Water is one of the biggest threats to road durability. When it collects on or under a pavement, it weakens the layers and leads to potholes and edge failures. On the 75 km Nasarawa urban roads project, ICCO built an extensive drainage and erosion control system to keep water away from the road and to stop the embankments from washing away.
Key facts
- Longitudinal drainage: More than 230 km of ditches, drains and pipes
- Median drain: Ø300 pipe along the full 75 km
- Box culverts: 8, each with two 2.0 × 2.0 m cells
- Pipe culverts: 6, with 810 m of Ø1200 RC pipe
- Grassing and hydroseeding: 900,000 m²
- Cascades and chutes: 60 on high embankments
Why water is a road's biggest enemy
A pavement's granular layers get their strength from tightly packed stone and soil. When water soaks in, that packing loosens and the layers lose much of their strength. Each heavy vehicle that passes over a wet patch then pushes the weakened material further out of shape, and a small crack can become a pothole within a single rainy season. Good drainage stops this cycle before it starts.
Planned from the start
Hydrological and hydraulic studies formed part of the design work. These studies estimate how much rainwater flows towards the road and where it needs to cross. Their results set the size and position of the drains and culverts, so that each one can carry the flows expected at its location.
Longitudinal drainage
Drains run along the length of the corridor, collecting water from the road and its surroundings. Together, the longitudinal drainage elements add up to more than 230 km of ditches, drains and pipes.
| Drainage element | Executed quantity |
|---|---|
| Unlined trapezoidal side ditches | 112,500 m |
| Median longitudinal drain pipe Ø300 | 75,000 m |
| RC lined side drains 1.0 × 1.0 m | 37,500 m |
| Mitre and outfall drains, stone-pitched or concrete-lined | 6,000 m |
| Access / driveway pipe culverts Ø600 | 1,500 m |
The median drain pipe runs the full 75 km, draining water from the centre of the six-lane carriageway. Mitre and outfall drains lead the collected water away from the road to natural watercourses, while driveway pipe culverts keep the side drains flowing where properties connect to the road.
Crossing culverts
Where water must pass under the road, ICCO built eight reinforced concrete box culverts, each with two cells of 2.0 × 2.0 m. Six crossing culverts with three lines of Ø1200 RC pipes added a further 810 m of pipe.
| Crossing culvert works | Executed quantity |
|---|---|
| RC box culverts, two cells of 2.0 × 2.0 m | 8 |
| Pipe crossing culverts, three lines of Ø1200 RC pipes | 6 (810 m of pipe) |
| C30 concrete in culvert barrels | 1,296 m³ |
| Concrete in headwalls, wingwalls and aprons | 400 m³ |
| High-yield reinforcement | 203.52 t |
Headwalls and wingwalls hold the embankment back at each end of a culvert and guide water in and out, while aprons protect the ground at the outlet from being scoured by fast-flowing water.
Erosion control and landscaping
Running water can also wash away embankment slopes. To prevent this, ICCO completed:
- 900,000 m² of grassing and hydroseeding, about 90 hectares;
- 3,000 m³ of gabion walls in PVC-coated galvanised mesh;
- 6,000 m² of 300 mm gabion and Reno mattresses;
- 12,000 m² of grouted stone pitching on slopes and channels; and
- 60 concrete cascades and chutes on high embankments.
Grass roots bind the soil on slopes, while gabions, pitching and cascades handle faster-flowing water. Cascades step water down high embankments in stages, so it cannot gain enough speed to cut channels into the slope.
Protecting the road for the long term
Together, these works protect the road structure and stabilise the corridor for the long term. A road that sheds water quickly needs less patching, stays open through the rainy season and keeps its smooth surface for longer, which is the real return on the drainage investment.
More on the Nasarawa 75 km urban roads
This article is part of a five-part series on the project:
- ICCO Completes Nasarawa's 75 km Six-Lane Urban Road Project – the scope, value, timeline and completion of the project.
- From Earthworks to Asphalt: Engineering Nasarawa's 75 km Road – the design studies, earthworks and layered pavement.
- Five 30 m Bridges Delivered on Nasarawa's 75 km Urban Road – the piles, beams, bearings and load testing of the five bridges.
- 1,878 LED Lights and More: Road Safety on Nasarawa's 75 km Road – street lighting, road markings, guardrails and pedestrian facilities.
Frequently asked questions
Why is drainage important for roads?
When water collects on or under a pavement, it weakens the layers and leads to potholes and edge failures. Drainage keeps water away from the road structure.
How long is the median drain on the Nasarawa road?
The Ø300 median longitudinal drain pipe runs the full 75 km of the corridor, or 75,000 m.
How many culverts cross under the road?
There are 14 crossing culverts: eight reinforced concrete box culverts with two 2.0 × 2.0 m cells, and six pipe culverts with three lines of Ø1200 RC pipes.
How were the embankment slopes protected from erosion?
With 900,000 m² of grassing and hydroseeding, gabion walls and mattresses, 12,000 m² of grouted stone pitching and 60 concrete cascades and chutes on high embankments.
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