Murray & Roberts Cementation

Less than a year ago, the future of East Rand Proprietary Mines (ERPM) Limited hung in the balance, with all indications pointing to the closure of the now 113 year old mine’s underground operations at Boksburg. However, thanks to careful planning and management, together with State assistance, mine executives are turning around the operation, with the prospect of extending the life of mine to 2011.

As part of its turnaround strategy, the mine has implemented an underground water management programme which will see the installation of five new high pressure mortar intruded water retaining plugs and the upgrading of three existing plugs at the South East and Far East Vertical Shafts respectively.

The project, which will cost R29,1 million, is being undertaken by Murray & Roberts Cementation. Work started in December last year and is expected to take 46 months to complete. The State has funded the first and second phase of the installation of the plugs.

Phillip Watters, general manager of ERPM, explains that as the last operational mine in the Central Rand Basin, ERPM has had its fair share of challenges, and one of the primary difficulties facing the mine in recent years being underground water management.

Explaining the rising underground water level, Watters says that in September 2004, the Central Rand Basin was already flooded above 24 Level, whilst the adjacent Hercules and South East Vertical Basins were flooded up to 50 Level.

By July 2005, water was expected to start cascading out of the Central Rand Basin into the Hercules and South East Vertical Basins into the Far East Vertical Basin on 42 Level.

“Mining activities in the Far East Vertical section were at risk with the influx of additional volumes of water. The situation called for the installation of five new high pressure water retaining bulkhead plugs on 42 Level at the South East Vertical shaft,” Watters says.

Murray & Roberts Cementation has previously installed bulkhead plugs on 58 and 68 Level at Far East Vertical, the only operational section of the mine, as part of the mine’s water management programme.

This project includes the upgrading of the previously installed 14 metre long bulkhead plug on 68 Level and two bulkhead plugs, one of 6 metres and one of 8 metres, on 58 Level.

Despite previous preventative measures, Watters explains that the mine has been pumping out water from the Central Rand Basin’s Rose Deep compartment from the South West Vertical shaft to prevent water cascading into adjacent shafts. This exercise will continue until construction of the bulkhead plugs on 42 Level at South East Vertical and on 58 Level and 68 Level at Far East Vertical are complete.

Watters says that even with this water management programme in place, the Far East Vertical Basin is experiencing an inflow of water of 1.6 Ml per day of extraneous water. Some 3.5 Ml per day of water is used for mining purposes. This water is being pumped into the South East Vertical Basin.

“Pumping costs are in the region of R 1,5 million per month and although this is subsidised to the extent of R1,0 million per month by the Department of Minerals and Energy, this figure does not take into account the costs of replacing pumps, an overhead which the mine is expected to carry,” Watters says.

“Once the bulkhead plugs are installed, the mine will stop pumping water altogether, resulting in massive long term cost savings both to the company and the State.”

Construction, by Murray & Roberts Cementation, of the five 22.5 metre long bulkhead plugs on 42 Level at South East Vertical shaft is being undertaken simultaneously to the upgrading of the two bulkhead plugs on 58 Level and the one bulkhead plug on 68 Level at Far East Vertical.

The high pressure water retaining bulkhead plugs have been designed to retain the inflow of water into underground working sections and secure future mining in areas that would otherwise be inaccessible.

Plug installations vary from 21 up to 40 metres in length. The plug on 68 Level, once completed, will be subject to the highest head in the mining sector. This bulkhead plug will withstand a maximum head pressure of 2841 metres (28,41 MPa).

Pete Ferreira, general manager – mining services at Murray & Roberts Cementation, explains that the installation of the bulkhead plugs is highly specialised construction work, and the position, design and quality of plugs is critical.

“In fact, a code of practice stipulates the acceptable area in which water retaining bulkhead plugs can be installed,” Ferreira explains. “According to this code, the site of the work should be situated in an area of homogenous rock. This infers that the ground for a distance of three plug lengths is free of structural weaknesses such as faults, fissures, shales, shists, friable or soft material or other mining excavations.”

Ferreira continues by explaining that this is to ensure safe anchoring of the plug and water tightness. Furthermore, the effect of stress changes as caused by mining should also be carefully assessed.

“The preparation of the proposed plug site is another critical area that is governed by the code of practice,” Ferreira says. “All surface rock, upon or against which the aggregate is to be packed, must be clean, solid, and free from standing water, objectionable coatings, such as oil, grease, loose rock fragments or grit which may impair the proper bonding of the cementitious materials to the rock. The rock formation at the plug site should also be sufficiently rough. It should also be sufficiently rough to ensure a satisfactory bonding with the concrete.”

Once the bulkhead plug site has been prepared, a 500 mm wide, 30 MPa strength concrete retaining wall is erected at the wet side of the bulkhead plug. The bulkhead plug is packed from the footwall up to the hanging wall with quartzite rock plums. This structure also contains a number of positioned intrusion and tightening pipes. The tightening pipes will be required to seal the rock/plug concrete interface once the structure is complete and cured. Pressure rated 316L stainless steel pipes and valves can also be installed in a plug construction for future dewatering requirements.

The plug’s front shutter is erected simultaneously with the construction and pipe installation. After construction mortar is intruded into the constructed plug via the intrusion pipes up to the last high point pipe.

The mortar used for intrusion is mixed locally at an underground plant which is especially erected for the duration of the bulkhead plug construction. The plant comprises an aggregate conveyor, high shear mixer, pumps and storage tanks.

Mortar cube samples are taken for compressive strength testing throughout the mortar placing.

Ferreira explains that water retaining bulkhead plugs must withstand and hold their design static heads and careful calculation of their length is a critical aspect of installation. He adds that bulkhead plugs are constructed in sections of no longer than 8 metres.

“This is a time consuming process and can take up to four months, depending on underground conditions,” Ferreira says. “The bulkhead installation process typically includes clearing of the plug site, construction, mortar intrusion, curing time and tightening (consolidation). The curing process alone takes 28 days.”

After construction and curing, the structure is tightened by means of pre-positioned tightening pipes, ensuring its permeability.

Watters says that installation of the bulkhead plugs is expected to help ERPM continue operations into the future. The exercise is expected to contribute to the cost effective running of the operation.

Murray & Roberts Cementation has installed bulkhead plugs at numerous mining operations across southern Africa. Given its experience and expertise in this field over the past four decades, the company is part of a working group convened by the South Africa National Standards (SANS) to review the current codes for the design and construction of high pressure water retaining bulkhead plugs.