1. Determination of the Permeability of Unsaturated Soils using Disk Infiltrometer
Monica Alves, Joshua Thomas
School 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 = 0.7014t + 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 Test
h/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/s
The slope of the graphs indicate the permeability of the soil as it becomes fully saturated, i.e. when the trendlines become constant.
Conclusion
The 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
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7
CumulativeInfiltration(cm)
t (hours1/2)
Suction Head = 2cm
BT-M1
BT-M2
BT-M3
BT-M4
BT-M5
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7
CumulativeInfiltration(cm)
t(hours1/2)
Suction Head = 5cm
BT-M1
BT-M2
BT-M3
BT-M4
BT-M5
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7
CumulativeInfiltration(cm)
t(hours1/2)
Suction Head = 10cm
BT-M1
BT-M2
BT-M3
BT-M4
BT-M5
10
100
0 0.5 1 1.5 2 2.5 3 3.5
HeightAboveDatum(cm)
Time (hours)
Falling-head permeability test for soil sample BT-M1
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Trial 6
Trial 7
Trial 8
Trial 9
Trial 10
10
100
0 0.5 1 1.5 2 2.5 3 3.5
HeightAboveDatum(cm)
Time (hours)
Falling-head permeability test for soil sample BT-M4
Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
Trial 6
(To see the tests that were performed for the project visit http://youtu.be/FADh4CWaP4k) Project Supervisor: Assoc. Prof. Leong Eng Choon