Two enemies, asphalt and heavy rain, met last week and the result was catastrophic. Bridges crumbled and roads swallowed vehicles as they caved in when downpours battered the province.
But how does water take down entire hills, leaving ravines where roads once were? And what can be done to prevent it?
University of KwaZulu-Natal civil engineering department's Prof Mohamed Mostafa said that during planning, engineers try to avoid flood paths because water is the enemy of asphalt.
The Umhlali N2 north bridge had moved about 50cm, dropped about 10cm on the north end and moved 20cm on the south end.
The R102 near the GJ Crookes Hospital had caved in, almost swallowing a taxi and a car.
A portion of Ntuzuma Road collapsed as the stream below grew into a raging river, removing all obstacles in its path.
— FORCE OF NATURE
“When it comes to roads, there is a misunderstanding that the 'black' layer is the 'road'. That is not correct. We have several types of roads in SA. The most common are called paved roads. These roads comprise a surface layer made of asphalt, and this layer is not a structural layer. It is mainly to provide the functionality of the road.
“This surface layer is based on other layers typically made of compacted natural soils or stabilised soils, with cement or something similar. These layers carry the loads from the vehicles to the natural soil. All infrastructure foundations are designed to do the same, as the load must go somewhere.
“If water comes from the top, it will cause potholes and can penetrate further to disturb and weaken the soil layers. If it comes from the side, as in the case of a flood, it can cause massive damage as the road layers will lose their structural integrity. This can be a localised collapse or a large-scale one. It will depend on where the water is attacking and what force this water has,” Mostafa said.
The time it takes to collapse depends on the intensity of the water.
“It can take only a few hours,” Mostafa said.
Wits school of civil and environmental engineering's Dr Imuentinyan Aivinhenyo said a collapse can occur through various mechanisms, “usually when the road becomes unable to bear the traffic load that it’s been subjected to, or when the foundation (natural ground) itself fails.
“The foundation can fail as a result of various factors, including excessive water infiltration which can cause the load-bearing capacity to weaken. So when the foundation weakens or gets washed away, whatever that foundation supports is bound to collapse.”
Was the force of that rainfall of 300mm over 24 hours in KwaZulu-Natal so calamitous that it just swept away bridges and roads?
“The failure of a road can occur through repeated traffic over a number of years,” Aivinhenyo said. “These kinds of failure are however usually gradual and manifest as cracks or other deformations on the road. Failure that results in sudden collapse can also be as a result of sudden unpredicted (non-traffic related) forces that causes disruption to the roadbed. The time it requires to cause damage is dependent on the extent of the generated forces.
“Water flowing under high pressure basically releases the pressure on whatever structure is hits. If the force is more than what the structure or foundation can withstand, failure or collapse will occur,” he said.
Zunaid Varyawa, CEO of Isigodi, a KwaZulu-Natal construction and engineering consulting firm, said as water becomes uncontrolled due to increased rainfall, it creates new pathways.
“Existing rivers or streams tend to get wider due to the amount of water that is contributing to the catchment. For roads that have been washed away, you would have to see if it was erosion or under design.
“Low level bridges naturally allow water to flow over, which will assist in a deluge. Other bridges are installed with a freeboard to achieve the same outcome.
“Water will have a pressure on a surface. This pressure is related to the slope of the land and depth of water column,” he said.
Mostafa said the roads damaged in KwaZulu-Natal could not be isolated from the bigger picture of infrastructure failure.
“This failure can be attributed to the lack of a comprehensive disaster management strategy that allows for pre-disaster actions and also quick interventions afterwards, poor maintenance plans, and poor plans aiming at upgrading the ageing infrastructure.”
Mostafa listed several solutions engineers should consider for coastal roads. They include using strong retaining walls to support the land slopes, using wave protections on the ocean sides and creating flood paths under the roads.
“For strategic roads, such as the port access roads, other solutions include proper planning, alternative access roads, and high strength pavements.”
Public works and infrastructure minister Patricia de Lille this week said her department and the department of defence had agreed to collaborate in the installation of modular steel bridges. Before the floods, 18 bridges were planned for the Eastern Cape and KwaZulu-Natal. Six more have now been added.
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