Scotland was booming, mostly in water-related schemes, said David Donaldson of Donaldson Associates, opening the meeting. Stirling was to be the scene for major investment by East of Scotland Water and these were described by the first speaker, Steven Boys. The £4.9M to be spent on the Forthside contract had first to meet future development needs at the new £80M Forthside retail development park. The contract had also to solve existing drainage problems of the Stirling wastewater treatment works pumping station, deal with Forthside overflow in dry conditions and cope with sewer problems. These included badly silted-up sewers, infiltration from the River Forth and structural defects in Forthside sewers.
Various simulations were run on the hydraulic model and it was concluded that an optimised sewer of 1.5m diameter at a gradient of 1:500 was required. Working with the council it was decided there would be three phases to the work, with the first tailored to the retail park.
A borehole survey identified past-glacial marine and estuarine alluvial deposits comprising fine silts, sands and organic silts and clays. Beds of peat were sometimes present together with a variety of cobbles and boulders. Water was generally found 9m below existing ground level and it was established that the tunnel would be through sandy gravelly clays with boulders. With the potential for a high water table, flotation of the shafts and ground stability of the shaft bases were the main design challenges.
An orginal 35m diameter, 18m pumping station was changed to 20m diameter and 22m deep after hydraulic modelling. It was formed with diaphragm walling extended deep enough to provide sufficient negative buoyancy. The other five intermediate shafts were all 6.5m in diameter and about 17m deep constructed by normal segmental lining techniques.
Stirling Council had not finalised its land deal with the MoD by the time Phase 1 was ready to go out to tender so as a precaution work was sub-divided into two packages. If necessary 1b could be held back if the land purchase agreements were delayed.
John Mowlem won the first $4.9M contract and its tunnelling tunnelling arm, Johnston Construction selected a Herrenknecht AVN 1600, full face slurry machine with a Bauer desanding plant. A soft ground cutter head used replaceable tungsten carbide bolted inserts and there was a stone crusher. Two hydraulic rams gave 1,000t of thrust.
With one shift working, an average production rate 13.1m per day was achieved on the first drive of 249m. On the second 183m drive with double shifting, the average production rose to 22.9m per day and a third 177m drive was completed at 19.6m per day with double shifting. Reverting to single shift, a fourth 140m was completed in 10 days at a rate of 14m per day despite a mainly clay face which was found to be difficult to slurry and caused delays at the de-sanding plant. Fifth and final drive at 346m was the longest and was completed in 18 days at just less than 20m per day.
Ground conditions were as investigated. The TBM coped best with mixed materials of gravel and clay and dealt with boulders up to 600mm. On only two occasions was it necessary to enter the face to remove wedged cobbles.
Stirling Council agreed land use for storage and drying of spoil slurry. This worked well during the summer months, although on occasions PFA was added prior to removal off-site. For finer slurry, temporary holding lagoons stored it until removal by tanker.
Remarkable progress was made on the 20m diameter pumping station. Ten diaphragm wall panels needed for its 22m depth were constructed in only three weeks with excavation taking another three weeks and the base slab a further two.
With one reportable accident and all tunnels meeting the ±20mm tolerance in levels, Phase 1a was completed on a revised programme and Phase 1b completed 10 weeks ahead of programme. Boys said the project was within budget and he was happy to report a best value for money solution.
Managing lined and unlined rock tunnels
A decade of tunnel management for Scottish & Southern Energy was next described by Neil Sandilands. His company owns and operates 66 conventional hydropower stations with an installed capacity of 1,100MW, plus the 300MW Foyers pumped storage scheme.
Among the schemes there are a total of 300km of tunnels, both concrete lined pressure tunnels and largely unlined, free flow aqueduct tunnels. The largest tunnel is the Upper Errochty at 13km. There are also 65 power tunnels, of which Clunie is the largest at 6.86m diameter and Deanie the longest at 9km.
Though generally less of a risk to public safety than dams and reservoirs, tunnels constitute a major business risk and have the capacity to cause major environmental damage.
Over the last 10 years, Scottish & Southern has adopted a risk based approach, which means predicting future performance and serviceability risk by identifying potential failure modes and putting in measures to manage them. Though still being developed the underlying principles are now widely applied.
Getting down to project details, Sandilands said that on 20 September 1990 the unthinkable happened in the North Lochay tunnel. A 2,000t fall completely blocked the tunnel at a fault that had only been partially supported during construction. The upstream catchment was cut off for power generation.
The rocks of the Ben Lawers Group, which are highly metamorphosed, are intensely crumpled and contorted into sharp steep folds. The tunnel supports were pre-cast concrete arch ribs at 1.5m centres with 50mm deep bank bars at irregular centres. There was rock packing behind the bank bars.
For repair a steel shield was fabricated from steel arch ribs at 600mm centres, tie-rods and bank bars. Steel plate, 12mm thick was welded over the arches. Inside an Eimco excavator loaded tracked dumpers as the shield was jacked forwards.
About 200m3 of concrete and 250m3 of grout were placed in the cavity to give about 10m cover to the tunnel. It was not certain to what height the cavity extended and it was never completely filled.
Unlined tunnels pose greater risks than lined tunnels and a four year programme of repair and improvements to unlined rock tunnels was completed in 1998.
Sandilands pointed out that although major repairs were undertaken in the Lochay Tunnels in 1991, many of then sound concrete arches had deteriorated significantly by 1998. This had added to the instability of a major junction where turbulence was a severe problem and a further remedial contract was needed in 1999. A total of 77t of concrete was used and a 75mm thick layer of gunite added to the arches to relieve the stresses and restrain the rock packing.
Repairs now taking place are aimed at preventing new rockfalls.
Concrete lined pressure tunnels were generally in good condition with only a few exceptions, said Sandilands. Regular patching of small holes had been carried out. But inadequate back grouting had caused some major problems in the Deanie and Culligram Tunnels. New concrete lining will be installed in 2001 in one tunnel at Nant following a risk assessment by Donaldson Associates of a 130m long crack in the crown which had been there almost since construction.
But last year saw two very sudden failures. At Foyers, on 11 December, debris was reported passing through a turbine. After emergency drawdown it was clear that a large section of invert slab had lifted in a single event.
A quick response was needed and repairs were initiated and completed by the term contractor within 10 days, with refilling started in time for Christmas. Sandilands said prompt action saved £1M for replacement generation at this busy time.
Failure of a bifurcation plate in the Luichart penstock led to installation of a new concrete surround to the existing pipeline and new concrete walls.
Despite focusing on problems, Sandilands said performance had been good in the 45 years or so since construction of the tunnel system. This was a major tribute to designers and contractors in British tunnelling industry.
Lochay aqueduct repair
Andy Sloan of Donaldson Associates described a £1.2M remedial job for Lochay aqueduct tunnels, part of Lochay hydroelectric scheme. Repair was carried out on both sections north and south of Glen Lochay. Donaldson was designer and the contractor was Ritchies, a specialist sub-division of Edmund Nuttall.
Dating back to 1959 the unlined tunnel system runs through schist and varying lithology. Faults and crush zones up to 1m thick cut across at various angles and some collapses occurred during construction.
Small sections of in-situ concrete lining were installed during construction but more commonly support was reinforced concrete or steel arches and bank bars. In general southern tunnels have concrete inverts but the northern tunnels have unlined inverts.
Several stability problems had been identified:
A repair list was prioritised and selected by type and location to reduce operating risk to Scottish & Southern Energy. The implementation of this system of working required close liaison and frank discussion between client, designer and contractor which related directly to the real value of engineering risk. It was additionally complicated because the full extent of the works could not be defined until after contract award when drawdown allowed access.
Work began with scaling and reinverting 4km of tunnel to provide good access. Loose blocks, caused by high in-situ stresses in the rock mass, were stabilised with untensioned stainless steel rock dowels.
Sprayed concrete was applied for protective coating to shear and fault zones, to areas of erosion where high levels of turbulence existed and to repair existing supports.
The North Lochay Junction at the confluence of the north and south tunnel systems was of critical importance and erosion control was provided by rock dowels and sprayed concrete.
Sloan concluded by saying that close liaison between client, designer and contractor working under the NEC conditions of contract led to the successful completion of this very demanding project to the satisfaction of all parties. A significant improvement in tunnels was achieved for a small sum.
Ayr sewerage scheme
Neil Hayes, of Byzak began by wryly pointing out that sewer tunnels were generally just “holes in the ground soon to be forgotten” unlike high profile contracts such as the Elbe tunnel and London Underground‘s Jubilee Line extension. Interceptor sewer phase 2 at Ayr, on the west coast was no exception.
Ayr’s outfalls discharge into the adjacent Firth of Clyde. A new 1,150m long, 4.4m id tank sewer at the end of a 4.5km interceptor was to pick up four of these overflows until they could be pumped 15km north to Irvine for treatment. Some shafts had been deleted by the contractor as an alternative proposal.
Ground comprised coal measures with a very high water table. Sands and gravels sat just above the tunnel and there was very low cover to the river. Siltstones and mudstones were also present and some sections of tunnel contained Dolerite, which a local described as “pretty strong” – they used it to make curling stones.
Byzak had considerable expertise with Herrenknecht tunnelling machines and found it the natural choice, particularly when a mix shield of the right size was available for the work. This was shipped directly to Ayr docks from Germany.
The treatment plant was designed to cope with 1,000m3 of spoil per hour and had two sets of vibrating screens, cyclones and four centrifuges. Site electric power came from three, 1,250kVA generators, auto-synchronised with load sharing facilities to provide the 1,700kVA for the machine and 500kVA for the other site equipment.
The segmental lining was a modified Jubilee Line ring. It had five segments and a key with a 24mm taper to cope with the curves. There were spear bolts but instead of grout holes, there were lifting sockets in the face of the segments. A tailskin grouting system proved very reliable when two pumps are used. This system provided the required 2.5m3 of PFA, cement and bentonite grout per ring with no undue blockages reported during the tunnelling.
Production rates achieved on the contract of a best weekly total of 74.4m, a best 24-hour advance of 22.8m and an overall average advance of 35m per week. The best production of all was in Dolerite.
