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2. Road Construction Practice

2.1 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 at their Web site which gives details of major road works being undertaken around Queensland.

The NSW Roads and Traffic Authority site includes a section which gives some details of major road building projects around NSW.

2.2 Pre-Construction Activities

2.2.1 Propery Acquisition

Property acquisition may take considerable time, and may be a very emotive issue for land owners involved.

2.2.2 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.

2.2.3 Cost Estimates

Detailed cost estimates are required for all construction jobs prior to commencement.

2.2.4 Pre-construction Investigations

These include geotechnical surveys and consideration of available labour, plant and materials.

2.2.5 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.

2.2.6 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.

2.3 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.

2.4 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.

2.5 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.

2.5.1 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.

2.5.2 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.

2.5.3 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.

2.5.4 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.

2.6 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.

Bridge construction is similar to other structural construction work (i.e. other steel, reinforced concrete or prestressed concrete work). This area is not dealt with in this course.

Page last modified 3 July 2003.