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E L S E V I E R Engineering Geology 44 (1996) 183-201

ENGiNEERiNG GEOLOGY

Engineering geological and geotechnical responses of schistose rocks from dam project areas in India

M.H.N. Behrestaghi a, K. Seshagiri Rao b,,, T. Ramamurthy h Department of Mining Engineering, Faculty of Engineering, Tarbiat Modaress University, Tehran, Iran

b Department of Civil Engineering, Indian Institute of Technology, Hauz Khasm, New Delhi-110016, India

Received 21 September 1995; revision 9 May 1996; accepted 29 May 1996

Abstract

For a rational and safer design of civil and mining engineering structures in or on rocks, a proper understanding of the quality of rock mass is required. To assess the rock mass quality, evaluation of physical and mechanical characteristics of the intact rocks is essential. Especially if the rock is anisotropic in nature, the genetic complexity associated with its petrofabric makes it more difficult to predict its behaviour. In this paper, a comprehensive study of the compositional, physical and geotechnical responses of four varieties of schists, i.e., quartzitic, chlorite, quartz mica and biotite schists obtained from two hydroelectrical project sites in the foot hills of Himalayas, India, has been presented.

Anisotropic strength behaviour of the schists has been brought out through the testing of specimens with varying orientation of schistosity with respect to the major principal stress under uniaxial and triaxial conditions. The significance of anisotropic response for consideration in the design is emphasized.

Key words: Geological responses; Geotechnical responses; Schistose rocks; Dam project areas; India

1. Introduction

The engineering behaviour of rocks has been found to depend on the intrinsic properties such as mineralogical composition, cementing material, grain size and shape, texture and porosity. The presence of discontinuities, such as macro- and microfractures, bedding planes, schistosity, and faults make the rock weaker and dictate its overall behaviour. Out of three generic categories of rocks, metamorphic rocks are mostly anisotropic due to the process of differentiation developing contrast-

* Corresponding author.

0013-7952/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0013-7952 (96)00069-5

ing layers of mineralogical composition. Large temperature and pressure gradients associated with tectonic activities lead to the development of schistosity, which is the main cause for the anisotropic behaviour of these rocks. For a rational and safer design it is essential to estimate the variation of strength and deformation behaviour and observe failure mechanisms of anisotropic rocks with orientation of stress.

2. Anisotropic behaviour

Study of anisotropic rocks involves preparation of rock specimens with planes of weakness inclined

184 M.H.N. Behrestaghi et aL/Engineering Geology 44 (1996) 183-201

Inhertnt

/ InducKI

! !

0 30 90 ~,degrce

Fig. 1. Possible variation of ac versus/3 for inherent and induced anisotropies (after Ramamurthy, 1993).

at angle fl with major principal stress O- 1 (Fig. 1). Many researchers have studied the effect of schistosity on the tensile strength of metamorphic rocks to demonstrate the tensile strength anisotropy in spite of the fact that consideration of this parame- ter is often neglected in the design of underground structures. Berenbaum and Brodie (1959), Hobbs (1963), and Chaberlain et al. (1976) studied the effect of orientation on Brazilian strength of different metamorphic rocks, and Broch and Franklin (1972) and Broch (1983) studied the point load strength anisotropy of metamorphic rocks. They observed that the tensile strength of metamorphic rocks increases as the orientation angle//changes from 0 to 90 ° with respect to the direction of loading.

Anisotropic strength in uniaxial compression of shales and slates was investigated by Donath (1964), Chenevert and Gatlin (1965), McLamore and Gray (1967), Hock (1968), AtteweU and Sandford (1974), and Brown et al. (1977). The studies were carried out on gneisses and schists (Deklotz et al., 1966, McCabe and Koerner, 1975), sandstones (Horino and Ellickson, 1970; Rao, 1984; Arora, 1987), chlorite and graphite schist (Akai, 1971), diatomite (Allirot and Boehler, 1979), coal (Pomeroy et al., 1971), and phyllite (Ramamurthy et al., 1988). The review of aforementioned works indicates that the failure strength is high at /~ =0 ° or 90 ° and is minimum when /~ varies from 20 to 40 ° . The shape of curve between the uniaxial compressive strength and the orientation angle//suggests one of the "types of anisotro-

pies" namely "U-shaped", "shoulder" shaped and "wavy" shaped (Fig. 1).

Study of modulus of elasticity, Et, under unconfined conditions with fl ranging from 0 to 90 ° for three schists, viz., Graywake schist I, Graywake schist II (Pinto, 1970) and Hast schist (Read et al., 1987), and three shales, viz., Moszczenica shale, Borynia shale and Wilchwy silty shale (Kwasniewski and Neuyen, 1986), Barnsley hard coal (Pomeroy et al., 1971) and diatomite (Allirot and Boehler, 1979), revealed the variation to be broadly "U-shaped" or "irregular shaped" with maximum value of modulus of elasticity at ~ = O" and minimum value in the range of fl varying from 30 to 60 °, and subsequent rise of modulus of elasticity at fl=75 °. Both types of variations are characterized with higher value of Et at ~ --- 0 ° than at ~ =90 °. Literature review on anisotropic schistose rocks reveals that the information on strength and deformation responses at different orientation angles (fl) and confining pressures (a3) of such rocks is scanty, and no definite approaches have been suggested for the prediction of the responses with minimum pre-evaluation data.

Keeping the above observations in view, a comprehensive investigation into the engineering behaviour of four varieties of schists is attempted. These schists were obtained from a tectonically active and complex geological sequence of rocks in the Himalayan region. These rocks are the foundation materials of two hydroelectrical pro- jects which are under different stages of construction in the region. In this paper it has been

M.H.N. Behrestaghi et al./Engineering Geology 44 (1996) 183-201 185

attempted specifically to study the basic lithology, texture and physical properties and strength indices of Indian schists and the effect of schistosity on these properties and also to investigate the triaxial compression behaviour of these intact rocks under high confining pressure at different specimen orientations.

3. Geology of the site and rocks tested

Quartzitic and chlorite schists were collected from the Uri hydroelectrical power project in Baramula District, Kashmir (Fig. 2). This project envisages construction of a concrete dam on the river Jhelum with a I0 km long and 8 m diameter of head race tunnel in quartzitic schist belonging to the Tanawal series (Precambrian). An underground power house with a tail race tunnel is being constructed in chlorite schist belonging to the Panjal traps (Precambrian to Eocene). In the eastern side, the Tanawals are overlaid by Panjal traps which are light green to greenish coloured lava flows, which have been chloritized and developed schistosity. In general the strike of the formations is N30°E-S30°W dipping 70-80 ° in a northerly direction.

The other two less competent schistose rocks, i.e., quartz mica schist and biotite schists were obtained from the foundation of the underground power house site at Nathpa-Jhakri hydroelectrical

project in Kinnaur District, Himachal Paradesh (Fig. 2). The scheme envisages construction of a 60.5 m high concrete dam on the river Sutlej at Nathpa with 27.78 km long and 10.15 m diameter head race tunnel and underground powerhouse at Jhakri, and a 280 m long tail race tunnel. The rocks of the area are quartz mica schist, biotite schist, granite gneiss, amphibolite and pegmatite belonging to Wangatu Jeori gneissic complex of Precambrian age. The foliation trend of these metamorphic rocks generally varies from N70°W-S70°E to N70°E-S70°W having an average dip of the order of 35 ° in the northerly direction.

4. Experimental work

In accordance with the objective mentioned earlier various tests were conducted on the four schistose rocks following Indian (IS: 10082, 1981) and international standards and practices. The tests are classified under the following three major categories: (i) petrography and petro-fabric; (ii) physical properties; and (iii) geotechnical properties.

4.1. Specimen preparation

Large size blocks were trimmed with their sides perpendicular to each other to facilitate coring at

36

32

2B

2~

68 72 76 80 8~

t / r I

/ Project site /'1~'

I

88 92 96 100

i 1 32

28

2&

Fig. 2. Location of Uri and Nathpa-Jhakri project sites.

186 M.H.N. Behrestaghi et al./Engineering Geology 44 (1996) 183-201

different angles, using special frames fitted to the base of the conventional laboratory drilling machine. About 500 specimens of l/d=2 (d= 3.8 cm) at different orientations of/3 (0, 15, 30, 45, 60, 75 and 90 ° ) were cored from the four rocks. Fig. 3 shows such specimens for uniaxial and triaxial compression tests at different orientations.

An attempt was made to drill cores with the same angles/3 from a single block to minimize the lithological differences. The polished specimens were first oven dried at 105°C for 24 h and kept in desiccators for cooling. For measuring axial (E,) and diametral (£d) strains under uniaxial and triaxial compression, electrical resistance strain gauges were fixed, two axially and two diametrally at opposite sides in the middle of the specimens. Uniaxial compressive strength of the schists was determined as per ISRM (1978) test procedure. Triaxial compressive tests were carried out using a 150 MPa capacity triaxial cell placed in a 5 MN capacity loading frame.

A 0.5 mm thick teflon sheet was introduced between the ends of the specimens and the loading platens for uniform distribution of axial stress and to minimise the end-restraints. The specimens were first subjected to the required confining pressure and then the axial load was applied until the specimen failed.

5.1. Quartzitic schist

This is a fine grained rock with very well developed schistosity. Quartz bands vary in thickness from 0.5 to 4 mm. The ground mass appearing in the form of schistose bands, and predominantly made up of crypto-crystalline to fine grained flaky micaceous (biotite) minerals, preferably oriented with fine grained recrystallised quartz which are in abundance. Subordinate and accessary minerals which are embedded in the schistose ground mass are the quartz porphyroblast, potash and plagioclase feldspars (Fig. 4(a)). The other accessaries present are some iron minerals and zircon. Based on the semi-quantitative estimation from X-ray diffractograms (Fig. 5(a)), the quartz constitutes about 43%, mica 15% and feldspar 12.6% of the rock, with clay minerals forming the rest. The predominant clay is kaolinite, and illite and chlorite are also present (Table 1).

The scanning electron micrographs presented in Fig. 6(a) obtained in perpendicular to the foliations show strong preferred orientation of the minerals, i.e., granular, irregular, fine quartz grains aligned in alternate arrangement with tabular flakes of micaceous minerals. This textural variation render the rock behaves anisotropically.

5.2. Chlorite sch&t

5. Petrographic and petro-fabric analysis

To evaluate the petrography and petro-fabric of the rocks, thin sections across and along the schistosity were prepared and observed under a high power microscope through polarized transmitted light. For detailed quantitative and semi-quantitative mineralogical composition of schistose rocks, an X-ray diffraction powder method (CuK~ target) has been adopted. The 20 values ranging from 10 to 110 ° were used for all the rocks. To get the surfacial grain to grain contacts and orientations, scanning electron micrography (SEM) was employed on the samples in both perpendicular and parallel directions (to schistosity). The results of these tests are presented in the following.

This is a very fine grained, highly chloritized basaltic rock. The rock is aphaneric, with well developed schistosity, having the quartz bands varying in thickness from 0.5 to 1 cm, highly altered chlorite in ground mass, with occasional porphyroblasts of augite. Thin sections show that the augites are pleochroic, weathered and chloritized along margins. Plagioclase varies in composition from labradorite to andesine. Flakes of muscovite and other ferromagnesium minerals have also been seen. The rock appears to be slightly over saturated with silica (Fig. 4(b)). It appears that the quartz bands are formed due to filling of the foliation planes. The rock contains 29% quartz, 25% chlorite, 11% mica, with clay minerals forming the rest of the constituents ( Table 1 ).

M. H. N. Behr estaghi et aL /Engineering Geology 44 (1996) 183-201 187

(a)

(b)

(c)

(d)

Fig. 3. Specimens prepared at different orientation angles: (a) quartzitic, (b) chlorite, (c) quartz mica and (d) biotite schists.


(al (b)

(c) (d) Fig. 4. Thin sections prepared parallel to foliation plane (× 100) for (a) quartzitic, (b) chlorite, (c) quartz mica and (d) biotite schists.

The X-ray diffractogram (Fig. 5(b)) indicates the presence of chlorite along with other minerals identified in thin section. The scanning electron micrographs (Fig. 6(b)) taken perpendicular to the foliation plane reveal the textural contrast among the grains.

5.3. Quartz mica schist

This rock contains quartzitic bands of varying thickness from 1 to 1.5 cm, whereas micaceous bands are 1-2 mm in thickness and at times show ptygmatic folding. It is a coarse grained rock with well defined schistose texture. Recrystallized and elongated quartz grains with sutured boundaries

show strong preferred orientation of the rock. Micaceous (biotite and muscovite) plates occurring in alternate order with respect to quartzitic bands also show strong preferred orientation. The presence of chlorite along with the aforementioned mineral assemblage suggests that the rock belongs to low grade regional metamorphic facies (Fig. 4(c)). Analysis of X-ray diffractogram of the rock reveals that quartz (31%), chlorite (26%) and mica (22%) are the most abundant minerals, followed by clay minerals such as kaolinite, sepiolite and illite forming the rest of its constituents (see Fig. 5(c) and Table 1).

The scanning electron micrographs presented in Fig. 6(c), shows the coarse nature of grains in this

M.I-LN. Behrestaghi et al.lEngineering Geology 44 (1996) 183-201 189

u~ C - Carbonate ~. O - Quartz t,. ::[ .i,., ~o ~ "~',,d Ch-Chl0r i te I ,.i 4,. =Ill~ = M- Mu.ovit,, I 1.1~111 ~ K - Kao.nit,

e4 " I . l ~ l Jill I F - Feldspar

l Jr "1 111 [ o v ~| I1 [ m ~ o ! ~ _ o S - Sepi01ite

.,: ~II i ~1.. Ill [ < < ~ V - Vermiculite a i l l~ , IIII111 / II1, I [ i ~ ( d ) BIOTITE SCHIST

- - - ~ ¢ ~ . ~

~tlll l: ~4 II I ,,,. ~, ,,, -g ~ l l i . I t+ . I t , 9 ÷ ~. C<+Qu.R+zMIc.

,5 i l..l). ,il 'tldl I l l 11 . I "7 SeXiST

f ° til - l o > o / l l l q i O II - - iOu . g ~ 0 _ 0 o - u l l l l l ~. ,4-.,. ] Z • I'~'.t r, J v ~ II o ! ~ ~ -~ ® , . ~ , ~ f N ¢,,i

• t11,~ ~ ¢ . v

' _ L . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . 5o

Di f f rac t ion angle ~ 2g (degreesl

Fig. 5. X-ray diffractograms of (a) quartzitic, (b) chlorite (c) quartz mica and (d) biotite schists.

Table 1 Mineralogical composition from X-ray diffractograms for the rocks tested

Rock type Minerals (%)

Quartz Mica Chlorite Feldspar Kaolinite Ilfite Sepiolite Carbonate Vermiculite

Qua~zitic schist 48.0 15.0 2.5 12.6 11.5 4.7 - - 4.7 Chlorite schist 29.2 11.3 25.4 3.8 8.8 1.1 - 6.4 14.8 Quartz mica schist 31.0 22.3 26.5 2.8 7.9 2.8 6.3 - - Biotite schist I0,0 56.0 - 3.0 6.0 5.0 20.0 - -


(a) (b)

(cl (d) Fig. 6. Scanning electron micrographs taken parallel to foliation plane for (a) quartzitic, (b) chlorite, (c) quartz mica and (d) biotite schists.

M. H.N. Behr estaghi et al./Engineering Geology 44 (1996) 183 -201 191

rock along with strong preferred orientation of mica.

5.4. Biotite schist

Megascopic study of this rock shows that it is characterized by thicker foliation planes than quartz mica schist. The quartz feldspathic bands varying in thickness from 2 to 7 mm and stretched to a length of 1-5 cm. Micaceous (biotite) flakes range in thickness from 2 to 5 mm. Thin section study of the rock (Fig. 4(d)) showed that Biotite schist is a coarse grained rock with a well developed perfect schistose texture. Quartz grains are deformed and elongated, occurring alternatively with biotite mica flakes. Garnet occurs as an accessary and also shows preferred orientation.

On the basis of analysis of X-ray diffractograms of the biotite schist, as shown in Fig. 5(d), it is found that biotite is the major constituent of the rock (56%), followed by quartz (10%), whereas clay minerals, especially sepiolite (20%), kaolinite (6%) and illite (5%), form the other constituents of this rock (Table 1 ). Coarse nature and preferred orientation of the grains is evident from the scanning electron micrograph shown in Fig. 6(d).

6. Physical properties

Following the standard test procedure outlined in ISRM (1972), various physical properties such as density, specific gravity, water absorption, porosity and void ratio, were determined for all four rocks. The mean values obtained out of ten tests for each property are presented in Table 2.

Table 2 summarizes the physical properties, and

indicates that the chlorite schist is characterized by a maximum density of 2.88 g/cc followed by the biotite and quartz mica schists with maximum densities of 2.74 and 2.72 g/cc, respectively. The higher densities of these three rocks can be related to the abundance of chlorite and mica minerals which have high specific gravity values in the range of 2.7-3.2, whereas quartzitic schist is found to have a density of 2.63 g/cc. The lower density of quartzitic schist is due to the presence of a higher quartz content (G= 2.65) than mica (G= 2.7-3.2), as shown in Table 2.

Quartz mica schist is the most porous (1.7%) of all rocks tested (maximum saturated water content, 0.64%), with chlorite schist being the least porous (0.26%; minimum saturated water content, 0.07%). The porosity and saturated water content of quartzitic schist and biotite schist vary between the maximum and minimum limits.

7. Geotechnical properties

Understanding of geotechnical behaviour of anisotropic rocks involves evaluation of the strength index properties such as tensile strength, uniaxial compressive strength, deformation behaviour and shear strength at different orientations and stress levels. An attempt has been made to correlate such strength index properties with physical and mineralogical properties of the four schists to illustrate their inter-relationship.

7.1. Tensile strength

7.1.1. Brazilian strength (atb) The results of the Brazilian tests at fl = 90 ° are

presented in Table 3, along with the other strength

Table 2 Physical properties of rocks tested

Rock type Properties

Saturated water Dry density Saturated density Specific gravity, Void Prosity content (%) (g/cm 3) (g/cm 3) G ratio (%)

Quartz mica schist 0.64 2.72 2.74 2.83 0.018 1.70 Quartzitic schist 0.26 2.63 2.65 2.66 0.007 0.81 Biotite schist 0.24 2.74 2.75 2.85 0.007 0.76 Chlorite schist 0.07 2.88 2.90 2.90 0.002 0.26


Table 3 Mechanical properties of rocks tested a t / /= 90 ~'

Rock type Properties

ac (MPa) Et × 104 (MPa) v at~ (MPa) ~rtp a (MPa) Ore d (MPa) Deere and Miller's Classification

Quartzitic schist 190 2.00 0.25 29.0 15.0 3.9 CM/CL BM/BL Chlorite schist 110 1.20 0.20 24.0 10.0 4.0 CM/CL BL Quartz mica schist 50 0.38 0.15 12.0 4.2 1.5 DM, DL Biotite schist 50 0.60 0.10 8.9 4.0 1.9 DM, DL

characteristics. The highest value of atb = 29 MPa was obtained for quartzitic schist and the lowest value, of 8.9 MPa, for biotite schist. Chlorite and quartz mica schists show atb values of 24 and 12 MPa, respectively. The Brazilian strength of these rocks increases as/~ increases from 0 to 90 °. A similar trend was observed by Hobbs (1963) and Chaberlain et al. (1976) for different anisotropic rocks. In most of the anisotropic rocks, the tensile strength is observed to be maximum when the tensile stress is applied parallel to the planes of anisotropy, i.e., O'tb9O.

As shown in Table 1, quartzitic schist is characterized by a maximum quartz content (48%), whereas biotite schist possesses minimum quartz content (10%): this is one of the main factors with regard to the difference in their tensile strengths. On the other hand, quartzitic and chlorite schists are made up of much finer grains with a higher degree of interlocking of grains, as seen in thin section (Figs. 4(a) and 4(b)) and scanning electron micrographs (Figs. 6(a) and 6(b)), in comparison with quartz mica and biotite schists, which are coarse grained and contain larger amounts of mica flakes (Figs. 4(c) and 4(d)). Generally, the presence of mica flakes leads to loss of interlocking between the grains and thus lowers the tensile strength.

It can be seen that quartzitic schist exhibits a lower tensile strength in the region of/~ <45 ° than chlorite schist, whereas in the regions where/~ > 45 ° quartzitic schist demonstrates higher tensile strength. This variation can be attributed to the fact that at lower /~ orientations tensile cracks follow the weak planes (i.e., incompetent micaceous bands between competent quartzitic bands), whereas at higher /~ orientations tensile cracks

have to rupture across many competent quartzitic layers.

Quartz mica schist possesses a higher tensile strength than biotite schist at higher orientations, i.e., /~>60 °, due to the higher quartz content of quartz mica schist, whereas both rocks have almost the same tensile strength in the region /~<45 °, where weak planes have a major effect on the failure of the specimens. It is also observed that the axial point load strength (6tpa) increases, whereas diametral point load strength (O'tr, d ) decreases, as /~ varies from 0 to 90 ° for all four rocks.

7.1.2. Uniaxial compressive strength (at) Results of uniaxial compressive strength tests at

/~ = 90 ° inclination are presented in Table 3 for all four rocks. The variation between compressive strength (ac) and the corresponding orientations is presented in Fig. 7. On the basis of average experimental data obtained from five tests for each orientation, the curves have been drawn. Compressive strength values for all the schists at /~ = 90 ° are found to be more than at other orientations. It is further observed that a number of compressive strength tests conducted at this orientation, under similar conditions, exhibit least scat- tering in their results as compared to all other orientations. This may be due to the averaging effect of the planes of anisotropy with fl = 90 ° as compared to when the foliations are inclined. The highest compressive strength is found to be at/3 = 90°; it is generally designated as the "representative compressive strength" value for the anisotropic rock and denoted as ~rc9o, while the strength at other orientations is generally designated simply a s O'cj.


O

X

200 - -

o Quortzitic schist • Chlorite schist

101

50~

Fig. 7. Variation of uniaxial compressive strength, ~ , with ft.

The representative values for quartzitic, chlorite, quartz mica and biotite schists are found to be 190, 110, 50 and 50MPa, respectively. As is demonstrated in Fig. 7, the variation of ao with fl shows minimum strength between 30 and 45 ° orientations with values of 80, 50, 25 and 30 MPa for quartzitic, chlorite, quartz mica and biotite schists, respectively.

7.2. The stress-strain response

The stress-strain curves obtained under uniaxial compression are presented in Fig. 8 for quartzitic, chlorite, quartz mica and biotite schists, respectively. These curves include both axial and diametral strains (%) plotted against axial stress for all the orientations, i.e., 0, 15, 30, 45, 60, 75 and 90 °. The stress-strain curves for all the schists show in general plastic-elastic-plastic behaviour in agreement with Miller's classification of stress-- strain curves. Quartzitic and chlorite schists show maximum axial deformation (0.8%) at representa-

tive strength orientation, i.e., fl=90 °, due to the fact that the foliation planes become compressed and undergo a larger axial strain at failure. Similarly, quartz mica and biotite schists experi- ence an axial strain of 0.7% at fl = 90 ° orientation.

7.3. Deformation modulus

From the stress-strain diagrams, the deformation modulus, Et, and the Poisson's ratio, v, were estimated at 50% of peak strength for all orientations of the four rocks. The E t and v values observed at t = 90 ° for these rocks are presented in Table 3. The variation of deformation modulus with fl is presented in Fig. 9. It is obvious from the figure that the deformation modulus for all four rocks at f l=0 ° is obviously higher than at other orientations. At this orientation there is less deformation, as a result of interlocking of the vertically oriented foliation planes when they are subjected to compression, whereas the values of deformation modulus at fl = 90 ° are less than those

194 M.H.N. Behrestaghi et al./rEngineering Geology 44 (1996) 183-201

'1"

~s

_I 1 I ~ 1 I l I i I l O J, 0.2 0.0 0.2 O.t 0.6 0.8 0.~, (12 0.0 • ~ O.t O.E 0.8

£d ~'/, F.. o , ' / . ~.d;/. Ea ,'/.

( a l ( b l

gO f160 ~ .9o ° •

M ~ / _ . : ¢ / / /

?o

l 0.0 0.2 0.;. 0.6 0.6

£o , ' / .

I 1 (~2

0.2 Ed ,'/" £d ,'/"

60

0.0 0.2 O.& O. S

£0 ,'/"

(c 1 (d I

Fig. 8. Stress strain curves at different fl values obtained from uniaxial compressive tests for (a) quartzitic (b) chlorite, (c) quartz mica and (d) biotite schists.

at/~ = 0 °, especially for the two incompetent schists (i.e., quartz mica and biotite schists), due to the compression of foliation planes. Quartzitic schist is characterized by maximum deformation modulus at all orientations, in comparison with the other three schists. The respective values of deformation modulus at/3 = 90 ° for quartzitic, chlorite, biotite and quartz mica schists are 2× 104 , 1.2 x 104, 0.6 x 104 and 0.38 x 104 MPa, respectively. The variation of Poisson's ratio with/~ for the schists does not show any systematic variation.

The data in Table 3 suggest that quartzitic schist is the most competent of all four rocks, followed by chlorite, quartz mica and biotite schists. These

results reflect the influence of mineralogy and physical properties on the compressive strength of schistose rocks.

7.4. Engineering classification of schists

The compressive strength, ac, and modulus values, Et, obtained from the uniaxial compressive strength test for these rocks are shown in Fig. 10 as per the Deere and Miller (1966) classification. Envelope "X" covers the major part of the high modulus ratio which has been related to the specimens tested at fl <45 °, where failure takes place along steeply dipping schistose layers. Whereas

M.H.N. Behrestaghi et aL/Engineering Geology 44 (1996) 183-201 195

o

IE

o

X

2.5jJ-- o Quartzitic schist

J • Chlorite schist J a Quartz mica schist

1.5q

1.0

0.5

0.0 1 I I I ~ ,1 0 ~S 3 0 ~;S SO 75 9 0

o o

Fig. 9. Variation of deformation modulus, Et, with ft.

envelope "Y" represents the specimens tested at fl > 45 ° where the strength is not much affected by the foliation planes. The lower value of deformation modulus in the latter zone is because of the compression or closure of microcracks parallel to the foliation planes. In Fig. 10, the scatter of the results is greater for quartzitic (CM/CL, BM/BL) and chlorite schists (CM/CL and BL) because of the continuous nature of their foliation planes which effect deformation modulus and strength. Whereas test results plotted for quartz mica and biotite schists show lesser scatter and fall in DM and DL categories. The limited scatter for quartz and biotite schists is due to the absence of continuous foliation planes, and thus a lesser effect of foliation planes on the strength and deformation modulus.

7.5. Strength behaviour in triaxial condition

High pressure triaxial tests were conducted at different orientations (fl) on the four schistose

rocks. The plots of compressive strength between tr~ and fl at different a3 of quarztitic, chlorite, quartz mica and biotite schists are presented in Fig. 11. The plots have been drawn taking the average experimental results into consideration. The overall strength behaviour of quartzitic, chlorite and quartz mica schists is similar as far as the shape of the anisotropy curve in the confined state is concerned. The shape of the anisotropy curve for these three rocks is "U-shape" over the entire range of tra adopted, whereas biotite schist, although exhibiting a "U-shape" anisotropy in the lower range of confining pressure, does not retain this shape but shows flattening of anisotropy curves at higher confinement.

It is clear from the plots that the maximum strength values are observed at fl = 90 ° for quartzitic and chlorite schists, throughout the range of confining pressure. Though in general the minimum strength values are obtained at fl= 30 °, it has been observed that the minimum of the curve shifted to fl=45 ° at confining pressures greater


o D. :z

I ,

o 2 x

~E

E D ¢. B A Very low Low Medium High Very high

20 sttengtla Strenqth Strenoth StrenQth Strenatn

0,5 o Biotite schist • Q u a r t z m i c a s ch i s t

X Tested /3 </,5 °

Y Tested p > / . 5 °

, I I I 0.2 10 20 50 100 200

Fig. 10. Deere and Miller's classification of schistose rocks tested.

, , , i l , I I , I t.O0

than 15 MPa. A similar observation has been reported by McLamore and Gray (1967) for slate beyond confining pressures of 276 MPa and by Singh et al. (1989) for quartzitic and carbonaceous phyllites beyond 70MPa confining pressure. Quartzitic schist shows a 30% strength improvement at fl=0 ° as a result of confinement (a~ = 100 MPa) and demonstrates an equal strength at fl=90 °. This improvement for chlorite schist is 15% and for quartz mica schist is only 10%. Quartz mica schist exhibits similar strength in unconfined state when compared with biotite schist, but shows higher rates of increase in compressive strength with increase of confining pressure owing to its higher quartz and lower mica content.

7. 6. Modulus in confined state

The tangent modulus, Et, and strain ratio, v, of the schistose rocks were obtained from their respective stress-strain curves. A study of the deformation modulus values reveals that their variation is similar to that of triaxial compressive strength as a function of/~ at all confining pressures. These values are the highest for quartzitic schist and the lowest for quartz mica schist. For chlorite and biotite schists the deformation moduli lie between the two extremes. Figs. 12(a) and 12(b) show the variation of modulus with fl at different confining pressures for quartzitic and chlorite schists following a U-shaped trend similar to their

M. H.N. Behrestaghi et aL /Engineering Geology 44 (1996) 183-201 197

~, MPO C~3 ,),4 Pa

6001 • 5 600{-- • 5 o 15 | a 15

• 35 5 o o 1 ~ ,~ so ~ m • 3s 500~ " 50

I " , ~ • ,oo ~ { \ • ,oo i -

300

2 0 0 ~ 200

1

I00 L i I , l I I • 0 0 15 30 /,5 60 75 90 0 ~5 30 ~5 60 75 90

/f ' /~°

(al (b)

L O ( ) ~ 4uu- 4 100 300,

b" 2

I00" I

0 15 )0 45 60 75 90 0 ~S 30 c5 60 75 90

(cl (d)

Fig. 11. Failure strength at different fl and a 3 values for (a) quartzitic, (b) chlorite, (c) quartz mica and (d) biotite schists.

O:: j ,M~ CT3'MP°

0 5 F 0 5 5.0 a 15 a 15

• 35

50 4.0 A 100

7"01 • 35 6.0 zx 50

• 100

n ° 5.0

~ o 4.04 :r 3.(

- i ta ?.0 x

~.0

1.0

1.0

I I I 1 1 O. 30 t, 5 60 75 90

~° (al

198 M.H. IV. Behrestaghi et aL/Engineering Geology 44 (1996) 183-201

! 0 . 0 I I 1 I I I 15 0 15 30 t,5 60 75 90

/9 °

(b) ~,MPQ ~ , M P o

s.ol Oo s ~'° rl o s 15 ~- a 15 • 35 3 5 • 35

o ~.0 a 50 • A 50 a. A 100 ~ a 100

o x x 2 . C ' ~

tff 2.0 G

1.5

1.0 1.0 .

0.0 0.5 C 15 30 t,5 60 75 90 0 15 30 45 60 75 90

F¢ t )° (c) (d)

Fig. 12. Variation of deformation modulus with fl and cr 3 for (a) quartzitic, (b) chlorite, (c) quartz mica and (d) biotite schists.

strength anisotropy curves (Figs. 11 (a) and 11 (b)). This behaviour can be attributed to the effect of foliation planes in the region 15 < fl < 60 ° where failure takes place along foliation planes and, as a result of low strength, modulus is also affected. Quartz mica schist exhibits minimum modulus at all orientations and confining pressures in compari-

son to the other three schists on account of its thinly laminated foliation planes and higher degree of porosity.

The curves demonstrating the variation of Et with fl at different tr 3 values for quartz mica and biotite schists (Figs. 12(c) and 12(d)) indicate higher values of modulus at ¢/= 0 ° which gradually


decreases as fl varies from 0 to 90 °. Such a variation is due to the presence of discontinuous foliation planes and a larger amount of mica planes in latter two rocks which do not produce general failure in the regions 15 < fl < 60 °. Quartz mica and biotite schists, on account of their higher degree of thinly foliated planes, undergo a greater deformation at / /=90 ° .

7. 7. Shear strength

To understand the shear strength generation of schistose rocks the triaxial compressive strength was utilized for constructing Mohr envelopes at different orientations. These envelopes were essen- tially nonlinear. Cohesion intercept (c) and angle of internal friction (~b) were determined from the envelopes to stress circles. It can be seen that the cohesive strength of the schists increases and the value of friction angle decreases with increase of confining pressure. Fig. 13(a,b) presents the variation between cohesive strength and fl for quartzitic and biotite schists. The shape of the anisotropy curve is similar to that observed for the failure strength (Figs. 11 (a) and 11 (d)), demonstrating a higher value of c at fl = 90 ° and minima at fl = 30 ° orientations at all confining pressures. The variation of ~ and fl is not significant. Figs. 14(a) and 14(b) show the variation of ~ with fl for quartzitic and biotite schists, respectively. A similar response was obtained in the case of phyllites (Ramamurthy et al., 1993).

8. Conclusions

A comprehensive study of compositional, physical and geotechnical responses of four schists (quartzitic, chlorite, quartz mica and biotite) from hydroproject construction sites in India has been carried out. The important findings are summa- rized in the following:

(i) Study of the thin sections and SEM photo- graphs of the four schistose rocks revealed that the constituent minerals show a strongly preferred orientation causing the rocks to respond anisotropically. (ii) Quartzitic and chlorite schists, on account of

~ ' , MF~

.o , a~3

o 100

5

0 15

(a)

0"~ j MPa I00 1 •0 05

80[- • iS

I :';o / , o,oo / /

0 0 15 30 ~5 60 75 90 B °

(b)

Fig. 13. Variation of c with ~ and a3 for (a) quartzitic and (b) biotite schists.

their being fine grained, highly interlocking and having lower amounts of mica, demonstrate higher tensile and compressive strengths than quartz mica and biotite schists at all orientations.

200 M. 1-1. N. Behres taghi et aL /Engineering Geology 44 (1996) 183-201

J

~ 3C u

. r - I.L

2C

0

1 0 I I I I I I 0 15 30 45 60 75 90

P (a)

O~ (MRa:

• 0

o 5

• 15

~, 35

• 50

o 100

L s° k . "

- o'-I" 1 0 - - dr 0

I I I I I I 0 15 30 60 75 90 45

P (b)

O~(MPa)

• 0

a 5

• 15

z~ 35

• 50

o 100

Fig. 14. Variation of ~ with fl and a 3 for (a) quartzitic and (b) biotite schists.

(iii) The representative uniaxial compressive strength (acgo) is found to be maximum in quartzitic schist followed by chlorite schist, and with similar values for both quartz mica and biotite schists. These schists exhibited U-shaped anisotropy. (iv) Variation of deformation modulus with/3 is either U-shaped or irregular characterized by continuous sharp reduction of deformation modulus in the range of 0 < fl < 60 °. (v) Quartzitic schist shows the lowest axial and diametral deformability in the direction parallel to stratification, and quartz mica schist shows

the highest deformation in the direction perpendicular to the stratification. Finer grain size and better interlocking of the grains and higher degree of Quartz content cause lower deformation and vice versa. (vi) Compressive strength and deformation modulus varied nonlinearly with confining pressure. Presence of continuous foliation planes in quartzitic and chlorite schists contribute to the U-shaped variation of deformation modulus. On the other hand, absence of such continuous planes in quartz mica and biotite schists, and the higher content of mica flakes, contribute to the decreasing values of deformation modulus with fl varying from 0 to 90 ° (vii) Maximum value of cohesive strength was observed at fl=90 °, with a minimum value at r = 30-45 ° for the schists at all confining pressures, similar to strength anisotropy. Therefore, for such schistose rocks one cannot assume con- stant values of c and ~b with variation of fl and a 3 in the analysis and design. The values of c increase with a3, while the values of ~b decrease.

References

Akai, K. 1971. The failure surface of isotropic and anisotropic rocks under multiaxial stresses. J. Soc. Mater. Sci., Japan, 20: 122-128.

Allirot, D. and Boehler, J.P. 1979. Evaluation of mechanical properties of a stratified rock under confining pressure. Proc. 4th Cong. ISRM, Montreux, 1: 15-22.

Arora, V.K. 1987. Strength and deformational behaviour of jointed rocks. Ph.D. Thesis submitted to Indian Institute of Technology, Delhi.

Attewell, P.B. and Sandford, M.R. 1974. Intrinisic shear strength of a brittle anisotropic rock - I; Experimental and Mechanical Interpretation. Int. J. Rock Mech. Min. Sci, 11: 423-430.

Berenbaum, R. and Brodie, I. 1959. The tensile strength of coal. J. Inst. Fuel, 32: 320-327.

Broch, E. and Franklin, J.A. 1972. The point load strength test, Int. J. Rock Mech. Min. Sci., 9: 669-697.

Broch, E. 1983. Estimation of strength anisotropy using the point load test. J. Rock Mech. Sci. Geomech. Abstr., 20: 181-187.

Brown, E.T., Richards L.R. and Barr, M.V. 1977. Shear strength characteristics of Delabole slates. Proc. Conf. Rock Eng., Newcastle upon Tyne, 31-51.

Chaberlain, P.G, Van Eeckhout and Podmieks, E.R. 1976. Four factors influencing observed rock properties, Soil Speci-


men Preparation for Laboratory Testing, ASTM STP 599: 21-36.

Chenevert, M.E. and Gatlin, C. 1965. Mechanical anisostropies of laminated sedimentary rocks. Soc. Petrol. Eng. J., 5: 67-77.

Deere, D.U. and Miller, R.P. 1966. Engineering classification and index properties for intact rock, Tech. Report no. AFWL-TR-65-116, Air Force Weapons Lab., New Mexico.

Deklotz, E.J., Brown, J.W. and Stemler, O.A. 1966. Anisotropy of a schistose gniess. Proc. 1st Cong. Int. Soc. Rock Mech., Lisbon, I: 465-470.

Donath, F.A. 1964. Strength variaton and deformational behaviour of anisotropic rocks. State of Stress in the Earth's Crust. In: W.R. Judd (Editor), Elsevier, New York, 281-298.

Hobbs, D.W. 1963. The tensile strength of rocks, Int. J. Rock Mech. Min. Sci., 1: 385-396.

Hock, E. 1968. Brittle failure of rock; Rock Mechanics in Engi- neering Practice. In: K.G. Stagg and O.C. Zienkiewicz (Edi- tors), J. Wiley and Sons, London, 99-124.

Horino, F.G. and EUickson, M.L. 1970. A method of estimating the strength of rock containing planes of weakness. US Bureau Mines, Report Investigation, no. 7449.

IS:10082 1981. Indian standard on method of test for determi- nation of tensile strength by indirect tests on rock specimens.

ISRM Committee on Laboratory Tests 1972. Suggested meth- ods for determining water content, porosity, density, absorption and related properties and swelling and slake-durability index properties, Document no. 2, 36 pp.

ISRM Committee on Laboratory Tests 1978. Suggested meth- ods for determining petrographic description of rocks. Int. J. Rock Mech. Min. Sci., 15: 43-45.

Kwasniewski, M. and Neuyen, H.V. 1986. Experimental studies

on anisotropy of the time-dependent behaviour of bedded rocks. Proc. of the Int. Syrup. on Eng. in Compl. Rock Foem. Beijing, China.

McCab¢, W.M. and Koerner, R.M. 1975. High pressure shear strength of an anisotropic mica schist rock. Int. J. Rock Mech. Min. Sci., 12: 219-228.

McLamore, R. and Gray, K.E. 1967. The mechanical behaviour of anisotropic sedimentary rocks. Trans, Am. Soc. Mech. Eng., Series B, 89: 62-76.

Pinto, I.J. 1970. Deformability of schistose rocks. Proc. Sec. Cong. Int. Soc. Rock. Mech. Belgrade, 491-496.

Pomeroy, C.D., Hobbs, D.W. and Mahmoud, A. 1971. The effect of weakness plane orientation on the fracture of Barns- ley hard by triaxial compression, Int. J. Rock Mech. Min. Sci., 8: 227-238.

Rao, K.S. 1984. Strength and deformation behaviour of sandstones. Ph.D. Thesis submitted to Indian Institute of Tech- nology, Delhi.

Ramamurthy, T. 1993. Strength and modulus response ofaniso- tropic Rocks. Comprehensive Rock Engineering, Chapt. 13. In: Prof. J.A. Hudson (Editor), Pergamon, UK, 1: 315-330.

Ramamurthy, T., Rao, G.V. and Singh, J. 1988. A strength criterion for anisotropic rocks. Proc. Int. Symp. Rock Great Depth, I: 37-44.

Ramamurthy, T., Rao, G.V. and Singh, J. 1993. Engineering behavior of phyllites. Eng. Geol. J., 33: 209-225.

Read, S.A.L., Perrin, N.D. and Brown, I.R. 1987. Measurement and analysis of laboratory strength and deformability characteristics of schistose rocks. Proc. 6th Int. Conf. Rock Mech. Montreal, 1: 233-238.

Singh, J., Ramamurthy, T. and Rao, G.V. 1989. Strength anisotropies in rocks. Indian Geotech. J., 19: 147-166.

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