8. Road Construction
8.1 The Road Construction Process
The type of road construction used varies from one
job to another. The type of construction adopted for a particular road depends
on:
- the volume and nature of traffic to use the road,
- the nature of the materials available,
- the topography,
- foundation conditions,
- type and availability of construction equipment,
and
- financing arrangements and timing.
Any road construction job consists of number of basic
steps, although the relevant importance and the interaction between these
steps will vary from job to job. These steps can be summarised as:
- planning, programming and pre-construction
activities;
- site clearance;
- setting out;
- earthworks;
- bridge construction;
- drainage structures;
- pavement construction;
- placement of road surfacing;
- placement of road furniture; and
- landscaping.
LINKS TO SITES ON ROAD CONSTRUCTION.
Main Roads
Queensland
has a List of Projects in "Road Projects" at their Web site which gives details of major road
works being undertaken around Queensland.
The
NSW Roads and Traffic Authority
site includes information on some of their major road building projects
around NSW.
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8.2 Pre-Construction Activities
8.2.1 Propery Acquisition
Property acquisition may take considerable time,
and may be a very emotive issue for land owners involved.
8.2.2 Public Utility and Property Adjustments
In rural areas public utility adjustments rarely
constitue major work and the main property adjustment work is fencing new
boundaries and restoring property accesses. In urban areas public utility
adjustments may be complex and extensive. Consequently they may be very costly,
take a long time, and require a lot of coordination.
8.2.3 Cost Estimates
Detailed cost estimates are required for all construction
jobs prior to commencement.
8.2.4 Pre-construction Investigations
These include geotechnical surveys and consideration
of available labour, plant and materials.
8.2.5 Work Planning
Construction activities are summarised onto some
form of job control diagram such as a bar chart or Critical Path Network.
This work must be done before the job commences and also periodically updated
as the job progresses.
8.2.6 Commencement
The first activities on site are usually the establishment
of site offices, the construction of compounds to ensure security for plant
and materials, and the provision of facilities (eg meal rooms, change rooms)
for the workforce.
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8.3 Site Clearance
The method used depends upon the extent of the task,
the type of country, the time available, and the equipment and labour available.
Clearing usually involves the removal of vegetation such as grass, brush,
trees and stumps, but may also include the removal of old buildings, structures,
etc.
The most commonly used item for clearing is the
bulldozer, although scrapers and graders may also be used. A dozer may affect
clearing by pushing but pulling with a rope, or the use of explosives, may
be required with large trees. Clearing by chainsaw can only be used if the
stumps can be left behind (which is not usually the case - stumps will rot
away leading to subsidence of overlying material).
Consideration must be given as to how cleared material
is to be disposed of. Timber may be burnt if environmental conditions allow,
but otherwise it has to be carted away and disposed of as landfill.
Following clearing operations, topsoil should be
stripped and stockpiled for future landscaping. Topsoil is unsuitable for
embankment or road building, and it is a valuable resource which should be
handled and used thoughtfully.
8.4 Setting Out
The object of setting out is to mark on the site
actual positions and ruling dimensions for the guidance of plant operators
and supervisors.
Setting out may involve two phases:
- establishment of a centreline (and suitable recovery
markers for use if the centre markers are destroyed) and marking of the limits
of the area to be cleared; and
- after clearing, the more detailed location of
the formation together with level control.
Setting out will therefore involve the placement
of a pegged centreline, placement of recovery pegs, placement of clearing
stakes, establishment of batter pegs (and batter profiles for larger cuts
and fills), and pavement control markers.
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8.5 Earthworks
The eventual aim of the earthworks phase of the construction
is to position the subgrade underlying the pavement layers in the right
location and at the correct level, and to provide drainage.
The operations to be performed are:
- formation of cuttings by excavating through
high ground,
- formation of embankments by filling over
low ground,
- shaping the finished surface to design levels,
and
- excavating for drainage works.
The earthworks is often the largest task in the road
building process and therefore careful planning and organisation are essential.
Speed and efficiency depend very much upon the quantity and types of earthmoving
plant available.
8.5.1 Sequence of Operations
The normal sequence of operations in cut and fill
work is:
- In Cut
- excavate to the depth necessary to reach
formation level,
- transport away from the site undesirable
material such as organic soils,
- haul suitable materials from cuts to fill
areas, and
- suitably dispose of any excess cut material.
- In Fill
- drain water from depressions and dispose
of any unsuitable underlying material,
- spread fill material in horizontal layers
not more than 250 mm thick, and
- thoroughly compact these layers to required
density.
8.5.2 Preparation of Fill Sites
Fill sites are prepared by removing any material
too weak to support the fill and then compacting the surface by rolling.Where
the natural surface slopes at more than 1 in 4, horizontal terraces will
need to be cut on which to bed the fill.
8.5.3 Excavation of Cuttings
Cuts in softer materials can be made by dozers, scrapers,
excavators and occasionally by graders. In hard materials excavation by
drilling and blasting may be required.
Haulage will usually be by scrapers or by trucks
loaded with a wheeled loader. In rocky materials special rock buggies may
be required. On shorter hauls (up to 100m) it is possible that a dozer may
be able to economically push material from cut to fill.
General principles to be observed in the excavation
of cuttings are:
- provide for drainage during construction;
- establish down-grades so that plant can work
downhill and be assisted by gravity while cutting;
- ensure that cuts are made to the full design
width;
- in through cuts keep the centre high; and
- in side hill cuts keep the inside edge low.
8.5.4 Construction of Fills
Transport of fill material will usually be accomplished
by bulldozers (short hauls), scrapers or trucks. Once the material is deposited
at the fill site it is spread (usually by dozer and/or grader) and, if necessary,
brought to its optimum moisture content by the incorporation of water from
a water cart. Compaction is then carried out using an appropriate roller or
compactor.
General principles to be observed when constructing
embankments:
- ensure that the base of the fill is the correct
width;
- place the material in layers and compact each
layer;
- check the suitability of the fill material
as it arrives;
- ensure that the fill material is at the correct
moisture content for compaction;
- keep the edges of the fill higher than the
centre to prevent material sliding out and to increase safety for plant; and
- in wet climates crown the fill at the end of
the days work so that any overnight rain will be shed from the work and will
not pond.
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8.6 Bridge Construction
Bridges form an integral part of the road system
and from a motorists viewpoint are often the most noticed feature. It is
often necessary to commence bridgeworks ahead of roadworks because bridges
are slower to build. Small bridges may take six months to build whereas large
bridges can take several years.
Compared to other parts of the road, bridges are
expensive. In urban areas the cost of a bridge would usually be at least
five times the cost of an equivalent length of road.
8.7 Drainage Structures
Many road failures can be attributed to the presence
of water. The soundness of a road is dependent upon adequate drainage being
built into the road and then adequately maintained.
Drainage structures can be classified into three
groups:
- Surface Drains:-
- Catch drains - to catch surface water and divert
it into the drainage system before it flows down the face of a batter or
up against a fill.
- Table drains - collect the surface water draining
from the road pavement.
- Kerb and gutter - the urban form of table drains.
- Subsoil Drainage - incorporated into the construction
wherever there is a risk of underground water causing damage to the road.
The drain is usually a trench into which a line of porous pipes is laid
and the trench backfilled with porous material.
- Culverts - which carry water from one side of
the road formation to the other. Types commonly used are precast concrete
pipes, precast concrete boxes, cast-in-situ concrete boxes, and corrugated
steel pipes and arches.
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8.8 Pavement Construction
Pavements are generally constructed of either gravel,
crushed rock, asphalt or cement concrete - each of which may be used in
its own right or in combinations with one or more of the others.
8.8.1. Gravel and Crushed Rock Pavements
- Source: pits,quarries.
- Haulage: trucks.
- Spreading: grader or paver.
- Compaction: higher compaction standard than
subgrade; different roller types used.
- Accuracy of levels important.
8.8.2. Asphalt Pavements
- Manufacture: fixed plants (up to 400 tonnes
per hour), or large mobile plants.
- Haulage: trucks.
- Placement: paving machine.
- Compaction: rollers - smooth vibrating drum
and pneumatic-tyred.
- Usually final layer thin to achieve good riding
surface.
8.8.3 Cement ConcretePavements
- Manufacture: ready-mixed batching plant.
- Haulage: agitator truck.
- Large quantities: site manufacture + normal
trucks.
- Placement: slip-form paver.
- Compaction: internal vibrators + external screeds.
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8.9 Placement of Road Surfacing
May be gravel, sprayed bituminous seal, asphalt or
cement concrete.
8.9.1 Construction of Sprayed Bituminous Surfacing
Pavement must be sound. For new work: compaction
and trimming of underlying gravel or crushed rock pavement. For existing
bituminous surface: patching.
Steps:
- Sweep surface - drawn rotary broom + hand sweeping.
- Prepare binder - transport to job by road or
rail tanker, or in drums.
- Flux and/or cutback binder.
- Incorporate adhesion agent.
- Prepare aggregate - stockpiles, precoating
- Spray binder - air temperature should be >
15 C; no rain should be threatening; mark out spray run- alignment and start
and finish.
- Apply aggregate - quick incorporation into
hot bitumen before excessive cooling; spreaders required; drag broom to correct
spreading inconsistencies.
- Roll surface - to bed stone; pneumatic-tyred
or static steel drum rollers.
- Drag broom
- Record work - quantities bitumen sprayed, aggregate
spread, etc.
- Traffic control - throughout steps 1 to 10;
minimise traffic delays.
LINK TO THE PICTURE GALLERY
The Picture Gallery contains pictures of the bitumen
sealing process on four pages:
8.9.2. Manufacture of Asphalt
Two types of plants:
LINK TO THE PICTURE GALLERY
The Picture Gallery contains photographs of
Asphalt Production
.
8.9.3 Asphalt Paving
Steps:
- Prepare surface
- Transport and deliver mix
- Place mix - warm weather, no rain; asphalt
temp > 110 C
- Compact mix
LINK TO THE PICTURE GALLERY
The Picture Gallery contains photographs of
Asphalt Paving
.
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8.10 Placement of Road Furniture
Protective barriers, guardrails, signposts, guideposts,
traffic signals, street lighting, emergency telephones, etc.
8.11 Landscaping
Includes:
- reshaping and contouring land;
- topsoiling, turfing and mulching;
- stabilising drains to prevent scour; and
- planting trees and shrubs.
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Page last modified 28 May 2009.