The main challenges of tunnel excavation have passed at Ingula pumped storage project in South Africa. The underground construction effort has switched to final lining. Concreting works are also underway for the powerhouse cavern foundations to hold four massive pump-turbine units.
"Tunnel and cavern excavation is virtually complete, with some excavations still ongoing," says John Sawyer, project engineering manager for the developer, South African energy utility Eskom. "Concrete and steel lining of the headrace and tailrace tunnels has now started, but is at an early stage."
The 1,332MW scheme will use two reservoirs. While the dams for both are complete, and the lower one – Bramhoek – is already full, the upper pond intake works at the top of the headrace still need to be completed before filling can be finished.
But the main challenge was the geology for the underground works. The geology comprises relatively weak sedimentary rock with some faults and fractures in the complex powerhouse area, long-term creep behaviour and the relatively low insitu stress at depth for the twin power tunnels.
Sawyer says the current schedule is for the generation units in the powerhouse, almost 400m below ground, to be commissioned at intervals throughout 2014. The Ingula scheme is to be completed in early 2015.
Contractor for the underground works on the scheme is a JV of Italian firms CMC and Impregilo with local company Mavundla. The design JV comprises Arcus Gibb, SSI (part of DHV), and Knight Piesold, operating as Braamhoek Consultant JV.
Project features
The Ingula pumped storage scheme has been designed to work under a rated head of 441m between two reservoirs, the upper Bedford and lower Bramhoek, both new impounded water bodes and positioned approximately 6km apart. The hydraulic system will hold 21GWh, or almost 16 hours of energy generation, although it would take about 20 hours of pumping to charge the system.
Driving the flows back-and-forth in the scheme, typically over a weekly cycle, will be the powerhouse which is being constructed underground, below under a prominent mountain ridge that features complex local geology between the reservoirs. The powerhouse will hold four large (333MW) Francis pump-turbines.
Despite the large reservoirs and dam construction on the scheme, the project could be viewed as being dominated by its significant and complex underground works – there are more than 40 tunnels and shafts. The main underground structures are the headrace, penstock, draft tube, tailrace and access tunnels; surge shafts and chambers; and, powerhouse and transformer caverns.
Running from Bedford reservoir are a pair of approximately 2.1km long twin headrace tunnels with surge shafts where they break gradient, mid-route, to run more steeply down to the powerhouse.
Each tunnel splits in two to take a feed to one of two adjacent pump-turbines in the 186m long machine hall, which has a span of 26m and is generally 49m high, and 55m at the deepest points. Adjacent, and sitting parallel, is the 176m long by 18m wide transformer hall. The 2.3km long, 9.4m i.d. concrete-lined tailrace tunnel, which features two 109m high surge chambers, takes the discharge from the pump-turbines to Bramhoek reservoir. It also is the initial conduit for flows being pumped back up, via the powerhouse and headrace tunnels, to the upper reservoir.
Ingula’s two dams are themselves major structures – the 38m high by 310m long roller compacted concrete (RCC) dam at Bramhoek, which required 10m deeper foundations in some areas due to heavily jointed dolerite, and has a 40m wide spillway to pass flows of 715m3/s; and, the 51m high by 810m long concrete faced rockfill dam (CFRD) at Bedford, which has a spillway to discharge 200m3/s.
The 11.5km2 catchment is small in relation to the size of the dam for the site, and runoff is low compared to live storage, but the spillway has been sized big enough to handle problems with the complex should the reservoir be already full. Generally, therefore, all normal floods will be discharged through the outlet works at flows of up to 50m3/s.
"Both dams are complete," says Sawyer. The upper reservoir "is waiting for completion of the headrace tunnel intake works before this area can be flooded."
Then, final filling of the reservoir will be done by pumping water up through the tunnel system from the Bramhoek pond – already fully impounded – during the commissioning phase, in 2014.
Geology
The area comprises horizontally bedded mudrocks, siltstones and sandstones of the Ecca and Beaufort Groups. The sedimentary rocks have been intruded by dolerites of the Karoo Dolerite Suite.
Powerhouse complex
The powerhouse caverns are located in Volksrust Formation mudrocks, overlaid by about 25m of Normandien Formation siltstones and sandstones of the lower Beaufort Group, and shale, then there is a 40m thick dolerite sill.
The thermal effects of the igneous intrusion altered the mudrock into the massive shale with relatively good uniaxial compressive strengths. Rock farther – deeper – from the sill was, therefore, relatively poor to fair.
Excavation of an exploratory tunnel to the area of the powerhouse complex was completed in early 2007 by Murray & Roberts (Concor). More than 60km of access roads were also built during the Stage One works.
While the undisturbed rock mass at powerhouse level is relatively unjointed, above the machine hall crown were a number of bedding parallel shear zones, identified during site investigation by boreholes and underground excavations in the area.
The infill to the planar contacts comprisied thin calcite stringers in a soft, brecciated, mylonitic matrix. The shears are offset where intersected by faulting. In the access tunnel excavations the intersected faults compised a zone of slickensided and striated joints, infilled with calcite and/or mylonitic, healed amorphous caclite. A few of the fault planes intersected the east end of the transformer cavern.
There is also a sub-vertical, sheared and faulted dolerite dyke that obliquely intersects the powerhouse area and main access tunnel.
Bedding and jointing was shown by modelling to have an overriding influence on stability and deformation of cavern excavations, requiring detailed investigations into strength, stiffness and orientation of the discontinuities to assist the detailed design.
Long-term creep tests were carried out on rock in uniaxial compression under a range of humidity and temperature conditions. The results confirmed that creep would occur in mudrocks due to changes in insitu stress, but the primary and secondary phases would be over relatively quickly. The results were later corroborated by plate bearing tests and monitoring data during tunnelling.
No groundwater issues were anticipated for the stability of the Ingula powerhouse cavern excavations.
Headrace and tailrace tunnels
In terms of engineering design of the underground works, one of the aspects of tunnel design at Ingula was insitu rock stress being less than water pressure in the headrace. Insitu stress measurements were taken by various techniques, including hydraulic jacking/ hydrofracturing, fracture opening and shut-in pressures. The specialist site investigations were conducted in 2006 and 2008 by Golder Associates and MeSy, and confirmed relatively low horizontal in situ stresses.
The rock stress conditions call for the steel lining to be used along the headrace as far up as the surge shafts. However, the sloping alignment helps to reduce the length of tunnel exposed to the overpressure and, hence, the tonnage of steel needed.
At Ingula, the steel-lined length of each of the two headrace tunnels approximately 1,060m long at 5.1m i.d., and the remainder, above, is 6.6m i.d. with SCL.
However, tunnelling on a slope is more challenging in comparison to the likes of the nearby Drakensberg pumped storage scheme, which was constructed by Eskom in the 1970s.
At the Drakensberg site, the insitu rock stresses are high enough to confine the high pressure from the water in the headrace, enabling most of the tunnel to be lined with concrete. Also, with the alignment having long vertical shafts and horizontal tunnels, the tunnelling work was relatively more simple.
Design and construction
The main civil package and main generation plant contracts were awarded in 2008. Early works, including advance infrastructure, more access roads and access tunnels – including a spiral extension of the exploratory tunnel to the operating floor level of the powerhouse – were completed between 2008 and 2009. Most tunnels were of horseshoe-shaped cross sections.
At a gradient of 1:10 and opened by drill and blast, the main access tunnel is up to 9.5m wide, and tunnel junctions have 10 to 14m spans. Five classes of support were used in mudrocks, three in dolerite, and comprised steel-fibre reinforced shotcrtete and 4m long rockbolts, typically, plus arches at the portal.
To check for delayed convergence during excavation, a number of points along the 5m wide spiral access tunnel, extended down to the powerhouse level, showed it took three to four months for primary and secondary creep to end at the crown. The data was in line with results from the plate bearing creep tests. Tests of intact cores had indicated much shorter periods, but they did not have the larger rockmass weaknesses of bedding and joints. The creep is relatively insignificant long term, after the first four months, but suspended floors on corbels were chosen for the machine hall to avoid load transfer. Also, the crane beams will rest on columns and not be held by ground anchors.
A curved profile was selected for the powerhouse cavern roofs – double-radius arches, like the project’s surge chambers which were of the same large span as the machine hall. The arch was chosen over a trapezoidal roof as modelling showed little difference in performance, but the latter induces bending moments in shotcrete, which would affect the timing and sequencing of construction work while convergence is awaited.
Modelling established the best plan for cavern excavations as a central heading with simultaneous side headings for the crown to be followed with a series of benches. But with delays to stabilising displacements in the adjacent spiral access tunnel, and the main access tunnel, the central headings were pushed ahead alone in the machine and transformer caverns, helping to confirm ground conditions.
Excavation support was provided by grids of primary rockbolts and cable bolts, and then secondary cable anchors further back from the face and with twice the load capacity at 60 tonnes. The cables were later grouted after some elastic convergence of the rockmass, and yielding of the primary support accepted with secondary support in place. Fibreglass dowels were installed in the floor and walls of the central headings to reduce heave and minimise effects on roof arch stability.
With permanent ground anchorages placed closer to the face than originally designed, convergence of the machine hall roof was less in the central heading, though in the side headings were in line with predictions.
In late 2009, an area of over break extended more than 2m as blasting progressed over a few days, and cracked some shotcrete areas too. Extra support measures included: full column grouting of installed rockbolts; installation and grouting of cable bolts up to the face; steel fibre reinforced shotcrete plus mesh; installation of cable anchors near to the face of side heading; and grouting of voids behind the cracked shotcrete.
Overbreak was generally to do with geology and not blasting.
The machine hall also has a central loading bay, added as buttress support the high rock walls in the cavern and minimise displacement between the turbine pits.
Distances between the large cavern, chambers and anchor galleries were large enough to prevent interaction and stress interference in the rockmass pillars seperating them. The distance between the machine and transformer caverns was increased from 35 to 40m to achieve this. But due to the many smaller tunnels and shafts in the layout around the large structures, extra strengthening works were undertaken around junctions, which saw steel arches placed at close centres.
The four draft tubes were raise bored as there were tunnels below for access and collection of spoil. The excavation commenced with 2m diameter pilot bores downwards, at 60 degrees inclines. Then the raised bores expanded the diameters to 6m as the debris fell down the pilot tubes to the tunnel below.
Raised bore excavation was also used for construction of the smoke extraction and ventilation shafts.
Six classes (A-F) of support were used for the tunnels and inclined shafts. The headrace was excavated uphill by drill and blast at a ‘challenging’ one in 2.2 slope.
As project construction got underway it was anticipated that the scheme could be finished in early 2013. By early 2011, in a presention to the BTS, the project parties were viewing the completion schedule as 2014. The latest view is a finish a few months later, early in the following year.
Final stages
Unusually for either a hydro or pumped storage project, much of the waters were impounded way before the tunnelling was even nearly finished, though not by design. Three years had been allowed for the Bramhoek reservoir to fill but, due to hydrological surprises, it was achieved in barely a few months.
Tunnel and cavern excavation is now virtually complete, with some shaft excavations still ongoing, says Sawyer. Construction works and plant installation in the machine hall will continue into 2013.
Concrete works and installation work have started at the draft tubes. He adds that the main cranes (two x 265 tonne) have been installed and are being commissioned, to be ready for installation of the generation plant.
