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Dublin Metro North An Bord Pleanala Hearing
Citation preview
Cormac Rabbitt 1st December 2009
Report download on www.darganproject.com
This report shows that the integrity of Metro North project is wanting in terms of safety and cost.
The authors would welcome an opportunity to directly present this proposal in greater detail and answer questions to An Bord Pleanala.
Dargan Project: Metro North, A Single Tunnel Solution
Contents
1. Executive Summary & Recommendation 1.1 Summary 1.2 Recommendation
2. Metro Operation 2.1 Safety 2.2 Service Areas
3. Development of Large Tunnels
4. Construction Issues 4.1 Geotechnical Risk 4.2 Cost 4.3 Timescale 4.4 Safety
For clarity this report: a) deals only with the tunnel section of Metro North between Albert Park Ballymun Road and St. Stephen’s Green; & b) rounds all figures.
1
Modern tunnelling technology is capable of far more than the current Metro North pro‐posals allow for. Tunnel technology, over the past decade, has improved construction safety and minimised cost by preferring larger single tunnels build by earth pressure tun‐nel boring machines over multiple hand built tunnels.
When Metro North was developed, there was no capital constraint ‐ neither on the State nor on private contractors’ bankers. This has since changed utterly. This submission shows that Metro North capital costs could be reduced by in excess of €180m.
This submission shows that replacing Metro North’s open excavation multiple tunnels, of which there are over 45, with what is now considered a medium size single 12m diameter tunnel provides; a significantly safer, cheaper and more sustainable solution.
Compared to Metro North’s multiple tunnels, it is apparent that a single tunnel is:
Safer; for patron evacuation, access by rescue services and ongoing infrastructure maintenance and operation;
Cheaper; because it is less complex to construct, has less geotechnical and pro‐gramme time risks which in effect reduces build cost by in excess of €180m, reduces build time by over one year and the likelihood of problems that could give rise to court proceedings, with the resultant implications for additional costs and time de‐lays;
More sustainable; on the grounds outlined above and because it avoids significant disruption to city streets during its construction.
1.1 Executive Summary
1. Executive Summary & Recommendation
It is recommended that An Bord Pleanala:
Recognise that:
a) The RPA’s Metro North proposal to build over 45 hand built tunnels of various sizes, joined together, with each join having a different cross‐section, has significant avoidable geotechnical risk, programme risk and cost risk; that, what is now consid‐ered a medium size single tunnel could right;
b) A single tunnel offers considerable environmental improvements to construction; safety, programme and cost, and in particular, facilitates significant patron and op‐erator benefits in terms of safety, comfort and operation.
Require that:
The RPA revisit their Metro North proposal, with a view to incorporating a single tunnel solution and put their proposal back on the drawing board.
1.2 Recommendation
Notice: This Report may be copied and reproduced but all rights remain the property of the Dargan Project
2.1 Safety
This section outlines how a single continuous tunnel, 12m in diameter, fa‐cilitates significant patron and operator safety benefits that are not possi‐ble with the RPA’s Metro North proposal.
Features of a single tunnel that facilitate safe evacuation of patrons, be‐tween stations, can be seen in figure 1. The figure shows four patron fire safety zones, where independent air flows can be maintained to help, in the event of a fire or breakdown, safe patron evacuation and, most impor‐tantly, to allow access by rescue services.
The four patron fire safety zones embrace the ability to include between stations at frequent intervals:
a) stairs connecting decks (shown in the figure);
b) multiple fire doors along and between passageways (not shown in the figure); and
c) multiple fire prevention equipment and room for new and updated equipment as they become available.
Barcelona’s Line 9*, 12m diameter, metro tunnel has two decks similar to the single tunnel proposed in this report. Features of Line 9 are illustrated in Figures 4, and 6 to 9.
Figure 4 also illustrates between twin tunnels; fire safety cross‐passages and vertical ventilation shafts as proposed for Metro North, ‐ which com‐pared to a single large tunnel’s features outlined above, can be seen to be more difficult and less safe:
d) to evacuate in the event of a fire or breakdown;
e) to provide coordinated independent air flow through multiple ver‐tical shafts;
f) importantly, to allow access by rescue services; and
g) to hand‐build, seal, maintain, etc.
Conclusion.
Provision of a single tunnel facilitates provision of significant patron and operator safety benefits that cannot be provided with the RPA’s proposal.
1
2
4 3
3
Figure 1. Four patron fire safety emergency areas
Cross‐section: between stations
2. Metro Operation
North Bound
South Bound
* Barcelona Line 9 Paper to Engineers Ireland: International Best Practice in Metro Related Tunnel Projects, by Ing. Nicola Della Valle Tunnelconsult SCP, www.iei.ie/media/engineersireland/community/whitepapers/International%20Best%20Practice%20in%20Metro%20Related%20Tunnel%20Projects.pdf
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2
11
7
63
8
9 10
2.2 Service Areas
This section outlines how a single continuous tunnel could enhance metro operation, that is not possible with the RPA’s Metro North proposal, by providing twelve service areas as shown figure 2.
The single tunnel offers an operator the space, between stations, to pro‐vide a safer and enhanced service at lower cost, by, for instance, facilitat‐ing access to ongoing line maintenance, servicing broken‐down trains, and allowing access to rail infrastructure such as power supply, parking trains, fire hydrants, etc.
In addition, a single tunnel provides room to facilitate auxiliary works such as crossovers between tracks through openings in the top floor slab (v fig‐ures 9 to 11). The openings do not affect the fire air zone safety require‐ments or the tunnel diameter.
A single tunnel offers flexibility to provide crossovers where required at a relatively low cost which is in stark comparison to twin tunnels where crossovers are relatively inflexible and costly.
Conclusion.
Provision of a single tunnel allows an operator to offer a safer and better level of patron service at reduced cost that cannot be provided with the RPA’s proposal.
4
Figure 2. Twelve service areas for ventilation, communications, power supply…
Cross‐section: between stations
2. Metro Operation ...continued
South Bound
North Bound
3.1 Development of large tunnel boring machines (TBMs)
In 1987 the Japanese made a major step in TBM size, see figure 3, when they designed and built a 14m. diameter slurry machine for the construction of the Konda River Tunnel in Tokyo, which was excavated through sands and gravels. The 2km long tunnel was advanced consistently at 5m/day. This project was the forerunner of the ambitious Trans Tokyo Bay Tunnel constructed between 1995 and 1999, where eight 14m. diameter machines were used to construct the 15km miles of twin tunnels for the channel crossing section of the project.
In 1998 a marginally larger 14.2m. diameter slurry machine was used for the construction of the 3rd Elbe tunnel in Hamburg, Germany. This 2.56km long tunnel was driven through sands, glacial drift material, silt, gravel and boulders with only 7m. of ground material separating the crown of the tunnel from the Elbe river bed. While peak production rates of up to 49m/week were achieved, the overall average was nearer to half this rate.
The mid‐1990s also saw the introduction of a wider range of conditioning mate‐rials and the use of foams. These allowed earth pressure boring machines (EPB) to handle coarser materials such as sands and gravels, thus further enhancing their capability. However, the maximum diameter of the EPBs was then limited by torque requirement as diameter increased.
Since 2000 there has been a continuation of the development of larger and more powerful TBMs. A significant technical development has been the intro‐duction of ever larger and more powerful torque machines and processes. Di‐ameters and tunnel depths are increasing. Tunnels are getting longer and con‐struction schedules ever more demanding.
In 2006 Madrid introduced the first 15.2m. diameter machines for the construc‐tion of its M30 Ring Road. The minimum cover to a tunnel crown at one of its structures was 6.5m. Madrid’s particular machines made unprecedented strides with the tunnel advance rate averaging 15m/day.
Shanghai introduced 15.4m. diameter machines in 2008, the largest in the world, to excavate two 7.5km parallel tunnels under the Yangtze River and op‐erated at a pressure of 6.5 bar. The TBMs averaged 13m/day and completed their tasks in 20 months.
Seattle has chosen a 16.5m. diameter for its 2.7km Alaskan Way replacement road tunnel which is due to commence construction in 2011.
Conclusion.
Over the past decade it is apparent that tunnel construction technology has im‐proved safety and minimised cost by preferring sophisticated large single tun‐nels over multiple smaller tunnels. The 12m. diameter TBM proposed in this re‐port now could only be regarded as a medium‐size as its’ cross‐sectional area is 53% that the current largest TBM.
3. Development of Large Tunnels
5
Figure 3: Development of Large TBMs
Large
Medium
Small
TBM Size Ranking
2009 16.8m
15.2m
13.7m
12.2m
10.7m
9.1m
7.6m
6.1m
Metres
USA
Ireland
Dublin Port Tunnel TBM, 12m. Diameter (regarded now as medium size tunnel — as large TBM cross‐sectional areas are now 60 to 90% greater)
4.1 Geotechnical Risk
This section considers avoidable geotechnical risk for station construction by comparing TBM methods with hand‐sequential excavation methods. Further geotechnical risks are considered in section 4.4 on construction.
The world’s most proficient tunnel builder Melis (ref. 1) states that tunnelling Tunnel sta‐bility fundamentally depends on one parameter of the soil (un‐drained shear strength) and that this in turn is based on very few soil samples. In effect, for a tunnel soil volume of some 1,000cu.m., tunnel face stability would be estimated by a 2.5 litre sample or by testing at most 0.00025% of soil affected by the tunnel, counting only the tunnel itself and not the soil above it.
Melis asserts that reliance on such a small sample is inappropriate as soil conditions can vary by the metre and that even with a pilot tunnel, geotechnical conditions can vary so substantially that any contract’s prior geotechnical information is of limited value.
If major problem were to occur, the RPA could face court proceedings, with the resultant implications for high costs and time delays.
The RPA relies on a multiplicity of tunnel types, as outlined in table 1, constructed by dif‐ferent methods that could be replaced by a single tunnel with one construction method.
The RPA’s O’Connell Street proposal (v box 1 and illustrated in figures 4 & 5) involves hand‐sequential excavation methods that could be replaced by a single tunnel.
A comparison of construction methods is as follows.
Consideration of station construction method: TBM or hand‐sequential excavation
The RPA’s proposal is to use what is called conventional tunnelling for stations, which in essence is an open face sequential excavation or sprayed concrete lining (SCL) method.
For tunnels in urban areas, limiting settlement is of paramount importance to avoid dam‐age to overlying structures. In order to limit settlement and ensure worker safety, the following SCL tunnel measures are used.
SCL tunnels are sequentially excavated and supported. The initial ground support is pro‐vided by reinforced shotcrete and often by additional ground reinforcement (e.g., soil/rock bolts). The permanent support is usually (but not always) a cast‐in‐place concrete lining. Excavation stages must be sufficiently short in dimensions and duration. Erection of the ‘‘full ring’’ of initial ground support must be completed immediately after excava‐tion.
Of primary safety consideration is that the periphery of the heading and the face remain unsupported until the sprayed concrete ring has become a structural support. Thus, sup‐port is delayed. Stresses are redistributed which has implications for settlement. Aus‐trian’s devised what is known as the ‘new Austrian tunnelling method’ (NATM) for rock tunnels. It is an SCL method that relies on rock stress redistribution (relax), which allows support to be minimized.
With SCL ground loss is undeniable and in particular must be prevented where a struc‐ture such as O’Connell Street bridge is concerned.
SCL relies on controlled deformation of the ground that mobilizes the ground’s strength whereas modern sophisticated earth pressure tunnel boring machines (TBMs) methods do not. Sophisticated TBMs compared to SCL methods are recognised by world geotech‐nical engineers as offering the best ground ‘fail‐safe’ option; in addition TBMs are inher‐ently safer, offer significantly better working environments, and are significantly faster to build and cost less.
The debate as to whether an open face sequential excavation or SCL method may be used in any particular ground case is not simply about whether the increased ground losses inherent in the approach can be accepted for that particular ground in that par‐ticular location; it is also about safety, time and cost. Melis concludes that experience shows that when open face methods are not used, construction runs smoothly and no collapses occur, regardless of how dangerous the ground is.
Key questions that arise, as to which alternative method SCL or TBM should be used, are:
1. Are the excavation and lining methods specified the most appropriate?
2. Do the methods of construction adopt the highest possible safety principles?
3. Which method provides the safest and most efficient working environment?
The tunnel method selected has to consider that, under O’Connell St. Bridge, the RPA propose to hand‐build three large oval shaped SCL caverns with minimum cover under the Liffey.
The answer to the three key questions, for Dublin’s urban centre, could only be the se‐lection of a sophisticated TBM.
Conclusion.
From the foregoing, it is totally apparent to the authors as to why the RPA needs to specify a sophisticated single TBM to undertake, where possible, its underground sta‐tion work. The RPA relying on anything other than a sophisticated TBM for its large sta‐tion tunnels such as those proposed for O’Connell St., could be seen as very suboptimal.
6
4. Construction Issues
Box 1. RPA O’Connell Street proposal includes: to hand mine 3 large tunnels, two of which are large oval shaped cavern sta‐tion platform tunnels (cross‐section 9.7m. high x 10.6m. wide) and between them a 70m. large cavern tunnel (varying cross‐section oval shape 12.8m. high x 14.4m. wide to a diameter of 12.7m) with 12 cross‐passages
Ref. 1: Professor Dr. Manuel J. Melis, Director General Infrastructure, Public Works Council, Regional Government of Madrid and President Metro de Madrid. CLIENT VIEWPOINT, MADRID METRO EXTENSION, Tunnel & Tunnelling International, March 1999, Volume 31 Number 3.
4.2 Cost Development of large Bore TBM Tunnels over the past two decades is outlined in section 2. It is apparent that tunnel construction technology has minimised cost by preferring large single tunnels over multiple smaller tunnels.
For instance, Seattle in 2008 (ref.2), for its 2.7km Alaskan Way replacement road tunnel, compared the provision of a single 16.5m. external diameter (ED) tunnel against twin 13.1m. ED. They concluded that the single tunnel option provides 23% cost savings (v. table 2) and time savings of 12 months on the tunnel alone and another 4 months on tunnel fin‐ishes.
Seattle’s cost saving of over €180m on twin 2.7km, could be seen as modest compared to replacing the RPA’s twin 5.3km complex of over 45 separate tunnels, of various sizes, joined together, with each join having a different cross‐section (v table 2 and illustrated in figure 4) ...by a single tunnel
Tunnel component costs are very much influenced by, a) construction complexity, b) time & number of teams, c) unit rate of material & excavation, d) space requirements for M&E, communications, power supply and auxiliary works.
a) Reducing programmed construction time by one year (see section 4.3) and a drasti‐cally smaller number of working teams reduces costs. For instance, reducing build time by 1/3 and the number of teams by 2/3 could save some 75% of team costs.
b) The unit rate costs of materials and excavations in a large tunnel are lower that those in smaller tunnels. Large tunnels make better use of larger machines with stronger hydraulics, etc. and the use of larger lining rings than small tunnels can, for instance:
i) It is significantly cheaper to purchase, to operate a large TBM with all it bits; con‐veyer belts, crews, etc, than to purchase and operate two smaller TBMs. Conse‐quently the unit cost of excavation & disposal are lower for a single large tunnel as against two smaller tunnels;
ii) The unit cost of tunnel lining materials and its placement is less for a single large tunnels than for smaller twin tunnels, as the former uses wider and longer ring segments and has, as can be seen from table 1, some 46k sq.m. less area to line, however, the saving is reduced by the cost of the deck and fire walls applicable only in the large tunnel.
c) Additional service areas available in a single tunnel (v figure 2) over that of the twin tunnels could reduce costs significantly by allowing flexibility in design for 38kv power supply, M&E, communications, auxiliary work such as line crossovers, etc.
When Metro North was developed, there was no capital constraint ‐ neither on the State nor on private contractors’ bankers. This has since changed utterly. Consequently, Metro North cost reduction could be seen for economic and other reasons to be very important.
Conclusion.
The forgoing substantiated how de rigueur cost saving in excess of €180m is achievable. Ref. 2: Washington State Department of Transport, Alaskan Way Viaduct & Seawall Replacement, Bored Tunnel Briefing, Dec. 2008 http://www.wsdot.wa.gov/NR/rdonlyres/200E4491‐47C7‐4DBF‐8810‐CB78663CD062/0/AWV_SAC_BoredTunnelBriefing_121608.pdf
4. Construction Issues ...continued
Table 2: Extracts from Seattle Bored Tunnel Briefing
Table 1: Differences in tunnel construction components
Single continuous tunnel
RPA’s multitude of complex tunnels
TBM length 5.3km 9.8km
TBM external diameter 12.0m 7.0m
TBM launches 1 12
TBMs dragged through stations 0 10 times
Cross‐passages at 5 stations 0 hand built: over 15
Cross‐passages & vent shafts between stations
included hand built: over 25
Station platform tunnels Included hand built: large oval shaped station tunnels each 9.7m. high x 10.6m. wide
O’Connell Bridge tunnel between platform tunnels*
Included hand built: 70m long, cross‐sect. varying 12.8m. high x 14.4m. wide to 12.7m. Dia.
Tunnel excavated materials (removed by city streets %)
610k cu.m (0%)
500k cu.m (30%)
Tunnel surface area Tunnel area of deck & walls
200,000sq.m 66,000sq.m
246,000sq.m ‐
7
Table 3: Indicative construction timescale for tunnel works
Single RPA’s multitude of
TBM: progress rate, & station launches
14m/day for 5.3km with 1 TBM, 0 drags & 1 launch
14m/day for 9.6km with 2 TBMs 10 drags & 12 launches; @ 10 days+ per drag & launch
Stations & cross‐passages progress rate
Included above 10 oval shaped station 94m. tunnels, rate 1m/day O’Connell St. oval 70m. tunnel rate 0.5m/day Station tunnels have to be excavated prior to the pas‐sage of the TBM (time involved) Excavation of the second station tunnel cannot start un‐til the first one has progressed some 40m Tunnel inverts must be concreted before the passage of the TBM ‐ completed at a rate of 12m/day Secondary lining construction rate 4m/day, + 1 month to set up the lining shutter and to remove it. 40+ cross passages of various lenghts, rate 1m/day
Ref. 3: Dublin Port Tunnel; http://en.allexperts.com/e/d/du/dublin_port_tunnel.htm http://www.dublinporttunnel.ie/about/building/pdf/tunnelling_its_effects.pdf
4.3 Timescale
This section outlines why a single continuous TBM tunnel is significantly faster and has a lower pro‐gramme time risk, as against the RPA’s proposal for a complex set of different hand‐built tunnels.
It is notable that world tunnelling time records are held by the larger TBMs. Consequently, subject to the type of TBMs selected, it could be expected that a 12.0m. ED TBM could make progress faster than twin 7.0m. ED TBMs.
Progress on the Dublin Port Tunnel (ref. 3) progress averaged over 12m/day, having to stop every 20 minutes to lay rings, and working limited hours. Modern TBMs don't need to stop to lay rings. Depend‐ing on the number of days & hours per day worked, a single TBM could complete 5.3km in 8 to 18 months.
Seattle, as stated in section 4.2, concluded that it could construct its 2.7km road tunnel road 16 months faster with a single large bore as against twin bores. Comparative time savings with Metro North single tunnel could be far greater as it is over twice Seattle’s length and could save on build sequences out‐lined below.
Analysis of the build sequential time restraints, to build the RPA’s set of different TBM and hand built tunnels, set out in table 3, highlight why it will take at least an extra year to build the RPA’s proposal with greater scheme programme risk than the single tunnel proposal has. Programme time risks in‐volve; geotechnical risk (section 4.1), time necessary for its sequential works (such as delays in tunnel launches while stations are hand mined), serious construction interaction with city streets, manage‐ment of a diverse number of construction teams, massive number of joints between all the tunnel seg‐ments (sealing problems), etc. In contrast a single tunnel avoids all the foregoing.
Conclusion.
One could only conclude from the above, and from world experience of larger tunnels that the time and programme risk to build a single continuous TBM tunnel is at least 20 months less than for the RPA’s proposal.
4.4 Safety
This section outlines why it is significantly safer to build a single continuous tunnel by a sophisticated TBM as compared to the RPA’s proposal to build the multitude of separate tunnels outlined in table 1.
Construction safety risks could be assessed by the following:
a) Construction safety ‘quality control’ risk issues associated with the RPA’s pro‐posal, above and beyond those which the single tunnel include, a) in excess of 45 small tunnel to large tunnel joins with each of the joining tunnels having a different cross‐section (size mismatched… seal quality problems, etc) ‐ see illus‐tration in figure 4, b) the need for many construction crews and working meth‐ods (dispersed; safety control, coordination, etc) and c) a much poorer working environment.
b) That it is safe to build a single tunnel 12.0m. external diameter (ED) as it is simi‐lar to the successfully completed Dublin Port Tunnel in terms of size (11.8m. ED, 5.2km TBM tunnel & 3.8km cut and cover, see ref. 3) and ground conditions.
c) That there are now many better/safer larger TBMs available than the TBM used for the Dublin Port Tunnel, with many of them having completed projects in far poorer ground conditions than Metro North will be. Recent proven develop‐ments in methods of tunnelling have far greater construction safety than here‐tofore. An outline of the development of large bore tunnels is given in section 2.
d) That world tunnel engineers accept that the geotechnical risks (section 4.1) as‐sociated with large hand‐built tunnels such as the RPA’s proposes have a much greater associated risk of ground movement than a single continuous tunnel built with a sophisticated TBM. For instance, the construction risk of cross‐passages is such that Consultants normally recommend that their number be minimised: the risks to builders during the construction of a cross‐passage they say could exceed the benefit provided by the cross‐passage during its operation.
Conclusion.
Construction safety gains in building Metro North with a single sophisticated TBM rather than by the RPA’s vast multitude of joined up hand‐ and TBM‐mined tunnels are such that it is illogical to ignore them.
4. Construction Issues ...continued
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4. Construction Issues ...continued
Note:
Single TBM Entry/Exit
at Albert Park
could be similar
...with a smaller footprint than the RPA’s twin tunnels
would have
9 Figure 4. Illustration: multiple of complex joined tunnel segments
RPA’s complex design of over 45 + separate tunnels, of various sizes, join together, with each join having a different cross‐section…… could better be accommodated within a single tunnel as shown in figures 1, 2 & 4 to 11.
Multiple TBM tunnel segments between stations. (TBMs have to be re‐launched at each stations.)
Multiple hand built station cross‐passage tunnels
Multiple air vent Tunnels
Multiple hand built Oval Station tunnels
Multiple hand built cross‐passage tunnels
Barcelona Metro Line 9 Tunnel — Diameter 12m.
Extracted from RPA’s EIS
4. Construction Issues ...continued
10
Alternat ive
One large machine built tunnel, which can be built safer, at lower cost and in a much faster time & with Reduced Compound Areas
Figure 5: O’Connell Street Station
Three large hand built tunnels & twelve hand built cross‐passage tunnels
Compounds
Temporary Bailey Bridge
O’Connell
St.
Westmoreland St.
Section A — A
Section D — D
Section B — B
A
A
B B
C C
Section C — C
D
D
Alternat ive
Base map extracted from
RPA’s EIS
Features
Tunnel cross‐section of 12 meter external diameter:
Platforms inside the tunnel;
One track above the other, separated by an intermediate slab;
Allows for future lengthening of platforms;
Suitable for construction in the high density urban areas.
Figure 7. Example: Existing Barcelona Line 9 tunnel has 12m. external diameter Figure 6. Example of how O’Connell St. Station could look
Looking North
Aston Quay
Eden Quay
2.9m
Dart Car Extract From Irish Rail: 8520
EMU
Barcelona Line 9
Figure 8. Confirmation that a DART sized car could operate in a Barcelona Line 9 size 12m. ED tunnel
11
Auxiliary type works: Interconnections between tracks The connection between the upper and lower level could be provided to facilitate intermediate terminals, partial services, etc. The foregoing requires an opening of the slab with a slope (circa 4 %) . The tunnel diameter is not affected.
Figure 9. Design flexibility. Interconnections between tracks and train parking
Section A A
Note: Four distinct air zone areas for patron fire safety and services could be maintained throughout the crossover (at mid‐point)
Section A A: Ramps
Going Down
Going Up
2.2m
2.2m
Fire Doors
Section A A
Fire Doors
Fire escape door
Figure 9: Barcelona Line 9 single level swop
Figure 10: Possible double level swop
Figure 11: Train parking
Cross‐over
Cross‐over
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