Engineering Geology and the Environment, Marinos, Koukis, Tsiambaos & Stournaras (eds) - pp. 2639-2645 © 1997 Balkema, Rotterdam, ISBN 90 54 10 877 0

Adverse geotechnical conditions in road construction. Sections of the new Egnatia highway across Pindos mountain range (N. Greece)

B. Christaras, N. Zouros, Th. Makedon, A. Dimitriou Lab. of Engineering Geology & Hydrogeology, School of Geology, Aristotle University of Thessaloniki, Thessaloniki, Greece. ABSTRACT: The investigation of the geological and geomechanical factors that affect the construction of important traffic arteries such as the Egnatia Highway, is of utmost importance. The problems arising in the construction of Egnatia Highway are due to the complex geotectonic regime of the Pindos area, which generates various instability conditions. The present paper’s purpose is to specify the tectonic and geomechanical features that are responsible for the creation of these problems in the construction of Metsovo-Malakasi-Panagia part. 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. The Egnatia

Highway in North-Western Greece 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 conditions, is highly anisotropic and in association with the morphology (high relief

Figure Σφάλµα! Άγνωστη παράµετρος αλλαγής.. Sketch showing the Egnatia highway section in Pindos mountain. The present investigation was performed in Metsovo - Malakasi - Panagia area.

and steep slopes), it presents serious geotechnical problems in the construction of high cut slopes, tunnels and bridges connecting the different sections of the highway. The part of Egnatia highway east of Metsovo includes an E-W section reaching Panagia village, changing subsequently to a N-NE direction. In this part the works that have been constructed so far include 3 tunnels and an 800m long 70m high cut slope. This part also includes the future construction of 3 more tunnels as well as a number of valley-bridges (Figure 1). Our former investigations of the Egnatia section Ioannina - Metsovo have dealt with the stability problems of the geological formations and have attempted to determine the mechanisms that induced them. These investigations have finally determined a series of tectonic and geomechanical features that in our opinion are characteristic of the Pindos area and are responsible for the creation of these problems. The purpose of this paper is to confirm that the above-mentioned features are also existing in the part Metsovo - Malakasi - Panagia and investigate the mechanical behaviour and the stability problems of the formations due to these features. The

geomechanical investigations and detailed studies, along with the field data and geological mapping are included in the research project entitled «Geological - Engineering Geological Research for the Egnatia Highway». GEOLOGICAL AND TECTONIC SETTING The study area is located in Northern Greece, in the Pindos mountain range. Two distinct geotectonic units dominate in the area, the Pindos zone nappe and the Pindos ophiolite nappe (Figure 2). 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 overthrusts towards WSW the flysch of Ionian and Gavrovo zones (Zouros 1993). The Pindos ophiolite complex represents fragments of the Neo-Tethyan oceanic lithosphere which was emplaced initially on the western margin of the Pelagonian zone (Cimerian micro-continent)

Figure Σφάλµα! Άγνωστη παράµετρος αλλαγής.. Schematic geological sketch map showing the main tectonic features in the study area.1. Pindos flysch (Fo), 2. Ophiolites, 3. Deep sea sediments with basic volcanics and the tectonic formation, 4. Tectonic contact between ophiolites and flysch, 5. Major strike-slip faults.

during Late Jurassic-Early Cretaceous (Mountrakis 1983) and subsequently over the Pindos flysch during Tertiary (Brunn 1956, Zouros et al. 1991). Pindos ophiolite consists of mafic and ultra-mafic rocks (upper mantle peridotites partly serpentinised, gabbros, mafic and ultra-mafic cumulates, sheeted dikes, massive lavas, pillow lavas and basic brecias), metamorphic rocks parts of the sole (amphibolites, schists and meta-sediments) as well as deep sea sediments and turbidites (pelagic limestones, sandstones, calcarenites and micro-breccias, siltstones, green and red ribbon and nodular radiolarites) (Brunn 1956, Jones & Robertson 1991, Mountrakis et al. 1992). 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 (Zouros & Mountrakis 1990, Mountrakis et al. 1992). 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 volcanics, 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 (Jones and Robertson 1991) and probably re-deformed during the tertiary emplacement of the ophiolites over the Pindos flysch (Mountrakis et al. 1992). 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 tectonic windows appearing in the study area, including the large semi-window of Malakasi confirms. 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 (Mountrakis et al 1992) show that several tectonic events took place. The Early Oligocene extentional tectonic event in an ENE-WSW direction, responsible for the emplacement of the ophiolites over the Pindos flysch, produced large scale normal faults trending

NNW-SSE in brittle conditions and other major extentional features typical of semi-ductile conditions both in the ophiolites and the underlying Pindos flysch.. Compressional deformation followed the previous event, with the maximum stress axes trending ENE-WSW. This event caused large strike-slip and reverse faults like the ones affecting the ophiolites and the flysch along Malakasiotikos river valley. A subsequent compressional event with the maximum stress axes trending N-S during late Miocene, produced congugate reverse and strike-slip faults and caused further imbrication of the tectonic units (Kemp & McCaig 1985). The neotectonic evolution of the study area seems to correspond with the deformation of the broader area. An extentional tectonism during Pliocene followed the compressional deformation. This event activated NNW-SSE trending normal faults while a subsequent early to middle Pleistocene extentional deformation formed ENE-WSW to E-W trending normal faults. This deformation is responsible for the present morphology of the area and appears to be still active as indicated by numerous geological and geomorphological features. GEOMECHANICAL INVESTIGATIONS AND ANALYSIS

From the approach mentioned earlier, the geomechanical and general stability problems in the study area, should arise from the presence of the tectonic formation, the existence of large scale strike-slip and normal faults, local rock mass wedging and sliding as well as possible combinations of these features.

As we already mentioned the dominating tectonic feature is the emplacement of the ophiolitic rocks over the Pindos flysch via a tectonic formation lying under the ophiolitic complex.

This formation has a significant horizontal extension under the ophiolitic complex and a varying thickness 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 Figure 3 while its physical and mechanical properties are included in Table 1.

The plasticity and compression index values show that the material presents high risk for settlement and sliding (Karlsson 1961, Lambe & Whitman 1979). In addition the correlation of its shear strength and moisture content presented in Figure 4, shows a very rapid decrease in shear strength with the increase of moisture content (Christaras 1991).

This formation and its mechanical properties are similar with an analogous tectonic formation along the thrust front of the Pindos tectonic nappe. The presence of the latter has been, in former studies (Christaras et al 1994 & Christaras et al 1995), related with serious stability problems in the Anilio - Anthohori - Votonosi area west of Metsovo.

Figure 3. Particle size distribution of the tectonic formation matrix. Table 1. Physical and mechanical properties of the tectonic formation.

Description Moisture content m (%)

Uniform. coef.

U

Permeability coef.

K (m/sec) Clayey sand

(SC-CH)

28 >50 10-7

LL (%)

PL (%)

Plastic. index

PI

Group index GI

Compres. index

Cc

Bulk density (t/m3)

60 35 25 13 0.45 1.94

Figure 4. Correlation diagram between the shear strength and moisture content of the tectonic formation. 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 various constructions. In the western part of the study area the road slopes cut mainly through the ophiolitic formations, leaving the tectonic formation 15-20 m deeper from the road level. Further to the east the thickness of the tectonic formation decreases and combined with the road altitude it should not create any stability problems. The existence of large scale normal and strike-slip faults, however, 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. In these situations severe construction problems must be expected and this is the case of the «Malakasi B» tunnel (Figure 5) which encountered large scale stability problems during the excavation through the tectonic formation.

Figure 5: Landslide inside the tectonic formation creating serious stability problems in the excavation of the "Malakasi B" tunnel. A second 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. As an example we can refer to the high cut slope (800m long, 70m high) following the northern exit of «Malakasi A» tunnel (Figure 6). The large scale landslides activated during the construction of the slope consisted of very strongly tectonized material resembling soil formations. 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. The extent of these zones reaches some times a width of 50-100m perpendicular to the strike of the faults. Further to the east close to Panagia village a major fault of the same direction bounds the contact between the Sub-pelagonian zone limestones and the accompanying sediments of the ophiolitic series. This fault zone lies next to the «Kokorelou» tunnel which will be constructed near Panagia. The stability problems related to the existence of these fault zones are, as mentioned earlier, large scale landslides which in the case of the high cut slope were studied in detail, showing a significant extension inward to the slope (Figure 7). This conception altered dramatically the assumption that these zones were surface loose materials created by erosion and weathering of the rock mass and that the problem could be eliminated by simply removing them.

Figure 7: Landslide of a fault zone tectonized material in the high cut slope area. Major E-W strike-slip faults are combined with low angle earlier tensile slip surfaces related with the emplacement of the ophiolites. The presence of the above mentioned faults along with the NW-SE to N-S normal faults, create additional stability problems due to strongly tectonized zones in the intersections of these faults (Figure 8).

Figure 6. High cut slope following the northern exit of «Malakasi A» tunnel. 1-3: Rock Mass Quality (Bieniawski 1974 & 1979). 1. Fair (RMR 41-60) 2. Poor (RMR 21-40) 3. Very Poor (RMR 0-20) 4. Unstable material of tectonized zone 5. Unstable material of landslide 6. Embankment 7. Earlier compressive tectonic planes 8. Earlier tensile tectonic planes 9. Normal faults with indication of dip 10. Strike-slip faults with indication of movement 11. Major fault zones 12. Recent open fissures 13. Landslides 14. Active landslides 15. Landslide toe 16. Slope stability analysis (stereographic projection)

Figure 8: Large normal fault cutting through the ophiolites in the high cut slope area.

A third category of stability problems arise in the cases where these large scale E-W faults are connected with the presence of the tectonic formation.

This is the case of the «Malakasi C» double tunnel. This tunnel was excavated in the tectonic formation and deep sea sediments accompanying the ophiolites. A large E-W trending strike-slip fault is cutting through the two parts of the tunnel. The fault plane activated a large scale detachment, creating a landslide that eventually led to the collapse of the northern branch of the tunnel (Figure 9).

Another category of stability problems involves the unfavourable orientations of the smaller scale discontinuities (fissures, joints, etc). These discontinuities affect mainly the parts of the road design which present better rock mass quality and combined with the various slope directions and slope angles can give rise to rock wedge sliding or planar failure.

The sizes of the rock wedges in the most critical parts of the road (800m long cut slope) were studied in detail and their safety factors were calculated (Figures 10, 11).

Figure 9: Collapsed branch of the "Malakasi C" tunnel, due to the presence of a large fault and the tectonic formation.

Figure 10: Large scale unstable rock wedge in the high cut slope area.

These calculations show that the formation of

such rock wedges can create serious construction problems and they should be taken into account in the road design.

Aside from the damages and stability problems sustained by the already performed construction works, a series of similar problems are expected to arise during the construction of future works included in the part Malakasi-Panagia. The majority of these problems will be associated with major faults that have been located crossing the tunnel positions and in some cases the foundations of the valley bridges. These faults are mainly E-W and NE-SW strike-slip faults but also NW-SE normal faults that have created strongly tectonized zones, and they are described in detail for every construction location in the research project mentioned earlier.

Figure 11: Stability analysis of rock wedge presented above (Markland 1972, Hocking 1976, Hoek & Bray 1981). CONCLUSIONS

Field observations and data analysis confirm the

presence of the factors affecting the geomechanical behaviour of the geological formations that have already been determined from the investigations in the Egnatia section west of Metsovo. These factors arise from the tectonic emplacement of the geological formations and the faulting of the broad Pindos area and have been summarised as follows:

The presence and the nature of the tectonic formation that lies under the ophiolites overlying the Pindos flysch. This formation has a «chaotic» structure with very poor mechanical properties and it is associated with the creation of landslides and stability problems affecting the surface and underground constructions along the road design.

The E-W strike-slip faults create tectonic zones of significant width. In these zones the rock mass presents a very poor quality and the tectonized material behaves like a non cohesive plastic soil, creating active landslides and rock falls affecting the road slopes, as well as the underground constructions.

The unfavourable orientation of the smaller discontinuities, which daylight on the slopes, create unstable rock wedges that in some cases pose serious construction problems mainly for the road slopes.

The combination of the above-mentioned factors aggravates the distinct stability problems, leading some times to the failure and collapse of constructions.

The nature of the problems mentioned in this paper makes their prediction essential for the better construction and integrity of the designed works. It is therefore necessary to perform detailed geotechnical investigations that will focus on the determination of the specific geological and tectonic aspects of the study area. Thus, it will be possible to adequately describe the mechanical behaviour of the geological formations and improve the design of the various constructions. ACKNOWLEDGEMENTS This work has been supported by the Ministry of Environment and Public Works in the frame of the project entitled «Geological - Engineering Geological Research for the Egnatia Highway» that has been carried out by the Laboratory of Engineering Geology & Hydrogeology - AUTH (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

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