Hydraulic Conductivity Tests for Soils

Hsin-yu ShanDept. of Civil EngineeringNational 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 rangeBoth very high and very low hydraulic conductivity values are difficult to be measuredHomogeneity 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 permeametersFlexible-wall permeameterRigid-wall permeameter

Compaction moldThin-wall tube

Consolidation cell

透氣閥門�(快速接頭)

頭水閥門

罩頂

鐵桿

試體

尾水閥門

鋼模

濾紙

透水石片

頭水閥門

尾水閥門圍壓閥門

罩頂

外罩

鐵桿

底盤

透水石片

濾紙

濾紙

透水石片

試體

橡皮膜

透氣閥門�(快速接頭)

上蓋

橡皮環

橡皮環

底座

Pressure/Flow Control Devices

Pressure control panel + (air compressor/pressurized gas bottle)Water columns/reservoirBoth can be used to run constant head or variable head tests

Pressure/Flow Condition

Constant Head MethodFalling Head MethodRising/Falling Head MethodConstant 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 stressHydraulic gradientDegree of saturationChemistry of permeation liquidVolume of flow

Non-representative samplesSample sizeFissures

Voids formed during sample preparationOnly becomes a problem for flexible-wall tests

Smear zonesNormally ~ 1/16 in

Growth of micro-organismsTemperature

Viscosity and density

Effective Stress

k

e

σ

Selection of Effective Stress

Based on the field conditionUnit weight of soil ~ 16 kN/m3 (130 pcf) Unit weight of solid waste ~ 5.5 kN/m3 (45 pcf)

Based on the test standardsNo specific stress level is specified in ASTM D5084

Hydraulic GradientLarge hydraulic gradient will cause:

Finer particles to migrate downstream and clogged the poresParticle distribution specimen becomes not uniform

Hydraulic gradient should be comparable to that in the field usually low

Using low hydraulic gradient is time-consumingASTM 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 flowApply backpressure to saturate the specimenASTM D5084 does not specify the magnitude of backpressureUsually apply backpressure up to 300 –400 kPa (~ 40 - 60 psi)

Chemistry of Pore Liquid

Effect of diffuse double layerConcentration of electrolyteValence of cationsDielectric constant of liquid

Importance of hydration liquid

Chemical Attack of Chemicals to Clays

Double Layer PrinciplesPermeation liquids

Solution of saltsAcid and Base

Dissolutioning of finer particlesSolutions of dilute organic chemicalsNAPLLandfill leachate

Negatively charged clay particle

T

T

T

Distance controlling k

Thickness of DDL

Flow

Principle of Diffuse Double Layer

D = dielectric Constant of liquidn0 = concentration of electrolytev = valence of cations

k = hydraulic conductivity

T Dn v

∝0

2

n v 02

k D ∝

Pore Volumes of Flow

Pore Volume, P.V. = total volume of voids of the specimenMust 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 resultsBasic requirements are:

Reasonable outflow/inflow ratio (qout/qin)[ASTM D5084: 0.75 - 1.25]Stable k over a certain period

Neither increasing nor decreasingASTM D5084: 2 to 4 consistent k values

In-Situ Hydraulic Conductivity Tests

Borehole k testPorous ProbesInfiltrometer

Open single/double ring infiltrometerSealed 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 infiltrationCan determine hydraulic conductivity in both vertical and horizontal direction

Two Stages of Testing

First stageCasing is driven to the bottom of the boreholeObtain hydraulic conductivity k1 by falling head test

Second stageThe casing is driven deeper and then the infiltrometer is reassembledObtain hydraulic conductivity k2 by falling head test

m

DmL

DmL

DL

DL

kk

•++

++=

])(1ln[

])(1ln[

2

2

1

2

Determine parameter m from k1 and k2

Determine hydraulic conductivity kv and kh

11 km

kv = 1mkkh =

Advantages

Inexpensive ( < US$2000 )Easy to installCan determine both vertical and horizontal hydraulic conductivityCan be used for soils of low hydraulic conductivity (≈ 10-9 cm/s)Can be conducted on slope

Disadvantages

The volume of soil tested is smallThe absorption of water by soil is not taken into account when the soil is unsaturatedLong 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 testsThe absorption of water by soil is taken into account (sorptive number α)

(a) Guelph permeameter (b) Bulb of saturation

Important assumptions:The soil is homogeneous and isotropicThe soil is saturatedNo volume change occurred during testing

The assumption of isotropy may lead to significant

Advantages

Inexpensive equipment ( < US$3000 )Easy to install and assembleThe absorption of water by soil is taken into accountRelatively short testing period (a few hours to a few days)Relatively good for measuring vertical hydraulic conductivityCan measure hydraulic conductivity of soil at a little deeper depth

Disadvantages

The volume of soil tested is smallNot suitable for determining horizontal hydraulic conductivityNot 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 timeBAT permeameter (Torstensson 1984) was designed for unsaturated, low permeability soilFlow rate and pore pressure are computed using Boyle’s law

Assumptions:Soils are homogeneous, isotropic, and incompressibleNeglect the adsorption of waterTemperature is constant through out the testHvorslev’s (1949) equations is valid

Advantages

Easy to installShort testing time for soils of higher hydraulic conductivity (usually a few minutes to a few hours)Pore pressure can be measured at the same timeCan be used for soils of low hydraulic conductivity (≈ 10-10 cm/s)Suitable for determining vertical hydraulic conductivityCan measure hydraulic conductivity of soil deeper below ground surface

Disadvantages

The equipment is relatively expensive ( > US$6000)The volume of soil tested is very smallNot suitable for determining horizontal hydraulic conductivityThe 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 surfaceAssumptions:

Soils are homogeneous, isotropic, and incompressibleSoils 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 smallThe 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 timeFour 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 infiltrometerAssumptions:

Soils are homogeneous, isotropic, and incompressibleSoils behind the wetting front are saturatedNo leakage between the ring and soil

The flow of water for single-ring infiltrometer is not one-dimensional over estimate hydraulic conductivityNot 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 installCan manufacture large-size infiltrometer to test larger volume of soilSuitable for determining vertical hydraulic conductivity

Disadvantages

Not suitable for soils with k < 10-7 – 10-6 cm/sNeed to correct for evaporationNeed to correct for non-one-dimensional flowRelatively 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 infiltrometerAssumptions:

Soils are homogeneous, isotropic, and incompressibleSoils behind the wetting front are saturatedNo leakage between the ring and soilFlow 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 conductivityThe 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 evaporationRelatively 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 infiltrometersThe inner ring is seal Do not need to correction for evaporationParticularly suitable for soils low hydraulic conductivityNeed to correct for non-one-dimensional flow

H

DA

B

Advantages

Relatively low cost ( < US$ 1000 )Only suitable for determining vertical hydraulic conductivitySuitable 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 mNeed to correct for the flow direction of infiltrating waterRelatively 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 infiltrometersDo not need to consider the volume change of soil before the flow rate becomes stableThe inner ring is seal Do not need to correction for evaporationParticularly suitable for soils low hydraulic conductivity

Measure vertical hydraulic conductivityDo 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-dimensionalDimension 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 measuredCollect water infiltrated through the soil to compute hydraulic conductivityOnly 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 smallWater in the soil can be assumed to be under positive pressure the hydraulic gradient is better defined

Advantages

Low equipment costApplicable for determining vertical hydraulic conductivityLarger volume of soil testedDoes not disturb the soil sample

Disadvantages

Need construction work for installationRelatively 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 testParticularly relevant for compacted soilsCan conveniently test with different boundary conditionsEconomical to performMany tests can be performed at the same time

Disadvantages of lab testSmall specimen sizeProblems with sample selection

Tend to select “good” sample for testingEffect of sample disturbanceFlow 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 testTest a large volume of soilMinimized sample disturbanceMore appropriate flow direction, more relevant results

Disadvantages of in-situ testExpensive to performTime consumingTest procedure is ill-defined

Problems with data reduction

Generalized Comments on kTests

Samples should be representativeOrient flow direction properlyConstant head test is preferable (constant volume during testing)Min. edge voids and smear zonesUse relevant pore liquid

Avoid getting air bubblesAvoid the growth of micro-organismUse appropriate hydraulic gradientMonitor stress-induced volume change

Hydraulic Conductivity of Compacted Soils

Earth damsLandfill liners (bottom liners and final covers)Surface impoundment linersLining of canals

Compaction Curves

Zero air voids curve

Modified Proctor

Standard Proctor

γd

w

70%50%Line of optimums

Zero air voids curveSr = 100%

γd

80%

w

Types of Compaction

ImpactProctor compaction test (lab)Dynamic compaction (field)

Kneading – RemoldedHarvard miniature compaction (lab)Sheepfoot roller (field)Padfoot roller (field)

Static – PistonSmooth wheel roller (field)Rubber tire roller

Vibratory - VibratorVibratory smooth wheel roller (field)

Effect on Undrained Shear Strength

γd

w%

w%

qu

wopt

w%u

(-)

w%

qu

wopt

Stress-Strain Behavior

woptA

B Cγd

w%

B

A Cσ

ε

γd

w%wopt

AB

σ

B

A

log

e

土塊擠壓變密

γd

w%wopt

k

w%