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 Material

Clay Silt, sandy silts, clayey sands, till Silty sands, fine sands Well-sorted sands, glacial outwash Well-sorted gravel

Intrinsic Permeability (darcy) 10-6 – 10-3

Hydraulic Conductivity (cm/s) 10-9 – 10-6

10-3 – 10-1

10-6 – 10-4

10-2 – 1

10-5 – 10-3

1 – 102

10-3 – 10-1

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

尾水閥門

透氣閥門(快速接頭)

罩頂

鐵桿

鋼模 試體

濾紙

透水石片

頭水閥門

透氣閥門(快速接頭)

罩頂 外罩 鐵桿 上蓋 橡皮環 透水石片 濾紙 試體 橡皮膜 濾紙 透水石片 橡皮環 底座 底盤

頭水閥門 尾水閥門

圍壓閥門

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. Compressor

T.W.

Tailwater 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 weight of soil ~ 16 kN/m3 (130 pcf) … Unit weight of solid waste ~ 5.5 kN/m3 (45 pcf) … Unit

„

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 Æ usually low

Using low hydraulic gradient is timeconsuming „ ASTM D5084 suggests a maximum hydraulic gradient of 30 for soils with k ≤ 1 x 10-7 cm/s „

Degree of Saturation

k

Sr

100%

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 controlling k

Flow 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 m

k h = mk1

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 (≈ 1010 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 Æ over estimate hydraulic conductivity „ Not suitable for soils with k < 10-7 – 10-6 cm/s due to the relative amount of evaporation „

Tensiometer

A

H 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 onedimensionally 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

A

H 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 Æ Do not need to correction for evaporation Particularly suitable for soils low hydraulic conductivity Need to correct for non-one-dimensional flow

A

H 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 Å 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 Æ 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 Æ flow from inner ring can be treated as one-dimensionally downward „

Tensiometer

A

H D

B

Advantages Moderate cost ( < US$ 2500 ) „ Suitable for low permeability soils (< 10-8 cm/s) „ Flow of inner ring can be treated as onedimensional „ 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 Æ errors caused by assumption of onedimensional flow is small „ Water in the soil can be assumed to be under positive pressure Æ 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 50%

70% 80%

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 C

B

γd A

w opt

w%

B σ

A

C

ε

γd

B A

w% w opt

log σ A 土塊擠壓變密

e

B

γd

w% wopt

k

w%