Determination of the Permeability of Unsaturated Soils using Disk Infiltrometer

Monica Alves, Joshua ThomasSchool of Civil and Environmental Engineering, Nanyang Technological University, Singapore

www.ntu.edu.sg

Background

Our objective was to measure the permeability of different soil samples to observe how moisture content of the soil affects the rate of water infiltration. We tested soil samples compacted between the range of 7% and 17.5% moisture content. The permeability of the samples compacted at each moisture content was performed at three different suction heads: 2, 5 and 10 cm. Following, these samples were completely saturated for determination of the saturated coefficient of permeability of the soil samples. The saturated coefficient of permeability was then compared to the permeability of the respective unsaturated samples.

Soil Sample Moisture Content BT-M1 7%BT-M2 10%BT-M3 13.8%BT-M4 16.5%BT-M5 17.5%

Results -Unsaturated: Disk Infiltrometer Test

The permeability of soils with lower moisture contents was shown to be higher than the soils with higher moisture contents. This was shown especially well by the results from the 2cm suction head. A trendline was fitted to each set of data and was used to determine the permeability. A sample calculation is shown below.

For BT-M1 sample at a suction head = 2cm Pressure: Cumulative Infiltration, I = C1√t + C2(√t)2 = 0.7014t + 1.0333t Permeability, k = C2/A where A is determined from table to be 10.9 k = 1.0333 / 10.9 (cm/h) = 2.633 x 10-5 cm/s

-Saturated: Falling-head Permeability Testh/hf = exp(-kA/aL* t)

For Saturated BT-M1:-0.0265 = kA/aL = gradient A = 19.635 cm2 a = 1.431 cm2 L = 5 cm k = 0.00966 cm/hr = 2.682 x 10-6 cm/s

BT-M1: k = 2.682 x 10-6 cm/s

BT-M4: k = 8.098 x 10-7 cm/sThe slope of the graphs indicate the permeability of the soil as it becomes fully saturated, i.e. when the trendlines become constant.

ConclusionThe permeability of the saturated soils exhibit a lower permeability for soils that were compacted at a higher moisture content. The corresponding coefficients of permeability for saturated and unsaturated soil samples were similar but not exact. We observed the coefficients of permeability for the corresponding saturated soils to be an order of magnitude smaller. These results were consistent throughout our data collected.

For more accurate k, A can be determined from the soil-water characteristic curve (SWCC) obtained from pressure plate test results. The SWCC is fitted with Equation 1 shown below to obtain a and n which are substituted into Equation 2 to obtain A.

van Genutchen equation: (1) qw = qs / (1 + (αh)n)m where m = 1 – (1/n) (2) n ≥ 1.9 A = (11.65(n0.1-1)exp[2.92(n-1.9)αho])/(αro)0.91

n ≥ 1.9 A = (11.65(n0.1-1)exp[7.5(n-1.9)αho])/(αro)0.91

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Falling-head permeability test for soil sample BT-M1

Trial 1Trial 2Trial 3Trial 4Trial 5Trial 6Trial 7Trial 8Trial 9Trial 10

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Falling-head permeability test for soil sample BT-M4

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(To see the tests that were performed for the project visit http://youtu.be/FADh4CWaP4k) Project Supervisor: Assoc. Prof. Leong Eng Choon