Engineering Geology 181 (2014) 38–47

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Engineering Geology

j ourna l homepage: www.e lsev ie r .com/ locate /enggeo

Geomechanical characterization, 3-D optical monitoring and numericalmodeling in Kirkgecit-1 tunnel, Turkey

A. Aydin a,⁎, A. Ozbek b, A. Acar c

a Department of Geological and Geological Engineering, The University of Mississippi, University, MS, United Statesb Department of Geological Engineering, Kahramanmaras Sutcu Imam University, K.Maras, Turkeyc Department of Geological Engineering, Cukurova University, Adana, Turkey

⁎ Corresponding author. Tel.: +1 662-801 1342.E-mail address: aaydin@olemiss.edu (A. Aydin).

http://dx.doi.org/10.1016/j.enggeo.2014.08.0100013-7952/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 September 2010Received in revised form 10 August 2014Accepted 12 August 2014Available online 21 August 2014

Keywords:Hard-rock tunneling3-D optical monitoringLongitudinal displacementDisplacement vectorRock mass classificationNumerical modeling

The geomechanical characterization of a twin-tube tunnel during exploration and excavation stages is presentedand used as a base (a) to interpret the monitored long-term displacements, (b) to numerically investigate if thedisplacement pattern at a given section could be predicted using an equivalent continuum model and (c) toassess adequacy of the NATM recommended excavation method and support systems. The pre-excavation rockmass classification by RMR and Q systems utilizing logs of two exploration boreholes lead to reasonably similarpredictions of the as-built rock mass quality.The resultant displacement magnitudes and angles are interpreted with reference to the as-built geological cross-sections of the tunnel. All displacement patterns monitored at five posts in each section are consistently effectivein providing advance warning for the presence of zones of contrasting conditions despite very low magnitudes ofdisplacements. Numerical simulations predict no imminent danger of stress-induced instability and that the twintubes could stand unsupported. A light support system as recommended by NATM (and similarly by RMR and Qsystems) is necessary to prevent structural instability, loosening and overbreaks. The numerical analysis using thecore-softening approach also exemplifies the sensitivity of tunnel convergence behavior to moving position of theexcavation face, and consequently supports the combined use of the displacement vector magnitudes and anglesto predict the presence and to appreciate the magnitude of changes in rock mass stiffness ahead of the excavation.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

One of the most important applications of broad-based geologicalengineering expertise is the design and construction of subsurfacemines, tunnels and underground caverns. Such projects demand alarge array of practical and theoretical considerations. In a conventionalengineering project, once its feasibility is accepted and a safe and eco-nomical design can be achieved for a given setting, the construction issimply to producewhat is designed. In subsurface engineering howeverthis is not possible due to incomplete characterization of the medium.This is particularly the case in linear structures such as tunnels. Inorder to accommodate this fact, tunnel alignment investigations donot generally invest much time and resources particularly when an in-teractive construction method is to be used.

In highly deformed rockmasses where the pre-excavation characteri-zation is often less successful, interactive tunneling methods based onmonitoring excavation-induceddisplacements, observationofmass struc-ture and advance probing have been developed. Of these methods, theNew Austrian Tunneling Method (NATM) (Rabcewicz, 1964) is based onan “as-built or during-excavation classification system” while tendering

documents are based on limited information gathered from surface map-ping and very widely spaced exploratory drillholes. Pre-excavation rockmass classification systems most commonly RMR and Q are utilized totranslate this information to provide an estimate of the required excava-tion methods and proportions of support system components duringtunneling. Numerical modeling techniques are often employed only as ameans of producing a framework to appreciate the range and variabilityof these estimates because in fractured rockmasses uncertainties pervadeevery aspect of modeling, including initial and boundary conditions (e.g.in-situ stress, pore pressure), fracture geometry and properties (e.g.stiffness, dilation), and pre-support relaxation percentage.

The Kirkgecit-1 tunnel was constructed as part of Ankara–PozantiHighway Project (Figure 1). The 503m long tunnel consists of two iden-tical three-lane tubes to accommodate southern and northern boundheavy traffic. The dimensions of the twin tubes and other structural fea-tures are shown in Fig. 2. The pillar width between the tubes is around15 m. The tunnel was excavated in two stages (topheading and bench)using smooth-blasting technique (KGM, 2002). The excavation se-quence and support system were selected according to NATM.

This paper focuses on geomechanical characterization of the tunnelalignment during exploration and excavation stages and demonstrateshow the gathered information can be used as a base (a) to interpretthe monitored long-term displacements, (b) to numerically investigate

ADANA

POZANTI

CIFTEHAN

Start

End

KIRKGECIT-1TUNNEL

0 10 km

Motorways

Carriageways

Town/City

N

EW

S

Ch: 370+000

Ch: 315+000I S T A N B U L

BLACK SEA

MEDITERRANEAN SEA

KAMISLI

20

ADANA

ANKARA

LEGEND

ANKARA

Fig. 1. Location of the study area.

39A. Aydin et al. / Engineering Geology 181 (2014) 38–47

if the displacement pattern around a selected tunnel section could bepredicted using an equivalent continuum and (c) to assess adequacyof the excavation method and support systems prescribed by NATM.

2. Engineering geology of the tunnel alignment

Prior to the excavation phase, an engineering geological map of thetunnel alignment was prepared based on the field studies (Ozbek,2004) (Figure 3A). The tunnel was excavated mainly throughmicrogabbro (Kgb) and biomicritic limestone (Kk) separated by about5–10 m thick mylonite zone dipping at 70°–80° (Figure 4). Limestonelayers display the same orientation as themylonite zone and are reddishmaroon in color, thin to moderately thick bedded and closely jointed,where the joints were filled by calcite. It proved to be quite difficult todrill intact cores due to the presence of sandy–silty laminations and

1.3 m

(-2.02)

(-1.07)

9.62

m

-2.5

-3.0

-

0.0

12 m

0.75

2.0

m

(+ 1.66)

R= 8.80

R= 6.0R= 4.80

6.0 m

400 Main drain

200 Sub-base drain

Tsixalennu

I

Fig. 2. Typical cross-section of Kirk

mudstone lenses. Microgabbro is green in color, moderately to locallyhighly sericitized, and also closely jointed. Physical and mechanicalproperties and elastic constants of the biomicritic limestone andmicrogabbro were determined from NX size cores drilled from theblock samples according to ISRM (2007) (Table 1).

During the excavation, systematic face mapping enabled prepara-tion of longitudinal as-built geological cross-sections of each tube(Ozbek, 2004) (Figure 3B). The first 100 m of the tunnel from thesouth portal is driven through an altered microgabbro with frequentgouge zones whereas the following 360mwas through a relatively uni-form section of unaltered microgabbro with occasional gouge zones.The tunnel excavation reached a thick subvertical mylonitic faultgouge oriented roughly normal to the tunnel axis (as expected fromthe geological map, Figure 3A), and ended within the biomicritic lime-stone unit.

The tunnel axis runs parallel to the nearby Kirkgecit River but the in-verts at both portals (Figure 4) are above the river channel, and not in-fluenced by the water level fluctuations in the river. Some minorseepages (1 lt/min) were observed at both portals due to weatheredand disintegrated rock structure. Similar seepages were observed alsoalong both contacts of the mylonite zone and other minor gougezones. The boreholes confirmed that the groundwater table is belowthe invert where the injection tests produced permeability valuesk N 10−4 m/s for limestone and k N 10−5 m/s for microgabbro (KGM,2002). Despite the generally dry nature of the tunnel excavation, an im-permeable membrane channeling water into the drainage system wasinstalled along the entire section of the tunnel.

2.1. Discontinuity survey

Discontinuity scanline surveys were carried out during the excava-tion of the tunnel. Various properties of rock mass discontinuities assuggested by ISRM (2007) were recorded on the full face of the tunnelexcavation after each blasting, mocking and scaling cycle, at about 3 mintervals. Orientation contour diagrams and averaged values of selected(aperture and spacing) discontinuity properties are given in Figs. 5 andTable 1. Note however that the scanline surveys were somewhat biased(toward more persistent or visible natural joints) because of the blast-

(-1.51)

(-0.65)

11.0

m

0.75

0.75

2.0

m

(+ 1.83)

R= 8.

40

R= 4.80

Theoretical excavation line

Shotcrete

Geomembrane

Concretelining

Foundation block

6.0 m 1.3 m

Cableditch

+2.80

nvert

4%

gecit-1 tunnel (KGM, 2002).

600

m

150

m

450

500

550

200

250

300

350

400

II

ANKARA

POZANTI

550

500

450

300

250

200

150

350

400

I

B3B2B3

B3 B2 B3

EXCAVATION

0 40 m

910

Kirkgecit creek

200

300

400

500

700

745m

FOREST

BH-2

920

930

94095096

0

97098099

0

1000

1010

1020

1040

1050

1040

103010

20

1010

1000

990

980

970

960

95094

0

93092

0910

90089

0

880

Qa

Kf

Qc

KkKgb

Km

BH-1

82

1030

200

300

400

500

700

745m

Kgb

Kgb

Qc

Qa

65

KmKf

25 m

8 m

0

A

B

LEGEND

Lithological boundary

Thrust fault

Bedding(strike & dip angle)

Qa Alluvium

Qc Colluvium

Kk

Km

Kgb

Kf

Borehole

Flysch

Limestone

Microgabbro(locally altered)

Melange

22

(biomicritic)

Gouge zone

SeepageJoint

Fig. 3. A) Engineering geological map and B) geological cross-section along the tunnel route.

40 A. Aydin et al. / Engineering Geology 181 (2014) 38–47

induced disturbance, accessibility and limited-time allowed at the exca-vation face.

Orientation contour diagram for microgabbro reveals the presenceof two dominant directions of jointing striking roughly normal and par-allel to the tunnel axis. These directions are consistent with the strike ofthe mylonitized zone along which microgabbro thrusts over limestone.Thus for the assessment of tunneling conditions, the rockmass structureis envisaged to consist of two major joint sets and subvertical randomjoints (Figure 5). Set 1 in microgabbro dips against the direction ofdrive (NW), and exposes the tunnel face to possible planar slidingwhile set 2 dips into the tunnel on the right sidewall also producing po-tential for planar sliding. Intersection of two dominant sets producedwedges dipping into the tunnel from the face and gravitational blockfailure potential on along the tunnel roof.

In limestone, the orientation contour diagram clearly shows twosets of subvertical discontinuities and minor concentrations of sub-

Kgb Kk

Kırkgecit Creek

Tunnel

Fau

lt go

uge

zone

Fig. 4.Views of (left) the gouge zone of a thrust fault forming the contact betweenmicrogabbro20 m above the Kirkgecit Creek.

horizontal joints. Set 1 is oriented roughly 40° to the tunnel axis and isalmost orthogonal to set 2 (representing the bedding planes). In termsof structural stability of the tunnel walls, intersection of the two domi-nant sets created free triangular column base of which were gently dip-ping into tunnel on the right sidewall and occasionally on the roof(when intersected sub-horizontal joints) resulting in potential for top-pling and gravitational failures. These potential failures were preventedby shotcreting, forepoling and systematic bolting (as prescribed in theselected NATM classes).

3. Rock mass classifications and NATM support classes

Different rock mass classification systems are available to facilitategeomechanical characterization and to foresee its technical implications(selection of excavation method and sequence, and support systems)(Aydin, 2004). In this study, RMR (Rock Mass Rating; Bieniawski,

Kirkgecit Creek

Kk

and limestone units and (right) the northern portal. Note the tunnel invert level is around

Table 1Average values of intact rock and discontinuity properties of the main lithologies. Elasticconstants were determined from a limited number (2) of tests.

Limestone Microgabbro

Inta

ct r

ock

Etan (GPa)

Eavg (GPa)

Esec (GPa)

Dis

con

t.

S (m)

2b (mm)

26.3 27.1

91.0 110.3

6.7 9.2

0.25 0.23

11.8 11.3

12.2 11.8

12.6 10.9

0.9 1.0

7.9 6.5

γ (kN/m3)

σc (MPa)

σt (MPa)

ν

γ: unit weight; σc & σt: uniaxial compressive & tensile strength.ν: poison ratio; Etan & Εavg & Esec: tangent, average & secant moduli; S: spacing; 2b:aperture.

41A. Aydin et al. / Engineering Geology 181 (2014) 38–47

1989) and Q (Rock Mass Quality System; Barton et al., 1974; Grimstadand Barton, 1993) are used to quantify tunneling quality of limestoneandmicrogabbro units (Table 2). Two separate sets of input parametersare gathered from two exploratory boreholes (Figure 3A) and from therecords of as-built conditions along (for the horizontal borehole BH1)and in the vicinity (for the vertical borehole BH2) of the borehole inter-vals intersecting the tunnel.

The data outlined in Table 2 represents average conditions withinthe two main lithological domains (microgabbro and limestone)along/around the relevant borehole intervals while some local varia-tions in structural characteristics (included in the records of as-builtconditions) are not considered in the classifications to enable directcomparisons especially considering availability of only two boreholes.Additionally the significant difference in the RQD values of the horizon-tal and vertical drillholes ismainly due to directional sampling bias rath-er than lithological or structural variations. As the discontinuities weresurveyed during the excavation over the blasted faces, aperture valuesare not taken as the decisive factor in assigning joint (discontinuity)condition ratings. RQD values for as-built conditions were determinedindirectly from the spacing measurements (Bieniawski, 1989). Stressreduction factor (SRF) is determined assuming b50 m overburdenthickness to be consistent with the setting of the vertical boreholeBH2. Total RMR values are determined only for the as-built records en-abling differentiation of major joint sets and their orientations. It can besaid that the horizontal borehole enabled reasonable prediction of theas-built conditions by both RMR and Q systems.

Fig. 5. Stereonet diagrams of the joints i

Subjectivity in assigning input parameters and rating factors in rockmass classification systems lead to a common practice of cross-checkingthe RMR and Q values using different relationships proposed by manyresearchers (e.g. Bieniawski, 1989; Goel et al., 1995). In this study, thefollowing relation by Goel et al. (1995) is adopted to estimate basicRMR and Q from each other (Table 2): RMR* = 8lnQ* + 30, whereRMR* is taken as basic RMR minus the rating for uniaxial compressivestrength and Q* is Q normalized by the stress reduction factor (SRF).The basic RMRvalues estimated from correspondingQ values are gener-ally similar but lower than the directly estimated values. The values of Qas estimated from RMR are also similar (considering the logarithmicscale of Q-system) but mostly larger. The direct estimates of basicRMR and Q values are therefore considered valid (although many un-certainties in assigning the input parameter values propagate in thesystem).

As the tunnel was excavated and supported according to the NATMprinciples, RMR and Q values are used to compare the recommen-dations of each rock mass class for excavation sequence and supportsystems. Implementation of NATM (as specified by O-NORM B2203-1994) requires prompt observations of as-built geological and geotech-nical conditions on the newly excavated face after each roundof excava-tion. The next step is to assign support classes in accordance with themonitored displacement trends (indicating similar, better or worseclass domains ahead of the face) and translate these classes into corre-sponding excavation and support methodologies.

The NATM support class for the portal excavation zones wasassigned to be B3 and the support class of the main tunnel excavationwas consistently designated as B2 as the excavation progressed(Figure 3B). Typical tunnel cross-sections where the excavation andsupport recommendations for these support classes are illustrated aregiven in Fig. 6. The tunnel was excavated by smooth blasting methodin two stages keeping the tunnel face at the top-heading section atleast 30 m ahead of the bench excavation. The excavated surfacesalong each round length of blastingwere quickly covered by a relativelythin shotcrete layer, reinforced by a combination of wiremesh andshotcrete and finally by systematic bolting as components of initial sup-port system. Along theportal sections (assigned B3 support class), in ad-dition to the above components, steel ribs were installed generally at 1mintervals while steel forepoling pipes (to prevent possible excessiveoverbreaks from the roof and provide temporary arching effect) weredriven where deemed necessary.

4. Monitoring and evaluating excavation-induced displacements

Displacement monitoring by various geodetic measurements is rec-ognized as an important part of tunnel construction process in recentyears (Schubert et al., 2002), especially in those tunnels designed andconstructed according to NATM. Monitoring displacement trends isvital to confirm the geotechnical design of the supported section, toprepare for unforeseen conditions that may require modification of

n A) microgabbro and B) limestone.

Table 2Summary of RMR and Q-system input parameters, ratings and mutual estimates as derived for the boreholes and for the corresponding chainages inside the tunnel.

Boreholes Rating

BH 1

(horizontal)

(North Portal)

BH–2 (vertical)

(South Portal)BH–1 BH–2

BH–1 (horizontal)

(North Portal)BH–1

Uniaxial compressive

strength (MPa)Very strong Very strong 12 12 110 12 110 12 Strong 10 90 10

RQD (%) 78 38 16 8 85 17 75 15 50 10 75 15

Joint spacing (m) Moderate Close 10 8 0.3 9 0.2 8 Close 8 0.2 8

Joint condition

Unaltered surfaces; some

slickensided; thin gouges

Slickensided surfaces; few thin

gouges10 10

Some slickensided

surfaces; few thin gouges

10Chlorite–coated

surfaces; >5 mm thick gouges

0

Smooth; locally filled by clay and silt; aperture 1–5

mm

10Smooth; locally filled; persistent bedding plane

10

Ground water

condition15 15 Locally damp 12 Damp 10

g/w table below tunnel invert

15 Damp 10

Joint orientation – – – –Strike // tunnel axis dip >45°

–12Strike // tunnel axis dip >45°

–12 – –Strike // tunnel axis

dip >45° –12

Basic RMR – – 48 38 48 – 35 – 38 – 43

Basic RMR (Q*) – – 41 36 52 – 29 – 55 – 45

Total RMR – – – – 36 – 23 – – – 41

RQD (%) 78 38 78 38 85 85 70 70 50 50 45 45

Joint set (Jn)Random discontinuities

Random discontinuities

2 2Two joint sets plus random

6Two joint sets plus

random6 One joint set 2

Two joint set plus

random 6

Joint roughness (Jr) 1.5 1.5

some

Slickensided

and mostly

smooth

2

Slickensided

and thick gouge

zones

1 Smooth 2 Smooth 2

Joint alteration (Ja) 10 10Occasional thin

clay filling8

Chlorite–coating; >5 mm thick

gouges (zones of crushed rock and

clay)

10Locally filled by

clay and silt8

Locally filled by

clay and silt8

Joint water reduction

factor (Jw)1 1 Locally damp 1

Local minor

seepages;

generally damp

1g/w table below

tunnel invert1

Local minor

seepages;

generally damp

1

Stress reduction

factor (SRF)2.5 2.5

Shallow tunnel* (low overburden stress)

1

Shallow tunnel (low overburden stress; thick clay–filled gouges)

2.5Shallow tunnel; favourable stress conditions

1Shallow tunnel; favourable stress conditions

1

Q – – 2.34 1.14 3.54 – 0.47 – 6.25 – 1.88

Q (RMR*) – – 2.12 0.61 2.12 – 0.42 – 0.78 – 1.45

Rating

Shallow tunnel* (low overburden stress; clay–filled gouges)

As–built

conditions

(along BH–1)

g/w table below tunnel invert

g/w table below tunnel invert

As–built

conditions

(around BH–2)

Rating

Slickensided

Thin swelling clay–filled gouges

Rmr–system

Q–system

ParametersRating

As–built

conditions

(along BH1)

Boreholes Rating

Microgabbro Limestone

RMR* = basic RMR − rating for UCS.Q* = Q / SRF.

B2

Bolts (4m)

Foundation beam (30 N/mm2)

0.00 m

+02.80

TOP HEADING

BENCH

Shotcrete(15cm)

Bolts (4m)

-0.75

TheoreticalExcavation line

B3

sBolt(4-6m)

Bolts(4m)

3.50%

Foundation beam (30 N/mm2) Invert concrete arch (5cm 30N/mm2)

0.00 m

+02.80

BENCH

-0.753.50%

TOP HEADING

Shotcrete(10cm)

Fig. 6. Basic support systems and excavation sequence based on O-NORM B2203 for rock classes B2 and B3.

42 A. Aydin et al. / Engineering Geology 181 (2014) 38–47

43A. Aydin et al. / Engineering Geology 181 (2014) 38–47

excavation methods and sequence, stabilization and support systems,and eventually to prevent significant deviations from estimated com-pletion costs and dates.

In contrast to the tape extensometer method, optical 3-D methodenables the determination of the horizontal, vertical and longitudinalcomponents of displacement vector at individual measurement posts,and the spatial and temporal variations of each component with greataccuracy. It therefore allows better prediction of relative conditionsahead of the face even when the magnitudes of displacements aresmall. Realization of these advantages and advances in the measure-ment systems led to a number of studies specifically concerned withutilization of 3-D monitoring data (Schubert and Budil, 1995; Schubertand Steindorfer, 1996; Steindorfer and Schubert, 1997; Sellner andSteindorfer, 2000).

Displacement monitoring in Tube I (Figure 3B) were carried out byoptical 3-D monitoring method in regular intervals for a period of upto 8–9 months in the top heading and about 5–6 months in thebench. Five measurement posts were installed in a typical array(Figure 7) at each section located at about 5–20 m intervals dependingon the ground conditions. The resultant displacement vector and itsangle from the vertical in the longitudinal section at each post(Figure 7) are separately presented in four profiles each representingsubsequent 1–3 monthly intervals (Figures 8 and 9). These two graphsare sufficient to calculate the magnitude of vertical and longitudinalcomponents of the displacement vector. Note that the scales of displace-ments in Figure 8 are different for the first three measuring posts at thetop-heading and the remaining twoposts at the bench level. The smallerdeformations at the bench can be attributed to the controlled deforma-tions due to the staged excavation in a weak rockmass but stress redis-tribution (as shown later by the numerical simulations) may provide amore reasonable explanation in rock masses with such small magni-tudes of displacement response.

Bench

Top heading2 3

1

Displacementcomponents

R: ResultantV: VerticalL: Longitudinal

4 5

LLVV

RR

Fig. 7. The arrangement of five displacement monitoring posts (see inset for a view of an ins

The resultant displacements and angles are interpreted with refer-ence to the as-built geological (longitudinal) cross-sections of bothtubes of the tunnel (Figure 3B). It should be noted that the monitoringposts at each new section can be installed after at least one blastingand mucking cycle is completed and components of initial support sys-tem is installed. Therefore themonitoring data does not reflect the totaldeformation response of the rock mass as the earlier phases of rockmass response to the approaching excavation face are inevitably lost(as discussed in detail in the next section). The most significant featureof themonitoring data presented is that all displacement patterns (bothin time and along the tunnel) at all five posts were perfectly consistentin reflecting the presence of zones of contrasting conditionsdespite verylow displacements and their different ranges in the top-heading andbench sections. This aspect of the data is reassuring against the possibil-ity of errors related to the equipment, operator and tunnelingconditions.

In order to interpret the trends presented in Figs. 8 and 9, it is neces-sary to establish a frame of reference for the displacement signatures ofvarious scenarios of changes in tunneling conditions. One such simpli-fied scenario (excavation through a zone of alternating softer andharder rock mass domains) was proposed by Steindorfer and Schubert(1997) for the resultant displacement angle which reflects relativechanges in the magnitudes of longitudinal and vertical components ofthe displacement vector (Figure 10). Fig. 3A and B shows that thereare several zones of such alternating conditions corresponding tomajor geological changes recorded along the tubes.

The magnitudes of resultant displacement vectors measured in thetop-heading (posts 1, 2 and 3) indicate the presence of a relatively loose(serpentinized) rock mass along the entrance (south) portal section,grading into generally better rock mass conditions which were sharplyinterrupted by a gouge zone of significantly lower rigidity (Figure 8). In-creasing displacement trend is reversed (despite approaching to the

Excavation

Measurement post

talled post) and the components of measured displacements in the longitudinal section.

0

1

2

3

4

5

6100200300400500600

0

1

2

3

4

5

6

0

1

2

3

4

5

6

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

1

Res

ulta

nt d

ispl

acem

ent (

mm

)

Chainage (m)

2

3

4

5

Fig. 8. Profiles of the cumulative resultant displacement vector as measured in each of thefive monitoring posts. Note the increasing temporal trend of displacements at individualposts.

Dis

plac

emen

t ang

le (

Δα°)

-80

-60

-40

-20

0

20

40

60

80

-80

-60

-40

-20

0

20

40

60

80

-80

-60

-40

-20

0

20

40

60

80

-80

-60

-40

-20

0

20

40

60

80

-80

-60

-40

-20

0

20

40

60

80100200300400500600

1

Chainage (m)

2

3

4

5

05.23.2002 08.20.2002 10.23.2002 11.15.2002

Fig. 9. Profiles of the angle the cumulative resultant displacement vector make with thevertical in the longitudinal section (see Figure 10).

44 A. Aydin et al. / Engineering Geology 181 (2014) 38–47

exit portal)when the excavation enter the limestone unit. Post 3 at 380mcorrectly signals the proximity of a local gouge zonewhich does not influ-ence displacements at the opposite post 2. Displacement patterns revealsimilar behaviors at two pairs (2–4 and 3–5) of monitoring posts oneach sidewall of Tube I. All monitoring posts display a significant shift atabout 200 m in response to the thick gauge zone lying more than 40 mahead. The temporal displacement patterns (expressed by the separationof the displacement profiles) show that rate of deformation did not de-crease when the monitoring ended, relying on the small magnitudes. Asdiscussed in the next section, however, the numerical simulations predict

that the vertical displacement component at post 1 was very close to itsultimate value.

The angle that the resultant displacement vector makes with thevertical is a sensitive indicator of relative changes in the rock massmodulus. Fig. 9 summarizes the spatial and temporal patterns of thevector angles corresponding to the magnitude patterns presented inFig. 8. The striking common features of these profiles at all posts arethat: a) the angles are essentially stationary in time (making them areliable indicator startingwith thefirst set ofmeasurements); b) the an-gles range between35 and 45° indicating similarmagnitudes of longitu-dinal and vertical displacement components; c) not all changes in the

E2 E1E1

E1

E1

E2

E2E2

VL

-+

>

Longitudinal Section E2

LR V

R

E2 E2

Fig. 10. Variations of the resultant displacement angle with changes in the rock massdomains with different rock mass moduli. Note that the resultant vector angle change istaking place within a relatively narrow range about zero degree (modified fromSteindorfer and Schubert, 1997).

(1)

(2)

(3)

(4)

(5)

Fig. 11. The boundaries and finite element mesh used in the simulations (1); the total dis-placement and strength factor contours for the unsupported (2& 4) and supported (3 & 5)sections. Note the displacement vectors and deformed excavation boundary aremagnifiedby a factor of 100. The location of the model cross-section is given in Fig. 3A.

45A. Aydin et al. / Engineering Geology 181 (2014) 38–47

direction of displacement vector correspond to a mappable feature orzone of weakness (Figure 3B) but certainly reflect a relative change inoverall rock mass conditions; d) as for the magnitude profiles (Figure8), themajor gouge zone is predicted at 200mbut unlike themagnitudeprofiles, this significant zone did not appear as a distinct feature.

5. Numerical modeling

In order to investigate the adequacy of the selected NATM supportclass, numerical simulations of excavation-induced stress redistributionand resulting displacements around a tunnel section are performed on amodel of equivalent continuum using a 2-D finite element code PHASE2

(Rocscience, 2013). Due to the relatively shallow depth of the tunneland significant topographical changes, the actual ground surface is cho-sen to represent upper boundary of the model (Figure 11). In-situstresses are assumed to be isotropic which is consistent with the isolat-ed setting of the hill through which the tunnel was excavated. Outerboundary geometry, boundary conditions and finite element meshstructure of the model as well as the patterns of total displacement(with displacement vectors) and strength factor (with principle stresstrajectories) are all presented in Fig. 11 for both unsupported and sup-ported sections.

The equivalent rock mass is assumed to be an isotropic, homoge-neous, elastic–plastic Hoek–Brown material. Estimates of the rockmass modulus and the parameters of the selected failure criterion aredetermined empirically (Hoek et al., 2002). The material strength σci

and theHoek–Brownmaterial constantmi are obtained from the triaxialtests on the microgabbro specimens. Table 3 summarizes the relevantrock mass properties of the microgabbro unit. As the input parametersare derived independently using the Hoek–Brown criterion (which isbased on a different rock characterization scheme, GSI), the results re-flect a combined influence of the adequacy of both the assigned GSIvalues and the NATM support classes. Therefore the simulations areconducted using a range of GSI values to inspect any significant influ-ence on stress and displacement patterns.

5.1. Overcoming shortcomings of 2-D simulation of tunnel excavation

As illustrated in Fig. 12, a given cross-section on a tunnel excavationroute starts converging long before the excavation face reaches that sec-tion and continues to deform at a high rate until the face moves behindthat section by a distance of at least several tunnel radius. This typicallytakes place in a stepwise manner as the rock mass responds to blastingcycles. This response patternmaybemodified to variable degrees by thetime-dependent behavior of the rock mass.

The influence of changing position of the face or the 3-D nature ofthe convergence means that the monitoring posts at a given sectioncan be installed only after some significant displacements have alreadytaken place, and that the 2-D numerical simulations of the convergence

do not capture this stepwise deformation pattern and cannot be directlycompared to themeasured displacements. This shortcoming can be off-set by the core-softening approach (Rocscience, 2013) simulating theeffect of moving excavation face in several stages. One of these stagesmust represent the installation of themonitoring systemat themodeledsection. The excavation face is usually one-round distance (2.5–3.0 mfor the NATM B2 class) behind that section (Figure 12).

Table 3The values of input parameters used in numerical modeling.

Rock mass properties Microgabbro

Uniaxial compressive strength (σci) MPa 110Intact rock parameter (mi) 25Geological Strength Index⁎ (GSI) 58Disturbance factor (D) 0Hoek–Brown rock mass constants

mb 6s 9.4 ∗ 10−3

Modulus of deformation⁎⁎(E) (MPa) 5364Unit weight MN/m3 0.027Poisson's ratio⁎⁎⁎ (νrm) 0.276

⁎ Estimated from RMR (Hoek and Brown, 1997).⁎⁎ Based on Hoek and Diederichs (2006).⁎⁎⁎ Taken as 1.2 ∗ νi (Kulatilake et al., 2004) (see Table 1).

46 A. Aydin et al. / Engineering Geology 181 (2014) 38–47

As the degree of core softening determines the amount of displace-ment, independent predictions of the total displacement and the dis-placement ratios corresponding to particular face positions are neededto determine the values of input parameters to achieve a reasonabletrend and degrees of softening. Based on a series of 3-D numerical simu-lations, Unlu andGercek (2003) determined two separate trends relatingthe elastic radial displacements of a circular tunnel to the face distance(ahead of and behind a monitored section) as a function of rock massPoisson's ratio. For an equivalent circular cross-section of about 6m radi-us, the ratios of radial displacements are predicted to be 0.25 and 0.69when the face is at the monitored section and when it is one roundlength past the section, respectively. According to this prediction, thesupport system is subjected to a fraction of initial (pre-excavation)load that will generate an additional 31% of the total displacements.

Thus, once the model is brought to equilibrium and the displace-ments are nullified, the response of microgabbro rock mass at a moni-tored section in Tube I (under the thickest overburden and steepestground profile along the tunnel) (Figure 3A) is simulated in threestages: two stages of core softening corresponding to these two face po-sitions and one stage for the full-face excavation when the support sys-tem (installed in the previous stage) is loaded and total displacementsare stabilized. The proximity of the twin tubes and the steep terrainalso require an analysis of the influence of Tube II excavation on the“final” deformations of Tube I.

5.2. Numerical results and comparison with the field data

Numerical models are necessarily simplification of the reality to var-iable extents, and numerical simulations predict the model behavior

0

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o (

%)

Distance from monitoring post to excavation face (in number of excavation round-lengths)

Ran

ge

of

Mo

nit

ori

ng

Baseline

Fig. 12. Elastic response of a tunnel section (with an equivalent radius of 6m) to changingposition of excavation face (based on Unlu and Gercek, 2003). The origin corresponds tothe position of the monitoring post, and the face distance at which monitoring starts isone round-length from the monitoring post (marked by dotted line).

under specified conditions. Though the question of non-uniquenesscannot be entirely removed, comparing the predicted and themeasured(if available) offers themost robustmeans of testingwhether themodelpossesses the dominant features and essential characteristics that cap-ture the equivalent system behavior. In this study, the measured resul-tant displacements (Figure 8) were compared with the predicted totaldisplacement distribution around Tube I (Figure 11). Considering thatthemeasured displacements represents about 31% of the total displace-ments, the magnitudes of the two displacement data are remarkablysimilar both in their distribution around the excavation boundary andtheir magnitudes.

The tunnel is located in a zone of moderate seismicity (Zone 3)according to the current earthquake zonation map of Turkey(http://deprem.gov.tr) and is in close proximity (around 12 km) tothe Ecemis Fault, activity of which is controversial. Therefore, seis-mic loading needs to be taken into account in the design and con-struction of the tunnel. The zonation map specifies the effectiveground acceleration coefficient for this zone to be 0.2 g. The numer-ical simulations with this equivalent static loading did not predictrock mass yield or noticeable changes in the patterns or magnitudesof displacement and strength factor.

Changes in the values of total displacement and strength factor(Figure 11) at all stages of simulation are presented in Fig. 13 for bothsupported and unsupported sections with reference to the position ofmonitoring post 1 in Tube I. Main observations from these two figurescan be summarized as:

a) total displacement vectors have a minor horizontal component, andaremuch smaller on the bench level (monitoring posts 4 and 5) thanin the crown (post 1) and top-heading sidewalls (posts 2 and 3)(which is perfectly consistent with the field data; Figure 8);

b) this difference in the displacement magnitudes can be explained bystress pattern (rather than staged excavation);

c) the displacement trends in Fig. 13 show that (i) a 20 mm thick reg-ular shotcrete layer increases strength factor slightly and decreasestotal displacement and (ii) excavation of Tube II (stages 5, 6 and 7)has almost no influence on strength factor but a minor influence oftotal displacements;

d) neither the rock mass nor the support system components yield inresponse to stress redistribution and convergence;

e) strength factor (comparing rock mass strength to excavationinduced-stresses and providing a measure of potential for yieldor stress-induced failure) is uniform around both tubes and rea-sonably above 1 even in unsupported section;

f) the central pillar (rib) between the twin tubes develops verysmall displacements; together with the stress trajectories, thedisplacement vectors show an effective arching action by thetwin tubes;

g) a stiffer or a thicker liner helps only slightly to limit the extents oflow factor zones.

Though the simulations show that the tunnel through the rockmassdesignated the NATM support class of B2 would stand unsupported,protection against loosening, block fall, etc. and containment of localoverbreaks and progressive failures require a light initial support.Thus, theNATMrecommendations of a thin protective layer of shotcrete(reinforced with wiremesh) and fully grouted rock bolting to securemovable blocks are deemed to be appropriate in the setting of this tun-nel for the B2 class. These observations hold their validity even if the ex-cavations were carried out as full-face rather than in staged manner asrecommended for the B2 class. It would however be more difficult tocontrol blasting and local overbreaks during prolonged excavation cy-cles. Thus it can be said that the NATM recommendations for bothstaged excavation and initial support system consider the practical as-pects of tunnel construction in addition to the possibility of stress-induced failures.

0

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gth

Facto

r

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ent

(mm

)

Stage Number

Displ - with support

SF - no support

SF - with support

Displ - no support

Fig. 13. Trends of total displacement and strength factor (Figure 11) at all stages of simu-lation for both supported and unsupported sectionswith reference to the position ofmon-itoring post 1 (Figure 7) in Tube I. Stagenumbers represent: 1= excavation face (EF) is farfrom the monitoring section (MS); 2 = EF at the MS; 3 = EF past the MS by one roundlength; 4 = EF past the MS by many rounds; 5, 6 and 7 = repeat of 2, 3 and 4 in Tube II.

47A. Aydin et al. / Engineering Geology 181 (2014) 38–47

6. Conclusions

This paper demonstrates how the monitoring data and numericalsimulations can be interpreted and compared within a framework de-lineated by a clear and concise description of 3-D nature of rockmass re-sponse during tunneling and of the numerical modeling procedure toproperly simulate this response. It also highlights the importance ofpre-excavation geomechanical characterization using rock mass classi-fication approach by RMR and Q systems, to provide predictions of therange of as-built rockmass quality, to generate input for preliminary de-sign using numerical and empirical methods and to guide the initialphases of tunneling when limited observations of as-built conditionsare made.

The resultant displacementmagnitudes and angleswere interpretedwith reference to the as-built geological cross-sections of the tunnel. Alldisplacement patternsmonitored atfive posts in each sectionwere con-sistently effective in providing advance warning for the presence ofzones of contrasting conditions despite very low magnitudes of dis-placements. Numerical simulations predicted no imminent danger ofstress-induced instability and that the twin tubes could stand unsup-ported. However a light support system as recommended by NATM(and similarly by RMR and Q systems) was necessary to prevent struc-tural instability, loosening and overbreaks. The numerical analysis usingthe core-softening approach also exemplifies the sensitivity of tunnelconvergence behavior to moving position of the excavation face, andconsequently supports the combined use of the displacement vectormagnitudes and angles to predict the presence and to appreciate themagnitude of changes in rock mass stiffness ahead of the excavation.

The interactive nature of NATM based on monitoring excavation-induced displacements, mapping and classifying as-built rock massstructure and advance probing is no doubt useful in any geological

setting but accommodating the flexibility of excavation and support re-lated decisions is not without cost. In order to assess and control thisadditional cost, possible rock mass conditions should be properly char-acterized at the exploration stage and support/excavation recommen-dations should be tailored for the local engineering geological setting.The full advantage of the interactive tunneling methods can be realizedat a lower cost if the 3-Dmonitoring can be fully integrated into the tun-nel excavation and support decisions. It is therefore more useful to pro-duce monitoring data at closely spaced tunnel sections during one ortwo rounds of excavation than long-term monitoring of widely spacedsections.

Acknowledgments

The authors are grateful to Tekfen Engineering A.S. and especially totheir chief engineerMr. Haldun Kahyaoglu for facilitating this study andsharing the monitoring data. The authors are also grateful for the finan-cial support provided by the Research Fund of Cukurova University(Grant FBE2002D182).

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