Digging Deep

The elevator trip down Twin Shafts at Gold Fields’ South Deep mine in South Africa takes between five and six minutes in a cage nine metres in diameter. At the bottom — a depth of three kilometres — temperatures at the rock face top 50 degrees Celsius.

It’s one of the most extreme environments in an industry known for them. Mining so far below the surface presents unprecedented technical challenges and requires unique considerations, from complex seismic monitoring to massive refrigeration plants and ventilation systems that keep tunnels at a constant 28.5 degrees Celsius.

South Deep is Gold Fields’ flagship growth project, mining one of the greatest undeveloped orebodies in the world. It’s expected to be a high-margin, high-volume gold mine for the next half-century.

“This is going to be the long-lasting standing gold mine in South Africa,” says Sarel Ferreira, senior manager of operations at South Deep’s South Shaft.

The Sandvik solution

Sandvik has been a supplier of underground equipment to South Deep for more than a decade. Sandvik commissioned the first DS210L-M, the only fully mechanized rock bolter available in the South African mining industry, in February 2012. The one-man electrohydraulic low-profile rig enables installation of several rows of bolts without moving the machine. Sandvik also supplies jumbos and LHDs to the mine, and assists in classroom and simulator training for operators and mechanics.

South Deep is the only South African mine that Gold Fields retained when it unbundled two legacy assets in February 2013. Since Gold Fields acquired full control of South Deep in April 2007, it has steadily transformed the mine into one of the world’s greatest deep-level mechanized operations, in a country where conventional mining methods still prevail.

Mechanization is key to both safety improvements and operational performance for a mine the size and depth of South Deep. The developing operation sprawls over more than 3,500 hectares of continuous mineral rights, 50 kilometres south-west of Johannesburg in the tiny province of Gauteng — “place of gold” in Sesotho, one of South Africa’s official languages. Its orebody comprises two thick reefs — the stacked Upper Elsburg conglomerates and the Ventersdorp Contact Reef. The Upper Elsburg reefs account for more than 90 percent of the mineral reserves of the mine.

Miners access the orebody through two shaft systems — the older South Shaft complex and, three and a half kilometres away, the Twin Shafts complex, which opened in 2005. The main shaft at the newer complex is one of the world’s deepest single-drop shafts at 2,995 metres — imagine flipping the Eiffel Tower upside down and stacking it one on top of another nine times. The steelwork in the shaft weighs more than the famous French monument, too.

South Deep produced 270,000 ounces of gold in 2012 and is forecast to produce just over that in 2013. The mine is ramping up to full production and hopes to more than double that total within the next few years.

The mine’s immediate focus is to open the orebody to enable that extra production, which requires increasing the rate of mechanized destressing, a unique mining method that is crucial for countering rock stress at extreme depths.

Destressing at South Deep means mining out select cuts some 2.2 metres high and 5 metres wide in order to relieve pressure above or below stopes. Backfilling the cavities with mined material increases stability.

The Sandvik fleet at South Deep

South Shaft:
4 LH514 LHDs
2 TORO 1400 LHDs
2 DS210L-M rock bolters
1 DS311 rock bolter
1 DD321-40 twin-boom jumbo drill rig
1 DD210 jumbo drill rig
Twin Shafts:
10 LH514 LHDs
3 TORO 1400 LHDs
1 DS210L-M rock bolter
14 LH514 LHDs
5 TORO 1400 LHDs
3 DS210L-M rock bolters
1 DS311 rock bolter
1 DD321-40 twin-boom jumbo drill rig
1 DD210 jumbo drill rig

“We’re very proud of our destress mining method,” Ferreira says. “We’re the only mining company in South Africa that’s currently doing this type of destress at such a deep level. We’re basically fooling the rock into thinking that we’re mining at 1,200 metres, instead of three kilometres underground.”

The majority of ore is extracted by long-hole stoping. Drilled and blasted ore is transported to internal orepasses using LHDs, hoisted to the surface through an expanded ventilation shaft and processed at a recently expanded central metallurgical plant.

South Deep’s Vision 2015 aims for a production rate of 330,000 reef tonnes per month, which would enable the mine to achieve its target production of 700,000 ounces of gold per year in a few years’ time — a rate it expects to maintain until 2057, with mine closure projected for 2080.

As Gold Fields works to position South Deep as one of the world’s safest and most efficient mines, the company believes building up to full production may also require increasing the number of ore passes from six to nine by July 2014, to help reduce underground ore accumulations and increase mill yield.

“We’ve cracked deep-level bulk mining at this depth with our mechanized equipment,” Ferreira says. “We’re really now focusing on implementation best practices, especially on the mechanized operation, to enable us to become more efficient with our machinery.”

Sandvik Mining has supplied much of the mine’s trackless equipment, including 14 LH514 LHDs and five TORO 1400 LHDs, the oldest of which is still in production after 23,000 hours.

South Deep also operates three Sandvik DS210L-M rock bolters. The one-man electrohydraulic low-profile rig enables installation of several rows of bolts without moving the machine and is the only fully mechanized rock bolter in South Africa.

“We’re using some high-tech equipment, and we believe we’re much more advanced in support at depth with the mechanized bolter than other mining companies,” Ferreira says. “We’re very proud to have this relationship with Sandvik at this point in time, with the new type of technology equipment that they bring in, especially in the old areas where we’re going to open up again.”

As part of its Vision 2015,South Deep completed a state-of-the-art mechanized training centre in December 2012. The centre accommodates more than 60 students daily and features four lecture rooms and an engineering workshop for basic maintenance training.

South Deep holds classroom and simulator training to introduce equipment operators to the trackless mining environment before they make their way underground. The centre takes miners who are accustomed to conventional methods and trains them for mechanized mining within six months.

“Our relationship with Sandvik is that they assist us with initial training, which consists of the theory component and also the practical components,” says Eddie Stonehouse, training superintendent at South Deep.On the theory component side they make use of our lecture rooms, we make use of their training material, they have qualified trainers and assessors come to our site and do the initial training.”

Adds Ferreira: “When it comes to our OEMs, we’re very proud to have the technical skills and the backup service as well. It’s just about selling the machines that’s important, but also giving us the backup on them as well.”

Eben Van Dyk, engineering manager for Twin Shafts, looks after the mechanized fleet, including planned maintenance, breakdowns and commissioning new machines.

“We started working with Sandvik to introduce various systems and controls at Twin Shafts two years ago in trying to improve our machine availabilities,” Van Dyk says.“I can gladly say that Sandvik really cares about the equipment on South Deep, and they share the failures and the successes supporting South Deep in doing what we do best.”

About Gold Fields and South Deep

Gold Fields produces around 2 million ounces of gold annually from six operating mines in Australia, Ghana, Peru and South Africa. The company has total attributable gold reserves of 54.9 million ounces and mineral resources of 125.5 million ounces. In addition to one of the world’s deepest mines in South Deep, Gold Fields also operates one of the highest gold mines in the world — Cerro Corona in Peru is located at an elevation close to 4,000 metres in the highest part of the Western Cordillera of the Andes Mountains.
South Deep is a fully mechanized mine 50 kilometres south-west of Johannesburg in South Africa’s Gauteng province. The mine employs 3,500 staff and mines ore from two thick reefs deep below the surface. The mine adopted a round-the-clock operating model in November 2012, embracing a long-time underground industry standard that hasn’t caught on in South African mines. South Deep’s 24/7 model has already shown a drastic increase in underground production.