Tunnelling has come a long way since the pioneering days of the 'Hydro Boys*', the drill-and-blast heroes of the Scottish Highlands. Construction of today's water power tunnels uses a sophisticated array of machines where virtually no underground obstacle is insurmountable. Chris Webb reviews some recent developments
It is Iceland’s largest ever industrial development. A 190m high concrete faced rockfill dam and 60km of tunnels feature at the heart of a US$1.3B project for national power company Landsvirkjun in the country’s east. It includes the Kárahnjúkar power station, which is being built to supply energy to US company Alcoa’s new aluminium smelter, to the tune of some 4600GWh/year.
More than 50 years have passed since the idea of harnessing the power of some of east Iceland’s great glacial rivers was first mooted. At last, Kárahnjúkar will make the vision a reality, the project’s network of tunnels utilising a head, or drop, of about 600m, producing 690MW of power from six generating units.
As lead consultant in a joint venture comprising Línuhönnun, Fjarhitun and HNIT, along with European companies coyne-et-bellier, SWECO and Norconsult, Mott MacDonald is overseeing the construction of the major works. The consultant is providing specialist expertise for the 7.6m diameter tunnel, 33km of which is being created using tunnel boring machines (TBMs), the remaining 27km using drill and blast techniques. Together the tunnels represent one of the most significant underground projects currently underway worldwide. The project as a whole is due for completion next year.
As this issue of IWPDC went to press, Italy’s Impregilo, main contractor for the dam and headrace tunnels, was adding to its workforce inside the headrace tunnel to make sure that a crucial deadline at the end of May will be met, when water will start flowing through the tunnel. In all, some 650 workers were toiling in the tunnel adding the finishing touches to this key element of the project. Three large rock traps, each 100m in length and 4m deep, were nearing completion, while the invert of the tunnel was in its final stages and concrete lining work was in progress.
Subterranean civil engineering works are traditionally fraught with uncertainty, as ground conditions – hard or soft, faulted or waterlogged – conspire to test the ingenuity of engineers to the absolute limit. But despite the difficult ground encountered during excavation of the tunnels, progress has been remarkable, in the main.
Only one of an original three Robbins TBMs now remains on site, where it has been reassembled after an earlier drive to excavate an additional 8.7km tunnel that will bring water from the Ufsarlón reservoir to the main headrace tunnel. A further TBM has been dismantled and prepared for transport out of the country, while another has already left the worksite.
It is now almost six months since the third and last TBM at the main headrace tunnel broke through its 11.5km section, on the morning of 5 December 2006. The breakthrough marked the completion of the 39.7km long bore. Ferruccio Borroni, of Robbins Europa, says the TBM performed remarkably well, averaging 520m per month and boring a best month of 1193m in exceptionally hard basalt, moberg and pillow lava geology with a compressive strength of up to 300MPa (44,000 psi).
The performance is all the more remarkable when considering the tough conditions the TBM experienced early on. ‘There were many unexpected problems at the outset,’ Borroni explains. ‘At adit 3, the TBM experienced a dramatic quantity of water inflows, but the problem was overcome because of diligence and good planning from Impregilo and the quality of the TBM and cutters.’ Another TBM had the task of boring its tunnel in sections by negotiating a tight turn with a 700m radius from an access adit to drive half of its leg of the headrace tunnel. The TBM was then pulled back and re-launched from the same adit to bore the other half of the tunnel.
TBMs also performed well elsewhere, finishing their sections of the headrace tunnel in the closing months of 2006. One machine went on to tie a world record for boring 92m in a single day, despite encountering extremely heavy water inflows and major fault lines in the middle of its bore. This broke through at the end of September 2006.
Hydro projects such as Kárahnjúkar highlight the global nature of the highly specialised tunnelling machine manufacturing business, where a relatively small number of mainly Japanese, European and US makers rule the roost. Recent years have also seen a significant increase in activity in developing markets, where water resource infrastructure calls for the deployment of such machines, particularly in India and China.
Just a year ago US-based Robbins and Jaiprakash Associates of India signed what the manufacturer claims was then the biggest TBM order in the industry’s history and for what is billed as the longest single tunnel without intermediate access in the world. The contracts include two Robbins 10m diameter hard rock double shield TBMs and related equipment, as well as all the conveyors, spares, cutters, and key personnel to bore the 43.5km long tunnel through hard rock.
The tunnel will bring irrigation water from a large existing dam and slake the thirst of thousands of acres of one of India’s most arid regions. The project is aimed at improving agriculture and thereby boosting employment in the region. The contract will have a duration of approximately five years.
The Asian sub-continent continues to provide opportunities for the tunnelling elite. Next month sees the deadline for prequalification for the turnkey construction of the 444MW Vishnugad-Pipalkoti hydroelectric project in India. Main civil engineering works include a 65m high gravity concrete dam, a 10m diameter diversion and spill tunnel, 12m diameter spill tunnel, intake structures, various desilting chambers and a 13.4km long 8.8m diameter horseshoe-shaped headrace tunnel. A 130m high, 22m diameter surge shaft, 351m long, 5.2m-diameter pressure shaft and an underground building housing four 111MW generating units are also included, as is an underground transformer cavern to accommodate 13 transformers and associated switchgear.
India has considerable hydro potential, which it continues to fulfill, notably with the 86MW Malana power project. Located on the River Malana in the Parbati basin in Kullu district of Himachal Pradesh, at the time of construction the project was one of the largest of such undertakings in the country. Construction began in 1999 and the project was commissioned in a record period of 30 months, half the time it was expected to take initially, and at a total cost of Rs350 crores (US$76.46M).
The project involved the construction of a dam and head-works structures which include a 3.5km long power tunnel, a surface penstock with a diameter of 2.2m and a surface power house complex. The power house complex includes two vertical 43MW Pelton turbines, each with a designed discharge of 10.5m3, operating under an average net design head of 480m.
From the reservoir, the water is conveyed through a 3.27km long, 3.4m diameter headrace tunnel and a 570m long, 2.2m diameter steel penstock to the surface power house located on the bank of the River Parbati. The water is discharged into the river through a 500m long tailrace channel.
Tunnelling machine manufacturers seeking to find new international markets for their wares are finding that China, especially, offers considerable scope. Germany’s Herrenknecht is a case in point. Traditionally, its core market has been in Europe, which still represents more than 50% of new orders. The Asian region, and China in particular however, have become the second most important sales and growth market of the Herrenknecht Group.
The company says orders are increasing for specially manufactured tunnelling machines with large and extra large diameters, and demand for Herrenknecht’s large-diameter tunnel boring machines from China has been on the increase since 2000. Orders received in 2006 included two 9m diameter mixshields for the underground crossing of China’s second largest river, the 5464km long Yellow river (Huang He) in the north of Zhengzhou. It is one of the world’s largest infrastructure projects.
After decades of planning, the giant water supply project known somewhat unpretentiously as the ‘south-to-north water diversion project’ is taking shape. As part of this unique project, abundant water reserves in the south of the country will be transferred to the north via three giant waterways, each 3000km long. In addition to the construction of dams a number of large tunnels, some several kilometres long, have to be driven below the Yellow river. The project will mean the provision of clean drinking water to 400 million Chinese in the north of the country.
Robbins TBMs also feature on water transfer tunnels in China. One of two machines boring on the Dahuofang Water Transfer project broke through into an intermediate adit at the end of last year. The breakthrough followed that of a sister TBM which reached its intermediate adit in September 2006. The TBM driven portions of tunnel are divided into three contracts. Tunnel excavation began in August 2005 on very long bores, one of 17km and a further tunnel 13.6km in length.
Machines were required to operate in a mixture of geological conditions ranging from migmatite, andesite, syenite and compound granites with UCS values up to 109 MPa (16,000 psi) to variable rock types, often in mixed face combination, of andesite, tufaceous brecciated lava, volcanic breccia, tuff, and tufaceous siltstone mixed with shale, sandstone, and mudstone. Though the types of rock vary widely, the TBMs 1 performed well, one machine achieving a monthly average of 614m, the other 508m per month.
Robbins says two of the TBMs utilised several technologies that contributed significantly to their rapid advance rates. Both use 19-inch back-loading cutters with front-loading optional, rather than 17-inch cutters (the size being used on the TBM 2 Contract). Larger cutter diameters increase cutter life and cutter load capacity, allowing the TBM to bore faster in harder rock.
The project is the first in China to use continuous conveyors for tunnel excavation. Steel cable belt has been used, rather than fabric belt, in order to accommodate the extremely long and straight tunnel paths. Steel cable has a higher strength than fabric, which allows the use of much more powerful conveyor drives. The 900kW drive motors are three times more powerful than typical drive motors and reduce the need for booster drives in the tunnel.
When boring is complete, the Dahuofang Water Transfer project, involving an 85km tunnel, will be one of the most extensive water transfer projects in the world. The tunnel will transfer water from the Huanren to the Dahuofang reservoir to supplement the arid and densely populated Shenyang region of China. Some 40% of tunnel is being excavated by drill and blast, while the remaining 60% is TBM-driven.
Water resources projects
By their very nature water resource projects in developing countries can be found in the most inhospitable – and most inaccessible – of locations. This is certainly true in the landlocked southern African state of Lesotho, where Mott MacDonald was appointed by the country’s Highlands Development Authority, in a joint venture with lahmeyer International, to provide consultancy services for the Muela hydro power station which forms the power generation component of the £1.5B (US$3B) Lesotho Highlands Water project.
The £70M (US$140M) hydro power station is the first electric power generator in the Kingdom of Lesotho, making the country self-sufficient in electricity. Water will reach the underground power station via a 45km headrace tunnel, which is the longest of its kind in the world. The maximum 300m hydrostatic head is used to drive three 24MW machines located in a cavern 60m long by 15m wide by 30m high, excavated in the Clarens sandstone. The water then discharges along a 1.4km tailrace tunnel to the Muela tailpond, formed by a 55m high double curvature arch dam across the Nqoe River.
Appropriately, perhaps, more than 60 years after the North of Scotland Hydro-Electric Board (NoSHEB) began its two-decade-long development plan (1943-65), tunnelling has returned to the Highlands. The Glendoe scheme, being built near Fort Augustus, Invernessshire, by Scottish and Southern Energy (SSE), is the first major conventional hydro power scheme to be constructed in the UK since 1957 and will generate up to 100MW of electricity.
But more on Eliza Jane, the UK£7M Robbins TBM specially refurbished by Herrenknecht, when IWPDC covers the construction of the £140M (US$280M) Glendoe project, near Loch Ness, next month. A 1km long dam and 8km of tunnels makes Glendoe one of the most significant civil engineering projects undertaken in Britain in recent years.
Early on in the project, both TBM 3 experienced heavy water inflows which temporarily slowed advance rates heavy water TBM 3 finished its 11.5km section at Karahnjukar on time despite difficult ground conditions early on ground conditions The outfall portal area for the AMR Project is in the early planning stages AMR Drill and blast is still an option for fast tunnel excavation. Face drilling equipment has grown in sophistication Boomer TBM 3 broke through at Karahnjukar in December 2006, averaging 520m per month TBM 3