Numerical Methods in Geotechnical Engineering – Hicks, Brinkgreve & Rohe (Eds) © 2014 Taylor & Francis Group, London, 978-1-138-00146-6

Modelling groundwater flow in Tikkurila area, Vantaa E. Lottanen City of Vantaa, Vantaa, Finland

L. Korkiala-Tanttu & I. Mataic Department of Civil and Environmental Engineering, Aalto University, Espoo, Finland

ABSTRACT: Starting on 2011 the construction of three new buildings with structures extending below the groundwater table have been started in Tikkurila area, Vantaa. There is a risk that these buildings would affect the groundwater flow and decrease permanently the level of the groundwater table negatively affecting functionality of the structures in the impact area. In this paper the impact the new buildings have on the level of the groundwater table in the long-term situation was studied by compiling a three-dimensional groundwater model of the area. It was also studied how a drainage layer built around the buildings would affect the level of the groundwater table. Finally, paper provides assessment of the effects of permanent groundwater level decrease on settlements of the street passing the area affected.

1

INTRODUCTION

From 2011 on the construction of three new underground spaces has been initiated in the Tikkurila area of city of Vantaa in Southern Finland. Locations of the new buildings as well as position of groundwater table prior to construction are presented in Figure 1.

Figure 1. Location of the new underground constructions and measured groundwater table in autumn 2010 in Tikkurila, Vantaa.

Figure 2 presents cross-sections of the construction area. Litostratigraphic conditions of the location are characterized for the most part of the soft soil sediments of varying thickness. Soft soil sediments are succeeded by silt and moraine sediment and finally by bedrock. Thickness of the soft soil formation alternating with hills of bedrock and moraine varies from several up to more than ten meters. The underground spaces are situated in the approximate vicinity (Figure 1) and their structures extend through the soil layers to the bedrock. From cross-sections 1 and 2 it can be observed that bedrock forms a sill in the central area where new underground spaces are situated. Thus from the aspects of stability, location of new constructions is favourably selected. However, there is a risk that new underground spaces would alter groundwater flow conditions, leading to the rise of groundwater table on one side and decrease on the other side of the construction area. Changes in groundwater flow

Figure 2. Cross-sections 1 and 2 from the construction area.

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conditions might negatively affect functionality of the existing structures at the area. This paper evaluates long-term effects of the new underground spaces on groundwater flow conditions and subsequent impact on functionality of the structures in the vicinity of construction area. For that purpose complex modelling task has been conducted including analyses of groundwater flow as well as associated settlement analyses of existing structures in the Tikkurila area. The site and the analyses are described in more detail in a master thesis (Lottanen 2013). Herein, the following analyses are addressed: 1. Simulation of groundwater flow conditions in the initial stage (before construction) 2. Analysis of long term effects of the new underground constructions on the groundwater flow conditions, 3. Effects of one meter thick highly permeable soil layer built around the underground spaces on groundwater flow in the long-term situation. 4. Effects of the groundwater flow modifications on settlement of the street passing the area affected.

2 2.1

MODELING Groundwater flow model

Analysis of groundwater flow conditions of the Tikkurila area were made on three dimensional flow model, created by software SoilVision SVFlux. SVFlux is a finite-element tool capable of solving linear and non-linear partial differential equations for groundwater flow (Thode & Gitirana 2012). The software provides the capability of taking into account both saturated and unsaturated flow component in modelling of seepage face conditions. Geometry characteristics of three-dimensional groundwater flow model are presented in the Figure 3. The model was compiled by defining borders of the soil layers as surfaces and by assigning adequate hydraulic properties to each layer. Position of the bedrock was modelled using the level information obtained from the in-situ survey of the outcrops, results of drilling tests, and correlations based on the results of laboratory tests. The drilling tests and the hydraulic conductivity tests identified upmost zone of the bedrock to be more fractured. On the basis of results of in-situ measurements, fractured zone of the bedrock is within the model represented by continuous 1.5 m thick layer with distinctively higher hydraulic conductivity. The soil layers were created using the level information obtained from the in-situ drillings and laboratory testing. In order to simplify the flow model characteristics, several simplifications are introduced. Within the in-situ area of interest, bedrock locally intersects and outcrops overlying formations. However, in those areas within the model, the soil layers are set to follow the shape of the bedrock. Thus, the soil strata are modelled as continuous layers extending throughout

Figure 3. The groundwater model scaled to 20x in the z-direction. a) three-dimensional model, b) projection of the model from the western border.

the model area. Associated with the sand deposit, formations of gravely sediment occasionally occur. Since the gravel occurred fairly infrequently, within the flow model sand and gravel sediments are represented as one continuous stratum. 2.2 Parameters The groundwater flow in the Tikkurila area was modelled by FEM steady-state flow analysis as the purpose was to study permanent effects that under-ground structures have on the level of the groundwater table. Input parameters necessary for defining hydraulic properties of soil layers in steady-state analyses are hydraulic permeability and volumetric water content. The hydraulic conductivities k for layers representing till, sand and gravel, were determined from the disturbed soil samples available from the studied area. From curves of particle size distribution of these samples the effective grain size d10 was determined and the hydraulic conductivity was evaluated using the Hazen (1892) equation (1).

where c is constant between 0,006…0,015 and θ is the temperature of the water (◦ C). The temperature of the groundwater was estimated to be 6◦ C. The water conductivities representing the soil layers are presented in Table 1. For the volumetric water content average values representing target materials in natural state were used. Values assigned are presented in Table 2. 2.3

Boundary conditions

The external borders of the model are defined to approximately follow natural boundaries of the environment. The eastern and western border areas of the

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Table 1.

Hydraulic conductivities. Hydraulic conductivity

Soil Sand & gravel Till

No of samples

Average m/s

Min m/s

Max m/s

37

6.554E–5

6.179E–7

8.692E–4

41

3.475E–6

1.32E–9

7.316E–5

Table 2. Hydraulic conductivities, volumetric water content and fluxes assigned at the northern border.

Soil

Calibrated hydraulic conductivity [m/s]

Volumetric water content θw

Flow from the north border [m3 /s/m2 ]

Clay Sand & Gravel Moraine Fractured bedrock Bedrock

1.0 × 10−8 1.0 × 10−5 1.0 × 10−7 1.0 × 10−6 1.0 × 10−10

0.5 0.35 0.4 0.1 0.1

5.0 × 10−11 5.0 × 10−7 5.0 × 10−9 5.0 × 10−10

Figure 4. Area modelled with location of piezometers and groundwater table monitoring results in the autumn 2010.

model are set parallel to the main direction of the groundwater flow. Thus, the flux on the eastern and western areas of the model is estimated to be negligible with flux equal to zero boundary conditions assigned. The groundwater inflows studied area for the most part from the north. Inflow is taken into account by adequate flow boundary conditions set at the northern border areas of the model. Flux values assigned to each layer at the northern side of the model are presented in Table 2. Geometry characteristics at the southern edge of the model are set to follow the natural outline of the Keravanjoki river. Alongside the river a dam is situated causing approximately a 5 meter water level difference. Since the southern upper edge of the model corresponds to the position of the river in-situ, the level of the groundwater table at the location complies with the water level in the river. Thus, at the southern border of the model boundary conditions of constant total pressures are set. Total pressures assigned correspond to measurements of the water level in the river obtained in the laser scanning survey. 2.4

Calibration

The groundwater model is calibrated using trial-anderror technique so as to adjust the initial groundwater level simulated with the measured groundwater level in-situ. The model was calibrated using the groundwater level monitoring measurements obtained in the period from 19th Oct to 22th Nov 2010. At the time the construction of the new underground spaces had not yet begun. Location of piezometers and initial position of groundwater table are presented in the Figure 4. In the calibration, the difference of one meter,

Figure 5. Comparison of simulated and measured groundwater level prior to construction.

corresponding to the natural annual variation of the groundwater table, was taken to be acceptable. The model is validated using the groundwater levels measurements performed in the period from 29th Oct to 4th Nov 2009. As presented in Figure 5, simulated initial position of the groundwater level complies well with the measured groundwater table. The calibrated and validated input parameters assigned to the flow model are presented in the Table 2.

3

RESULTS

3.1 The impact of the new underground spaces on the groundwater table The long-term impact of the construction activities is analysed by using the calibrated and validated flow model representing initial groundwater conditions and subsequent activation of the underground spaces as

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Figure 6. The long-term impact of the new underground spaces have on the groundwater table.

Figure 7. The long-term situation when a one meter thick drainage layer is built around the buildings.

impervious regions penetrating the soil layers. The fractured zone below the new buildings was grout injected, and within the model this area is set to have the same permeability properties as the compact bedrock. Results of the seepage analysis addressing longterm situation are presented in Figure 6. Construction impact in the long term would lead to the decrease of the groundwater table downstream from the construction sites. In fact, groundwater level would decrease 0.3 m within the area 10–50 m away from the buildings, while at distance of 200–300 m decrease would lessen to 0.1 m. According to the results obtained, upstream from the construction sites groundwater table raises. Nearby the underground spaces the groundwater table would rise approximately 0.3–0.4 m. Within the area approximately 100 m east and 200 m north-east from the new buildings, the rise of the groundwater table would be 0.1 m.

3.3 The reliability of the flow model

3.2 The impact of the drainage layer built around the buildings The long-term impact of the construction activities is re-evaluated by including one meter thick drainage layer built around the buildings as remediation measure. The highly permeable layer is modelled to extend through all the soil layers except bedrock. Permeability of the drainage material is set to 0.1 m/s. Results of the seepage analysis considering impact of the drainage layer are shown in Figure 7. The highly permeable drainage layer increases flow in the vicinity of the buildings. Nevertheless, compared to the situation prior the construction, long-term groundwater level would rise on the south-west side, and decrease on the north-east side of the new underground spaces. The change of the groundwater table level would amount 0.3–0.4 m in the vicinity of the buildings and approximately 0.1 m at 100–150 m distance.

Some simplifications in the modelling were necessary as the soil has been noted to be heterogeneous and the modelled area was considerably large. The soil layers were modelled to continue across the modelled area although the layers are not continuous at some locations. Also the hydraulic conductivity of the soil layers in the model was chosen to be constant in different areas and directions. For the modelling a considerable number of soil investigations and groundwater level measurements were available. The amount of soil investigations available from the area varied and the interval of the soil investigations was denser in the area where the new underground spaces are located. No direct measurements of hydraulic permeability were available. Hydraulic permeability was estimated on the basis of relatively simplistic approach given by Hazen (1892). Reliability of the results of the analyses needs to be re-evaluated by additional calculus using laboratory measurements of hydraulic conductivity or alternative approaches for hydraulic conductivity estimations such as the Kozeny-Carman formula (Kozeny 1927, Carman 1938, 1956, Sanzeni et al. 2013). Present seepage analyses do not consider effects of water content change resulting from groundwater flow modifications. 3.4 The impact of the groundwater level change on the settlement of the ground Long-term impact of the groundwater flow modifications on the functionality of the existing structures in the vicinity of the construction area was evaluated by settlement analyses of the street passing the area affected. Here presented settlement analyses address impact of the groundwater level decrease, i.e. the area at the south-west side of the underground spaces. The settlements were calculated with NovaPoint GeoCalc software using tangent modulus

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Table 3. Settlement parameters defining normally consolidated soil response. PreconSoil solidation Module Tension Consolidation layer pressure number exponent coefficient β1 Cv1 no σc [kPa] OCR m1 1 2 3 4

55 55

1 1

50 7.5 50 100

0.5 0.1 0.25 0.5

1 0.5 2 50

Table 4. Settlement parameters defining over consolidated soil response. Soil layer no

Module number m2

Tension exponent β2

Consolidation coefficient Cv2

1 2 3 4

100 15 100 200

0.5 1 0.25 0.5

5 2 30 100

Figure 8. The impact of the groundwater table level on the street settlement, a) groundwater table stays constant, b) decreases permanently by 0.3 m after 25 years of building the street. Table 5. The impact of the change of groundwater level on the amount of settlement.

method (also called Ohde-Janbu method). The software calculates the primary consolidation based on the Terzaghi’s one dimensional consolidation theory (Vepsäläinen & Takala 2004). The parameters used in the settlements analyses are presented in the Tables 3 and 4. Soil response in normally consolidated stress range was defined by module number m1 and tension exponent β1 . Thus, normally consolidated response was characterised by exponential increase in compressibility with strain level. Same set of parameters, yet significantly less in magnitude, characterise soil behaviour in over-consolidated stress range, i.e. parameters m2 and β2 . (Aalto et al. 2004). Consolidation characteristics were defined by separate parameters as well, with Cv1 and Cv2 dependent upon stress history. In consolidation analysis, influence of permeability change on consolidation rate was defined as being both stress and strain level dependent (Ravaska & Vepsälainen 2001). The street has been built approximately 25 years prior the construction of the new underground spaces. The loads induced by the structural layers of the road were taken into account in the load history. The load of structures was given as an extending load which began from the start of the calculation. The load caused by 0.1 m decrease of groundwater level was estimated to be 1 kN/m2 . According to the results of the analysis, if the groundwater table remains unaffected by the underground constructions, street is expected to settle 11 mm in the period of next 10 years. If the level of the groundwater table permanently decreases 0.1 m, street settlement would increase for additional 11 mm during the next 10 years. If the permanent decrease

Decrease of groundwater m

10 years mm

20 years mm

0.0 0.1 0.3

11 22 40

17 27 52

of the groundwater level is 0.3 m, the settlement of the ground in the vicinity of the under-ground spaces would increase by 29 mm during the next 10 years. Magnitudes of settlement calculated for different scenarios are presented in the Figure 8 and Table 5 (Lottanen 2013).

4

CONCLUSIONS

A three dimensional flow model of the Tikkurila area was compiled using SoilVision SVFlux. With the model the long-term impact of the three new underground spaces on the groundwater flow is analysed. Results of the seepage analyses are used for analyses of settlement in the vicinity of the construction site using NovaPoint GeoCalc software. Construction of the new underground spaces modifies the groundwater flow conditions in relatively large densely constructed urban area. According to the result of the analyses by the composed groundwater flow model, the new underground spaces would decrease the level of the groundwater table by 0.3 m on the downstream side, if no actions to compensate the flow of groundwater around the buildings are carried out. On the upstream side the level of the groundwater table would rise approximately 0.3–0.4 m. If one meter thick drainage layer with permeability of 0.1 m/s is built around the buildings, groundwater would start to rise

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on the downstream side and decrease on the upstream side of the buildings. The permeability of the material affects how effectively the drainage layer increases the flow of groundwater around the buildings. Backed by the evaluation of the long term settlements of the street passing the affected area, modifications of the groundwater flow significantly influence extent of long-term settlements of the existing structures in the urbanized Tikkurila area. If no actions are taken estimated long-term settlements of street considered would increase four times in the period of next 10 years. The settlement of the ground has already induced visible damages on the street structures and caused the settlement of some of the buildings founded on the ground. Significance of the negative effects of the analyses presented herein shows necessity for investigating optimum remediation measures. Reliability of the results of the analyses presented herein can be increased by improving the performance of the models used. Groundwater flow model performance could be improved by implementing measured hydraulic conductivities as well as taking into account effects of stress and flow caused water content modifications on the magnitude of hydraulic conductivity. Settlement calculations presented herein could be reevaluated by alternative 3D FEM model calculus accounting for long-term creep settlements.

REFERENCES Aalto, A., Lojander, M. & Ravaska, O. (2004). On the stress dependence of settlement parameters of Finnish clays. XIV Nordic Geotechnical Meeting (NGM04),Ystad, Sweden, Vol. 2, I27–I38. Carman, P. C. (1938). The determination of the specific surface of powders. J. Soc. Chem. Ind. Trans., 57, 225. Carman, P. C. (1956). Flow of gases through porous media, Butterworths Scientific Publications, London. Hazen, A. (1892). Some physical properties of sands and gravels, with special reference to their use in filtration. 24th Annual Rep., Massachusetts State Board of Health, Pub. Doc. No. 34, 539–556. Kozeny, J. (1927). Ueber kapillare Leitung des Wassers im Boden. Wien, Akad. Wiss., 136(2a), 271. Lottanen, E. (2013). Effect of building underground spaces on groundwater flow in the centre of Tikkurila. Master’s thesis. Aalto University, Espoo. Ravaska, O. & Vepsälainen, P. (2001). On the stress dependency of consolidation parameters. Proc. XV Int. Conf. Soil Mech. Geotech. Engng. vol. 1, Istanbul, 205–210. Sanzeni, A., Colleselli, F., & Grazioli, D. (2013). Specific surface and hydraulic conductivity of fine-grained soils. J. Geotech. Geoenviron. Engng.,139, No. 10, 1828–1832. Thode, R. & Gitirana, G. (2012). SVFlux Theory Manual. SoilVision Systems Ltd. Saskatoon, Saskatchewan, Canada. Vepsäläinen, P. & Takala, J. (2004). Program SETTLE. Theoretical Principles. p. 19.

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