Tutorial_14_Cofferdam_Seepage.pdf

Groundwater Flow in a Coffer Dam 14-1

Groundwater Flow in a Coffer Dam

In this tutorial, finite element groundwater seepage analysis is used to

determine the quantity of seepage entering a cofferdam. The example is

based on problem 2.4 from Craig (1997). The problem is constructed and

solved entirely with Slide 5.0 .

Topics Covered

• Seepage analysis

• Multiple materials

• Discharge sections

• Groundwater only calculation

• Flownets

Geometry

The finished product of this tutorial can be found in the Tutorial 14

Cofferdam Seepage.sli data file, located in the Examples > Tutorials

folder in your Slide installation folder.

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Groundwater Flow in a Coffer Dam 14-2

Model

Start the Slide Model program.

Project Settings

Open the Project Settings dialog from the Analysis menu and make

sure the General tab is selected. Define the Units of Measurement as

being “Metric”.

Click the Groundwater tab. Under Method choose Finite Element

Analysis. This enables steady state finite element analysis of

groundwater flow. Close the Project Settings dialog by pressing the OK

button.

Boundaries

First add the external boundary. Select the Add External Boundary

option from the toolbar or the Boundaries menu and enter the following

coordinates:

0 , 0

27 , 0

27 , 10

18.2 , 10

18.2 , 13

18 , 13

18 , 7.5

9.2 , 7.5

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Groundwater Flow in a Coffer Dam 14-3

9.2 , 13

9 , 13

9 , 10

0 , 10

c (to close the boundary)

Hit Enter to finish entering points. This defines the soil surface and the

parts of the thin sheet pilings that protrude above the soil to hold back

the ponded water (see figure at the start of the tutorial).

Now we need to add boundaries to delineate the rest of the impermeable

sheet pilings. To create the right sheet pile, go to the Boundaries menu

and select Add Material Boundary . Enter the following points:

18 , 7.5

18 , 5

18.2 , 5

18.2 , 10

Hit Enter to finish entering points.

TIP: when you are entering boundary points, the cursor should snap to

existing points. Therefore you do not need to enter the first and last

points but simply click on the existing points on the external boundary.

If your cursor does not snap to existing points, right click with the mouse

when you are creating a boundary and select Snap from the popup menu

to turn on the snapping option.

To create the left sheet pile, go to the Boundaries menu and select Add

Material Boundary . Enter the following points:

9 , 10

9 , 5

9.2 , 5

9.2 , 7.5

Hit Enter. Your model should now look like this:

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Groundwater Flow in a Coffer Dam 14-4

Material Properties

Select Define Materials from the Properties menu. You will see the

default material properties for Material 1. In this tutorial we don’t care

about the strength of the solid material therefore leave all the default

values. Change the name of Material 1 to Soil. Now click on the Material

2 tab. Change the name of Material 2 to Sheet Pile. Click OK to close the

dialog.

We now need to define the fluid flow properties of the soil. To do this, we

first need to switch to the groundwater mode. Go to the Analysis menu

and select Switch to Groundwater . Now go to the Properties menu

and choose Define Hydraulic Properties . Click on the Soil tab at the

top of the dialog. Enter 4e−7 for Ks. Ks is the saturated permeability in

m/s (also called hydraulic conductivity). You may specify anisotropic

permeability by specifying K2/K1 ≠ 1 and an angle to indicate the

directionality. However we will assume isotropic permeability so do not

change the default values.

The Model option at the top of the dialog refers to the function used to

calculate the permeability in the unsaturated zone as a function of matric

suction. Different models may be chosen, including a user-defined model.

However we will use the default Simple option. See the Slide Help for

more information on permeability models. Your dialog should now look

like this.

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Groundwater Flow in a Coffer Dam 14-5

Now select the Sheet Pile tab. The sheet piling is assumed to be

essentially impermeable. We wish to set the permeability to a very low

value, however we cannot choose 0 since this will lead to numerical

instability. Therefore set the permeability, Ks, to 1e−20. Click OK to

close the window.

Assign Material Properties

By default, the entire model is assigned the properties of Soil (material

1). We wish to assign the Sheet Pile properties to the sheet pilings. From

the Properties menu, select Assign Properties . Select Sheet Pile from

the Assign dialog and click inside the two narrow sections representing

the two sheet pilings (zoom in if necessary). Close the dialog.

Mesh

Now generate the finite element mesh. Select the Mesh Setup option in

the Mesh menu. Leave the default element type (3 Noded Triangles) and

the number of elements (1500). Click the Discretize button followed by

the Mesh button.

Close the Mesh Setup dialog by selecting the OK button. Your model

should now appear as shown.

Slide v.5.0

Tutorial Manual

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