4. Subgrade : Foundation Considerations
1. Introduction
The performance of any structure is strongly dependent on the foundations. Road
pavements are no exceptions and their foundations need to be fundamentally
modelled using sound geotechnical engineering principles. Thus, it is very important
to have sufficient information about the subgrade, which is obtained through
investigation and accumulation of test results. In this way the risks regarding the
subgrade can be estimated. Variation in the subgrade directly below the pavement
cannot be prevented (this is a fact of nature), so it is important not to overload the
subgrade at any place. The primary structural task of the pavement is to prevent too
high stresses in the subgrade as shown in Figure 4.1.
Sound pavement
Poor pavement
Low E
Surfacing
High E
Low E
Granular
Base
High E
HIGH
stresses
in S/G
Subgrade
LOW
stresses
in S/G
Figure 4.1 Load spreading abilities of different pavements
Every pavement design method requires some knowledge of the subgrade quality as
an important input parameter in analysis procedure. Until recently the most common
design methods were based on the CBR value of the materials in the pavement, as
determined in the CBR test. Based on CBR values of succeeding layers design
charts were developed. This is called the CBR cover design method, as it aim to
identify the thickness and bearing capacity of different layers above the subgrade
that are needed to provide enough load-spreading to protect the subgrade. Examples
of the CBR cover design are provided in the figures below.
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ESALs (80kN x106)
E0 <0.2
0.2< E1 <0.8
0.8< E2 <3
3 < E3 <12
12 < E4 <50
Figure 4.2 CBR Cover Design Curves adapted for 80kN axles (Freeme, Marais
and Walker)
The procedure for calculating the CBR is shown below. Material is first compacted to
densities that are representative of field density and soaked for 4 days before testing.
The name CBR comes from Californian Bearing Ratio because a high quality
Californian gravel was used as the reference material. Ironically, California does not
use CBR any longer (but many other countries still do).
Reference material (Ref)
Force
F
F0.1’ Ref
Actual material
F0.1’
∆P
Displacement ∆P
0.1’’
0.2’’
0.3’’
Figure 4.3. CBR Test Procedure
CBR (%) = F0.1’/F0.1’ Ref x 100
Typical values :
Sand CBR = 10 to 15%
Crushed stone CBR = 40 to 100%
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Researchers have tried to characterise the subgrade stiffness by using a correlation
with CBR values. Shell, for example, tested in situ stiffness of sands by measuring
wave propagation velocity through the sand and comparing that to laboratory
measured stiffness, as shown in the Figure below. They developed an average
relation as follows:
E [MPa] = 10 x CBR [%]
Such a correlation not very reliable, as can be seen by the variability of the
relationship. In addition, it is material specific and cannot be applied to other
gradations.
Figure 4.3 Relationship between CBR and Dynamic Modulus (EDINAMIES) for
sands (Shell)
2. Material depth
In TRH4 in South Africa (Technical Recommendations for Highways: Structural
design of Flexible Pavements for Interurban and Rural Roads), the subgrade is
shown to play a very important role in the structural design of a road (Paragraph 6.2 6.8). The catalogue design method in the TRH4 can be used only if CBR of the
subgrade is at least 15% for a depth of 150mm and decreasing gradually with depth.
This is the depth below which very little influence of the loads on the subgrade can
be expected.
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Table 4.1 Subgrade Classification for Structural Design (TRH4)
Class Subgrade CBR (%) Comment
SG1
>15
Good quality material, just rip and recompact
SG2
7 to 15
Moderate quality, needs 150mm of CBR>15 above
SG3
3 to 7
Fair quality, needs 150mm of CBR>15 + 150mm of
CBR>7 above
SG4
<3
Poor material, special treatment
Note: It should be remembered that the standard CBR is determined after specimens
have been soaked for 4 days (quite extreme but necessary to ensure that dramatic
failure doesn’t occur when a material is exposed to moisture).
Uniform sections
In South Africa Road Categories are used to designate the importance of the road
and the level of reliability for the design i.e. Categories A, B, C and D. In order to
identify the classification of the subgrade one should first identify the uniform
sections. There are two methods that one can use in identifying the uniform sections:
a) Graphical method
This method uses a graphical plot of CBR values over the centreline distance of the
road. Visual distinction can help to identify the uniform sections as shown in the
figure below.
CBR
(%)
Section 1
Section 2
Section 3
Centreline Distance (m)
Figure 4.4 Graphical Identification of Uniform Sections
b) Cumulative Sum Method
This method uses the equation below to calculate the cumulative sum value for the
CBR data over the length of the road. The cumulative sum values are then plotted
against centreline distance and the limits of the uniform sections are identified as the
inflection points of the lines.
Si = xi – x + Si-1
Where
Si = cum sum of deviations of mean CBR values
xi = CBR at point i
x = mean CBR
Si-1 = cum sum of previous point
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Cum
Sum
CBR
(%)
Section 1
Section 2
Section 3
Centreline Distance (m)
Figure 4.4 Cumulative Sum Identification of Uniform Sections
Design Reliability
For each of the uniform sections, the CBRdesign value must be calculated using the
reliability of the road category. There are two basic methods that can be followed for
such analyses:
a) Standard normal distribution method
This method uses the assumption that the CBR results in a uniform section are
normally distributed. This is a reasonable assumption for most cases of subgrade
conditions. Thereafter the method uses a basic statistical approach.
CBRdesign = x – kα. S
Where
CBRdesign = CBR design value given the reliability of the uniform section
x = average CBR value of the subgrade in the uniform section
kα = statistical coefficient for a given level of reliability
S = standard deviation for the CBR values in the uniform section
Table 4.2 Design reliability for Roads in South Africa (TRH4)
Category
A
B
C
Description
Major
Interurban
Lightly traffic
interurban
collectors and rural roads &
freeways
major rural rds strategic rds
Approx design 95
90
80
reliability (%)
1.695
1.282
0.842
kα
D
Light
pavements,
rural access
50
0
b) Cumulative less than frequency method
This method uses the distribution of only the values in the uniform section being
evaluated with fitting any standard normal distribution. The reliability value can be
read of a plot cumulative frequency versus CBR value, as shown in the example
below for a Category B road.
Example: CBR values in the uniform section: 5, 6, 7, 7, 8, 5
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Cum Frequency
(%)
CBR
Freq
60
4
0
0
50
5
2
2/7 = 28.5
40
6
1
3/7 = 42.8
7
3
6/7 = 85.7
8
1
7/7 = 100
30
Cum (%)
20
Cat B
10
CBRdesign
0
4
5
6
CBR (%) 8
Figure 4.5 Cumulative Less than Frequency Plot (example)
From such a plot the CBRdesign would be identified as 4.4% for the given uniform
section.
Seasonal Changes
The CBR test makes provision for extreme conditions i.e. 4 days of saturation. In
reality, the subgrade in the outer wheelpath changes in moisture content with time
i.e. from the dry season to the wet season, as shown in the figure below. The
influence of climatic effects will be discussed in a separate chapter.
wet
season
dry
season
Figure 4.6 Seasonal variations in moisture and CBR under a road (TRH4)
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3. Compaction
In all aspects of material behaviour, the compaction plays a very important role. The
process of compaction increases the density of a soil by packing the particles closer
together with a reduction in the volume of air. Compaction influences stiffness (load
spreading), shear strength, bearing capacity, permeability, porosity etc of a material.
How is the compactibility of a material tested and which parameters are important?
There are various standard compaction methods used in different parts of the world.
Two commonly used methods are Proctor Compaction and Modified AASHTO (or
Modified Proctor) Compaction (see method A7 in TMH 1 of SA). These methods are
used in a laboratory to provide a benchmark for the levels of compaction that can be
achieved in the field. Each method uses a standard amount of energy that is
imparted on a material in a special way (a falling weight over a known height for a
certain number of blows, and compaction of 5 layers in the mould). From the
Modified AASHTO method, density requirements for the material in the field after
compaction are determined. One of the reasons for the evolution from Proctor
compaction (lower energy) to Modified AASHTO compaction (higher energy) was to
keep pace with the developments and improvements in roller technology. In this way
the specifications were kept more realistic.
The relationship between the moisture content and achievable density during
compaction is of particular importance. Based on the results of the Mod AASHTO
test, the optimum moisture content necessary to achieve a certain required dry
density for the material in the road, is determined. The achievable density and
resulting material properties such as stiffness and shear strength etc, will determine
the behaviour of the material during service life in a road pavement.
Density is specified as ρdry (dry density) in kg/m3 so that it is independent of moisture
content. But bulk density (aggregate plus moisture) is what is measured, so the
moisture content needs to be removed mathematically from the calculation. This can
be achieved using the formula below, where w = moisture content represented as a
fraction.
ρdry = ρbulk / (1+w)
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Dry Density
ρd (kg/m3)
Proctor-compaction of
5 layers of equal blows
Max ρd
OMC=x%
Moisture Content (%)
Figure 4.7 Moisture versus density relationship
The achievable compaction is dependent on numerous factors including aggregate
grading, particle shape, particle angularity, plasticity, moisture content, compaction
energy etc. If the compaction energy is increased, not only will the dry density
increase but the optimum moisture content reduces. A “family of curves” can be
developed for a specific material that shows the changes in the moisture-density
relationship relative to the compaction energy, as shown in the figure below.
The influence of material gradation on compaction can be shown in a similar fashion.
In Figure 4.9 it is apparent that the highest density is achieved for well graded gravel
(GW), followed by well graded sand (SW), then low plasticity silt (ML), followed by
low plasticity clay (CL) and then high plasticity clay (CH).
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DD=(1-Va/V).1000/(1/Gs+w)
Figure 4.8 Influence of
relationship (Craig)
Compaction
Energy on
the
Moisture-Density
From the relationship given in Figure 4.8, it can be seen that the dry density is a
function of the aggregate specific gravity (Gs) and moisture content (w) when the air
voids are zero (Va = 0%).
Improve grading
Coarser grained
Figure 4.9 Influence of
relationship (Craig)
Compaction
Energy on
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Moisture-Density
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It can be shown using the compaction moisture curve in an elementary fashion that
with increasing density it is possible to maintain the strength of a plastic soil material
in certain service conditions. Using Figure 4.10, it can be seen that 2 samples of the
same material compacted at 17% moisture content with different compaction efforts,
the material at lower density (less compaction) exceeds the plastic limit when
saturated and will lose strength-supporting properties. The same material compacted
to a higher level will probably remain stable. This also indicates that relationships can
be found between CBR, compaction level and void content. Dry density requirements
are so very important for practice, therefore.
A similar situation can be ascribed to asphalt mixes. Instead of water, bitumen is
used in these materials. The bitumen cannot evaporate or leach out of the material
as is the case with water in granular materials and soils. However, the same
principles in terms of fluids will be used in order to understand what happens during
compaction. This is dealt with in another chapter.
ZAV
Wetting after compaction
to saturation
Dry
Density
High
Compaction
PL
Plastic Limit
Low
Compaction
17%
PL
25%
Moisture Content
Figure 4.10 Advantage of higher compaction in terms of material plasticity
(Oglesby)
Field Compaction
Achieving optimum compaction in the field requires judicious selection of rollers. The
roller selection (weight and type) is primarily judged on the gradation of the material
and the depth of the layer to be compacted. Figure 4.11 provides a guideline for roller
selection. In addition to this selection, rolling techniques for subgrade usually involve
high amplitude, low frequency initial compaction to achieve greater depth of
penetration followed by low amplitude, high frequency compaction to densify the
upper part of the layer.
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Figure 4.11 Guide to Roller Selection (Wirtgen)
References
Oglesby. Highway Subgrade Structure
Wirtgen. Cold Recycling Manual.
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