N3: Why all the repairs?

2008-08-12 00:00

Work has started on repairing the slope that damaged a section of the south-bound N3 highway on Town Hill during a storm months ago. However, authorities have warned that the unstable geology of the area means that the hillside will continue to move and damage the road from time to time. The only way to avoid this would be to build a tunnel through Town Hill.

A section of the right-hand lane of the N3 between the Hilton and Peter Brown off-ramps has been cordoned off with traffic cones since about March. It is now also isolated with concrete bollards to allow repairs to proceed. This section of the N3 was originally built more than 30 years ago. The south-bound carriageway was completed in 1971 and the north-bound in 1962.

According to Gunyaziwe Makaula, regional manager of the South African National Roads Agency Limited (SANRAL), the road surface was damaged by a shallow and localised “slough” or section of the hillside that slipped and caused the asphalt road surface to lift. Makaula emphasises that this is a purely geo-technical phenomenon that has nothing to do with the volumes of heavy traffic that the highway carries.

He stressed that because of the geology of Town Hill, damage to the highway caused by sloughs or slips will continue to occur, “to a greater or lesser degree. For as long as the highway is in place, we will need to repair it periodically. The only other option, which was considered during the eighties, would be to build a tunnel through Town Hill from about Cedara to Chase Valley.”

Michel Benet of Drennan, Maud and Partners, specialist consulting engineers and engineering geologists explained: “The geology of Town Hill is such that talus [landslip debris] deposits are common and movement of the rock material is generally a slow, ongoing process. Occasionally the movements are more rapid as for example in 1996 when there was a slip 300 metres long and 100 metres wide above the north-bound carriageway after a spell of heavy rain.

“The Townhill escarpment is underlain by the Vryheid Formation of the Ecca Group, comprising a sequence of thinly interbedded sandstone, shale and mudstone. Intruded into this formation are numerous dolerite sills and dykes. The talus is predominantly made up of dolerite blocks in a distinctive reddish-brown clay matrix. Between the Hilton off-ramp and the Peter Brown off-ramp these ancient deposits litter the surface as a result of gravitational soil and rock falls as the Townhill escarpment retreats naturally.

“The rock formations in the area are also very deeply weathered and, when they become saturated with water, tend to slough or slip readily. The highway crosses about nine areas of known instability that have been moving very slowly since long before the highway was built.”

Benet said many geotechnical techniques have been used to deal with problems on the roads caused by the area’s geology. These include minor repairs and earthworks and the building of the Rickivy Bridge over the unstable area adjacent to the Peter Brown off-ramp. “In the case of the current damage, the key is to keep the balance between the north-bound and south-bound carriageways and the median slopes.”

Drennan, Maud and Partners were appointed as a specialist consultant to investigate the damage and recommend solutions. The partnership has worked on the Town Hill highway for about 20 years, so has considerable information and experience to draw on. They are acting as sub-consultants to the existing routine road maintenance consultants, UWP Consultants. After extensive investigations, including a full survey and historical analysis, they came up with four design solutions, all involving different versions of retaining walls. Makaula said that in choosing a solution, SANRAL adopted an approach that looked at not only cost-effectiveness but also traffic disruption. Once the design was selected, the contract was put out to invited tender.

The approximately R5-million contract was awarded to Inhlanhla Civils, a Hillcrest-based civil engineering contractor. The company has started work on the slope above the highway to stabilise it in some areas and to cut into it to create space to construct a retaining wall. This will shore up the slope between the two roads and prevent it from slipping further. Like an existing section of retaining wall in the vicinity, the new structure will be built of gabions or wire baskets filled with hand-picked rock. Each gabion is two metres long, one metre high and one metre wide and holds two cubic metres of rock. When completed, the wall of stacked gabions will be 150 metres long and between four and six metres high. Rock is being brought in from a nearby quarry to fill the gabions. The material dug out of the slope will be used on site and, apart from the rock for the gabions, no other material will be shipped in. The work is expected to take three months, but will not require any additional closure of the highway or disruption to traffic.

Grant Shimwell of Inhlanhla Civils said the project will create about 24 jobs for the duration of the contract as local labour will be employed to do things like fill gabions. He said the biggest challenge of the project is the speed with which it has to be done to limit traffic disruption. “Three months is not a long time to carry out work of this nature,” he said.

SANRAL’s appointed routine road maintenance contractor, Superway Construction, will later move onto the site to repair the damaged highway. This will involve milling out the damaged section and replacing the asphalt road surface. If the highway’s drainage system was also damaged by the slough, this will be replaced.

Makaula hopes that this can be done at the same time as the retaining wall is finished so as not to prolong the disruption to traffic.

Frequently asked questions:

• What agency is responsible for the highway?

The South African National Roads Agency Limited (SANRAL).

• What caused the damage to the highway?

A “slough” — a section of the slope above the highway that slipped and caused the asphalt road surface to lift.

• Does it have anything to do with the volumes of traffic the highway carries?

No — it is a purely geotech-nical problem.

• What is being done to fix it?

The bank is being excavated to build a retaining wall to prevent further slippage. The asphalt road surface will be repaired later.

• Who is doing the work?

Contractors: Inhlanhla Civils is excavating the bank and will build the retaining wall; and Superway Construction will repair the damaged highway.

• How long will the work take and will it cause more disruption?

Three months, and no, only the right-hand lane will continue to be out of action.

• How much will it cost?

Approximately R5 million.

• Is this likely to happen again in future?

Yes — the rock material on Town Hill is unstable, so sloughs will continue to occur.

• Is the highway inspected regularly?

Yes, by a contractor responsible for routine maintenance.

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