The dewatering tunnel, some 60 m underground, circumvents the lateral walls of the Kimberley diamond-mine's Big Hole for about 3,5 km.
It was built after the mine closed down as a maintenance mechanism to slow down movement, and perhaps the eventual collapse of the lateral walls of the mine shaft due to water seepage, De Beers business development manager Gary Joseph tells Engineering News.
The tunnel creates ideal conditions for mushroom growing below the harsh dry climate of the Northern Cape Province.
As air flows through the tunnel, its moisture content increases to humidity levels of between 75% and 90% compared to the 15% water content in the outside air.
Hot summer temperatures are cooled in the tunnel due to the high water content to between 18 ˚C and 25 ˚C.
Similarly, the tunnel's water content moderates the winter temperature to within the same range, says Joseph.
The oyster mushrooms are grown on 300-mm-diameter plastilon bags made from recycled plastic.
These bags are tied at the base to vertical poles and filled with a mixture of growth medium and mushroom spawn before being tied at the top, explains Joseph.
The growth substrate is made up of wheat-straw, bought in bales and passed through a hammer mill to bring it to the desired size.
This straw is then passed through a mixing drum – converted from an old cement mixer – and mixed with agricultural gypsum and lime to bind the substrate. Fungicide and insecticides are also added.
About 20 kg of this mixture is placed into each bag, mixed with wheat germ and mushroom spawn, before being compressed to a certain density. This mixture also contains all the water content that the mushrooms will need throughout the growing process, so no further watering is needed.
The bags are then placed underground to stabilise them.
The following day a series of 20-mm-long holes, 100 mm apart, are punctured into the bags where the mushrooms will grow out of.
It takes between four and five weeks for the bags to begin to bear fruit, and each bag can produce for up to six months before being replaced.
The bags produce on a cyclical basis, with the first harvest being the largest, and the subsequent harvests gradually decreasing in size.
The mushrooms are hand-picked and placed into crates that are brought to the surface, and are cooled before being cleaned, cut, wrapped and packed.
The project currently employs 11 full-time employees, with its main customer being Highveld Mushrooms.
A percentage of the production is also marketed through the local produce market under the brand name Big Hole Oyster Mushrooms.
Mushroom farming around the Big Hole was started by a private company in 1998, after seeing the concept developed in Canada and Australia, where mushrooms were grown in old mine shafts De Beers supported this initial venture by supplying an interest-free loan to the company for tunnel infrastructure, and allowed the company to use the tunnel.
A fruit-fly infestation of the crop in September 2000 resulted in the company not being able to fulfil a contract to supply Denny Mushrooms, and the directors opted to shut down the operation.
Due to the unusual nature of the venture, De Beers decided to continue with the project, with the vision of eventually selling it to a black economic empowerment (BEE) company.
Since then De Beers has investigated various job-creation strategies for the project, including on-site processing and packaging, and the company aims to increase production to 500 kg a week in order to make the project a lucrative business opportunity.
However, routine maintenance activities in the tunnel that produce noxious gases greatly reduce the production of the crop.
De Beers is currently investigating ways to decrease the impact of these maintenance activities on the mushroom-production process.
The diamond-miner has also joined in partnership with the Northern Cape Manufacturing Advisory Centre to help identify an appropriate BEE partner with a good business plan and ideas on production improvements to decrease the impact of tunnel maintenance, as well as downstream job-creating activities, says Joseph.
Another large challenge facing the project is that up to 40% of production cannot be sold into the consumer market for aesthetic reasons, although in all other respects the product meets required specifications.
De Beers is also investigating possible added-value processes that could make use of the lower quality product, such as finely chopping the mushrooms, or dehydrating them for soups or canned foods.
Speaking about the oyster mushroom market in South Africa, Joseph says the market is relatively small compared to the traditional button mushroom market.
However, with the increase in health-consciousness among South Africans mushrooms' popularity could increase significantly as a good alternative to red meat, as some 30% of mushroom content is protein.
Besides mushroom farming, De Beers is also in the developmental stages of investigating possible projects in other nondiamond-related activities, such as abalone farming and the conversion of mines into tanks for fish farming.