Not such a slippery slope

2008-08-19 00:00

Julia Denny-Dimitriou’s article “N3: Why all the repairs?” on August 12 provides a lengthy but inaccurate explanation of the damage done to the southbound carriageway of the N3 on Town Hill when a few localised slip-circle failures occurred after the latest heavy rains. The article heading refers to “geological forces” which “caused the road surface to lift”, which is an exaggerated and misleading representation of the facts. There are so many horror stories doing the rounds about this area, and about Victoria Country Club Estate, that her article, if left unchallenged, will only serve to fuel further misinformation.

A slip-circle failure is a localised phenomenon caused when an active groundwater condition combines with a steep slope to overcome cohesive forces between the soil particles. This results in a slump of the unstable section in a somewhat circular shape, particularly where the toe of the embankment is not restrained, leaving a hole in the slope and a raised pile of soil at the bottom. Although in this case one would have assumed that the roadway itself would have provided some sort of restraint to the failure, it did not, and the steep embankment above the road displays exactly the characteristics of this mode of localised failure, limited to small slip areas of relatively minor height and width. That is why gabion baskets are being put in place to stabilise the toe of the embankment. Presumably additional subsurface drainage measures are also being considered by the engineers to manage the groundwater in this area, and to reduce the risk of repeat failures.

Of interest is the fact that the damage to the roadway is limited to the right-hand lane directly adjacent to the areas of embankment failure. Were the failures caused by “geological forces” and generalised down-slope movement of the talus material, as implied in the article, one would have assumed that the entire width of the carriageway in this area would have been affected, but this is not the case.

The two sections of the freeway which have been problematic are well known. The one is the area of the now famous “Rickivy Slip” where the freeway crosses a deep valley. Here the large, engineered fill prisms of the two carriageways were originally placed across an obvious and ancient drainage valley, causing instability and settlement. The need to make adequate provision for the traversing of this valley should have been obvious to the original freeway designers and contractors, but in any event the recent construction of the new bridge spanning the valley appears to have solved the problem.

Interestingly, as one crosses this bridge a quick glance up the hill reveals a number of houses nested at the top of this same valley, presumably constructed on conventional unreinforced strip-footing foundations, which have remained happily and blissfully unaware that they should be sliding down the “faultline” or “unstable landslide” to catastrophic failure, as the prophets of doom would have us believe.

Similarly, the area of current attention occurs in a section where the freeway traverses a particularly steep section of the Town Bush Valley south slope, on what is predominantly a more aggressively eroded outside bend of the river valley. Major cut-and-fill embankments have been engineered on this steep section, and one has to wonder whether adequate provision was made for the particularly challenging aspects of this local topography.

As the developers of the Victoria Country Club Estate, we have had to pay close attention to the geology of the valley, particularly to the geo-technical characteristics of the northeast facing slopes of the escarpment, which combine unconsolidated water-deposited talus material, deeply weathered residual soils, dolerite rock and sandstone in a seemingly random configuration.

Having invested significantly on research and reports from a number of geological engineers, we have a reasonable idea of the complexity of the area in so far as stability and founding conditions are concerned. The results of the research are neither unanimous nor simple. There is general consensus, however, that conventional foundations are inappropriate for new developments in these areas, and on the estate we therefore insist on an appropriate foundation for each building designed by a structural engineer.

Rumours abound about the exorbitant costs of these foundations, one even being that despite the fact that the foundations cost more than the houses, they are cracking. This is absolutely false. The stiffened raft foundations used on the estate should take the foundation costs from the normal 10% of the total house cost up to a maximum of 20%, therefore adding at most 10% to the building cost, and we have had no evidence of structural failure or distress in any of the buildings completed or under construction.

If one considers that huge areas of Pietermaritzburg have been developed on colluvial talus slopes, including Oak Park, Montrose, Wembley, Northdale, Sweetwaters, Blackridge, Athlone, Clarendon, Claridge and Edendale, and virtually all of these homes have been built on conventional unreinforced strip footings, the incidence of structural failure is minimal. And where it occurs it is generally due to differential settlement across the footprint of the dwelling typical of these soils, and not to downslope movement.

The Town Hill area is no different, and alarmist talk about geological forces causing things to lift and go bump in the night, or to slip uncontrollably down the slope, do nothing more than escalate the hype around what should be no more than normal engineering issues. And although a tunnelled route from Cedara to Chase Valley would take the noisy heavies off the slopes and remove the risk of further instability, we are as likely to see that happen as to be greeting each other from our rafted balconies while floating down Town Bush Stream.

Join the conversation! encourages commentary submitted via MyNews24. Contributions of 200 words or more will be considered for publication.

We reserve editorial discretion to decide what will be published.
Read our comments policy for guidelines on contributions. publishes all comments posted on articles provided that they adhere to our Comments Policy. Should you wish to report a comment for editorial review, please do so by clicking the 'Report Comment' button to the right of each comment.

Comment on this story
Comments have been closed for this article.

Inside News24

Traffic Alerts
There are new stories on the homepage. Click here to see them.


Create Profile

Creating your profile will enable you to submit photos and stories to get published on News24.

Please provide a username for your profile page:

This username must be unique, cannot be edited and will be used in the URL to your profile page across the entire network.


Location Settings

News24 allows you to edit the display of certain components based on a location. If you wish to personalise the page based on your preferences, please select a location for each component and click "Submit" in order for the changes to take affect.

Facebook Sign-In

Hi News addict,

Join the News24 Community to be involved in breaking the news.

Log in with Facebook to comment and personalise news, weather and listings.