Proc. 30th Int'l. Geol. Congr., Vol. 23, pp. 423-435 Wang Sijing and P. Marinos (Eds) VSP 1997

Common mechanism of landslide creation along the under construction Egnatia highway, in Pindos mountain range (W. )

CHRISTARAS B., ZOUROS N., MAKEDON Th. & DIMITRIOU An. Lab. of Engineering Geology & Hydrogeology, School of Geology, Aristotle University of Thessaloniki, GR-54006, Thessaloniki, Greece, Tel/FAX: 3031-998506

Abstract

The part of the under construction Egnatia highway, that crosses Pindos mountain-range (W. Greece), can be characterized as one of the most difficult parts for the construction, because of the high relief, the mountain slopes are steep and the flysch, which is the do- minating geological formation in the area causes important landslides, occurred almost from the beginning of the road construction. According to our investigation the more important landslides occurred in the area are due: 1. to the geometry and the activity of the discontinuities (faults and important joints). 2. to the nature of the tectonic formation that lies under the Pindos nappes overthrusting the Ionian flysch (on the west of Metsovo tunnel), 3. to the nature of the tectonic formation that lies under the ophiolites overlying the Pin- dos flysch (on the East of Metsovo tunnel). These “tectonic formations” consist of shailes and pelites, containing detached blocks of limestones and deep sea sediments. Mechanically the above materials behave differently in dry and in west conditions. In dry conditions they behave like a rock, while in wet conditions they loose rapidly their cohesion and their original structure behaving like a sutured soil causing landslides along an important part of the road.

Keywords: Landslides, Egnatia highway, flysch, Pindos zone.

INTRODUCTION

The Egnatia highway is the most significant traffic artery in Greece. It is also the main artery linking trade and commerce from Western and Central Europe to the Middle East, as part of the traffic network of the European Union (Fig. 1). The Egnatia Highway in North- is crossing Pindos mountain range and is considered to be one of the most difficult parts for its construction. It includes complex geological formations that have undergone multiple tectonic deformations. The rock mass, under these condi- tions, is highly anisotropic and in association with the morphology (high relief and steep Landslides along Egnatia highway (Greece) slopes), it presents serious geotechnical problems in the construction of high cut slopes, tunnels and bridges connecting the different sections of the highway. The landslides are abundant along the road and occurred also at sites where the geome- try of the discontinuities in the flysch are not favourable. The landslides present similar

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Egnatia highway and Metsovo tunnel area, in Pindos mountain range characteristics, making necessary an investigation regarding the common mechanical behaviour of the geological formations along the road. The geomechanical investigations and detailed studies, along with the field data and geological mapping are included in the research project entitled «Geological - Engineer- ing Geological Research for the Egnatia Highway».

GEOLOGICAL SETTINGS

The study area is located in Northern Greece, in the Pindos mountain range. Two dis- tinct geotectonic units dominate in the area, the Pindos zone nappe and the Pindos ophiolite nappe. Pindos zone represents the passive margin of the Neo -Tethyan ocean, composed of Mesozoic carbonate and silisiclastic rocks and the Tertiary Pindos flysch, which forms the main outcrops of the Pindos zone in the area. Pindos zone consists of a sequence of Tertiary thrusts including the Pindos nappe which over-thrusts towards WSW the flysch of Ionian and Gavrovo zones [10]. Three lithostratigraphic groups of flysh sendiments have been distinghuished in the study area. Politses group is very well exposed in the area, represents the nappe of the Pindos flysch and overthrusts the group sendiments. The last one represents the younger sendiments of the flysch of the Ionian zone. The Metsovon group appears as a tectonic window under the thrust-sheets of the Pindos Flysch nappe. Pindos flysch (Politses group) is divided into four formations, from base to top these are: the "red flysch", alternation of red shales, pelites and sandstones with maximum Christaras B., Zouros N., Makedon Th. & Dimitriou An. thickness 100 m, the second formation comprises thin grey micaceous sandstones alter- nating with grey shales and marls with an average thickness 70 m, the third formation comprises thick massive sandstones and interbedded grey shales and marls with an max- imum thickness 350 m and the last one characterized as "wild flysch" composed of strongly tectonized grey siltstones and sandstones. Beneath the Pindos nappe, along the thrust front appears a "tectonic formation" consist- ing a melange of strongly tectonized rocks, pelites, sandstones and blocks of limestones [11]. The Pindos ophiolite complex represents fragments of the Neo-Tethyan oceanic litho- sphere which was emplaced initially on the western margin of the Pelagonian zone (Ci- merian micro-continent) during Late Jurassic-Early Cretaceous and subsequently over the Pindos flysch during Tertiary [1, 12]. Pindos ophiolite consists of mafic and ultra-mafic rocks (upper mantle peridotites partly serpentinised, gabbros, mafic and ultra-mafic cumulates, sheeted dikes, massive laves, pillow laves and basic brecias), metamorphic rocks parts of the sole (amphibolites, schists and meat-sediments) as well as deep sea sediments and turbidites (pelagic lime- stones, sandstones, calcarenites and micro-brecias, siltstones, green and red ribbon and nodular radiolarites) [1, 5, 9]. A tectonic formation containing blocks of all the above mentioned lithologies occurs along the tectonic contact between the Pindos ophiolite nappe and the Pindos flysch [11, 9]. In the northern part of the study area, molassic type sediments of the Meso-Hellenic Trough, were deposited during Oligocene-Early Miocene over the ophiolites and the Pindos zone sediments. The general attitude of the contact between the Pindos ophiolite nappe and the Pindos flysch seems to be horizontal to slightly eastward dipping, as the large number of tec- tonic windows appearing in the study area, including the large semi-window of Malakasi confirms.

TECTONICS

Although several studies have been carried out and different explanations have been given on the emplacement of the Pindos ophiolites over the Pindos flysch, we believe that it took place during an important early Oligocene extentional tectonic event that caused a re-deformation of the tectonic melange along the ophiolite-flysch contact. The tectonic evolution of the area is complicated. Structural analysis carried out in the area [9] show that several tectonic events took place. Successive tectonic events arise from the structural analysis in the area. The sense of movement was established by using shear criteria and kinematic indicators. Using the methods of quantitative analysis it was possible to provide a quantitative interpretation in terms of strain from the striations observed on the fault planes [10]. Landslides along Egnatia highway (Greece)

Tertiary evolution started in Late Eocene times with a D compressional event (max- 0 imum stress σ1 axes ENE-WSW) which caused detachment, folding and thrusting of the Pindos flysch before the emplacement of the ophiolite over the flysch.

D0 compressional event caused NW-SE to NNW-SSE trending inverse faults which are the dominant tectonic features in the area and bound the tectonic slices with a movement direction towards SW. Strike slip faults with remarkable displacements of the deformational front of the Pindos nappe along them, are closely related with the above mentioned compressional features. These faults are either dextral or sinistral The largest exists along river. It is a major transverse fracture zone, known as Kas- taniotikos fault [8] that interrupts the continuation of the Pindos zone. D0 event was followed by an important D1 extensional event (minimum σ3 axes ENE-WSW) in Early Oligocene times, which caused a semi-ductile to brittle defor- mation in the area and the emplacement of the ophiolites over the Pindos flysch. Two younger successive compressional events D2 and D3 are responsible for the refolding, imbrication and final shape of the Pindos nappe, with the maximum stress axes trending E-W and N-S respectively, took place during the Middle-Late Miocene (the second probably evolutionary to the first).

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Landslide in the tectonic formation, west of Metsovo tunnel Christaras B., Zouros N., Makedon Th. & Dimitriou An.

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Landslides in the tectonic formation east of Metsovo tunnel

THE TECTONIC FORMATIONS

The Tectonic formation between Pindos and Ionian flysch (to the east of Motsovo tun- nel, Fig. 2) Beneath the Pindos nappe, along the thrust front appears a "tectonic formation" consist- ing a melange of strongly tectonized rocks, pelites, sandstones and blocks of limestones [11]. Different thrusting planes have been distinguished along the thrust front of he Pin- dos nappe, within the tectonic formation. This formation is widely extended throughout the study area. It concerns a tectonic melange having a "chaos" structure and an appear- ance that reminds a "wild-flysch" formation . The matrix of the melange is mainly grey shales and sandstones in most cases com- pletely sheared. Detached blocks of limestones and deep sea sediments such as thin bed- ded pelagic limestones, radiolarian cherts, and Late Cretaceous neritic limestones with dimensions from several centimetres up to several hundred meters, are observed within the matrix. The blocks are particularly tectonized and generally fault bounded.

Landslides along Egnatia highway (Greece)

The tectonic formation between the opiolites and the flysch (to the west of Metsovo tun- nel, Fig.3) A tectonic formation containing blocks of all the above mentioned lithologies occurs along the tectonic contact between the Pindos ophiolite nappe and the Pindos flysch [11, 9]. This formation resembles a tectonic melange which presents a "chaotic" structure. The matrix of the melange consists mainly of multicoloured shales, siltstone and fine grained sandstones and appears completely sheared. Detached blocks of serpentinites, basic vol- canics, cherts, pelagic limestones and deep sea sediments derived from the ophiolite complex can be observed within the matrix. These blocks are strongly tectonized and fault bounded. This tectonic formation was initially created during the Jurassic subduction-accretion evolution [5] and probably re-deformed during the tertiary emplacement of the ophio- lites over the Pindos flysch [9].

GEOMECHANICAL INVESTIGATION

The region to the west of Metsovo tunnel The more important landslides occurring in the area are not only due to the geometry of the discontinuities of the flysch, in relation to the cut slopes directions, but mainly to the nature of the specific geolog- Particle size distribution ical formation in which Egnatia project (Tect. formation matrix) landslides are determined. 100 After our investigation, 90 80 the more important geologi- 70 cal formation considered to 60 be responsible for landslides 50 creation is the "tectonic for- 40 mation" that lies under the 30 20

Pindos nappe overthrusting 10 the Ionian flysch; it can be 0 0.001 0.01 0.1 1 10 100 observed in many places Diameter (mm) along its front [11]. The TF1 TF2 TF3 formation has a significant thickness of 20 to nearly 80 Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Particle m, depending on the site, and size distribution of three samples collected from the tectonic extends under Pindos flysch, formation creating important foundation problems. The particle size destribution of the matrix of the tectonic formation is given in Fig. 4. Christaras B., Zouros N., Makedon Th. & Dimitriou An.

Mechanically the material behaves differently in dry and in wet conditions. In dry conditions it behaves like a rock, having a dry density of 2.56 gr/cm3 and unaxial strength of 350 Kg/cm2. In wet conditions it loses rapidly its cohesion and its original structure and behaves like a saturated soil. It is a fine grained material of intermediate plasticity. The small plastic range between plastic (PL) and liquid limits (LL), given in Table 1 determine the ability of the material to change rapidly from the semi-solid to the liquid state, improving the significant decrease of the cohesion, angle of internal friction and bearing capacity after raining [7]. The Group Index (IG), given in Table 1, is rather high determining poor foundation conditions.

Table 1. Physical characteristics of three representative samples from the tectonic formation.

Property TF1 TF2 TF3 Liquid limit (LL, %) 36 39 38 Plastic limit (PL, %) 25 25 25 Plasticity index (PI, %) 11 14 13 Pass No 200 sieve (%) 58 78 73

Group Index (IG) 5 11 9

One other important parame- Schear strength - Fall cone penetrometer ter which was investigated, Egnatia highway - Tectonic formation was the change of the shear 27 y = 6208809.1623e -0.4629x 24 TF1 strength in relation to the r=-0.974 moisture content. For this 21 y = 2910826.3489e -0.4238x 18 TF2 purpose three representative r=-0.971 15 samples from the area were -0.4378x TF3 y = 4929868.7625e investigated using a desk top 12 r=-0.984 fall cone penetrometer [6, 2]. 9 The shear strength was meas- 6 ured for 10 artificially differ- 3 ent moisture contents. The 0 25 28 31 34 37 water contents in our tests covered almost the whole Moisture (%) TF1 TF2 TF3 range of moisture, from the plastic limit to the liquid limit percentage. According to the Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Correla- tion diagram betwee the shear strength and the moisture. correlation diagram of Fig. 5, Samples were collected from the tectonic formation (between an exponential relationship Pondos and Ionian flysch)and moisture changes was mad can express the changes of the artificially above properties confirming that a small quantity of water can cause a significant de- crease of the shear strength. Landslides along Egnatia highway (Greece)

This observation can be related to our previous observation, that a small quantity of wa- ter can change rapidly this material from the semi-solid to the liquid state, improving that in the humid conditions of Pindos mountain chain a light rain can easily create landslides on the hill-slopes. As the material is fine grained and so present low permeability, the above described change of the mechanical behaviour could not influence the stability of the formation at significant depth without the presence of important faults and closely spaced multiple open join sets, which drive the rain water dipper, in the mass of this tectonic formation.

The region to the east of Metsovo tunnel Having already the experience of the western side (already described), the geomechani- cal and general stability problems in this area, should arise mainly from the presence of the tectonic formation (lying between the ophiolites and the flysch). Furthermore the existence of large scale strike-slip and normal faults, local rock mass wedging and slid- ing as well as possible combinations of these features, strengthen the instability phe- nomena along the road.

Table 2. Physical and mechanical properties of the tectonic formation. Description Moist. Uniform. Permeab. LL PL Plastic. Group Compres. Bulk content coef. coef. K (%) (%) index index index density m (%) U (m/sec) PI GI Cc (t/m3) Clayey sand 28 >50 10-7 60 35 25 13 0,45 1,94 (SC-CH)

The tectonic formation has a significant horizontal exten- sion under the ophiolitic complex and a varying thick- ness which in most cases lies between 10 to 20 meters. The thickness decreases from the west to the east. The particle size distribution of the matrix of the tectonic formation is presented in Fig. 6 while its physical and me- chanical properties are in- cluded in Table 2.

The plasticity and compres- sion index values show that Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Particle size distribution of the tectonic formation matrix the material presents high risk for settlement and sliding [6, Christaras B., Zouros N., Makedon Th. & Dimitriou An.

7]. In addition the correlation of its shear strength and moisture content presented in Fig. 7, shows a very rapid decrease in shear strength with the in- crease of moisture content [2]. This formation and its me- chanical properties are similar with the analogous tectonic formation along the thrust front of the Pindos tectonic nappe (west of Moetsovo tun- nel, [3, 4]). Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Corre- lation diagram between the shear strength and moisture contnt of the tectonic formation.

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. Landslides at the entrance of Malakasi B tunnel constructed in the tectonic formation between the ophiolites and the flysch (east of Met- sovo tunnel) The poor mechanical properties of the tectonic formation can lead to a series of stability problems depending on the position of the formation in relation to the road design and the location of the construction (Fig. 8). In some cases the road slopes cut mainly through the ophiolitic formations, leaving the tectonic formation 15-20 m deeper from the road level. The existence of large scale normal and strike-slip faults, however, Landslides along Egnatia highway (Greece)

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. A representative tectonic zone, crossing per- pendicularly the ax of the road. The material in the zone is broken behaving like a soil. changes the position of the tectonic formation, bringing it to the road level and in some cases, to the level of major constructions like tunnels and bridges.

Figure Συάλμα! Άγνωστη παράμετρος αλλαγής.. The tunnel is constructed in the tectonic formation, east of Metsovo tunnel. An E-W strick slip fault collapsed the left branch. Another category of similar stability problems, mainly landslides and strongly tectonized mass movements, were encountered in cases where large scale fault zones cut through the road design. The large scale landslides activated at the slopes during the construction of the road consisted of very strongly tectonized material resembling soil formations. Christaras B., Zouros N., Makedon Th. & Dimitriou An.

The zones of this material present a general E-W orientation and our investigations have shown that they are actually large scale fault zones created by the E-W strike-slip faults (Fig. 9). Combined stability problems are also related to important E-W faults, cutting the tec- tonic formation and collapsing probably the constructing works (tunels etc., Fig.10). Concerning the geometry of the discontinuities, the sizes of the rock wedges in the most critical parts of the road were studied in detail and their safety factors were calculated. These calculations show that the formation of such rock wedges can create serious con- struction problems and they should be taken into account in the road design.

CONCLUSIONS

According to our investigation the more important landslides along the under construc- tion Egnatia highway are mainly related: 1. to the nature of the tectonic formation that lies under the Pindos nappes overthrusting the Ionian flysch (on the west of Metsovo tunnel), 2. to the nature of the tectonic formation that lies under the ophiolites overlying the Pin- dos flysch (on the East of Metsovo tunnel). These “tectonic formations” consist of shailes and pelites, containing detached blocks of limestones and deep sea sediments. Mechanically the above materials behave differently in dry and in west conditions. In dry conditions they behave like a rock, while in wet conditions they loose rapidly their cohesion and their original structure behaving like a sutured soil causing landslides along an important part of the road. Furthermore, important E-W strike slip faults either menace to collapse tunnels and oth- er constructing works or create wide tectonic zones (>100 m width), in which the rock- mass is totally broken, behaving like a soil. These zones are usually perpendicular to the road axis, causing unstable high cut-slopes.

ACKNOWLEDGEMENTS

This work has been supported by the Ministry of Environment and Public Works in the frame of two engineering geological research projects that has been carried out by Laboratory of Engineering Geology & Hydrogeology - AUTH (a) DMEO/D3b/97/1- I/29-3-94 and b) DMEO/d/1430/16-10-95). Furthermore the authors would like to thank Prof. D. Moundrakis of the AUTH, for his many helpful suggestions during all phases of this study.

REFERENCES Landslides along Egnatia highway (Greece)

1. Brunn, J.H. Contribution a l' Etude Geologique de Pinde Septentrional et de la Ma- cedoine Occidentale. Ann. Geol. Pays Hellen., 7, 1-358 (1956). 2. Christaras, B. Casagrande and Fall cone Penetrometer Methods for Liquid Limit Determination. Application on Marls from Greta /Greece. J. Eng. Geol. Elsevier, 31, pp. 131-142 (1991). 3. Christaras, B., Zouros, N. & Makedon, Th. Slope Stability Phenomena along the Egnatia Highway. The Part - Metsovo, in Pindos Mountain Chain, Greece. Proc. 7th Int. Congr. Iaeg, Lisboa, in Balkema, Roterdam, 3951-3958 (1994). 4. Christaras, B., Zouros, N. & Makedon, Th. Behaviour of the Votonosi Formation in Pindos Mountain (Greece). Proc. XI ECSMFE Copenhagen ’95, 7, 7.23-7.28, (1995). 5. Jones, G. & Robertson, A. H. F. Tectono-Stratigraphy and Evolution of the Meso- zoic Pindos Ophiolite and Related Units, Northwestern Greece. J. Geol. Soc. Lond., 148, 267-288 (1991). 6. Karlsson, R. Suggested Improvements in the Liquid limit Test with Reference to Flow Properties of Remoulded Clays. Proc. 5th ICSMFE, Paris, 1, 171-184 (1961). 7. Lambe, T.W. & Whitman, R.V. Soil Mechanics, SI Version, John Wiley & Sons, New York (1979). 8. Lyberis, N., Chrowicz, J. & Papamarinopoulos, S. La paleofaille transformante du Kastaniotikos (Grece): Telediction, donees de terrain et geophysiques., Bull. Soc. Geol. France, 7, XXIV, 1, 73-85 (1982). 9. Mountrakis, D., Kilias, A. & Zouros, N. Kinematic Analysis and Tertiary Evolution of the Pindos-Vourinos Ophiolites (-, Creece)., Proc. 6th Congr., Bull. Hell. Geol. Soc. (1992). 10. Zouros, N. Study of the Tectonic Phenomena of Pindos Nappe Overthrust, in Epirus Area. Ph.D. Thesis, AUTH, 407p. (1993) (in Greek). 11. Zouros, N. & Mountrakis, D.. The Pindos thrust and the tectonic relation between the external geotectonic zones in the Metsovon-Eastern Zagori area (Northwestern Greece). Proc. 5th congress, Bull. geol. Soc. Greece, XXV/1, 245-262 (1990). 12. Zouros, N., Mountrakis, D., Kilias, A. & Pavlides, S. Tertiary Thrusts and Associ- ated Structures in the Pindos Nappe, Epirus, Nw Greece. Int. Proc. Intern. Symp. "Thrust Tectonics In Albania", Tirana., Bul. Shk. Gjeol., 1, .69-79 (1991).