2. SUBGRADE SOILS
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The sub-grade is the undermost layer of a pavement and
as such is one of the main concerns of a pavement design.
Many pavement failures could be traced to insufficient
consideration given to the natural sub-grade material,
especially in the case of problematic soils, the
identification of which is of paramount importance and
half the solution towards the mitigation measures.
Subgrades usually consist of fine grained cohesive or non
cohesive soils.
All these materials exhibit a stress dependent behavior
implying that both the stiffness and the shear strength
increase with increasing confinement.
3. SUBGRADE SOILS
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Soil is the most important foundation and
construction material for pavement structures.
Foundation material for all pavements as undisturbed in situ
sub grade material or transported and reworked embankment
material
Construction material for pavement structures either in its
natural form (sand and gravel) or in a processed form as
stabilized layer.
Soil investigation is, thus, an integral part of the
location, design and construction of highways.
4. Soil Surveys and Investigations
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Along with traffic and economic criteria, the design
of a road and of a road’s pavement in particular, is
based on the surface and sub soils conditions, and
the characteristics and quality of construction
materials used.
Soil survey for highway purposes involve the
exploration of the soils along the highway routes and
the identification of suitable soils for use as sub base
and fill materials.
5. Soil Surveys and Investigations
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The results of soil investigation provide pertinent
information about soil and rock for a decision on one
or more of the following subjects
Selection of roadway alignment
Decision of the need for sub grade or embankment foundation
treatment
Investigation of slope stability in cuts and embankments
Location and design of ditches and culverts
Selection and design of the roadway pavement
Location and evaluation of suitable borrow and construction
materials, and
Design of foundations for bridges and other structures
6. Soil Surveys and Investigations
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In selecting the alignment of a new highway the first step
is normally to define a number of conceivable corridors
between the end termini of the road.
The next step is to select the best corridor for the
proposed road and define within it one or more different
alignments.
These alignments are compared, and a final selection is
made for design purposes.
The process involves continuous searching and selecting,
using increasingly more detailed knowledge of sub-grade
soils at each decision-making stage.
7. Soil Surveys and Investigations
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Before a field investigation is carried out at the site, preliminary
information regarding soil condition can often be obtained from the
following sources
i. Geological and agricultural soil maps. These often indicate
the types of soil or geological formation that cover the area being
investigated.
ii. Aerial photographs. Terrain information visible on air photos
can be used for identification of most of the common bedrock types
associated residual soils, transported soils, and organic soils.
iii. Satellite images. Satellite images are employed as a
supplemental to air photos or as a substitute for air photos for
geological studies and soil investigations.
iv. Area reconnaissance. Reconnaissance survey aids in securing
broad understanding of soil conditions and associated engineering
problems that may be encountered.
v. The visual examination of vegetation cover, roadway cuts, and
valleys in the area can give clue. The depth of water level in
adjacent wells may indicate the elevation of the groundwater table.
8. Soil Surveys and Investigations
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Field investigations and sample collection for laboratory tests
are commonly carried out by the following four methods.
Geophysical methods (seismic or electrical). The seismic
refraction method relies on the principle that the velocity of sound in
soils and rocks is different for different materials. It is particularly useful
in predicting the depth to bedrock. The electrical resistivity mainly
depends on the content of clay minerals, moisture content, and type and
concentration of electrolyte in the soil-water. An increasing content of
clay, water or electrolyte causes decreasing the resistivity of soils.
Test pits or trenches: suitable for shallow depths only to sample soils
and rocks and register soil profiles.
Hand augers: suitable for shallow depths only to obtain disturbed or
mixed samples of soils.
Boring test holes and sampling with drill rigs: the principal
method for detailed soils investigations.
9. Depth of investigations
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The design depth is defined as the depth from the finished
road level to the depth that the load bearing strength of the
soil no longer has an effect on the pavement’s performance in
relation to traffic loading.
Properties of soil below the design depth may indirectly affect
pavement performance, but are generally unrelated to traffic
loading.
The depth of test pits and borings should in no case be less
than 1.5m below the proposed sub grade level unless rock
material is encountered.
A preliminary vertical alignment may be required at the time
of the soil survey in order to ensure that soil samples are
actually taken at levels that fall within the design depth of the
road.
10. Depth of investigations
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For ordinary work, it is quite sufficient to go to a depth of
about 3m below the proposed foundation level in areas of
cut and 3m below the existing ground in areas of fill
investigations in cuttings deeper than 3m could be
impractical and special equipment may have to be
required.
If possible, postponement of sampling until the time of
construction should be considered under such
conditions.
Evaluation of sub grade strength in embankment areas
should be based on the best possible information about
likely sources of fill materials for use within the design
depth.
11. SAMPLING AND FREQUENCY
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Common investigations should cover basic data collection, such as depth
and nature of soils (subgrade and embankment materials), and should be
limited to test pits and hand augers.
The common investigations can be for new roads and/or existing roads.
Once the alignment of a new road is finalized, investigations for soil
sampling along the alignment can be initiated. The frequency of sampling
depends on the field conditions.
As a standard guideline, at least one representative soil sample should be
collected per kilometer of the proposed roadway alignment, with more
frequent samples where there are significant changes in soil type.
Significant changes are those which affect the general classification of the
soils as well as their bearing strength (CBR).
The sampling location may be alternatively on the left and right edge of the
proposed roadway. Table 4-1 gives a recommended sampling frequency and
the corresponding tests. This frequency may be altered depending on the
variations in soil types along the alignment.
12. SAMPLING AND FREQUENCY
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Table 4-1 gives a recommended sampling frequency and the
corresponding tests which may be altered depending on the
variations in soil types along the alignment.
Spacing in specific locales may be increased where the
subgrade exhibits a fair degree of homogeneity, and
conversely be decreased where variations become evident, or
when problem soils or design problems are encountered.
Table 4-1 Design depth (Tanzania pavement Design manual, 1999)
13. SAMPLING AND FREQUENCY
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The recommended approximate quantity (mass) of sample
required may be determined by verifying the tests required
and referring to Table 4-2 below.
It is simpler, and generally preferable, to retrieve in the field
each sample large enough to conduct all required tests in the
laboratory.
This allows for a better selection of representative samples in
the laboratory prior to compaction and CBR testing.
If logistics preclude the taking and transportation of large
quantities of samples, careful examination of the soils in the
field must be conducted and judgment must be exercised to
select truly representative samples for compaction and CBR
testing.
15. SAMPLING AND FREQUENCY
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For the purpose of taking representative samples, pits shall be
dug mostly in anticipated cut areas (since these cuts will
expose the subgrade support of the future pavement and
provide embankment materials), if possible down to at least
30 cm below the expected subgrade level.
Further, in the case of a new alignment, the depth of any pit
should in no case be less than 1.5 m unless rock or other
material impossible to excavate by hand is encountered.
The position (in plan and elevation) of each test pit must be
accurately determined and recorded.
This implies that geotechnical and topographical tasks must
be coordinated in the field. In every test pit, all layers,
including topsoil, shall be accurately described and their
thicknesses measured.
16. SAMPLING AND FREQUENCY
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All layers of more than 30 cm (except topsoil) shall be
sampled. This will promote a proper assessment of the bulk of
the materials excavated in cuts and to be used in
embankments.
Care shall be taken, when retrieving samples, to secure and
preserve a small but sufficient quantity of soil for the purpose
of measuring the moisture content in the laboratory.
Measuring the in-situ moisture content is particularly
desirable at the anticipated subgrade level.
The log of each test pit shall be accurately drawn and included
in the Soils and Materials Report.
Photographs should also be taken of the test pit location, as
well of the soils horizons in the test pit which will help for
reporting the investigations and interpreting the results.
17. RECOMMENDED TESTS ON SOIL SAMPLES
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For new road alignments, the following tests shall
normally be conducted, as a minimum, on the
collected soil samples:
Grain Size Analysis (AASHTO T88)
Atterberg Limits (AASHTO T89, T90)
Moisture Content (AASHTO T265)
Compaction Test (AASHTO T180)
CBR and Swell (AASHTO T193)
18. RECOMMENDED TESTS ON SOIL SAMPLES
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The compaction test requires preparation of at least four (preferably
five) molds for compaction and at least one mold for CBR.
The CBR shall normally be measured after four days of soaking,
except in arid areas (annual rainfall less than 500 mm).
In arid areas, the CBR may be measured at OMC (optimum
moisture content) or after a reduced soaking period, depending on
the equilibrium moisture content predicted under the pavement in
the area (see ERA Pavement Design Manual - 2002).
The compaction tests shall be conducted on samples compacted to
95% of the MDD achieved by AASHTO Test Method T180.
The CBR tests shall be conducted at three levels (normally 90%,
95%, and 100%) of compaction, and at each level, two conditions of
moisture.
This procedure is to determine design CBR and to know the effect of
the relative compaction and moisture.
19. CLASSIFICATION OF THE SUBGRADE SOILS
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The results from the above testing, combined with the
relevant field observations, will enable a classification of the
subgrade soils to be made.
A category of soil should include the soils of the same type
having fairly consistent geotechnical characteristics (Grading,
Atterberg Limits, Compaction and particularly CBR).
A soil map should be prepared indicating the area limits of
each soil type.
Usually, the number of soil categories will not exceed 4 or 5
for a given road project.
It is advisable to avoid introducing short sections along the
alignment with numerous changes in the soil categories as
this can make the construction operations overly complicated.
20. CLASSIFICATION OF THE SUBGRADE SOILS
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For pavement design, the road sections must be defined in
accordance with subgrade strength classes, as follows
Table 4-3: Subgrade Strength Class vs. CBRs
21. CLASSIFICATION OF THE SUBGRADE SOILS
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Almost all types of soil, ranging from sandy clays through to
broken rock, can be used for embankment construction and
pavement support, the main limitation being the ease with
which the material can be handled and compacted. However,
materials with CBRs less than 2 are usually very difficult to
work and, as subgrade, would lead to uneconomical pavement
structures. Such soils, except if unavoidable, are usually
considered unsuitable. If they must be used, they must be
covered by select subgrade materials or capping layers.
The ERA Pavement Design Manual - 2002 also gives
guidance regarding estimated subgrade strength classes
depending on the plasticity of soils and the depth of the water
table. This estimated correlation can usefully be referred to as
a means to ascertain the results of the above tests.
