Rapid Drawdown

GEO-SLOPE International Ltd, Calgary, Alberta, Canada www.geo-slope.com

SEEP/W Example File: Rapid drawdown.docx (pdf) (gsz) Page 1 of 5

Rapid drawdown

1 Introduction

The objective of this illustration is to show how to model the rapid drawdown of a reservoir. It presents

two cases – one with instantaneous drawdown and another with drawdown over a period of time.

This example also shows how to create a plot of factor of safety with time during and after the drawdown.

2 Problem configuration

The following diagram shows the problem configuration.

Figure 1 The problem configuration

3 Analysis tree

The analysis tree explains the steps used in the analyses.

The first step is to establish the long term steady-state conditions with a full reservoir.

The second step is to look at the case of instantaneous drawdown and how the factor of safety changes

over a period of 30 days after the drawdown.

The analysis is then repeated for the case where the reservoir is drawdown over a period of five days.

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2 2

4 m

8 m

Rapid Drawdown Example

Core

Toe drainmetres

0 5 10 15 20 25 30 35 40 45 50

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vation

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4 Instantaneous drawdown

The instantaneous case is modeled with boundary conditions. The Point at the upstream toe is assigned a

head-type boundary (H = 0.0). The remained of the upstream face below the initial reservoir level is

assigned a potential seepage face type of boundary condition.

The changing position of the piezometric line is shown Figure 2.

Figure 2 Changing positions of the piezometric line after instantaneous drawdown

Figure 3 shows the condition at the end of Day-1. Note how the excess pore-pressure remaining in the

embankment after the reservoir has been removed is seeping out of the upstream face. Also, note the

colored circles on the upstream face. These symbols show which nodes are H nodes and which Q nodes.

The H nodes are red and the Q nodes are blue. Flow emerges from the upstream face at the H (red).

These will change with each time step.

Figure 3 Conditions at the end of Day-1

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2 2

4 m

8 m


Core

Toe drain

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These results can now be used in SLOPE/W to look at the changing factors of safety with time after the

drawdown. The SLOPE/W stability analysis becomes a child of the SEEP/W transient analysis as you

can see in the Analysis Tree

The key to computing factors of safety for all the time steps is to select (all) in the Time edit box as

shown in the follow Analysis Dialog box.

Using The Draw Slip Surface command in SLOPE, we can now create the following plot (Figure 4).

Note how that removing the reservoir instantaneously causes the factor of safety to fall immediately

below 1.0 but then recovers with time as the excess pore-pressure dissipates.

Figure 4 Factor of Safety versus time after instantaneous drawdown

5 Drawdown over time

We can now repeat the process but simulate drawing down the reservoir over a five day period. This can

be done with a boundary function such as in Figure 5. The reservoir head or reservoir level is specified as

an H versus time function.

Factor of Safety vs Time

Fa

cto

r o

f S

afe

ty

Time (days)

0.8

1

1.2

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Figure 5 Reservoir drawdown curve

Now the stability looks dramatically different (Figure 6). The margin of safety against failure drops as

the reservoir is drawdown but then bottoms out at a safety factor above 1.0.

Figure 6 Factor of Safety versus time with drawdown over five days

Drawdown curveT

ota

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Factor of Safety vs Time

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Time (days)

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0 10 20 30



6 Commentary

This example illustrates the power of the product integration by using the transient SEEP results in

SLOPE to create factor of safety versus time plots.

The example also illustrates that the instantaneous case is unrealistic and likely over conservative.

Drawing down the reservoir over time is much more realistic and gives much more realistic results.

Documents

Rapid Drawdown