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Goline 2005 Lyon, France 23rd

- 25th, May/Mai 2005

Assessment of groundwater inflow into a metro tunnel (Ankara)

DOYURAN VEDATDepartment of Geological Engineering, Middle East Technical University, 06531Ankara, [email protected]

AbstractThe metro tunnel between Ulus and Kecioren (Ankara) is about 9685 m long. Approximately 42% ofthe tunnel alignment consists of volcanic series and 58% sedimentary units. The volcanic series iscomposed of a chaotic mixture of andesites, dacites, agglomerates, and tuffs (42%). The sedimentaryunits include Hancili formation (marl, siltstone, sandstone, conglomerate, and tuffite alternation : 2%),Ankara clay (20%), and alluvial deposits (36%).

Along the tunnel alignment a total of 43 geotechnical boreholes have been drilled. In each borehole

permeability tests, constant head and/or water pressure tests, have been conducted and theboreholes are equipped with perforated PVC pipes for groundwater level monitoring. The constanthead tests have revealed that the hydraulic conductivity of the alluvium ranges between 1.5x10

-7

m/sec and 6.4x10-4

m/sec. Ankara clay is found to be practically impervious. The water pressure testsperformed within the boreholes tapping volcanic series revealed 1.17 and >25 Lugeon units.

The groundwater inflows into tunnel and cut-and-cover sections are estimated for steady andunsteady flow conditions. The estimated flow rates are then compared with the inflows measuredduring tunnel construction.

Keywords: metro tunnel, groundwater inflow, steady-state flow, transient flow

Introduction

Estimating groundwater inflow into tunnels is a difficult art, even if done carefully. The difficulties arisefrom several sources. The geology of the site may not be adequately understood. This is generally thecase for metro tunnels in urban areas where the surface may entirely be covered by buildings andpaved roads. The equations governing groundwater flow may not adequately represent the conditions.Particularly in fractured rock aquifers the uncertainties are more than those of porous media. Thecollected hydrogeological data may have limitations that are not accounted for. Due to densesettlement and heavy traffic of the urban environment subsurface investigations, both geotechnicaland hydrogeological, are rather limited. In areas with complex geology, widely spaced boreholes canonly provide general information about subsurface conditions.

This paper presents methods for evaluating hydrogeological data and estimating groundwater inflowinto Ulus-Kecioren metro tunnel located in the densely populated part of Ankara, Turkey. The totallength of double tube metro tunnel is 9685 m, has an excavated diameter of 6.70 m, and a maximum

depth of about 35 m below the ground surface. About 27% of the alignment will be constructed as cut-and-cover and the rest as tunnel.

GeologyDue to dense settlement the geology of the tunnel alignment is entirely based on borehole data. Somelocal rock exposures are also studied for the evaluation of rock mass characteristics.

The bedrock of the project area consists of volcanic series and various sedimentary units. Thevolcanic series comprise a chaotic mixture of andesites, dacites, agglomerates and tuffs. Theyconstitute the bedrock along 4035 m segment (42%) of the total alignment. The sedimentary unitscomprise Hancili formation represented by limestone, marl, siltstone, sandstone, conglomerate, andtuffite alternations (2%), Ankara clay consisting of silty clay/clayey silt with occasional sand and gravelbands and lenses (20%), and alluvium (36%).

Hydrogeology
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The positions of the groundwater table throughout the site and the hydraulic conductivities of thefoundation rocks have been determined from the exploratory boreholes. Along the tunnel alignment 43boreholes totaling 1170 m have been drilled. In boreholes penetrating soft sedimentary rocks (Ankaraclay and alluvium) a total of 73 constant head permeability tests have been performed. In the volcanicseries and Hancili formation 41 water pressure tests have been conducted. The boreholes are thanequipped with perforated PVC pipes for groundwater level measurements.

The alluvial deposits are composed of clay, silty clay, gravelly clay, clayey silty sand, and sandygravel. Distribution of hydraulic conductivity within alluvial deposits is given in Figure 1. From thefigure it is seen that the average hydraulic conductivity of the alluvium is 3.3x10

-6m/sec.

0

5

10

15

20

25

30

35

40

45

50

10E-8 10E-7 10E-6 10E-5 10E-4

Hydraulic Conductivity (m/sec)

Percent

Figure 1. Distribution of hydraulic conductivity within alluvium.

In Ankara clay a total of 22 constant head permeability tests have been performed and the test resultshave shown that Ankara clay is practically impervious. In fact in the previously opened metro tunnelselsewhere in Ankara within Ankara clay deposits no significant groundwater occurrences had beenreported.

The results of 39 water pressure tests performed within the volcanic series have yielded a wide rangeof Lugeon values ranging between 1.17 Lugeon and >25 Lugeon. Figure 2 depicts Rock QualityDesignation (RQD)-Lugeon relationships. As it is seen there is almost no relationship between the twoparameter. This may be attributed to the heterogeneity and anisotropy of the fractured rocks and alsoto the limitation of the RQD concept. Although RQD indicates the degree of fracturing of the rock massit does not, however, take aperture, infillings, persistence, etc into consideration. Figure 2, however,shows that the Lugeon values are grouped between 1.17 and 10.25 Lugeons and at 25 Lugeons.Thus, a value of 4x10

-7m/sec is assigned as an average hydraulic conductivity of the jointed rocks

and a value of 10-5

m/sec for the highly jointed and/or sheared zones.

0

20

40

60

80

100

120

0 5 10 15 20 25 30

LUGEON

RQD

Figure 2.RQD-Lugeon relationship of the volcanic series

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The water level measurements are taken on a monthly basis from each observation well. The watertable roughly follows the topography and it fluctuates within 2 m to 10 m below the surface. Within thevolcanic series the hydrostatic pressure at the invert level of the tunnel ranges between 3.5 bar and1 bar. In the alluvium, however, the hydrostatic pressure is generally less than 2 bar.With the beginning of the tunnel construction the water levels within the observation wells located inclose vicinity have undergone rapid drawdown (Figure 3). Wells UK-32 and UK-33 became dry due to

tunnel drainage.

880,00

890,00

900,00

910,00

920,00

930,00

940,00

950,00

10.10.2003

29.11.2003

18.01.2004

08.03.2004

27.04.2004

16.06.2004

05.08.2004

24.09.2004

13.11.2004

02.01.2005

GroundwaterLevel(m)

UK-32

UK-30

UK-33

UK-34

Figure 3.Fluctuation of groundwater level in wells affected from construction works.

Groundwater Inflow Into Tunnels Under Steady-State or Transient ConditionsA tunnel normally acts as a drain. In tunnel drainage steady-state approach is applicable as long asthe water table is not drawn down by the existence of the tunnel. However, for rock formations withlow porosity and low specific storage, it is unlikely that steady-state conditions could be maintained. Itis more likely that a transient flow system will develop with declining water tables above the tunnel. Inthat case the initial steady-state inflow rate Qoper unit length of tunnel will decrease as a function of

time (Freeze and Cherry, 1979).

The only theoretical analyses that can be found in the literature for the prediction of groundwaterinflows into tunnels are those of Goodman et al. (1965). They show that for the case of a tunnel ofradius (r) acting as a steady-state drain in a homogeneous isotropic media with hydraulic conductivity(K), the rate of groundwater inflow (Qo) per unit length of tunnel is given by:

Qo= (2KHo)/2.3 log(2Ho/r) .........................................(1)

Under transient conditions the inflow Q(t) per unit length of tunnel at any time (t) after the breakdownof steady flow to be given by:

Q(t)= {(8C/3)KHo3Syt}

..............................................(2)

where: K is the hydraulic conductivity of the medium; S y is the specific yield; and C is an arbitraryconstant. Goodman et al. (1965) suggested a value of 0.75 for the constant C. Heuer (1995) based onhis observations in hard and fractured rock tunnels, suggested that groundwater inflow into tunnels isonly 1/8 of those found from Goodman equation and introduced Heuer reduction factor:

Qo= (2KHo)/2.3 log(2Ho/r)(1/8) ................................. (3)

Lei (1999) proposed the following equation for groundwater inflow in to a tunnel:

q=(2Kh)/ln{(h/r)+(h/r)2-1}....................................... (4)

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where: q=leakage(m3/m/sec); K=hydraulic conductivity (m/sec); h=distance from tunnel to

equipotential (m); and r=tunnel radius (m). Karlsrud (2001) proposed the following equation to predict

groundwater inflow into a tunnel:

q= (2Kh)/ln{2(h/r)-1} ................................................ (5)

Groundwater Inflows Into ExcavationsPrediction of groundwater inflow into cut-and-cover sections of the metro alignment can beaccomplished through the method proposed by Ibrahim and Brutsaert (1965). The method is based onthe following assumptions: (1) the excavation face is vertical; (2) the excavation is emplacedinstantaneously; (3) the geological stratum is homogeneous and isotropic; (4) the excavation is longand lineal in shape, rather than circular, so that the two-dimensional Cartesian symmetry is applicable.In spite of these restrictive assumptions, results are proven to be quite satisfactory for the estimationof transient response of more complex aquifer system. Ibrahim and Brutsaert (1965) introduced a

dimensionless time () and dimensionless discharge given by:

= (KH/SyL2)t ....................................... (6)

=(SyL/KH2)q ....................................... (7)

where H is the initial saturated thickness of the aquifer; L is the radius of influence; K and S yare thehydraulic conductivity and specific yield of the aquifer; and t is time. The outflow q=q(t) is the rate offlow (with dimensions L

3/T) into the excavation from seepage face, per unit length of excavated face.

Prediction of Groundwater Inflows Into Cut-and-Cover Sections within AlluviumBased on the in-situ permeability test results the average hydraulic conductivity of the alluviums isestimated as 3.3x10

-6m/sec. In this section the hydraulic heads range between 2m and 20m. The

radius of influence (L) is not known. Thus, in the analyses different L values ranging between 50 mand 150 m are adopted. The result of analyses is given in Figure 4.

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

H (m)

Q

(m3/day/m) L=150 m

L=125 m

L=100 m

L=75 m

L=50 m

Figure 4.Groundwater inflow into excavations under different

hydraulic heads and radius of influences.

From Figure 4 it is seen that even under most unfavorable conditions the maximum groundwaterseepage into excavation is about 350 m

3/day/m. This rate will gradually decrease as hydraulic head

decreases (Figure 5).

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0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Days

Discha

rge(m3/day/m)

Figure 5.Groundwater inflow versus time (Ho=20 m; L=150 m)

Groundwater Inflows Into Tunnels within Volcanic SeriesDue to heterogeneous nature of the volcanic series it is difficult to assign a realistic hydraulicconductivity value. Thus, based on two clusters of water pressure test results hydraulic conductivitiesof 4x10

-7m/sec and 10

-5m/sec are adopted for the rock mass showing varying fracture systems.

Table 1 summarizes the results of groundwater inflow into tunnels under various hydraulic heads. Thesteady-state solutions yield almost similar results excepting Heuers (1995) approach, which is highlyconservative. In transient flow case, however, the inflows show more than four-fold increase and thedischarges rapidly decrease with decreasing hydraulic heads. Groundwater inflows into tunnels understeady-state and transient flow conditions for K=4x10

-7m s

-1are given in Figures 6 and 7.

Table 1.Groundwater inflow from the volcanic series under different hydraulic heads (K=4x10-7

m/sec).

Q (m3/day/m)

Ho(m) Goodman,v.d(1965)

(*)Heuer (1995) Lei (1999) Karlsrud (2001) Goodman,v.d(1965)

(**)

30,00 2,260263 0,282533 2,260179 2,303594 9,659814

27,50 2,136331 0,267041 2,136781 2,182955 8,477854

25,00 2,010598 0,251325 2,011705 2,061224 7,348469

22,50 1,882932 0,235366 1,884873 1,938549 6,274233

20,00 1,75321 0,219151 1,756262 1,815266 5,258137

17,50 1,621361 0,20267 1,625978 1,692095 4,303719

15,00 1,48746 0,185932 1,494454 1,570623 3,41526

12,50 1,351991 0,168999 1,362991 1,454593 2,598076

10,00 1,216669 0,152084 1,235575 1,354248 1,859032

7,50 1,087615 0,135952 1,127057 1,306701 1,207477(*)

Steady-state flow;(**)

Transient flow

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30 35

Hydraulic Head (m)

Discharge(m3/da

y/m)

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35

Hydraulic Head (m)

Discharge(m3/day

/m)

Figure 6. Steady-state drain from volcanic series (K=4x10-7

m/sec)

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Figure 7. Transient drain from volcanic series (K=4x10-7

m/sec)

0

2

46

8

10

12

0 5 10 15 20 25 30 35

Hydraulic Head (m)

Discharge(m3/day/m)

Table 2 summarizes the groundwater inflow into tunnels from regions of greater hydraulic conductivity(K=10

-5m/sec). It is seen that the inflow rates show considerable increase. However, the flow rates

are still manageable with conventional sump-and-pump method. The flow rates given in Table 1 andTable 2 suggest that in fractured rocks where the hydraulic conductivities show wide ranges, the inflowat one place in a tunnel may be several fold greater than the inflow at other place, a commonlyobserved phenomenon in hard-rock tunnels.

Table 2.Groundwater inflow from the volcanic series under different hydraulic heads (K=10-5

m/sec).

Q (m3/ day /m)

HO(m) Goodman,v.d(1965)

(*)Heuer (1995)

Lei(1999) Karlsrud (2001) Goodman,v.d(1965)

(**)

30 56,50659 7,063323 56,50448 57,58985 48,29907

27,50 53,40828 6,676035 53,41954 54,57388 42,38927

25 50,26496 6,28312 50,29262 51,5306 36,74235

22,50 47,0733 5,884162 47,12183 48,46373 31,37117

20 43,83025 5,478781 43,90655 45,38164 26,2906817,50 40,53403 5,066753 40,64946 42,30239 21,5186

15 37,18649 4,648312 37,36134 39,26558 17,0763

12,50 33,79977 4,224971 34,07477 36,36482 12,99038

10 30,41672 3,80209 30,88939 33,8562 9,29516

7,50 27,19037 3,398796 28,17643 32,66753 6,037384(*) Steady-state flow; (**) Transient flow

Figures 8 and 9 depict groundwater inflows under steady-state and transient conditions from volcanicseries having K=10

-5m/sec. Here, it is noteworthy to mention that the steady-state case yields slightly

greater inflows than those of transient state.

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Hydraulic Head (m)

Discharge

(m

3/day/m)

Figure 8.Steady-state drain from volcanic series (K=10-5

m/sec)

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0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Hydraulic Head (m)

Discharg

e(m3/day/m)

Figure 9. Transient drain from volcanic series (K=10

-5m/sec)

DiscussionsAlluviums and volcanic series forming the bedrock along Ulus-Kecioren metro alignment constitutepoor-to-medium aquifers having hydraulic conductivities ranging between 4x10

-7 m/sec and 10

-5

m/sec. The Ankara clay is practically impervious and no significant groundwater inflows had been

reported from the metro tunnels opened elsewhere in this formation.

Although the excavations in the sedimentary sequence had not yet been started, one would normallyexpect uniform inflows through these porous media. Where sand and gravel dominant layers and/orlenses are encountered within the alluvium, significant increase of inflows should be expected.

The volcanic series form a fractured rock aquifer characterized by high heterogeneity and anisotropy.Thus, the hydraulic conductivity of the fractured rock may not be adequately characterized. The rangeof permeability of the rock mass may be even higher than that determined from the water pressuretests. Normally the longer and more open fractures will capture most of the flow and channelize ittoward the tunnel (Raymer (2001). This will result in non-radial flow paths.

In hard-rock tunnels, most of the inflow comes from a few places, some of the inflow comes from

many places, and much of the tunnel is dry. During initial stages of excavation water is removed fromstorage at the immediate vicinity, where the hydraulic gradient is also high. Therefore, initial inflowrates may be about 30% larger then computed values. As time elapses the steady-state condition willbe reached and the inflow will take place at a constant rate.

The groundwater inflow into the tunnel takes place in the form of seeps and leakages (Figure 10)located in a random fashion. Due to chaotic mixture of the rocks of volcanic series and randomdistribution of discontinuities the location of discharge points could not be predicted. However,measured discharge rates within the tunnel agree very well with the calculated values. The measureddischarges of individual seepages and leakages from volcanic rocks generally lie within 1.5 l/sec and0.03 l/sec.

(a) (b)

Figure 10.Groundwater inflow into the tunnel at (a) Km: 7+970 and (b) 7+935

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ReferencesFreeze, R.A., Cherry, J.A., 1979. Groundwater. Prentice-Hall, Inc., New Jersey, 487-491.Goodman, R.E., Moye, D.G., van Schalkwyk, A., Javandel, I., 1965. Ground water inflows duringtunnel driving. Engineering Geology 2, 39-56.Heuer, R.E., 1995. Estimating rock-tunnel water inflow. Proceeding of the Rapid Excavation and

Tunneling Conference, June 18-21, 41p.Ibrahim, H.A., Brutsaert, W., 1965. Inflow hydrographs from large unconfined aquifers. Journal ofIrrigation and Drainage Division, Proceeding of American Society of Civil Engineers 91,(IR2),21-38.Karlsrud, K., 2001. Control of water leakage when tunneling under urban areas in the Oslo Region.Norwegian Tunneling Society Publication 12, 27-33.Lei, S., 1999. An analytical solution for steady state flow into a tunnel. Ground Water 37, 1, 23-26.Raymer, J.H., 2001. Groundwater inflow into hard-rock tunnels: Tunnels and Tunneling International,50-53.

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Geoline2005 s7 Pp 1-8 Vedat