: 14 August 2009 Springer-Verlag 2009
Abstract In the present study a new classification method for the assessment of ease of excavation of rock masses is proposed, based on the Geological Strength Index and the point load strength of the intact rock. The data originate from excavation sites in Greece in sedimentary and metamorphi morphicc rock rock masses masses.. A wide wide variety variety of rock rock structu structures res were considered, ranging from blocky to disintegrated, and differ different ent excava excavatio tion n method methodss have have been been used used (blast (blasting ing,, hydraulic hydraulic breaking, breaking, ripping and digging). The proposed proposed method method cannot cannot be applied to heterogeneo heterogeneous us rock masses and soft rocks/hard soils. Keywords GSI Excavatability Rockmass Rippability Rock strength
Introduction
Predicting the ease of excavation of rock and rock masses is very significant in earthworks for highway construction or other civil engineering works, in surface mines and also for founda foundatio tions. ns. In order order to descri describe be the excava excavation tion of rocks, different terms have been used, related to the principle of excavation and the mechanics of fracture. These include include cuttability, cuttability, rippability, rippability, excavatabili excavatability, ty, diggability diggability and drillability. In the present work, the term excavatability is used as a broad term that refers to the ease of excavation of rock rock and and rock rock masse massess and and incl includ udes es the the meth method odss of (a) diggin digging, g, when when easy/v easy/very ery easy easy excava excavatio tion n condit condition ionss
G. Tsiambaos (&) H. Saroglou Geotechnical Engineering Department, School of Civil Engineering, National Technical University of Athens, 9 Iroon Polytechniou str., 157 80 Athens, Greece e-mail:
[email protected] [email protected]
exist, exist, (b) rippin ripping, g, for modera moderate te to difficu difficult lt excava excavatio tion n conditions, conditions, and (c) blasting for very difficult excavation excavation conditions. The knowledge of the physical and mechanical characteristics as well as the behavior of the geo-materials to be exca excavat vated ed is vita vitall for for the the selec selecti tion on of the the most most effective effective method of excavation. excavation.
Previous research
Assessment Assessment of rock excavatabil excavatability ity All the methods used for the assessment of excavatability or rippability of rock take into account the uniaxial compressive strength, weathering degree and spacing of discontinuities. Some of them also include seismic velocity, as well as the continuity, aperture, orientation and roughness of joints joints.. A detaile detailed d review review of the princip principal al excava excavatio tion n methods is given in MacGregor et al. (1994 (1994)) and Basarir and Karpuz (2004 (2004). ). Duncan (1969 (1969)) states that the assessments to determine the the ease ease or diffi difficu culty lty with which which a rock rock mass mass may may be excavated are based upon the consideration of: (a)
the rock rock material material formin forming g the rock blocks blocks withi within n the in situ rock mass—becau mass—because se excavation excavation entails fragmentation and rupture of the rock materials when the block volume is large, (b) the nature, nature, extent and orientat orientation ion of the fractures fractures,, and (c) the geolog geologica icall struct structure ure with respec respectt to folding folding and faulting. Initia Initially, lly, Franklin Franklin et al. (1971 1971)) proposed a method to asse assess ss the the exca excava vati tion on of rock rock base based d on the the poin pointt load load strength of intact rock, Is50, and on the fracture spacing index, index, If , whic which h is the the mean mean spac spacin ing g of join joints ts alon along g a
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scanline. Atkinson (1971) suggested that the ease of excavation can be predicted using the velocity of longitudinal waves in the rock mass for different rock types. Scoble and Muftuoglu (1984) proposed a classification 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 presented a detailed chart, which is similar to that proposed by Franklin et al. (1971) but with a more detailed categorization 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 classification system for Coal Measures and marls for use in lignite 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 rippability 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 environment. 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.
G. Tsiambaos, H. Saroglou
parameters based on Barton et al. (1974) Q system. Fowell and Johnson (1982), Smith (1986), MacGregor et al. (1994) and Hadjigeorgiou and Poulin (1998) have also developed
Fig. 1 Layered marble corresponding to the blocky rock mass type
Rock mass classification for estimation of excavatability Rock mass classification systems have also been used for the assessment of excavatability. Weaver’s (1975) classification 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 excavatability index (N), which is determined by the use of several
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Fig. 2 a Sandstone and b limestone, both corresponding to the very blocky rock mass type
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 corresponding to the blocky/disturbed/seamy rock masses
grading classification systems for the assessment of rock rippability. Additionally, Abdullatif and Cruden (1983) presented an assessment of ease of excavation and productivity in relation to rock mass quality using the RMR system. Recently, Hoek and Karzulovic (2000) used the data from Abdullatif and Cruden (1983) to estimate the Geological Strength Index, GSI and strength of these rock masses and suggested a range of GSI for different excavation methods. They proposed that rock masses can be dug up to GSI values of about 40 and rock mass strength values of about 1 MPa, 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.
Fig. 5 Studied rocks superimposed on the Franklin chart
In the present study the Geological Strength Index (GSI), as proposed by Marinos and Hoek (2000) was used in order to describe the rock masses and correlate each rock mass type with the applicability of the available excavation methods. In this approach, the intact rock strength was taken into account and the properties of the discontinuity sets and fracture spacing (controlling the size of rock blocks) were carefully evaluated. The advantage of the proposed classification is that it is a qualitative tool for easy and quick assessment of excavatability.
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G. Tsiambaos, H. Saroglou
Table 1 Range of point load strength and rock mass classification for different geological formations
Rock mass type
GSI
Rock structure
Discontinuity surface
Is50 (MPa) average
Is50 (MPa) range
I f (cm)
I f (cm)
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
Schist
15–70
S2, S3, S4, S6
D2, D3, D4
2.20
0.80–4.60
49
23–160 a
Limestone
20–65
S2, S3, S5
D2, D3, D4
2.45
0.70–4.00
45
20–80 b
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
25
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 The Geological Strength Index (GSI) was introduced by Hoek et al. (1992), Hoek (1994) and Hoek et al. (1995). This index was subsequently extended for weak rock masses in a series of papers by Hoek et al. (1998) and Marinos and Hoek (2000). Later, Marinos and Hoek (2001) proposed a chart of the Geological Strength Index for heterogeneous rock masses, such as flysch, which is frequently composed of tectonically disturbed alternations of strong and weak rocks (sandstone and siltstone, respectively). This chart was modified by Marinos et al. (2007). The GSI relates the properties of the intact rock elements/blocks to those of the overall rock mass. It is based on an assessment of the lithology, structure and condition of discontinuity surfaces in the rock mass and is estimated from visual examination of the rock mass exposed in outcrops, surface excavations such as road cuts, tunnel faces and borehole cores. It utilizes two fundamental parameters of the geological process (blockiness of the mass and condition of discontinuities), hence takes into account the main geological constraints that govern a formation. In addition, the index is simple to assess in the field.
therefore built on the linkage between descriptive geological terms and measurable field parameters such as joint spacing and roughness. The rock mass type is a controlling factor in the assessment of the excavation method, as it is closely related to the number of discontinuity sets and reflects the rock mass structure. The Geological Strength Index, in its original form, was not scale dependant, thus the rock block size is not directly related to the rock mass type. Nevertheless, each rock type has a broad correlation to the rock block size, i.e., a blocky rock mass has larger blocks than a very blocky rock mass or a disintegrated rock mass which is made up of very small rock fragments. This correlation is only informative, however, and is not applicable to certain rock mass types, e.g., sheared schist rock masses, as the spacing of the schistosity planes equates to the discontinuity planes and hence the concept of block volume is not applicable. For this reason, the present classification for the assessment of excavatability is based on the original GSI charts (2000 version), but specific reference to the block volume is made.
Characteristics of investigated rock masses
Quantification of GSI classification—block volume of the rock mass
Field investigation—methodology
According to Palmstro¨m (2000), block size and discontinuity spacing can be measured by means of the Volumetric Joint Count J v, or the mean block volume, V b. Sonmez and Ulusay (1999) quantified block size in the GSI chart by the Structure Rating coefficient (SR) that is related to the J v coefficient. Cai et al. (2004) presented a quantified GSI chart and suggested that the block size is quantified by the mean discontinuity spacing S or by the mean block volume V b. The structure was quantified by joint spacing in order to calculate the block volume, and the joint surface condition was quantified by a joint condition factor. The GSI is
The field investigation was carried out at highway construction sites in Greece. In general, the rocks involved were sedimentary (limestone, sandstone and siltstone) and metamorphic (gneiss, schist and marble). The most predominant rock types were sandstone and limestone. The field investigation in sixty-one (61) selected locations included the determination of rock mass properties, the excavation method and its performance in terms of production against time. In order to describe and classify 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) (b) (c) (d) (e) (f)
rock type, joint set number, joint spacing, joint orientation, joint surface condition, degree of weathering.
Laboratory testing of the block samples from each site included determination of unit weight and point load strength in accordance with the methods suggested by ISRM (1985). All the rock masses examined were rated according to the Geological Strength Index.
Rock mass classification The rock masses studied generally have a blocky (18 sites) and very blocky structure (29 sites). The discontinuity conditions of the blocky rock masses are fair, good and very good. For the very blocky rock masses, the discontinuities are poor, fair and good. Some rock masses (7 sites) have a blocky/disturbed/seamy structure and good to fair discontinuity surface conditions. Finally, a few disintegrated (5 sites) and laminated/sheared rock masses (2 sites) were found with fair to poor joint surface conditions.
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G. Tsiambaos, H. Saroglou
The sandstone, limestone, gneiss, marble and schist (amphibolitic and micaceous) rock masses have a blocky structure, as shown in Fig. 1. Gneiss, limestone and sandstone rock masses were also found to have a very blocky structure (Fig. 2a, b). Blocky/disturbed/seamy rock masses were found in folded thinly bedded limestone (Fig. 3a) and in folded schist environments (Fig. 3b). Finally, some heavily fractured limestones affected by tectonic activity appear totally disintegrated and broken (as shown in Fig. 4). The laminated/sheared structure was encountered only in the schists. The point load index (Is50) of the different rocks ranges between 0.5 and 5.0 MPa. The lower values originate from weathered rocks. The range of point load strength, Is50, and fracture spacing, I f, of discontinuities as well as the rock classification of the different geological formations are given in Table 1. The fracture spacing ( I f) had a relatively wide range. The average fracture spacing is higher for the gneiss and marble rock masses with a blocky and very blocky structure. The limestone, schist and sandstone rock masses with a blocky/disturbed/seamy and disintegrated structure have lower average fracture spacings. It should be emphasized that a realistic determination of fracture spacing is often difficult. The three-dimensional development of discontinuities should not be underestimated when calculating the fracture spacing. Moreover, fracture spacing in laminated/sheared schist rock masses expressed by the schistosity planes (acting as the predominant discontinuity) and in disintegrated limestones, which are brecciated by faults, is not meaningful.
Assessment of excavatability using existing methods
Fig. 7 Relationship between point load strength and excavation method
Fig. 8 Plot of point load strength versus GSI for different excavation methods
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Franklin et al. (1971) method The oldest graphical indirect rippability assessment method is that of Franklin et al. (1971). It considers two parameters: the fracture spacing, I f, and strength values of intact rock. Franklin’s method has been re-evaluated and modified by many researchers; the most well known being Pettifer and Fookes (1994). Although this graph allows excavatability to be assessed rapidly, the subdivisions have become outdated as more powerful, more efficient equipment has become available. The Franklin et al. (1971) chart shows that most of the rock masses encountered in the selected sites would have to be excavated with blasting to loosen the rock mass and some (9 of the 61) with ripping. However, as shown in Fig. 5, most of the rock masses (29) were excavated using rippers, indicating that the chart is quite conservative and predicts more difficult excavation conditions than is actually the case with modern machinery. Pettifer–Fookes (1994) classification method Pettifer and Fookes (1994) emphasized the value of a threedimensional discontinuity spacing index as this provides a more realistic assessment of the average block size. With Pettifer and Fookes’ chart (Fig. 6), the evaluation of excavatability is simple and hence the chart is still commonly used (Kentli and Topal 2004; Gurocak et al. 2008). However, the rock mass data from the present study indicate that it underestimates the difficulty of excavation.
Excavatability assessment of rock masses using GSI
For material falling in the region of the chart where D6 and D7 rippers are proposed, in four sites D8 rippers were required and in six sites D9 rippers were used. In only three sites were the D7 rippers appropriate. In ten sites the predicted D8 equipment was used, but in six sites heavier (D9) rippers were necessary. In eight sites where D8 or D9 rippers were predicted, hydraulic breaking, or rippers and hydraulic hammers were used. This deviation from the predicted conditions could be attributed to the accuracy of measuring the fracture index of the predominant joint sets, which is somewhat
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subjective, and also to the fact that in many sites other construction matters may have been involved in the decision to use heavier equipment. Prediction using the RMR and Q rock mass classification systems Abdullatif and Cruden (1983) proposed that a rock mass can be dug up to Rock Mass Rating (RMR) values of 30 and ripped up to RMR values of 60 while a rock mass rated as ‘‘good’’ or higher would require blasting. They also state
Fig. 9 GSI classification for tested rocks with intact rock strength (Is 50 \ 3 MPa)
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G. Tsiambaos, H. Saroglou
Table 2 Detailed rock mass data and excavation methods used on study sites (point load strength of intact rock Is 50 \ 3 MPa)
Site number
Rock type
Structure/ discontinuity
GSI
B5
Schist
S2D3
60
B6
Limestone Sparitic
S2D2
B7
Marble
B8
Is50 (MPa)
Excavation method
80
2.6
Blasting
65
40
1.7
Blasting
S2D1
75
40
2.5
Blasting
Marble
S2D1
70
40
1.8
Blasting
B9
Marble
S2D1
65
50
2.7
Blasting
B10
Sandstone
S2D2
60
100
2.7
Blasting
H4
Amphibolitic Schist
S2D2
70–75
36
1.8
Hammer
H5
Amphibolitic Schist
S2-3D3
50–55
26
1.2
Hammer
H6
Mica schist
S2D2
65
70
1.3
Hammer
H7
Mica schist
S2D3
55
72
1.3
Hammer
H8
Amphibolitic Schist
S3D2
55–60
30
1.4
Hammer
H9
Limestone micritic
S2D3
55
80
2.9
Hammer
H10
Gneiss
S3D2
60
150
2.2
Hammer
R11
Sandstone
S2D2
50–55
50
1.7
Ripper D8
R12
Sandstone
S2D3
50
80
–
Ripper D8
R13
Sandstone
S2D2
50
40
2.3
Ripper D8
R14
Sandstone
S4D2
45
–
1.3
Ripper D8
R15
Sandstone quartzitic
S2D2
50–55
50
1.7
Ripper D8
R16
Sandstone quartzitic
S4D3
40
20
2.8
Ripper D8
R17
Sandstone quartzitic
S4D3
35
30
–
Ripper D8
R18
Sandstone quartzitic
S3D3
40–45
30
0.9
Ripper D8
R19
Sandstone silty
S3D3
40
30
2.2
Ripper D8
R20
Mica Gneiss
S3D4
35
30
1.3
Ripper D8
R21
Gneiss
S2D3
50
100
–
Ripper D8
R22
Gneiss
S2-3D3
45
100
1.7
Ripper D8
R23
Limestone micritic
S3D3
45
30
0.7
Ripper D9
R24
Mica Gneiss
S3D4
35
30
0.6
Ripper D9
R25
Mica Gneiss
S3D4
35–40
30
1.4
Ripper D9
R26
Granitic Gneiss
S3D3
40–45
30
1.7
Ripper D9
R27
Sandstone
S3D1-2
55–60
50
1.9
Ripper D9
R28
Sandstone
S3D2
55–60
–
0.8
Ripper D9
R29
Sandstone
S3D2
55
–
2.0
Ripper D9
R30
Schist
S4D2
40–45
23
2.2
Ripper D10
R31
Sandstone
S3D3
40–45
20
0.7
Ripper D7-Digger
R32
Sandstone
S3D4
30
–
2.9
Ripper D7-Digger
R33
Sandstone–Siltstone
S3D4
30–35
–
0.9
Ripper D7-Digger
R34
Sandstone
S3D3
40–45
30
1.1
Ripper D7-Digger
D3
Siltstone
S4D4
30
–
0.5
Digger
D4
Mylonitic limestone
S5D4
25
–
–
Digger
D5
Schist
S6D4
15
0.8
Digger
D6
Limestone
S5D4
20
0.7
Digger
D7
Calcareous schist
S6D4
15
0.9
Digger
that rocks with a Q value up to 0.14 can be dug but those with Q values above 1.05 require ripping. However, they pointed out that the use of Q as a guide to excavation methods presents problems, as there is an overlap where
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Fracture spacing I f (cm)
rocks with Q values between 3.2 and 5.2 can be ripped and/ or require blasting. The present study found Abdullatif and Cruden’s (1983) ranges for digging, ripping and blasting are in good
Excavatability assessment of rock masses using GSI
agreement with the methods actually used at the investigated sites but the use of the Q system was less consistent with field practice.
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(a)
(b) Guidelines concerning I f and Is50 From the evaluation of the data from this study using the classification methods of Franklin et al. (1971) and Pettifer and Fookes (1994), the following conclusions can be drawn concerning fracture spacing and point load strength of intact rock.
(c)
Rock masses that have a joint spacing, I f, greater than 0.3–0.5 m and a point load strength of intact rock greater than 1 MPa have to be excavated using either hydraulic breaking or blasting. Rock masses with fracture spacing of less than about 100 mm (close to very close spacing according to ISRM 1981) can be excavated by rippers or diggers irrespective of the point load strength of the intact rock. 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 (Is 50 C 3 MPa)
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G. Tsiambaos, H. Saroglou
spacing ( I f). No data from rock masses with intact rock strength lower than 0.5 MPa were available. A point load strength value equal to Is50 = 3.0 MPa and fracture spacing of I f = 0.3 m proved to be threshold values below which ripping was performed in the majority of the sites. The intact rock strengths obtained were analyzed for the different excavation methods and the results are presented in the bar chart in Fig. 7. In summary, (a)
(b)
(c)
Rock masses excavated with blasting had an intact point load strength of between 2 and 5 MPa, with a mean value of 3 MPa. Rock masses excavated using a hydraulic hammer in conjunction with ripping are characterized by point load strengths between 1.2 and 3 MPa (mean strength 2.3 MPa). Rock masses excavated using rippers have point load strengths in the range of 0.5–5 MPa with a mean value of 2 MPa.
Proposed classification
General An assessment of the excavatability of the rock masses encountered on the selected sites, based on the most
commonly used prediction methods, proved that the selection of the excavation method depends on the parameters which are taken into account. In the RMR and Q classification systems, ground water and joint orientation will influence the total ranking, while in both the Franklin and Pettifer–Fookes classification charts, the correct assessment of the fracture spacing is significant. The study has shown that the GSI classification in con junction with the intact rock strength can produce a qualitative categorization of excavation methods for rock masses. In this procedure, the rock structure and the joint surface conditions are important. For example, if the joints in a rock mass are tight or very tight (separation of discontinuity surfaces less than 0.5 mm) it is most probable that the rock blocks cannot be detached and thus the rock mass will not be rippable, although, a joint spacing in the range of 0.1–0.5 m would allow ripping in most circumstances. If the joints are open (separation is between 2.5 and 10 mm) or very wide (between 10 and 25 mm), either empty or filled with soft material, and their spacing is between 0.5 and 1.0 m, rippers are commonly used as the rock blocks are separated relatively easily. However, the strength of the intact rock in the individual rock blocks is also important as excavation with rippers entails fragmentation and rupture of the rock itself. Sedimentary rocks which are well-bedded and jointed or closely interbedded strong and weak rocks can be excavated by ripping or digging.
Table 3 Detailed rock mass data and excavation methods used on study sites (point load strength of intact rock Is 50
Site number
Rock type
Structure/ discontinuity
GSI
B1
Schist
S2D3
60
B2
Schist
S2D2
B3
Marble
B4
MPa)
Is50 (MPa)
Excavation method
90
3.9
Blasting
70
160
4.2
Blasting
S2D2
65
70
4.2
Blasting
Sandstone
S3D2
55–60
4.8
Blasting
H1
Schist
S2D3
50
35
4.6
Hammer
H2
Crystalline limestone
S3D2
55
50
3.1
Hammer
H3
Crystalline limestone
S3D2
55–60
3.1
Hammer
R1
Limestone
S3D3
45
10
3.7
Ripper D9
R2
Limestone
S3D3
40–45
20
4.0
Ripper D9
R3
Limestone
S3D4
35
30
3.4
Ripper D9
R4
Mica Gneiss
S3D3
40
54
4.7
Ripper D9
R5
Sandstone
S3D4
40
4.8
Ripper D8
R6
Sandstone
S4D4
30
20
4.1
Ripper D8
R7
Sandstone
S4D3
35
20
3.9
Ripper D8
R8
Mica Gneiss
S3D4
35
30
3.1
Ripper D8
R9
Gneiss
S3D2
50
100
4.8
Ripper D8
R10
Mylonitic limestone
S5D3
30
–
–
Ripper D7
D1
Mylonitic limestone
S5D4
20
–
–
Digger
D2
Siltstone
S5D3
25
–
–
Digger
1 3
Fracture spacing I f (cm)
C 3
Excavatability assessment of rock masses using GSI
23
Fig. 11 Proposed GSI chart for the assessment of excavatability of rock masses (Is 50 \ 3 MPa)
A first assessment of the excavation methods in the study sites based on a GSI classification of the excavated rock mass and the point load strength of the intact rock is presented in Fig. 8. It is evident that three distinct regions exist in the GSI-Is50 chart, which correspond to the different excavation methods (blasting and/or use of hydraulic hammer, ripping and digging). For a given strength of rock, the ease of excavation increases as the rock mass quality decreases (lower GSI values), thus blasting can be substituted by ripping or even digging. The study also indicated the threshold value of strength of an intact rock, beyond which the rock mass requires blasting, is equal to 3 MPa. This value is similar to the
threshold values proposed in the literature; most researchers suggesting a UCS of 70 MPa, equivalent to a point load strength of 3 MPa (Bell 2004; McLean and Gribble 1985; Bieniawski 1975). Two classification charts are proposed for the assessment of excavation method based on GSI: (a) (b)
For rock masses with a point load strength (Is50) between 0.5 and 3 MPa; For rock masses with a point load strength (Is50) equal to or above 3 MPa.
In order to correlate the excavatability method with GSI classification, categories of rock mass types were
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G. Tsiambaos, H. Saroglou
Table 4 Excavation method for different rock mass types (Is 50 \ 3 MPa) Intact rock
Method of
strength
excavation
Drill & Blast
Rock mass type based on GSI (Structure-Discontinuity condition) 1 D 1 2 S S
X
X
2 D 2 S
3 D 2 S
X
X
4 D 2 S
5 D 2 S
1 D 3 S
2 D 3 S
3 D 3 S
4 D 3 S
X
X
X
X
X
X
X
X
5 D 3 S
1 D 4 S
2 D 4 S
3 D 4 S
X
X
4 D 4 S
5 D 4 S
X
X
1 D 5 S
2 D 5 S
3 D 5 S
4 D 5 S
5 D 5 S
X
X
4 D 5 S
5 D 5 S
X
X
or hammer Is 50
