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Differences between computed and calculated HK equivalent


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I want to simulate a Sheet Pile with permeable holes through the whole acquifer.

  • STEP 1

The simplified model is a 10x10 m grid, with only two Boundary Conditions: from CHD 1.0 m (left side) to CHD 2.0 m (right side).

As I've the HK for an equivalent Slurry Wall (width 1 m), I insert 3 polygon features:
1) HK representing the Slurry wall (width = 1 m): HK = 0.0041 m/day
2) HK representing the Slurry wall's holes, filled with gravel (width = 1 m): HK = 864 m/day
3) HK representing the aquifer: HK = 1.2 m/day

I check the Flow Budget for all the model (run 1): 30.6 m3/day.
The head value in the cells just before the HK is 1.45 m.

  • STEP 2

Now I need to set an HK equivalent rapresenting the Slurry wall with holes. 
I use the parallel approach (Figure_3, in the xy plane): HK eq  = (HK sw x Lenght sw + HK holes x Lenght holes) / (Lenght sw + Lenght holes) = 43 m/day

I replace the Slurry wall with holes polygon with a new polygon with HK equivalent (43 m/day)

I check the Flow Budget for all the model: 41 m3/day. Why is not the same value obtained from run 1?
The head value in the cells just before the HK is 1.23 m, less than 1.45 (run 1).

  • STEP 3

I change the HK  equivalent for the Slurry wall: 0.38 m3/day
The Flow Budget for all the model: 30.8 m3/day. 
The head value in the cells just before the HK is 1.44 m.

Why the HK equivalent are so different (100 times smaller)?
What's wrong in my exercise?

Thank you
Daniele

Figure 1: HK representing the Slurry wall (width = 1 m): HK = 0.0041 m/day
Figure 2: HK representing the Slurry wall's holes, filled with gravel (width = 1 m): HK = 864 m/day
Figure 3: Formula for calculate the Effective conductivity parallel to layers

Figura-3.png

Figura-1.png

Figura-2.png

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