In pursuit of perfection

Master Drilling’s horizontal raise boring projects at Petra Diamonds’ Cullinan mine is a giant leap for the mechanisation of underground mines in South Africa, writes Leon Louw, editor of Mining Mirror and African Mining.

Seven hundred and seventeen metres below surface, 30km east of Pretoria, a large reamer head — in reverse gear — is gradually grinding its way through metres and metres of notoriously fragile Kimberlite material. Although progress seems painfully slow, the reamer head’s successful retreat through the rich blue ground of Cullinan will fast-track the pace at which South Africa’s underground mines transform into modern and more efficient operations. The reamer head was hooked onto the drill string after the bit created a pilot hole 349mm in diameter and just short of 180m long. The pilot hole, lined with a steel casing, protects the drill bit as it now pulls back and causes the massive rock eating reamer to rotate back towards Master Drilling’s Horizontal Raise Boring (HRB) machine.

Fochville based Master Drilling and Petra Diamonds teamed up almost three years ago at Petra’s Cullinan Diamond mine to test run the drilling trailblazers’ revolutionary HRB machine. The project is now rapidly nearing completion as the reamer head gains traction and increases the speed at which it excavates its way back towards the HRB unit. When the monstrosity reaches the mothership it will not only leave behind an almost perfectly round 4.5 diameter horizontal tunnel, but also a legacy that might just prove to be a giant leap in a rapidly changing mining world.

If Master Drilling and Petra Diamonds pull it off — and there is no reason why they wouldn’t — it will be a first for underground mining, not only in South Africa, but also in the world. The fact that the entire operation was carried out without water using a unique vacuum system, and that the pilot hole was drilled in a direct straight line for almost 180m, makes the project even more remarkable. And that is enough reason to justify an endeavor that was originally scheduled to take only six months, but eventually has dragged on for close to three years.

Making an unknown entity work

Horizontal raise bore drilling — unlike vertical raise boring — is an unknown entity in underground mining. Various experiments, ideas and technologies geared at improving performance and removing people from the rock face fell by the wayside over the years. In their pursuit to mechanise mining operations, Master Drilling and Petra Diamonds might just have stumbled upon something that could be extremely useful — even if only as a case study — in the underground mine of the future.

Horizontal boring is mostly used in urban construction projects where drilling and blasting are restricted or forbidden and tunnel-boring machines are too bulky. The tricky part is that in all instances the method is used in areas where the ground and rock conditions are extremely stable and where water is available to cool the drill bit and suppress the dust. It has never been tested in underground kimberlite applications. However, in typical fashion, Master Drilling turned the project into a challenge to overcome these perceived hurdles. But it came at a price.

The nature of the geology at Cullinan and the instability of the loose Kimberlite rocks have been tormenting the project team. Although a trial project, it has taken way longer than originally envisaged, and has placed more strain on financial and human resources than initially expected.

Valuable lessons have been learnt and the team does acknowledge that. “We never expected it to be plain sailing, but it’s been a massive learning curve. We knew there were going to be many challenges and difficulties, but we managed to overcome all of them. It is only a trial run so we didn’t spend capital equipment investment on the project; however, in the future we will look at many improvements, including the logistics and the cleaning methods,” says Koos Jordaan, technical director at Master Drilling.

When they started deploying their equipment on site a little more than two and a half years ago, Master Drilling expected the excavation to be done in less than a year, but it took them almost six months just to open up and develop the area where the HRB machine would be deployed. If there are any negatives that can be taken away from the undertaking, it would probably be that the machine is cumbersome as well as difficult to manoeuvre and get into position. But once in place, it didn’t take long for the bit to start drilling away into the Kimberlite.

With raise bore drilling a pilot hole is required to enable the operators to hook up the reamer head on the other side, which is then reamed back towards the machine. The most important requirement when doing horizontal raise bore drilling is that the project team needs to have access to both sides of the material they intend drilling — to develop or construct access can take a fair amount of time though. Nonetheless, in Cullinan this was not a major concern, as a ring road was already in place. The challenge, however, was to drill the pilot hole as accurately as possible through extremely friable material. The pilot hole then lies empty for a while so it collapses. Therefore, Master Drilling decided to case the pilot hole with steel tubing to prevent it from closing, which would have resulted in a major problem when the reamer had to start excavating. “We decided to just cut the tubing away with the reamer on the way back, and it worked,” says Jordaan.

In addition, there was plenty of ground movement, which caused further unease among the drilling team. “Despite the steel casing in the pilot hole and the fact that the hole didn’t collapse, the steel casing actually squeezed shut — it looked like a banana,” says Jordaan. “Look, there are massive challenges in undertaking a project like this: the drill is not straight, the hole moves, and halfway through the hole the bit bends; however, the method needs to be robust enough to deal with these challenges,” says Jordaan.

Headwinds in the tunnels

During the first three weeks of drilling the pilot hole everything went according to plan. The initial goal was to drill to a distance of 220m; all the way, accurately and on line — but that was the first major challenge. Just when everybody thought that it was all too good to be true, the drill bit got stuck at about 180m. And it remained stuck. “It took us a year to recover the drill string so that we could use the hole. Then it took another six months to do the preparations to start drilling again,” says Jordaan.

When the drill bit got stuck, Jordaan and his team didn’t know what had triggered it or what was causing the seizure. They resorted to pushing down a camera into the hole to check every rod connection, but couldn’t find anything. “Remember, this method was originally designed to do civil construction, and drilling utility holes are normally limited to 100m. We knew we were pushing the envelope on this one,” says Jordaan. Eventually Jordaan and Steven Kloppers, mining manager production at Petra Diamonds, decided to develop another tunnel. To develop the tunnel they had to drill and blast, which caused more ground collapse around the drill string. “Eventually we brought in another rig and drilled over the bit to recover all the drill rods. We fixed the drill bit into a steel casing and grouted around that, and eventually had a pilot hole again,” says Kloppers. At last it was decided to hook the reamer head at 174m — 46m short of the initially planned 220m. This was another risky decision, as the area where the reamer head would initially have been hooked would have been solid host rock, as opposed to the Kimberlite where the reamer had to be fitted now. This of course required a lot more preparation work, which wasted more time.

When I visited the tunnel the reamer was advancing at about 2m per day, but Jordaan is convinced that this rate will increase gradually over the next few weeks. “We actually planned for 3m a day on the original schedule, but we are aiming to get 6m a day. Nothing really prevents us from eventually getting to 9m or 12m in the near future,” says Jordaan. The one advantage of the soft Kimberlite is the fact that it is gentle on consumables and the reamer advance will thus be much quicker. “The best development crews on the mine can do 40–50m a month using traditional drilling and blasting methods to excavate a tunnel. In comparison, the HRB can potentially get through the total ore body in one month,” says Jordaan. Taking into account that a metre in tonnes equals about 50 tonnes of material, then advancing at 6m per day will mean that more than 300–500 tonnes of waste material will have to be moved daily — that, in anybody’s book, is a lot of work. “It then becomes less of a cutting problem and more of a logistics problem, especially in cleaning up the waste material and supporting the tunnel,” says Jordaan.

Endless benefits

Currently, underground crews at Cullinan advance at between 2m and 6m per day, very similar to what Master Drilling is aiming to achieve with the HRB. However, according to Ben Swarts, group manager projects at Petra, the benefits of using the machine are endless. “The ability to do dry drilling is unheard of, but we achieved it here. The accelerated advance and the optimisation of the entire mining sequence will generate great value. To add to this, the trial run has created a great deal of knowledge and capacity,” says Swarts. “The bottom line,” says Kloppers, “is that I need a stable, safe tunnel to be excavated from one side of the ore body to the other (220m), and I need to do it as quickly and safely as possible. The HRB allows me to remove people from dangerous areas such as the face; that in itself makes it worth using the machine,” says Kloppers.         

According to Swarts, De Beers tested a Roadheader in 2006 at Cullinan, but the machine was too big and the geology too tricky to make it a success. The Kimberlite material is not homogeneous and has many hard rock intrusions in the ore body. Although the HRB technology has not been perfected yet, there are many advantages in using equipment like Master Drilling’s machine. “For one, the shape and the integrity of the tunnel make it one of the strongest holes that you can put in the ground. It is a lot more stable than conventional tunnels, and the consistency of the tunnel shape makes it a lot safer. In addition, the rate at which you can move forward holds many benefits,” says Swarts.

“Other mines will make use of these methods soon — it is the next phase of mining. How long can we prevent it? In South Africa we have been mining tabular bodies by hand for the past 80 years; yet, we say our mines are mechanised. This machine is a step in the right direction, although I’m sure the next phase in mining will be cutting edge technology,” says Swarts.  

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