Shani Wallis, TunnelTalk

Part 1: Construction of the Metro de Santiago started like most systems in the world with open cut stations and cut-and-cover running tunnels. Today, construction in the urban areas is completely underground in NATM tunnels and caverns. A report from Chile in early 2003 explained the path of progress from cut-and-cover to mined construction and the details of the NATM design and sequences.

Caverns of 150m² excavated just 7m-9m beneath the city streets, intersected at right angles by caverns of equal size (17m wide x 14m high) form the NATM mined stations for Santiago's new metro extensions in Chile. These impressive spaces, with 60m²-70m² single-tube double-track running tunnels between, represent some of the most advanced NATM design and excavation underway in the world today. Not only is the method sympathetic to

Fig 1. Current extent of the Santiago Metro system (2010)

the hectic traffic on Santiago's city streets, the NATM approach is highly cost-effective at a current US$6.5 million/km of running tunnel and about US$12.5 million/km for station tunnels (2003 prices). For Line 4, currently in design, advanced value engineering is being introduced to reduce costs further, increase productivity and improve durability.

Designers of these structures are JVs between local firms and experienced international NATM engineers. Ingendesa, one of the largest engineering consultants in Chile, proposed and designed the first-ever mined running tunnel for Metro in 1993 enabling development of the current underground works. Today, in JV with ARA of Chile and the Dr Sauer Corporation (DSC) of Austria as specialist NATM subconsultant, the team is the detailed designer and site supervision team for the Line 5 West and Line 2 South extensions, as well as for the Tobalaba connection station with Line 1 and the Vicuna Mackenna and Vicente Valdes underground connections with Line 5 on the new Line 4 project (Fig 1, Table 1).

Table 1. Details of the new works projects in 2003

Geoconsult of Austria as subconsultant to local company Cade Idepe Ingenieria, the largest consulting firm in Chile, was detailed designer of the first purpose-designed fully NATM running tunnels in Santiago in the late 1990s. Since then the team has completed basic design for all underground works on the current Line 2 and Line 5 extension projects and for the new Line 4 alignment. Today it is detailed designer and construction supervisor for Line 2's North extension and for three detailed design lots of mined underground works on Line 4.

NATM of extraordinary size and complexity for the Cerro Blanco Station on the Line 2 North extension

Bureau de Projetos e Consultoria Ltda of Brazil is the specialist NATM designer with Chilean consulting group Arcadis Geotecnica. The team was design checker of the first NATM designs by Cade Idepe-Geoconsult in the late 1990s and is now detailed designer and site supervisor for a 4km long section of NATM works with four NATM underground stations on Line 4.

Rapid evolution
One of the most remarkable aspects of the Metro de Santiago development is the speed by which construction methods have evolved. Construction started in the mid-1970s with the assistance of Sofretu (now Systra) of France, based on Sofretu's operating system and its rubber-tyred rolling stock. Of the existing 40km network more than 23km is underground, mainly in open cut works, with some 6km, elevated and another 5km at grade. It was mounting objection to open cut disruption and the increasing cost of relocating services that persuaded Metro in the late 1980s that non-disruptive methods would be needed in future.

With that in mind, the last 2km of the Phase 1 Line 5 construction in the north was converted, by Metro and consultant Ingendesa, from open cut to a mined tunnel alternative. Located under a public park, this was a suitable location to experiment with the mined alternative and at 2km long, any risk to schedule would be limited.

All of Santiago's subsurface construction is based on the axiom that the ground is as near 'perfect' as possible. The ground water table at up to 80m below ground surface, is well below the deepest level of excavations and the quaternary conglomerate, deposited by ancient glacial and river action, is an amazingly stable matrix of smooth rounded stones, held by a small but highly effective percentage of bonding fines. Known as 'Ripio de Santiago', it is suggested that deposits of ground water salts provide partial cementation in the gravel conglomerate and that earthquake activity has contributed to its excellent compaction and interlocking.

When excavated, the immediate impression is that the ground would be unstable, but experience proves otherwise. The average ground stiffness is 250MN/m²-300MN/m² on the Young's modulus at tunnel horizon. The densely compacted ripio is up to ten times the rigidity of soft clay and weighs about the same as concrete at 23kN/m³-24kN/m³. The friction angle is 45°-55° and cohesion is 30kN/m²-40kN/m². Slopes of up to 55° and 18m-20m deep for the early open cut works remained open for months at a time without any applied support. Today, the same is true in mined headings. With due consideration, remarkably large faces can be exposed and left unsupported without risk of ravelling or collapse.

Early open cut works in the Ripio de Santiago

Once the shear strain limit is reached however, the ground can fail dramatically. There were slope failures on the early open cut works, some resulting in fatalities, but as lead site supervisor Alexandre Gomes of Geoconsult explained: "It is incredible what this ground will allow you to do. Where ground losses in clay would be 1.5%-2% for an equivalent tunnel, volume losses in the ripio are as low as 0.2%-0.5%. In fact the strength of the ground increases when excavated and frictional forces are mobilised."

For this ground, deformation is more a result of exposing the roof of the tunnel than the face. When the face arrives at a surface instrumentation monitoring station, about half the total expected settlement is registered. The rest arches back over the course of 3-4 weeks as the tunnel advances. Deformations are also very low at 5mm or less for running tunnels. "The stability of the ground is taken somewhat as a given," said Gomes. "Nevertheless no-one on recent works has been allowed to push the behaviour to the limit. Hence there have been no critical situations or NATM emergencies to date."

This favourable geology plus the alignment under the park and the precedent set by the initial open cut works, led to the selection of NATM and of a 55m² (6.5m high x 9.6m wide) single-tube, double-track configuration for the 2km Line 5 mined running tunnel experiment in 1993. Designed by Ingendesa, the tunnel was excavated by two consortia of mainly Chilean contractors - DPS (Desco-Preson-Salfa) and IMS (Ingecol-Mendes Junior (of Brazil)-Sical) - on a full face heading and with a flat invert. "Basically one of our senior engineers, Patricio Aodrigues, with experience obtained designing hydro power rock tunnels, developed a design for the urban ripio situation," said Leonardo Bustamante, a senior tunnel engineer with Ingendesa who worked also as a junior engineer on the first metro tunnel experiment. Design support comprised 300mm of dry mix shotcrete reinforced to full load-bearing requirements using lattice girders and rebar rather than wire mesh, and finished with a 200mm thick, strongly reinforced in-situ concrete final lining. The experiment was largely a success. It proved that, despite being about 20% more expensive/km than cut and cover in direct construction costs, mined tunnels were a safe, technically feasible, cost effective alternative when including cost savings in surface reinstatement, social costs and service diversions.

The one drawback was that attention to settlement control across the park and under the shallow cover fixed by the original cut-and-cover design was limited. No registered settlement readings were logged and settlement of up to 100mm was estimated. For the next extension - a 2.8km continuation of Line 5 west from this first mined tunnel experiment, beneath a main city street and adjacent to the city's 300 year old Cathedral and other old, historic buildings - anything less than tight control on surface settlement would be unacceptable. Instrumentation expertise, ground movement monitoring, and the implementation of design criteria, construction methods and construction sequences, to limit settlement and prevent damage to adjacent structures, was required.

This initial expertise was provided by Geoconsult. As subconsultant to Cade Idepe, engaged to design the new extension in 1997, Geoconsult developed the NATM design, excavation sequences and a comprehensive instrumentation programme. Regular monitoring stations comprised 3D-deformation points, pressure cells and strain meters in the tunnel and sliding micrometers, extensometers, inclinometers, piezometers and levelling points on the surface.

Metro also engaged Geodata of Austria as an independent geotechnical consultant to install the instrument arrays as specified by Geoconsult, take all the readings as required, and make the collected data available to the consultant, contractor and Metro management team according to schedules. "All instrumentation readings at that time were manual," explained Silvano Pozza, project manager for Geodata. ''There was no real-time data logging. Also, under our contract, we were required to make the data available but in no way to make interpretations. That was the responsibility of the design engineers."

First mined running tunnels between open cut stations on the first Line 5 extension in the heart of historic Santiago

In early 1998 work started on the 2.8km extension of single-tube, double-track running tunnels between three open-cut stations. Working in the very heart of the city, a robust design was adopted for the NATM tunnels. The 35m² full-face top headings of the running tunnels were excavated in 800mm-1m rounds with the bench and closed invert no more than seven rounds, or 10m maximum, behind the top heading. Immediate support comprised an initial layer of dry mix shotcrete followed by installation of lattice girders at 600mm-1.4m centres and covered by additional layers of wire mesh reinforced shotcrete to a minimum thickness of 200mm. Primary reinforcement totalled some 20kg/m³, with lattice girders of 28mm/32mm diameter rebar. Following excavation, a final lining of reinforced 400mm-450mm thick in-situ concrete was cast to the invert and arch. In addition micro pile curtain walls were installed next to the foundations of more sensitive and historic buildings and extensive underpinning was carried alongside open-cut station boxes.

"At the time we were intentionally applying a conventional NATM soft ground tunnel design with its inherent redundancies," said Kurt Laubbichler, Project Director for Geoconsult. "The engineers at Metro, as well as the contractors' staff and labour, were using a new construction technique for the first time in practical terms, in the heart of the capital and adjacent to major monuments. In addition local engineers and contractors had, and still have, strong opinions about engineering theory and practice based on extensive experience of foundations and open cut techniques in the ground conditions in the city. Yes, the scope for advancement was evident, but acquiring the confidence to undertake these developments would take time. It would take more experience, more geotechnical data to confirm ground behaviour, and improvements of NATM construction techniques - steps that I believe have been achieved remarkably fast by the tunnelling community here in Santiago."

In early 2001, following successful completion of the Line 5 extension, newly elected President Aicardo Lagos adopted a plan to extend Lines 2 and 5 and to construct a 33km long, 27-station Blue Line 4. The estimated US$2 billion investment would almost double the network and bring services to another 1.8 million (or 40%) of Santiago's population. Approval came with the political promise to have the Line 2 and 5 extensions in operation during 2004, and Line 4 in service by the third quarter of 2005 - within the new President's first six-year term.

With no time to lose, new managers within Metro established two new departments to undertake the new works - first, construction of the Line 2 and Line 5 extensions under programme manager Carlos Mercado, and secondly, implementation of Line 4 under programme manager Vicente Acuna. Today, from preliminary designs completed in 2001 by Cade Idepe and Geoconsult, construction work on all three extension projects is on-going and early works contracts for Line 4 have started.

Mined stations
For these new projects, Metro introduced several major advances. One of the most significant is the selection of mined underground stations. Although some of the adjacent open-cut access points are large, there are no more open cut station boxes in the city streets.

Shotcrete as the composite initial and final lining in stations and running tunnels provided significant construction savings

Another major change is that, except in special cases, there are no in-situ concrete final linings. In both the running tunnels and in station caverns, shotcrete is the initial and final permanent finish as part of a composite single shell support and lining concept. ''This has significantly reduced the lining thicknesses," said Augusto Lucero, Metro's design coordinator for the extension works. "Where before we had 250mm of primary shotcrete lining and support in running tunnels, with a 400mm thick heavily reinforced in-situ concrete lining, we now have 150mm of initial lining and 250mm-300mm of final lining for a totaI400mm-450mm."

Another significant development is instrumentation optimisation. The comprehensive instrumentation programme implemented on the previous Line 5 extension amounted to about US$1 million/km of tunnel according to Lucero. Today, the instrumentation requirement is about US$200,000-$300,000/km. As Pozza of Geodata explained: "So much is now known about this ground. We don't need to prove what we already know. We need more to monitor the situation, to check that all is as we expect. Also, movement is so very small - less than 5mm as a general rule in running tunnels. Where in the past we had main instrumentation stations every 50m-100m in the tunnels and around every shaft, these are now placed only in potentially critical areas, and not in shafts in normal conditions. Even for convergence arrays, where we installed cross section arrays every 10m-15m on the original Line 5 extension running tunnels, today they are installed, in normal conditions, at 25m-30m intervals."

Readings taken daily at 3pm are made available by the following morning. "Any less than that and you begin to lose the value of instrumentation and monitoring as a legal tool in the event of settlement damage claims. I prefer 20m intervals as this also relates the deformation field of influence to the tunnel diameters," said Pozza.

The use of expensive slide micrometers and of pressure cells has also been eliminated. "Slide micrometers we don't need anymore, the information already existing from previous experience in the same ground conditions, and intense research proves that tangential pressure cells in shotcrete do not provide reliable results. Radial pressure cells to measure the contact pressure between ground and shotcrete lining are used; occasionally and give better results," said Pozza. "Cells also usually need re-pressurising just at the same moment that you need their data. What we have added to the instrumentation programme for the new works are tangential strain meters in the shotcrete lining from where stresses can be recalculated. We have also made provision for selected instrumentation arrays to remain accessible for long term monitoring."

Twin side wall drift for a station access adit

By contrast, the ARA-Ingendesa-DSC, detailed designer of the first mined stations on the Line 5 west extension, uses ground pressure cells in combination with concrete pressure cells to monitor loading of, and compression in, the lining - one set of readings verifying the other. "Using these pressure cells in conjunction with in-tunnel convergence measurements and surface settlement measurements in specific monitoring cross sections are the most reliable form of instrumentation," said Kurt Egger, Project Manager for DSC. "The cost is by far off-set by the results."

Financial statements
From being constituted in 1968 as a department of the Ministry of Public Works, Metro became an independent government-owned organisation in 1990, mandated to own, operate and maintain the system, extend the network and manage its finances. Since then it has become one of the ten most respected organisations in Chile. In the last financial year it had an annual income of about US$17 million with ticket sales accounting for 90% and 10% from advertising. With this revenue, and without selling land or air space rights to private developers, Metro covers all operating and maintenance costs and is able to contribute 30%-50% to its new capital investment projects. Self-financing is helped in that the relatively new system has low maintenance expenses and Metro is exempt from paying back the initial network construction costs. Still, the financial situation is enviable. Ticket prices are low at an average 47c to travel anywhere on the system, and ridership on the current 40km long operating system is some 58,300 passenger trips/hr average with near 80,000 passenger trips/hr during peak times.

During an interview with Line 4's Programme Manager Vicente Acuña, it was said that Metro paid about 50% of the civil works, M&E, and rolling stock costs of the US$234 million Line 5 extension in the late 1990s with the government meeting the remainder. Today government contribution to Line 4 is about two-thirds with some US$600 million of the estimated US$1.1 billion cost (excluding finance charges) borrowed via a favourable loan with the BNP National Bank of France.

Acuña also said that the metro to date has cost about US$33 million/km, excluding finance costs, including administration fees, import taxes and the 18% domestic sales tax, This he explained is substantially less than the US$80 million-plus cost/km in operation of other systems such as in Hong Kong and Singapore. On producing a copy of the final account for the previous US$234M 2.8km, three-station Line 5 extension in the late 1990s, civil works accounted for some 40%, with: 23% for rolling stock; 3% for administration; 3% for design; 2% for construction supervision; 3% legal services; 3% land and property expropriation; 1.6% for service diversions; 1.4% for imported M&E equipment; 1% for local M&E supplies; and 9% set aside for contingencies. Cost of the 60m²-70m² o.d. Single-tube double-track tunnels was around US$7,000/m, with an average of US$21,000/m for tunnels and mined stations combined.

Junction zone between a lateral access adit and a perpendicular double-track running tunnel

Procurement and cost
To date, procurement of metro civil works has been via 100% detailed design, bid and build contracts with supervision awarded as an extension of the detailed design contracts and different consultants engaged to cover construction inspection. Construction contracts are tendered among a Metro register of 21 pre-qualified contractors and are awarded to the lowest responsible bid once an initial technical tender from each interested contractor has been accepted. Foreign contracting involvement to date is via Chilean registered subsidiaries of large international companies such as Sacyr of Spain and Mendes Junior of Brazil.

In general, construction prices were said to have been going down slowly over the past three years in Chile but the trend now is up in line with other economic indicators. But for Metro, major economies have been realised through design and construction evolution. Today, Carlos Mercado, Programme Manager of the current extension projects, explained that the cost of building the current Line 5 extension is some 60% that of building the previous Line 5 extension. Both are about the same in length, about 3km, and both have three new stations. The differences he said are that the current extension is through better ground conditions than the first; it is through an area of mostly low rise residential buildings; and - the most significant change of all - the stations on the new extensions are all mined as opposed to expensive open boxes.

The second part of this report investigates how design value engineering processes have resulted in major NATM design and construction savings and even greater potential economies on the coming Line 4 contracts.

References

Santiago Metro withstands massive earthquake - TunnelTalk, Mar 2010
Optimized NATM designs for Santiago Metro - TunnelTalk, Apr 2003

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