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ORIGINAL PAPER

Surface subsidence induced by twin subway tunnellingin soft ground conditions in Istanbul

Yılmaz Mahmutoglu

Received: 2 February 2010 / Accepted: 18 April 2010 / Published online: 1 May 2010

� Springer-Verlag 2010

Abstract Unlike the symmetrical surface settlement

trough of a single tunnel which can be described using the

Gaussian function, surface settlement over twin tunnels can

be symmetric with respect to the mid-point between two

tunnels or asymmetric. The paper reports the settlement

troughs which developed when earth pressure balance

(EPB) machines were used to excavate twin tunnels at

shallow depth in the soft ground conditions beneath a

developed part of Istanbul. An attempt is made to evaluate

the effects of different factors on the surface subsidence.

Detailed monitoring was undertaken when one tunnel was

advanced ahead of the other and when only one tunnel was

being driven. It was found that the shapes of the subsidence

troughs over the two tunnels were different and varied with

the excavation of the second/subsequent tunnel. It is con-

cluded that changes in the subsidence trough are related to

disturbance in the geo-material when an excavation is

advanced ahead, as well as the nature and thickness of the

overburden.

Keywords Soft ground � Subsidence �Surface monitoring � Twin tunnels

Resume La cuvette symetrique de tassement au-dessus

d’un tunnel peut etre decrite par une fonction gaussienne,

mais le tassement au-dessus d’un bi-tube peut etre syme-

trique par rapport au point median entre les deux tubes ou

asymetrique. L’article presente les cuvettes de tassement

qui se sont developpees quand un tunnelier a pression de

terre (EPB) a ete utilise pour creuser deux tubes a faible

profondeur dans des conditions de sols mous sous une

partie de la ville d’Istanbul. Une tentative est faite pour

evaluer les effets de differents facteurs sur le tassement de

surface. Une instrumentation de precision a ete mise en

œuvre lorsque l’un des tubes etait en avance par rapport a

l’autre et lorsque seulement un tube etait fore. On a trouve

que les formes des cuvettes de tassement au-dessus de deux

tubes etaient differentes et se modifiaient avec le creuse-

ment du deuxieme tube. On a conclu que les modifications

de la cuvette de tassement resultent d’une part des defor-

mations des materiaux geologiques lorsqu’une excavation

prend de l’avance sur l’autre et d’autre part de la nature et

de l’epaisseur du recouvrement.

Mots cles Sol mou � Tassement �Instrumentation de surface � Bi-tube

Introduction

Urban transport systems are structured in an extremely

complex way. The level of infrastructure provided and the

way it is managed affect the demand for travel in Istanbul,

the largest urban settlement area in Turkey with commer-

cial, cultural and historical significance. Between 1960 and

c. 1990, the average rate of population growth was 4.3%

(Gercek et al. 2004) and although this has dropped to 3.2%

in the last decade, as a result traffic congestion is a major

problem.

The municipality of Istanbul already has an 18 km long

light rail transit (LRT) system and twin-tube subway lines

between Taksim and Levent on the European side of the

city. Other subway lines are under construction, including a

Bosphorus crossing between Sarayburnu and Uskudar

Y. Mahmutoglu (&)

Department of Geological Engineering, Faculty of Mines,

Technical University of Istanbul,

34469 Maslak, Istanbul, Turkey

e-mail: [email protected]

123

Bull Eng Geol Environ (2011) 70:115–131

DOI 10.1007/s10064-010-0289-8

(Lykke and Belkaya, 2005; Sakaeda, 2005). It is antici-

pated that some of these lines will be integrated with the

existing system (Fig. 1).

The most serious problem caused by the excavation of

shallow underground openings is surface subsidence,

which can cause detrimental effects on adjacent structures

near the tunnel alignment (Leca and New 2007) and

seriously interrupt city life (Mahmutoglu et al. 2006).

Although earth pressure balance (EPB) machines were

used for the construction of the twin subway tunnels in

Istanbul, surface subsidence occurred between Esenler and

Kirazli. Many structures along the routes were seriously

affected resulting in a considerable increase in the project

cost.

This paper discusses the problems encountered with the

subway connection between Esenler and Kirazli, part of

the second phase of the Istanbul Subway Project, and

considers the implications for similar works in soft ground

conditions.

Tunnelling method

In view of the difficult ground and sensitive environmental

conditions between Esenler and Kirazli, EPB machines

with closed mode were chosen to minimise the possibility

of surface subsidence. The two subway tunnels are at the

same elevation and almost parallel; the distance between

the centrelines varying from 14.8 to 15 m. Generally, the

right hand tunnel (RHST) advanced ahead of the left hand

drive (LHST). They pass beneath a heavily developed area

at a very shallow depth, particularly where streams/rivers

are present. A Lovat EPB machine was used in the RHST

and a Herrenknecht EPB for the LHST. The outer diameter

of the shields was 6.52 and 6.45 m, respectively, and the

lengths of the shields 9.30 and 7.68 m, respectively

(Table 1).

The machines constructed a precast concrete lining

composed of six 0.6 m thick, 1.4 m long segments. As the

outer diameter of the pre-cast lining was 6.3 m, there was a

physical gap (Lee et al. 1992) of 150 mm at RHST and

220 mm at LHST. As the shield advanced, pressurized

grout was injected into the tail void. In view of the shallow

overburden, the face pressure was limited. However, the

rate of tunnel advancement in the main line tunnels aver-

aged 10 m/day.

Ground characterization along the subway line

Much of western Istanbul is underlain by the Palaeozoic

Thrace Formation, which is composed of sandstone,

Fig. 1 Location map and the integration of the phases of the Istanbul Subway Project

116 Y. Mahmutoglu

123

siltstone and claystone alternations with rare conglomerate

layers. Andesitic dykes of varying thicknesses are com-

mon. In the southern part of the European side the subway

lines crosses the Kirklareli Formation, composed of thickly

bedded limestone with marl and clay and some weak

mudstone. Unconformably overlying the Kirklareli For-

mation are the weak Cukurcesme, Gungoren and Bakirkoy

Formations of Upper Miocene age (Aric 1955). Approxi-

mately 97% of the tunnel drive was in these weak forma-

tions (Fig. 2).

As seen in Fig. 3a, dense Miocene sands were one of

main units encountered. These sands are overlain by the

Gungoren clay which changes laterally with braided-river

gravels of meta-sandstone to gneiss, ophiolite and different

volcanic, quartz and limestone pebbles present as well as

minor intercalations of marls and clays with thin coal

seams. Large-scale planar and trough-type cross-bedding is

the principle sedimentary structure (Sayar, 1976). Although

the thickness of the unit is about 30 m in the type area,

along the subway line it varied from only 10 m thick. At

the excavation level, the green clays are highly plastic and

overconsolidated, containing a significant amount of illite

and some silt fraction (Fig. 3b). Sand and marl lenses are

common at some localities. This formation is conformably

overlain by the thinly bedded limestone and rare marl

lenses of the Bakirkoy Formation (Fig. 3c).

At the feasibility stage a large number of boreholes were

drilled along the line with SPTs and pressuremeter tests

undertaken at 1.5 m intervals. In addition to the in situ tests

many laboratory tests were performed on undisturbed

borehole samples (see Guven, 2008 and Table 2). The

geological cross-sections undertaken at problematic loca-

tions where significant subsidence occurred are given in

Fig. 4, which also details the N30 values (number of blows

for 30 cm penetration) at the four problematic chainages

(referred to as 1, 2, 3a and 3b).

Surface subsidence on the Esenler–Kirazli subway line

The main parameters involved in the vertical movement of

a structure were described by Attewell et al. (1986) who

identified the differential subsidence and the length of

structural elements in the direction of the settlement trough

Table 1 Technical properties

of Herrenknecht and Lovat EPB

machines employed in the

subway line

Herrenknecht (Left tube) Lovat (Right tube)

Bore diameter (m) 6.50 6.56

Outer diameter of shield (m) 6.45 6.52

TBM Length (m) 7.68 9.30

Back up Length (m) 80.0 65.0

Weight (tons) 567 534

Cutter head rotation speed (rpm) 0–2.5 0–6

Total installed power (kW) 963 1622

Cutting head type Mixed ground Mixed ground

Cutting head power (kW) 600 900

Maximum applicable torque (kNm-rpm) 2350–2.5 4450–1.9

Maximum thrust force (kN) 32 54

Number of thrust cylinders 32 30

Maximum thrust rate (mm/min) 80 150

Face pressure (kPa) 300 300

Screw conveyor inner diameter (mm) 700 851

Screw conveyor power (kW) 110 225

Screw conveyor rotation speed (rpm) 0–19 0–18

Screw conveyor capacity (m3/h) 275 400

Maximum particle size (mm) 250 300

Erector type Rotary type Rotary type

Segment outer diameter (m) 6.3

Segment inner diameter (m) 5.7

Segment length (m) 1.4

Ring configuration 5 Segment ? 1 Key segment

Injection type By back shield nozzles

Tail seal Wire brushers

Surface subsidence over twin tunnels 117

123

as the basic parameters (Fig. 5). As a study of the cracking

of walls and structural members has shown that damage is

most often due to distortional deformation, angular dis-

tortion (b) was selected as the critical index of settlement.

The following limiting values of angular distortion are

recommended for frame buildings (Peck 1976; Skempton

and MacDonald 1956).

b = 1/150, structural damage probable;

b[ 1/300, cracking of load bearing or panel walls

likely;

b\ 1/500, safe level of distortion.

The method of calculating forces in buildings subjected

to bed deformations induced by underground excavations

and the ways of protecting buildings were discussed by

Pushilin et al. (2007) and Isaev (2008). Burland et al.

(1977) and Mair et al. (1996) described essential parame-

ters in assessing building damage. Leu and Lo (2004);

Leblais et al. (1996); and Neaupane and Adhikari (2006)

emphasise the effects of the geotechnical characteristics of

the region, the depth and size of the underground opening,

the distance between tunnel centrelines and the tunnelling

methods on settlement. In view of this, prior to the com-

mencement of tunnel excavations between Esenler and

Kirazli, an extensive monitoring network was established

to record the subsidence over a wide area. The number and

frequency of the monitoring points was increased where

the overburden was least and where the effects of subsi-

dence on surface structures would be most severe. Never-

theless, structural damage still occurred (Fig. 6) notably at

Esenler, Cincin and Tavukcudere, which resulted in a c.

18% increase in the project cost.

The longitudinal subsidence trough

In some localities between Esenler and Kirazli, surface

settlements exceeded the permissible value of 25 mm

accepted for the project (Fig. 2). The monitoring indicated

the subsidence was mainly over the LHST (Herrenknecht

drive) while the settlement over the RHST (Lovat) did not

exceed the allowable 25 mm.

The maximum subsidence (Smax) measured at monitor-

ing points installed on the centrelines of both tunnels is

given in Table 3, which also shows the ratio between the

LSmax (left hand tunnel) and RSmax (right hand tunnel)

varies from 1.46 to 4.00 at Esenler.

The Smax values measured in the Esenler region are

correlated with the ratio (Z0/D) between equivalent depth

Fig. 2 Longitudinal geological section along the left hand side tunnel and problematic localities where design value of surface subsidence has

been exceeded by tunnelling

118 Y. Mahmutoglu

123

(Z0) and tunnel diameter (D = 6.50 m) in Fig. 7. As gro-

uting was undertaken prior to the drive at Tavukcudere,

these values are excluded. The relationship has an r value

of 0.84. The highest surface subsidence was recorded

where the tunnel passed from the very dense sands into the

thickly bedded limestone (Fig. 4c); once the tunnel entered

the limestone there was no significant surface subsidence.

Examples of the ground response curves are given in

Figs. 8 and 9. In Fig. 8, the Herrenknecht EPB machine on

the left side was advancing toward monitoring points BMP-

60S06 and BMP-60S08 while the Lovat EPB machine had

stopped 80 m behind these points, hence the curves in this

figure show the subsidence resulting from the effect of the

LHST excavation. The two monitoring points are almost at

the same position (km 1 ? 975) with Point BMP-60S06

very close to the tunnel centreline and BMP-60S08 offset

from the centreline by some 5 m. Subsidence was recorded

at both points when the excavation was 20 m from these

points.

Figure 9 shows the variation in surface displacement at

SMP 251 on the RHST centreline at km 4 ? 366. The

effect of the shield passing these points is very evident. It

can also be seen that there is significantly more subsidence

on the LHST, even though the monitoring point is 15 m

from the centreline of this tunnel. The horizontal parts of

this curve indicate ending of subsidence after ring closure

behind the shield.

The transverse subsidence trough

The shape of the surface trough above an underground

mine excavation was examined by Martos (1958) who

proposed that surface settlement could be represented by

a Gaussian or normal distribution curve. Schmidt (1969)

and Peck (1969) suggested a similar form of transverse

trough occurs above single tunnels. O’Reilly and New

(1982) developed the Gaussian model by making the

assumptions that ground loss could be represented by a

radial flow of material toward the tunnel and that the

trough could be related to ground conditions through an

empirical ‘‘trough width parameter’’. They made an

analysis of case history data and developed the equations

below for the calculation of the vertical and horizontal

displacements (Fig. 10).

Sðx;zÞ ¼ Sðmax;zÞexp� x2=2 Kz0ð Þ2

Vs ¼ 2pð Þ1=2Kz0Sðmax;xÞ

Hðx;zÞ ¼ Sðx;zÞx=z0

where S(x,z) and H(x,z) are the vertical and horizontal com-

ponents of displacement, respectively, at the transverse

distance x and the vertical distance z from the ground

surface above the tunnel axis; S(max,z) is the maximum

surface settlement (at x = 0); z0 is the vertical distance

from the tunnel axis; K is an empirical constant related to

the ground conditions; Vs is the settlement volume per unit

advance; Kz0 defines the width of the trough and corre-

sponds to the value of x at the point of inflexion of the

Fig. 3 The geological formations representing the Miocene sequence

on the subway line (a Cukurcesme, b Gungoren, c Bakirkoy

formations)


123

curve. In practical applications it is recommended that the

total trough width can be taken as 6Kz0. Arioglu et al.

(2002) discussed the point of inflexion of the surface set-

tlement curves in the Bakirkoy Formation in Istanbul; as

noted above, this is found as a thin cover on the hill tops in

some areas along the Esenler–Kirazli line.

Changes in the transverse trough were examined

individually for each problematic section of the Esenler–

Kirazli line. Firstly, it was assumed that settlement

associated with the operations at the face and settlement

that occurred at the tail should be differentiated and

the variation of vertical displacements versus time were

Table 2 Geo-material properties obtained from site investigation in problematic locations

Geo-material properties Distances to the starting point of the project (location name)

km 0 ? 850–1

? 000 (Esenler)

km 1 ? 850 ? 2

?100 (Cincin)

km 4 ? 250–4

? 460 (Tavukcudere)

Unit weight, cn (kN/m3) 17–20.5 17.5–22.5 19.2–22.1

Liquid limit, LL (%) 42–68 31–56 47–77

Plasticity index, Ip (%) 29–38 24–31 23–44

Moisture content, wn (%) 20.9–42.1 17.3–26.1 12.0–29.6

Permeability, k (m/sec) 1.3–3.9.10-7 4.1.10-4–3.8.10-7 –

Limit Pressure, Lp (MPa) 0.58–3.2 0.57–4 1.1–3.6

Pressiometric Modulus, Ep (MPa) 5.9–57.5 4.2–60 9.2–200

Penetration strength (N30) [70 [67 [73

Compressive strength, qu (kPa) 105–484 120–294 128–157

Cohesion, c (kPa) 50–200 60–140 43–78

Friction angle, U (�) 5–21 2–11 2

Lithology Dense-very dense sand

and hard clay

Fine grained very dense

sand, hard clay

Dense-very dense sand

and stiff clay

Overburden thickness, z (m) 11–34.5 11–15 16-18

Fig. 4 Detailed geological cross-sections of problematic locations at kilometres corresponding to the monitoring arrays (1, 2, 3a and 3b)

120 Y. Mahmutoglu

123

correlated using subsidence curves corresponding to the

series of monitoring points at the problematic locations.

This is shown schematically in Fig. 11 which indicates

the locations of the monitoring points relative to the

tunnel axes and the directions of the selected monitoring

arrays (1, 2, 3a and 3b). The offsets of the monitoring

points from the LHST centreline and the maximum sub-

sidence at these points after each tunnel excavation are

given in Table 4. Negative offsets refer to the points at

the left hand side of LHST centreline while the measured

values of total subsidence after excavation of both tunnels

(RTHS and LHST) are given in columns SR and SL,

respectively.

The Esenler region

Esenler is the location of the eastern portals. In this area,

the RHST was advanced 80 m ahead and the distance

between the tunnel centrelines was 14.8 m. The ratio of

equivalent depth to tunnel diameter (Z0/D) is lower than 2.

The overburden was mainly composed of hard clay, dense

sand and man-made fills on top (Fig. 4a) and the tunnels

Fig. 5 Definition of settlement terminology for building (Wahls

1981). L construction length in the direction of subsidence trough,

qVA absolute settlement at point A, dqVAB differential settlement

between A and B, dqmax maximum differential settlement, x tilt, UBC

rotation of segment BC, bBC = UBC-x angular distortion of

segment, aC angular distortion at point C

Fig. 6 Cracking of walls and

floors related to the surface

subsidence around Cincin


123

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12

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1.8

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?0

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?9

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14

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?9

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80

?9

60

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13

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SM

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Z0

(Eq

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td

epth

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122 Y. Mahmutoglu

123

were driven in saturated conditions as the groundwater

level was 3 m above the tunnel crown.

Subsidence versus time at each monitoring point on

cross line 1 (km 0 ? 907) is shown in Fig. 12a. In this area

the monitoring was not perpendicular to the tunnel but

there was angle of 34� between the tunnel axis and cross-

line 1. As seen from the figure, there are immediate

increases in the subsidence curves related to the face of the

tunnels, i.e. the two deflection points in the subsidence

curves are related to the passing of the shields.

Measured values of subsidence corresponding to total

vertical displacements were interpolated with theoretical

Gaussian curves (Fig. 12b), which represent both situa-

tions after the excavation of the RHST and LHST. The

relative positions of the tunnel centrelines are shown asFig. 7 Relationship between maximum subsidence on the centreline

of left hand side tunnel and Z0/D ratio in Esenler region

Fig. 8 The effect of single

tunnel excavation on subsidence

curve corresponding to two

different monitoring points and

face positions of tunnels

Fig. 9 The variation of surface

subsidence by time at a

monitoring point on RHST

centreline


123

dot-dash lines in the figure. It can be seen that there is a

significant increase in surface subsidence after the LHST

excavation, with the maximum subsidence occurring on

the centreline and the transverse trough shifting towards

the centreline of this tunnel. As a result, the point of

inflexion (KzL) corresponding to the fitting curve obtained

for the LHST moves away from centreline. The points of

inflexion representing the RHST and LHST (KzR and KzL,

respectively) are given in Table 4. Although the equiva-

lent depths of the tunnels (Rz0 and Lz0) are almost the

same and both tunnels were driven in similar ground

conditions, the points of inflexion are different: 5.41 m

for the first excavation and 12.06 m for second one. In

other words, the empirical trough factor (K) obtained for

the LHST is 2.23 times higher than that obtained for the

RHST in this location.

The Tavukcudere region

Excessive surface subsidence took place between km

4 ? 300 and 4 ? 500 in the Tavukcudere area, which is

located on a small stream bed filled by artificial material.

The shield positions and the distance between the tunnel

centrelines are almost the same as in the Esenler region.

The Z0/D varies from 2.97 to 3.20 and the overburden is

mainly composed of dense sand, clayey sand and a thin

man-made fill cover. The ground water conditions are

similar to those in the Esenler region.

Fig. 10 Idealized transverse

settlement trough

Fig. 11 The locations of monitoring points and the numbers and directions of cross lines according to tunnel axes

124 Y. Mahmutoglu

123

Ta

ble

4C

ross

lin

esal

on

gw

hic

hsu

bsi

den

cep

rofi

les

are

iden

tifi

edin

Fig

s.9

–1

2an

dth

ep

oin

to

fin

flex

ion

s(K

ZR

and

KZ

L)

and

mat

eria

lco

nst

ant

Kco

rres

po

nd

ing

toea

chst

age

of

tun

nel

exca

vat

ion

s

Cro

ssli

ne

nu

mb

erM

on

ito

rin

g

po

ints

on

Off

set

fro

mle

ft

tub

ece

ntr

elin

e(m

)

Aft

erR

HS

Tex

cav

atio

n

(ex

cav

atio

nb

ein

gah

ead

)

Aft

erL

HS

Tex

cav

atio

n

(ex

cav

atio

nb

ein

gb

ehin

d)

Kz L

/Kz R

Z0(m

)Z

0/D

SR

(mm

)K

Kz R

(m)

Z0(m

)Z

0/D

SL

(mm

)K

Kz L

(m)

1(k

m0

?9

07

)E

sen

ler

SM

P-2

61

.75

14

.65

2.2

5–

0.3

85

.41

14

.89

2.2

93

0.0

0.8

21

2.0

62

.23

SM

P-3

15

.13

–2

8.6

SM

P-3

27

.20

–2

7.5

SM

P-3

38

.96

6.0

24

.0

SM

P-3

41

1.9

87

.02

0.1

SM

P-3

51

5.6

09

.01

4.3

SM

P-3

61

5.0

01

0.0

18

.8

SM

P-3

71

4.6

01

0.0

13

.3

SM

P-3

81

6.1

21

4.0

11

.0

SM

P-3

91

6.3

5–

10

.0

SM

P-4

02

0.3

84

.07

.6

SM

P-4

12

4.1

83

.05

.0

2(k

m1

?8

52

)C

inci

nS

MP

-12

7-

18

.95

RH

ST

hav

eb

een

sto

pp

edin

this

reg

ion

18

.45

2.8

42

.00

.49

9.3

4–

SM

P-1

28

-5

.70

14

.0

BM

P-1

29

0.2

01

7.0

SM

P-1

30

8.1

01

3.0

SM

P-1

31

14

.80

9.0

SM

P-1

32

21

.40

4.0

BM

P-1

42

6.7

01

.0

BM

P-1

51

6.4

82

.0

BM

P-1

83

.95

18

.0

BM

P-2

0-

8.1

51

1.0

3a

(km

4?

42

5)

Tav

uk

cud

ere

BM

P-Y

SM

A1

04

-1

7.8

71

9.3

12

.97

0.0

0.5

19

.95

19

.90

3.0

61

4.0

0.8

61

7.3

11

.74

BM

P-Y

SM

A1

03

-8

.01

0.0

34

.0

BM

P-Y

SM

A1

00

-7

.23

1.0

39

.0

SM

P-2

57

2.2

11

1.0

41

.0

SM

P-2

58

7.8

92

0.0

32

.0

BM

P-Y

SM

83

12

.28

17

.03

5.0

BM

P-Y

SM

A1

01

14

.80

24

.04

2.0

BM

P-Y

SM

82

21

.35

14

.01

8.0

BM

P-Y

SM

81

22

.96

12

.01

7.0

BM

P-Y

SM

80

26

.24

12

.01

3.0


123

The same construction procedure was followed for the

twin tunnels. The subsidence curves corresponding to

monitoring points and transverse troughs along cross-lines

3a and 3b (Fig. 11) at km 4 ? 425 and 4 ? 460 are given

in Figs. 13 and 14. Similar theoretical curve fitting to the

measured data was undertaken. This indicated the Smax

value coinciding with the centreline of the left tube (LHST)

is higher than that in the Esenler region while the ratios

between points of inflexion (KzL/KzR) have values of 1.74

and 1.42 at the points noted above.

The Cincin region

As in Esenler and Tavukcudere, the main line tunnels were

at shallow depths in Cincin. The ratio of Z0/D is 2.84 and

the overburden consists of saturated sand and Gungoren

clay covered by a thin layer of man-made material. In this

part of the subway line, the right tube excavation was

stopped (mainly related to surface settlement over the

RHST) and only the left hand side tunnel excavation was

advanced. As a consequence, the surface subsidence is

lower than that measured for the twin tunnels and the data

recorded fit well with a normal distribution curve (Fig. 15).

The point of inflexion and K value for this single tunnel

excavation are included in Table 4.

Discussion

Peck (1969) recommended that if two tunnels are driven

adjacent to one another, the construction of the second

tunnel would generate significantly greater movements

because of the stress relief resulting from the excavation

of the first tunnel. It occurs as a result of disturbance in

the primary state of stresses and the soil movement (flow)

toward to the underground opening. Xu et al. (2003)

discussed soil disturbance during EPB tunnelling, high-

lighting the degree of stress disturbance and significant

decrease in the mechanical properties of the soil after

tunnelling.

Cording and Hansmire (1975) observed ground move-

ments occurring over twin tunnels during the Washington,

D.C. Metro project. They suggested that the asymmetric

trough after the second shield passed could be caused by

the interaction of the two tunnels. Suwansawat and Einstein

(2007) used a superposition technique to describe surface

settlement troughs over twin tunnels. Using extensive data

from the Bangkok Subway Tunnel project, they combined

settlement curves induced by the first and second shields

using the Gaussian function and proposed a total settlement

trough for twin tunnels. The trough width parameter (Kz)

obtained from the settlement curve is in agreement with

O’Reilly and New (1982) work when the tunnel is locatedTa

ble

4co

nti

nu

ed

Cro

ssli

ne

nu

mb

erM

on

ito

rin

g

po

ints

on

Off

set

fro

mle

ft

tub

ece

ntr

elin

e(m

)

Aft

erR

HS

Tex

cav

atio

n

(ex

cav

atio

nb

ein

gah

ead

)

Aft

erL

HS

Tex

cav

atio

n

(ex

cav

atio

nb

ein

gb

ehin

d)

Kz L

/Kz R

Z0(m

)Z

0/D

SR

(mm

)K

Kz R

(m)

Z0(m

)Z

0/D

SL

(mm

)K

Kz L

(m)

3b

(km

4?

46

0)

Tav

uk

cud

ere

BM

P-Y

SM

A1

06

-1

6.4

32

0.5

13

.15

0.0

0.5

51

1.1

42

0.8

03

.20

23

.00

.76

15

.80

1.4

2

BM

P-Y

SM

93

-5

.54

9.0

87

.0

SM

P-2

61

-0

.94

6.0

75

.0

BM

P-Y

SM

91

4.7

22

4.0

84

.0

BM

P-Y

SM

85

12

.04

37

.07

6.0

BM

P-Y

SM

90

20

.97

31

.04

2.0

BM

P-Y

SM

A1

08

26

.96

15

.01

8.0

BM

P-Y

SM

87

37

.06

6.0

8.0

126 Y. Mahmutoglu

123

within the sand layer. It was found that the material con-

stant (K) falls within the bound of 0.4 and 0.5, which is

similar to most cases of tunnelling in clay layers.

Interactions between twin tunnels and effects of the

relative positions of the shields were recently discussed

by Chehade and Sharour (2008). They used a finite ele-

ment solution and analysed a similar geometrical con-

figuration to that of the twin tunnels with a horizontal

alignment described in this study. Their results from the

analysis of horizontally aligned twin tunnels with a ratio

spacing Sx/D = 2 (Sx, D denote the distance between

tunnel axes and tunnel diameter, respectively) indicate

the maximum subsidence occurs between the two

tunnels.

Chehade and Sharour’s (2008) findings are not consis-

tent with the monitoring data obtained over a long period

between Esenler and Kirazli, which indicate that the

maximum settlement took placed on the centreline of the

LHST which was advanced behind the RHST and the

transverse troughs obtained from interpolation of moni-

toring data are quite similar along cross lines 1, 3a and 3b.

It should be noted, however, that the point of inflexion is

Fig. 12 Surface subsidence

versus time a at monitoring

points and transverse troughs

b after tunnel excavations in

Esenler region


123

not at the same distance from the centrelines of both tun-

nels and the trough factor K (Glossop 1979; O’Reilly and

New 1982) varies after the second tunnel excavation.

While it falls within the ranges proposed by O’Reilly and

New (1982) in the case of a single tunnel, it takes higher

values after excavation of the second tunnel. Thus the point

of inflexion Kz moves away from the centreline of the first

tunnel and the maximum subsidence occurs on the centr-

eline of the second tunnel (Figs. 12, 13, 14). In other

words, the geo-material properties have been adversely

affected by the advance excavation and as a result of the

pre-disturbance in the geo-environment there is a signifi-

cant enlargement in the area affected by the excavation of

the second tunnel.

In view of the above, it is clear that the disturbance in

the geo-environment as a result of the first step of tunnel

excavation should be taken into consideration when esti-

mating surface settlement and construction procedures for




b along cross line 3a after tunnel

excavations in Tavukcudere

region (km 4 ? 425)

128 Y. Mahmutoglu

123

twin tunnels in order to minimize subsidence over shallow

tunnels in a soft ground environment.

Conclusions

The paper reports the data obtained from surface moni-

toring along the Esenler–Kirazli subway line where twin

tunnels were driven through soft ground conditions.

Although the measured values of subsidence support a

theoretical normal distribution curve in the case of a single

tunnel, the results are not in good agreement with the

recent finite element solutions for twin tunnelling in similar

conditions. The monitoring data showed that the maximum

subsidence associated with the twin tunnelling coincides

with the centreline of the second tunnel.

It is concluded that there is a significant increase in

surface settlement after the excavation of a second tunnel




b along cross line 3b after

tunnel excavations in

Tavukcudere region

(km 4 ? 460)


123

advancing behind the first. In this study the empirical geo-

material constant K changed after the passing of the first

tunnel and does not fall within the limits proposed in

previous work. In the studied section, the point of inflexion

(Kz) moved away from the centreline of the first tunnel and

the maximum subsidence occurred on the centreline of the

second tunnel.

From this study it is considered that the geo-environ-

mental properties were adversely affected by the excava-

tion of the first tunnel, such that the second tunnel,

advancing behind, was driven in extremely weak (dis-

turbed) geo-mechanical conditions, i.e. some grain sepa-

ration will have taken place and pore water distribution will

have been affected. As a consequence, there was a signif-

icant enlargement in the overall area affected by the twin-

tunnelling.

Acknowledgments The author would like to express his deep

gratitude to Prof. Erdogan Yuzer and Prof. Nuh Bilgin for their

valuable comments on this manuscript, and to the authorities of the

Gulermak-Dogus Joint Venture for providing the monitoring data.

Fig. 15 Measured surface

subsidence versus time a at

monitoring points and the

transverse trough b along cross

line 2 after single tunnel

excavation in Cincin region (km

1 ? 852)

130 Y. Mahmutoglu

123

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