Chapter 4. Material Characterization of Hillslope Soils
4.2. Geotechnical Soil Properties
4.2.4. In–Situ Infiltration Characteristics
In-situ tests, using Mini-disk infiltrometer and Guelph Permeameter, have been conducted for assessing the of in-situ infiltration characteristics.
The Mini-Disk infiltrometer is ideal for field measurements, due to its compact size and quick procedure. Significant number of observation spread across an area can be conducted. The Mini Disk Infiltrometer is a tension infiltrometer with an adjustable suction of 0.5 cm to 7 cm, and it measures the hydraulic conductivity of the medium it is placed on at different applied tensions (Figure 4.5).
Figure 4.5 Mini-disk Infiltrometer
The upper and lower chambers of the Infiltrometer are both filled with water up to some level. The top chamber (or bubble chamber) controls the suction. The lower chamber contains a volume of water that infiltrates into the soil at a rate determined by the suction adjusted in the bubble chamber. The lower chamber is labelled like a graduated cylinder with volume shown in ml. The bottom of the infiltrometer has a porous sintered stainless-steel disk does not allow water to leak in open air. The small diameter of the disk allows for undisturbed measurements on relatively level soil surfaces. Once the Infiltrometer is placed on a soil surface, water begins to leave the lower chamber and infiltrate into the soil at a rate
determined by the hydraulic properties of the soil. As the water level drops, the volume is recorded at specific time intervals attuned to the rate of infiltration observed during the test.
The method proposed by Zhang (1997) is used for estimating the hydraulic conductivity. The method requires the cumulative infiltration measurements versus time and fitting the results with the function as given by Equation (4.1)
1 2
I C t C t (4.1)
where, C1 is the parameter related to hydraulic conductivity and C2 is related to the soil sorptivity. The hydraulic conductivity for the soil (k) is then given by
C
1k A
(4.2)where, C1 is the slope of the curve of the cumulative infiltration versus the square root of time, and A is a value relating the van Genuchten parameters for a given soil type to the suction rate and radius of the Infiltrometer disk, and is given by
0.1
0.91
11.65 n 1 exp 2.92 n 1.9 ah A
ar
(4.3)or,
0.1
0.91
11.65 n 1 exp 7.5 n 1.9 ah A
ar
(4.4)where, n and a are the van Genuchten parameters for the soil (Fig 4.5), r is the disk radius (2.25 cm), and h is the suction at the disk surface (-1 cm and -2 cm). The formulation of the method is provided as a Microsoft Excel spreadsheet macro by the manufacturer of the Mini- disk infiltrometer, for different soil types and is used for the determination of the in-situ hydraulic conductivity.
95 tests are conducted in all around major 11 sites. Table 4.1 gives a summary of the test results.
Table 4.1 Summary of Mini-disk infiltrometer test results Site name
Maximum rate of infiltration {×10-6(m/s)}
Minimum rate of infiltration {×10-6(m/s)}
Average rate of infiltration {×10-6(m/s)}
Chunsali hill 8.68 4.42 6.55
Noonmati hill 1 3.51 0.21 2.01
Noonmati hill 2 3.06 1.41 2.23
Kailash Nagar hill 1 3.14 0.51 1.67
Kailash Nagar hill 2 0.81 0.61 0.43
Kailash Nagar hill 3 2.93 0.91 1.92
Punnya Nagar hill 6.33 1.52 4.84
Jyoti Ban 8.4 2.87 2.53
Indupur Kharghuli 12.9 3.08 7.82
Kamakhya hill 9.46 2.68 5.78
Shantipur hill 17.9 2.74 9.91
The Guelph Permeameter (Figure 4.6) is Constant-Head Permeameter (Guelph Permeameter Operating Instruction, 2008), employing the Marriotte Principle (McCarthy, 1934). As shown in the diagram (Figure 4.6), a stoppered reservoir is supplied with an air inlet and a siphon. The pressure at the bottom of the air inlet is always the same as the pressure outside the reservoir, i.e. the atmospheric pressure. If it were greater, air would not enter. If the entrance to the siphon is at the same depth, then it will always supply the water at atmospheric pressure and will deliver a flow under constant head height, regardless of the changing water level within the reservoir. The method involves measuring the steady-state rate of water recharge into unsaturated soil from a cylindrical well hole, in which a constant depth (head) of water is maintained. The constant head level in the well hole is maintained at the level of the bottom of the air tube by regulating the position of the bottom of the Air Tube, which is located in the centre of the Permeameter (Figure 4.6). As the water level in the reservoir falls, a vacuum is created in the air space above the water. The vacuum can only be relieved when air of ambient atmosphere pressure, which enters at the top of the air tube, bubbles out of the air inlet tip and rises to the top of the reservoir. Whenever the water level in the well begins to drop below the Air Inlet Tip, air bubbles emerge from the tip and rise into the reservoir air space. The vacuum is then partially relieved and water from the reservoir replenishes water in the well. The size of opening and geometry of the Air Inlet Tip is designed to control the size of air bubbles in order to prevent the well water level from fluctuating. When a constant height of water is established in the cored hole, a bulb region of soil surrounding the hole undergoes saturation. The outflow of water from the permeameter
the hole, and the height of water maintained in the hole is used to determine the field hydraulic conductivity. The analysis of steady-state discharge from a cylindrical hole into unsaturated soil, measured using the Guelph Permeameter, accounts for the force for gravity, and the suction on the water out of the hole and into the surrounding soil.
Figure 4.6 (a) Guelph Permeameter employing the Marriotte Principle (Guelph Permeameter Operating Instruction, 2008); (b) Guelph Permeameter test conducted in-situ
The field saturated hydraulic conductivity is calculated as given by
2 2
2 2 *
saturated
K CQ
H r C H
(4.5)
0.754
2.074 0.093 H C r
H r
(4.6)
where, r is the hole radius in cm, C is shape factor parameter, Q is rate of outflow of water from the reservoir tube of the permeameter in cm3/min, α* parameter depends on soil texture- structure categories (Elrick et al., 1989).
The detailed methodology and the empirical equations adapted from Zhang et al., 1998 is outlined in the manual, along with Microsoft Excel spreadsheet macro provided by the manufacturer for the determining the in-situ hydraulic conductivity of different types of soil.
Total 66 tests are conducted in and around major 11 sites. Table 2 gives a summary of the test results.
Table 4.2 Summary of Guelph Permeameter test results Site name
Maximum rate of infiltration {×10-6(m/s)}
Minimum rate of infiltration {×10-6(m/s)}
Average rate of infiltration {×10-6(m/s)}
Chunsali hill 0.96 0.87 0.91
Noonmati hill 1 1.75 0.16 0.95
Noonmati hill 2 7.36 6.70 4.02
Kailash Nagar hill 1 2.12 1.83 1.97
Kailash Nagar hill 2 0.83 0.64 0.72
Kailash Nagar hill 3 0.57 0.46 0.54
Punnya Nagar hill 4.59 4.48 4.53
Jyoti Ban 17.5 11.1 1.43
Indupur Kharghuli 11.3 9.00 10.1
Kamakhya hill 0.66 0.58 0.63
Shantipur hill 1.59 1.08 1.33