Soil loss and road construction

ROMA – ONE of the most overlooked causes of soil erosion is road construction, according to Dr ’Mamohau Thamae.

“Even here in Lesotho, we see a lot of attention given mainly to hillside erosion controlled with prevention methods called lifato-fato,” Dr Thamae said.

Dr Thamae is an Applied Environmental Scientist with focus on Geomorphology and is a lecturer at the National University of Lesotho (NUL). Dr Thamae and her peers took it upon themselves to study the actual causes of road construction-related soil erosion and the best ways to deal with it.

She took her studies in Lesotho and some parts of South Africa. Let’s imagine this. Suppose a road is built on a sloppy land surface.

“Engineers will use what is called Cut and Fill methods,” she said.

As the name itself suggests, the engineers will cut the soil from the upper side of the slope and use it to fill the lower side where the road is being levelled. Both the cut sites and fill sites have a problem.

On the cut side, the soil is left bare and the slope gets sharper. When the rain comes, it may just wash away the soil, sometimes creating severe erosion.

In fact, as she and her peers have observed, “this kind of erosion is responsible for between 70 to 90 percent of the total soil loss from the affected roadway area”. On the other hand, the soil on the fill side is shaky.

It has been removed from its natural area to a new area. It experiences disturbances in the process and it may not be as compacted where it is now.

So if the rains come, it’s easier to pick away the soil, causing erosion. There is yet another problem, she said.

More often, the place over which a road is constructed was once a land with its own soil and vegetation. However, once we have a road constructed, now you have a hard surface over which rain water can barely go through. It’s a closed land.

So water finds its way towards the fill side of the road, worsening the problem. As a way to reduce these challenges, engineers have constructed various forms of waterways that collect water from the road surface and the fill side down to a specific place where the water will be discharged.

This solution is creating other problems. For instance, the water, which once scattered, with reduced impact, has now been concentrated into one place. Now it is more forceful.

“That is why gullies erupt where these waterways are discharged,” she said.

In order to fully understand the nature of the problem, Dr Thamae made trips to several roadway areas in Lesotho and South Africa. More importantly, she could trace the roads and associated infrastructure along it for many kilometres using Google Maps and its Street Views.

For instance, she once traced the Main North 1 road that runs from the heart of Maseru down to Mokhotlong on Google Maps. She examined and documented its nature, evidence of soil erosion and its possible causes along the way.

She did the same thing in several places in South Africa, including some parts of the Eastern Cape. In the end, she noticed patterns of soil erosion caused by road construction in a many cases.

Also interesting was the efforts made by the road engineers to try to prevent erosion, even if they fell short in many cases.

“In South Africa, engineers there go out of their way to try to prevent soil erosion. In Lesotho, not that much.”

That could be related to the depth of the pockets of the two neighbours. A number of efforts were observed which included the following: Gabions: These are rectangular baskets fabricated from a mesh steel wire.

The baskets are filled with rocks and stacked over each other to prevent soil sliding. Retaining walls: These are relatively rigid walls used for supporting soil along the slope side so that it can be stable.

“I found these to be some of the most effective methods to prevent soil erosion,” she said.

Farrows above the slope: Some interesting efforts were observed where engineers created some farrows on top of roadside slopes to redirect the water away from the slope surface.

This, in her view, proved to be the worst approach since the water just found a way to dig into the soil, creating soil pipes that made the slopes even more fragile. The thing with all these efforts is that they have to be maintained.

“You don’t build a retaining wall and go, you have to keep monitoring it and taking appropriate measures if there are signs it might crack,” she said.

“However, maintenance rarely happens and sometimes accidents occur.”

She observed that engineers often didn’t appreciate the nature of soil and geomorphology. They ended up with one-size-fits-all approaches. She recommends they should rather work with soil scientists and geomorphologists to approach these solutions better.

Own Correspondent