TECHNIQUES FOR MINIMISING THE COST OF RURAL ROADS AND ACCESS
TRACKS CONCEPTUAL
16:1 DRIFT CONSTRUCTION
Climatic area:
Seasonally wet
Problem:
Carriageway periodically submerged
Waterlogged carriageway
Solution :
Construct drift / clear downstream vegetation
1.
BACKGROUND
Wherever water crosses a road, damage can occur (Figure 1). When that water is in flood, the fast
flows can cause serious damage. Access can be lost in a matter of hours, and repair costs can be high.
Even when the water is flowing slowly, damp soil is normally weaker than dry soil, and will rut
rapidly. All such crossings must therefore be protected.
Steep incline can
erode and become
slippery
Damp soil beginning to rut
Water causing erosion –
increased during periods
of flooding
Figure 1: Drift site before construction
The most common form of water crossing for small flows (a maximum flow of around 5 to 10 m 3 per
second) is a culvert of one or more pipes. Water passes through the pipes; vehicles travel above the
pipes; the two do not come into contact.
However, culverts have some disadvantages and in certain situations, a drift (bed-level causeway) may
be more appropriate. Box 1 provides guidance on making selection decisions between culverts and
drifts.
Disadvantages
Advantages
Box 1
Culverts
Water and traffic not normally in contact
Road is less prone to short term closure
Where terrain permits, the crossing does not
affect comfort of travel
Drifts
Easy to inspect
Easy to maintain
Drifts will rarely block
Water will remain at ground level for
easier onward disposal
Are suitable for dispersed flows
In flat terrain, road must be raised locally
(expensive and unsafe?) or water must be
lowered below ground level with the risk of
siltation and blockage of the outlet
Culverts tend to concentrate flows and may
generate problems in doing so
Takes longer to inspect and maintain
Culverts can block easily
A dry culvert may harbour snakes
Undulation as road dips slightly
through the drift
Prone to short term closure
Often prove unacceptable to
authorities, contractors and users
MINIMISING THE COST OF SUSTAINABLE BASIC RURAL ROAD ACCESS
TECHNIQUES FOR MINIMISING THE COST OF RURAL ROADS AND ACCESS
TRACKS CONCEPTUAL
16:1 DRIFT CONSTRUCTION
2.
BEFORE CONSTRUCTION
A drift normally consists of a strengthened base slab and two inclined approach slabs along the line of
the road. The natural stream cross section should be disturbed as little as possible. Stone protection is
often laid up- and down-stream of the slabs to prevent erosion and local plant species should be planted
wherever possible. As with many steep slopes, and particularly as vehicles will carry water on their
tyres, the higher slopes should be surfaced – normally with gravel. Guide posts at the corners (and
along the slabs of long drifts) will guide vehicles when the water is flooding. Figure 2 identifies
elements of the drift that require design attention.
Figure 2. A generic drift
Box 3 summarises the typical construction sequence that takes place in the building of a drift.
The following boxes are typical proformas for the engineer to use / check before construction begins.
Approvals checklist
Tasks
Approval
Flow
AP1
Diversion
AP1.1
Bed level
AP2.2
Cut-off walls
AP3.4
Running Surface
AP3
Concrete
AP4
Reinforcement
AP7.1
Reno matresses AP7.4
Gabions
AP7.2
Apron
AP3.3
Signage
AP8
Vegetation
AP7.8
Data required for costing
Unit
Unit costs
Quantity
Cost
Gravel
x (m3)
Masonry
x (m3)
Concrete
x (m3)
Aggregate
Gabion (2mx1mxm1)
Gravelling unit
Structures unit
Skilled workers
Unskilled workers
x (m3)
x cages
x days
x days
x days
x days
MINIMISING THE COST OF SUSTAINABLE BASIC RURAL ROAD ACCESS
Total cost
TECHNIQUES FOR MINIMISING THE COST OF RURAL ROADS AND ACCESS
TRACKS CONCEPTUAL
16:1 DRIFT CONSTRUCTION
3.
DURING CONSTRUCTION
The drift in Figure 3 has finished stage 4 (construction of cut-off walls); the soil is being compacted
prior to the construction of a rock and concrete base.
Water flow
Water diversion
Downstream cut-off wall
Upstream cut-off wall
Compacted soil prior to rock and
concrete infill
Figure 3: Drift during construction
Box 3 – Construction stages of a drift
Action
Comment
1. Clear site
2. Dig a water diversion with access bridge

If cannot be diverted build in two sections using sand
bags or bunds and water either pumped or carried
away
3. Establish level of upstream edge of slab

Slightly above average existing stream bed
4. Construct cut off walls

Impervious walls should have an upstream cut off wall
 0.7m below the stream bed. To effectively key in the
structure the downstream wall should be 1m below
the bed level.
5. Provide stable / level base for the drift

Compact sub-base to 96% AASHTO T180
The drift will be either be reinforced concrete 0.25m
deep or a masonry, concrete or handpacked stone
deck 0.3m.
A concrete with a ratio of 1:2:4 with crack inducers
7. Pour concrete slab ramps and approach slab 
every 5m is suitable for this application.
Additional protection upstream will improve the drifts
8. Place apron or gabions downstream
 durability. For details on gabion construction refer to
Section 15.9
Warning signs placed 50m away from approach ramps
9. Place necessary signage and marker posts
 and observable from another 60m
10. Re-vegetate surrounding site / Close water diversion / Allow traffic over drift
6. Construct drift

MINIMISING THE COST OF SUSTAINABLE BASIC RURAL ROAD ACCESS
TECHNIQUES FOR MINIMISING THE COST OF RURAL ROADS AND ACCESS
TRACKS CONCEPTUAL
16:1 DRIFT CONSTRUCTION
4.
AFTER CONSTRUCTION
Gravelled
incline
Loose rip-rap
upstream
Planted
vegetation
Gabion
mattresses
downstream
Guide posts
on all four
corners
Figure 4: Completed drift
5.
MAINTENANCE OF DRIFTS
In addition to routine maintenance, drifts will often require repair after periods of flooding.
Maintenance will include:
 Structure - repair / replacement of any plucked gravel / masonry
 Warning signs, marker posts, and depth indicators - cleaned and or replaced
 Carriageway - clear silt, regrading where necessary
Silt can be discouraged from settling by having a step 0.3m down to the apron. Aprons if not installed,
can lead to erosion downstream and so should always be fitted. Some drifts will have culverts, which if
not large or steep enough will cause silt to build upstream. These will need regular checking and
improvement if necessary. For detailed information on these techniques refer to box 4.
Box 4 Maintenance guidance can be found in the following sections:
14.9 Bioremediation
15.9 Gabion and Reno mattress construction
17.8 Culvert improvement
6.
REFERENCE DATA
Box 5 gives typical dimensions for the parts necessary for the construction of a drift.
Box 5 – Typical design dimensions
Maximum flow (m3/s)
2
5
5
10
Deck
Pitched stone
ORN9 / MRP 1992
Concrete
ORN9 / MRP 1992
Concrete and Stone
ORN9 / MRP 1992
Crossfall (%)
2
2
Thickness
0.3m
0.3m
Cutoff wall
Downstream
Masonry
0.5m thick
1m deep
Concrete
0.2m thick
1.2m deep
Concrete
0.2m thick
0.6m deep
2
0.15m concrete
0.25 to 0.3m hand
packed stone
Gabion curtain
1m thick
1m deep
Gabion curtain
1m thick
1m deep
Reinforced concrete
ORN 16 / MRP
1992
5
Cutoff wall
Upstream
Apron
Reinforcement for
concrete
Gradient of
approaches
Not necessary
Rip rap
ORN9 Table 6.1
4m paved
Reno mattress
230mm thick
0.25m thick
Gabion curtain
1m thick
1m deep
RE-concrete
0.7m deep
0.25m thick
Rip rap
ORN9 Table 6.1
Slab – top/bottom – T10@ 200 EW
Edge beams – Links – R10 @ 200
Edge beams - Longitudinal reinforcement – 6 No T10
1 in 12 (8%) if the drift is less than 10m long and
1 in 7 (15%) if longer. The approaches should be of the same construction as the deck.
MINIMISING THE COST OF SUSTAINABLE BASIC RURAL ROAD ACCESS
TECHNIQUES FOR MINIMISING THE COST OF RURAL ROADS AND ACCESS
TRACKS CONCEPTUAL
16:1 DRIFT CONSTRUCTION
7.
USEFUL SOURCES OF INFORMATION
The following titles provide guidance in the construction and maintenance of drifts.
Overseas Road Note 1,2,7,9,16
Government of Zimbabwe, Road Design for Low Volume Roads
International Road Maintenance Handbook, PIARC Volume 1
Earth Roads Their Construction and Maintenance, Jack Hinds, IT Publications
Engineering In Emergencies – Jan Davis and Robert Lambert, IT Publications
MINIMISING THE COST OF SUSTAINABLE BASIC RURAL ROAD ACCESS