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7/21/2019 Project Pp t

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Methods Used to Determine

Hydraulic Conductivity

By

Josh Linard



Background

• Hydraulic Conductivity, K, is essential to

understanding flow through soils.

– Darcy’s Law

– Richards’ Equation

– Advection-Dispersion-Equation

• Soil characteristics that determine K – Particle size

– Porosity

– Bulk density



More about K

• K is a function of pressure or moisture

content

– low matric potential = high moisturecontent = high K

• Want to know either

– Saturated hydraulic conductivity, Ks, or – Unsaturated hydraulic conductivity, K.



Other considerations

• What should the sample size be?

• Where to conduct experiment?

• How is the water applied?

• Sample size – Contemporary soil core devices.

– Representative Elementary Volume (REV).



Experiment location• Field

– Advantages

• Soil is undisturbed.

– Disadvantages

• Can’t control the environment.• Logistics.

• Laboratory

– Advantages• Highly controlled environment.

– Disadvantages

• Sample can be aggravated during transport.

• Facilities



Water Application

• Ideally, the soil should be wetted from

the bottom up.

• Should use a deaerated 0.005 MCaSO4 solution to limit air retention.

• What volume of water is required and

what volume is available.



Determining Ks

• Laboratory Methods

– Constant head

– Falling head

• Field Methods

– Test basins

• Note: for each method….

– good contact must be made at the lateral

boundaries of the core.

– Evaporation must be measured.



Constant Head Method

• Wet the column from the bottom up.

– Can be a problem depending on sample size.

• Add water until it’s at the desired height.

– Hydraulic gradient = 1 (Figure 10.1a)

– Macropore collapse? Need a different gradient.

– (Figure 10.1b)

• Capture the outflow, when it’s rate becomes

constant Ks is obtained.

L y x

L

H

L



Constant Head Apparatus



• L is length through the soil

• y is the height of ponded water

• x is the height of water required to lower

the gradient so that y can be maintained.

• Note: if the gradient is 1 then Ks = q as

per Darcy’s Law.

L y x

L

H

L



Falling Head Method

• Wet the column from the bottom up.

• Fill a burette to above the height of the soil

column and allow it to drain.

• Drain until the rate of head loss is constant.

• (Figure 11.1)

K aL A t t

H H

s

( ) log

2 1

2

1



Falling Head Apparatus



• a is the cross-sectional area of the

burette

• A is the cross-sectional area of the soilcolumn

• t2 – t1 is the time required for the head

to drop from H1 to H2.

K aL

A t t

H

H s

( )

log2 1

2

1



Test Basin Method• Isolate a column of soil

– Usually much larger than a core to be used

in the laboratory.

• Seal the lateral faces of the column

• Ensure the column is saturated

• Apply a constant head of water at rate P.

• Obtain Ks

using a mass balance

approach: I = P - E where, Ks is equal to I

since the soil is saturated.



Ks Method Summary

• The constant head method is used for

soil with a high Ks (> 0.001 cm/s).

• The falling head method is used forsoils with lower Ks (10-3 - 10-6 cm/s).

• Laboratory experiments can obtain Ks in

each dimension.



Determining Unsaturated K

• Field methods

– Ring infiltrometer.

• Laboratory methods

– Instantaneous profile method.

• Note: ensure that all instruments make

good contact with soil.



Ring Infiltrometer

• Used either in the field or laboratory.

• Can use either one or two rings.

– Scale dependent on ring size.

– 2 rings allows vertical K to be isolated.

• Can measure K when the matric

potential, ym, is 0.

– When ym is 0 a surface crust of a known

potential can be used.



Ring Infiltrometer Method

• Isolate soil column as in other methods.

• Place the infiltrometer on the soil,

ensuring good contact.

• Water is ponded on the soil and the

infiltration rate recorded.

• Unsaturated K is determined using the

Richards’ equation.



Ring Infiltrometer

Double Ring

Water Supply



Instantaneous Profile Method

• Uses tensiometers and gamma ray

absorption to measure matric potential, f,

and moisture content, q, respectively.• Pond water until the outflow is constant

and then start the experiment when the

last of the water has entered the soil.• K is obtained using q

f

t z K

z



Instantaneous Profile Method

TDR’s

Gamma RayEmitter

Gamma

RayDetector

Tensiometers



Unsaturated K Method

Summary• Ring infiltrometer

– Different sample sizes require different

rings and sometimes infiltrometers.

– Water can be hard to provide depending onthe sample size.

– Have to ensure good contact with soil.

• Instantaneous profile method – Expensive to operate and hard to set up.

– Have to ensure good contact with the soil.



Conclusion/Recommendations

• Methods described allow for determining

K in most settings.

• It’s hard to account for macropore flow.

• There is no method for determining

horizontal K in situ.

• Scales of measure are subject to criticism.

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