Shani Wallis, TunnelTalk

Part 2: Current NATM construction for the Santiago Metro, Chile, is costing some 40% less than NATM running tunnels constructed five years ago. Value engineering processes are creating even greater potential economies on the coming Line 4 contracts. How these economies have been achieved and where Metro goes from here on its path of design and construction evolution is investigated.

Technical feasibility of NATM construction in Santiago, as demonstrated by projects in 1994-5 and 1997-2000, gave Santiago Metro's management the confidence to take major leaps forward for the current Line 2 and Line 5 extensions and for the underground sections of the new Line 4.

Station platform tunnels and adit junctions at La Cisterna Station

First, all the underground stations and connecting running tunnels are mined. There are no open boxes, to cause the disruption and closure of public roads endured during earlier open-cut Metro works. Secondly, a combi-shell lining system has been adopted for all underground spaces. Previous NATM running tunnels were finished with 400mm thick heavily reinforced in-situ concrete linings over 200mm-250mm of primary shotcrete support. Running tunnels on the current works are lined with 150mm-200mm of initial reinforced shotcrete and finished with a 250mm-300mm secondary lining, for a total 400mm-450mm shotcrete finish.

These two strategic decisions represent a large part of the reported 40% cost reduction of subsurface metro construction for the current extensions, compared to the 1997-2000 Line 5 extension. In addition, directives for new works require that construction be kept as economical as possible and that underground alignments be "as shallow as possible but as deep as necessary".

As a result, most current excavations are less than one tunnel diameter deep. With top of rail at 17 -18m below ground surface, the crowns of 17m wide x 14m high (150m²) station caverns lie just 6-7m below surface, with 10m-12m cover above 60m²-65m² single-tube double-track running tunnels.

Fundamentally, it is the favourable Santiago Ripio conglomerate that affords the opportunity to excavate such structures but seeing first hand the scale of the openings and the method by which they have been achieved, is impressive. The junction of 150m² mezzanine access adits, with 150m² station platform caverns and 60m²-70m² double-track running tunnels either end make for spectacular NATM excavations and all beneath limited cover.

Quinta Normal Station, Line 5 Extension
Quinta Normal Station at the end of the Line 5 extension is a massive affair. Underground works include a vast 180m long x 25m wide x 27m deep cover and cut excavation, through the bottom of which will be the station of a new suburban rail service. Across the middle of this huge box lies the mined NATM Quinta Normal Metro station.
Designed by the Ingendesa/ARA/DSC JV, concurrent excavation of the metro station and rail station box has been a logistical challenge, when T&TI was on site in February 2003, excavation of the box, between driven-pile supported walls, had split the 140m long NATM metro station in half. Contactor Inela was progressing sidewall drift excavation of the 150m² station paltform cavern to the east of the continuing box excavation.
Excavation of the station platform cavern west by CIL was already complete. Precast concrete elements forming the roof of the box also form the road deck of Matucana Street above. Large machine excavated, reinforced concrete piles supporting the roof will also support the concrete deck that will bridge the metro station and carry the track bed across the open box structure.

These impressive structures are designed by joint ventures between Chilean consulting engineers, and specified a minimum 10% contribution from international NATM designers to incorporate state-ofthe-art expertise. The JVs are Cade-Idepe with Geoconsult of Austria; the Ingendesa-ARA JV with the US and UK offices of the Dr Sauer Corporation (DSC); and the Arcadis Geotecnica-Minmetal group with Bureau de Projetos e Consultoria of Brazil.

NATM excavations for the current Line 2 and Line 5 extensions, comprising about 6km of running tunnels and seven underground stations, lie in good quality ripio above the local ground water table. Line 4 however, takes the underground alignment into a soil configuration dominated by clays, with intercalations of poorly graded (and sometimes completely cohesion less) sand and gravel lenses. In other areas, particularly close to the Maipo River in the south and near the San Carlos Canal on the east, identified ground water in the ripio strata increases the potential for less stable conditions and possible water seepage.

These issues were considered during basic design of the new works by Cade-Idepe/Geoconsult, and detailed designs have since introduced refinements and different philosophies for NATM structures in these conditions. Such differences have considerable influence on constructability and tunnelling crews admit to having favourites among the different design specifications.

La Cisterna Station, Line 2 South Extension

Fig 1 - (clockwise from top) Details of the junctions zones at La Clstema Station, illustrating the increased size of the adits for breakout of lateral platform tunnels; a plan of La Cisterna Station shows its location under Gran Avenida main street; detail of the combi-shell primary and final shotcrete lining system that represents part of the reduction in subsurface metro extension costs

La Cistema Station at the end of the Line 2 south extension, connecting with Line 4, is the 1argest station on the current works from a passenger access point of view. From a large access box, three 150m² passenger access adits extend laterally some 7m under the adjacent Gran Avenida main street to intersect the 150m² x 140m long station platform cavern. Designed in detail by DSC, the large station caverns are being excavated by contractor DPS using the two sidewall drift and central core excavation sequence. Work on this large station contract started in September 2001. When TunnelTalk was on site, excavatin of the two outer access adits was complete, work had started on the middle access adit, and the tunnelling crews had broken into excavation of the perpendicular station platform tunnels. La Cisterna Station with its 500mm of final combi-shell finish (300mm primary + 200mm secondary) is scheduled to be handed over to M&E contractors in September 2003. The two new stations and 2.2km of track on the Line 2 south extension is due to open in December 2004. Subsequent surface alignment construction-of Line in this area will run over the top of the mined Line 2 station structures.

Design differences
A major difference in design is the division of multiple headings for excavation of large faces. For 150m² access adits Cade-ldepel/Geoconsult favours full span top headings, followed by a bench and invert sequence, with a wedge of unexcavated ground in the top heading providing face support and worker access. Under shallow cover, full span top headings require pre-support using spiling or mini-pile forepollng to stabilise conditions and control potential surface settlement. Larger boulders in the ripio had caused drilling accuracy and rod jamming problems for long, small diameter pre-support systems. These were overcome using shorter, larger diameter pipe roof methods, with 140mm diameter pipes installed and filled with concrete prior to excavation advance.

TBM alternative study As testimony to Metro's policy of investigating and introducing new techniques, it commissioned a comprehensive study into the probable costs and feasibility of using TBMs on the 8km-9km of Line 4 running tunnels between Rotonda Grecia and Tobalaba Stations. This was primarily initiated to address excavation options for a section of this route through deposits of clay, which (although stiff) is described as two to three times weaker than the ripio. It was also thought that EPBM technology may be more appropriate through the potentially water bearing ripio close to the San Carlos Canal. Cade Idepe and Geoconsult, as basic designer of Line 4 and detail designers of the clay zone section, completed and presented the most in-depth report of those commissioned. "We started by assuming that the single-tube, double-track NATM running tunnels - pre-supported in the crown and lined with a single 350mm-450mm thick combi-shell shotcrete lining, reinforced with lattice girders on 1m centres and welded wire mesh and/or rebar in between - represented 100% of the cost/m," explained Martin Böfer,Project Engineer for Geoconsult. "Against that we studied three TBM alternatives - first using one 10.3m diameter Mixshield TBM to excavate a single-tube, double-track tunnel in open and closed mode as necessary; secondly using two 10.3m diameter TBMs, an EPB machine for the clay and potentially water bearing ripio zone and an open faced machine for the more stable ripio section south of a Tobalaba portal; and thirdly, two 7.5m diameter Mixshields to advance two single track tubes from the south end" he said. The data studied included TBM delivery and assembly lead times; the cost of the precast concrete segmental lining and production plant; scheduling; station layout design - a major consideration for the TBM alternatives; and the risks to the overall project schedule. Set against the 100% cost/m NATM baseline, all three TBM alternatives came in more expensive and with greater risk to schedule. The single large TBM option was the nearest competitive price at 120% of the NATM approach, but where NATM presented a potential eight-month margin on project schedule, the one large TBM had no margin at all. The next most competitive price was the two large diameter TBMs. at 135% and presented a time margin of about three months. The most expensive option was the two smaller TBM alternative, at 140%, but it did present a time margin of 6.5 months. "To estimate time schedules we assumed advance rates of 14.5m/24h day/machine for the large diameter single-tube, double-track TBM options and 3m/24h day/heading for the NATM approach," said Böfer, Having commissioned the study, Metro satisfied itself that with NATM it has selected the most appropriate approach for all underground works in the geological conditions beneath Santiago. For the first detailed design of 150m2 underground station caverns DSC introduced the two side-wall and central core drift sequence, with each drift excavated on a top heading, bench and invert sequence (Fig 1). This approach eliminates the need for forepoling or spiling under shallow cover and allows rotation of equipment and crews between concurrent faces of advance. Cade-ldepe/Geoconsut has also adopted the twin sidewall drift sequence for the 150m2 station platform caverns for the Line 2 north extension stations. For its Line 4 detail designs, Bureau de Projetos with Arcadis Geotécnica-Minmetal has adopted single side-wall drift sequences for the large 150m2 station caverns (Fig 2), with face support wedges, systematic forepoling or spiling pre-support in zones of potentially unstable ground, and 'elephant's foot' foundations for lattice girder arches. A particular detail design refinement concerns the invert area in the junctions between lateral access edits and perpendicular station platform tunnels. Initially, the two inverts were at the same elevation, requiring partial breakout of the access adit invert to allow for a continuous closed invert in the subsequent platform tunnel. To reduce demolition efforts, DSC increased the size of the access adit in the 17m-18m long junction zone to 165m2 to provide a permanent invert in the adit and to create space on the adit sidewalls for breakout of lateral platform tunnels with permanent inverts of their own (Fig 1). The extra space between the lowered access adit invert and the platform tunnel cross section will be backfilled either with compacted earth or with concrete creating the eventual track bed and station platform structures. Geoconsult on the other hand, has designed a permanent invert for the access adit and a step down to the closed invert section of the lateral platform station tunnels (Fig 3). Bureau de Projetos has adopted the same ptinciple. A more fundamental difference in NATM philosophy relates to the installation of lattice girders and their contribution to long term support and stability. DSC and Bureau de Projetos design lattice girders with the wide edge of the triangle against the tunnel wall, while Geoconsult places the pointed side against the wall. Each method is preferred for the same reason - to help the nozzle man or robot boom to spray at various angles to prevent shotcrete shadows or voids at the girder locations.

With regard to the long-term stability, DSC believes that lattice girders contribute minimal load bearing support and could be omitted from the primary lining, with steel fibre providing extra strength or load bearing support where necessary. Engineers in other design groups agree in certain instances. As Martin Bôfer. Senior Design Engineer with Geoconsult explained: "The arguably limited contribution of lattice girders to short term load bearing capacity of the primary lining is an advantage which is highly appreciated. We must also design for the possibility of earthquake activity during construction, which requires a margin of stability to allow primary lining to stand for 1-2 years before the final lining goes in. If you remove the lattice girders, their contribution to the load bearing safety factor would have to be substituted with extra layers of wire mesh or steel bar reinforcement. Steel fibre has been considered for Line 4 contracts and in principle, whenever structural efforts do not require wire mesh or steel bar reinforcement, steel fibre is preferred, but we would use it only in the running tunnels. In our opinion bending moments would be too great for its application in station cavern excavations."

Fig 2. Design and excavation sequence of 150m² platform tunnel caverns for Line 4

Engineers within Metro are yet to be convinced and girders remain in all NATM designs. In addition to their arguably limited contribution to load bearing requirements, girders also help maintain a correct tunnel profile. Although this could be achieved by other means, the purpose is important in Santiago. "If the distance between the girders is increased from just 1m to 1.2m centres in this ground," said Otto Gerstgrasser, NATM site supervision engineer for DSC on the Line 2 south extension, "the shape of the profile suffers; and shotcrete applied to fill excavation outside the design fine is a cost to the contractor."

To date, the largest percentage of shotcrete by far in Santiago is dry mix with nearly all contractors on current contracts using Aliva dry mix rotor units and hand-held nozzles. According to Pedro Honorato, one of two Sika/Aliva agents in Santiago, there are some 30 Aliva units working on current metro contracts. Things may change however on Line 4, where wet mix shotctete with steel fibre reinforcement may be specified to improve quality, durability, water tightness, air quality during application, and production cycles. To date Sacyr is the only contractor using wet mix shotcrete, working with two Putzmeister shotcreting rigs.

Permeability of shotcrete is a significant concern, particularly on Line 4 where water ingress and seepage is expected to be higher than on previous NATM contracts. In theses areas Metro was advised to include positive waterproofing systems such as PVC geotextile membranes or spray on waterproofing barriers such as MBT's Masterseal 340F. Metro however, in efforts to control costs, believes that water ingress levels will be manageable as a maintenance issue, that the high content of fines and particles in the ground water will self-heal leaks and seepage points, and that the quality of concrete linings can be specified to prevent heavy ingress or seepage. To this end, designers have specified the use of microsilica in shotcrete mixes to meet an average maximum water ingress of 20mm on probes when exposed to 7 bar water pressure. The in-situ concrete lining on the first 1997-2000 Line 5 NATM extension, close to the Mapocho River and in the ground With perched water lenses, was designed by Cade-ldepe/Geoconsult to meet a watertight requirement. The 400mm-450mm thick lining, reinforced with 100-120kg/m³ of rebar, limited tensile strain due to shrinkage to within O.5mm-6mm/m instead of the O.3mm/m requested by the standards.

Workmanship, quality, productivity
Productivity and rates of advance are a concern for all those working on, and anticipating construction contracts on, the 33km long, 27-station, Line 4 (some 15km and 10 stations of which are underground in NATM excavations). Vicente Acuña, Metro's Line 4 Programme Manager, described the work to be achieved to meet the promised third-quarter 2005 in-service opening date as "epic".

To meet the tight construction schedules, there is a push from contractors to relax NATM specifications and allow excavation rounds of up to 2m. This is resisted by designers, who might in general agree with increasing round lengths, but on a progressive approach and only if reasonable in view of the ground behaviour. As Gerstgrasser, DSC site supervising engineer explained: "Production could be increased by as much as 50% by leaving round lengths as they are and making improvements in construction methods and logistics." For example, he suggested, the excavation cycle could be improved by using purpose designed tunnel excavators. At present contractors are using standard backhoe machines that have no 360° articulated buckets and require profiling by hand. There are also problems with muck handling. Muck gets stockpiled behind tunnel faces waiting to be lifted to the surface for permitted off site muckhauling schedules. "There is little point in increasing production without first addressing muck handling issues," said Gerstgrasser.

NATM stations in clay

Fig 4. Binocular configuration design for the Plaza Egaña and Los Orientales stations located in a clay zone near the Mapocho River

For the two underground stations located in the clay dominated deposits on Line 4 (Plaza Egañ and Los Orientales, Cade-idepe and Geoconsult have designed a binocular configuration for the two 140m long stations located in the clay zone. Volume losses and surface settlements are expected to be higher then in the ripio, at up to 30mm on the runing tunnels. To control deformation in the stations, sequenced excavation will start with a 10m high x 6m wide pilot heading, into which a series of 1.1m diameter permanent central support columns on 4.7m centres will be cast, working back to the access point.

This will be followed by excavation of the binocular tubes either side, progressing together on a staggered advance and on a top heading, with temporary invert and bench/invert excavation sequence. Beneath an average cover of 8m-9m, the station cavern will be supported initially with 300mm of reinforced shotcrete and finished with a 200mm thick in-situ concrete lining.

Production cycles would also improve by using higher output wet-mix shotcrete nozzle booms and mobile units. In conjunction with purpose designed excavator machines, more headings could be developed. At present, advance in many cases is limited by logistics of one or two headings.

While there are these areas presenting room for production improvement, it must be said that the quality of the workmanship on display in Santiago is impressive, with contractors undertaking NATM work with very little or in some cases no previous tunnelling experience.

Commenting on the differences between detail designers, Acullña said that it is from these differences that Metro will establish its own set of design standards and norms for future works, "We believe we have made a good compromise between speed of advance, application of modern technology, cost and quality," he said, "and that we have achieved good optimisation between construction needs and permanent works requirements. Our efforts to research cost savings and improve quality and construction rates, however, will continue as we progress network expansion projects."

References

Santiago Metro withstands massive earthquake - TunnelTalk, Mar 2010
Santiago Metro goes underground with NATM - TunnelTalk, Apr 2003

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