Hydraulic Conductivity Tests of Soils

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					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
         Material        Intrinsic     Hydraulic
                       Permeability   Conductivity
                          (darcy)        (cm/s)
  Clay                  10-6 – 10-3    10-9 – 10-6
  Silt, sandy silts,    10-3 – 10-1    10-6 – 10-4
  clayey sands, till
  Silty sands, fine      10-2 – 1      10-5 – 10-3
  sands
  Well-sorted sands,     1 – 102       10-3 – 10-1
  glacial outwash
  Well-sorted gravel    10 – 103        10-2 – 1
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 Panel
                       Cell P. H.W.   T.W.        Tailwater

  Compressor                                       Headwater

                                                    Water

                                                       PID
  Vacuum
                                                    Permeant




  Deaired
  Water



                                                  Permeameter
            Control Panel
                                             Cell pressure
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




                   σ
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 Gradient
   Large 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
                                           Thickness of DDL
                                T

                Negatively charged clay particle
                                T
Distance
                            Flow
controlling k
                                 T
Principle of Diffuse Double
Layer
 D = dielectric Constant of liquid
 n0 = concentration of electrolyte
 v = valence of cations
  T∝       D
         n0 v2
 k = hydraulic conductivity
      n0 v 2
   k∝
        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
Determine parameter m from k1 and k2
Determine hydraulic conductivity kv and kh
                 L       L 2
             ln[ + 1 + ( ) ]
       k2        D       D
          =                   •m
       k1       mL       mL 2
            ln[    + 1+ ( ) ]
                D        D

           1
       kv = k1          k h = mk1
           m
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 α)
(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
Tensiometer

              H
      A
              D
      B
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
Tensiometer


              H
     A
              D
     B
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
A
    D
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
Tensiometer

              H
        A
              D
        B
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 k
Tests
 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

        Modified Proctor



                                    Zero air voids curve

   γd


                 Standard Proctor



                                                 w
                  Zero air voids curve
                  Sr = 100%

γd

             70% 80%
     50%
       Line of optimums

                           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

      γd




                       w%
               wopt
       q
        u




                       w%
         wopt
q
 u




                 w%

                w%
     u


 (-)
Stress-Strain Behavior
                B              C
       γd

            A
                        w
                         opt           w%

                    B

        σ
                A
                                   C



                                        ε
γd                      B
         A


                                            w%
             w
              opt                    log σ

                            A
                             ¤g¶ôÀ½ £ÅܱK

     e              B
γd



                w%
         wopt




     k



                w%