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10 - Hydroconsolidation

Topics Covered in this Tutorial:

    • Hydroconsolidation/Collapsible Soils

    • Swelling

Finished Product:

The finished product of this tutorial can be found in the Tutorial 10 Hydroconsolidation.s3z file. All tutorial files installed with Settle3 can be accessed by selecting File > Recent Folders > Tutorials Folder from the Settle3 main menu.

1.0 Introduction

In California, it is common for housing estates to be constructed on top of thick fill layers used to fill in steep canyons. These fills generally perform well when dry but significant wetting can occur due to irrigation and other urban activities. Depending on the material used for the fill, the introduction of a large amount of water can cause considerable settlement due to the collapse of the soil structure.

This tutorial describes a simplified model of a house that is part of the Villa Trinidad subdivision north of San Diego. The subdivision was built on 70 feet of fill and settlements of up to 12 inches were observed over a ten-year period. The case study is described in Brandon et al. (1990).

2.0 Model Setup

Start the Settle3 program.

2.1 Project Settings

Open the Project Settings dialog from the Analysis menu.

Project Settings icon Select Analysis > Project Settings

Make sure the General tab is selected. Set the Stress units = Imperial, stress as ksf, and the Settlement units = Inches.

Project Settings dialog - General tab

Click on the Stages tab. Set the Number of Stages = 8.

Project Settings dialog - Stages tab

Select the Soil Profiles tab. Select the 'Depth below Ground Surface' option.

Click on the Groundwater tab. Select the Groundwater Analysis checkbox. Click OK to close the dialog.

2.2 Assigning Water Table

Add Piezometric Line icon Select Groundwater > Add Piezometric Line

Enter a depth of 70 ft. Click OK to close the Assign Piezometric Line to Soils dialog. Add more piezometric lines using the same steps. Assign depths of 60, 50, 40, 30, 20, 10, and 0 ft.

Edit Piezometric Lines icon Select Groundwater > Edit Piezometric Lines

The dialog should look as follows, and you should have a total of 8 piezometric lines defined.

Edit Piezometric Lines dialog

Click OK to close the dialog.

Soil Groundwater Icon Select Groundwater > Soil Groundwater Properties

Click on the Stage Piezo Lines checkbox. Assign the piezo lines to each stage as shown below.

Define Soil Groundwater Properties dialog

Click on the Apply to All button, and click OK to close the dialog.

2.3 Soil Properties

We will assume the fill layer is placed on top of stiff rock, therefore we will only model the fill layer. The fill is a compacted clayey sand.

Soil Properties icon Select Soils > Soil Properties

Change the name of Material 1 to Fill. Set the Unit Weight to 0.115 kips/ft3 and the Saturated Unit Weight = 0.125 kips/ft3 .

For this example, we will only consider hydroconsolidation settlement. Turn OFF the Primary Consolidation checkbox, so that all settlement types are disabled (immediate, primary, and secondary). The dialog should look as follows.

Soil Properties dialog

Click OK to close the dialog.

2.4 Soil Layers

The thickness of the fill at this site is ~70 feet.

Soil Layers icon Select Soils > Soil Layers

Change the thickness of the soil layer to 70 as shown.

Soil Layers dialog

Click OK to close the dialog.

2.5 Adding a Load

We will simulate building a house in Stage 1. From the Loads menu, choose Add Rectangular Load.

Add Rectangle icon Select Loads > Add Rectangular Load

Set the length to 30 feet and the width to 20. Leave all other values as default as shown.

Define Load dialog

Click OK. Now enter the coordinates 0 0 in the prompt line at the bottom right of the screen, to place the centre of the rectangular load at the 0,0 coordinate in the Plan View.

Click in the Plan View and select Zoom All (or press the F2 function key) to center the load in the view.

Zoom All Icon Select View > Zoom > Zoom All

The model should look as follows.

2D and 3D View of Model

Notice the water table at the bottom of the fill layer. Select the stage tabs 1 to 8, and you will see the water table rise in 10-foot increments to the ground surface.

TIP: You can use the Page Down / Page Up keys to increase or decrease the viewing stage.

3.0 Hydroconsolidation

Soil collapse, or hydroconsolidation, occurs when a loose clayey sand is exposed to water and the clay bonds break causing significant volume reduction. The amount of collapse depends on the stress. At very low stresses, these soils may actually experience swelling.

To quantify the hydroconsolidation behaviour, it is common to perform a series of oedometer tests in which the sample is loaded to a certain stress state and then saturated. The change in void ratio (or strain) due to wetting is then measured (see εw in the figure below). From a series of such tests, a curve of compaction versus stress can be created.

Curve of Compaction versus Stress

Alternatively, one could simply run two oedometer tests: one with ‘dry’ material and one with wet. The difference in the two curves can then be used to get a curve of compaction versus stress (see εw in the figure below).

Curve of Compaction versus Stress

3.1 Hydroconsolidation Properties

A series of single oedometer tests were conducted on the fill material from the Villa Trinidad site and strain changes due to wetting at different stresses were measured. The results are shown in the figure below with a best-fit curve drawn ‘by eye’.

Best Fit Curve Figure

Axial strain due to wetting of fill material at different stresses. Data from Brandon et al. (1990).

To enter this data in Settle3:

Select Groundwater > Hydroconsolidation Regions > Hydroconsolidation Properties

Make sure the Fill tab is selected. Select the Consider Hydroconsolidation checkbox. Here you can enter numbers to give the relationship between the amount of collapse (strain) and the applied pressure – usually obtained from laboratory testing. Relationships from published literature can be found by clicking the property assist icon shown to the right. You will see a table of possible soil types to choose from. Select sandy-clay fill.

Published Hydroconsolidation Curves dialog

Click OK and you will see the following chart:

Hydroconsolidation Properties dialog

Negative strain refers to swelling while positive strain refers to compaction. Click OK to close the dialog.

TIP: For a non-linear material, you can enter changes in strain OR changes in void ratio. For a linear material you can only enter changes in strain because you do not specify an initial void ratio.

3.2 Hydroconsolidation Region

Hydroconsolidation occurs due to a localized increase in moisture content possibly due to irrigation, raising of the water table, broken water pipes, etc. The region over which this wetting occurs must be specified in Settle3.

Add Hydroconsolidation regiona icon Select Groundwater > Hydroconsolidation Regions > Add Hydroconsolidation Region

In the dialog, click on the Wetting Stage combo box, and choose the Use Water Table Only option. This means that hydroconsolidation will only occur due to raising of the water table. We will assume the entire fill depth is a collapsible soil, so leave the top and bottom depth as 0 and 70.

Hydroconsolidation Region Properties dialog

Click OK to close the dialog. You now need to draw a polygon in the Plan View to delineate the wetted region. We want to fully enclose the house so you may need to zoom out (use the middle mouse wheel). Enter the following coordinates to define the hydroconsolidation region:

(20, 15)

(-20, 15)

(-20, -15)

(20, -15)

c (for close)

The model should look like this:

2D and 3D View of Model

4.0 Computation and Results Visualization

To obtain results, we will add a point query to the center of the load.

Add query point icon Select Query > Add Query Point

Leave the default Automatic setting, and select OK in the dialog. Now click on the center point of the load to add the query to the model.

Select Hydroconsolidation Settlement from the drop-list of data types in the toolbar. Click on the stage tabs 1 to 8, and observe the progress of the hydroconsolidation settlement, as the water table is raised. For stage 8 the results should look as follows.

3D Model of Hydroconsolidation Settlement

The maximum settlement is 13.45 inches, which is close to the 12 inches of settlement observed at the actual site. The discrepancy may be due to several factors:

  • The entire depth of 70 feet may not have been wetted at the site.
  • The fill may not have become completely saturated at the site.
  • The best-fit line chosen to represent the strain versus stress laboratory data may not be the best choice.The lab data showed considerable scatter so it would probably be better to run two models using upper and lower bound curves for the lab data to get an estimate of maximum and minimum possible settlements (see Brandon et al., 1990).
  • The lab data may not be perfectly representative of the soil properties in-situ.

Nevertheless, Settle3 gives a reasonable estimation of settlement due to hydroconsolidation at the site.

Now let’s graph the results. Right-click on the point query and select Graph Query from the popup menu. Choose the Select All button to plot all stages. Select OK.

Plot Query Points dialog

You should see the following plot:

Hydroconsolidation Settlement versus Depth Graph

This clearly illustrates the progress of the hydroconsolidation settlement as the water table is raised. It is interesting to note that at the final stage, near the ground surface, swelling occurs, rather than consolidation, due to the low-stress level (recall that the hydroconsolidation curve for this material indicates negative strains at low-stress levels).

5.0 Further Analysis

To simplify this example, we turned OFF all other settlement types in the Soil Properties dialog. Therefore the Total Settlement is equal to the Hydroconsolidation settlement. You can verify this by viewing the Total Settlement results.

For collapsible soil, you can still define soil properties for immediate settlement, primary or secondary consolidation. This allows you to simultaneously analyze all modes of settlement. This is left as an optional exercise to explore.

Finally, note that hydroconsolidation settlement can only occur ONCE for a given material, the first time it is wetted. If the water table is raised, lowered and then raised again, hydroconsolidation will only occur once.

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

6.0 References

Brandon, T.L., Duncan, J.M., Gardner, W.S., 1990. Hydrocompression settlement of deep fills, Journal of Geotechnical Engineering, 116, 1536-1548.

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