The earthquake which shook Johannesburg earlier this week, sparked a new round of speculation on the safety of Kariba Dam Wall. It is true that the Kariba dam site experiences a considerable amount of seismic activity – almost daily – most of which is never even felt by us mere humans. That the dam wall itself is built to withstand this activity is fact, and it is monitored electronically from deep within constantly. Ironically, any threat to the wall comes from a completely unrelated source.Kariba Dam Wall’s Legacy
In the last issue’s editorial of African Fisherman (Vol.24 No.6), mention was made of the ongoing maintenance and refurbishment on the Kariba dam wall. It has long been known that the stilling pool – the area directly below the wall on the downstream section of the river – suffers erosion when the gates are opened to release water, and if left unchecked, could result in the wall’s foundations being undermined. The authorities also continually make vague references to other monitoring, surveys, maintenance, and up-grades… so vague in fact, they often lead to rumors of the wall’s immanent collapse spreading like wild fire.
In recent years, work has been undertaken on the flood gates themselves, with several of the gates being “tested’ prior to any possible flooding. There is a real fear Kariba could experience the kind of floods seen during the building of the wall when, first a “100 year” flood, following by the “thousand year” flood hit. At that point, the proposed number of flood gates was increased from four to six. However, concerns still exist that a major flood in the Zambezi’s catchment could overload Kariba’s ability to successfully discharge the water before it overtops the wall.
A recent report carried in the New Civil Engineer magazine revealed that EU funding had been made available to begin blasting some 300 000 cubed metres of rock from the stilling pool in order to create a series of steps on the downstream side of the pool, which it is believed will bring the erosion under control. Originally about 10 meters deep, the pool is now some 80 meters deep as successive flood waters from the open gates have broken off pieces of rock within the stilling pool. Most of this occurred within the first 20 years of the dam’s construction. These chunks of rock are further circulated in the pool, and as they swirl around, they cause greater erosion. The report states “Currently the dam operators are limited to opening a maximum of three gates at once to minimize scour. Fissures in the riverbed gneiss allow the dam’s discharge to break away large masses of rock. These boulders then circulate in the plunge pool, causing more fragmentation.”
Apparently, such blasting work to modify the pool is likely to be undertaken in the dry, behind a temporary cofferdam. An international researcher and Technical Director for International Dams and Hydro power- Peter Mason – said “…the scour problem is controllable and doesn’t threaten the dam’s stability”.
Of greater concern though, is a process known as Alkalai Silica Reaction (ASR). Simply, this is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica, which is found in many common aggregates, and causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate. This gel increases in volume with water and exerts an expansive pressure inside the material. Little understood in the 1950s when Kariba was built, it is estimated that about a hundred dams worldwide suffer ASR, and it is also responsible for damage to bridges, roads, runways and the like.
Cement contents in Kariba’s construction was relatively low, and formed by pouring blocks measuring up to 20m in length, by 15m wide, by 2,3m deep with the gaps between blocks pressure grouted. In 1989, ASR was detected in Kariba and Quartz embedded in the gneiss coarse aggregate used at the time of construction was blamed. Although extensive monitoring equipment has been installed, in the early 90’s the process began interfering with the smooth operation of the flood gates as they scrape on the swelling concrete. Interestingly, the wall is some 80mm higher than when it was first built due to the ASR process. In the greater scheme of things however, this is relatively minor if attended to, though could mean the modification or replacement of floodgate systems. According to the New Civil Engineer article, the Zambezi River Authority (a partnership between Zambia and Zimbabwe to manage Kariba) has plans to add an emergency flood gate to be brought into use should any of the existing flood gates malfunction. Peter Mason states “Not a lot can be done about ASR, we just have to understand what future expansion to expect. The structure is essentially sound, particularly as Kariba is an arch dam.”
A little understanding of Kariba’s intricate challenges is enlightening and reinforces the ZRA press release which stated “…the wall is not in immediate danger and is constantly monitored”, a process which is assisted too by a team of French engineers.