Modelling the Discharge Rate and the Ground Settlement produced by the Tunnel Boring

Transcription

1 Modelling the Discharge Rate and the Ground Settlement produced by the Tunnel Boring Giona Preisig*, Antonio Dematteis, Riccardo Torri, Nathalie Monin, Ellen Milnes, Pierre Perrochet *Center for Hydrogeology and Geothermics (CHYN) University of Neuchâtel Piacenza,

2 Contents Socio- Economic Scope This presentation aims to show the interest and some predictive tools solving for the problematic of groundwater in tunnels. page 2

3 Below the water table, underground excavations (tunnels) behave as draining structures (atmospheric pressure). This results in discharge rates into the underground excavation and in decreasing water pressures. High water inflows into the tunnel are a major cause of slowing the tunnel progression (e.g., Rawyl Tunnel: Q > 1000 L/s), and modify the natural hydrodynamic behaviour of surface water / groundwater systems. Moreover, an important decrease in groundwater pressures due to the drainage results in increasing effective stresses and in the consolidation of the aquifer, ground settlement (e.g., Zeuzier arch dam: 12 cm of settlement for ~230 m of drawdown). (a) before tunnel excavation (b) after tunnel excavation (c) Illustrative hydraulic head fields and infiltration / exfiltration zones of an alpine valley (a) before and (b) after the excavation of a tunnel, and (c) resulting ground settlement (Preisig et al., Ground Water, 2012). page 3

4 Hydrogeological conceptual Schematic cross section showing the main hydrogeological situations encountered during the drilling of a tunnel into a mountain system (Preisig et al., RMRE, under revision). page 4

5 : Analytical steady flow rate in tunnel I: Goodman formula Left: seminal steady state solution of Goodman et al. (1965), where the symbols stand for hydraulic conductivity K, initial hydraulic head in tunnel H 0 (drawdown at the tunnel), tunnel depth d, tunnel length L, tunnel radius r 0 and discharge rate Q. If the tunnel is drilled through different geological zones, the total flow rate in tunnel is obtained by the summation of each sector discharge rate. Since the solution of Goodman other specific and practical formulas for the steady state case have been developed. For example considering the detailed geometry of the aquifer system (above, Dematteis et al., 2005) or limiting the flow rate in tunnel to a maximum corresponding to the local recharge. In the example above: a is the lateral spacing of the aquifer system perpendicular to the distance d between the tunnel and the surface via the aquifer. page 5

6 : Analytical steady flow rate in tunnel, II: effective stress consideration Deep tunnels (depth > m) possible reduction in hydraulic conductivity due to (1) increasing effective stresses (fractures compression) and (2) decreasing fracture occurrence. To avoid overestimation, the calculated flow rates in tunnel are multiplied with a reduction factor. Estimation of the reduction factor via numerical analysis: (a) simulated steady water flow rates in tunnel and (b) reduction factor as a function of initial pressure head in tunnel (Preisig et al., under review). page 6

7 : Analytical transient flow rate in tunnel Perrochet and Dematteis (GW, 2007) proposed an analytical solution solving for the discharge rate produced during the drilling of a tunnel in an heterogeneous massif. (c) (c) (b) (a) Cross section along a LTF exploratory adit (LTF, 2007), (b) parametric and geologic information for the analytical simulation (Perrochet et Dematteis, 2007), and (c) observed and simulated discharge rates in tunnel with the excavation progression. page 7

8 : Analytical consolidation, ground settlement solving for the ground settlement based on the Jacob (1940, 1950) compressibility of aquifers: where e is aquifer thickness, C v is aquifer compressibility, s 0 is the drawdown at the tunnel, and ΔV z is vertical consolidation (ground max settlement). (a) (a) Illustrative cross section perpendicular to the tunnel axis showing the temporal evolution of the drawdown cone (dashed lines) and the resulting ground settlement (dotted lines). (b) Application of the solution of Preisig et al. (RMRE, under review) solving for the transient drawdown and ground settlement for a LTF exploratory adit. page 8

9 : Numerical 3D regional hydrogeological scale: FEM, FDM The tunnel is introduced in the as a time-varying inner boundary. According to a time function describing the boring/drilling progression, tunnel nodes become active as Dirichlet boundary condition at constant atmospheric pressure (elevation head) (Figure below). A detailed geological can be used in the flow and consolidation simulation, but more the geology is detailed more the introduction of the tunnel in the mesh become difficult. The role of geomechanical stresses on hydrogeological parameters can be taken into account. The computed pressure head distributions are then used in a deformation to calculate the aquifer consolidation (ground settlement) due to the excavation drainage. Results in the form of maps. approaches are time consuming compared to analytical analysis. 3D finite element for the simulation of the flow rate in tunnel produced by the excavation of a LTF exploratory adit (Preisig et al., RMRE, under review). page 9

10 : Numerical Some illustrative examples (b) (a) Flow rate in tunnel as a function of excavation time for a LTF exploratory adit: measured data (red line), simulated data (blue line) (Preisig, RMRE, under review). (b) Simulated settlement map for the Zeuzier area due to the excavation of the Rawyl exploratory adit (Preisig et al., Modelcare 2011, in press). page 10

11 Conclusion This presentation has dealt with some quantitative tools specific to the problematic of groundwater inflows in underground excavations (tunnels). Both analytical and numerical approaches are able to correctly reproduce hydrogeological processes involved in excavations. formulas are simple to use and rapidly lead to results. The accuracy of equations can be improved using some factors, e.g., reduction factor to consider the effect of effective stresses on hydrogeological parameters. simulations allow detailed calculations but are time consuming. Moreover at regional scales, 3D geological s usually have a low reliability. Excavation Phase Operation Phase page 11