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Geotechnical rock-mass evaluation of the Anamur dam site, Turkey
ENGINEERING GEOLOGY ELSEVIER
EngineeringGeology42 (1996) 65-70
Geotechnical rock-mass evaluation of the Anamur dam site, Turkey A. Ozsan a, C. Karpuz b , a University of Ankara, Faculty of Science, Geological Engineering Department, 06100 Ankara, Turkey b Middle East Technical University, Mining Engineering Department, 06531 Ankara, Turkey Received 21 March 1995; accepted 14 September 1995
Abstract
This paper describes a feasibility-level geotechnical evaluation carried out at Anamur dam site, in terms of stability analysis of dam foundation, excavation slopes and a diversion tunnel. The proposed Anamur dam will be built across the Anamur river on the Alanya metamorphic series which consists of phyllite, schist and slate units at the site. Geoteelmical investigations included drilling for core, pumping tests, sampling for laboratory testing, and a detailed discontinuity survey. Uniaxial compressive strength, density, triaxial compressive strength and direct shear tests, have all been carried out in the laboratory. Discontinuity data are processed for the determination of dominant sets and for the rock mass characterization in terms of classification systems. Evaluation concentrated on kinematic analysis in slope stability, Goodman's bearing capacity approach in foundation stability and rock mass classification for diversion tunnel stability assessment. The geotechnical evaluation of the proposed dam and reservoir site established that there will be no stability problems. Detailed geotechnical investigations are required for the final design of this project.
1. Introduction
The proposed Anamur dam will be built across the Anamur river in southernmost Turkey (Fig. 1 ). The dam will be approximately 105 m high with a crest length about 650 m and base width of 450 m and is planned to be rock-fill dam. The purpose of the dam is to control and store water for an irrigation project being developed near Anamur river and to generate electricity. The design of the dam is under the joint direction of Electricity Research Department and State Water Work. * Correspondingauthor. E-Mail: karpuz @ rorqual.cc.metu.edu.tr 0013-7952/96/$15.00© 1996ElsevierScienceB.V. All fights reserved SSDI 0013-7952(95)00065-8
This paper explains some preliminary guidlines for the safe design of the dam foundation, stability of excavation slopes, as well as potential displacement of natural slopes into the reservoir and stand-up conditions for a diversion tunnel.
2. Geology The investigated area known as the Alanya metamorphics make part of the Toros Mountains tectonic group (Ozgal, 1976) and consists of schist, slate and phyllite (SSP), quartzite lenses, Palaeozoic-aged. It crops out around the Anamur
A. Ozsan, C] Karpuz/Engineering Geology 42 (1996) 65- 70
66
N
~\
Erm~ek
plagioclase and opaque mineral. Slate is green to greenish black in color and shows lepidoblastic texture with a composition of quartz, albite and chlorite. Quartzite also crops out in the area, as small to larger lenses within the SSP. They are grey, white and cream like in color, and are dominantly composed of quartz minerals with a Granoblastic texture.
2.1. Structural geology
MEDITERRANEAN "v///.-~ ~ ,
~" = '
SEA o
~
m
4o ~ o ~
Fig. 1. Location map of the dam site.
dam site (Fig. 2). Phillites are light grey in color, made up of quartz, plagioclase, chlorite and muscovite, with traces of opaque mineral and carbonate. Texture is lepidoblastic. Schist occur in the forms of interlayers, 30-40 cm in phillites. Green grey to dark grey in color. They show porphyroblastic texture and are composed of chlorite,
Two dominant folds are distinguished in the area. The first axis lies 250 m SW of borehole BH-1 (Fig. 2), and a strike of N W - S E with a wave length of 1800 m. A second fold is more or less parallel to the Anamur creek ( N W - S E direction) and its length is around 4000 m.
2.2. Methodology Discontinuity survey including orientation, aperture, infilling and spacing, has been carried out in accordance with the working party report (Anon, 1977) and the International Society for Rock Mechanics ISRM Suggested Method N
EXPLANATION oo,z.
Schist, Slate, Phyllite
~
s~l~ ond ~ ~ j~nt Foliation Contoct Anticlt~l axis Syndinol o~is
0 B;-;-I o/D'
(~)re~ob, ( k)co~l (~'ld numi~br) Dora axis
z ~-=--'~
Tunnel oligr~te~l
--~ O
~O
~
~00m.
Fig. 2. Geological map at the site area.
Expec~s~ped~-~,c~on
A. Ozsan, C Karpuz/Engineering Geology 42 (1996) 65-70
length (meter) of a borehole at a unit time (minute) under the effect of 10 atm pressure. The total 185 pumping tests were performed at the SSP unit and one performed at the quartzite unit in the 5 boreholes. The distribution of those Lugeon values and their descriptions are given below:
(ISRM, 1978). In total, 922 discontinuities, 506 on the left bank and 416 on the right bank, were measured. Orientations are processed utilizing a computer program based on equal-area stereographic representation and the following dominant discontinuity sets were distinguished on left and fight banks (Fig. 2). On the left bank: D2; N45E/82 NW, D3; N46W/84NE, D4; N42W/ 49NE, D1; N22E/62NW, Ds; N46E/66SE On the fight bank: DI; N80E/50SE, D2; N35E/83NW, Da; N52W/ 78NE, D4; N34W/54SW, Five boreholes were drilled ranging from 54.6 m to 125 m in depth at the site. Both the tunnel alignment and the dam axis cut across all rock units (Fig. 3). During the site exploration, pumping tests were carded out, and rock samples were taken for laboratory testing. The conductivities are expressed in terms of Lugeon values. A Lugeon unit is the loss of a liter of water along a unit
mD
Lugeon values <1 1-5 5-25 >25 The Lugeon value of
#of Tests 122 46 16 1 the quartzite unit
D'
~"
SWr
Domoxi. ~N40E
I
I
m
. ow
t
BH - 4
I
I
!
~7 0
,-. ~
Description Impermeable Slightly permeable Permeable Highly permeable is greater than 25.
Rock-mass properties: SSP: moderately to highly weathered, 70-180 cm in discontinuity spacing. Discontinuity aperture is 1-5 mm and mostly filled with kaolinite, mica and chlorite. Rough to planar surface roughness. SSP is generally impermeable and has an average of 44% (21-74%) RQD. Quartzite: slightly to moderately weathered,
.~
~ - 4
67
50
15Ore
Schist,slote, phyllite
t BH-I Bonthole ( Io~fionand
numtxtr)
Tunnel alignment
Fig. 3. Geological cross-sections along the tunnel alignment and dam axis,
68
.4. Ozsan, c Karpuz/Engineering Geology 42 (1996)65 70
200-600 mm discontinuity spacing. Discontinuity aperture is mostly less than 1 mm, but sometimes it is as wide as 3 mm and highly healed with quartz and epidote secondary mineralization. It is highly porous and has an average of R Q D 58% (49-66%). Uniaxial compressive strength, density, triaxial compressive strength and direct shear tests were carried out in the laboratory in accordance with ISRM suggested Methods. The test results are given in Table 1.
program K A N R O S (Ocal, 1994) is based on the kinematic technique suggested by G o o d m a n (1989). The program handles kinematic analysis for slopes having several designated discontinuity sets and/or individual discontinuities. Several slopes in multiple structural domains can be included and maximum safe slope angles can easily be determined. One of the main features of the program is that it can accomodate a designated scatter in dip and dip direction of the discontinuities, as well as variations in the angle of friction for each discontinuity.
3. Kinematic analysis for slopes 3.2. Left bank 3.1. Introduction
Stability analysis for slopes are usually carried out with the limit-equilibrium technique, the analysis of forces acting on the most plausible unsafe portion of the slope. In this analysis, geometry of slope and relative orientations of critical features, possibly type of failure and the shear strength parameters of these features must be well defined. Kinematic analysis is carried out first to identify and eliminate safe features for further analysis. For example, a wedge whose line of intersection does not daylight into free face will never cause instability. Kinematic analysis is performed using equal-area spherical projection techniques. After distinguishing the dominant discontinuity sets on both left (Region 1 ) and right (Region 2) banks of the reservoir area, the kinematic analysis was carried out. First, the possible left and right bank slope orientations were determined as N 50 W (220 °) and N 50 W NE (040°), respectively (Fig. 2). Then the F O R T R A N coded computer
Considering the center value of the pole concentration for all discontinuity sets, the most critical face angle is the 67.8 ° due to the discontinuity set (D4) then 75.5 ° intersection axis comes into play due to the wedge formed by the intersection of D1 and D3 (Ix.3). Considering the effect of pole concentration scattering, the critical cut-slope angles decrease to 64.6 ° and 64.5 ° respectively. These are rather steep angles so kinematically it can be said that the left bank is almost safe but will yet require engineered reinforcement. The right bank is fully safe due to the wedge and plane failure point of view. The expected instability is toppling due to discontinuity set D3. The analytical stability analysis was not carried out since the kinematic analysis indicates stability at higher slope angles than are present. Generally, the faces on both sides of the river can be regarded as safe. Because discontinuities are not continuous and moderately spaced (70-180 cm), toppling failure is not expected to
Table 1 Laboratory test results Rock u n i t
SSP Quartzite *Mean value.
Uniaxialcomp. str. (MPa)
9.34 77.65 (43.97)* 95.77 112.80
Density (kN/m3)
Triaxial test
Direct shear (parallel to joints)
Int. friction angle
Cohesion MPa
Int. friction angle
Cohesion MPa
2.57 2.83
46'
3.9
30.1-34.0
0.01-0.06
2.65
48'
12
A. Ozsan,C. Karpuz/EngineeringGeology42 (1996)65-70 occur. Secondly, for the steeply dipping discontinuities, dominant joint-bounded rock bodies are naturally buttressed by other blocks which may prevent the toppling failure and the slope may be stable. On the other hand, for all potential slope failure geometries, water is present and decreases the stability factors. However, in the reservoir area, hydraulic pressure provided by stored water increases the normal force acting on the slope face hence increases overall stability below the water line.
Calculation of a bearing capacity by the limitequilibrium method for a footing under load must respect the complexity and variety of failure modes such as cracking, crushing, wedging, punching and shearing. As Goodman (1989) pointed out, no universal formula for bearing capacity of rock can be given. Several simple approaches can be utilized. In this study, bearing capacity of the rock mass beneath the dam axis is carried out by Goodman's (1989) approach for homogeneous, discontinuous rock masses. In this analysis, both Mohr-Coulomb and Hock and Brown's empirical failure criteria are utilized Bearing capacity in terms of Mohr-Coulomb failure criteria: qf=2 Cr tan(45 +$r/2) + 2 Cptan(45+$p/2) tan2(45+$r/2)
instability) can be approximated as not less than the unconfined compressive strength of the rock mass around the footing, and this can be taken as a lower bound. For the safest condition, St, and Cr are zero and qf=2Co. If the minimum uniaxial compressive strength value of the SSP is considered, qf= 2 × 9.34 = 18.68 MPa, or average uniaxial compressive strentgh of SSP is used qf= 56.38 MPa is found. Hock and Brown (1980) failure criteria:
qf=Co{(S)U2 +[mr(S)U2+ Sr]U2t
3.3. Bearingcapacity calculation
(1)
where, qf= bearing capacity, MPa; Cr =residual cohesion, MPa; Cp =peak cohesion, MPa; Sr = residual internal friction angle, degree; Sp= peak internal friction angle. Kulhawy and Goodman (1980) suggested that it is necassary to use rock-mass strength parameters in the evaluation of bearing capacity and for simplicity, rock mass properties are about 50-70% of intrinsic (Laboratory Material [M]) rock properties. So if Sm =23 and Cr = 1.9 MPa is used; qf is calculated as 26.95 MPa ($=46 and C=3.9 MPa, Table 1). Mohr-Coulomb failure criteria leads to the conclusion that the bearing capacity of a homogeneous, discontinuous rock mass (without kinematic
69
(2)
where:Co=uniaxial comp. strength, MPa; S = intact material constant; mr = rock mass constant; St=rock mass constant, when S = I , mr= 0.317, St=0.00001, co=28.19 MPa is replaced into Eq. (2), qf is determined as 43.97 MPa (mr and Sr is calculated from the table (p. 176) given in Hock and Brown (1980)). If the R M R data are processed in accordance with Hock (1990), the calculated bearing capacities are 32.90 MPa and 31.00 MPa for the average and lowest R M R values respectively. The safest condition (mr=0, S t = 0 ) and the uniaxial comp. strength of SSP as 28.19 MPa or minimum strength of 9.34 MPa is considered, the allowable bearing capacities are calculated as 28.19 MPa and 9.34 MPa respectively. As can be seen from the calculated values of both failure criteria, the bearing capacities of the rock unit SSP is greater than the distributed load exerted by the dam. (The maximum load exerted by axis is calculated as 105 m x l m x 5 m x3 tons/m3=1575 tons or 1575 tons/5 m2=315 tons/m 2 or 3.15 MPa). Therefore dam the foundation will be safe from the bearing capacity point of view. However, when determining the safe bearing pressures on a footing on rock, it is never permissible to use the bearing capacity as calculated without consideration of scale effects. There is an element of uncertainity associated with the variability of the rock and a significant size effect in strength under compressive loads. However, even with a factor of safety of 5, the allowable loads yet tend to be higher than the allowable bearing capacities. It should also be
70
A. Ozsan, C Karpuz/Engineering Geology 42 (1996) 65-70
Table 2 Engineering classification of rock masses and proposed support requirements in tunnels Rock unit
RSR
RMR
Q
Proposed support requirement
SSP
31
13-37
0.12-1.08
Quartzite
52
33-56
1.00-8.21
2.75 m long tensioned rock bolts on a grid spacing of 1 m with a 50 mm thick shotcrete reinforced with a welded steel mesh No support requirement indicated
*Bolt length is calculated using the equation L=2+0.15(B/ESR), where B=5 m, ESR (Excavation Support Ratio)= 1. stressed here t h a t d i s c o n t i n u i t y s p a c i n g / f o u n d a t i o n w i d t h r a t i o will be smaller a n d will help to increase b e a r i n g c a p a c i t y o f r o c k unit a n d hence the stability o f the d a m f o u n d a t i o n .
investigations will be r e q u i r e d for the s u p p l e m e n tary structures such as the l o c a t i o n a n d the design o f the spillway a n d p o w e r h o u s e as well as the m a i n structures. T h e q u a r t z i t e lenses will require g r o u t i n g to seal t h e m f r o m the p a s s a g e o f groundwater.
4. Rock mass classification for tunnel design Several r o c k m a s s classification systems have been p r o p o s e d to s u m m a r i z e key geological a n d geotechnical d a t a a n d to p r o v i d e a t o o l for decision m a k i n g d u r i n g c o n s t r u c t i o n . These systems p r o vide a m o r e q u a n t i t a t i v e m e a s u r e o f r o c k m a s s a n d hence minimizes j u d g e m e n t a l bias. T h e systems c o n s i d e r e d a p p r o p r i a t e to the diversion tunnel ( h a s 5 m d i a m e t e r ) o f A n a m u r D a m were W i c k h a m et al. (1974), R o c k Structure R a t i n g ( R S R ) , B a r t o n et al. (1974) Q - s y s t e m a n d Bieniawski (1989), R o c k M a s s R a t i n g ( R M R ) systems. T h e collected d a t a were processed in a c c o r d a n c e with the classification systems a n d the d e t e r m i n e d R S R , R M R a n d Q values are p r e s e n t e d in Table 2.
5. Conclusions A n a m u r D a m will be l o c a t e d on the A l a n y a m e t a m o r p h i t e s , o f only a fair r o c k m a s s quality. G e o t e c h n i c a l investigations, testing o f m a t e r i a l s a n d the c o m p u t a t i o n s indicate t h a t rock-fill d a m can be safely f o u n d e d at the p r o p o s e d site. T h e only p o t e n t i a l p r o b l e m is the relatively higher h y d r a u l i c c o n d u c t i v i t y o f the quartzite lenses. F o r the final design purposes, a d d i t i o n a l geotechnical
References Anon, 1977. The description of rock masses for engineering purposes. Q. J. Eng. Geol., 10: 355-388. Barton, N., Lien, R. and Lunde, J., 1974. Classification of rock masses for design of tunnel support. Rock Mech., 6(4): 189-236. Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. Wiley, New York, 251 pp. Hock, E. and Brown, E.T., 1980. Underground Excavations in Rock. Inst. Min. Metall., London, 527 pp. Hoek, E., 1990. Estimating Mohr-Coulomb friction and cohesion values from the Hock-Brown failure criterion. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., pp. 227 229. Goodman, E.R., 1989. Introduction to Rock Mechanics, 2nd ed. Wiley, New York, 562 pp. ISRM 1978. Suggested methods for quantitative description of discontinuities in rock masses. Int. J. Rock. Mech. Min. Sci. Geomed~..,Abstr., 15: 319-368. Kulhawy, F.H. and Goodman, R.E., 1980. Design of foundations on discontinuous rock. In: P.J.N. Pells (Editor), Int. Conf. Structural Foundations on Rock, Sydney. A.A. Balkema, Rotterdam, pp. 209-220. Ocal, A., 1994. Computer-aided Kinematic Analysis for Jointed Rock Slopes. M.Sc. Thesis, Middle East Tech. Univ., Ankara, 121 pp. Ozgt~l, N., 1976. Some geological aspects of the Taurus Orogenic Belt (Turkey). Bull. Geol. Soc. Turkey, 19: 65-78. Wickham, G.E., Tiedemann, H.R. and Skinner, E.H., 1974. Ground Support Prediction Model-RSR Concept II. In: Proc. Rapid Excavation Tunneling Conf. AIME, New York, pp. 691-707.
EngineeringGeology42 (1996) 65-70
Geotechnical rock-mass evaluation of the Anamur dam site, Turkey A. Ozsan a, C. Karpuz b , a University of Ankara, Faculty of Science, Geological Engineering Department, 06100 Ankara, Turkey b Middle East Technical University, Mining Engineering Department, 06531 Ankara, Turkey Received 21 March 1995; accepted 14 September 1995
Abstract
This paper describes a feasibility-level geotechnical evaluation carried out at Anamur dam site, in terms of stability analysis of dam foundation, excavation slopes and a diversion tunnel. The proposed Anamur dam will be built across the Anamur river on the Alanya metamorphic series which consists of phyllite, schist and slate units at the site. Geoteelmical investigations included drilling for core, pumping tests, sampling for laboratory testing, and a detailed discontinuity survey. Uniaxial compressive strength, density, triaxial compressive strength and direct shear tests, have all been carried out in the laboratory. Discontinuity data are processed for the determination of dominant sets and for the rock mass characterization in terms of classification systems. Evaluation concentrated on kinematic analysis in slope stability, Goodman's bearing capacity approach in foundation stability and rock mass classification for diversion tunnel stability assessment. The geotechnical evaluation of the proposed dam and reservoir site established that there will be no stability problems. Detailed geotechnical investigations are required for the final design of this project.
1. Introduction
The proposed Anamur dam will be built across the Anamur river in southernmost Turkey (Fig. 1 ). The dam will be approximately 105 m high with a crest length about 650 m and base width of 450 m and is planned to be rock-fill dam. The purpose of the dam is to control and store water for an irrigation project being developed near Anamur river and to generate electricity. The design of the dam is under the joint direction of Electricity Research Department and State Water Work. * Correspondingauthor. E-Mail: karpuz @ rorqual.cc.metu.edu.tr 0013-7952/96/$15.00© 1996ElsevierScienceB.V. All fights reserved SSDI 0013-7952(95)00065-8
This paper explains some preliminary guidlines for the safe design of the dam foundation, stability of excavation slopes, as well as potential displacement of natural slopes into the reservoir and stand-up conditions for a diversion tunnel.
2. Geology The investigated area known as the Alanya metamorphics make part of the Toros Mountains tectonic group (Ozgal, 1976) and consists of schist, slate and phyllite (SSP), quartzite lenses, Palaeozoic-aged. It crops out around the Anamur
A. Ozsan, C] Karpuz/Engineering Geology 42 (1996) 65- 70
66
N
~\
Erm~ek
plagioclase and opaque mineral. Slate is green to greenish black in color and shows lepidoblastic texture with a composition of quartz, albite and chlorite. Quartzite also crops out in the area, as small to larger lenses within the SSP. They are grey, white and cream like in color, and are dominantly composed of quartz minerals with a Granoblastic texture.
2.1. Structural geology
MEDITERRANEAN "v///.-~ ~ ,
~" = '
SEA o
~
m
4o ~ o ~
Fig. 1. Location map of the dam site.
dam site (Fig. 2). Phillites are light grey in color, made up of quartz, plagioclase, chlorite and muscovite, with traces of opaque mineral and carbonate. Texture is lepidoblastic. Schist occur in the forms of interlayers, 30-40 cm in phillites. Green grey to dark grey in color. They show porphyroblastic texture and are composed of chlorite,
Two dominant folds are distinguished in the area. The first axis lies 250 m SW of borehole BH-1 (Fig. 2), and a strike of N W - S E with a wave length of 1800 m. A second fold is more or less parallel to the Anamur creek ( N W - S E direction) and its length is around 4000 m.
2.2. Methodology Discontinuity survey including orientation, aperture, infilling and spacing, has been carried out in accordance with the working party report (Anon, 1977) and the International Society for Rock Mechanics ISRM Suggested Method N
EXPLANATION oo,z.
Schist, Slate, Phyllite
~
s~l~ ond ~ ~ j~nt Foliation Contoct Anticlt~l axis Syndinol o~is
0 B;-;-I o/D'
(~)re~ob, ( k)co~l (~'ld numi~br) Dora axis
z ~-=--'~
Tunnel oligr~te~l
--~ O
~O
~
~00m.
Fig. 2. Geological map at the site area.
Expec~s~ped~-~,c~on
A. Ozsan, C Karpuz/Engineering Geology 42 (1996) 65-70
length (meter) of a borehole at a unit time (minute) under the effect of 10 atm pressure. The total 185 pumping tests were performed at the SSP unit and one performed at the quartzite unit in the 5 boreholes. The distribution of those Lugeon values and their descriptions are given below:
(ISRM, 1978). In total, 922 discontinuities, 506 on the left bank and 416 on the right bank, were measured. Orientations are processed utilizing a computer program based on equal-area stereographic representation and the following dominant discontinuity sets were distinguished on left and fight banks (Fig. 2). On the left bank: D2; N45E/82 NW, D3; N46W/84NE, D4; N42W/ 49NE, D1; N22E/62NW, Ds; N46E/66SE On the fight bank: DI; N80E/50SE, D2; N35E/83NW, Da; N52W/ 78NE, D4; N34W/54SW, Five boreholes were drilled ranging from 54.6 m to 125 m in depth at the site. Both the tunnel alignment and the dam axis cut across all rock units (Fig. 3). During the site exploration, pumping tests were carded out, and rock samples were taken for laboratory testing. The conductivities are expressed in terms of Lugeon values. A Lugeon unit is the loss of a liter of water along a unit
mD
Lugeon values <1 1-5 5-25 >25 The Lugeon value of
#of Tests 122 46 16 1 the quartzite unit
D'
~"
SWr
Domoxi. ~N40E
I
I
m
. ow
t
BH - 4
I
I
!
~7 0
,-. ~
Description Impermeable Slightly permeable Permeable Highly permeable is greater than 25.
Rock-mass properties: SSP: moderately to highly weathered, 70-180 cm in discontinuity spacing. Discontinuity aperture is 1-5 mm and mostly filled with kaolinite, mica and chlorite. Rough to planar surface roughness. SSP is generally impermeable and has an average of 44% (21-74%) RQD. Quartzite: slightly to moderately weathered,
.~
~ - 4
67
50
15Ore
Schist,slote, phyllite
t BH-I Bonthole ( Io~fionand
numtxtr)
Tunnel alignment
Fig. 3. Geological cross-sections along the tunnel alignment and dam axis,
68
.4. Ozsan, c Karpuz/Engineering Geology 42 (1996)65 70
200-600 mm discontinuity spacing. Discontinuity aperture is mostly less than 1 mm, but sometimes it is as wide as 3 mm and highly healed with quartz and epidote secondary mineralization. It is highly porous and has an average of R Q D 58% (49-66%). Uniaxial compressive strength, density, triaxial compressive strength and direct shear tests were carried out in the laboratory in accordance with ISRM suggested Methods. The test results are given in Table 1.
program K A N R O S (Ocal, 1994) is based on the kinematic technique suggested by G o o d m a n (1989). The program handles kinematic analysis for slopes having several designated discontinuity sets and/or individual discontinuities. Several slopes in multiple structural domains can be included and maximum safe slope angles can easily be determined. One of the main features of the program is that it can accomodate a designated scatter in dip and dip direction of the discontinuities, as well as variations in the angle of friction for each discontinuity.
3. Kinematic analysis for slopes 3.2. Left bank 3.1. Introduction
Stability analysis for slopes are usually carried out with the limit-equilibrium technique, the analysis of forces acting on the most plausible unsafe portion of the slope. In this analysis, geometry of slope and relative orientations of critical features, possibly type of failure and the shear strength parameters of these features must be well defined. Kinematic analysis is carried out first to identify and eliminate safe features for further analysis. For example, a wedge whose line of intersection does not daylight into free face will never cause instability. Kinematic analysis is performed using equal-area spherical projection techniques. After distinguishing the dominant discontinuity sets on both left (Region 1 ) and right (Region 2) banks of the reservoir area, the kinematic analysis was carried out. First, the possible left and right bank slope orientations were determined as N 50 W (220 °) and N 50 W NE (040°), respectively (Fig. 2). Then the F O R T R A N coded computer
Considering the center value of the pole concentration for all discontinuity sets, the most critical face angle is the 67.8 ° due to the discontinuity set (D4) then 75.5 ° intersection axis comes into play due to the wedge formed by the intersection of D1 and D3 (Ix.3). Considering the effect of pole concentration scattering, the critical cut-slope angles decrease to 64.6 ° and 64.5 ° respectively. These are rather steep angles so kinematically it can be said that the left bank is almost safe but will yet require engineered reinforcement. The right bank is fully safe due to the wedge and plane failure point of view. The expected instability is toppling due to discontinuity set D3. The analytical stability analysis was not carried out since the kinematic analysis indicates stability at higher slope angles than are present. Generally, the faces on both sides of the river can be regarded as safe. Because discontinuities are not continuous and moderately spaced (70-180 cm), toppling failure is not expected to
Table 1 Laboratory test results Rock u n i t
SSP Quartzite *Mean value.
Uniaxialcomp. str. (MPa)
9.34 77.65 (43.97)* 95.77 112.80
Density (kN/m3)
Triaxial test
Direct shear (parallel to joints)
Int. friction angle
Cohesion MPa
Int. friction angle
Cohesion MPa
2.57 2.83
46'
3.9
30.1-34.0
0.01-0.06
2.65
48'
12
A. Ozsan,C. Karpuz/EngineeringGeology42 (1996)65-70 occur. Secondly, for the steeply dipping discontinuities, dominant joint-bounded rock bodies are naturally buttressed by other blocks which may prevent the toppling failure and the slope may be stable. On the other hand, for all potential slope failure geometries, water is present and decreases the stability factors. However, in the reservoir area, hydraulic pressure provided by stored water increases the normal force acting on the slope face hence increases overall stability below the water line.
Calculation of a bearing capacity by the limitequilibrium method for a footing under load must respect the complexity and variety of failure modes such as cracking, crushing, wedging, punching and shearing. As Goodman (1989) pointed out, no universal formula for bearing capacity of rock can be given. Several simple approaches can be utilized. In this study, bearing capacity of the rock mass beneath the dam axis is carried out by Goodman's (1989) approach for homogeneous, discontinuous rock masses. In this analysis, both Mohr-Coulomb and Hock and Brown's empirical failure criteria are utilized Bearing capacity in terms of Mohr-Coulomb failure criteria: qf=2 Cr tan(45 +$r/2) + 2 Cptan(45+$p/2) tan2(45+$r/2)
instability) can be approximated as not less than the unconfined compressive strength of the rock mass around the footing, and this can be taken as a lower bound. For the safest condition, St, and Cr are zero and qf=2Co. If the minimum uniaxial compressive strength value of the SSP is considered, qf= 2 × 9.34 = 18.68 MPa, or average uniaxial compressive strentgh of SSP is used qf= 56.38 MPa is found. Hock and Brown (1980) failure criteria:
qf=Co{(S)U2 +[mr(S)U2+ Sr]U2t
3.3. Bearingcapacity calculation
(1)
where, qf= bearing capacity, MPa; Cr =residual cohesion, MPa; Cp =peak cohesion, MPa; Sr = residual internal friction angle, degree; Sp= peak internal friction angle. Kulhawy and Goodman (1980) suggested that it is necassary to use rock-mass strength parameters in the evaluation of bearing capacity and for simplicity, rock mass properties are about 50-70% of intrinsic (Laboratory Material [M]) rock properties. So if Sm =23 and Cr = 1.9 MPa is used; qf is calculated as 26.95 MPa ($=46 and C=3.9 MPa, Table 1). Mohr-Coulomb failure criteria leads to the conclusion that the bearing capacity of a homogeneous, discontinuous rock mass (without kinematic
69
(2)
where:Co=uniaxial comp. strength, MPa; S = intact material constant; mr = rock mass constant; St=rock mass constant, when S = I , mr= 0.317, St=0.00001, co=28.19 MPa is replaced into Eq. (2), qf is determined as 43.97 MPa (mr and Sr is calculated from the table (p. 176) given in Hock and Brown (1980)). If the R M R data are processed in accordance with Hock (1990), the calculated bearing capacities are 32.90 MPa and 31.00 MPa for the average and lowest R M R values respectively. The safest condition (mr=0, S t = 0 ) and the uniaxial comp. strength of SSP as 28.19 MPa or minimum strength of 9.34 MPa is considered, the allowable bearing capacities are calculated as 28.19 MPa and 9.34 MPa respectively. As can be seen from the calculated values of both failure criteria, the bearing capacities of the rock unit SSP is greater than the distributed load exerted by the dam. (The maximum load exerted by axis is calculated as 105 m x l m x 5 m x3 tons/m3=1575 tons or 1575 tons/5 m2=315 tons/m 2 or 3.15 MPa). Therefore dam the foundation will be safe from the bearing capacity point of view. However, when determining the safe bearing pressures on a footing on rock, it is never permissible to use the bearing capacity as calculated without consideration of scale effects. There is an element of uncertainity associated with the variability of the rock and a significant size effect in strength under compressive loads. However, even with a factor of safety of 5, the allowable loads yet tend to be higher than the allowable bearing capacities. It should also be
70
A. Ozsan, C Karpuz/Engineering Geology 42 (1996) 65-70
Table 2 Engineering classification of rock masses and proposed support requirements in tunnels Rock unit
RSR
RMR
Q
Proposed support requirement
SSP
31
13-37
0.12-1.08
Quartzite
52
33-56
1.00-8.21
2.75 m long tensioned rock bolts on a grid spacing of 1 m with a 50 mm thick shotcrete reinforced with a welded steel mesh No support requirement indicated
*Bolt length is calculated using the equation L=2+0.15(B/ESR), where B=5 m, ESR (Excavation Support Ratio)= 1. stressed here t h a t d i s c o n t i n u i t y s p a c i n g / f o u n d a t i o n w i d t h r a t i o will be smaller a n d will help to increase b e a r i n g c a p a c i t y o f r o c k unit a n d hence the stability o f the d a m f o u n d a t i o n .
investigations will be r e q u i r e d for the s u p p l e m e n tary structures such as the l o c a t i o n a n d the design o f the spillway a n d p o w e r h o u s e as well as the m a i n structures. T h e q u a r t z i t e lenses will require g r o u t i n g to seal t h e m f r o m the p a s s a g e o f groundwater.
4. Rock mass classification for tunnel design Several r o c k m a s s classification systems have been p r o p o s e d to s u m m a r i z e key geological a n d geotechnical d a t a a n d to p r o v i d e a t o o l for decision m a k i n g d u r i n g c o n s t r u c t i o n . These systems p r o vide a m o r e q u a n t i t a t i v e m e a s u r e o f r o c k m a s s a n d hence minimizes j u d g e m e n t a l bias. T h e systems c o n s i d e r e d a p p r o p r i a t e to the diversion tunnel ( h a s 5 m d i a m e t e r ) o f A n a m u r D a m were W i c k h a m et al. (1974), R o c k Structure R a t i n g ( R S R ) , B a r t o n et al. (1974) Q - s y s t e m a n d Bieniawski (1989), R o c k M a s s R a t i n g ( R M R ) systems. T h e collected d a t a were processed in a c c o r d a n c e with the classification systems a n d the d e t e r m i n e d R S R , R M R a n d Q values are p r e s e n t e d in Table 2.
5. Conclusions A n a m u r D a m will be l o c a t e d on the A l a n y a m e t a m o r p h i t e s , o f only a fair r o c k m a s s quality. G e o t e c h n i c a l investigations, testing o f m a t e r i a l s a n d the c o m p u t a t i o n s indicate t h a t rock-fill d a m can be safely f o u n d e d at the p r o p o s e d site. T h e only p o t e n t i a l p r o b l e m is the relatively higher h y d r a u l i c c o n d u c t i v i t y o f the quartzite lenses. F o r the final design purposes, a d d i t i o n a l geotechnical
References Anon, 1977. The description of rock masses for engineering purposes. Q. J. Eng. Geol., 10: 355-388. Barton, N., Lien, R. and Lunde, J., 1974. Classification of rock masses for design of tunnel support. Rock Mech., 6(4): 189-236. Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications. Wiley, New York, 251 pp. Hock, E. and Brown, E.T., 1980. Underground Excavations in Rock. Inst. Min. Metall., London, 527 pp. Hoek, E., 1990. Estimating Mohr-Coulomb friction and cohesion values from the Hock-Brown failure criterion. Int. J. Rock Mech. Min. Sci. Geomech. Abstr., pp. 227 229. Goodman, E.R., 1989. Introduction to Rock Mechanics, 2nd ed. Wiley, New York, 562 pp. ISRM 1978. Suggested methods for quantitative description of discontinuities in rock masses. Int. J. Rock. Mech. Min. Sci. Geomed~..,Abstr., 15: 319-368. Kulhawy, F.H. and Goodman, R.E., 1980. Design of foundations on discontinuous rock. In: P.J.N. Pells (Editor), Int. Conf. Structural Foundations on Rock, Sydney. A.A. Balkema, Rotterdam, pp. 209-220. Ocal, A., 1994. Computer-aided Kinematic Analysis for Jointed Rock Slopes. M.Sc. Thesis, Middle East Tech. Univ., Ankara, 121 pp. Ozgt~l, N., 1976. Some geological aspects of the Taurus Orogenic Belt (Turkey). Bull. Geol. Soc. Turkey, 19: 65-78. Wickham, G.E., Tiedemann, H.R. and Skinner, E.H., 1974. Ground Support Prediction Model-RSR Concept II. In: Proc. Rapid Excavation Tunneling Conf. AIME, New York, pp. 691-707.