Technical University
DCE 2301 TRANSPORTATION
ENGINEERING I
PAVEMENT DESIGN
FUNCTIONS OF PAVEMENT LAYERS
THE WEARING COURSE:
The wearing surface may range in thickness between 25mm for light traffic to 150mm or more
of asphalt concrete used for heavily travelled routes. The surface must be capable of
withstanding the wear and abrasive effects of moving vehicles and must possess sufficient
stability to prevent it from shearing and rutting under traffic loads entrance of surface water
into the base, subbase and subgrade.
BASE:
This is a layer (or layers) of very high stability and density.
Its major purpose is to spread or distribute the stresses created by wheel loads acting on
the wearing surface so that the stresses transmitted to the subgrade will not be sufficiently
great to result in excessive deformation or displacement of the foundation layer.
The base must also have a characteristic that it cannot be damaged by capillary water and
frost action.
Locally available materials are extensively used for base construction and the materials
preferred vary widely in different sections of the country.
SUB-BASE:
A sub-base of granular material or stabilized material may be used:
, in areas where frost action is severe,
in locations where the subgrade soil is extremely weak or
where a construction working table (embanking) is needed. It may be used
in cost reduction where suitable subbase materials are cheaper than base materials of
higher quality.
SUB-GRADE:
This is the foundation layer which must eventually support all the load that comes onto the
pavement.
In most cases, it consists of natural earth surface material.
In other cases it is a compacted soil existing in a cut section or the upper layer of an
embankment section (improved subgrade).
In the fundamental concept of the action of flexible pavements, the combined thickness of
subbase (of used) base, and wearing surface must be great enough to reduce the stresses
occurring in the subgrade to values that are not sufficiently great to cause excessive
distortion or displacement of the subgrade soil layer.
PRINCIPLES OF DESIGN
Flexible pavements are those pavements that reflect the deformation of the subgrade and of
subsequent layers on load distribution characteristics of the composite layers.
The main function of the pavement is to carry heavy traffic loads and to transfer the same over
a wide area of the underlying subgrade soil making the deformation within the allowable
range.
The flexible pavement structure mainly consists of:
(i) Soil subgrade
(ii) Sub-base course
(iii) Base Course
(iv) Wearing Course.
Depending on site conditions, the pavement structure may be placed on embankment or
cutting.
SOIL SUBGRADE AND ITS EVALUATION
This is a layer of natural soil which is prepared to receive other layers of pavement placed over
it. The loads on the pavement are ultimately received by the subgrade for dispersal to the earth
mass. It is essential that the soil subgrade be not overstressed at any time. This means that the
pressure transmitted on top of a subgrade is within the allowable limit not to cause excessive
stress conditions or deformation, beyond the elastic limit.
DCE 2301 TRANSPORTATION
ENGINEERING I
PAVEMENT DESIGN
FUNCTIONS OF PAVEMENT LAYERS
THE WEARING COURSE:
The wearing surface may range in thickness between 25mm for light traffic to 150mm or more
of asphalt concrete used for heavily travelled routes. The surface must be capable of
withstanding the wear and abrasive effects of moving vehicles and must possess sufficient
stability to prevent it from shearing and rutting under traffic loads entrance of surface water
into the base, subbase and subgrade.
BASE:
This is a layer (or layers) of very high stability and density.
Its major purpose is to spread or distribute the stresses created by wheel loads acting on
the wearing surface so that the stresses transmitted to the subgrade will not be sufficiently
great to result in excessive deformation or displacement of the foundation layer.
The base must also have a characteristic that it cannot be damaged by capillary water and
frost action.
Locally available materials are extensively used for base construction and the materials
preferred vary widely in different sections of the country.
SUB-BASE:
A sub-base of granular material or stabilized material may be used:
, in areas where frost action is severe,
in locations where the subgrade soil is extremely weak or
where a construction working table (embanking) is needed. It may be used
in cost reduction where suitable subbase materials are cheaper than base materials of
higher quality.
SUB-GRADE:
This is the foundation layer which must eventually support all the load that comes onto the
pavement.
In most cases, it consists of natural earth surface material.
In other cases it is a compacted soil existing in a cut section or the upper layer of an
embankment section (improved subgrade).
In the fundamental concept of the action of flexible pavements, the combined thickness of
subbase (of used) base, and wearing surface must be great enough to reduce the stresses
occurring in the subgrade to values that are not sufficiently great to cause excessive
distortion or displacement of the subgrade soil layer.
PRINCIPLES OF DESIGN
Flexible pavements are those pavements that reflect the deformation of the subgrade and of
subsequent layers on load distribution characteristics of the composite layers.
The main function of the pavement is to carry heavy traffic loads and to transfer the same over
a wide area of the underlying subgrade soil making the deformation within the allowable
range.
The flexible pavement structure mainly consists of:
(i) Soil subgrade
(ii) Sub-base course
(iii) Base Course
(iv) Wearing Course.
Depending on site conditions, the pavement structure may be placed on embankment or
cutting.
SOIL SUBGRADE AND ITS EVALUATION
This is a layer of natural soil which is prepared to receive other layers of pavement placed over
it. The loads on the pavement are ultimately received by the subgrade for dispersal to the earth
mass. It is essential that the soil subgrade be not overstressed at any time. This means that the
pressure transmitted on top of a subgrade is within the allowable limit not to cause excessive
stress conditions or deformation, beyond the elastic limit.
