SESSION 3. SUBGRADE
Concrete Pavement Design Details and Construction Practices—Instructor’s Guide 41

SESSION 3. SUBGRADE
42 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

SESSION 3. SUBGRADE
Instructional Time: 60 minutes
Presentation File: ses_3.ppt
Technical Digest Cross Reference: Chapter 3, “Subgrade,” p. 11-22
Overview
This session describes the characterization of subgrade soils for use in the structural design of concrete pavements. Methods of determining the k-value are presented, with recommendations provided on how to determine an appropriate k-value for design. Subgrade preparation and remediation measures are also discussed.
Learning Objectives
1.
Be able to characterize the subgrade for concrete pavement design purposes.
2.
Develop an appropriate k-value for use in design given either soil properties or deflection data.
3.
Describe ways of improving subgrade soils, including appropriate means of addressing swelling soils and frost heave.
Participant Review Questions And Answers
1.
Why is the dense liquid model favored for subgrade characterization in concrete pavement design?
The dense liquid model has traditionally been favored because natural soils and untreated bases of low shear strength tend to behave more as the dense liquid model suggests than the elastic solid model. Critical edge and corner stresses and deflections are more realistically estimated using the dense liquid model than the elastic solid model. Other advantages to the dense liquid model are the availability of reliable correlations with soil properties, and easier computation than with the elastic solid model.
2.
What is a static elastic k value and why is it the appropriate parameter for concrete pavement design?
The static elastic k value is the appropriate parameter for concrete pavement design because the concrete pavement performance models are calibrated to the static elastic k value of the
Concrete Pavement Design Details and Construction Practices—Instructor’s Guide 43

SESSION 3. SUBGRADE soils of the pavements on which the performance models are based. The static elastic k value from a standard plate bearing test is representative of the response of full-sized concrete slabs under static loading.
3.
What is a realistic range of static k value for fine-grained soils (A-4 through A-7)? For coarse-grained soils (A-1 through A-3)?
A realistic range of static k value for fine-grained soils is 6.8 to 69.2 MPa/m (25 to 255 psi/in). A realistic range of static k value for coarse-grained soils is 40.7 to 135.7 MPa/m
(150 to 500 psi/in).
4.
What is the purpose of determining a seasonally adjusted k value for design?
The purpose of determining a seasonally adjusted k value for design is to identify a single constant value of k which corresponds to the same amount of damage (life consumption) to the pavement over the course of the year as the seasonal variation in k value.
5.
What causes frost heave? What can be done to minimize it?
Frost heave is caused by the formation of ice lenses when water freezes in unbound pavement layers and the foundation. Silts and low-plasticity clays are most susceptible to frost heave.
The main options for minimizing frost heave include removing and replacing frostsusceptible material, or providing a cover of pavement thickness and non-frost-susceptible material, to a total depth equal to the expected depth of frost penetration. Improving foundation drainage may also be beneficial.
6.
What causes soil swelling? What can be done to minimize it?
Soil swelling is caused by an increase in moisture content and/or a reduction in overburden pressure (i.e., excavation) in swelling-susceptible soils (certain clays and shales). It is difficult to prevent soil swelling, but methods which may minimize its likelihood include: avoiding excavating swell-susceptible soils, avoid overcompaction of such soils on the dry side of the optimum moisture content, stabilizing with lime, and minimizing moisture variation with the use of moisture barriers or geomembranes.
Pitfalls
None.
Discussion Points
Before beginning to describe k value methods, make sure the participants understand elastic versus plastic deformation, and static versus dynamic loading.
44 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

SESSION 3. SUBGRADE
Ask if anyone in the class has ever witnessed or participated in plate load testing. Don’t be surprised if some have; some DOTs do occasional plate testing, and the Air Force and Corps of
Engineers do.
Discuss the geologic characteristics of the State where the course is being held: the types of natural soils, the regions where bedrock is exposed or shallow. Under what soil conditions is a fill layer of improved material required?
Also discuss the climatic characteristics of the State which may influence foundation properties: precipitation, frost penetration, etc.
If either frost heave or soil swelling is a concern in the State, discuss the participants’ experience in dealing with these problems.
Areas To Reduce If Time Constraints Exist
Skip 1986/1993 Guide k value methods if necessary. Ask during Objectives slide whether either frost heave or swelling is a concern in the State; if not, skim over these topics to save time.
Associated Workshop
Workshop 1, Selection of Design k-value.
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SESSION 3. SUBGRADE
Presentation Graphics and Instructor Notes
Slide
1
This module presents the concepts and methods of characterizing the subgrade for the purpose of concrete pavement design.
It also highlights some practical aspects of preparing a subgrade for construction.
Slide
2
The foundation upon which the pavement and base are constructed
Concrete slab
Base
Embankment
Natural soil
Rigid layer
}
Subgrade
The term “subgrade” is used commonly to refer to the foundation upon which the base and concrete layers are constructed.
The foundation consists of the natural soil at the site, possibly an embankment (fill) of improved material, and possibly a rigid layer
(e.g., bedrock, hard clay) at a depth sufficiently shallow (within 3 m [10 ft]) that it may be significant.
46 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
3
• Characterize subgrade for concrete pavement design purposes
• Select appropriate subgrade preparation methods
• Identify subgrade remediation measures for protection against frost heave and soil swelling
SESSION 3. SUBGRADE
The objectives of this module are to:
--Describe the concepts and methods of subgrade characterization for concrete pavement design purposes
--Describe subgrade preparation for construction purposes, including protection against frost heave and soil swelling.
Slide
4
Dense liquid ( k ) model
Real soil
Elastic solid ( E ) model
Dense liquid (k) model of elastic soil response:
--deflection under the plate equals pressure divided by a k value (spring constant)
--deflection = 0 beyond edge of plate
--deflection is the same for rigid and flexible plates
--for a given pressure and deflection, k is independent of plate size
Elastic solid (E) model of elastic soil response:
--deflection depends on soil elastic modulus, plate size, and distance
--deflection basin is continuous and infinite
--rigid and flexible load plates produce different deflections
Between these two idealized extremes lies the elastic response of real soils (unbound sands, silts, and clays) of relatively low shear strength:
--plate punches down somewhat, producing a discontinuous deflection basin
--some surface deflection occurs beyond edge of load plate
--deflection = 0 at some finite distance
--for a given pressure and deflection, k varies with plate size
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SESSION 3. SUBGRADE
Slide
5
• Elastic response (k or E)
• Plastic (permanent) deformation
• Time-dependent response
• Standardized tests have been developed to differentiate the elastic response from the plastic and time-dependent components
Slide
6
• Static k: the elastic portion of a soil’s response to a static load
• Dynamic k: the elastic response to a dynamic load
- a fast-moving wheel load
- an FWD load
Both the dense liquid (k) and elastic solid (E) models attempt to describe the elastic portion of soil response. Real soils also exhibit plastic response (permanent deformation), and timedependent response: slow dissipation of porewater pressures under sustained static loading results in larger deflections than rapid dynamic loading
Standardized plate bearing test methods were developed to differentiate the elastic response of soils to static loading from the plastic and time-dependent components.
Also, in real soils, deflection is a function of plate size for small plate sizes. This is because real soils do possess some (albeit low) shear strength, and with small plates, the shear resistance around the perimeter of the plate contributes significantly to the total resistance to deformation. As the plate size becomes larger (the perimeter-to-area ratio decreases), the k asymptotically approaches a value independent of plate size.
These two terms are used a lot and it’s important to understand the difference between them. The static k is the elastic portion of soil response to static loading, as in a plate bearing test or under a load moving at creep speed. The dynamic k is the elastic portion of soil response to dynamic loading, as under a fast-moving wheel load or an FWD load impulse.
Both are elastic (that is, recoverable) but the dynamic k is higher than the static k, because under dynamic loading, porewater pressures cannot dissipate and permit further consolidation. This is especially true of finegrained soils, although coarse-grained soils also exhibit higher dynamic k values than static k values.
The phenomenon of soil stiffness being greater under dynamic loading is explained by
Terzhagi, for example. The idea of a higher k value being used to characterize this stiffer response goes back as far as Westergaard. It did not, however, become a practical concern in characterizing k value until deflection testing became common in recent decades.
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Slide
7
• K of unprotected subgrade soil
• Composite (top-of-the-base) k
• Adjustment for rigid layer
• Seasonal adjustment
• Loss-of-support adjustment
SESSION 3. SUBGRADE
These are the five steps in k value determination according to the 1986
AASHTO Guide. The differ in some significant ways from the k value methods which have been evolving since the time of
Westergaard, the Arlington Road Tests, and the Corps of Engineers field studies.
-K for unprotected soil: the equation in the
AASHTO Guide (k = M r
/19.4) is flawed and we do not recommend its use. We recommend instead the methods presented here and in the
1998 Supplement (correlations, backcalculation, or plate bearing tests).
--Composite k (from nomograph in 1986
AASHTO Guide, or its equations which we have put into the AASHTO Calculator spreadsheet): the composite k concept is not really valid. On the other hand, there’s no other way in the 1986/93 AASHTO method to reflect the potentially beneficial effect of a base on the performance of a concrete slab.
The nomograph can produce unrealistically high k values in some cases. We recommend
190 MPa/m (700 psi/in) as the upper limit on static k value.
-Adjustment for rigid layer: conceptually okay, not a practical concern unless the rigid layer is within 3 m (10 ft) depth. More significant practical problem is that the
1986/93 method does not provide an additional adjustment for fill material on top of the natural soil.
-Seasonal adjustment: Conceptually okay, except one thing – the adjustment calculates a seasonally weighted annual average k value, but the AASHTO model for rigid pavements was derived for the AASHO Road Test springtime k value, not the weighted annual average.
-Loss-of-support adjustment: A serious conceptual flaw in the 1986/93 AASHTO design procedure, and if used, produces drastic reductions in k which are unwarranted.
The AASHTO performance model is calibrated to the performance of AASHO
Road Test pavements which experienced substantial loss of support. We strongly recommend against using any LOS factor other than 0.
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SESSION 3. SUBGRADE
Slide
8
• K value methods
correlation with soil type and properties
backcalculation
plate bearing tests
• Adjustment for fill and/or rigid layer
• Seasonal adjustment
These are the three steps in the determination of k value according to the 1998 AASHTO
Supplement.
K value methods:
--correlations between soil k values and soil classes, CBR, density, degree of saturation.
Note also, Dynamic Cone Penetrometer (DCP) correlates well to CBR for fine-grained soils
(CBR less than 12).
--backcalculation from deflection tests on inservice pavements
--plate bearing tests
Adjustment for fill and/or rigid layer :
Needed if k is estimated based on soil properties only, or if, for backcalculation and plate bearing test methods, the pavement being tested and pavement being designed are different in terms of (i) soil characteristics, (ii) fill height, and (iii) rigid layer depth.
Seasonal adjustment: determination of a damage-weighted annual average k value.
The revised extended AASHTO model given in the 1998 Supplement is calibrated to this damage-weighted annual average k value for the AASHO Road Test site.
50 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
9
Slide
10
SESSION 3. SUBGRADE
Direct measurement of static elastic k value
new alignment on subgrade soil
on test embankment
existing alignment remove slab and base
• Repetitive loading test
ASTM D 1195, AASHTO T221
k = slope of pressure to elastic deformation
760-mm (30 in) plate required k = mean p /
 e
Photo of stacked plates positioned for plate bearing test. Loading plate is 760 mm (30 inches) diameter. Successively smaller plates are stacked on top to minimize plate bending, thus ensuring uniform deflections. Plate deflection is measured at 3 points around loading plate. A heavy load (e.g., water tank or heavy construction vehicle) is needed to produce sufficient deflection to determine the k value.
Plate load testing is the direct method of determining the static, elastic k value used in concrete pavement analysis and design.
According to decades of testing by the Bureau of Public Roads and the Corps of Engineers, plate bearing test methods as standardized produce static k values which are consistent with the loading response of soils under fullsize concrete slabs.
However, it is labor-intensive, timeconsuming, and costly, and therefore rarely done today; instead, correlations are often drawn between k and other soil properties.
Thus, most concrete pavement design procedures characterize the subgrade with a property that is rarely measured directly on soils in the field.
In a repetitive plate loading test, loads of increasing magnitude are slowly applied and slowly released, and the total and plastic deformations are measured each time. The elastic deformation is the total minus the plastic. The k value is defined as the average ratio of pressure to elastic deformation for the series of loads applied.
 p
 e
Deflection,

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SESSION 3. SUBGRADE
Slide
11
• Nonrepetitive loading test
ASTM D 1196, AASHTO T222
k = pressure/deformation ratio at 1.25 mm (0.05 in)
760-mm (30 in) plate required k = p /


= 1.25 mm (0.05 in)
Deflection,

A nonrepetitive test takes less time than a repetitive test, but without going through an unloading cycle to determine the plastic deformation, how does one define the k value?
Extensive testing by the Corps of Engineers established that the ratio of pressure to deflection at a deflection level of 1.25 mm
[0.05 in] yields a k value consistent with that which would be obtained in a repetitive test.
However, sometimes in the literature you will come across references to defining k at a pressure of 68.9 kPa (10 psi) instead of 1.25 mm (0.05 in) deflection. Where did the idea of defining k at 68.9 kPa (10 psi) rather than
1.25 mm (0.05 in) come from? The PCA, when it conducted plate bearing tests on cement-treated bases in the 1960s. Since it was too difficult to produce a 1.25-mm (0.05in) deflection on a cement-treated base with the available loading equipment, a pressure level of 68.9 kPa (10 psi) was selected as the defining criterion. This does not appear, however, in either the ASTM or AASHTO standard test methods.
52 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
12
Slide
13
Soil Class Density CBR k
A-1-a, well graded 125 - 140 60 - 80 300 - 450
A-1-a, poorly graded 120 - 130 35 - 60 300 - 400
… … … ...
A-2-4 or 5, gravelly 130 - 145 40 - 80 300 - 500
A-2-4 or 5, sandy 120 - 135 20 - 40 300 - 400
… … … …
A-4, silt 90 - 105 4 - 8 25 - 165
A-4, mix 100 - 125 5 - 15 40 - 220
… … … …
Degree of Saturation Affects k of Fine-Grained Soils
250
200
150
100
50
0
50 60 70 80
Degree of saturation (percent)
A-6
A-7-6
A-7-5
A-5
A-4
90 100
SESSION 3. SUBGRADE
Table 4 , page 13 in the Technical Digest presents the correlations of k to all soil classes, as well as soil descriptions, CBR ranges, and dry density ranges. This table summarizes the correlation recommendations of the Bureau of Public Roads, Corps of
Engineers, Portland Cement Association, and the Zero-Maintenance design manual.
The bearing capacity of coarse-grained cohesionless soils (A-1 and A-3) is primarily a function of the shear modulus G, which is in turn a function of elastic modulus, Poisson’s ratio, void ratio, and all-around confining pressure. The bearing capacity of these materials is relatively insensitive to moisture variation.
Coarse-grained soils with high fines (the A-2s) are diverse in their gradation characteristics and difficult to characterize. Some are stresshardening while others are stress-softening.
The nature of the fines is not a good indicator of k value. Field data indicate that in terms of bearing capacity, A-2 soils tend to behave like
A-1 or A-3 soils of comparable density.
The bearing capacity of fine-grained cohesive soils (A-4 through A-7) is strongly influenced by their degree of saturation, which is a function of the dry density, water content, and specific gravity. This is shown on the next slide.
This figure shows the effect of degree of saturation on k value of fine-grained soils.
There is a band of approximately + 275.6 kPa
( + 40 psi) around each line.
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SESSION 3. SUBGRADE
Slide
14
Slide
15
Dynamic Cone Penetrometer
(DCP)
Falling Weight
Deflectometer (FWD) existing pavement new alignment on similar soil
One increasingly popular method of determining subgrade soil properties is through the use of a dynamic (or drop) cone penetrometer (DCP). This device, which is used on in place soils, has a weight that is dropped, driving the rod (which is tipped with a metal cone) into the soil. The dropping of the weight is repeated, and the penetration of the device is measured using a graduated rod.
The penetration rate (mm/blow) is then used in established correlations to determine the CBR of the subgrade. The CBR can then be used to estimate a static, elastic k-value.
Automated DCPs are also now available.
Major advantages of this test is that it is done on in place soils, it can be performed relatively quickly and inexpensively, and it thus it can be used to provide more extensive testing on a project.
Photo of falling weight deflectometer, with load masses visible, and load plate and deflection sensors lifted up off the pavement.
Briefly explain the testing sequence: lowering the plate and sensor bar, a small load drop to seat the plate, and then a series of loads applied by lifting the masses up to one or more heights and dropping them one ore more times. The load impulse is about 15-20 milliseconds. All load and deflection data are recorded by computer, along with a distance measurement for stationing. Loads in the range of 25-70 kN (5,650-15,750 lbs) are typical for highway testing. Heavyweight
FWDs (for airport testing) can produce loads up to 250 kN (56,250 lbs).
Deflection measurement should be done on slabs which are uncracked, at least within the radius of the deflection basin measurements.
Slab size corrections may be necessary for deflections measured on small or cracked slabs.
54 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
16
SESSION 3. SUBGRADE
Photo of FWD showing load plate and deflection sensors lowered onto pavement.
Not visible is the sensor at the center of the load plate which measures the maximum deflection.
Slide
17
Westergaard’s interior deflection equation:
P

= ----- {

( a /

) } k
 2

= radius of relative stiffness:
E h

=
4 
3
12 ( 1 -

2 ) k
All k-value backcalculation equations and algorithms really boil down to this simple technique: using Westergaard’s equation for deflection (as a function of load, k, radius of relative stiffness, and load radius) and rearranging it to solve for k from a measured deflection.
To do this you need to know the measured deflection D, the load P, the load radius a, and the radius of relative stiffness. But you don’t need to know the slab modulus and thickness and subgrade k to know l, because, for a given load radius and sensor configuration, there is a unique relationship between the deflection basin AREA and the radius of relative stiffness.
AREA is the cross-sectional area of the deflection basin within the radii of the measured deflections (e.g., 0 to 36 inches), calculated by the trapezoidal rule, with all deflections normalized with respect to the maximum deflection to eliminate the effect of load level. AREA thus has dimensions of length, not area. See equation 1, page 15,
Technical Digest .
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SESSION 3. SUBGRADE
Slide
18
Slide
19
AREA = f (  ), for given sensor configuration
• Slab size adjustment usually needed
• Static k value needed for design:
approximately = dynamic k / 2
• Different backcalculation equations for deflections measured on AC-overlaid
PCC
• Variations in embankment thickness and/or rigid layer depth affect k
Load, P radius, a
Figure 3, page 15 in the Technical Digest.
This chart, which appears in the 1993
AASHTO Guide, is simply a graphical representation of Westergaard’s interior deflection equation, for a load level of 40 kN
(9,000 lbs), a load radius of 150 mm (5.91 in)
(the FWD load plate) and a sensor configuration of four sensors, at 0, 305, 610, and 914 mm (0, 12, 24, and 36 in) from the center of the load plate.
Variations on the main theme:
--AREA - r other specific sensor configurations (SHRP, Air Force seven-sensor, outer AREAs excluding D
0 ,
for
AC-overlaid pavements and very thick slabs, etc.)
--Solutions using Westergaard’s edge and corner deflection equations
--Corrections for finite slab size
--Decomposition of the backcalculated E of the pavement into the component E values : concrete and base, or concrete and overlay
Adjustments needed to backcalculated k results in order to obtain appropriate static k value for concrete pavement design:
--slab size adjustment
--divide dynamic k by two to obtain reasonable estimate of static k
--use appropriate backcalculation equations when testing on pavement with existing AC overlay
--adjust if necessary for design situation with embankment thickness and/or rigid layer depth significantly different than those of pavement tested
56 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
20
12
10
8
6
4
2 psi/in
600
T h ickn ess o f fill (ft) D en sity o f fill (lb /cu ft)
90 100 110 120 130 140 150
400
D ep th to rig id layer
< 10 ft
> 10 ft
200
200 400
A d ju sted k valu e
600 psi/in
200
E n ter w ith k fo r n atu ral su b g rad e
400
1 ft = 0.305 m ,
1 p si/in = 0.27 kP a/m m ,
1 lb /cu ft = 159 N /cu m psi/in
SESSION 3. SUBGRADE
Adjustment for embankment and/or rigid layer. Figure 2, page 14, Technical Digest . K may increase by factor of up to 2 when rigid layer is present at depth within 3 m (10 ft). At greater depths, no significant effect.
Embankment adjustment depends on embankment thickness and embankment material, which is conveniently represented by its dry density.
Slide
21
• 1998 AASHTO Supplement:
- seasonal movement of water table
- seasonal precipitation levels
- winter frost depths
- freeze-thaw cycles
- frost protection
• 1986/1993 AASHTO Guide
- annual average, or springtime?
Factors which the designer should consider when assigning k values to the distinct seasons of the year.
Should a seasonally adjusted k value be used in the 1986/1993 procedure, or the springtime k value? It’s an interesting philosophical point (the springtime value is probably more appropriate), but not a huge practical concern, because the procedure is not very sensitive to k value, at least for k values of 27 MPa/m
(100 psi/in) or more.
Slide
22
• Foundation must provide:
– Assumed stiffness
– Uniformity
– Long-term stability
– Stable construction platform
• Has significant influence on smoothness
• Typically achieved by monitoring density and moisture content during compaction
To ensure satisfactory concrete pavement performance, the subgrade must be prepared so as to provide the stiffness which was assumed in design, uniformity, long-term stability, and a stable platform for construction of the base and slab.
The uniformity of the foundation has a significant influence on the smoothness achieved in construction of the slab.
In the United States, these objectives are usually achieved by monitoring density and moisture content during compaction.
Dynamic cone penetrometer (DCP) testing is also used sometimes. In some other countries, other methods such as small-plate testing and proof rolling are used.
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SESSION 3. SUBGRADE
Slide
23
• Excavation and recompaction with moisture density control
• Mechanical improvement (mixing in coarser material)
• Excavation and replacement with select fill
• Stabilization (with lime, cement, limeflyash, asphalt)
• Reinforcement with geosynthetics
Measures which can be employed to improve a subgrade to achieve the desired stiffness and stability, roughly in order of increasing cost.
Slide
24
• Formation of ice lenses in frostsusceptible soils
- fine sands and silts
- low-plasticity clays
• Both winter frost heave and subsequent spring thaw can cause pavement cracking
Slide
25
• Replacing frost-susceptible soil with non-frost-susceptible within depth of frost penetration
• Covering frost-susceptible soil with sufficient thickness of non-frostsusceptible soil
• Factors to consider: drainage, change of grade, side slopes and ditches
Soils freeze when freezing temperatures penetrate down into the pavement structure and encounter moisture in the unbound layers.
Frost heave is a problem in areas with significant winter frost penetration, such as the northern United States. Silts are perhaps the most frost-susceptible soils because of their capacity to pull and hold water to considerable heights above the water table. Sands have much less suction potential because they are low in fines, and highly plastic clays, although fine-grained, have very low permeability
(moisture movement is inhibited).
Both winter frost heave and subsequent settlement due to spring thaw can cause cracking.
Frost protection options: remove and replace, or cover, depending on grade restrictions.
Cross-section features which will help drain the foundation will also help protect against frost damage.
58 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide

Slide
26
• Some clays and shales are susceptible to swelling (significant volume increase) when sufficient moisture is available, especially when an overburden pressure is removed
- southern and western US
- dry climates, low soil moisture contents
- pavement inhibits evaporation from soil
- excavation reduces overburden
• Swelling causes heaving and cracking
SESSION 3. SUBGRADE
Swelling is a natural response of consolidated clays to a reduction in applied pressure, when moisture is available to permit an increase in void ratio. Some clays and shales (layered sedimentary rock) are susceptible to to rather large volume changes. Soil swelling is particularly problem in the southern and western United States, where these soils are prevalent, and the climate is arid (so soil moisture contents are low).
A reduction in overburden pressure due to excavation, and/or an increase in soil moisture content, can cause soil swelling.
Slide
27
• Avoid cut sections in soils with known swelling potential
• Avoid overcompaction on dry side of optimum moisture content
• Lime stabilization to adequate depth may be useful
• Minimize moisture variation (moisture barriers or geomembranes may help)
Methods for attempting to control soil swelling are described here.
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SESSION 3. SUBGRADE
Slide
28
Slide
29
• Soils experiencing large decrease in volume with increases in water content
• Treatment methods
– Modest depths: compaction with rollers, wetting or inundation, and overexcavation and recompaction (with lime or cement)
– Thicker deposits: ponding, flooding, dynamic compaction
• Foundation: soil, embankment, rigid layer
• k value model works well for concrete pavements
• Real soils exhibit some shear strength, elastic and plastic behavior, time-dependent response
• Various methods for determining design k
• Prepare subgrade to achieve stiffness, uniformity, long-term stability, stable construction platform, protection against frost and swelling
Collapsing soils are soils with a large void content in the dry state, but undergo a very large decrease in volume if their water content increases significantly. Susceptible soils are loessial soils, weakly cemented sands and silts, and certain residual soils. Many collapsible soil deposits are associated with dry or semiarid climates, while others are commonly found on flood plains.
Treatment methods for collapsible soils depend on the depth of the soil deposits:
--For modest depths, compaction with rollers, wetting or inundation, and overexcavation and recompaction, sometimes with lime or cement stabilization.
--For thicker deposits, ponding or flooding are ordinarily very effective, as is dynamic compaction.
In summary: in this module we’ve described what makes up the foundation of a pavement, and how we characterize that foundation by a k value for the purpose of concrete pavement design. We recognize that real soils are not ideal dense liquids, but the dense liquid model suits our purposes far better than the elastic solid model.
We also described some practical considerations in preparing a subgrade so that it will provide the stiffness, stability, and uniform support desired: compaction and density control during construction, protection against frost heave, and protection against swelling.
60 Concrete Pavement Design Details and Construction Practices—Instructor’s Guide