A slope with a water table

Add a piezometric line, see the pore pressure it gives along the slip surface, and how ponding works.

This tutorial uses a published example with a water table, from Arai & Tagyo (1985). Open it from Help › Benchmarks and click Arai & Tagyo (1985) in the Explorer.

1. The model

A 20 m high slope in one soil (γ = 18.82 kN/m³, c = 41.65 kPa, φ = 15°), with a water table that falls from 3 m below the crest to the ground surface at the toe.

A slope with a blue water table falling from below the crest to the toe — The water table (blue, with ▽ symbols) and the search ranges on the crest and at the toe.

2. The piezometric line

Open Model › Water Pressure › Piezometric Lines. The model has one line, Water Table, with five points.

The Piezometric Lines window with a line of five points — The piezometric line and its points. Free water above ground is on.

To make your own: click Add for a new line across the model, then edit its points in the table or drag them on the canvas. Right-click the line on the canvas to add a point.

Where the line is above the ground surface, Free water above ground (ponding) lets the water stand on the slope and load it. Here the line meets the ground at the toe, so no water is ponded.

3. Run

Click ▶ Run.

Result mode with the critical circle crossing the water table — The critical circle. The water table lowers the effective normal stress on the slip surface.

cSlope finds a factor of safety of 1.112 with Morgenstern–Price. The published values are 1.138 (Arai & Tagyo) and 1.118 (from a commercial program's verification manual).

4. Pore pressure along the slip surface

Choose Results › Plots › Results Plot and set Y axis to Pore pressure:

A plot of pore pressure against slice number, rising to about 106 kPa in the middle — Pore pressure at the base of each slice, highest where the circle is deepest below the water table.

The plot window can show any quantity along any analysed surface, not only the critical one.

5. Try it