The initial article on Taiwan’s first BOT project was published in the November 2001 issue of T&TI with a follow up article on the design of the tunnel lining in September 2002. To sum up, the basic features of the project are; a high speed rail line stretching 345km almost the length of the 394km long main island between terminal stations in Taipei the capital of Taiwan in the north and Kaohsiung in the south. Initially six intermediate stations are planned with more to be added later (Figure 1). The high speed rail link is designed for trains travelling at 350kph and will have an operational speed of 300kph.
Commencement of operations is planned for October 2005 and the system is expected to become fully operational by 2006.
The civil works contracts include:
These elements account for approximately US$5.3bn of the total project construction cost of US$12.3bn, excluding land acquisition and financing costs.
For the civil works portion of the project, the design-build project delivery system was used to construct the mainline guideway. This work was divided into 12 contracts, let as self-certification contracts, and executed by JV teams comprised of both national and international designers and contractors. Typical contract periods range from 36 to 42 months. Six of the twelve contracts have mined tunnels and are generally located within the northern half of the project.
The civil works contracts were awarded from March 2000 with track work scheduled to commence approximately 30 to 36 months following contract award. As of September 2004 all tunnel construction for civil works are completed with no delay to the overall project schedule.
This is the largest known Design-Build BOT project, which has successfully demonstrated that Design-Build contracts can be used as an efficient delivery method for underground construction works.
Project summary
There are 42 mined tunnels on the project ranging from approximately 92m to 7360m in length, with overburden up to approximately 115m and medium to high groundwater tables. Typically 40-50 of the approximately 80 planned tunnel drive faces were being excavated at any given time during the project. The 42 mined tunnels result in a total of 39,050m of mined tunnel excavation. The number of tunnels may be subdivided into four groups as indicated below.
For completion of the works, THSRC required that the JV contractors prepared a baseline critical path programme supported by a requirement for time-chainage schedules. These schedules allowed visibility of how the linear tunnelling operations interact with time and access thus providing robust plans to execute the construction.
Accordingly, for the tunnel works, the management of the work for the main drives resulted in approximately 5.5Mm³ of excavation which, allowing for a bulking factor, corresponds to an estimated 7.3Mm³ of spoil to be handled. These estimated volumes do not account for works associated with the ancillary adits and shafts.
Tunnel excavation began in November 2000 and essentially all tunnel headings were completed by July 2003, 21 months later. This was achieved at an approximate average project rate of 77.6m per day or approximately 2m per day per drive face. Peak rates of up to 10m-12m per day were achieved on some drives on occasion.
Accordingly, approximately an equivalent 40,160m² of sprayed concrete lining (shotcrete) was placed per month to support the tunnel heading excavations.
The excavation of the drive was typically advanced in three stages; heading, bench and invert. Table 1 provides the percentages of drive excavation per stage and the project rates.
To maintain the project schedule, the permanent reinforced concrete lining began in November 2001 and continued through until March 2004 for the main drives. In some cases more forms were ordered by the contractors to meet the schedule. This resulted in casting approximately 1.1Mm³ of reinforced concrete tunnel lining. An average production rate of casting 67,530m³ per month was achieved during the prime execution of the work. This resulted in an approximate 12m long block pour of one block per day per form on some contracts where the contractor had an efficient system for lining production.
The concrete mix design strengths were typically 50% greater than that required by specification of 280kg/cm² (f’c). Form stripping was typically allowed once the concrete achieved 15% of f’c (42kg/cm²), which allowed the form striking to take place between approximately 12-16hrs after casting.
Concrete strengths were normally determined by in-situ or laboratory testing prior to form striking. Twenty eight day concrete strengths were often in excess of 400kg/cm².
Generally, tunnel excavation and lining installation followed the proposed and approved contractor schedules which were based on the overall project programming requirements to meet specified completion dates for trackwork contracts to commence. THSRC reduced the risk to programme overruns by adopting a milestone performance strategy, which required the contractors to meet production milestone dates for payment. Of particular note is the fact that the tunnel excavation generally exceeded planning expectations.
Tunnel geometry and construction method
Approximately 48km of the railway line is constructed underground in mined tunnels, cut and cover tunnels, or underground stations. The tunnels are twin-track, with track centres 4500mm apart, typically having a finished area above rail level of 90m².
A standard finished tunnel cross-section profile was typically used throughout. This comprises a circular upper arch (“vault”) with a constant inner radius to the walkway level, and a lower arch (“invert”) below the walkway. In the case of the most common tunnel size (90m²), as shown in Figure 2, the inner radius of the vault is 6250mm with its centre point located at a height of 2450mm above rail level. The finished span and centreline height is approximately 12,500mm and about 10,500mm for the largest tunnel.
To provide the finished tunnel area and the design thickness of the inner and outer linings, as well as an allowance for deformation of the ground following excavation, the theoretical excavation area varied from about 135m² to 155m², depending on the lining designs. Accordingly, the sequential excavation and support (SES) construction method was selected for the advancement of the tunnel. For such an area, excavation with a TBM, although allowed under the contract, was considered impractical and uneconomical given the number of shorter tunnels.
In view of the large excavation sizes, the tunnel cross-section was excavated in stages. Initially a top heading with an invert on or just above the tunnel spring line was excavated and stabilised, if necessary with a temporary invert of shotcrete to form a closed support ring (Figure 3).
At a variable distance behind the face of about 40m to 150m, a bench was taken out below the top heading and supported. The final excavation of the floor, or invert, was taken out and supported, fairly close to the bench face, depending on the ground conditions. The closed support ring around the full tunnel section comprises the outer lining, which must stabilise the excavation before the inner lining is placed.
During excavation, survey displacement monitoring stations were installed close to the excavation face, typically at intervals of 10m to 20m, to measure the settlement and convergence of the outer lining and provide verification of the design of the outer lining. In poor ground conditions the spacing was reduced to as little as 5m.
A review of the completed top headings and the corresponding monitoring data recorded, observed that in general, on most of the drives, the tunnel crown settlements and top heading convergence has been of the same order as that predicted. In a typical medium overburden case of 20m-40m with timely and correct installation of support and relatively dry conditions, crown settlements have stabilised within 60m from the top heading face at about 20mm-40mm displacement of the outer lining. Convergence has, in general, been small and of the order of 20mm-30mm maximum.
However, in some headings, movements have been relatively very high and of the order of 100mm-200mm, typically associated with higher overburden and/or lower rock mass or material strengths as well as the effectiveness of groundwater control measures.
Geology and outer lining support
Taiwan sits on the convergent boundary of crustal plates and is very seismically active. The central and eastern, more mountainous, parts of Taiwan result from the folding and thrusting that continues to occur where the plates collide. The tunnels of the THSR Project are confined to the northern half of the island and are located along the lower lying western strip of the country, which represents the foothills of the mountain range.
The majority of materials encountered in the tunnels are relatively young (Miocene to Quaternary) sedimentary materials ranging from silts and clays, through sands and gravels, to bouldery conglomerates. These materials generally represent the depositional products, both terrestrial and marine, of materials eroded from the mountains. They have been variably compacted and locally lightly cemented, and generally range from dense or stiff soils to ‘soft’ rocks of up to a few MPa in strength.
None of the tunnels intersect faults that are known to have been active in the last 10,000 years (Holocene), but some tunnels do intersect faults which are known to have been active in the last 100,000yrs, or are classified as being “suspect active faults”.
Groundwater has a significant impact on the behaviour of the excavation and particularly within soil tunnels. Dewatering, was typically achieved using face probes, however surface drilled wells, typically spaced at 25m to 50m intervals on either side of the tunnel along with advanced inclined horizontal drainage probes installed at the face, have been necessary on occasion to substantially dewater some tunnels.
Standard immediate support comprises a combination of plain shotcrete reinforced with one or two layers of welded wire mesh, lattice girders, and untensioned grouted bolts (“dowels”), termed herein as the “outer lining”. If required by local conditions, additional (“auxiliary”) measures comprising spiles and forepoling, long large diameter forepoling (“pipe roofing”), top heading shotcrete footing widening (“elephant footings”), footing micropiles, jet grouting, face bolting with fiberglass bolts, face sealing with shotcrete, leaving a supporting core at the inner part of the face, and placement of a temporary shotcrete invert in the top heading, were also implemented.
The designed thickness of the outer lining shotcrete varies, depending on the ground mass strength in relation to the overburden, from a minimum of 175mm to 350mm in the heaviest support classes. Bolt lengths typically vary from 4m to 6m, and lattice girders were spaced at one per round length, which varies from about 1.8m in the best ground to about 0.5m in the unfavourable ground conditions. Lattice girders comprise three-bar types with depths between 70mm and 130mm, depending on the ground class.
Response to loss of drive stability
As with all tunnel construction, there is always an element of risk due to unknown conditions and/or workmanship. Typically these events usually correspond with weekend/holiday or evening/night shifts when the more experienced personnel are absent from the face or the contractors are in the early stages of understanding the particular nature of the ground currently being driven and its response to the stress relief resulting from the excavation.
This project was no exception. The magnitude of the recoveries required varied from small localised events to larger occurrences, which required methods to encapsulate the drive to prevent further risk of breech to the previously installed outer lining works. Methods included general ground stabilisation techniques incorporating ground injection with Ordinary Portland Cement (OPC), polyurethane and silica gels to complete drive isolation utilising double or triple pipe roofing or horizontal jet grouted columns (dowels) to provide a canopy over the recovery portion of the drive.
Recovery methods were generally developed in joint cooperation between the contractor, designer and owner’s representative to develop an agreed approach geared for both safe resumption of the excavation works and project program requirements. Generally this was achieved by “fast-tracking” the design and was typically accomplished by breaking the design process into discrete steps, which closely mirrored the construction activity using similar equipment and the work force on site allowing the remedial construction works to proceed without undue delay.
Considering the number of tunnels undertaken in the short time frame and in particular, the number of drive faces open at any given time, the number of events that occurred is considered small in relation to the size of the project.
Of particular note, once a remedial approved drive recovery method was identified and progressed, there was no subsequent loss or instability of the drive or to sections in proximity to the recovery works. All tunnels were completed on or ahead of programme.
Inner final (permanent) linings
All tunnels were required to be provided with a final (“inner”) cast-in-place concrete or segmental lining. All the contractors opted for the former. Reinforcement is required in the inner lining for crack control according to ACI224, even if not required for structural reasons. An additional benefit of such minimum reinforcement is the containment of the inner surface concrete layer during a seismic event. The minimum reinforcment could be placed in two layers, one at the inside face and one at the outside face, or as one layer at the inside face only. Spacing of the bars was to be at 150mm maximum.
Depending on ground conditions, drainage and groundwater pressure, the thickness of the final lining varies considerably, from 300mm to 600mm for the vault and from 450mm to 1500mm for the invert lining. Design standards and codes of practice that were generally used for design of the linings were ACI 318, the Eurocode EC2 or the British Standard BS8110.
The design specifications required that “The tunnels shall be constructed to be completely dry” and the majority of the designers have used the German code DS 853 Class 3 as the objective criteria for evaluating compliance with this requirement.
There are essentially two types of tunnel lining designs for the project; drained and undrained. Drained tunnels were required to have longitudinal groundwater drain pipes along each side of the tunnel with maintenance access at 50m intervals and a waterproofing membrane over the vault area. Undrained tunnels were required to have a waterproofing system around the full circumference of the tunnel (fully tanked). For either type of tunnel the waterproofing membrane was 2.5mm PVC and isolating waterstops were required at bulkhead locations to prevent water leakage from extending beyond the space between the bulkheads.
Where contractors proposed drained tunnels, all used perforated pipe with either a gravel pack or no fines concrete for collection of the groundwater. The pipes are located near, but always below, the longitudinal construction joint at the interface between the crown lining and the invert ‘corner’.
Related Files
Figure 3 – Standard outer lining schematic cross-section with short longitudinal section for excavation and support sequence
Figure 1 – Taiwan map and general alignment
Figure 2 – Standard tunnel cross-section with lining
