Geo Slope - Lysimeter Behavior

7/27/2019 Geo Slope - Lysimeter Behavior

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

SEEP/W Example File: Lysimeter behavior.docx (pdf) (gsz) Page 1 of 8

The Behavior of Lysimeters

1 Introduction

This steady state SEEP/W example illustrates how to model a lysimeter from construction of the model to

interpretation of the results. Lysimeters are used to measure flow through caps and liners, but their correctdesign is critical to their ability to function under field conditions. If they are not specifically designed for

each case, considering material properties, likely flow conditions, and physical dimensions, they arelikely to not provide useful field data.

A lysimeter is a physical container (i.e., shallow pan, barrel, cup etc.) that is installed at depth within asoil profile and is designed to collect water that has percolated vertically through the soil. At regular intervals, the water that has entered the lysimeter is measured, and the amount of water collected over a

given time frame is interpreted to be equal to the infiltration rate that has occurred through thesurrounding soil profile. The measured infiltration rate can then be used to evaluate situations such as theeffectiveness of an engineered soil cover system or to predict the movement of contaminants with time.

Because a lysimeter is buried at depth, it is not possible to visually evaluate and witness its performance.Interpretation of performance depends entirely on having an understanding of the processes involved and

the properties that control flow. In the past, questions were raised about the effectiveness and reliability of shallow pan lysimeters in particular as appropriate monitoring devices. The use of a finite element modelis instrumental in learning more about how a lysimeter works and in helping understand the dominant

processes involved, ensuring that future designs are effective.

When a lysimeter is installed in the ground, it should be backfilled with the same soil as the surrounding

material.

Brenda Bews, S. Lee Barbour, G. Ward Wilson and Mike A. O’Kane drew attention to the behavior of

lysimeters in a paper presented at the 50th Canadian Geotechnical Conference in Ottawa, Ontario in 1997.

The illustrative model used here is in part based on what was presented in this paper.

2 Configuration

The following figure shows the model configuration. A clay layer overlies six metres of sand. There is a

watertable at depth in the sand. Also, it is also assumed that there is sufficient water on the surface so thatthe pore-pressure remains zero at all times.

The conductivity of the clay will be used to illustrate the effect of infiltration rates. A low conductivity

means less infiltration and vice versa.

The intent is to model both a shallow pan and a deep pan. This can be conveniently be done by adding or

removing material from the wall of the container.

Advantage can be taken of symmetry in this case to reduce the file sizes and computing time. Only theleft half of the problem is used in the analysis.

Water can potentially exit the pan at the center. It is flagged as a Potential Seepage Point.





Figure 1 Problem configuration

3 Material properties

The conductivity functions used are shown in fig. Worth noting is that the conductivity is orders of magnitude less than the sand. Three cases will be considered; one with K = 1 x 10-7 m/s, another with K

= 1 x 10-6 and a third with K = 1 x 10-5 m/s. These different values give different infiltration rates.

These conductivity functions are estimated from the sample volumetric water content function for clay

and sand given in GeoStudio.

Lysimeter Behavior

Clay liner

Sand

Distance - m

-1 0 1 2 3 4 5 6 7 8 9 10

E l e v a t i o n - m

-1

0

1

2

3

4

5

6

7





Figure 2 Conductivity functions used

4 Convergence

This example is somewhat like a vertical infiltration problem which usually requires many iterations to

obtain a converged solution. For this example the Under-Relaxation minimum rate needs to be reduced to0.01 (1%) to achieve converged solutions.

Sand K

Clay K function

X - C o n d u c t i v i t y ( m / s e

c )

Matric Suction (kPa)

1.0e-02

1.0e-13

1.0e-12

1.0e-11

1.0e-10

1.0e-09

1.0e-08

1.0e-07

1.0e-06

1.0e-05

1.0e-04

1.0e-03

0.01 10000.1 1 10 100





5 Shallow pan – high infiltration

The following figure shows that under high infiltration (relatively speaking) the leakage through the clayis all collected by the pan. In other words the lysimeter is functioning as anticipated.

Figure 3 Shallow pan with high infiltration

Lysimeter Behavior

Clay liner

Sand





6 Shallow pan – medium infiltration

If we now tighten up the conductivity of the clay by a factor of 10, only a portion of the leakage iscollected by the lysimeter as is evident in Figure 4. A good portion of the leakage is siphoned out of the

pan and is lost from the collection system.

Figure 4 Shallow pan with medium infiltration

Lysimeter Behavior

Clay liner

Sand





7 Shallow pan – low infiltration

As shown in Figure 5, if we now decrease the infiltration even further, all the leakage through the clay bypasses the lysimeter. This highlights the issue with the design and installation of these types of leakage

collection systems.

Figure 5 Shallow pan with low infiltration

Lysimeter Behavior

Clay liner

Sand





8 Deep pan – low infiltration

As Bews and others have pointed out, one solution to this problem with lysimeters is to make the wallsrelatively high. As can be seen in Figure 6, with high walls all the leakage is collected even for very low

flow rates.

Note the high was case was obtained by simply removing the material from the region representing thewall.

Figure 6 High walls with low infiltration

Lysimeter Behavior

Clay liner

Sand





9 Judging the solution

In problems like this where it is difficult to achieve convergence it is very important to examine theresults very carefully. One of the best ways to judge the results is to make a K versus suction graph such

as in Figure 7. It reveals that the K values used in the solution fall on the K-function which they must ina converged solution.

Figure 7 Conductivity versus suction for the computed solution

10 Reference

Bews, B.E., Barbour, S.L., Wilson, G.W. and O’Kane, M.A. (1997). The Design of Lysimeters for a Low Flux Cover System over Acid Generating Waste Rock , Proceedings – 50th Canadian GeotechnicalConference, Ottawa, Ontario, Canada, pp. 26 – 33.

K- suction

Actual - Sand

K-Function -Sand

Actual - Clayliner

K-Function -Clay liner

X - C o n d u c t i v i t y ( m / s e c )

Matric Suction (kPa)

1.0e-08

1.0e-02

1.0e-07

1.0e-06

1.0e-05

1.0e-04

1.0e-03

0.1 1001 10

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Geo Slope - Lysimeter Behavior