Shaft Sinking Technology: The Long and the Short of It
European Editor Kyran Casteel reports on the progress at Anglo Platinum’s Paardekraal 4,000-ft-deep No. 2 shaft project in South Africa, and Editor-in-Chief, Steve Fiscor, introduces a Southeast U.S. company that specializes in blind bore drilling
The shaft project is designed to restore Merensky Reef output at Paardekraal, mining this reef, which has a higher unit value, as the base operating horizon. This should ensure sustained profitability. The UG2 Reef horizon could be used to fill spare shaft hoisting capacity but not at the expense of Merensky production.
The shaft project, which is Anglo Platinum’s largest in the Rustenburg area for more than 10 years, is estimated to cost R2.5 billion ($261 million). It incorporates an 8.7-m-diameter downcast main people and materials shaft and a 6.5-m-diameter upcast ventilation shaft, with both to be concrete lined. Also included in the project is the extension of the existing declines from Paardekraal No. 1 shaft some 4.5 km away. The main shaft will have shaft steel work for hoisting and there is provision for shaft stations at the 28, 32 and 33 levels. The planned final shaft depth is approximately 1,200 m below surface.
Within Anglo American, the Anglo Technical Division (ATD) in South Africa has a wealth of experience in the design, project management, testing and commissioning of shafts, not least deep vertical shafts. The unit was involved with several conceptual decisions in the early stage of the PK2 project. These required preparatory trade-off studies of the shaft sinking methodology for both shafts, evaluating the head gear construction concepts and tradeoffs on the winder selection, explained ATD’s Hendrik Oosthuizen.
Another ATD contribution during the early stages of the project involved the Quality Assurance department, which assisted the definition and finalization of quality controls for the early introduction of an agreed quality management system for the whole project. This system proved not only necessary on occasion but also made an invaluable contribution to the project’s success.
In order to benefit equally from South African contractors’ considerable shaft sinking expertise, the shaft sinking contractor was selected early in the project feasibility study phase. A thorough tender process led to the choice of Murray & Roberts (M&R Cementation) whose contribution in the early stages considerably assisted the design process.
Although hot-dip galvanizing, rather than painting, appeared to be a more cost-effective way to protect the hear gear, Anglo Platinum determined that the galvanizing process should first be tested on the most complicated section of the head gear. The test program was planned by staff from ATD, the Hot Dip Galvanizing Association of South Africa and a galvanizing company. Although the test piece was protected by galvanizing, it was also distorted by the process to an unacceptable extent. Taking time and other constraints into account it was decided to paint the box girder sections, although all other steel work was hot-dip galvanized. ATD expects that controlling fabrication methods for box girder sections as well as controlling cooling and handling of the sections will allow successful use of this process in future.
Another issue for the head gear installation design team was the complicated angular foundations required. These dip in two directions and are difficult to construct. Experience suggested that matching the base plates of the head gear legs perfectly for erection would be the key to success. Following consultation with the project surveyors and a review of the options, the one thought most likely to succeed—a unit for inserting the foundation bolts which became known as the “rocket launcher”—was implemented. Not withstanding the office jokes, it worked perfectly.
The project team sought other innovative techniques that would add value. One of these was the replacement of the traditional steel plate bunton shutter boxes that are cast-in into the concrete shaft lining by lighter materials: man-handling these heavy structures in the shaft sinking environment poses numerous injury risks for the personnel involved. Trials were conducted at the Murray and Roberts’ premises to determine the technical feasibility and construction practicality of various polystyrene grades and construction angles and this idea was successfully implemented.
A second innovation was the redesign of the conventional water ring. Used to drain water from the shaft barrel, this ring is difficult to clean and maintain. The new design uses a spiral channel cast into the lining which will self-clean the water-ring. Time will tell how effective this is.
Another challenging task for the core ATD and Murray and Roberts’ project team has been the design of an electrohydraulic (EH) drill rig for shaft sinking. Despite the uncertainty element involved in using an unproven piece of equipment, the potential benefits were rated sufficient to justify the risk. Using an EH drill was expected to improve excavation cycle times and to reduce the number of workers in the shaft bottom. The final design is sufficiently promising that the project team envisages that it should be the future of shaft sinking in South Africa and perhaps elsewhere too.
SDI Sees a Bright Future
for its Blind Shaft Boring Business
One of the more well known shaft development methods, blind shaft development, uses reverse circulation technology. In the mining districts of the southeastern United States, Shaft Drillers International (SDI) leads the way with the use of blind shaft development and, over the course of the last 25 years, the company has gained a considerable amount of experience. Through a series of acquisitions SDI broadened its capabilities to include geotechnical reinforcement. In 2006, Scott Kiger and Charlie Riggs, natives of the western Pennsylvania coalfields, purchased SDI through their own geotechnical firm, Coastal Drilling East. At that time SDI was a holding company for North American Drillers and Zeni Drilling. Today, SDI is the holding company for North American Drillers, Zeni Drilling and Coastal Drilling East.
The acquisition gave SDI access to a group of geotechnical engineers and working together they have been able to enhance the services that SDI can provide. As an example, the geotechnical division of Coastal Drilling East performs pre-grouting for shaft installation and they also have a well-plugging division, which has become increasingly important to longwall coal miners.
SDI considers itself a geotechnical construction company— one that is uniquely positioned to deliver both large and small diameter shafts with the appropriate ground treatment and stabilization to minimize surface and strata risk. They are routinely drilling shafts as large as 20 ft in diameter with depths approaching 2,000 ft deep. Through its experience in the coalfields, the company also has a keen understanding of the new regulatory demands that underground mine operators face.
SDI developed large diameter drilling capabilities in the Appalachian coalfields and has perfected its techniques over time. Theoretically, the technology has no depth limitations. “As far as the shaft development business and this particular method, SDI has been successfully completing these projects in the U.S. since the 1980s,” said Tim Bruner, project manager–shaft construction, SDI. “We accomplish this drilling in advance of mining by drilling down into a virgin seam of coal, removing all of the cuttings, and installing the liner. When the miners reach the shaft, they have the ventilation they need.”
The shaft drilling rig has a rotary table connected to a reamer body (or cutting tool) by a heavy double walled rod or drill string. The rotary table provides the torque or turning action for the reamer. To create up flow, the shaft is filled with water which is maintained at a constant level throughout the entire shaft development. The water fills both the shaft and the hollow drill string creating two independent columns of water. Compressed air is injected into the water column of the drill string displacing the fluids and creating a much lighter column. The heavier water column inside the shaft pushes down and across the development face (bottom of the shaft). The water is then forced through a small opening called a pick-up on the reamer body, which displaces the lighter water column in the drill string. The displacement of the lighter drill string fluid results in an upward flow or reverse circulation.” The volume of fluids being displaced, typically 2,000 to 3,000 gpm, creates a tremendous vacuum at the pick-up point that literally sucks the cuttings from the face.
Once fully developed, the shafts are typically lined prior to dewatering. Depending on application—and to maximize efficiency— the final liner is available in steel, concrete or composite material. “Most mines prefer the steel liners, which have multiple advantages,” Bruner said. “There are no water rings and the liner is guaranteed water tight. Any project up to 15-ft diameter is lined with steel. The cost of the steel becomes prohibitive for larger diameter shafts. Larger shafts are usually concrete lined.”
Shaft development rates are a function of diameter and ground conditions. For 8- to 10.5-ft-diameter shafts, which is the vast majority of what SDI does, they can complete a 500- to 800-ft shaft in four to six months. For 12-to 15-ft-diameter shafts, the development rate would be between five and seven months.
Bruner said he has seen the business change recently, mainly due to regulatory requirements being placed on the coal industry these days. “We have seen more activity with the larger diameter shafts due to the additional ventilation required underground,” Bruner said. “We have also completed several secondary escapeway projects which were required as part of the company’s emergency response plans.”
Raise bore shaft development requires access to the bottom of the shaft in order to connect the reamer head to the drill rod as well as to remove the cuttings. SDI is developing shafts using raise bore technology, which produces faster advance rates and requires fewer excavation personnel.
“We’re providing surface and underground setup with the raise bore division,” said Bruner. “The disturbance to surrounding rock is minimized and we’re able to limit the interruptions to traffic and services.”
With the previously mentioned acquisitions, SDI has realized synergies in ground stabilization technologies and other capabilities, such as well plugging. Coastal Drilling East originally started as a civil firm. Through its association with SDI, however, it has discovered new opportunities in the mining business.
“Coastal’s geotechnical division is a specialty geotechnical construction company, which means we deal with all types of ground improvement, such as pre-grouting work for shafts, grouting of poor soil, water leaks into the mine, etc.” said Dean Dibert, project manager–geotechnical construction, SDI. “In development, we get into all types of earth retention work, shoring systems, foundation work for crushers and conveyor systems. For difficult ground conditions, we offer solutions to help the mines with anything from ground improvement to foundation support.”