Road construction
The history of road
construction
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
Very broadly, the construction of roads includes
three processes:
§ Setting out.
§ Earthworks.
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 traveller in 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 can be laid out in a similar manner.
Earthworks involve
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 earthworks 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.
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.
Once the subgrade has
been prepared and drainage or
buried services installed,
the paving construction 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 a better ability to
expand and contract with temperature and
so do not need expansion joints.
Flexible paving
Flexible paving consists of materials applied
in layers directly
over the subgrade onto 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 the surfacing is laid which is made up of the base layer and
the wearing course.
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). Porous asphalt 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 cross falls and gradients.
Sub-base
This is placed in a layer usually
not exceeding 150 mm over the subgrade after waterproofing is complete.
Various materials can be used but it is common for
crushed stone or dry lean concrete (such
as 1:15) laid and compacted by heavy rollers. Cellular Concrete (Foamed Concrete)
is another material that can be used for the sub-base,
the thickness and strength required
depends upon the type of road.
Rigid paving
Rigid paving consists of a reinforced or
unreinforced insitu concrete laid
over a thin granular base. 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 typically made up of the
following layers (from bottom to top):
§ Subgrade.
§ Sub-base of
thick crushed stone. Usually to a thickness of 80 mm.
§ Anti-friction membrane normally
made of polythene sheeting.
This also prevents grout loss from
the 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:
§ The
hickness of the slab.
§ Whether
there is reinforcement in the slab or
not.
§ The
expected traffic load and
flow rate.
§ The temperature at
which the concrete is laid.
For more information,
see
§ Road joints.
§ Kerbs.
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