Abstract. Introduction

Transcription

1 Invited Paper The modelling of saline intrusion during the construction of submerged tunnels T. Roberts', J. White, Z. Mohammed' "WJ Engineering Resources Ltd. Civil Engineering, Queen Mary and Westfield College, London University, London, UK ^Department ofhydraulics and Irrigation, Alexandria University, Abstract This paper describes the procedure that may be used to apply the USGS two dimensional numerical model SUTRA to predict groundwater flow and salinity distribution for the boundary conditions that are likely to be encountered during the construction of a submerged tube tunnel. The paper discusses the method of calibration that may be adopted to assess relevant parameters and boundary conditions. These include coefficients of permeability, dispersion and leakage, together with the distribution of undisturbed groundwater levels and salinity at the boundary of the model. Case study examples are included. Introduction Submerged tube tunnels are suited to the crossing of rivers, estuaries or shallow water channels where ground conditions or environmental considerations are unsuitable for conventional tunnel or bridge solutions. A number of submerged tunnels have now been constructed around the world and the technique is relatively well understood. Culverwell/ records details of 67 immersed- tube road and rail tunnels world-wide. These have been constructed between 1910 and 1986 with 16 constructed during the latter ten years of that period. Two more recent submerged tunnels in the UK are the Conwy Crossing in North Wales and the Medway Tunnel at Chatham in Kent. A feature of the construction procedure for immersed tube tunnels is that it often relies heavily on groundwater control. The tunnel consists of prefabricated sections which are floated out and then sunk into position in a preformed trench across the bed of the channel. The prefabrication of the tunnel sections may be carried out in a casting basin on site. This consists of a deep excavation which may penetrate an aquifer and, because of its proximity to open water, be constructed below the water table. The casting basin is subsequently

2 32 Water Pollution flooded in order to float the tunnel sections out into position. Temporary dewatering of the casting basin will therefore be required. Furthermore there will be a need to dewater and excavate for the two cut and cover approach roads at either end of the tunnel. See figure 1. The effect of such extensive groundwater control procedures is to draw down the water table locally, and possibly to such an extent that there will be a significant effect on the regional or 'far field' groundwater regime. Thus throughout the construction period the groundwater system acts like a large well drawing towards it groundwater over large distances. Since the site is next to a large water body which is likely to be saline, the region of groundwater flow that is influenced by the dewatering procedure is almost bound to contain a significant source of saline water. This will be induced into fresh water zones of the aquifer and may be so extensive that it will threaten any regional sources of groundwater supply that may be in the area. See figure 2. Besides the influence of the dewatering system, there are two other procedures which may adversely effect the quality of the groundwater. Prior to floating out the prefabricated tunnel units a trench is excavated across the channel in order to receive the units. During this time the dewatering system will have reduced pore pressures in the aquifer below the trench and increased porepressure gradients between the channel and the aquifer. The excavation for the trench may significantly reduce the leakage path from the channel to the aquifer thus increasing the leakage of saline water into the aquifer system. A further adverse effect may arise as the result of flooding the casting basin to float out the tunnel units. When this happens a relatively large surface area of the aquifer becomes inundated with saline water for the first time. This provides the potential for a large source of recharge saline water which again may adversely effect the quality of the regional groundwater resource. This paper describes the procedure that may be used to model the groundwater flow and salinity distributions for the boundary conditions that are likely to be encountered during the construction of a submerged tube tunnel. The procedure is illustrated with examples from the Medway Tunnel Project. Model Structure The computer modelling work for the Medway Tunnel Project was carried out using thefiniteelement computer program SUTRA produced by the US Geological Survey. SUTRA simulates groundwater flow and the transport of dissolved substances in two dimensions. The model employs a hybrid finite element and integrated finite difference method to approximate the governing equations that describe the interdependent processes, Voss/ The input data required to define the boundary conditions for the model are as follows, drawdown potential at boundary chloride concentration at boundary aquifer transmissivity distribution

3 channel leakage coefficients channel chloride concentration dispersion coefficient extraction flows from supply wells in the modelled area extraction flows from the dewatering wells. Water Pollution 33 Whilst some of this information is available prior to construction from pre-contract pumping tests, Water Authority records and existing hydrogeological mapping, the most detailed information arises shortly after the commencement of dewatering operations when the most significant changes in the groundwater regime and salinity distributions will be recorded. The plan area covered by the SUTRA model at Medway is shown in figure 3. The area is 4000m by 3000m. The modelled area was selected to cover the area of influence of the tunnel dewatering works based on data from various remote monitoring points. The computer mesh used by the SUTRA programme is shown in figure 4. The density of the mesh has been increased in the area of the tunnel works to improve the accuracy of the simulation of the dewatering wells, trench works and casting basin flooding. Boundary Conditions a) Drawdown Potentials The boundary potentials used for the model runs are shown in figure 4. The estuary and river were taken as OmOD with the potential rising to +2.5mOD south of the river and to +lmod to the west. This distribution is consistent with the hydrogeological maps of the area and also with limited data available before commencement of pumping. The model takes no account of tidal fluctuations. b) Transmissivity At the Medway Tunnel site the aquifer is confined consisting of approximately 300m of soft, fine grained, fissured chalk overlain with a narrow band of gravel which in turn is overlain by clay. It is thought that the groundwater flow takes place mainly in an upper fissured region of the chalk no more than 50m in depth. The transmissivity distribution used for the model runs is shown in figure 5. This is based on three factors. (i) An analysis of early drawdown data from the dewatering system using a simple Theis analysis showed an average permeability of 1.8 x 10 m /sec. (ii) The hydrogeological map of the area indicates that the chalk becomes confined just to the south of the tunnel site and that a spring line is present to the west. Historic leakage/springs at the confining boundary may well have lead to a narrow zone of high transmissivity. (iii) The transmissivity distribution was modified following repeated model runs and comparison with the steady-state drawdown pattern obtained after the dewatering system had been operating from about 9 months. This

4 34 Water Pollution particular drawdown pattern, together with the associated chloride data was used throughout to calibrate the model. c) River/Estuary Leakage Initially it was thought that leakage from the river and estuary would not be of great significance because of a sealing layer of alluvial silt. However, it was found that incorporating a leakage algorithm into the SUTRA model gave an improved fit to the calibration data. Such an algorithm was also necessary to provide the facility to simulate leakage from the cross-channel trenching and the casting basin during flooding. The leakage algorithm simulates inflows or outflows at relevant nodes on the finite-element mesh. The flows are proportional to the difference between the computed potential and a reference potential. The constant of proportionality is termed the leakage factor and is the ratio of the permeability to the thickness of the river bed silt. This suggests a range for leakage factor values of 10 sec to 10 sec. Higher values might occur as the result of regular dredging for shipping whereas lower values will arise when the seal between the river and the aquifer is improved by the presence of the overlying clay. The reference potential applied to the river/estuary nodes was OmOD. In the undisturbed state, where the potential in the chalk aquifer is above this leakage will occur from the aquifer into the river which acts as a sink. However, where the potential in the chalk aquifer is reduced due to the dewatering works leakage will occur out of the river/estuary which then becomes a source which may contain a high chloride content. The distribution of the river/estuary leakage factor used for the Medway model runs is shown in figure 6. Initially a uniform leakage factor was assumed but this was modified following repeated model runs and comparison with the calibration data. d) Chloride concentration The chloride distribution for the Medway model boundary and for the river/estuary leakage is shown in figure 7. Leakage from the estuary was taken as 14,000 mg/1 chloride, and leakage from the river as 10,000 mg/1 falling to 3,000 mg/1 from north to south. Fresh water entering across the south/east boundary and the west boundary was taken as 60 mg/1. The effect of molecular diffusion and dispersion is small in comparison with advection. The Medway model does not take account of diffusion but a dispersion coefficient of 70m has been used. e) Extraction Flows At Medway there were a total of 47 wells providing a total extraction flow of 400 litres/second. These were grouped into 26 nodes each with an extraction flow in the range 10 to 25 litres/second. Where necessary the nodes were moved from the rectangular grid array to give a better representation of the well distribution.

5 Water Pollution 35 f) Trench and casting basin leakage Modelling of the trench excavation is achieved by increasing the leakage factor at the trench excavation nodes across the channel. The trench leakage factor used at Medway was 2.2x10 sec" which gave a trench leakage flow of 100 litres/second. This was considered to be a likely upper value for the leakage in practice. Modelling of the flooding of the casting basin is achieved by introducing leakage at appropriate nodes. For the Medway model a leakage factor of 7.5x10 sec was used. For both the trench excavation and casting basin the leakage water chloride concentration was taken as 9,000 mg/1. Some results and observations The steady state solution computed by SUTRA giving both drawdown and chloride distribution for the situation shortly after the commencement of dewatering is shown in figures 8 and 9. The comparison between observed data and spot heights computed by SUTRA is shown in figures 10 and 11, where it can be seen that it is possible to obtain a good match. The possibility of obtaining an even better match by taking into account vertical anisotropy and density effects using a three dimensional model SWICHA has been investigated and the improvement is also shown on figures 10 and 11. In the case of the three dimensional model the match with observed data was obtained with very much reduced leakage coefficients. This indicates that the introduction of the leakage component in the two dimensional model has to some extent compensated for the two dimensional approximation which does not model vertical anisotropy or density effects. This matter is now the subject of further study by the authors. The increased accuracy of a three dimensional model, whilst desirable, may not be justified in view of the attendant increased time and cost. Despite the approximations of the two dimensional model its application to the Medway Tunnel Project has made a useful contribution to the tunnel dewatering strategy. References 1. Culverwell. Immersed tunnel techniques. Proc. of conf. Institution of Civil Engineers Manchester April Voss, C.I. A finite-element simulation model for saturated-unsaturated, fluid-density-dependent groundwaterflow with energy transport or chemically-reactive single-species solute transport. US Geological Survey Water-Resources Investigations Report , Acknowledgment The authors are grateful for the cooperation and assistance of Kent County Council (Client for the Medway Tunnel Project), Travers Morgan (the Client's Engineer), Tarmac-HBM Joint Venture (Design and Build Contractor) and Mott MacDonald (Medway Tunnel designers).

6 .--V Casting basin Deep well ground water control A. Construction of tunnel units in casting basin. B. Float out: Casting basin flooded. Trench excavated. C. Tunnel units installed with cut and cover approaches. Fig. 1 Different stages for installation. Fig. 2 Flow lines illustrating seepage induced from far field and local saline sources. 'idr-a _aw wp-» Fig. 3 Area Covered by the Model. Fig. 4 Boundary conditions - Potentials.

7 Water Pollution Fig. 5 Boundary conditions - transmissivity » Fig. 6 Boundary conditions - leakage coefficients fswwwtf^ k 600 " i ( 4-i.il 7 S /' \ ' J-^" I i we L ' -^^ C14OOO P/Af ^ 1000 s/\/1 r= ;r>oofl VA/ ; gggggj C= 9OOO P /Af.' ' 1 csooo /A/ P : 1 c = 3ooo p /A/ J mo Fig. 7 Boundary conditions - chlorides levels.

8 38 Water Pollution Fig. 8 Drawdown Distribution Fig Chloride Distribution.

9 Water Pollution 39 3-D 2-D -2-1 X Fig. 10 ^ Observed Head(m) Potential Head Results for 3-D and 2-D Model. 3-D 2-D ex, 8 1 c, O 1 I a Observed Chloride Concentration ppm Fig. 11 Chloride Concentration Results for 3-D and 2-D Model.