Assessment of rock slope stability for a segment of the Ankara–Pozantı motorway, Turkey

Engineering Geology 74 (2004) 73 – 90 www.elsevier.com/locate/enggeo

Assessment of rock slope stability for a segment of the Ankara–Pozantı motorway, Turkey B. Kentli, T. Topal * Department of Geological Engineering, Middle East Technical University, Ankara 06531, Turkey Received 5 August 2003; accepted 10 March 2004 Available online 26 April 2004

Abstract The Cßiftehan – Pozantı segment of the Ankara – Pozantı motorway is problematic due to the existence of lithological units with variable characters. Ten cut slopes are planned along Km 352 + 350 – Km 358 + 000 of the proposed motorway. The purpose of this study is to determine engineering geological properties of the rocks exposed along the motorway and to assess excavatability and stability of the cut slopes. Both field and laboratory studies were carried out during this research. Field studies involved detailed discontinuity surveys. Laboratory tests were carried out to determine unit weight, point load strength index and shear strength parameters of the discontinuities. In the study area, recrystallized limestone, dolomite – limestone, microgabbro, reefal limestone, conglomerate and Quaternary deposits are exposed. However, the cut slopes are located within microgabbro, reefal limestone, dolomite – limestone and recrystallized limestone. Hard to extremely hard ripping with local blasting for the fresh inner part of the rocks is recommended for the excavatability. In eight cut slopes, wedge failure is expected. Based on the field observations and stability analyses of the cut slopes, slope flattening with various angles, wire mesh and drainage ditches are suggested. D 2004 Elsevier B.V. All rights reserved. Keywords: Ankara – Pozantı motorway; Excavatability; Kinematic analysis; Limit equilibrium; Rock slope

1. Introduction As part of Trans-European Motorway (TEM) project, the construction of Istanbul –Ankara and Pozantı – Gaziantep sections of the motorway in Turkey is completed. It is also essential to establish a better connection between Central Anatolia and the Mediterranean coast due to increasing traffic density. Therefore, the Ankara – Pozantı section of the motorway is planned to be constructed. The Ankara – Pozantı mo* Corresponding author. Fax: +90-3122101263. E-mail address: [email protected] (T. Topal). 0013-7952/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2004.03.002

torway is 2
3-lane autoroad with a total length of 366 km. Site investigations almost completed for this section of the motorway revealed that more study should be carried out for Cßiftehan –Pozantı segment of the project. In this segment, various lithological units having different engineering geological properties are exposed in narrow and steep valleys. Several slopes and/or road cuts also exist. It is important to assess the engineering geological properties of the lithological units, excavatability and stability of slopes for the safety of the motorway. The purpose of this study is to investigate the material and mass characteristics of the lithological

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to the proposed motorway. No negative effects of the stream exist for the motorway because the elevation of the motorway is 10 – 50 m above the stream channel. Additionally retaining walls will be constructed to protect the fills, where the proposed motorway is close to the stream bed. Continental climate prevails at the site vicinity. The study area is generally covered with dense pine trees. However, the pine trees are rarely observed above an elevation of 2000 m.

units exposed along Km 352 + 350 –Km 358 + 000 of the Cßiftehan – Pozantı segment of the proposed Ankara – Pozantı motorway. In this section, 10 cut slopes with dimensions indicated in Table 1 will be constructed. The cut slopes are evaluated from the excavatability and slope stability points of view, and practical remedial measures for the cuts slopes are suggested. In order to accomplish these tasks, field work, including geological mapping, discontinuity surveying and rock sampling, was performed. Laboratory tests for the determination of unit weight, point load strength index and shear strength parameters of the discontinuity surfaces by direct shear tests were performed on samples collected from the cut slope areas. The method suggested by Pettifer and Fookes (1994) was used for the assessment of rock excavatability. The cut slope stability was evaluated through kinematic and limit equilibrium analyses. The study area is located along the Cßiftehan – Pozantı segment of the Ankara –Pozantı motorway (Fig. 1). The topography is steep along the segment. However, the site is accessible throughout the year by the Ankara –Adana E-90 highway and rural asphalt paved and/or stabilized roads. Narrow valleys and high topography with elevations ranging between 800 and 2424 m characterize the site vicinity. A dentritic drainage pattern is developed in the study area. Most streams are intermittent and flow only after heavy rains. Cßakıt stream is one main stream, flowing with high amounts of discharge throughout the year, especially in the spring season. The Cßakıt stream flows parallel

2. Geology 2.1. Site geology Geological mapping with a scale of 1:5000 was conducted for 1-km-wide motorway corridor along the study section of the Ankara – Pozantı motorway. The field study reveals that the main lithological units from older to younger are recrystallized limestone, dolomite – limestone, microgabbro, reefal limestone, conglomerate and Quaternary deposits (Figs. 2 and 3). In the study section, the recrystallized limestone is light gray to white, massive, moderately jointed and strong. This unit is observed along 830 m of the motorway between Km 357 + 220 and Km 358 + 050 of the section (Fig. 3). The recrystallized limestone overthrusts the dolomite –limestone unit at the northern boundary and is unconformably overlain by the conglomerate at the southern boundary (Figs. 2 and 3). The age of the limestone is Permian (Demirtaslı et al., 1986).

Table 1 Dimensions of the cut slopes Rock type

Microgabbro Reefal limestone Dolomite – limestone

Recrystallized limestone

Cut slope

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10

Number of cut slopes created

Location

2 2 1 1 1 1 1 1 1 1

Km Km Km Km Km Km Km Km Km Km

352 + 334 – Km 352 + 823 – Km 354 + 026 – Km 354 + 388 – Km 355 + 000 – Km 355 + 540 – Km 356 + 000 – Km 356 + 410 – Km 356 + 700 – Km 357 + 792 – Km

Length (m)

352 + 534 353 + 139 354 + 123 354 + 621 355 + 354 355 + 700 356 + 192 356 + 470 356 + 892 357 + 918

200 316 97 233 354 160 192 60 192 126

Slope height (m) Maximum

At axis

51.8 74.8 51.6 22.6 29.5 51.9 33.4 28.7 52.8 25.3

23.3 42 7 7.3 13.3 27.6 18.5 3.5 22.5 6.5

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Fig. 1. Location map of the study area.

The dolomite –limestone unit consists of dark bluish gray, thick-bedded dolomites and light gray, medium to thick-bedded recrystallized limestones. The unit is observed along 2330 m of the motorway between Km 354 + 850 and Km 357 + 180 of the section. The unit overtrusts reefal limestone in the study area. The limestone is overthrusted by the recrystallized limestone and has a tectonic contact with conglomerate (Figs. 2 and 3). The age of the dolomite– limestone is Late Triassic (Demirtaslı et al., 1986). The microgabbro is greenish dark gray, fine grained, slightly weathered and very strong. Staining and calcite infilling are observed along the joint surfaces. The microgabbro is observed along 1110 m of the motorway between Km 352 + 350 and Km 353 + 460. The unit has a tectonic contact with reefal limestone and is overthrusted by the dolomite– lime-

stone unit (Figs. 2 and 3). The emplacement age of the microgabbro is given as pre-Campanian (Demirtaslı et al., 1986). The reefal limestone consists of light gray, medium- to thick-bedded, fossiliferous limestone and red conglomerate, composed of pebbles derived from ophiolitic rocks at the base of the formation, and yellowish white foliated schist intercalated with the limestone. The formation is observed along 1430 m of the motorway between Km 353 + 460 and Km 354 + 890. The age of unit is Late Maastrichtian – Early Paleocene (Demirtaslı et al., 1986). The reefal limestone has a tectonic contact with the microgabbro and is overthrusted by the dolomite – limestone. The conglomerate consists of dominantly conglomerate with local sandstone and lacustrine marl layers. The conglomerate is reddish green, fine

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Fig. 2. Geological map of the site vicinity (modified from Demirtaslı et al., 1986).

grained with subangular shape, slightly weathered and medium strong. The grains are mainly derived from older rocks, such as microgabbro, recrystallized lime-

stone, dolomite– limestone and reefal limestone. The conglomerate is not observed along the motorway. It crops out N/NE of the recrystallized limestone (Fig.

B. Kentli, T. Topal / Engineering Geology 74 (2004) 73–90 Fig. 3. Geological map, cross-section and engineering geological properties of the units exposed along Km 352 + 350 – Km 358 + 000 section of the Ankara – Pozantı motorway.

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3). Based on the regional lithological correlations, the age of the conglomerate is given as Oligocene – Early Miocene (Demirtaslı et al., 1986). Quaternary deposits are observed in the form of alluvium. The deposits are commonly seen along the Cßakıt stream (Fig. 3). They are composed of clay, silt,

sand and gravel. The Quaternary deposits are observed for a total length of 520 m (Km 352 + 600 – Km 352 + 820 and Km 535 + 230 – Km 353 + 530). The thickness of the Quaternary deposits varies along the motorway. It ranges between 5 and 10 m. No cut slopes exist in this unit.

Fig. 4. Distribution of the earthquake epicenters with magnitudes greater than 3 in the last 101 years and the major fault zones in Nigde – Pozantı region (USGS, 2002).

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2.2. Faults and seismicity of the study area In the study area, there are two overthrusts that can be traced at Km 354 + 900 of the motorway between the reefal limestone and the dolomite –limestone and at Km 357 + 180 of the motorway between the dolomite –limestone and the recrystallized limestone. No seismic activity is observed along the overthrust faults.

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The only seismically active fault of the region, the so-called Ecemis¸ fault, which has a left lateral strike – slip character (Yetis¸, 1984; Koc¸yig˘it and Beyhan, 1998), is located at the southeast (outside) of the motorway. It does not interfere with the motorway segment based on the information presented by Demirtaslı et al. (1986). A total of 18 earthquakes with instrument-measured magnitudes greater than 3.0 have been recorded in the study area between January 1900

Fig. 5. Plots of (a) pole, (b) contour, (c) rose and (d) dominant discontinuity sets for C-1 cut slope.

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and July 2001 (USGS, 2002). Their spatial distributions are shown in Fig. 4. Based on the seismic acceleration zones of the study area proposed by Gu¨lkan et al. (1993) and GDDA (1996), the study section falls in the third-degree earthquake zone. This represents a ground acceleration ranging between 0.2g and 0.3g.

3. Engineering geological properties of the rocks Engineering geological properties of the rocks exposed along the study section were determined on the basis of field observations/measurements and laboratory tests. The description of rock material and mass characteristics were based on ISRM (1981). A total of 25 oriented block samples were collected from the field for laboratory testing. Unit weight, point load strength index and shear strength parameters of the discontinuities by direct shear tests were determined in accor-

dance with ISRM (1981, 1985). In the field, however, scan-line surveys on natural rock exposures were performed following methods suggested by ISRM (1981). The directional data collected from scan-line surveys are evaluated using DIPS 5.0 computer program (Rocscience, 1999a). The data are plotted on Schmidt nets using lower hemisphere projections. Diagrams for pole plot, contour, rose and dominant joint sets were prepared for each cut slope. Fig. 5 is an example of the diagrams prepared for C-1 cut slope. In the study area, microgabbro, reefal limestone, dolomite –limestone and recrystallized limestone daylight where the cut slopes will be formed (Fig. 6). Material and mass properties of the units are presented in Table 2, whereas the shear strength parameters of the dominant discontinuity sets are summarized in Table 3. The unit weights of the units have similar values; however, point load strength index, Is(50), of the reefal limestone is low compared to the other units. The rocks contain at least three discontinuity sets which control

Fig. 6. Photographs showing some of the slopes where stability analyses are done. (a) cut slope C-1 in microgabbro, (b) cut slope C-10 in recrystallized limestone (c) cut slope C-8 in dolomite – limestone, (d) cut slope C-4 in reefal limestone.

Table 2 Material and mass properties of the units Rock type

Microgabbro

Dolomite – limestone

Recrystallized limestone

Unit weight (kN/m3)

Is(50)a (MPa)

27.57

6.45

26.54

26.45

26.41

1.94

4.53

3.97

Cut slope

Number of joints measured

C-1

105

C-2

91

C-3

80

C-4

72

C-5

124

C-6

67

C-7

75

C-8

101

C-9

107

C-10

92

Mass properties Dominant bedding plane (B)/joint set (J) (dip/dip direction)

Aperture

Spacing

Persistence

Roughness

Weathering

GWb

RMRbc

J-1 (45/210) J-2 (78/313) J-3 (46/022) J-1 (69/244) J-2 (40/080) J-3 (86/146) B-1 (65/119) J-1 (14/350) J-2 (53/322) B-1 (69/120) J-1 (35/026) J-2 (49/322) B-1 (71/140) J-1 (48/030) J-2 (46/203) B-1 (73/132) J-1 (37/209) J-2 (40/046) B-1 (71/110) J-1 (50/206) J-2 (44/033) B-1 (75/143) J-1 (55/281) J-2 (44/011) J-3 (69/077) B-1 (58/157) J-1 (88/315) J-2 (62/281) J-1 (44/274) J-2 (64/185) J-3 (79/076)

tight to moderately wide (0 – 8 mm)

close to moderate (10 – 60 cm)

medium to high (3 – 11 m)

undulating smooth to rough (JRC = 11)

slightly weathered

dry

very good rock (82)

tight to moderately wide (0 – 6 mm)

moderate (20 – 40 cm)

low (1 – 2 m)

undulating smooth (JRC = 16)

slightly weathered

dry

very good rock (89)

tight to open (0 – 2 mm)

close to moderate (10 – 40 cm)

low (2 – 3 m)

undulating smooth to rough (JRC = 12)

fresh to slightly weathered

dry

very good rock (84)

tight to open (0 – 2 mm)

moderate to wide (30 – 100 cm)

medium (3 – 4 m)

undulating smooth to rough (JRC = 12)

fresh to slightly weathered

dry

good rock (79)

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Reefal limestone

Material properties

a

Point load strength index. GW – groundwater condition. c RMRb – Basic Rock Mass Rating (Bieniawski, 1989). b

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Table 3 Shear strength parameters of the discontinuities Rock type

Microgabbro

Laboratory

Cut slope

Cohesion, c (MPa)

Friction angle, / (j)

Peak (cp)

Residual (cr)

Peak (/p)

Residual (/r)

0.55

0.22

35

33

C-1 left C-1 right C-2 left C-2 right

Reefal limestone

1.49

1.03

28

27

Dolomite – limestone

0.61

0.32

38

36

C-3 C-4 C-5 C-6 C-7 C-8 C-9

Recrystallized limestone a

1.59

1.23

28

25

C-10

Joint set number

Instantaneous Normal load, rn (MPa)

Cohesion, ci (MPa)

Friction angle (/i) (j)

J-1 J-2 J-1 J-2 J-1 J-2 J-1 J-2 – – J-1 J-2 J-1 J-2 J-1 J-2 J-1 J-2 J-1 J-2 J-1 J-2

3.728 1.125 0.152 1.380 1.133 0.332 3.240 0.744 – – 1.485 0.131 3.508 –a 0.976 –a 4.324 5.817 2.698 2.476 3.795 1.341

0.367 0.131 0.023 0.156 0.131 0.046 0.327 0.092 – – 0.209 0.027 0.434 –a 0.146 –a 0.519 0.668 0.347 0.323 1.281 0.707

32 36 44 35 36 41 32 38 – – 38 49 35 –a 40 –a 34 33 36 36 24 35

Not calculated because failure is not controlled by the specified joint set.

the excavatability and slope instability. Although there exists some variations in aperture, spacing and persistence of the joints, the rocks have similar roughness, weathering category and basic RMR values (Table 2).

The field observations also reveal that the cut slopes are dry. The laboratory direct shear box tests (Table 3) indicate that the peak cohesion values of the units range between 0.55 and 1.59 MPa. The residual cohesion

Table 4 Mean discontinuity spacing, discontinuity spacing index (If), point load strength index (Is(50)) and excavatability classes of the cut slopes Rock type

Cut slope

MDSa (m) J1

J2

J3

Microgabbro

C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10

0.80 0.75 0.60 0.50 0.50 0.65 0.45 0.70 0.55 0.95

0.60 0.70 0.45 0.50 0.35 0.40 0.20 0.30 0.25 1.10

0.20 0.40 0.25 0.30 0.25 0.15 0.35 0.40 0.20 0.50

Reefal limestone Dolomite – limestone

Recrystallized limestone a

MDS – mean discontinuity spacing. (If) – discontinuity spacing index. c Is(50) – point load strength index. b

(If)b

Is(50)c (MPa)

Excavatability class

0.379 0.570 0.380 0.409 0.339 0.280 0.298 0.413 0.277 0.757

4.42 6.16 1.95 2.05 4.62 3.96 5.91 7.08 6.20 4.23

very hard ripping extremely hard ripping hard ripping hard ripping very hard ripping hard ripping very hard ripping extremely hard ripping very hard ripping extremely hard ripping

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values are found to be slightly lower than the peak ones. The peak internal friction angles found from the laboratory tests vary between 28j and 38j, whereas the residual values range between 25j and 36j.

4. Assessment of rock excavatability In many construction projects, the method of overburden removal is a crucial factor affecting the

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safety and cost of operations. Blasting, ripping and digging are the main excavation methods. The excavatability of rocks mainly depends on the geotechnical properties of the material, on the excavation method and on the type and size of the excavating equipment being used (Pettifer and Fookes, 1994). A number of methods are suggested in the literature to assess the rock excavatability (Franklin et al., 1971; Weaver, 1975; Kirsten, 1982; Minty and Kearns, 1983; Scoble and Muftuog˘lu, 1984; Smith, 1986; Singh et al., 1987;

Fig. 7. Excavatability assessment chart (after Pettifer and Fookes, 1994) of the rocks in the study area.

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Caterpillar, 1988; Karpuz et al., 1990; Hadjigeorgiou and Scoble, 1990, MacGregor et al., 1994; Pettifer and Fookes, 1994). In this study, the revised excavatability chart proposed by Pettifer and Fookes (1994) was used for the assessment of rock excavatability because it considers the types of excavating equipment and requires engineering geological parameters (discontinuity spacing index [If] and point load strength index [Is(50)]) which are very easy to obtain through field and laboratory studies. Discontinuity spacing index (If) was calculated based on the procedure recommended by ISRM (1981). For the point load strength index, Is(50), a total of 96 samples were tested in accordance with ISRM (1985), and their average values were used for each station of the cut slope. The parameters used are tabulated in Table 4. The plotting of the data in the revised excavatability chart for each cut slope is shown in Fig. 7. Based on the excavatability analysis of the rocks, the excavatability category of C-3, C-4 (reefal limestone) and C-6 (dolomite – limestone) is hard ripping. However, it is very hard ripping for C-1 (microgabbro), C-5, C-7 and C-9 (dolomite –limestone) cut slopes. The cut slopes falling into extremely hard ripping category are C-2 (microgabbro), C-8 (dolomite –limestone) and C-10 (recrystallized limestone).

5.1. Kinematic analysis For the kinematic analyses, the lower hemisphere stereographical projection method described by Hoek and Bray (1981) and Goodman (1989) was used. Based on the motorway project requirements, the cut slope faces were considered to be parallel to the motorway trend. Planar, wedge and toppling failure modes were investigated through DIPS 5.0 computer program (Rocscience, 1999a). The friction angles (/) found from the laboratory tests for each cut slope (Table 3) were used for the analyses. The cuts having two slopes (C-1 and C-2) facing each other are named as left slope and right slope regarding the slope at the left-hand and the right-hand sides, respectively, following the increasing trend (kilometers) of the motorway. During the kinematic analyses, the optimum slope angle for each cut slope was determined by considering the safest slope angle for any kind of failure. For remedial measures, only slope flattening was considered during the kinematical check since the route of the motorway was finalized and not subject to change. The kinematic analyses of each cut slope are shown in Figs. 8 and 9. The summary of the analyses and the proposed slope angle for each cut slope are given in Table 5. Based on the results of the kinematic analyses, no failure is expected for C-3 and C-4 cut slopes. However, wedge failure is expected for all the other cut slopes. The safe slope angles range between 36j and 90j.

5. Assessment of slope stability 5.2. Limit equilibrium analysis Assessment of slope stability in rocks is usually done through kinematic and limit equilibrium analyses. Kinematic refers to the motion of bodies without reference to the forces that cause them to move. If the kinematic analysis indicates that the failure controlled by discontinuities is likely, the stability must be evaluated by a limit equilibrium analysis, which considers the shear strength along the failure surface, the effects of pore water pressure and the influence of external forces such as reinforcing elements or seismic accelerations (Turner and Schuster, 1996). In this study, the kinematic analysis is done for all of the cut slopes. Nevertheless, the limit equilibrium analysis is only performed for the slopes which are likely to fail from the kinematics point of view.

Permanent stability of the cut slopes to be created along the motorway is an important issue for safe transportation. The kinematic analysis does not consider forces acting on a slope forming material. Therefore, the stability of slopes is usually analyzed in engineering practice by the limit equilibrium analysis which provides a direct measure of stability in terms of the factor of safety. Due to very good rock mass quality, no rotational failure is expected to occur at the cut slopes. The slope instability is totally controlled by the discontinuities. In this study, limit equilibrium analysis is carried out for the cut slopes having wedge failure potential assessed during the kinematic analyses. Regarding the recommendations of General Direc-

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Fig. 8. Kinematical analysis of the cut slope C-1, C-2, C-3, C-4 and C-5. Dashed lines indicate recommended slopes.

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Fig. 9. Kinematical analysis of the cut slope C-6, C-7, C-8, C-9 and C-10. Dashed lines indicate recommended slopes.

B. Kentli, T. Topal / Engineering Geology 74 (2004) 73–90 Table 5 Modes of failures and safe slope angles for the cut slopes based on kinematic analyses Rock type

Microgabbro

Cut slope Probable Safe slope angles (j) failure mode Left Right slope slope

C-1 C-2 Reefal limestone C-3 C-4 Dolomite – C-5 limestone C-6 C-7 C-8 C-9 Recrystallized C-10 limestone

wedge wedge none none wedge wedge wedge wedge wedge wedge

42 68 90 90 47 41 58 36 39 41

49 38 – – – – – – – –

torate of Highways for this motorway, bench height and width are considered to be 10 and 5 m, respectively. For the limit equilibrium analyses of the wedge failure potentials, a computer program called SWEDGE 3.0 (Rocscience, 1999b) is used. The stability method used in the program is the limit equilibrium method for wedge failure suggested by Hoek and Bray (1981). It is based on the stereographical projection method including some geotechnical parameters such as slope height, friction angle and cohesion of the rock and unit weights of the rock and water with some other stereographical relations between the two discontinuities by the following formula:   3cA 3cB c F¼ Xþ Y þ A  w X tan/A cH cH 2c   c þ B  w Y tan/B 2c

gather these values at first, every slope was modeled in SWEDGE, and average normal loads on each joint set forming the wedge was obtained. Then, by employing these normal load values on graphs representing the nonlinear failure criterion for every cut slope, instantaneous cohesion ci and instantaneous friction angle /i values were obtained for each joint set of every cut slope showing the wedge failure. For the analysis, residual shear strength values were used as a conservative approach. This condition could be the case especially during and after excavation, which causes disturbance. A seismic acceleration coefficient of 0.15g, which is 1/2 PGA (Peak Ground Acceleration) of the region as suggested by Marcuson and Franklin (1983), is employed for the analysis. The limit equilibrium analysis is carried out using two approaches which can be described as conservative and nonconservative. The difference between these two approaches is that in conservative approach, instantaneous cohesion ci value is taken as zero, whereas in nonconservative approach, instantaneous cohesion ci value is the value found from the graph representing the nonlinear failure criterion. For both approaches, instantaneous friction angle /i is assumed to be the same. The summary of limit equilibrium analyses of the wedges formed in the cut slopes is presented in Table 6. For all cut slopes, the use of the instantaneous cohesion and friction angles suggests that the cut slopes even with vertical benches have a factor of

Table 6 Results of limit equilibrium analyses of the cut slopes Cut slope

where, cA and cB are the cohesive strengths of planes A and B; /A and /B are the angles of friction on planes A and B; c is the unit weight of the rock; cw is the unit weight of water; H is the total height of the wedge; X, Y, A and B are dimensionless factors which depend upon the geometry of the wedge. In the limit equilibrium analyses of the critical cut slopes found from the kinematic analyses, instantaneous cohesion ci and instantaneous friction angle /i values of the rocks (Table 3) were used. In order to

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c = 0, / = /I

c = ci, / = /I

Bench slope Factor of Bench slope Factor of angle (j) safety angle (j) safety C-1 (left slope) C-1 (right slope) C-2 (left slope) C-2 (right slope) C-5 C-6 C-7 C-8 C-9 C-10

43 50 69 39 90 90 59 90 90 42

0.7 0.7 0.4 0.9 1.1 1.2 0.9 1.3 1.1 0.9

90 90 90 90 90 90 90 90 90 90

7.6 4.2 5.3 7.6 8.3 16.6 5.6 22.1 10.7 39.7

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safety greater than 4.2. However, the conservative approach (c = 0) yields lower factor of safety. Although the factor of safety values are generally lower than 1.1 (the lowest long-term acceptable value for motorways in Turkey) for the conservative case, a slight modification of the slope angles will produce kinematically safe slopes.

6. Discussion Along the Km 352 + 350– Km 358 + 000 section of the proposed Ankara – Pozantı motorway, 10 cut slopes occur in four different lithologies, namely, microgabbro, reefal limestone, dolomite – limestone and recrystallized limestone. The units have variable engineering geological properties. They are slightly weathered and jointed. Each rock unit has at least three discontinuity sets. The excavatability category of rock cut slopes along the motorway for ripping ranges from hard to extremely hard ripping. However, the spacing of rock joints were measured on the surface, and this may increase towards the inner parts of the rock mass. Thus, blasting may be necessary to loosen the rocks especially in the extremely hard ripping category cut slopes. In practice, smooth blasting may be required during excavation phases in order to obtain welldefined slope geometry.

In any of the excavation techniques, the rock mass and rock joints are disturbed. Thus, the use of residual shear strength parameters for slope stability analysis should be preferred in order to be on the safe side. The microgabbro, where the cut slopes C-1 and C2 (Fig. 3) will be made, is jointed. During the field studies, old cut slopes of a stabilized road formed within the microgabbro were also examined. Detachment of small rock fragments and wedge failures were observed in the slopes with slope angles ranging between 60j and 90j. The kinematic and limit equilibrium analyses performed in this study also gave similar failure modes. The recommended slope angles for left and right slopes are 42j and 49j, respectively (Table 7). Slope flattening, wire mesh and ditches are recommended as remedial measures for C-1 and C-2 slopes (Table 8). The motorway route between Km 353 + 460 and Km 358 + 000 (Figs. 2 and 3) runs parallel to the E90 highway. This highway passes through the same formations (reefal limestone, dolomite – limestone and recrystallized limestone) as the motorway. Along the E90 highway, there are many cuts exceeding 10 m high and with 90j slope angles in these rock units. No planar wedge or toppling failures were observed along the E90 highway. However, the kinematic and limit equilibrium analyses of the cut slopes in these units indicate that excluding C-3 and C-4, wedge failure

Table 7 Comparison between safe slope angles for kinematic and limit equilibrium methods and recommended final slope angles Rock type

Microgabbro

Reefal limestone Dolomite – limestone

Recrystallized limestone a

Cut slope

C-1 (left slope) C-1 (right slope) C-2 (left slope) C-2 (right slope) C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10

Safe slope angle (j) Kinematic analyses

Limit equilibrium analyses

42 49 68 38 90 90 47 41 58 36 39 41

–a –a –a –a –a –a 90 90 –a 90 90 –a

Recommended bench slope angle (horizontal/vertical)

Overall slope angle (j)

42j 49j 68j 38j 90j 90j 90j 90j 58j 90j 90j 41j

32 49 49 32 64 66 71 64 43 71 65 33

(1/1) (1/1) (1/5) (1/1) (0/1) (0/1) (0/1) (0/1) (1/2) (0/1) (0/1) (1/1)

Factor of safety z 1.1 could not be satisfied for the slope angles greater than the safe slope angles gathered from kinematic analyses.

B. Kentli, T. Topal / Engineering Geology 74 (2004) 73–90 Table 8 Recommended remedial measures for the cut slopes Rock type

Cut slope

Remedial measures Flattening Wire mesh Ditch

Microgabbro

C-1 (left slope) C-1 (right slope) C-2 (left slope) C-2 (right slope) Reefal limestone C-3 C-4 Dolomite – C-5 limestone C-6 C-7 C-8 C-9 Recrystallized C-10 limestone

U U U U

U U U

U U U U U U U U U U U U

U U U U U U U U U U U U

89

this study are based on the field observations and laboratory test results. Therefore, new information obtained during the excavation stage should be used, and some practical modifications may be considered in case the engineering properties of the units significantly vary.

Acknowledgements The authors gratefully acknowledge PETRA and Yavuz Ergintav for their helps during the field and office studies.

References exits in the cut slopes. Nevertheless, the wedge failure may easily be eliminated by slope flattening (Table 7). In addition to this, due to the existence of highly jointed rock, small rock falls may occur in the future. Therefore, wire mesh and ditch may be used as additional remedial measures (Table 8).

7. Conclusion The purpose of the study is to provide the stability analyses of ten cut slopes between Km 352 + 350 and Km 358 + 000 of the proposed Ankara – Pozantı motorway to assess the excavatability and safe slope angle of the cut slopes. The cut slopes are located within microgabbro, reefal limestone, dolomite– limestone and recrystallized limestone. The excavatability analysis of the rocks reveals that the excavatability of the units ranges from hard to extremely hard ripping. However, blasting may locally be necessary for loosening especially the inner parts of the rock masses for ease of excavation. Kinematic analysis of the slopes indicate that wedge failure exists in eight cut slopes. However, no failure mode is detected in two cut slopes. Evaluation of the long-term limit equilibrium analyses of the cut slopes and kinematic analyses together show that the overall slope angles should range between 32j and 71j with the bench slope angles of 41j – 90j. Flattening, wire mesh and ditches are recommended as remedial measures. The recommendations given in

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