News & Media

6 March 2010

GENERATING ADDITIONAL POWER WITH BRAMHOEK AND BEDFORD DAMS

Almost 20 months after moving onto site, lead contractor Concor Roads & Earthworks is on schedule with the contract to construct the Ingula Pumped Storage Scheme’s Bramhoek and Bedford dams, straddling the provincial boundary of the Free State near Harrismith and KwaZulu-Natal near Ladysmith. The dams are 4.6 km apart and are connected by underground waterways through an underground powerhouse which will house four 333-megawatt pump turbines.

The contract, which is being undertaken by the Braamhoek Dam Joint Venture (BDJV), comprising WBHO, Concor Roads & Earthworks, Edwin Construction and Silver Rock, is planned for total completion in May 2011.

Bedford Dam is located in the eastern Free State at an elevation of 1 710 metres above sea level while the Bramhoek Dam is in KwaZulu-Natal at an altitude of 1 300 metres above sea level.

When completed, a total concrete volume of 90 000 m3, which includes roller compacted concrete (RCC) and conventional concrete, will have been poured.

 

Bramhoek Dam
The Bramhoek Dam is a 337 metre long roller compacted concrete dam and at its deepest point the dam wall reaches 37.2 metres to foundation level, allowing for a water depth of 31.1 metres, a capacity at full supply level of 26.3 mil m3 and a surface area at 240 hectares.

This dam is being constructed by a workforce of 350 (excluding 50 management and administrative staff), using a combination of the JV’s own as well as hired-in equipment. Included in the contract were the bulk excavations and the construction of an intake tower with attached outlet works.

It was necessary to begin with the intake tower outlet works’ 162 064 m3 bulk excavation, including soft excavation as well as hard excavation through drill and blast operations. This was because the construction of the intake tower outlet works was on the critical path of the project, with this structure leading to the balance of the dam wall where the RCC works were to be constructed.

The intake tower with outlet works consists of 12 000 m3 of conventional structural and mass concrete and two flood release outlets at the bottom, each with a diameter of 2.8 metres, as well as a 1 metre outlet for environmental releases.

“The upstream section was founded on good dolorite, however the downstream section was founded on a baked mudstone, which necessitated an additional 6 metres of excavations, making this foundation a total depth of approximately 7 metres,” Sarel van der Walt, site agent for the BDJV, says.

 

Because the tower incorporates hydro mechanical components and internal structures, it required a high structural integrity, which was provided by 350 tons of reinforced steel and conventional concrete. When completed, the structure will be 32 metres high with a 20 metre by 34 metre footprint.

“The hydro mechanical items, especially the large outlet pipes, were challenging to install and keep in position while casting the concrete because of the uplift forces of the concrete,” van der Walt says. “This was overcome by securing the components in place using a combination of nylon and steel straps to ensure no movement would take place during the concrete placement.”

Van der Walt points out that this was just one of the challenges faced by the JV. The lifting and rigging of the two 2.8 metre diameter butterfly valves, each weighing approximately 16 tons, required the use of a 100 ton mobile crane used in combination with a specialised rigging team. “We managed to accomplish this task over a three day period. These valves are installed in the flood release pipes in the lower section of the intake tower outlet works and are apparently the largest such valves ever to be manufactured in South Africa.”

The project was characterised by large volumes of concrete poured for each lift, reaching record levels of 1 000 m3 for the biggest lift, using two 36 metre boom placing pumps.

“Another challenge we faced was keeping the concrete placing temperature below the specified 23˚C. This was done by pouring at night and by cooling the aggregates using two chiller plants to cool the water down to 4˚C and then spraying it onto the aggregates and using it as batching water,” van der Walt explains.

 

The first tower pour was done in March 2009 and the final pour was competed in October 2009. “We had to do the pours on a very tight programme which required careful planning. We achieved this through instituting double shifts during the initial periods of tower construction and by using a formwork climbing system whereby the formwork was gang formed into 2.5 metre sections to facilitate shorter formwork erection time,” van der Walt says.

“A 10 ton capacity portal crane will be installed at the top of the intake tower to operate the maintenance gate and fine screens and a 16 ton gantry crane will be installed in the outlet house for maintenance purposes on the both the flood release valves and the butterfly valves,” van der Walt adds.

The main dam wall consists of 77 000 m3 of roller compacted concrete with the right bank wall having a length of 180 metres and the left one being 140 metres long.

The grout enriched roller compacted concrete (GE-RCC) method is being used on the upstream and downstream faces to increase the impermeability of the dam wall. Gaining fast renown as a construction method, GE-RCC replaces normal skin concrete and offers a number of advantages. “The method increases productivity on site because it allows the use of only one batch plant as opposed to the two batch plants used for a normal skin concrete face,” van der Walt says.

“The overall construction period is shortened and the bond strength between layers increases because the delay between placing subsequent layers is reduced. This will increase the shear and tensile capacity of the dam,” he adds. “The GE-RCC has a far higher bearing capacity when wet than the high slump conventional concrete used in this application, which enables far more effective use of compaction plant.” Production of conventional concrete is minimised and the size of the construction crew is reduced.  

 

With GE-RCC, cementitious grout is poured over about a 0.4 metre strip of the RCC lift surface along the vertical formwork, using a grout pump. Once the grout soaks into the RCC, the mixture can be successfully consolidated with internal vibrators to form a homogenous, impervious RCC facing.

“The lower section of the right bank had poor ground conditions which meant that the concrete founding level had to be approximately 12 metres deeper than originally anticipated,” van der Walt says.

The dam has a non overspill crest which is 5 metres wide and will be approximately 6.5 metres high. This will also act as a road to the portal crane on the intake tower. On the left bank there is a 40 metre wide Ogee spillway with a design capacity of 170 m3 per second. “This spillway is designed to accommodate flooding, should this occur, and at the bottom of the spillway there is a concrete stilling basin which will dissipate the energy in the event of a flood, to prevent downstream scouring of the river bed,” van der Walt adds.

The dam’s 2.5 metre wide, 3.3 metre high and 180 metre long gallery in effect cuts this section of the dam wall in half and affects the productivity levels for the placement of the RCC. “We purchased a truck mounted conveyor belt to assist with the placement of the RCC in this confined area to mitigate this,” van der Walt says.
 
“There were some steep sections in the gallery with gradients up to 1 in 3.5 and it is not considered best practice to place RCC at such a steep angle so, in yet another challenge, it was decided to use conventional concrete to form the gallery in these sections,” van der Walt explains.

Bedford Dam
The Bedford Dam site is five times larger than the Bramhoek Dam site and has a workforce and office staff complement of 674 people, almost double that of Bramhoek Dam’s.

Bedford Dam is a 49 metre high, 577 metre long concrete faced rock filled dam incorporating an 80 metre spillway. Requiring 982 000 m3 of rock fill and 27 000 m3 of conventional concrete, the dam wall will reach 50.9 metres from the lowest founding level to the top of the parapet wall.

The capacity of the dam at full supply level is 22.43 million cubic metres, covering 255 hectares and it will have a 42 metre high intake tower with a 1 metre diameter reinforced concrete outlet conduit and a 15 x 45 metre energy dissipating basin.

Neels Du Buisson, production manager, says that the intake tower consists of a circular structure with two square sections to house the two intake pipes and the inlet to the conduit. Purpose made gang formwork was used as climbing formwork for the 2 metre lifts. The intake tower is well advanced and only four of the 19 lifts are outstanding.

“The left retaining wall is completed and we were fortunate that the foundation for the plinth did not deviate too much from the anticipated level, except for the last section of the right bank were we had to go 11 metres deeper than anticipated. Due to the founding levels the plinth alignment was changed to suit accordingly and the hole of the plinth is completed with the exception of the last section to the right bank,” Neels Du Buisson adds.

“The placing of the upstream filters and rock fill are currently in progress. The kerbs between the filter and concrete face slab were done with a mechanical kerb extruding machine and we were able to complete 400 metres on a double shift,” Du Buisson says.

Careful management of the quarry on site is required to ensure constant supply of the different grades of rock fill for the dam wall placing. “The geology of the quarry is intricate as mudrock and sandstone occur in layers. Mudrock may not be used and contamination needs special attention,” Du Buisson explains.

“Despite all the challenges we have faced on the dam to date, the project is proceeding steadily, any lost time has been recovered and we still intend on completing it on time” Du Buisson affirms.

 

Environmental issues
The Braamhoek Dams Project represents a partnership that balances development with environmental guardianship. The Braamhoek Dams Partnership (comprising Eskom, BirdLife South Africa and Middelpunt Wetland Trust) has its origins in the pump storage scheme named after a local farm. This led to the purchase of a number of farms in the area which will make up a nature reserve in the area surrounding the two dams. Known as the Bedford Wetlands Nature Reserve, this area consists of 8 500 ha of former agricultural land, scarps, rocky outcrops and wetlands.

The need for Eskom to take great care in the manner in which the dams are constructed and to prevent environmental damage has created an opportunity to better protect the Bedford and Chatsworth marshes.

Initially 80 potential sites were identified for this Eskom hydro driven scheme. “Of these, six were selected as both geologically correct from a ground perspective and environmentally correct from a climatic perspective,” van der Walt says.

“From an environmental perspective this was not considered an ideal site but other geotechnical factors weighed in favour of this site and the ensuing environmental impact was ultimately going to be managed through strict environmental compliance as per the approved Environmental Management Plan and recommendations through the Environmental  Authorisation,” Mark Gentle, environmental manager, says.

“There is no doubt that the environment will be irrevocably changed and we know that the affected environment will eventually adapt in years to come, but we still took all the necessary aspects and impacts into account as well as the specific conditions from the record of decision which gave us permission to proceed with the project subject to specific recommendations,” Gentle clarifies.

The Bedford Dam is being constructed in an existing wetland and is also the upper catchment area for the Wilge River so maintaining the water quality is of paramount importance. “Water was tested to establish a baseline and daily testing is done to ensure that water quality remains within this envelope and that the wetland is not impacted as a result of our activities. Bio, micro and chemical water analysis is carried out on a monthly basis by a SANAS accredited laboratory which adds much value in keeping up the quality of the works in progress and ensuring that the construction works do not impact negatively upon the wetland,” Gentle adds.

“It was necessary to deviate the water through the site without affecting the water quality to the downstream wetland. A diversion channel was constructed between the outlet conduit situated in the left bank of the river and the riverbed consisting of a 5-metre saturated peat layer,” Gentle explains.

“Once this channel was completed a pre-cofferdam was placed across the river to deviate the water through the channel. As rockfill was used for this cofferdam the contamination of the water with sedimentation was limited. At the downstream side of this pre cofferdam the main cofferdam was constructed to commence with the peat excavation,” he adds.

The peat excavation was on the critical path and proved to be a major challenge due to the high water table. It was eventually decided to drill four dewatering holes to draw down the water table. All of this unpolluted water was returned to the downstream wetland. “In the end, the best solution was to utilise a 3.5 m3 FEL in conjunction with ADTs with tailgates to remove the peat that eventually became sludge. “Considerable time was lost on this operation,” Gentle says. 

“Once this operation was completed to the mudrock foundation some significant palaeontology findings were discovered and fondly called ‘Jurassic Park’. While these findings caused some further delays, we acknowledge that this is a vital part of our history and 350 000 m3 of rockfill has already been placed since then,” Gentle says.

The excavation of the intake structure, conduit and outlet works was done concurrently with the peat excavation as the conduit has to be completed to enable the rock fill placing to continue. The conduit was a challenging aspect in terms of form work as it starts off as a rectangular section at the inlet tower, changing to a circular shape diverting into two horseshoe conduits and ending as a rectangular section at the radial gates. Specialist sub-contractors were used to design and manufacture the formwork and the conduit is now finished and covered with the rock fill.

“It’s obvious that we cannot construct without water as it is a critical element within the construction programme. It is used for concrete, dust suppression, curing and green cutting, sluicing and for grouting, so water treatment systems have been put in place using an inline dosing system which is used to bring the pH of the water down to normal levels before it is released back into the river system,” Gentle explains.
 
Water is abstracted from the river systems and, in order to work in the basin where the excavations are below the original peat level, the JV has installed large pumps to dewater the area and this water is then released back into the wetland.
 
“All water used on the site is recycled either for further reuse in processes on site such as dust suppression, or it is further treated and tested before being released back into the environment,” Gentle says.
 
“We have identified and mitigated against a number of potential risk areas such as oil or diesel contaminating water.  This is a high risk problem and includes minor spills from, for example, hydraulic pipes bursting or diesel spilling during filling, or even minor collisions between equipment or plant, together with the oil and grease used in the workshop,” Gentle says.

“We react immediately on site and the contaminated soil is removed to a bio remedial area where it is treated with enzymes and other active ingredients to break down the hydro carbon in the soil. We then test the soil after a ninety day cycle and then when we are satisfied with its quality we release it back into the environment,” Gentle explains. “It is commonly known that one litre of oil or diesel can contaminate one million litres of water so we are extremely cautious and we use oil spill kits on site.” 

Refuse is another major challenge which is being overcome through interventions and training.  “We put all of the labour force through an induction course and we create ongoing environmental awareness through regular toolbox talks,” Gentle says.

“Due to the sheer size of our labour force we tend to generate a lot of litter on site. In an average month both Bramhoek and Bedford Dam sites generate approximately
250 m3 of general waste, not including hazardous waste or scrap metal. Bramhoek site generates about 90 m3 of general waste and at the compound we generate 120 m3 of general waste, so a waste management plan is in place for both sites as well both compounds and entails refuse removal by a sub-contractor in accordance with best practice,” Gentle says.

Gentle points out that there is always a risk of fire so a fire regime programme is in place and fire breaks are burnt both by the JV and the locals around the sites to safeguard the sites and compounds.

“At Bramhoek Dam the environmental water quality specification is lower because the area is not deemed as ecologically sensitive; however the protection of the Bedford Wetland, which had special water quality specifications, was a specific condition of the contract and the project has, in fact, had a positive impact as the EIA highlighted the degradation of the wetland and led to the institution of protection measures for the area, including restoration and erosion control,” Gentle says.

Safety
Safety remains a high level priority on both sites with induction training providing the necessary introduction to acceptable OHS practice on site.

On the Bramhoek site a safety manager and four safety officers are responsible for the project and their active interaction with and education of the workforce has resulted in a DIFR (disabling injury frequency rate) of 0.34.  “Daily safe task instruction encapsulates a behavioural-based programme which entails selecting high risk tasks and performing planned task observation (PTO),” van der Walt says.

On the Bedford site a safety manager and four safety officers are available on the day shift and one safety officer is allocated to the night shift when hauling operations are being undertaken. This site also has a low DFIR with 1 116 000 injury free shifts achieved since May 2009. “A hands-on approach to safety, combined with an excellent team of supervisors and foremen and the commitment of our people on site, have been the contributing factors,” van der Walt concludes.