Mining & Mined Caverns - Parsons Brinckerhoff

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Mined Caverns APRIL 2012 http://www.pbworld.com/news/publications.aspx 52 Large hydro-electric projects often comprise mined caverns to house the turbines and generating equipment. The cavern becomes the powerhouse and must be linked to the surface by several tunnels, adits and shafts, depending upon the configuration. Parsons Brinckerhoff is currently involved in a number of hydro-electric projects and has experience in hydropower caverns from the civil engineering, geotechnical engineering and mechanical-electrical engineering perspectives. For the latter, Parsons Brinckerhoff is the owner’s engineer for the 1,332 MW capacity Ingula Pumped Storage project in South Africa, having undertaken tender stage designs for the mechanical-electrical components of the scheme, most of which are housed within twin caverns (see Figure 1). The Parsons Brinckerhoff hydropower team is also engaged as lenders’ technical advisor (LTA) for two pumped storage hydropower schemes in Israel, each with significant amounts of tunnelling and each with a pair of mined caverns housing turbines and generation equipment. The team is also pursuing an LTA position for a medium-sized conventional hydropower scheme in Laos with 10km of tunnels and a large underground cavern complex. Network Mined Caverns for Hydro-electric Projects by Andrew Noble, Sydney, Australia, +61 2 92725427, anoble@pb.com.au Figure 1 - Powerhouse cavern layout for the 1,332 MW Ingula pumped storage project in South Africa The future of cavern engineering for hydropower, both conventional and pumped storage, appears to be solid, as developers need to go further afield to explore the hydropower potential in remote and mountainous areas and, in steep terrain, creating a large cavern is more cost effective than the alternative of stabilising an external slope for the construction of a surface powerhouse. Pumped storage hydropower schemes Pumped storage hydropower is becoming increasingly prevalent in countries where other forms of renewable energy, such as wind power, have expanded. The grid requires stability which is offered by the near instantaneous generation of electrical power when a pumped storage scheme starts up, and can be used to balance the supply when other supply schemes have fluctuating outputs. The concept is to use a closed system of water (excluding evaporation from the reservoirs) to generate electricity by allowing water to flow by gravity from an upper reservoir to a lower reservoir through tunnels, shafts and turbines. The same water is later pumped up from the lower to upper reservoirs through the same facilities. While some schemes use a silo-type powerhouse (basically a deep shaft near the lower end), the majority comprise underground caverns. The selection of suitable rock conditions deep within the mountain is a challenge for the geological team to assess, including a ‘Plan B’ in the event that rock strata are not aligned as expected or unfavourable joint orientations are encountered. A key aim of the designer for pumped storage schemes is to develop as much head (H) over as little horizontal distance (L) as possible, in other words to minimize the L/H ratio. This is principally for economic reasons and means that the tunnel waterways designer needs to understand the economic driver for the optimum layout. However, the greater the operating head usually means the greater the constructability challenges. Deep
shafts are required in such schemes and an obvious optimization is to consider reducing the total waterway length by making a steeply inclined shaft and shortening the lower tunnel length; conceptually this all makes good sense providing that the time and cost and risks are appropriately considered with experienced constructors during the design phase. A vertical raise bore shaft 300m deep is far more straightforward than an inclined 60-degree shaft, although there can be significant cost savings in shortening the overall waterway lengths, especially if the shaft and lower tunnel are to be steel lined, and there are plenty of successful precedents where the steeply inclined shaft was selected. Access adits, services tunnels and cavern complexes All hydropower schemes that contain headrace tunnels, shafts or caverns require access adits. For schemes with a powerhouse cavern, there could be several kilometres of tunnels spiraling down from the surface to cavern level, resulting in a network of adits and tunnels. Some key issues with caverns are: Fire-and-Life-Safety (FLS) This is related to the means of egress in the event of emergency. The transformers are normally housed within a separate and adjacent cavern to the main powerhouse cavern. This is partly because a single span cavern would be very large but more importantly to create a separation for safety issues. A dedicated fire fighting network is necessary to deal with fires and this often requires gravity fed water mains. Given that these power stations usually require permanent staffing, the whole safety issue is a key aspect of initial layout design, requiring close interaction between the designers for rock mechanics, hydraulics, constructability, FLS, and geotechnics. Complex hydraulics of the waterways connections Depending upon the final cavern orientation, the final arrangement of waterway (headrace) tunnel connections may need to be adjusted. The dominant decision in cavern design is often the long axis orientation to suit the encountered geological jointing. Proximity of caverns As mentioned above, normally there are at least two caverns in any major underground powerhouse complex; the main powerhouse cavern and a smaller transformer cavern. These need to be as close as possible to minimise the costs of the high voltage busbars connect- Network ing the generators to the transformers, but that offset must be weighed against cavern stability and costs for support. This is an area where the tunnel designer has freedom to optimise the layout in conjunction with the other disciplines. Confinement and in-situ stresses The majority of caverns for hydropower schemes are located in hard rock and most remain unlined, i.e., without a secondary permanent concrete lining. If the rock is not sufficiently sound to stand with only conventional rock support, then the economics of the cavern option may be in doubt. Also, the complex array of adjacent openings would realistically only be manageable in a rock mass that is basically sound. Confinement, however, is another issue. Although the cavern is not a water filled space, the connecting waterways on the upstream and downstream sides contain high and low pressure water, respectively. Those waterways, tunnels or shafts, will be designed so that the internal water pressure is contained safely within the confines of the mountain and, unless expensive steel linings are used extensively, the location of the cavern is partly dictated by the confinement design. The cavern itself is analysed for the known in-situ stress regime deep in that part of the mountain. Geotechnical investigations for hydropower caverns Most caverns for hydropower are located deep into the mountainside and access to the ground surface directly above the cavern location is often very difficult. In remote locations, the challenges will be the terrain, which is likely to be very steep as the caverns are frequently located where there is a significant height difference, and the dense vegetation cover. It is not uncommon for inclined bore holes to be drilled to reach the planned cavern location, sometimes extending to several hundreds of metres depth. For the pumped storage schemes in Israel, the developer was constrained from accessing many parts of the mountainside above the proposed cavern location due to the presence of a national park. This required the investigation team to make the best use of those areas where access was granted, and this resulted in deep vertical boreholes, up to 780m deep, to recover rock core samples (see Figure 2). This task in itself was a major undertaking, with a 20-foot long container required just to house the core boxes for each hole. The author also has experience of feasibility studies for various projects in Africa where the drill rigs Mined Caverns APRIL 2012 http://www.pbworld.com/news/publications.aspx 53
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Page 9 and 10: Figure 5 - Leverage of Early Work o
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Page 13 and 14: Stage 2: Geological modelling Const
Page 15 and 16: Network The Relevance of Internatio
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Page 21 and 22: Network Reshaping Social Impact Ass
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Page 33 and 34: Figure 2 - RopeCon® cross section
Page 35 and 36: Network Parsons Brinckerhoff Global
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Page 39 and 40: Figure 2 - Slurry strength failure.
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Page 43 and 44: Network Mined Cavern Stability Surv
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Page 63 and 64: With reference to Muir Wood (2009)
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Mining & Mined Caverns - Parsons Brinckerhoff

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