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

Transcription

1 Bull Eng Geol Environ (2010) 69:13 27 DOI /s 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 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 metamorphic rock masses. A wide variety of rock structures were considered, ranging from blocky to disintegrated, and different excavation methods have been used (blasting, hydraulic breaking, ripping and digging). The proposed method cannot be applied to heterogeneous 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 foundations. In order to describe the excavation of rocks, different terms have been used, related to the principle of excavation and the mechanics of fracture. These include cuttability, rippability, excavatability, 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 and rock masses and includes the methods of (a) digging, when easy/very easy excavation conditions G. Tsiambaos (&) H. Saroglou Geotechnical Engineering Department, School of Civil Engineering, National Technical University of Athens, 9 Iroon Polytechniou str., Athens, Greece gktsiamb@central.ntua.gr exist, (b) ripping, for moderate to difficult excavation conditions, and (c) blasting for very difficult excavation conditions. The knowledge of the physical and mechanical characteristics as well as the behavior of the geo-materials to be excavated is vital for the selection of the most effective method of excavation. Previous research Assessment of rock excavatability 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. A detailed review of the principal excavation methods is given in MacGregor et al. (1994) and Basarir and Karpuz (2004). Duncan (1969) states that the assessments to determine the ease or difficulty with which a rock mass may be excavated are based upon the consideration of: (a) (b) (c) the rock material forming the rock blocks within the in situ rock mass because excavation entails fragmentation and rupture of the rock materials when the block volume is large, the nature, extent and orientation of the fractures, and the geological structure with respect to folding and faulting. Initially, Franklin et al. (1971) proposed a method to assess the excavation of rock based on the point load strength of intact rock, Is 50, and on the fracture spacing index, I f, which is the mean spacing of joints along a

2 14 G. Tsiambaos, H. Saroglou 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. 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 Fig. 2 a Sandstone and b limestone, both corresponding to the very blocky rock mass type

3 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.

4 16 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 Is 50 (MPa) average Is 50 (MPa) range I f (cm) average I f (cm) range Gneiss S2, S3 D2, D3, D Weathered gneiss 35 S3 D Schist S2, S3, S4, S6 D2, D3, D a Limestone S2, S3, S5 D2, D3, D b Sandstone S2, S3, S4 D1, D2, D3, D Marble S2 D1, D Siltstone S4, S5 D3, D 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. Quantification of GSI classification block volume of the rock mass According to Palmström (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 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 Field investigation methodology 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):

5 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.

6 18 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 (Is 50 ) 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, Is 50, 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 )hada 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 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. Fig. 7 Relationship between point load strength and excavation method Fig. 8 Plot of point load strength versus GSI for different excavation methods

7 Excavatability assessment of rock masses using GSI 19 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 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)

8 20 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 Fracture spacing I f (cm) Is 50 (MPa) Excavation method B5 Schist S2D Blasting B6 Limestone Sparitic S2D Blasting B7 Marble S2D Blasting B8 Marble S2D Blasting B9 Marble S2D Blasting B10 Sandstone S2D Blasting H4 Amphibolitic Schist S2D Hammer H5 Amphibolitic Schist S2-3D Hammer H6 Mica schist S2D Hammer H7 Mica schist S2D Hammer H8 Amphibolitic Schist S3D Hammer H9 Limestone micritic S2D Hammer H10 Gneiss S3D Hammer R11 Sandstone S2D Ripper D8 R12 Sandstone S2D Ripper D8 R13 Sandstone S2D Ripper D8 R14 Sandstone S4D Ripper D8 R15 Sandstone quartzitic S2D Ripper D8 R16 Sandstone quartzitic S4D Ripper D8 R17 Sandstone quartzitic S4D Ripper D8 R18 Sandstone quartzitic S3D Ripper D8 R19 Sandstone silty S3D Ripper D8 R20 Mica Gneiss S3D Ripper D8 R21 Gneiss S2D Ripper D8 R22 Gneiss S2-3D Ripper D8 R23 Limestone micritic S3D Ripper D9 R24 Mica Gneiss S3D Ripper D9 R25 Mica Gneiss S3D Ripper D9 R26 Granitic Gneiss S3D Ripper D9 R27 Sandstone S3D Ripper D9 R28 Sandstone S3D Ripper D9 R29 Sandstone S3D Ripper D9 R30 Schist S4D Ripper D10 R31 Sandstone S3D Ripper D7-Digger R32 Sandstone S3D Ripper D7-Digger R33 Sandstone Siltstone S3D Ripper D7-Digger R34 Sandstone S3D Ripper D7-Digger D3 Siltstone S4D Digger D4 Mylonitic limestone S5D4 25 Digger D5 Schist S6D Digger D6 Limestone S5D Digger D7 Calcareous schist S6D 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 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

9 Excavatability assessment of rock masses using GSI 21 agreement with the methods actually used at the investigated sites but the use of the Q system was less consistent with field practice. Guidelines concerning I f and Is 50 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. (a) (b) (c) Rock masses that have a joint spacing, I f, greater than 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)

10 22 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 Is 50 = 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 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 conjunction 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 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 C 3 MPa) Site number Rock type Structure/ discontinuity GSI Fracture spacing I f (cm) Is 50 (MPa) Excavation method B1 Schist S2D Blasting B2 Schist S2D Blasting B3 Marble S2D Blasting B4 Sandstone S3D Blasting H1 Schist S2D Hammer H2 Crystalline limestone S3D Hammer H3 Crystalline limestone S3D Hammer R1 Limestone S3D Ripper D9 R2 Limestone S3D Ripper D9 R3 Limestone S3D Ripper D9 R4 Mica Gneiss S3D Ripper D9 R5 Sandstone S3D Ripper D8 R6 Sandstone S4D Ripper D8 R7 Sandstone S4D Ripper D8 R8 Mica Gneiss S3D Ripper D8 R9 Gneiss S3D Ripper D8 R10 Mylonitic limestone S5D3 30 Ripper D7 D1 Mylonitic limestone S5D4 20 Digger D2 Siltstone S5D3 25 Digger

11 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-Is 50 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) For rock masses with a point load strength (Is 50 ) between 0.5 and 3 MPa; (b) For rock masses with a point load strength (Is 50 ) equal to or above 3 MPa. In order to correlate the excavatability method with GSI classification, categories of rock mass types were

12 24 G. Tsiambaos, H. Saroglou Table 4 Excavation method for different rock mass types (Is 50 \ 3 MPa) Intact rock strength Method of excavation Rock mass type based on GSI (Structure-Discontinuity condition) S1 S2D1 S2D2 S2D3 S2D4 S2D5 S3D1 S3D2 S3D3 S3D4 S3D5 S4D1 S4D2 S4D3 S4D4 S4D5 S5D1 S5D2 S5D3 S5D4 S5D5 Is50<3 MPa Drill & Blast or hammer X X X X Ripper (D8, D9) X X X X X X X X X Ripper (D7) X X X Digging X X X X 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 Table 5 Excavation method for different rock mass types (Is 50 C 3 MPa) Intact rock strength Method of excavation Rock mass type based on GSI (Structure-Discontinuity condition) S1 S2D1 S2D2 S2D3 S2D4 S2D5 S3D1 S3D2 S3D3 S3D4 S3D5 S4D1 S4D2 S4D3 S4D4 S4D5 S5D1 S5D2 S5D3 S5D4 S5D5 Drill & Blast or hammer X X X X X Is50 3 MPa Ripper (D8, D9) X X X X X X X X Ripper (D7) X Digging X X X 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 surface conditions of discontinuities. Each rock mass type is given a code in the form of S (number) for rock mass structure and D (number) for discontinuity condition. For example, the intact/massive structure is defined as S1 and the laminated/sheared rock mass as S6, while discontinuities with a very good condition are defined as D1 and those with a very poor condition D6. Thus, a rock mass that has a very blocky structure and good condition of discontinuities would be described with S3 and D2 (S3D2). Excavatability assessment using GSI Samples with a rock strength lower than 3 MPa are classified in the GSI chart shown in Fig. 9; the detailed data concerning the rock mass characteristics and excavation method are presented in Table 2. It is evident that blasting was required in blocky rock masses with a fair to very good discontinuity condition (S2D1 to S2D3). Hydraulic breaking was used in similar rock conditions and in some cases in very blocky rock masses. Most of the rock masses excavated with rippers (D8, D9 and D10) have a very blocky structure with poor to good joint surface conditions (S3D2 to S3D4), while some have a blocky/disturbed/seamy structure (S4D2 to S4D3). To some extent ripping was also successful in blocky rock masses with fair joint surface conditions (S2D3). Easy ripping conditions (D7 rippers) were encountered in very blocky rock masses with poor joint conditions (S3D4) while rocks with a seamy, disintegrated and laminated/sheared structure and poor joint (or schistosity) surface conditions (S4D4 to S6D4) were excavated with digging equipment. The GSI classification for rock masses with strengths above 3 MPa is shown in Fig. 10 and their relevant rock mass characteristics and the excavation method used are summarized in Table 3. Blasting was used for rock masses with a blocky structure and fair to good joint surface conditions (S2D2 to S2D3) and for rocks with a very blocky structure with good joint conditions (S3D2). Hydraulic breaking was used in some very blocky rock masses while heavy ripping equipment (D8, D9) was used to excavate the very blocky and blocky/disturbed/seamy rock masses with poor to fair joint surface conditions (S3D3 to S3D4 and S4D3 to S4D4). Diggers were only used in the disintegrated limestone rock masses (S5D3 to S5D4).

13 Excavatability assessment of rock masses using GSI 25 Fig. 12 Proposed GSI chart for the assessment of excavatability of rock masses (Is 50 C 3 MPa) Proposed excavatability charts using GSI Based on the GSI classification of the rock masses, the following excavation charts are proposed: GSI excavation chart with Is 50 \ 3 MPa The proposed excavation method categories in the GSI chart for rock masses with intact rock strength less than about 70 MPa (Is 50 \ 3 MPa) are shown in Fig. 11. Blasting is necessary for rock masses with GSI [ 65 and blocky or very blocky rock structures. Hydraulic breaking is required for the loosening of rock masses with GSI between 55 and 65 while ripping is successful in rock masses with GSI \ 55. The lower margin for ripping depends on the rock structure, thus for very blocky rock masses it is around 25 but in blocky/disturbed/seamy and disintegrated material it is 35. Rock masses with GSI up to 25 (or 35) can be dug, obviously with increasing difficulty. The applied excavation method in relation to rock mass type for material with Is 50 \ 3 MPa is presented in Table 4.

14 26 G. Tsiambaos, H. Saroglou Fig. 13 Overall assessment of excavatability of rock masses GSI excavation chart with Is 50 C 3 MPa Figure 12 shows the proposed excavation method categories in the GSI chart, for rock masses with intact rock strengths greater 70 MPa (Is 50 C 3 MPa). It can be seen that blasting is required when GSI [ 60 (the rock structure is blocky or very blocky). The transitional zone where hydraulic breakers should be used to loosen the rock mass is applicable to rock masses with a blocky, very blocky or seamy structure and GSI between 45 and 60, although in some cases blasting might be necessary in this zone of the chart. Although the rock material itself is not rippable due to its high strength, the fractured rock mass indicates a low block volume which would allow ripping. Heavy rippers (D8 and heavier) can be used up to GSI of between 20 and 45 for very blocky rock masses and 30 for seamy and disintegrated rock masses. It can be seen from the chart, however, that for rock masses with a disintegrated and laminated/sheared structure, digging is only applicable for GSI \ 30. The applied excavation method in relation to rock mass type for intact rock strength higher than 3 MPa is presented in Table 5. Heterogeneous rock masses (flysch and molasses) and soft rocks The proposed classification cannot be used for the assessment of the excavation method/ease of excavation in heterogeneous rock masses, as the flysch or molasse formation (alternations mainly of siltstone or clay shales and stronger sandstone layers) and in bimrocks (blocks in matrix rocks) such as ophiolitic complexes with strong blocks in weak surrounding material, as well as volcanic formations, i.e., agglomerate tuffs. However, the proposed method of excavatability assessment is appropriate in the case of flysch formations with thick beds of sandstones. It is also not applicable to hard soils/soft rocks, especially those characterized as very weak to moderately weak rocks (Hawkins 2000) with intact rock strengths between 1.25 and 10 MPa. In this case, the discontinuities have a secondary and minor role in the behavior of the rock mass (i.e., marly formations). Excavation in these formations should always be undertaken using conventional methods, e.g., shovels and bulldozers. An overall assessment of excavatability of rock masses is presented in the decision chart in Fig. 13. Conclusions The Geological Strength Index (GSI) was used to assess the ease of excavation of rock masses. The 61 sites investigated included sedimentary (limestone, sandstone and siltstone) and metamorphic (gneiss, schist and marble) rock masses with a variety of rock structures and discontinuity surface conditions. The majority of the rocks exhibited a blocky to very blocky structure with a significant number of blocky/disturbed/seamy and disintegrated rock masses. The proposed classification method takes into account the point load strength of the intact rock and the rock mass structure. Two GSI classification charts are proposed: (a) for rock masses with Is 50 \3 MPa, and (b) for rock masses with Is 50 C 3 MPa. It was found that blasting is required when GSI values are greater than 65 when Is 50 C 3 MPa and 60 when Is 50 \ 3 MPa, hence blasting is usually required in massive, blocky and very blocky rock masses or when joints are tight. Successful ripping is generally achieved for rock masses with GSI values between 20 and 45. However, as the

15 Excavatability assessment of rock masses using GSI 27 strength affects the ripping, the GSI range is between 20 and 45 for rock masses with point load strength of intact rock Is 50 C 3 MPa and between 25 and 55 for those with Is 50 \ 3 MPa. In the transitional zone between the ripping and blasting areas of the GSI charts, excavation with hydraulic breakers is necessary. It is emphasized that the proposed classification is applicable only for rock masses where discontinuities control the excavation, thus is should not be used for the assessment of excavation in heterogeneous rock masses (i.e., sheared flysch, bimrocks and soft rocks). Acknowledgments The contribution of Athanasiou J., Makrinikas A. and Zalachoris G., graduate students of the Geotechnical Engineering Department, NTUA in the fieldwork is gratefully acknowledged. References Abdullatif OM, Cruden DM (1983) The relationship between rock mass quality and ease of excavation. 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