REPUBLIQUE DU BENIN
MINISTERE DE L’ENSEIGNEMENT SUPERIEUR ET DE LA
RECHERCHE SCIENTIFIQUE
ECOLE SUPERIEURE DES TECHNOLOGIES AVANCEES ET DE
MANAGEMENT
Filière: Génie Civil Licence:1
THEME :
PROCESS OF BUILDING A ROAD
Réalisé par : Nom du professeur:
COUGBE ZOGAN Roland Mr WEINTA Massama Crespin
Année académique : 2020- 2021
Plan
Introduction
I – Setting out
II –Earthworks
III – Paving construction
Conclusion
Introduction
The methods of constructing roads have changed a lot since the
first roads were built around 4,000 BC – made of stone and timber. The first
Roman roads were stone paved, built in North Africa and Europe for
military operations. Road construction techniques were gradually improved
by the study of road traffic, stone thickness, road alignment, and
slope gradients, developing to use stones that were laid in a regular,
compact design, and covered with smaller stones to produce a solid layer.
I. Setting out
This is carried out following the dimensions specified in layout drawings.
A commonly used setting out procedure is the profile board method. A
series of boards that show the exact level 1 metre above
the completed construction level are placed at intervals along the proposed
line of the road. A profile board with a fixed height, called the traveller, is
used for controlling the excavated levels between these profile boards. By
placing the travellerin the sight-line between two level boards, it can be
seen whether or not the excavation has been carried out to
correct levels and adjusted accordingly.
The level of each profile board is controlled using a line level which is a
short spirit level hung from a nylon string. The line operator moves the
string up or down until the bubble is centred.
Junctions, hammer heads, turning bays and intersecting curves are laid out
in a similar manner.
II. Eathwork
Earthwork is one of the major works involved in road construction. It
involves the removal of topsoil, along with any vegetation, before scraping
and grading the area to the finished ‘formation level’. This is usually done
using a tractor shovel, grader or bulldozer. Below the formation level,
the soil is known as the ‘subgrade’. It is essential that the strength of
the subgrade is tested prior to earthwork beginning.
Most earthworks are formed by cut-and-fill, and the type of
‘fill’ material must be considered, not only in terms of its
physical properties, but on the conditions in which it is to be used, and the
methods of compaction.
Depending on its quality, compressible subsoil may be removed or
stabilised. If the cost of full or partial excavation of subsoil is uneconomical
and would be likely to result in consolidation, sand wicks
or sand drains may be used. Sand wicks are sand-filled boreholes beneath
the road embankment that give greater stability to the soil by decreasing
the length that water has to travel in a drainage path, so
dissipating water pressure. Sand drains alongside the road are used to
intercept ground water.
Subsoil drainage should be provided to deal with seepage
through pavements and verges, from higher ground and a result of the
seasonal rise and fall of the water table.
Subgrade strength
The required thickness of the pavement is determined by
the subgrade strength, so it is desirable to make the subgrade as strong as
possible.
The strength of the subgrade can be achieved by using the following
techniques:
Removal of poor material in cuttings and replacing with selected fill.
Compacting subgrade to a high dry density.
Providing adequate subsoil drainage.
Soil stabilisation methods such as the use of cement,
bituminous materials or chemicals.
The subgrade strength will decrease as moisture content increases so
protection may be required if it is to left exposed for any length of time.
Protection covering can be either:
Medium gauge plastic sheeting with 300 mm laps.
Sprayed bituminous binder with a sand topping.
III. Paving construction
Once the subgrade has been prepared and drainage or
buried services installed, the pavingconstruction can begin. Paving can be
either flexible or rigid. There are pros and cons to each type, with one being
selected over the other depending on the specific needs of a project.
Rigid pavements tend to have lower maintenance costs, a longer design
life and higher flexural strength; but flexible pavements tend to have
lower construction costs and have a higher ability to expand
and contract with temperature and so do not need expansion joints.
A. Flexible paving
Flexible paving consists of materials applied in layers directly over
the subgrade to which the traffic loads are distributed. To prevent
permanent deformation, and therefore an uneven running surface, the
thicknesses of individual layers must be capable of distributing such loads.
The subgrade is compacted with the sub-base on top of it. On top of this is
laid the surfacing which is made up of the base layer and the wearing
course.
i. Surfacing
The wearing course is the upper layer of bituminous material, often denser
and stronger than the base layer. The thickness depends on
the material specification and the amount of wear that is expected.
Desired properties are good non-skid capabilities, minimal glare and
acceptable durability.
The main materials that are used are hot rolled asphalt (HRA),
dense bitumen macadam(DBM), dense tar macadam (DTM) and porous
asphalt (PA). PA is especially suitable as it is an open-graded material that
is designed to allow rapid drainage of surface water, thereby reducing
spray as well as tyre noise.
The base will typically have a minimum thickness of 60 mm and is usually
made of dense bitumen macadam or asphalt. It is laid with the appropriate
crossfalls and gradients.
ii. Sub-base
This is placed in a layer usually not exceeding 150 mm over
the subgrade after waterproofingis complete. Various materials can be used
but it is common for crushed stone or dry leanconcrete (such as 1 : 15) laid
and com pacted by heavy rollers.
B. Rigid paving
Rigid paving consists of a reinforced or unreinforced insitu concrete
slab laid over a thin granular base course. The rigidity and strength of
the pavement enables the loads and stresses to be distributed over a
wide area of the subgrade.
Rigid paving is made up of the following layers (from top to bottom):
Subgrade.
Sub-base of thick crushed stone. Usually to a thickness of 80 mm.
Anti-friction membrane normally made of polythene sheeting. Also
prevents grout loss from freshly laid concrete.
Insitu concrete paving slab. Reinforcement in the form of
either steel fabric or re-bar may be used.
Asphalt or similar topping if required.
Longitudinal and transverse joints are required in rigid paving between
the slabs, limiting the stresses applied due to subgrade restraint (friction
between the pavement and subgrade), and
providing room for expansion and contraction movements. The spacing
of road joints is determined by:
Thickness of the slab.
Whether there is reinforcement in the slab or not.
The expected traffic load and flow rate.
The temperature at which concrete is laid.
Conclusion
Very broadly, the construction of a road can be described by three
processes. Those are setting out, earthworks and paving construction.
Modern roads tend to be using asphalt and/or concrete. They also
participate the development of a country if they are everywhere because
the development of a country goes through that of road they say.
