Appendix A3.1g
SEEP/W Model Analysis
Summary Design Note of
Seepage/Groundwater analysis
SEEP/W (SEEPage for Windows) is a finite element software product that can be used to model the movement and pore-water pressure distribution within porous materials such as soil and rock. It is formulated on the basis that flow of water through saturated soil follows Darcy’s Law.
SEEP/W model is constructed to solve 2-Dimensional flow situations with multiple soil layers. Flow directions of groundwater can be analyzed. Under steady state conditions, the difference between input flux and output flux is zero at all times.
For finite element calculation, the SEEP/W model is divided by nodes. The elevation of water level at each node is calculated.
In
the analysis, the geometry in the SEEP/W models is configured as the following
layers of materials in descending order :
i)
Fill (F) / Pond (P) deposits,
ii)
alluvial clay/ silt (A1/A2) and alluvial sand/ gravel (A3/A4)
materials, and
iii)
completely decomposed volcanic tuff (V), and
iv)
Bedrock of volcanic tuff.
In the SEEP/W models, the following assumptions have been made.
i) The aquifer is heterogeneous and isotropic.
ii) The aquifer is partly confined and unconfined. Hydraulic conductivity used in the model are as follows:-
|
Soil |
Permeability |
|
Fill |
7.67 x 10-6 m/s |
|
Alluvial Clay/Silt |
5.32 x 10-6 m/s |
|
Alluvial Sand/Gravel |
4.36 x 10-5 m/s |
|
Completely Decomposed Tuff |
8.51 x 10-7 m/s |
iii) Total head 100m upstream of the tunnel assumed to be at +4 mPD. Total head below the center of the rivers assumed to be +2mPD.
(NB the 2m adopted also equates with measured piezometric levels to date and can be sensibly considered to be a worst case. Any greater differential would involve upstream head levels above ground, i.e. flooding.)
iv) The Twin Tunnel linings are totally impermeable.
v) The models were run in steady state with and without the implantation of the twin tunnels.
Four geological sections across Long Valley were modeled using SEEP/W. and the findings show introduction of the tunnels has no meaningful effect, given the design decision for the tunnels to avoid the main aquifer.
As a sensitivity analysis a special case has been run for the River Sutlej crossing. Here, the 2-D analysis has been used to investigate the groundwater level variation due to implantation of the twin tunnel and an impermeable concrete stabilizing block.. In 2D, total blockage of all aquifers is checked and the maximum ground water differential could be as large as 640mm. The same 2D model with competent drainage installed across the concrete block reduces the damming effect to a 10mm differential head. However it is also noted that in practice a 3-D drainage condition will prevail and lateral flow/drainage in the main alluvial material will actually occur to restore the pressure differential. The provision of a drainage layer across the concrete block above the twin tunnels together with the prevailing 3D condition ensures that locally the groundwater will not be adversely impacted even when the tunnels rise above the alluvial /CDV horizon.