Hydraulic Conductivity Tests for Soils. Hsin-yu Shan Dept. of Civil Engineering National Chiao Tung University

Transcription

1 Hydraulic Conductivity Tests for Soils Hsin-yu Shan Dept. of Civil Engineering National Chiao Tung University

2 Purpose Why do we need to know the hydraulic conductivity of soil?

3 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

4 Ranges of Hydraulic Conductivity Clay Material Intrinsic Permeability (darcy) Hydraulic Conductivity (cm/s) Silt, sandy silts, clayey sands, till Silty sands, fine sands Well-sorted sands, glacial outwash Well-sorted gravel

5 Laboratory Hydraulic Conductivity Tests Types of permeameters Flexible-wall permeameter Rigid-wall permeameter Compaction mold Thin-wall tube Consolidation cell

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8 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

9 Pressure/Flow Condition Constant Head Method Falling Head Method Rising/Falling Head Method Constant Rate of Flow

10 Pressure/Flow Control Panel Cell P. H.W. T.W. Tailwater Compressor Headwater Water Vacuum PID Permeant Deaired Water Permeameter Control Panel Cell pressure

11 Constant-Head Method

12 Falling Head Method

13 Influencing Factors of Lab Test Effective stress Hydraulic gradient Degree of saturation Chemistry of permeation liquid Volume of flow

14 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

15 Effective Stress k e σ

16 Selection of Effective Stress Based on the field condition Unit weight of soil ~ 16 kn/m 3 (130 pcf) Unit weight of solid waste ~ 5.5 kn/m 3 (45 pcf) Based on the test standards No specific stress level is specified in ASTM D5084

17 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

18 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

19 Degree of Saturation k S r 100%

20 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 kpa (~ psi)

21 Chemistry of Pore Liquid Effect of diffuse double layer Concentration of electrolyte Valence of cations Dielectric constant of liquid Importance of hydration liquid

22 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

23 T Thickness of DDL Distance controlling k Negatively charged clay particle T Flow T

24 Principle of Diffuse Double Layer D = dielectric Constant of liquid n 0 = concentration of electrolyte v = valence of cations T D n v 0 2 k = hydraulic conductivity k n v 0 2 D

25 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

26 Termination Criteria The test should be conducted long enough in order to obtain reliable results Basic requirements are: Reasonable outflow/inflow ratio (q out /q in ) [ASTM D5084: ] Stable k over a certain period Neither increasing nor decreasing ASTM D5084: 2 to 4 consistent k values

27 In-Situ Hydraulic Conductivity Tests Borehole k test Porous Probes Infiltrometer Open single/double ring infiltrometer Sealed single/double ring infiltrometer Lysimeter

28 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

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30 Two Stages of Testing

31 First stage Casing is driven to the bottom of the borehole Obtain hydraulic conductivity k 1 by falling head test Second stage The casing is driven deeper and then the infiltrometer is reassembled Obtain hydraulic conductivity k 2 by falling head test

32 Determine parameter m from k 1 and k 2 Determine hydraulic conductivity k v and k h k k 2 1 = L ln[ D ml ln[ D + + L 2 1+ ( ) ] D ml 2 1+ ( ) D m ] 1 k v = k1 k 1 m h = mk

33 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

34 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)

35 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)

36 (a) Guelph permeameter (b) Bulb of saturation

37 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

38 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

39 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)

40 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

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42 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

43 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 ( cm/s) Suitable for determining vertical hydraulic conductivity Can measure hydraulic conductivity of soil deeper below ground surface

44 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

45 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

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48 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 ( cm/s) Suitable for determining vertical hydraulic conductivity

49 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

50 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

51 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

52 The flow of water for single-ring infiltrometer is not one-dimensional over estimate hydraulic conductivity Not suitable for soils with k < cm/s due to the relative amount of evaporation

53 Tensiometer A B H D

54 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

55 Disadvantages Not suitable for soils with k < 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 < cm/s) Cannot be performed on steep slope

56 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

57 Not suitable for soils with k < cm/s due to the relative amount of evaporation Use the flow rate of inner ring to compute infiltration rate and hydraulic conductivity

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59 Tensiometer A B H D

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63 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

64 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 < cm/s) [shorter than single-ring infiltrometer] Cannot be performed on steep slope

65 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

66 A B H D

67 Advantages Relatively low cost ( < US$ 1000 ) Only suitable for determining vertical hydraulic conductivity Suitable for soils low hydraulic conductivity ( cm/s)

68 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

69 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

70 Measure vertical hydraulic conductivity Do not need to correct for direction of flow flow from inner ring can be treated as one-dimensionally downward

71 Tensiometer A B H D

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74 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

75 Disadvantages Relatively long testing time (a few weeks to a few months) Not applicable on sloping ground surface

76 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

77 Large area of water ponds on the soil errors caused by assumption of onedimensional flow is small Water in the soil can be assumed to be under positive pressure the hydraulic gradient is better defined

78 Advantages Low equipment cost Applicable for determining vertical hydraulic conductivity Larger volume of soil tested Does not disturb the soil sample

79 Disadvantages Need construction work for installation Relatively long testing time (a few days to a few weeks for soils with k < cm/s)

80 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

81 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

82 Grain shape and orientation can affect the isotropy or anisotropy of a sediment

83 Advantages of in-situ test Test a large volume of soil Minimized sample disturbance More appropriate flow direction, more relevant results

84 Disadvantages of in-situ test Expensive to perform Time consuming Test procedure is ill-defined Problems with data reduction

85 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

86 Avoid getting air bubbles Avoid the growth of micro-organism Use appropriate hydraulic gradient Monitor stress-induced volume change

87 Hydraulic Conductivity of Compacted Soils Earth dams Landfill liners (bottom liners and final covers) Surface impoundment liners Lining of canals

88 Compaction Curves Modified Proctor Zero air voids curve γ d Standard Proctor w

89 Zero air voids curve S r = 100% γ d 70% 50% 80% Line of optimums w

90 Types of Compaction Impact Proctor compaction test (lab) Dynamic compaction (field) Kneading Remolded Harvard miniature compaction (lab) Sheepfoot roller (field) Padfoot roller (field)

91 Static Piston Smooth wheel roller (field) Rubber tire roller Vibratory - Vibrator Vibratory smooth wheel roller (field)

92 Effect on Undrained Shear Strength γ d q u w opt w% w%

93 w opt q u w% u w% (-)

94 Stress-Strain Behavior γ d B C A w opt w% B σ A C ε

95 γ d A B w opt log w% σ A e B

96 γ d w opt w% k w%