Subbase Construction

1. HIGHWAY CONSTRUCTION METHODS
SUBBASE
ENG. S.A.S.T SALAWAVIDANA
CEng FIESL, MCIHT, MEng (Highway & Traffic), BSc.Eng (Hons)

2. INTRODUCTION
• Basically, all hard surfaced pavement types can be
categorized into two groups, flexible and rigid.
Flexible Pavements are those which are surfaced
with bituminous (or asphalt) materials.
• These can be either in the form of pavement
surface treatments (such as a bituminous surface
treatment (BTS) generally found on lower volume
roads) or, HMA surface courses (generally used on
higher volume roads such as the Interstate highway
network).
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3. INTRODUCTION
• These types of pavements are called
"flexible" since the total pavement
structure "bends" or "deflects" due to
traffic loads.
• A flexible pavement structure is generally
composed of several layers of materials
which can accommodate this "flexing".
• On the other hand, rigid pavements are
composed of a PCC surface course.
• Such pavements are substantially "stiffer"
than flexible pavements due to the high
modulus of elasticity of the PCC material.
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4. INTRODUCTION
• Further, these pavements can have reinforcing
steel, which is generally used to reduce or
eliminate joints.
• Each of these pavement types distributes load
over the subgrade in a different fashion.
• Rigid pavement, because of PCC's high elastic
modulus(stiffness), tends to distribute the load
over a relatively wide area of subgrade.
• The concrete slab itself supplies most of a rigid
pavement's structural capacity.
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5. INTRODUCTION
• Flexible pavement uses more flexible surface
course and distributes loads over a smaller
area.
• It relies on a combination of layers for
transmitting load to the subgrade.
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6. BASIC STRUCTURAL ELEMENTS
• A typical flexible pavement structure consists of the surface
course and the underlying base and subbase courses.
• Each of these layers contributes to structural support and
drainage.
• The surface course (typically an HMA layer) is the stiffest (as
measured by resilient modulus) and contributes the most to
pavement strength.
• The underlying layers are less stiff but are still important to
pavement strength as well as drainage and frost protection.
• A typical structural design results in a series of layers that
gradually decrease in material quality with depth.
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7. CONSTRUCTION OF SUBBASE
• The subbase course is between the base
course and the subgrade.
• It functions primarily as structural support
but it can also:
– Minimize the intrusion of fines from
the subgrade into the pavement
structure.
– Improve drainage.
– Minimize frost action damage.
– Provide a working platform for
construction.
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8. CONSTRUCTION OF SUBBASE
• It enables traffic stresses to be reduced to
acceptable levels in subgrade.
• It acts as a working plate form for the construction
of upper pavement layers.
• Acts as a drainage layer, by protecting the subgrade
from wetting up.
• It intercepts upward movement of water by
capillary action.
• It acts as a separating layer between subgrade and
road base.
• By this it prevent the two layers from mixing up.
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9. CONSTRUCTION OF SUBBASE
• The subbase generally consists of lower quality materials
than the base course but better than the subgrade soils.
• A subbase course is not always needed or used.
• For example, a pavement constructed over a high quality,
stiff subgrade may not need the additional features offered
by a subbase course so it may be omitted from design.
• However, a pavement constructed over a low quality soil
such as a swelling clay may require the additional load
distribution characteristic that a subbase course can offer.
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10. CONSTRUCTION OF SUBBASE
• In this scenario the subbase course may consist of high quality fill used to replace poor quality
subgrade (over excavation).
• Soils used for subbase must be naturally occurring or blended gravels and sands or mixtures
thereof and must not include highly plastic clays, silts, peat or other organic soils or any soil that
is contaminated with top soil vegetation and other deleterious matter.
• The material used for the top 150mm of embankment must conform to the requirements of
Type I material, and the material for lower layers of embankment must conform to the
requirements of Type II materials as given below.
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11. Type I
• The 4 day soaked CBR of the soil at 100% maximum
dry density under standard conditions of
compaction must not be less than 20%.
• The PI & LL of the soil must be less than 15% and
40% respectively. This condition may however be
relaxed at the discretion of the engineer if the
portion of materials finer than 75µm is small.
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12. Type II
• The 4 day soaked CBR of the soil at 100%
maximum dry density under standard
conditions of compaction must not be less
than 8%.
• The PI & LL of the soil must be less than 15%
and 40% respectively.
• This condition may however be relaxed at
the discretion of the engineer if the portion
of materials finer than 75µm is small.
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13. CONSTRUCTION OF SUBBASE
• Normally the construction of the subbase must not
commence in any section of the road until the
drainage works in that section have been completed.
• When this is not practical, temporary cross drains
must be provided suitably to drain the subgrade.
• Immediately prior to spreading of subbase material,
the subgrade that has been already prepared must be
cleared of any extraneous matter and the surface
must be suitably moistened.
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14. CONSTRUCTION OF SUBBASE
• If the subbase is to be placed on an existing paved road,
the bituminous crust must be first removed by
scarifying.
• The exposed surface must be suitably compacted prior
to the laying of the subbase.
• The subbase material must be normally spread in layers
not exceeding 225mm for compaction using 8-10 tonne
smooth wheeled roller or any other roller of
comparable compactive effort.
• If a heavier roller is used, the materials may be spread
in thicker layers as appropriate.
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15. CONSTRUCTION OF SUBBASE
• In case the subbase is built up of more than one
layer, the layers to follow must be placed only after
the degree of compaction of the previous layer has
been tested and found satisfactory.
• The moisture content must be at the time of
compaction at the frequency of 1 test for 250m2.
• If the material is too wet, it must be dried by aeration
and if the material is too dry, it must be sufficiently
wetted prior to compaction.
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16. CONSTRUCTION OF SUBBASE
• The rolling must start at the edge and proceed towards
the centre longitudinally except at superelevated sections.
• In such sections, the rolling must start at the lower edge
and proceed towards the higher edge.
• The subbase must be compacted to a density not less
than 100% of the maximum dry density as determined by
the standard compaction test.
• Following tests must be carried out in given frequencies to
ensure the quality of the construction.
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18. Eng. S.A.S.T Salawavidana