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The Project Control Framework
This framework sets out how we, together with the Department for Transport, manage and deliver major road improvement projects.
A417 Cowley to Brockworth Bypass Improvement
8 ECONOMIC COMPARISON OF OPTIONS
8.1 Economic appraisals of Options 1, 2 and 3 have been undertaken to evaluate the comparative economic value of constructing the different layouts taken forward for assessment.
8.2 The appraisals have been undertaken for the standard 30 year assessment period, using the cost-benefit analysis computer program COBA in accordance with Volume 13 of the Design Manual for Roads and Bridges (DMRB), Economic Assessment of Road Schemes. A predicted scheme opening year of 2010 has been used and therefore the final appraisal year is 2039.
8.3 The objective of an economic evaluation is to assess the value for money of the scheme by comparing those elements of scheme benefits that can be expressed in monetary values against the total cost of having the scheme in place. The benefits of a scheme that are included in an economic assessment are time savings to travellers, savings in accident costs and vehicle operating cost savings.
8.4 The scheme costs, which form the basis for the calculation of the value of Present Value of Cost (PVC) includes works, land, statutory utilities and future preparation and supervision. The scheme costs are discounted to 1998 to provide a comparison with the Present Value of Benefits (PVB) which are also discounted to 1998.
8.5 The currently estimated scheme costs (without applying COBA discount values to obtain the PVC in 1998 prices) are approximately £40 Million for Option 1, £55 Million for Option 2 and £200 Million for Option 3. The Option 3 estimated cost of £200M includes the tunnelling operations, disposal of spoil and the associated roadworks at either end of the tunnel. As the options have only been developed to the stage where outline design elements of the routes have been identified, the detailed design has not been undertaken and therefore the cost estimates are based on the preliminary information currently available. Land costs, future design fees and site supervision costs are excluded from the above costs.
8.6 The difference between the PVB and PVC is known as the Net Present Value (NPV). A positive NPV indicates that a scheme is economically worthwhile. A further measure of a scheme's economic worth can be obtained by dividing the PVB by the PVC to obtain the Benefit to Cost ratio (BCR). The BCR thus gives the benefits per unit cost. A BCR of more than 1.0 indicates a scheme that is economically worthwhile.
8.7 An economic appraisal for each option has been calculated using both high and low traffic growth forecasts. A summary of the economic results obtained, for both high and low growth scenarios, is included in Table 8-1 below.
| SCHEME OPTION | ||||||
|---|---|---|---|---|---|---|
| Option 1 | Option 2 | Option 3 | ||||
| Low Growth | High Growth | Low Growth | High Growth | Low Growth | High Growth | |
| Time Benefits (£M) | 58.3 | 75.5 | 101.9 | 138.6 | 113.0 | 148.9 |
| Vehicle Operating Benefits (£M) | 0.4 | 0.8 | 1.4 | 1.9 | 7.3 | 8.7 |
| Accident Benefits (£M) | 5.6 | 6.7 | 10.8 | 12.4 | 7.5 | 8.7 |
| Total Benefits (£M) | 64.2 | 83.0 | 114.1 | 152.9 | 127.8 | 166.3 |
| Cost (£M) | 26.9 | 26.9 | 36.9 | 36.9 | 144.3 | 144.3 |
| Benefit / Cost Ratio | 2.39 | 3.09 | 3.09 | 4.14 | 0.88 | 1.15 |
| Net Present Value (£M) (Benefit minus Cost) |
37.4 | 56.1 | 77.1 | 116.0 | -16.6 | 21.9 |
| Notes Costs and Benefits are in 1998 prices Costs and Benefits are in multiples of a Million Pounds Figures based on COBA | ||||||
Discussion of Economic Assessment
8.8 The results indicate that, with the exception of the low growth scenario for Option 3, the schemes would represent value for money with positive NPV's.
8.9 The primary economic benefits for each of the options are from the time savings resulting from the removal of congestion along the route. Each of the options would also result in accident benefits due to the predicted reduction in accidents.
8.10 Option 1 would be the least expensive option but has the least benefits, primarily due to the low time saving benefits because of the traffic signals at the Air Balloon roundabout.
8.11 Option 2 represents the best value for money as it has the highest NPV's and BCR's. The NPV's are more than twice those for Option 1.
8.12 Option 3 has the highest total benefits but, due to the costs of the tunnelling operations, is also the most expensive option and therefore has the lowest NPV's and BCR's. Due to the high cost of the tunnelling operations the low growth scenario for Option 3 would not be economically viable. Although the tunnel route is considerably shorter than Option 1 and 2, this would only benefit approximately 60% of the traffic as traffic using the A436 would continue to use the existing route.
8.13 To reflect an overall benefit from the low growth and high growth economic scenarios, a weighted average of benefits (60% of low growth benefits plus 40% of high growth benefits) can be used. The weighted average of benefits results in a BCR of 2.67 for Option 1, 3.51 for Option 2 and 0.99 for Option 3. Using the weighted average of benefits Option 3 would have a poor return because the benefits are less than the costs, and it would therefore be difficult to justify a case in economic terms.
8.14 Overall, Option 2 is considered to represent better value for money.
Estimated Tunnel Costs
8.15 A large and complex project such as this is likely to produce a variation in the bid prices. Assumptions on the chosen method of construction have been made here, but other methods are possible which could alter the programme and the bid price. For example, the contractor may price the hard rock section assuming a computer controlled, three four-boom drilling rig to advance the full tunnel face without a heading. The Laerdal Road Tunnel in Norway for example adopted 500 kg of explosive and 100 drill holes per 5 m deep round, giving 500 m3 of spoil and an advance rate of 60 to 70 m/week. Similarly a Perforex type system could be used to excavate the Lias clays and silts, landslip material and Fullers Earth, again as a full face without the need for a heading and bench.
8.16 To produce a reasonable estimate of the tunnel construction costs it would be necessary to estimate the labour, engineering, equipment and material quantities and to make some allowance for site facilities, office, overhead, profit, contingency and inflation. This could be done by adapting the rates given in Spon's, Wessex, Laxton, etc to current inflation prices. At this early stage, it is probably sufficient to make a less detailed assessment based on contractor's advice. To this end both Skanska and Amec were asked what in their opinion would be the likely construction costs of the project. Skanska suggested a sum of £100 to £120 million for the construction of the twin, 2 and 3 lane tunnels, exclusive of the finishing works (for the excavation and lining, but not for the lighting, road surface, ventilation etc). With contingency, they suggested £150 million. By comparison, the 4.5 km Dublin Port Tunnel (TBM) was estimated to cost £204 million, equating to approximately £136 million for the Cowley - Brockworth tunnel at current prices. The A3 Hindhead, twin tube, twin carriageway 1.5 km tunnel was estimated at ~£75 million (1995), which is equivalent to £150 million for Crickley Hill (£193.5 million at current prices). Amec suggested a rate of between £12,000 and £13,000/lin.m for the excavation, support and lining; based on their current estimate for the Stonehenge Tunnel. This is equivalent to a much lower figure of ~£75 million for the Cowley - Brockworth tunnel if two, twin carriageway tunnels are selected (approximately £94 million if one, three lane carriageway is included). Stonehenge is a relatively straightforward tunnel in relatively good ground and this would be reflected in their estimate for Cowley - Brockworth.
8.17 The only comprehensive analysis of recent road tunnel construction costs that we are aware of was undertaken by STUVA in Germany. They made detailed cost analyses for both cut and cover, and bored road tunnels of various standard profiles, construction methods, ground types and lengths in Germany (described in English in ref. 13, reproduced as a guideline in ref. 15). A comparison could be made using the data presented by STUVA, but this would reflect the German market and some allowance would also need to be made with contemporary (1996) and current UK prices. There is also no English language version of this report.
8.18 Considering German road tunnels as an example; four twin bore, twin carriageway road tunnels were opened on the A71 German Federal Motorway in 2001/2002. The Rennsteig (7.8 km) and Alte Borg (0.865 km) Tunnels are in a combination of hard lava flows, siltstone and clay. The Hochwald (1.06 km) and Berg Bock (2.74 km) Tunnels are in granite. The tubes are 25 m apart and connected by cross passages at 300 m intervals. The final lining is 300 mm of unreinforced concrete. The cost of these four tunnels averaged 22,000 DM/linear metre/tube. This is equivalent to about £7,000/lin.m/tube, or approximately £42 million for the Cowley - Brockworth Tunnel. This is much less than suggested by Skanska and Amec. The excavation and support of the two running tunnels through Castle Hill averaged £3,900/lin.m/tube (1992). The lining similarly averaged 4,000/lin.m/tube. The excavation and support (concrete segments) of the 1.5 m diameter pilot tunnel cost £1,100/lin.m. This suggests an equivalent value of about £48.5 million for the excavation and lining of Crickley Hill at 1992 prices, £68 million at current prices, which is similar to the price suggested by Amec.
8.19 Recent information developed by the Highways Agency gives a bored cost of £40 million per kilometre for twin bore two lane tunnels. This would give a cost of £112 million for the Cowley -Brockworth tunnel.
8.20 Given the wide range of possible construction prices, it is suggested that a price of £100 million be taken for the excavation and lining at this initial stage. To this would be added road construction through the tunnel, access roads and links, M&E, signage and lighting, and construction of the control buildings and reservoir. With contingency, a figure of £150 million should be adopted. This excludes waste disposal costs which would increase the overall cost to approximately £200 million.
Cost Comparisons with Other Tunnels
8.21 The Pen y clip Tunnel on the A55 in North Wales is a single bore tunnel of 930 m in length, 9.95 m width and 7.1 m height. The heading broke through after 33 months and the tunnel was completed in 48 months. The tunnel was excavated by drill and fire, with 130 holes per face, pulling an average 3.5 m advance. The initial and final thirds of the tunnel were in highly fractured rock, requiring steel arch support. The central third was less broken and was supported with steel mesh-reinforced shotcrete and rock bolts.
8.22 The Trattenbach Tunnel in Austria is a twin lane, single tube road tunnel of 419 m in length, about 85m2 face area and 2.5% gradient. The tunnel was excavated by NATM using a heading and bench. A 250 mm thick concrete lining with a membrane was installed. The project included some 750 m of slope stabilisation. The rock types are metamorphic sandstones and phyllites. Constructed took 24 months (1988 to 1990) at a cost of 261 million Aus.sch. This is roughly equivalent to £255 million at current value for a twin tube tunnel of the length of Crickley Hill.
8.23 Zell am See Tunnel in Austria is a twin lane, single tube road tunnel of 5109 m length and about 80 m2 face area. This project also has a 35 m long ventilation cavern, 480 m deep ventilation shaft, 652 m ventilation tunnel, 682 m of interchange tunnels and 375 m emergency exit tunnel. There is also a 4.7 km, 3.2 m diameter pilot tunnel, built by TBM. Also in metamorphic sandstones and phyllites, the project was constructed over 42 months (1992 to 1996) by NATM at a cost of 1200 million Aus.sch. This is roughly equivalent to £79 million for Crickley Hill at current prices.
8.24 Karawanken expressway tunnel on the Austria/Yugoslavia border is a twin tube, twin carriageway tunnel of 7,864 m and a face area of 86 to 100 m2. The tunnel has a gradient of 0 to 1.35% and was constructed by NATM through hard and soft rock. The tunnel ventilation is divided into two, about 3.5 km long sections for semi-transversal ventilation at each side. The central 1.2 km has only longitudinal ventilation. There are ventilation stations at each portal, but no ventilation shafts. The project was constructed between 1986 and 1991 at a cost about 2,600 million Aus.sch. This is roughly equivalent to £68 million for Crickley Hill at current prices.
8.25 Four, twin tube, three lane motorway tunnels were built along the Adana Toprakkale Gaziantep motorway in Turkey. These tunnels (Tasoluk Tunnel is 361 m, Ayran Tunnel is 600 m, Kizlac Tunnel is 520 m and the Aslanli Tunnel is 1360 m) were built through sedimentary and metamorphic rock by NATM and have face areas of 126 to 152 m2. The tunnels are lined with 0.4 m concrete. The tunnels were constructed at a cost of $150 million between 1990 and 1996 (equivalent to $189 million at current prices).
8.26 A twin tube, three lane tunnel of 3.24 km and 126 to 152 m2face area was built through sedimentary and metamorphic rocks by NATM along the Anatolian Motorway in Turkey. The tunnel has a 0.4 m thick concrete lining and longitudinal ventilation. The tunnel was built between 1992 and 1996 at a cost of $270 million. This is roughly equivalent to $340 million at current prices.
