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)
Surface subsidence over twin tunnels 119
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
Surface subsidence over twin tunnels 121
123
Ta
ble
3M
easu
red
surf
ace
sett
lem
ent
atth
em
on
ito
rin
gp
oin
tso
ntu
nn
elce
ntr
elin
esan
dth
eZ
0/D
rati
os
atre
late
dk
ilo
met
res
Lo
cati
on
san
dk
mO
nth
ece
ntr
elin
eo
fri
gh
th
and
sid
etu
nn
el(R
HS
T)
On
the
cen
trel
ine
of
left
han
dsi
de
tun
nel
(LH
ST
)In
terv
al
(km
)
LS
max/R
Sm
ax
Mo
nit
ori
ng
po
ints
km
Rz 0
(m)
RS
max
(mm
)
Rz 0
/DM
on
ito
rin
g
po
ints
km
Lz 0
(m)
LS
max
(mm
)L
Z0/D
Ese
nle
rk
m0
?8
42
–k
m
1?
17
5
SM
P-1
10
?8
42
13
.42
35
2.0
6S
MP
-20
0?
85
11
2.2
65
11
.88
0?
84
7–
0?
85
12
.55
SM
P-1
20
?8
47
11
.92
20
1.8
3S
MP
-21
0?
85
61
2.3
05
51
.89
0?
86
7–
0?
86
91
.46
SM
P-2
80
?8
69
11
.78
39
1.8
1S
MP
-22
0?
86
11
1.6
66
21
.79
0?
87
5–
0?
87
81
.58
SM
P-2
90
?8
75
11
.87
28
1.8
3S
MP
-23
0?
86
71
2.0
45
71
.85
0?
90
3–
0?
90
53
.22
SM
P-3
00
?8
78
12
.21
29
1.8
8S
MP
-24
0?
87
51
1.8
64
61
.82
0?
94
7–
0?
96
03
.20
SM
P-3
50
?9
03
12
.74
91
.96
SM
P-2
50
?8
80
12
.10
28
1.8
61
?0
75
–1
?0
77
4.0
0
BM
P-2
70
?9
28
14
.16
72
.18
SM
P-2
68
?8
86
12
.15
30
1.8
7–
SM
P-4
90
?9
50
15
.54
62
.39
BM
P-1
90
?9
05
12
.18
29
1.8
7–
BM
P-5
80
?9
60
15
.70
52
.41
SM
P-4
80
?9
34
13
.64
21
2.1
0–
SM
P-5
91
?0
16
22
.47
83
.46
SM
P-5
00
?9
47
15
.48
16
2.3
8
SM
P-7
11
?0
75
29
.35
14
.51
SM
P-5
71
?0
20
22
.06
83
.39
–
SM
P-7
61
?1
26
35
.29
05
.42
SM
P-6
61
?0
77
29
.09
44
.47
–
SM
P-8
11
?1
75
40
.60
06
.26
SM
P-7
41
?1
20
34
.98
05
.38
–
Cin
cin
km
1?
83
9–
km
2?
04
2
RH
ST
hav
eb
een
sto
pp
edin
this
reg
ion
SM
P-1
29
1?
85
81
8.2
51
72
.81
–
SM
P-1
35
1?
89
11
7.2
91
42
.66
–
BM
P-2
71
?9
54
15
.55
82
.39
–
SM
P-1
41
1?
95
71
5.3
47
2.6
5–
BM
PIN
-06
2?
02
01
4.3
11
12
.43
–
SM
P-1
47
2?
04
21
4.5
58
2.2
4–
Tav
uk
cud
ere
km
4
?2
18
–k
m4
?5
03
SM
P-2
36
4?
16
22
6.9
51
14
.15
SM
P-2
40
4?
21
82
0.5
31
43
.16
4?
21
8–
4?
22
12
.14
SM
P-2
42
4?
22
12
0.3
51
43
.13
BM
P-Y
SM
10
94
?2
68
20
.16
30
3.1
04
?3
08
1.6
4
SM
P-2
43
4?
25
41
8.5
51
53
.16
SM
P-2
46
A4
?3
08
16
.94
18
2.6
14
?4
52
–4
?4
55
3.0
4
BM
P-Y
SM
13
4?
27
81
7.5
31
62
.69
SM
P-2
46
D4
?3
13
17
.51
18
2.6
9–
SM
P-2
46
4?
30
81
6.9
51
12
.61
BM
P-Y
SM
79
4?
40
31
9.7
64
83
.03
–
BM
P-Y
SM
16
4?
29
71
7.1
51
72
.63
SM
P-2
57
4?
42
01
9.4
25
52
.98
–
BM
P-Y
SM
36
4?
32
61
6.3
51
72
.51
SM
P-2
61
4?
45
22
0.3
47
33
.13
BM
P-Y
SM
63
4?
37
81
8.1
23
22
.78
BM
P-Y
SM
A1
07
4?
47
12
0.5
31
09
3.1
6–
SM
P-2
54
4?
39
31
8.9
57
2.9
1S
MP
-26
44
?5
03
22
.03
13
3.3
9–
BM
P-Y
SM
A1
01
4?
45
52
0.3
02
43
.12
–
Z0
(Eq
uiv
alen
td
epth
of
tun
nel
)=
Z?
D/2
Zd
epth
of
tun
nel
cro
wn
,D
tun
nel
dia
met
er
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
Surface subsidence over twin tunnels 123
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
Surface subsidence over twin tunnels 125
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
Surface subsidence over twin tunnels 127
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
Fig. 13 Surface subsidence
versus time a at monitoring
points and transverse troughs
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
Fig. 14 Surface subsidence
versus time a at monitoring
points and transverse troughs
b along cross line 3b after
tunnel excavations in
Tavukcudere region
(km 4 ? 460)
Surface subsidence over twin tunnels 129
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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