Assessment of groundwater inflow into a metro tunnel (Ankara)

Transcription

1 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 Assessment of groundwater inflow into a metro tunnel (Ankara) DOYURAN VEDAT Department of Geological Engineering, Middle East Technical University, 6531Ankara, Turkey vedat@metu.edu.tr Abstract The metro tunnel between Ulus and Kecioren (Ankara) is about 9685 m long. Approximately 42% of the tunnel alignment consists of volcanic series and 58% sedimentary units. The volcanic series is composed of a chaotic mixture of andesites, dacites, agglomerates, and tuffs (42%). The sedimentary units include Hancili formation (marl, siltstone, sandstone, conglomerate, and tuffite alternation : 2%), Ankara clay (2%), 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 the boreholes are equipped with perforated PVC pipes for groundwater level monitoring. The constant head tests have revealed that the hydraulic conductivity of the alluvium ranges between 1.5x1-7 m/sec and 6.4x1-4 m/sec. Ankara clay is found to be practically impervious. The water pressure tests performed 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 and unsteady flow conditions. The estimated flow rates are then compared with the inflows measured during 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 arise from several sources. The geology of the site may not be adequately understood. This is generally the case for metro tunnels in urban areas where the surface may entirely be covered by buildings and paved 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. The collected hydrogeological data may have limitations that are not accounted for. Due to dense settlement and heavy traffic of the urban environment subsurface investigations, both geotechnical and hydrogeological, are rather limited. In areas with complex geology, widely spaced boreholes can only provide general information about subsurface conditions. This paper presents methods for evaluating hydrogeological data and estimating groundwater inflow into Ulus-Kecioren metro tunnel located in the densely populated part of Ankara, Turkey. The total length of double tube metro tunnel is 9685 m, has an excavated diameter of 6.7 m, and a maximum depth of about 35 m below the ground surface. About 27% of the alignment will be constructed as cutand-cover and the rest as tunnel. Geology Due to dense settlement the geology of the tunnel alignment is entirely based on borehole data. Some local 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. The volcanic series comprise a chaotic mixture of andesites, dacites, agglomerates and tuffs. They constitute the bedrock along 435 m segment (42%) of the total alignment. The sedimentary units comprise Hancili formation represented by limestone, marl, siltstone, sandstone, conglomerate, and tuffite alternations (2%), Ankara clay consisting of silty clay/clayey silt with occasional sand and gravel bands and lenses (2%), and alluvium (36%). Hydrogeology

2 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 The positions of the groundwater table throughout the site and the hydraulic conductivities of the foundation rocks have been determined from the exploratory boreholes. Along the tunnel alignment 43 boreholes totaling 117 m have been drilled. In boreholes penetrating soft sedimentary rocks (Ankara clay and alluvium) a total of 73 constant head permeability tests have been performed. In the volcanic series and Hancili formation 41 water pressure tests have been conducted. The boreholes are than equipped with perforated PVC pipes for groundwater level measurements. The alluvial deposits are composed of clay, silty clay, gravelly clay, clayey silty sand, and sandy gravel. Distribution of hydraulic conductivity within alluvial deposits is given in Figure 1. From the figure it is seen that the average hydraulic conductivity of the alluvium is 3.3x1-6 m/sec. Percent E-8 1E-7 1E-6 1E-5 1E-4 Hydraulic Conductivity (m/sec) 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 results have shown that Ankara clay is practically impervious. In fact in the previously opened metro tunnels elsewhere in Ankara within Ankara clay deposits no significant groundwater occurrences had been reported. The results of 39 water pressure tests performed within the volcanic series have yielded a wide range of Lugeon values ranging between 1.17 Lugeon and >25 Lugeon. Figure 2 depicts Rock Quality Designation (RQD)-Lugeon relationships. As it is seen there is almost no relationship between the two parameter. This may be attributed to the heterogeneity and anisotropy of the fractured rocks and also to the limitation of the RQD concept. Although RQD indicates the degree of fracturing of the rock mass it does not, however, take aperture, infillings, persistence, etc into consideration. Figure 2, however, shows that the Lugeon values are grouped between 1.17 and 1.25 Lugeons and at 25 Lugeons. Thus, a value of 4x1-7 m/sec is assigned as an average hydraulic conductivity of the jointed rocks and a value of 1-5 m/sec for the highly jointed and/or sheared zones RQD LUGEON Figure 2. RQD-Lugeon relationship of the volcanic series 2

3 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 The water level measurements are taken on a monthly basis from each observation well. The water table roughly follows the topography and it fluctuates within 2 m to 1 m below the surface. Within the volcanic series the hydrostatic pressure at the invert level of the tunnel ranges between 3.5 bar and 1 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 in close vicinity have undergone rapid drawdown (Figure 3). Wells UK-32 and UK-33 became dry due to tunnel drainage. 95, Groundwater Level (m) 94, 93, 92, 91, 9, 89, 88, UK-32 UK-3 UK-33 UK Figure 3. Fluctuation of groundwater level in wells affected from construction works. Groundwater Inflow Into Tunnels Under Steady-State or Transient Conditions A tunnel normally acts as a drain. In tunnel drainage steady-state approach is applicable as long as the water table is not drawn down by the existence of the tunnel. However, for rock formations with low porosity and low specific storage, it is unlikely that steady-state conditions could be maintained. It is more likely that a transient flow system will develop with declining water tables above the tunnel. In that case the initial steady-state inflow rate Q o per 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 groundwater inflows into tunnels are those of Goodman et al. (1965). They show that for the case of a tunnel of radius (r) acting as a steady-state drain in a homogeneous isotropic media with hydraulic conductivity (K), the rate of groundwater inflow (Q o ) per unit length of tunnel is given by: Q o = (2πKH o )/2.3 log(2h o /r)...(1) Under transient conditions the inflow Q(t) per unit length of tunnel at any time (t) after the breakdown of steady flow to be given by: Q(t)= {(8C/3)KH o 3 S y t} ½...(2) where: K is the hydraulic conductivity of the medium; S y is the specific yield; and C is an arbitrary constant. Goodman et al. (1965) suggested a value of.75 for the constant C. Heuer (1995) based on his observations in hard and fractured rock tunnels, suggested that groundwater inflow into tunnels is only 1/8 of those found from Goodman equation and introduced Heuer reduction factor: Q o = (2πKH o )/2.3 log(2h o /r)(1/8)... (3) Lei (1999) proposed the following equation for groundwater inflow in to a tunnel: q=(2πkh)/ln{(h/r)+ (h/r) 2-1}... (4) 3

4 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 where: q=leakage(m 3 /m/sec); K=hydraulic conductivity (m/sec); h=distance from tunnel to equipotential (m); and r=tunnel radius (m). Karlsrud (21) proposed the following equation to predict groundwater inflow into a tunnel: q= (2πKh)/ln{2(h/r)-1}... (5) Groundwater Inflows Into Excavations Prediction of groundwater inflow into cut-and-cover sections of the metro alignment can be accomplished through the method proposed by Ibrahim and Brutsaert (1965). The method is based on the following assumptions: (1) the excavation face is vertical; (2) the excavation is emplaced instantaneously; (3) the geological stratum is homogeneous and isotropic; (4) the excavation is long and 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 estimation of transient response of more complex aquifer system. Ibrahim and Brutsaert (1965) introduced a dimensionless time (τ) and dimensionless discharge given by: τ = (KH/S y L 2 )t... (6) γ =(S y L/KH 2 )q... (7) where H is the initial saturated thickness of the aquifer; L is the radius of influence; K and S y are the hydraulic conductivity and specific yield of the aquifer; and t is time. The outflow q=q(t) is the rate of flow (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 Alluvium Based on the in-situ permeability test results the average hydraulic conductivity of the alluviums is estimated as 3.3x1-6 m/sec. In this section the hydraulic heads range between 2m and 2m. The radius of influence (L) is not known. Thus, in the analyses different L values ranging between 5 m and 15 m are adopted. The result of analyses is given in Figure 4. Q (m3/day/m) H (m) L=15 m L=125 m L=1 m L=75 m L=5 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 groundwater seepage into excavation is about 35 m 3 /day/m. This rate will gradually decrease as hydraulic head decreases (Figure 5). 4

5 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai Figure 5. Groundwater inflow versus time (H o =2 m; L=15 m) Groundwater Inflows Into Tunnels within Volcanic Series Due to heterogeneous nature of the volcanic series it is difficult to assign a realistic hydraulic conductivity value. Thus, based on two clusters of water pressure test results hydraulic conductivities of 4x1-7 m/sec and 1-5 m/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. The steady-state solutions yield almost similar results excepting Heuer s (1995) approach, which is highly conservative. In transient flow case, however, the inflows show more than four-fold increase and the discharges rapidly decrease with decreasing hydraulic heads. Groundwater inflows into tunnels under steady-state and transient flow conditions for K=4x1-7 m s -1 are given in Figures 6 and 7. Table 1. Groundwater inflow from the volcanic series under different hydraulic heads (K=4x1-7 m/sec). H o Q (m 3 /day/m) (m) Goodman,v.d(1965) (*) Heuer (1995) Lei (1999) Karlsrud (21) Goodman,v.d(1965) (**) 3, 2,26263, , , , ,5 2,136331, , , , , 2,1598, ,1175 2, , ,5 1,882932, , , , , 1,75321, , , , ,5 1,621361,2267 1, , , , 1,48746, , , , ,5 1,351991, , , , , 1,216669, , , , ,5 1,87615, , ,3671 1,27477 (*) Steady-state flow; (**) Transient flow Days Hydraulic Head (m) Figure 6. Steady-state drain from volcanic series (K=4x1-7 m/sec) 5

6 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 Figure 7. Transient drain from volcanic series (K=4x1-7 m/sec) Hydraulic Head (m) Table 2 summarizes the groundwater inflow into tunnels from regions of greater hydraulic conductivity (K=1-5 m/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 and Table 2 suggest that in fractured rocks where the hydraulic conductivities show wide ranges, the inflow at one place in a tunnel may be several fold greater than the inflow at other place, a commonly observed phenomenon in hard-rock tunnels. Table 2. Groundwater inflow from the volcanic series under different hydraulic heads (K=1-5 m/sec). Q (m 3 / day /m) H O (m) Goodman,v.d(1965) (*) Heuer (1995) Lei (1999) Karlsrud (21) Goodman,v.d(1965) (**) 3 56,5659 7, , , , ,5 53,4828 6, , , , , , , ,536 36, ,5 47,733 5, , , , ,8325 5, , , , ,5 4,5343 5, , , , , , , , ,763 12,5 33, , , , , , ,829 3, ,8562 9, ,5 27,1937 3, , , ,37384 (*) Steady-state flow; (**) Transient flow Figures 8 and 9 depict groundwater inflows under steady-state and transient conditions from volcanic series having K=1-5 m/sec. Here, it is noteworthy to mention that the steady-state case yields slightly greater inflows than those of transient state Hydraulic Head (m) Figure 8. Steady-state drain from volcanic series (K=1-5 m/sec) 6

7 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai Hydraulic Head (m) Figure 9. Transient drain from volcanic series (K=1-5 m/sec) Discussions Alluviums and volcanic series forming the bedrock along Ulus-Kecioren metro alignment constitute poor-to-medium aquifers having hydraulic conductivities ranging between 4x1-7 m/sec and 1-5 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 normally expect uniform inflows through these porous media. Where sand and gravel dominant layers and/or lenses 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 range of permeability of the rock mass may be even higher than that determined from the water pressure tests. Normally the longer and more open fractures will capture most of the flow and channelize it toward the tunnel (Raymer (21). 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 from storage at the immediate vicinity, where the hydraulic gradient is also high. Therefore, initial inflow rates may be about 3% larger then computed values. As time elapses the steady-state condition will be 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 1) located in a random fashion. Due to chaotic mixture of the rocks of volcanic series and random distribution 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 measured discharges of individual seepages and leakages from volcanic rocks generally lie within 1.5 l/sec and.3 l/sec. (a) (b) Figure 1. Groundwater inflow into the tunnel at (a) Km: 7+97 and (b)

8 Géoline 25 Lyon, France 23 rd - 25 th, May/Mai 25 References Freeze, R.A., Cherry, J.A., Groundwater. Prentice-Hall, Inc., New Jersey, Goodman, R.E., Moye, D.G., van Schalkwyk, A., Javandel, I., Ground water inflows during tunnel driving. Engineering Geology 2, Heuer, R.E., Estimating rock-tunnel water inflow. Proceeding of the Rapid Excavation and Tunneling Conference, June 18-21, 41p. Ibrahim, H.A., Brutsaert, W., Inflow hydrographs from large unconfined aquifers. Journal of Irrigation and Drainage Division, Proceeding of American Society of Civil Engineers 91,(IR2), Karlsrud, K., 21. Control of water leakage when tunneling under urban areas in the Oslo Region. Norwegian Tunneling Society Publication 12, Lei, S., An analytical solution for steady state flow into a tunnel. Ground Water 37, 1, Raymer, J.H., 21. Groundwater inflow into hard-rock tunnels: Tunnels and Tunneling International,