When was the last time the Earth moved for you? A strange question, perhaps, given that South Africa is not widely recognised as being prone to earthquakes. However, when you consider the fact that low-intensity tremors are a daily occurrence in our country, the question develops relevance.
The kind of low intensity tremors experienced in South Africa are mining induced and are not dangerous to life and limb. They rarely cause serious structural damage to buildings but they do cause problems, especially with low-rise structures such as residential houses, leading to cracks and the kind of damage that, when left unattended over a protracted period can add up to major problems.
Because South Africa is considered an earthquake-free region, technical research into the loading of its building structures is concentrated specifically on ‘static” loading — in other words the load a building can withstand while not moving.
Until recently there were no testing facilities available to enable research to be conducted into how buildings react to ‘dynamic” loading — how they hold up while being shaken and moved around as in an earthquake.
But a project now in its second year at the University of the Witwatersrand’s department of civil engineering has changed this by developing and constructing its own ‘shake table” for testing how structures handle seismic activity.
The shake table is a means to an end, as the project, under the leadership of Nigerian-born Doctor Herbert Uzoegbo, is primarily aimed at testing the structural viability of low cost housing solutions throughout Africa.
The project’s industry partner is Hydraform Africa, the Boksburg-based manufacturer of Hydraform blocks and the Hydraform building system.
The Hydraform system is a dry-stack system, with interlocking blocks placed one on top of the other without mortar. Blocks are only bonded to the foundations of a Hydraform building, and depend on the interlocking system to provide structural resistance.
Uzoegbo and his team of Andy Hofmeyr from Wits, PhD research student Joseph Ngowi from Tanzania, final year undergraduates Mario Lopez and Luca Pellegrini and Hydraform’s Quintin Booysen set out originally to test the Hydraform system for static loading.
However, because of its low cost and ease of construction, the building system was planned for use across Africa, especially in East African countries such as Tanzania and Uganda.
East Africa is prone to major tectonic earthquakes, which occur as a result of seismic activity along the Rift Valley fault system where two of the Earth’s major tectonic plates meet, and as a result it was felt that testing should be done on how the Hydraform system measures up to Earth tremors.
The project then began to aim its research at refining and improving Hydraform’s method of construction and examining the effect if low- intensity tremors.
To do this Uzoegbo had to design and build Africa’s first and only shake table. This was not an easy thing to do, but was more cost-effective than buying one. And now being used to run tests on small-scale Hydraform structures and how they react both to the major tectonic quakes that can occur in other parts of Africa, and the low-intensity quakes experienced in parts of South Africa.
Most destructive earthquakes are of tectonic origin and the epicentres of more than 90% of global natural earthquakes occur at the boundaries of the major plates.
The seismicity of Southern Africa with its sporadic, scattered shallow earthquakes belongs to the type of intra-plate activity which may occasionally reach critical values such as the earthquake that occurred in Ceres in the Cape in 1969 (6,3 on the Richter scale) and another in 1976, in Welkom that measured 5,5 on Richter scale.
However, most seismic events in the region measure below five and originate in the mining areas. Earthquakes of intensity less than five can normally be resisted by well-designed structures.
The characteristics of mine-induced earthquakes are different from the tectonic type. They are usually low intensity and affect a smaller area but can cause serious damage and casualties in the mines and structural damage to low-rise buildings.
The immediate objective of the project is to investigate the effect of mine-induced tremors on low-rise structures, with particular emphasis on masonry structures.
‘Testing is a slow process,” explains Uzoegbo. ‘To build the test structure can take anywhere up to six weeks, then there is the implementation of the instruments, which takes another three or four weeks. On top of this we have to add the full three months it takes to perform the actual tests. So you have to allow anywhere between four and five months to run one test, all things considered,” he says.
‘It’s a slow but very thorough process,” he adds.
‘That’s the thing with research of this nature, you don’t always see immediate results.”
Uzoegbo adds that his team is working in collaboration with Spoornet, using the parastatal’s laboratory facilities.
His aim is to build similar facilities at Wits and to encourage more interest by students in technically based careers.
He points out that his particular field can have unexpected humanitarian rewards.
‘This project is a case in point. We set out to discover how new design can improve a structure’s seismic resistance, and this means that low cost housing in developing countries is being improved and made safer as a result of the work we are doing.”