Engineering Geological Considerations in

Engineering Geological Considerations in Tunnelling through Major Tectonic Thrust Zones – Cases along the Egnatia Motorway,Northern GreeceVassilios P. Marinos1, Georgios Aggistalis2, and Nikolaos Kazilis11National Technical University of Athens, Faculty of Civil Engineering9 Iroon Politechniou str., 157 80 Zografou, Greecevmarinos@central.ntua.grTel: +30 210 7722442,Fax: +30 210 77237702Egnatia Odos S.A., 6th km Thessaloniki-Thermi, 57001, Thessaloniki, Greece {nkazilis,gaggistalis}@egnatia.grAbstract. Tunnelling through tectonic shear zones is always a challenge due to the geomet-rical complexity and the presence of poor quality rock masses with high variability in their occurrence. Based on the experience from the Egnatia motorway, in Northern Greece, the paper aims to give some general concepts, on the main failure mechanisms of the deformed materials and discuss guidelines for the design. Two case studies are presented, the first when the rock mass is brecciated derived from brittle rocks and the second when the rock mass is sheared from the compression of weak rocks with plastic characters. In the first case the rock mass is composed of interlocking angular pieces of strong rock and can ex-hibit satisfactory stability under confined conditions. Unless immediate support is provided, the cohesionless rock mass does ravel in tunnel excavation. In the second case heavily sheared clayey flysch exhibits squeezing behaviour even under tens of meters of overbur-den. In this case there is a variability of the material and it is not suggested to provide a number of support categories for each of the erratically alternating various geotechnical conditions that may be present but it is preferable to design an excavation and support system and sequence which are capable to deal with the worst anticipated conditions. Keywords: tunnelling, thrusts, weak rock masses, ravelling, squeezing, design, Egnatia, Greece.1 IntroductionEgnatia Highway, today under construction, composes a very important and mod-ern infrastructure for the communication of Greece with Europe, the Balcans, and the East. It starts from Igoumenitsa at the west, runs across the northern part of the country, ending to the east to the Greek-Turkish borders (Fig.1.1). The Motorway will be 680Km long, providing to the European Union access to the east with no border crossing and belongs to the 14 projects of a Trans-European network. Until know 449Km have already been constructed. When complete, the Egnatia Motor-Robert Hack,Rafig Azzam,and Robert Charlier(Eds.):LNES104,pp.527–537,2004.c Springer-Verlag Berlin Heidelberg2004528 Vassilios P. Marinos, Georgios Aggistalis, and Nikolaos Kazilisway will have a total of 76 road tunnels of an overall combined length of 99km. The majority will be bored tunnels. The excavation diameter of the tunnels is about 12m and among those already finished, most were opened by the method of top heading and bench. The Egnatia Motorway runs across the entire width of Greece traversing almost perpendicularly the main geotectonics units of the coun-try crossing perpendicularly major tectonic contacts and thrusts. Thus, there is a great variety of geological situations and each geotectonic unit displays different particularities in terms of occurrence of weak rock masses, tectonic structure, and therefore the possibility of unstable arrangements of rocks. The weak rock masses produced by such thrusts are originated from both strong or weak intact rock. They are present to a large scale and may cover the whole tunnel length. In that case, the discrimination of the different engineering geological units and qualities with specific characteristics and parameters along the tunnel is very difficult. The quality of the rock mass in such environment can change dramatically in few me-ters and in such cases; a swift to a radical change of the support can be dangerous if temporarily a better quality of the rock mass appears.Fig. 1.1. Location of the Egnatia motorway and its connections to the rest of Europe (in an informative article, Greeman 2001).2 Geological Model in Major Tectonic Thrusts2.1 GeneralThe geological model is the basic initial step on which the design of the tunnel will be based. As a result, the geological conditions to be encountered in terms of quality of the ground can be defined and the type, location, and size of potential hazards can be identified. In the case of major tectonic thrusts the alignment, even crossing them perpendicularly, deals with a rock mass disturbed in a very wideEngineering Geological Considerations in Tunnelling 529 zone often extended by satellite shears. An important factor for the establishment of the geological model is the site investigation. This investigation has to be de-rived into several phases: a) review of the regional geology, b) surface explora-tion, c) subsurface investigation and testing. Many problems arise during the geo-logical investigation when searching for the specific location, the extent, and the frequency of the tectonic zones. This becomes more true in flat areas or in urban environment. Many times tectonic zones cannot be determined exactly with boring because of low core efficiency. In such cases the disturbed material, when clayey, is washed out and these sections cannot be quoted. Moreover, when fault zones, mainly normal, are in a vertical orientation, boreholes cannot detect them. In this kind of investigation, further information on the rock mass properties can be ob-tained by the use of down hole cameras, air photographs, appropriately inclined boreholes, geophysical surveys and, in case of areas close to surface, trial shafts (Dalgic, 2000). In mountainous areas due to morphological changes it may be possible to map tectonic zones such as thrusts or faults based on differences of vegetation, while the existence of a “pilot” strata affected by shearing and displac-ing is of great help.2.2 Complex Geological Models in Egnatia MotorwayEgnatia is crossing perpendicularly almost all the main geological zones of Greece. These zones have been differentiated by their lithology and tectonic his-tory and many of them are placed one beside the other by big thrusts. The rock mass consists mainly of limestones and flysch in the western part whether ophio-lites, gneiss, marbles and phyllites are present in the central and eastern part. In this paper, two case studies of different geological structure in a thrust environ-ment are presented. The first when it is derived from the brecciation of brittle rocks and the second when the rock mass is derived from a sheared weak rock mass such as flysch. These cases were chosen because their behaviour in tunnel-ling is completely different, based on structurally control stability in the first case and stress controlled stability in the second.Case Study 1: Tunnelling in heavily fractured and brecciated limestones.The area is at the edge of the cliff of a “balcony” overlooking the plain of Para-mythia and dominated by the mountain of Khionistra in the western part of the motorway (fig.2.1). The rock mass of the area is a heavily fractured and brecciated limestone resulting from thrusting and from tectonic rugs. The Greek Geological Survey and the Institut Francais du Petrol, studied the broader area, during the early 60s, and have produced an excellent publication (1966). In figure 2.1 a block diagram, is shown, illustrating the geological model in which a series of tunnel are bored.Case Study 2: Tunnelling in heavily sheared flysch.The motorway runs through a contact of the huge overthrust of the Pindos Unit formation over the Ionian Flysch in Metsovo area. The base of the overthrusted-530 Vassilios P. Marinos, Georgios Aggistalis, and Nikolaos Kazilis sheared material is mainly in the Pindos flysch. This flysch is characterized bydifferent alternations of siltstones and sandstones. Within the area of one of the tunnels, the flysch is of a more clayey nature and often exhibits a chaotic structure. The main thrust movement is associated with satellite shears within the thrusted body. These shears are generally marked by a reddish siltstone sequence of the Pindos flysch, which acted as a ’soap layer’ for these internal thrusts and when vegetation is absent, these shears can be mapped. The flysch has thus suffered from large compression and very weak rocks masses have been produced. A con-ceptual model has been drawn to picture the geological structure of the area in figure 2.2. Inside the body of the sheared mass, it is reasonable to expect less deformed sandstone flysch with more rock-like behaviour. However, this material is likely to be cut by small shears. Consequently, frequent shifts from good to weak flysch with a significant presence of sheared and very weak siltstone or clay shales are a common practice. Another tunnel, in the same area, crosses a series of thrusts and faults associated with weak flysch material. The programme except of the geological mapping and the lab tests involved geophysics and boreholes. Moreover, the kinematic analysis of the thrusts was verified by the tectonic analy-sis of the fold axis (Sotiropoulos and Mourtzas, 1999).Fig. 2.1. The geological model for the Khionstra-Paramythia thrust and the Petoussi fault (from IGME – IFP, 1966). Western part of Egnatia Motorway.Engineering Geological Considerations in Tunnelling 531Fig. 2.2. Geological model in a tectonically sheared zone with weak rock masses (flysch).3 Rock Mass Model and Mechanisms of Failurein Tunnelling in an Environment of Thrusts3.1 GeneralHaving defined the basic geological model, design can start on a sound basis and selection of rock mass properties is the next step. Estimation of rock mass proper-ties can be achieved by one of the following methods: a) laboratory testing, b) in situ testing, c) back analysis, and d) use of rock mass classifications (RMR, Q, GSI etc.). However, the outcropping geological materials do not permit high qual-ity sampling and consequently performance of laboratory tests to derive the design geotechnical parameters is difficult. Moreover, sampling is not representative of the rock mass due to the disturbance, jointing, and the heterogeneity of most for-mations. Additionally, it is often not realistic or always feasible to carry out in situ tests. On the other hand, back analysis, although the best way to estimate the geo-technical parameters can be done only during construction and by evaluation of the deformation measurements. Thus, the remaining solution in order to estimate reasonable geotechnical parameters is to rely upon the use of the rock mass classi-fications schemes that are correlated with the basic parameters needed for the design. However, someone must always have in mind their limitations; their532 Vassilios P. Marinos, Georgios Aggistalis, and Nikolaos Kazilisproper use is assured if the engineering geological behaviour during tunnelling is understood.If the problem is stress dependant, having defined the parameters of the Geo-logical Strength Index, GSI, (mainly used in Egnatia project- Marinos and Hoek2001) the unconfined strength of the intact rockci and the material constant mi,are defined for the assessment of the mechanical properties of the rock mass. On the contrary, when failures are structural dependant the persisting influencing discontinuities have to be defined and their properties accessed.Regarding groundwater, its most basic impact is upon the mechanical proper-ties of the intact rock, when this is of clayey nature. Furthermore, when the water is not drained it reduces the effective stresses and finally, in all cases, the strength of the rock mass. This is the case of clayey flysch. In more permeable masses (e.g. limestones, fractured sandstone flysch) free drainage helps but groundwater in-flows must be controlled.3.2 The Cases of Egnatia MotorwayCase Study 1: Tunnelling in heavily fractured and brecciated limestonesThe predominant rock mass of the area is a heavily fractured and brecciated lime-stone. Under confined conditions this tightly interlocked, highly frictional but cohesionless material behaves in a stable manner. This is evident in many of the steep slopes in such material that were cut in the site. However, when confinement is released, as for example in the roof of a tunnel, the rock mass dilates and indi-vidual pieces fall under gravity loading. If left unconfined, this ravelling will con-tinue and a chimney will be formed, until, and if, arching is achieved. As a result, the local stability of the rock mass immediately above the roof has to be dealt with in an entirely different manner since the failure mechanism involved is driven by gravity rather than by stress. In these conditions the “blind” application of rock mass classifications for support selection can be very misleading since these clas-sifications were developed for different failure mechanisms. The rock mass classi-fications remain a useful qualitative tool for communication between those in-volved in the project but the selection of the excavation method and the choice and sequence of support installation must be based on a clear understanding of the mechanics of rock mass failure due to loss of confinement. In this kind of material it is very conservative to give low geotechnical parameters and to use a rather “heavy” support system. The prime concern of the design of the supporting shell has to cope with a mass that must first be protected against disintegration and ravelling.Case Study 2: Tunnelling in heavily sheared flysch.As it is described in the geological model, flysch has suffered from large compres-sion-shearing and very weak rock masses are likely to be found close to the shear zones. The original structure is no longer recognisable and blockyness is lost. The rock mass may contain small pieces of strong rock (sandstones) floating in the surrounding weak material. However, in the scale of the tunnel, these boulders do not contribute significantly to the overall strength of the rock mass and addition-Engineering Geological Considerations in Tunnelling 533ally, due to the heterogeneity in both horizontal and vertical direction these better conditions may not encounter to the whole tunnel section (e.g. foundation area of the elephant foot of the top heading). The normal tunnel design process, in which a number of support categories are provided for the different geological and geo-technical conditions predicted or observed in the tunnel, may not be applicable to this case due to the heterogeneity of the mass and the rapid changes from better to worst rock mass. It is preferable to design an excavation, support system and se-quence, which are sufficiently robust to deal with the worst anticipated geological-geotechnical conditions and which are therefore practically independent of the precise geological conditions encountered. The problem that has to be anticipated in this weak rock mass is the expected large deformations when the tunnel is under high overburden (in the Egnatia motorway, up to 250m). An indicator of potentialtunnel squeezing problems is the ratio of uniaxial compressive strength cm of the rock mass to the in situ stress p o . Its relation to the “strain” of the tunnel is shown in figure 3.1. To proceed to such an analysis the correct geotechnical parametersshould be chosen. The Geological Strength Index (GSI), ci and m i ,of the pre-dominant sheared clayey siltstone, suggested for this purpose ,for these tunnels,were GSI=15-20, ci =5-8 MPa and m i =5-7. These parameters can be validated or revaluated from the deformation measurements during construction and support to be optimally adjusted to the rock mass structure. Thus, monitoring is of essential importance.Fig. 3.1. Squeezing conditions depending on the ratio of rock mass strength to in situ stress related to strain. The example of Anthochori and Anilio tunnel in Egnatia highway (based on Hoek and Marinos 2000).534 Vassilios P. Marinos, Georgios Aggistalis, and Nikolaos Kazilis4 Design Guidelines4.1 Design Features in Tectonic Zones with Applicationin Egnatia HighwayIn this paragraph it is not attempted to give precise design categories and support measures for the rock mass qualities here discussed, but to focus on guidelines and describe the features from the design philosophy based on the understanding of the behaviour of these particular rock masses.Case Study 1: Tunnelling in fractured and brecciated limestonesFrom a practical point of view it is believed that it is essential that the rock mass ahead of and above the advancing tunnel face should be treated by grout injection in order to provide sufficient cohesive strength to prevent the ravelling process described in paragraph 3.2. This would create a stable arch in the pervious brecci-ated limestone and this would permit tunnel advance with shotcrete and rock bolt support only. An alternative solution could be the excavation of an annular pe-ripheral slot that is shotcreted in the beginning of every excavation cycle. Shot-crete is a very effective support medium in this type of rock mass since it prevents the start of the ravelling process. Furthermore, where this pre-conditioning of the rock mass is not absolutely necessary, the use of self-drilling rock bolts with im-mediate grout injection could be sufficient. Additionally, it is very important to excavate in a small advance step in order not to disturb the surrounding rock mass and to use an excavation method that induces as little disturbance as possible. In case water is present, ravelling is easier to take place by flowing process. In order not the shotcrete to wash out, accelerating admixtures should be used or the thick-ness of the shotcrete should be increased. In this tunnel the ground water table is below the tunnel and problems of this type were absent. Usually, it is very difficult to identify the exact place of the more brecciated zones that will more easily ravel during excavation. More tectonised zones may be found during construction by drilling ahead of the face with probe-holes.Case Study 2: Tunnelling in heavily sheared flysch.The main issue that has to be faced in dealing with this support design is how to control the large deformations that occur from the squeezing of the tunnel. In order to control these deformations, it is necessary to install a heavy temporary support in the form of steel sets, shotcrete or rock bolts or a combination of these systems. Face stability is also a main issue. The addition of forepoles is very effec-tive in controlling the deformation ahead of and at the tunnel face but of course has little influence upon the deformations behind the face and the shell has to carry the load of the poles too. In such a support system, in a weak rock mass and high overburden, it is very important to make a good foundation of the support cell and to close the excavation phase with a temporary invert. If, however, these large deformations cannot be accommodated by simply making the support system stronger and more rigid, particularly for a top heading and bench method of exca-vation where it is impossible to achieve a closed support ring in one step, a muchEngineering Geological Considerations in Tunnelling 535 more effective approach is to allow the support system to yield while, at the same time, retaining sufficient capacity to support the loads imposed by the overburden. This flexible support system can be provided by different sophisticated yielding systems. The theoretical basis for this approach is illustrated in Figure 4.1 (from Hoek and Marinos, reports to Egnatia S.A.1998-2003). The “soft” support system follows the reaction curve that starts with yielding at very low load. The amount of this yielding can be controlled and the support system is locked and will react in the same way as a “stiff” system when the pre-determined tunnel deformation is achieved.Fig. 4.1. Rock-support reaction curves for the top heading of a tunnel subjected to squeez-ing conditions. The “stiff” support system fails due to overloading while the “flexible” system yields and achieves stability at a displacement of about 0.3 m in this example (from Hoek and Marinos,reports to Egnatia S.A.1998-2003).5 ConclusionsThe paper discusses some general concepts in tunnelling through thrusts zones, from the definition of the geological model to the main failure mechanisms and the guidelines for the design. Tectonic zones produced by big thrusts or satellite shears within the thrusted body are very difficult to access as far as the conceptual model is concerned. Due to high geometrical complexity, heterogeneity and big scale tectonic disturbance, it is difficult to precise specify by the zones of weak rock masses. The rock masses are heavily deformed in a greater scale, producing536 Vassilios P. Marinos, Georgios Aggistalis, and Nikolaos Kazilis either heavily fractured, brecciated mass from brittle rocks or sheared melanges and chaotic masses if the initial rocks were soft or heterogeneous. The site investi-gation programme must be soundly based on the guidelines provided by the knowledge of the regional geology. To specify the geotechnical parameters of these rock masses to use for the design, the use of the rock mass classifications offer the main tool. Given their limitations their proper use depends on the under-Table 1. Tunnelling through tectonic thrust zones. Summary of the two cases of weak rock mass derived from either brittle or soft initial rock.Rock Mass TypeBrecciated limestones Sheared siltstones or shales Rock Mass Characteristics Interlocking angular pieces of strong rock. No clayey material No sign of initial structure.Shear surfaces-slickensides.Interlocking of rock piecesis absent. Boulders of morecompetent rock floatinginto the chaotic clayey massWater Effect If present, reduction of effective stresses. Possi-ble inflows into the tunnel Reduction of the mechani-cal properties of the intactrock and of the condition ofthe discontinuities. Reduc-tion of effective stressesFailure Mechanism Ravelling, driven by gravity rather than by stressesLarge deformations.Squeezing conditionsPossibility in predicting rock mass quality ahead of the face (when inside the thrust zone) Probe drilling Limited options due toheterogeneity (rapidchanges and alternations inshort lengths)Design concept Keep confinement Control deformations ofperimeter and faceDesign guidelines Pre-grouting of the sur-rounding rock massthrough perforated spiles. Alternatively, createperimeter slot ahead theface associated withspiles. Immediate appli-cation of shotcrete. But-tress on the face and short advance step. Then apply required temporary shellForepoles or spiles, face nailing, heavy shell (e.g. steel sets embedded inshotcrete). Temporary invert necessary. Short advance step. Alternatively flexible support system with yielding elements (for the cases of large deforma-tions)Measures to prevent ad-verse respond of the sur-rounding rock mass inorder to provide assis-tance to the shell of tem-porary supportGrouting Anchoring well beyond theplastic zone if necessarymay assistEngineering Geological Considerations in Tunnelling 537 standing of the behaviour of the rock mass during tunnelling. Rock masses tecton-ically disturbed but with different lithology may have similar geotechnical pa-rameters. However their behaviour in underground excavation can be very differ-ent. Thus, support measures should not only consider the stress problem but also the mechanism of failure of the rock mass especially in brecciated brittle rocks. In other cases of sheared chaotic rock mass, subject to plastic deformations, it is preferable to design an excavation, support system and construction sequence, which are capable to deal with the worst anticipated geotechnical conditions and which are therefore practically independent of the geological conditions encoun-tered. However, this conclusion must not be misunderstood because this inde-pendency from the geological conditions is imposed just because of these geologi-cal conditions. In table 1 are summarized the two cases, discussed in this paper, as far as the main failure mechanisms, the water effect, the intervention to the sur-rounding rock mass and the guidelines for the design are concerned. ReferencesDalgic S (2003) Tunnelling in fault zones, Tuzla tunnel, Turkey. Tunnelling and Under-ground Space Technology, 18:453-465.Greeman A (2001) Ancient route to the East revived. Tunnels and Tunneling International, 33, (5): 26-30.Hoek E, Marinos P (2000) Predicting tunnel squeezing problems in weak heterogeneous rock masses. Tunnel and Tunneling International, 32 (11):45-51, part one and 33(12):33-36, part two.Marinos P, Hoek E (2001) From the Geological to the rock mass model. Driving the Eg-natia Highway through difficult geological conditions.Proc. 4th Congress of the Hellenic Society for Soil Mechanics and Foundation Engineering.Sotiropoulos I, Moutzas N (1999) Anilio tunnel: Geological structure and classification of the rock mass.Proc.1st Conference on Tunnels of Egnatia Highway, Ioannina, Greece.。