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%