Hydraulic Conductivity Tests of Soils
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Transcript of Hydraulic Conductivity Tests of Soils
Hydraulic Conductivity Tests for Soils
Hsin-yu Shan
Dept. of Civil Engineering
National Chiao Tung University
Purpose
Why do we need to know the hydraulic conductivity of soil?
Challenges with Hydraulic Conductivity Measurement
Hydraulic conductivity of soil/rock varies over a very large range
Both very high and very low hydraulic conductivity values are difficult to be measured
Homogeneity and anisotropy have huge influence
Ranges of Hydraulic Conductivity
10-2 – 110 – 103Well-sorted gravel
10-3 – 10-11 – 102Well-sorted sands, glacial outwash
10-5 – 10-310-2 – 1Silty sands, fine sands
10-6 – 10-410-3 – 10-1Silt, sandy silts, clayey sands, till
10-9 – 10-610-6 – 10-3Clay
Hydraulic Conductivity
(cm/s)
Intrinsic Permeability
(darcy)
Material
Laboratory Hydraulic Conductivity Tests
Types of permeameters
Flexible-wall permeameter
Rigid-wall permeameter
Compaction mold
Thin-wall tube
Consolidation cell
�
�
Pressure/Flow Control Devices
Pressure control panel + (air compressor/pressurized gas bottle)
Water columns/reservoir
Both can be used to run constant head or variable head tests
Pressure/Flow Condition
Constant Head Method
Falling Head Method
Rising/Falling Head Method
Constant Rate of Flow
Pressure/Flow Control PanelTailwaterCell P. H.W. T.W.
Cell pressure
Headwater
Permeameter
Water
Permeant
Compressor
Vacuum
Control Panel
DeairedWater
PID
Constant-Head Method
Falling Head Method
Influencing Factors of Lab Test
Effective stress
Hydraulic gradient
Degree of saturation
Chemistry of permeation liquid
Volume of flow
Non-representative samples
Sample size
Fissures
Voids formed during sample preparation
Only becomes a problem for flexible-wall tests
Smear zones
Normally ~ 1/16 in
Growth of micro-organisms
Temperature
Viscosity and density
Effective Stress
k
e
Selection of Effective Stress
Based on the field condition
Unit weight of soil ~ 16 kN/m3 (130 pcf)
Unit weight of solid waste ~ 5.5 kN/m3 (45 pcf)
Based on the test standards
No specific stress level is specified in ASTM D5084
Hydraulic GradientLarge hydraulic gradient will cause:
Finer particles to migrate downstream and clogged the pores
Particle distribution specimen becomes not uniform
Hydraulic gradient should be comparable to that in the field usually low
Using low hydraulic gradient is time-consuming
ASTM D5084 suggests a maximum hydraulic gradient of 30 for soils with k 1 x 10-7 cm/s
Degree of Saturation
k
100%Sr
Air bubbles reduce the effective area to conduct flow
Apply backpressure to saturate the specimen
ASTM D5084 does not specify the magnitude of backpressure
Usually apply backpressure up to 300 –400 kPa (~ 40 - 60 psi)
Chemistry of Pore Liquid
Effect of diffuse double layer
Concentration of electrolyte
Valence of cations
Dielectric constant of liquid
Importance of hydration liquid
Chemical Attack of Chemicals to Clays
Double Layer Principles
Permeation liquids
Solution of salts
Acid and Base
Dissolutioning of finer particles
Solutions of dilute organic chemicals
NAPL
Landfill leachate
Negatively charged clay particle
T
T
T
Distance controlling k
Thickness of DDL
Flow
Principle of Diffuse Double Layer
D = dielectric Constant of liquid
n0 = concentration of electrolyte
v = valence of cations
k = hydraulic conductivity
T Dn v
02
n v 0
2k
D
Pore Volumes of Flow
Pore Volume, P.V. = total volume of voids of the specimen
Must allow enough liquid to flow through the specimen to be sure that the interaction between the soil and the pore liquid has stabilized
Termination Criteria
The test should be conducted long enough in order to obtain reliable results
Basic requirements are:
Reasonable outflow/inflow ratio (qout/qin)
[ASTM D5084: 0.75 - 1.25]
Stable k over a certain period
Neither increasing nor decreasing
ASTM D5084: 2 to 4 consistent k values
In-Situ Hydraulic Conductivity Tests
Borehole k test
Porous Probes
Infiltrometer
Open single/double ring infiltrometer
Sealed single/double ring infiltrometer
Lysimeter
Two-Stage Borehole Test
Developed by Boutwell (Soil Testing Engineers, 1983)
Two testing stages, each its own bulb of saturation
Obtain different rate of infiltration
Can determine hydraulic conductivity in both vertical and horizontal direction
Two Stages of Testing
First stage
Casing is driven to the bottom of the borehole
Obtain hydraulic conductivity k1 by falling head test
Second stage
The casing is driven deeper and then the infiltrometer is reassembled
Obtain hydraulic conductivity k2 by falling head test
m
D
mL
D
mL
D
L
D
L
k
k
])(1ln[
])(1ln[
2
2
1
2
Determine parameter m from k1 and k2
Determine hydraulic conductivity kv and kh
1
1k
mkv 1mkkh
Advantages
Inexpensive ( < US$2000 )
Easy to install
Can determine both vertical and horizontal hydraulic conductivity
Can be used for soils of low hydraulic conductivity ( 10-9 cm/s)
Can be conducted on slope
Disadvantages
The volume of soil tested is small
The absorption of water by soil is not taken into account when the soil is unsaturated
Long test period required (it takes several days to weeks for the flow to become steady when k < 10-7 cm/s)
Constant-Head Borehole Permeameter
Guelph Permeameter (Reynolds and Elrick 1985, 1986; Soilmoisture Equipment Corp.)
Similar to borehole tests
The absorption of water by soil is taken into account (sorptive number )
(a) Guelph permeameter (b) Bulb of saturation
Important assumptions:
The soil is homogeneous and isotropic
The soil is saturated
No volume change occurred during testing
The assumption of isotropy may lead to significant
Advantages
Inexpensive equipment ( < US$3000 )
Easy to install and assemble
The absorption of water by soil is taken into account
Relatively short testing period (a few hours to a few days)
Relatively good for measuring vertical hydraulic conductivity
Can measure hydraulic conductivity of soil at a little deeper depth
Disadvantages
The volume of soil tested is small
Not suitable for determining horizontal hydraulic conductivity
Not suitable to be used for soils of low hydraulic conductivity (k < 10-7 cm/s)
Porous Probe
Porous probes have been used to measure in-situ k for quite some time
BAT permeameter (Torstensson 1984) was designed for unsaturated, low permeability soil
Flow rate and pore pressure are computed using Boyle’s law
Assumptions:
Soils are homogeneous, isotropic, and incompressible
Neglect the adsorption of water
Temperature is constant through out the test
Hvorslev’s (1949) equations is valid
Advantages
Easy to install
Short testing time for soils of higher hydraulic conductivity (usually a few minutes to a few hours)
Pore pressure can be measured at the same time
Can be used for soils of low hydraulic conductivity ( 10-
10 cm/s)
Suitable for determining vertical hydraulic conductivity
Can measure hydraulic conductivity of soil deeper below ground surface
Disadvantages
The equipment is relatively expensive ( > US$6000)
The volume of soil tested is very small
Not suitable for determining horizontal hydraulic conductivity
The absorption of water by soil is not taken into account when the soil is unsaturated
Air-Entry Permeameter
The test is performed on the ground surface
Assumptions:
Soils are homogeneous, isotropic, and incompressible
Soils behind the wetting front are saturated
Advantages
Moderate cost ( < US$ 3000 )
Short testing time (reached equilibrium within a few hours to a few days)
Can be used for soils of low hydraulic conductivity ( 10-9 - 10-8 cm/s)
Suitable for determining vertical hydraulic conductivity
Disadvantages
Volume of soil tested is relatively small
The wetting front is within a few centimeters below the ground surface
Cannot be performed on slope
Ring Infiltrometer
Has been used to determine hydraulic conductivity of shallow soil for a long time
Four types of setup:Open single- or double- ring infiltrometer(most frequently used)
Sealed single- or double- ring infiltrometer
Hydraulic gradient is often assumed to be 1
Open, Single-Ring Infiltrometer
Most simple infiltrometer
Assumptions:
Soils are homogeneous, isotropic, and incompressible
Soils behind the wetting front are saturated
No leakage between the ring and soil
The flow of water for single-ring infiltrometer is not one-dimensional over estimate hydraulic conductivity
Not suitable for soils with k < 10-7 – 10-6
cm/s due to the relative amount of evaporation
H
DA
B
Tensiometer
Advantages
Low equipment cost ( < US$ 1000 )
Easy to install
Can manufacture large-size infiltrometer to test larger volume of soil
Suitable for determining vertical hydraulic conductivity
Disadvantages
Not suitable for soils with k < 10-7 – 10-6 cm/s
Need to correct for evaporation
Need to correct for non-one-dimensional flow
Relatively long testing time (a few weeks to a few months for soils with k < 10-7 – 10-6 cm/s)
Cannot be performed on steep slope
Open, Double-Ring Infiltrometer
Most often infiltrometer
Assumptions:Soils are homogeneous, isotropic, and incompressible
Soils behind the wetting front are saturated
No leakage between the ring and soil
Flow of water from inner ring is one-dimensionally downward
Not suitable for soils with k < 10-7 – 10-6
cm/s due to the relative amount of evaporation
Use the flow rate of inner ring to compute infiltration rate and hydraulic conductivity
H
DA
B
Tensiometer
Advantages
Inexpensive equipment ( < US$ 1000 )
Suitable for measurement of vertical hydraulic conductivity
The flow of water from inner ring can be treated as one-dimensional
Disadvantages
Not suitable for soils of low hydraulic conductivity (< 10-7 cm/s)
Need to correct for evaporation
Relatively long testing time (a few days to a few weeks for soils with k < 10-7 – 10-6
cm/s) [shorter than single-ring infiltrometer]
Cannot be performed on steep slope
Sealed, Single-Ring Infiltrometer
Same basic assumptions as those for open ring infiltrometers
The inner ring is seal Do not need to correction for evaporation
Particularly suitable for soils low hydraulic conductivity
Need to correct for non-one-dimensional flow
H
DA
B
Advantages
Relatively low cost ( < US$ 1000 )
Only suitable for determining vertical hydraulic conductivity
Suitable for soils low hydraulic conductivity (10-9 – 10-8 cm/s)
Disadvantages
Volume of soil tested is still small the diameter of the ring is less than 1 m
Need to correct for the flow direction of infiltrating water
Relatively long testing time (a few weeks to a few months)
Not suitable for sloping ground surface
Sealed Double Ring Infiltrometer, SDRI
Same basic assumptions as those for open ring infiltrometers
Do not need to consider the volume change of soil before the flow rate becomes stable
The inner ring is seal Do not need to correction for evaporation
Particularly suitable for soils low hydraulic conductivity
Measure vertical hydraulic conductivity
Do not need to correct for direction of flow flow from inner ring can be treated as
one-dimensionally downward
H
DA
B
Tensiometer
Advantages
Moderate cost ( < US$ 2500 )
Suitable for low permeability soils (< 10-8
cm/s)
Flow of inner ring can be treated as one-dimensional
Dimension of outer ring is relatively large
Disadvantages
Relatively long testing time (a few weeks to a few months)
Not applicable on sloping ground surface
Underdrain
Installed underneath the soil of which hydraulic conductivity is to be measured
Collect water infiltrated through the soil to compute hydraulic conductivity
Only suitable for test pad constructed of compacted soil
Large area of water ponds on the soil errors caused by assumption of one-dimensional flow is small
Water in the soil can be assumed to be under positive pressure the hydraulic gradient is better defined
Advantages
Low equipment cost
Applicable for determining vertical hydraulic conductivity
Larger volume of soil tested
Does not disturb the soil sample
Disadvantages
Need construction work for installation
Relatively long testing time (a few days to a few weeks for soils with k < 10-7 – 10-6
cm/s)
Lab Test vs. In-Situ Test
Advantages of lab test
Particularly relevant for compacted soils
Can conveniently test with different boundary conditions
Economical to perform
Many tests can be performed at the same time
Disadvantages of lab test
Small specimen size
Problems with sample selection
Tend to select “good” sample for testing
Effect of sample disturbance
Flow may be in the direction that is not the most critical
Grain shape and orientation can affect the isotropy or anisotropy of a sediment
Advantages of in-situ test
Test a large volume of soil
Minimized sample disturbance
More appropriate flow direction, more relevant results
Disadvantages of in-situ test
Expensive to perform
Time consuming
Test procedure is ill-defined
Problems with data reduction
Generalized Comments on kTests
Samples should be representative
Orient flow direction properly
Constant head test is preferable (constant volume during testing)
Min. edge voids and smear zones
Use relevant pore liquid
Avoid getting air bubbles
Avoid the growth of micro-organism
Use appropriate hydraulic gradient
Monitor stress-induced volume change
Hydraulic Conductivity of Compacted Soils
Earth dams
Landfill liners (bottom liners and final covers)
Surface impoundment liners
Lining of canals
Compaction Curves
Zero air voids curve
Modified Proctor
Standard Proctor
d
w
70%50%
Line of optimums
Zero air voids curveSr = 100%
d
80%
w
Types of Compaction
Impact
Proctor compaction test (lab)
Dynamic compaction (field)
Kneading – Remolded
Harvard miniature compaction (lab)
Sheepfoot roller (field)
Padfoot roller (field)
Static – Piston
Smooth wheel roller (field)
Rubber tire roller
Vibratory - Vibrator
Vibratory smooth wheel roller (field)
Effect on Undrained Shear Strength
d
w%
w%
qu
wopt
w%
u
(-)
w%
qu
wopt
Stress-Strain Behavior
wopt
A
B C
d
w%
B
AC
d
w%wopt
AB
B
A
log
e
土塊擠壓變密
d
w%wopt
k
w%