Expert Modelling: Hands-on help from Rocscience Engineers. Register Here
 

Search Results

4 - Wick Drain Tutorial

1.0 Introduction

Topics Covered in this Tutorial:

  • Embankments
  • Multiple Materials
  • Time-Dependent Consolidation Analysis
  • Wick Drains

Finished Product:

The finished product of this tutorial can be found in the Tutorial 04 Wick Drains.s3z file. All tutorial files installed with Settle3 can be accessed by selecting File > Recent Folders > Tutorials Folder from the Settle3 main menu.

2.0 Model

If you have not already done so, run Settle3 by double-clicking on the Settle3 icon in your installation folder. Or from the Start menu, select Programs > Rocscience > Se!le3D 4.0 > Settle3.

Note that when the Settle3 program is started, a new blank document is already opened, allowing you to begin creating a model immediately.

2.1 Project Settings

  1. Open the Project Settings dialog from the toolbar or the Analysis menu and make sure the General tab is selected.
  2. Set the Stress units = Metric, stress as kPa, and the Settlement units = Centimeters.
  3. Select the Time-dependent Consolidation Analysis checkbox.
  4. A message will appear informing you that the groundwater analysis option will be turned on because it is required for consolidation analysis. Click OK to close the warning.
  5. Set the Time Units = Days and Permeability Units = meters / day.
  6. Project Settings dialog - General tab

  7. Click on the Stages tab. Set the Number of Stages = 7 and fill in the stage times as shown.
  8. Project Settings dialog - Stages tab

  9. Select Soil Profiles tab. Select 'Depth below ground surface' option.
  10. Click OK to exit the dialog

2.2 Assign Piezometric Line

Piezometirc Line icon Select: Groundwater > Add Piezometric Line

Set the Depth to 0 m, and click OK. Choose Select All to assign the piezo line to all materials and click OK to close the dialog.

2.3 Adding the Embankment

Add Embankment icon Select: Loads > Add Embankment

The embankment will be built in three layers. The first is a sand blanket to enable drainage from the wick drains. The other layers are fill for a road bed.

In the Embankment Designer dialog, set the Number of Layers to 3 and the Base Width to 40 m.

For Layer 1 you can leave the default values.

For Layer 2, change the stage to “Stage 2 = 30 d” and set the Left Bench Width and Right Bench Width to 1 m and the Height to 2 m.

For Layer 3, set the stage to “Stage 3 = 60 d” and the Height to 1.2 m. The dialog should look like this:

Embankment Designer dialog

Click OK. You will see cross hairs and will be prompted to pick the near point of the embankment centerline. You can either click in the Plan View or manually enter the coordinates in the bottom right of the screen.

Enter {0,-50} then press Enter. Now enter {0,50} and press Enter. Select Zoom All (or press the F2 function key) and you should now see the embankment in both the Plan View (left) and in the 3D View (right).

Note that you will only see the bottom layer if you are looking at Stage 1. Click through the other stages to observe the rest of the embankment construction.

Embankment Designer

2.4 Soil Layers

The 3D view shows a soil layer with a default thickness of 20 m. It is assumed that rigid bedrock lies below the soil. In this section we will change the soil properties and layer geometry.

2.4.1 Soil Properties

Soil Properties Icon Select: Soils > Soil Properties

The first layer shall be named Weathered Clay. Fill in the following soil properties:

  • Change unit weight and saturated unit weight to 16 kN/m3 (the non-saturated unit weight is ignored since the water table is at the surface).
  • K0 = 0
  • Immediate Settlemement is OFF
  • Primary Consolidation Settlement is ON with Non-Linear Material Type
  • Set Cc = 0.78, Cr = 0.16, e0 = 0.8 and OCR = 5
  • For time-dependent consolidation, use the permeability, K and set K = Kr = 0.0026 m/d

The dialog should look like this:

Click on the tab for Soil Property 2. Change the name to “Very Soft Clay” and fill in the parameters as shown.


Now fill in Soil Property 3 as shown


Click OK to close the dialog.

2.4.2 Layer Thickness

To change the thickness of the soil layers:

Soil Layers Icon Select: Soils > Soil Layers

Here you can add layers of different material and change their thickness. Click the Insert Layer Below button twice, to create a total of three layers, and enter the thickness as shown.

Soil Layers

Click on the Drainage Conditions button, and click the Drained Ground Surface checkbox in the Soil Drainage Conditions dialog.

Soil Layers Drainage Conditions

Click OK to close the dialog.

2.5 Field Point Grid

Select: Grid > Auto Field Point Grid

Enter number of grid points = 500, expansion factor = 2. Select OK.

 Auto Field Point Grid

A grid will be generated and stresses and settlements will be automatically computed throughout the 3-dimensional volume. By default, contours for Total Settlement will be shown. This will take a little time, depending on the speed of your computer. Click through the stages to observe settlement with time. Your model for Stage 7 (400 days) should now look like this:

 Auto Field Point 3D grid

3.0 Results Visualization

3.1 Query Point

You can see from the contour plot that the maximum settlement after 400 days is 87.7 cm. What we now need to determine is whether or not the soil has finished consolidating. To do this we will use a Query Point to look at the excess pore pressures at depth.

Add Query Point Icon Select: Query > Add Query Point

You will see the Query Point dialog as shown:

Add Query Point dialog

  1. Leave the default choice of Automatic. This will generate subdivisions such that the discretization is denser near the ground surface where the high stress gradients are likely to be.
  2. Click OK and the cursor will become a cross-hairs in the Plan View. You now need to specify the location of the Query Point. Enter the coordinates 0,0 in the bottom right of the screen and hit Enter to place the Query Point at the centre of the embankment.
  3. Now right-click on the query point and select Graph Query.
  4. For Plot Type choose Depth vs. Data.
  5. For Data to Plot choose Excess Pore Water Pressure.
  6. For Stages to Plot choose Select All.
  7. Plot Query Points

  8. Click OK to draw the graph. The graph should look like this:

Excess Pore Water Pressure vs. Depth

You can see that at 400 days, excess pore pressure has not dissipated from the lower layer, indicating that the embankment has not finished settling. To speed up the settlement, we will add an array of wick drains.

4.0 Wick Drains

Wick drains or prefabricated vertical drains (PVDs) can be driven into the soil at regular spacings to speed up consolidation. Pore water flows horizontally towards the drains and then water flows freely up the drains to the surface. This speeds up consolidation in two ways: i) horizontal permeability is usually higher than vertical permeability so horizontal flow is faster, ii) the lengths of the flow paths are significantly shortened. In this tutorial we will show how wick drains can be used to speed up consolidation under the embankment load.

4.1 Horizontal Flow Properties

Before we install wick drains, we need to specify the horizontal flow behaviour in the different materials.

  1. Click the tab at the bottom to go back to the Plan/3D View.
  2. Select Soil Properties from the toolbar or the Soils menu.
  3. For the Weathered Clay, click on the Wick Drains button. We will assume that the material is twice as permeable in the horizontal direction as it is in the vertical so set the ratio of horizontal to vertical permeability to 2 as shown.
  4. Wick Drain Material Properties

  5. Click OK.
  6. Repeat for the other 2 materials so that all material layers have a horizontal permeability that is double the vertical.
  7. Close the Soil Properties dialog.

4.2 Wick Drain Properties

To add an array of wick drains:

Add Wick Drain Icon Select: Groundwater > Wick Drains > Add Wick Drain Region

Enter the following wick drain properties:

Wick Drain Material Properties

Note:

  • The length of the drains is only 12 m, so they do not extend to the bottom of the Soft Clay layer.
  • We are assuming there is a smear zone. The smear zone results from the driving of the drains into the soil. This generally causes disturbance close to the hole and a decreased permeability.
  • We have selected strip (band) drains, with an effective 10 cm width and 5 mm thickness. An equivalent circular drain would have diameter given by d = 2(b+t)/pi = 0.067 meters.

Click OK to close the dialog. You must now enter the coordinates of the wick drain region in the Plan View. We want the wick drain region to completely cover the embankment. You can use the mouse to graphically enter the coordinates, or enter the following points in the prompt line:

{22,52}, {-22,52}, {-22,-52}, {22,-52}, "c" (to close the region)

Your model should look like this.

Wick Drain mode;

Tip: You can display the drains in the 3D View by clicking on the Display Options button, selecting the 3D View tab and selecting the Show Wick Drains checkbox.

5.0 Results

Choose Compute from the Analysis menu to re-compute the contours. Your model should look like this for Stage 7:

2D and 3D View of Stage 7 Model

You can see that the maximum settlement is now 268 cm.

Go back to the graph of excess pore pressure for the query point and it should look like this:

Excess Pore Water Pressure versus Depth Graph

Here you can see that at 400 days, the excess pore pressure is close to zero throughout the model, except in the bottom 4 meters where the wick drains do not penetrate. It is likely therefore that the soil has mostly finished consolidating.

To check this, close this graph and draw another graph for the query point by right clicking on the point in the 2D view and selecting Graph Query. Chose Plot Type: Data vs. Stage Time and choose Total Settlement at Depth: 0.

In the Sidebar under Axes, change the horizontal and vertical values to the following:

Axes Control pane

Your plot should look like this:

TIme versus Total Settlement Graph

Here you can see that the rate of settlement is decreasing with time and that the graph is nearly flat at 400 days, indicating that there should not be much more settlement.

This concludes the Wick Drain tutorial; you may now exit the Settle3 program.

Account Icon - click here to log in or out of your account Shopping Cart icon Click here to search our site Click here to close Learning Tech Support Documentation Info Chevron Delete Back to Top View More" PDF File Calendar Location Language Fees Video Click here to visit Rocscience's LinkedIn page Click here to visit Rocscience's YouTube page Click here to visit Rocscience's Twitter page Click here to visit Rocscience's Facebook page Click here to visit Rocscience's Instagram page Bookmark Network Scroll down for more Checkmark Download Print Back to top Single User Multiple Users CPillar Dips EX3 RocFall RocPlane RocSupport RocTopple RS2 RS3 RSData RSPile Settle3 Slide2 Slide3 SWedge UnWedge Commercial License Education License Trial License Shop safe & secure Money-back guarantee