ABSTRACT
Base course permeability plays an important role in many pavement failures which are subjected to moisture-related problems. The Mack-Blackwell National Rural Transportation Study Center and the Arkansas Highway and Transportation Department (AHTD) sponsored the 'Permeability of Pavement Base Course' research to measure the permeability of some granular base course materials. Limestone, sandstone, igneous rock, and Razorrock chert were the materials tested.
The permeameter, which was obtained from the U.S. Bureau of Reclamation, was used during the permeability test to contain a 19" diameter by 9" thick base course specimen. A laboratory procedure was developed to build the specimen in 3 layers by using a mechanical compactor. The AHTD Class-7 base course gradation was used to construct the base course specimens. Specimens with 3%, 6.5%, and 10% fines were tested to identify the change in permeability due to the variation of fines.
Limestone is the most permeable aggregate tested for all gradations. The permeability of limestone ranges from 5.52 x 10-3 cm/sec at 3% fines to 2.49 x 10-3 cm/sec at 10% fines. The least permeable aggregate at 3% fines content is Razorrock chert which has a permeability of 2.91 x 10-3 cm/sec. At 10% fines, sandstone has the lowest permeability of 1.86 x 10-4 cm/sec. Samples with 3% fines has an average decrease of 74% in permeability when fines were increased to 10%. For all gradations, permeability results obtained from the DRAINIT spreadsheet program are approximately 100 times less permeable than the results obtained from the permeability tests.
Permeability of Pavement Base Course
Final Report
Permeability of Pavement Base Course
Submitted
to
Arkansas Highway and Transportation Department
Little Rock, Arkansas
and
Mack-Blackwell National Rural Transportation Study Center
University of Arkansas, Fayetteville
Prepared
by
SAM I. THORNTON
CHIN LEONG TOH
Civil Engineering Department
University of Arkansas
May 1995
TABLE OF CONTENTS
Page
Abstract ........................................................................................................................ 1
CHAPTER I Introduction .............................................................................. 2
CHAPTER II Literature Review .................................................................... 4
CHAPTER III Permeability Testing
── Equipment .............................................................. 14
── Materials ............................................................... 19
── Sample Preparation ............................................... 20
── Testing Procedures ................................................ 24
CHAPTER IV Results and Analysis
── Permeability Test Results ..................................... 27
── DRAINIT Results ................................................. 32
CHAPTER V Conclusions ............................................................................. 42
References .................................................................................................................. 43
Appendices
── Appendix A ................................................................................................ 46
── Appendix B ................................................................................................ 61
── Appendix C ................................................................................................ 65
LIST OF FIGURES
Figure Page
1. Constant Head Permeability Test and Falling Head Permeability Test ........... 11
2. Permeameter for Containing Base Course Specimen ...................................... 15
3. Head Tank ........................................................................................................ 15
4. WACKER Mechanical Compactor ................................................................. 16
5. Sponge Rubber Liner Attached to Inside Wall of Permeameter ..................... 16
6. Mixing Pan for Mixing Aggregates ................................................................ 18
7. Sieve Machine for Separating Soil Aggregates into Desired Gradations ....... 18
8. Rock Grinder for Grinding Soil Aggregates into Fines .................................. 19
9. AHTD Class-7 Gradation Limit
and Gradation Used for Base Course Specimens ................................ 22
10. The Set-up of Permeability Apparatus ............................................................ 25
11. The Flow Chart of Sample Preparation
and Permeability Testing Procedures .................................................. 26
12(a). Semi-log Plot of Permeability Coefficient, k (cm/sec)
vs Percent Fines Obtained from Permeability Tests ........................... 30
12(b). Semi-log Plot of Permeability Coefficient, k (ft/day)
vs Percent Fines Obtained from Permeability Tests ........................... 31
13(a). Semi-log Plot of Permeability Coefficient, k (cm/sec)
vs Percent Fines Obtained from DRAINIT ......................................... 35
13(b). Semi-log Plot of Permeability Coefficient, k (ft/day)
vs Percent Fines Obtained from DRAINIT ......................................... 36
14(a). Semi-log Plot of Permeability Coefficient, k (cm/sec)
vs Percent Fines ................................................................................... 37
14(b). Semi-log Plot of Permeability Coefficient, k (ft/day)
vs Percent Fines ................................................................................. 38
15(a). The Comparison of Averaged Permeability Test Results
and DRAINIT Results (cm/sec) ........................................................ 39
15(b). The Comparison of Averaged Permeability Test Results
and DRAINIT Results (ft/day) .......................................................... 40
LIST OF TABLES
Table Page
1. AHTD Class-7 Base Course Grading ............................................................. 21
2. Weight Distribution of Aggregate Gradations
for A Base Course Specimen (1 Layer) .............................................. 23
3(a). Permeability Test Results (cm/sec) ................................................................ 28
3(b). Permeability Test Results (ft/day) .................................................................. 28
4. DRAINIT Results ........................................................................................... 33
5(a). Equations of Plotted Best Fit Lines (cm/sec) ................................................. 34
5(b). Equations of Plotted Best Fit Lines (ft/day) ................................................... 34
CHAPTER I
Introduction
Because many pavement failures are subjected to moisture-related problems, the Mack-Blackwell National Rural Transportation Study Center and the Arkansas Highway and Transportation Department funded a research project to measure the permeability of pavement base course materials. The research project, 'Permeability of Pavement Base Course' (AHTD No. TRC-9119 and MBTC No. 1010), was also to compare the measured permeability with an often used empirical formula which is built into the 'DRAINIT' spreadsheet program.
A positive drainage system to remove free water from pavement structures is necessary in order for the pavement to have a long service life. The presence of excessive water in a paving system is known to be responsible for failures of both rigid pavement and flexible pavement. Water can cause premature rutting, cracking, faulting, increased roughness, and a decrease in the level of serviceability (Baldwin, 1987). Without a good drainage system, the pavement may suffer rapid deterioration under the action of pumping caused by dynamic traffic loading, and face the risk of frost damage.
Subsurface drainage design is a part of the pavement structural design procedure (Manual of Pavement Design Principles and Practices, 1987). Most pavement design procedures include some means of adjusting thickness or pavement life based upon the pavement drainage system. If drainage is poor, the base or wearing course thickness must be increased, resulting in a more costly pavement.
One of the best methods to evaluate the internal drainage of pavement is to measure the permeability of the least permeable layer. In a pavement, the least permeable layer is most often the base course.
Base course permeability is dependent on the density, composition, and gradation of the aggregates. However, there is no sufficiently reliable relationship for predicting permeability from the grading characteristics. Therefore, it is necessary to understand the effect of the mixture design and the construction variables on permeability in order to select the best aggregate combination (Forsyth, Wells, and Woodstorm, 1987).
The U.S. Bureau of Reclamation has developed a procedure for preparing coarse aggregate specimen like base course material and measuring the permeability. The USBR permeameter, which is a 19 inches diameter by 16 inches deep steel cylinder, is used to accommodate a specimen with particles up to 3 inches in diameter.
Objectives
The four major objectives of this study are:
P to develop a laboratory procedure for preparing base course specimens for permeability testing,
P to determine the coefficient of permeability for bases made from limestone, sandstone, igneous rock, and Razorrock (chert) aggregates,
P to find the changes in the permeability due to the variation of percent fines used, and
P to compare laboratory results with the estimated values determined from the empirical formula, which was built into the DRAINIT spreadsheet program.
CHAPTER II
Literature Review
Permeability
Permeability is a property of soil that permits the passage of fluid under a gradient of force. The coefficient of permeability is defined as the rate of discharge of water at 20EC under conditions of laminar flow through a unit cross sectional area of a soil medium under a unit hydraulic gradient (Parker and Thornton, 1977). The coefficient of permeability, which is mostly used as hydraulic conductivity, has the same units as velocity. The units of the permeability coefficient are expressed in cm/sec and ft/day in this thesis. The computation of the permeability coefficient is based on Darcy's Law, which in turn, is derived from the velocity and flow rate equations introduced by H. Darcy in 1856.
Darcy's Law
In Darcy's experiments in the 1850's, he found that for laminar flow, the quantity of water flowing through the soil in a given period of time is proportional to the soil area and the difference in piezometer levels, and inversely proportional to the length of soil between piezometers (Darcy, 1856):
Q / t % (ªh)(A) / L
where, Q = flow volume
t = time of flow
ªh = head difference
A = area of soil
L = length of flow in soil
Darcy's equation for velocity (Dunn, Anderson, and Kiefer, 1980) is shown as:
v = k (hL /L) = k i
where, v = Darcy velocity (superficial velocity)
k = coefficient of permeability
hL = head loss
L = length of flow
i = hydraulic gradient
In soil flow problems, the total cross-sectional area is most often used as the area of flow. The volume flow rate equation is shown as:
q = v A
where, q = volume flow rate
v = Darcy velocity (superficial velocity)
A = area of total cross section
Darcy's Law is valid for most types of fluid in soils except when the fluid flows at high velocity. Also, Darcy's Law may not be valid when turbulent flow condition exists in coarse sand and gravels. Therefore, with low hydraulic gradient, Darcy's Law is bounded by the following assumptions (Leonards, 1962):
P laminar flow,
P steady-state flow condition,
P homogeneous porous medium,
P no change in voids of porous medium,
P homogeneous and incompressible fluid, and
P continuous (saturated), two dimensional flow.
Base Course Permeability
In an article, Harry R. Cedergren (1994) pointed out that the life of a poorly drained pavement is reduced to one third or less of the life of a well-drained pavement. He believes the permeability of the pavement base course should be between 10,000 ft/day (3.53 cm/sec) and 100,000 ft/day (35.28 cm/sec). Cedergren also stated that the permeability increases 40,000 times if the drainage layer material is coarse, open-graded aggregates of 0.5 to 1.0 inch instead of sand.
In another paper, Jones and Jones (1989) suggested that a granular sub-base or capping layer which is to function as a drainage layer must have the following properties:
P stability,
P adequate strength and stiffness,
P adequate permeability,
P the ability to maintain its function throughout its service life, and
P be non-frost susceptible.
Jones and Jones also stated that the method of test, state of compaction, and range of hydraulic gradients should be included in any specification of permeability.
Factors Affecting Permeability
The coefficient of permeability of soil is mostly dependent on the hydraulic gradient, grain size distribution, fluid viscosity, void ratio, and degree of saturation (Das, 1994).
Hydraulic Gradient
In Darcy's equation, the head loss used to calculate the hydraulic gradient,
i = hL/L, includes the loss of pressure head, elevation head, and velocity head. The flow rate, q, in a given sand is directly related to the hydraulic gradient when the flow is laminar. When the velocity is high and the flow is turbulent, the flow rate is no longer varying in direct proportion with the hydraulic gradient; the flow rate is increased by about 1.5 times when the hydraulic gradient is doubled (Edward E. Johnson, Inc., 1966).
In 1989, C. J. Baker emphasized that Darcy's Law is only valid at low hydraulic gradient (less than 0.05) for base course material. Baker also pointed out that it is not sufficient to assume that permeability can be specified simply by a characteristic value of particle size, usually D10 , (the diameter in the particle size distribution curve corresponding to 10% finer). In reality, permeability is a function of grading, moisture content, compaction and flow direction as well as particle size. The results from Baker's study indicated that Darcy's Law is invalid at high hydraulic gradients, and there was a variability in closely controlled laboratory test results for k due to variations in porosity and degree of saturation.
Grain Size
An increase in grain size increases the coefficient of permeability. However, the relationship between permeability and grain size only exists for fairly coarse soils with rounded grains (Hazen in Das, 1994). Hazen's experiment proposed an empirical relationship for the coefficient of permeability for fairly uniform sand:
k (cm/sec) = c (D10 )2
where, c = a constant that varies from 1.0 to 1.5
D10= diameter of 10% finer (mm)
Because the formula does not include other factors like grading, moisture content, compaction, and flow direction, it is simplistic and does not accurately predict permeability (Baker, 1989).
Fluid Viscosity
From the Kozeny-Carmen (Parker and Thornton, 1977) equation, the permeability is directly proportional to the unit weight of water, γw , and inversely proportional to the viscosity of soil water, μ:
k = Ds2 (γw /μ) (e3/1+e) C
where, k = coefficient of permeability
Ds = diameter of spherical grain
γw = unit weight of water
μ = viscosity of water
e = void ratio of soil
C = a composite shape factor
The above formula, which includes the parameter of spherical grain diameter, is not appropriate for determining the coefficient of permeability for open well-graded base course material.
Temperature
Water viscosity changes primarily with temperature because unit weight and other properties remain constant. Since permeability is inversely proportional to viscosity, permeability increases when the temperature is higher (Parker and Thornton, 1977).
Twenty degrees Celsius is the most convenient temperature for laboratory permeability tests:
k20°C = kT (μT /μ20°C )
where, k20°C = permeability at temperature 20°C
kT = permeability at temperature T°C during test
μT = viscosity of water at temperature T°C during test
μ20°C = viscosity of water at temperature 20°C
Void Ratio
A decrease in void ratio decreases the permeability of soil. An equation which gives the relationship between void ratio and permeability is suggested by Casagrande as (Terzaghi and Peck, 1968):
k = 1.4 (e2) (k0.85 )
where, k = coefficient of permeability
e = void ratio
k0.85 = coefficient of permeability at void ratio of 0.85
Degree of Saturation
A decrease in the degree of saturation, Sr, of soil decreases the permeability. Darcy's Law is valid when the degree of saturation is 85% and higher because much of the air in soil is held in the form of small occluded bubbles (Parker and Thornton, 1976). The permeability is significantly decreased when the degree of saturation is less than 85% because the bubbles block some of the pores and much of the air is continuous through the voids.
McEnroe (1994) emphasized that the best measure of the granular base of a pavement is the minimum degree of saturation that can be achieved through gravity drainage in the field.
Measurements of Permeability
The permeability of soil can be measured in either the laboratory or the field. The laboratory permeability test is much easier to conduct and is most commonly used. However, the field test is necessary for in-situ soil because the laboratory test may be inadequate as a result of differences in soil structure and stress conditions between the laboratory and the field (Fang, 1991).
There are two direct methods to conduct permeability test in the laboratory: constant head permeability test and falling head permeability test.
Constant Head Permeability Test
Figure 1(a) shows the set-up of a constant head permeability test. The coefficient of permeability may be found by applying Darcy's Law as:
k = Q L / t hL A
where, k = coefficient of permeability
Q = flow volume in time t
L = length of flow
t = time of flow
hL = head loss
A = area of total cross section
Falling Head Permeability Test
Figure 1(b) shows the set-up of a falling head permeability test. The coefficient of permeability may again be computed from Darcy's equation as:
k = 2.303 (aL/At) log10 (h1 /h2 )
where, k = coefficient of permeability
a = area of graduated cylinder
L = length of flow
A = area of total cross section
t = time of flow
h1 = initial head
h2 = final head
Empirical Formulas
In 1974, Amer and Awad introduced an empirical formula to estimate the coefficient of permeability by including the effective grain size, uniformity coefficient, and the void ratio:
k = C2 D102.32 Cu0.6 (e3/1+e)
where, C2 = a constant, 3.5 x 10-4
D10 = diameter of 10% finer
Cu = uniformity coefficient, D60 / D10
D60 = diameter of 60% finer
e = void ratio
Another empirical formula presented in the Highway Subdrainage Design (1980) was developed from data on granular basses and subbases:
Permeability Coefficient, k (ft/day) = [(6.214x105)(D10 )1.478(1-γd /62.4xGs )6.654]
) (P200 )0.597
where, D10 = diameter of 10% finer (mm)
γd = dry unit weight (lb/ft3)
Gs = specific gravity
P200 = percent passing #200 sieve (%)
This formula was then used in the DRAINIT spreadsheet program to estimate the permeability of base course material.
A report from the Federal Highway Administration (Longitudinal Edge Drains in Rigid Pavement Systems, 1986) illustrated the effect of fines variation on permeability. The permeabilities of limestone, silica, sitlt, and clay were determined to be 0.07, 0.085, 0.001, and 0.0006 ft/day respectively.
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FIGURE 1. Constant Head Permeability Test and Falling Head Permeability Test.
CHAPTER III
Permeability Testing
Equipment
The following equipment was used for specimen preparation and permeability testing:
Permeability Apparatus
The permeability apparatus consist of the permeameter, the head tank, and the porous disk. The permeability apparatus was obtained from the Bureau of Reclamation, United States Department of The Interior. The permeameter (Figure 2) is a 19 inches diameter by 16 inches deep steel cylinder designed to contain a 9 inches thick base course specimen. The head tank (Figure 3) is a 6 inches diameter by 40 inches long plexiglas cylinder which contained water of 425.85 cm3/in. The porous disk was placed at the inside bottom of the permeameter. The disk is made of a 19 inches diameter by 2 inches thick coarse grade carborundum porous material.
Compaction Machine
A vibratory hammer (Figure 4), made by Wacker Corporation in Wisconsin (Model EHB 10/110), was modified and used for compacting the sample aggregates. The compactor was fitted with a 4 inches diameter compaction head and operated at 60 Hz frequency.
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FIGURE 2. Permeameter for Containing Base Course Specimen.
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FIGURE 3. Head Tank.
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FIGURE 4. WACKER Mechanical Compactor.
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FIGURE 5. Sponge Rubber Liner Attached to
Inside Wall of Permeameter.
Sponge Rubber Sheet and Rubber Cement
A 1/2 inch thick closed cell medium density sponge rubber sheet was used as a liner for the inside wall of the permeability cylinder to prevent flow between the sample and cylinder wall. A rubber cement adhesive was used for attaching the sponge rubber liner to the inside wall of the permeameter (Figure 5).
Geofilter Fabric
A geofilter fabric, made by Phillips Fibers Corporation (Type SUPAC 5-P), was placed between the base course specimen and the porous disk for preventing the fines from clogging the porous disk.
Tubing and Connector
Tubing was needed to connect water from the water supply to the head tank, from the head tank to the permeameter, and from the permeameter to a drain. The clear plastic tubing used was 5/8 inch diameter and approximately 20 feet long. A T-connector was used to inflow water from the head tank to both sides of the bottom of the permeameter.
Mixing Pan
A mixing pan (Figure 6) with a size of approximately 3 feet by 2 feet by 6 inches deep was used to mix the aggregates.
Sieves and Sieve Machine
The sieve machine (Figure 7), made by Gilson Screen Company, Inc. in Ohio (Model TS-1), was used to separate the soil aggregates into the desired gradations. Sieve screens 11/2", 3/4", #4, #40, and #200 were used to meet the AHTD specification for Class-7 base course grading.
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