Earthworks relate to pre-construction site clearance, excavation and the movement of soil and rock. Earthworks should be kept to a minimum and an optimum vertical alignment will ensure that material from cuttings can be placed in adjacent embankments with the minimum of haulage. Due adjustment should be made for the
compaction factor cut/ill.
Material can be hand-cast up to a few metres, or wheel-barrowed, head-loaded, stretchered or panniered up to about 100 metres. Carts or equipment haulage are usually required for longer haulage
distances. Signiicant earthworks will add substantially to the works’
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Embankments should be speciied to be constructed in layers
appropriate to the compaction equipment to be used, the standard of compaction required (e.g. 95%–98%) and the optimum moisture content (OMC) for the material.
Figure 6.12 – Labour based earthworks (photo courtesy: P. H Bentall).
Figure 6.13 – Applying water to the road formation to achieve the OMC before compaction (photos courtesy: P. H Bentall).
In-situ compaction testing is possible if appropriate apparatus is
available. An alternative is a method-speciication based on the type
of equipment and local knowledge of the behaviour of the material, otherwise compaction until no observed movement under the roller
Transport chapter 6 – Speciic design aspects
6.5 Drainage
Well-designed drainage is essential to ensure that damage to the road surface and structure is minimised, and to avoid problems at points of discharge to the adjacent land. Prevention of damage to the sub- grade should be a major priority since structural damage to the road will require extensive works to rectify.
Road surface
Rainfall should be drained away from the road surface as shown in Figure 6.14.
Figure 6.14 – The relationship between roads and the water cycle (ILO, 1990).
Camber / crossfall
Road surface drainage is affected by constructing the carriageway with a camber or crossfall. Initially this should be 6% or 7% for an
earth or gravel surface, which will reduce gradually under trafic
action and weathering. The camber should be maintained between 3% and 7%. For paved roads, less crossfall is required and this is
typically speciied as 2.5%–3%.
Roadside drainage
Side drains carry water away from the road and prevent surface water from the surrounding ground from reaching the road as shown in Figure 6.15.
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Ditches constructed by grading equipment are usually V-proile.
However, using labour-based techniques it is easier to construct ditches of trapezoidal section. This is also preferable to a V-section
for better low and erosion characteristics.
Minimum ditch gradients should be 2% to minimise silting and on steep sections if > 4% then scour checks are required to minimise erosion as shown in Figures 6.16 and 6.17. Spacing of these decreases from 20 m to 5 m with gradients of 4%–10%.
Figure 6.16 – (above) scour checks in a ditch using stones (ILO, 1990).
Figure 6.15 – Side drains collect surface water from the road and embankments to be discharged at suitable locations or through culverts (ILO, 1990).
Figure 6.17 – (right) scour checks installed on site (photo courtesy: P. H. Bentall).
Transport chapter 6 – Speciic design aspects
Turnout drains are speciied
to lead water from the side drains to adjacent land at convenient points where no erosion damage will result as shown in Figure 6.19. Where possible, these should be constructed at 20 metre intervals on the basis that ‘little and often’ will minimise erosion risks or siltation on adjacent land, and the risk of land users blocking them off.
Figure 6.18 – Catchwater drain situated above the cutting (ILO, 1990).
Figure 6.19 – Turnout drain (ILO, 1990).
Catchwater (intercepting) drains of trapezoidal section may be required above the road on sidelong ground and in cuttings to prevent overground water reaching the road as shown in Figure 6.18. However, these can promote slip circles in the cutting face. An alternative technique is to construct a bund back from the top of the cut face to intercept runoff water and direct it to a safe discharge point.
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Cross drains are required to accommodate existing water courses and where water in side drains needs to be transferred as shown in Figure 6.20. Pipe sizes and culvert openings are designed on normal hydraulic principles, with minimum pipe diameter/openings of 60 cm to allow easier maintenance.
Culverts may be of pre-cast pipe concrete, cast in-situ concrete,
stone masonry, treated timber or ired clay brick. Drifts can be used to discharge water across LVR where there is insuficient depth for
a culvert and adequate cover under the road surface, or on bedrock.
Standard designs and speciications exist for pipe and box culverts;
inlets and outlets; short-span bridges; and they should be consulted e.g. the Small Structures for Rural Roads Guideline and Overseas Road Note 9.
Small bridges for rural roads can be concrete, stone masonry,
ired clay brick or timber; single or multi-span depending upon the water low and trafic volume. An example of a single span bridge
structure is shown in Figure 6.21.
Figure 6.20 – Cross drainage (ILO, 1990).
Transport chapter 6 – Speciic design aspects
Larger structures in steel or reinforced concrete should be subject
to a full rigorous design process by qualiied engineers to reduce the
risk of structure failure as shown in Figure 6.22.