Often called ‘The most isolated city in the world’, Perth lacks many of the characteristics of a metropolis that desperately needs to exploit its underground space. The capital of Western Australia, its suburbs are characterised by large houses and wide, relatively empty roads.

But there are many reasons to invest in public transport. In the case of Perth, a new rail line from the CBD out to the airport and the city’s eastern extremities will service a growing population. This AUD 1.86bn (USD 1.27bn) Forrestfield- Airport Link will also help to revitalise an area that has been economically disadvantaged in recent years. The line itself will spur off the existing Midland Line, near Bayswater Station, and run to Forrestfield through twin bored tunnels.

As for the reasons to go underground, the most important of these is environmental. Perth’s iconic Swan River, called ‘Derbarl Yerrigan’ by the local Aboriginal community, is considered a sacred, spiritual place. Another bridge was not acceptable.

This is where the Salini Impregilo-NRW joint venture comes in, and at the time of Tunnels and Tunnelling’s visit in October 2019, the first TBM had completed its river crossing.

“This was thought to be a critical part of the project by local media, but in reality, the water table is so high, we have been under water for the entire alignment anyway,” says Richard Graham, project director for Salini Impregilo-NRW.

Scope of works

  • 7.5km of 7.1m excavated diameter running tunnels
  • 1.2km of dive ramp structures
  • Three stations, two underground with a length of 150m each
  • 12 cross passages
  • Three egress 9m-diameter shafts with a maximum depth of 40m
  • Track work (2x9km)
  • Traction Power & OLE
  • Full MEP works both for stations and tunnels
  • SCADA and all other systems
  • Signalling and communication systems
  • Integration and testing
  • 10 years of maintenance

Project timeline

Project approval: August 2014 
Contract award: April 2016  
Start of tunnelling: mid-2017 
Arrival at Airport Central Station: mid-2018  
Tunnelling under runways: early-2019  
Arrival at Redcliffe Station: mid-2019  
Passing under Swan River: late-2019  
Arrival at Bayswater Junction: mid-February  
Fit-out: 2020-2021  
Commissioning: H2 2021

The two 7.1m-diameter tunnels run for 7.5km through the sedimentary deposits and rocks of the Swan Coastal Plain and Perth Basin. The height of the water table along the alignment varies between 1.5 to 3m below ground level, with seasonal fluctuations of potentially as much as 2 to 3m.

The main geological units encountered (in order of excavation) along the tunnel alignment are (see box 3 for further details):

  • Guildford Formation (incorporating three different sub-units): clayey sand, silty sand and fine to coarse grained sand, medium dense to very dense.
  • Gnangara Sands: silty sand and sand, loose to dense.
  • Ascot Formation: sand, sandy gravel and gravelly sand with shell fragments, medium dense to dense.
  • Osborne formation (Mirrabooka Member): sand, silty sand and clayey sand, medium to coarse grained, dense to very dense.
  • Osborne Formation (sand dominated): silty sandstone, fine grained, layered, moderately weathered to fresh, extremely low to medium strength.
  • Perth Formation (sand): sand to silty sand, fine to coarse grained, medium dense to dense.
  • Perth Formation (clay): clay, some silt and sand generally low to medium plasticity and generally stiff to hard.

For excavation, the client specified two Herrenknecht Variable Density dual mode (slurry EPB) TBMs. There is a contract requirement to operate the machines in slurry mode using highdensity slurry while inside the airport footprint (approx. 50% of tunnel).

Readers will probably be aware that Herrenknecht’s Variable Density machines were used the first time for karstic ground conditions. But while the Ascot Formation was expected to have some voids and very high d10 parameter, these have not been encountered. Nevertheless, consultants engaged by the stakeholders saw this TBM as a good and safe option. The use of the High Density Support Medium (HDMS) provides advantages in general, not only for karstic limestone formations or a network of crevices and fissure, but also in highly permeable ground conditions. The thicker suspension and the higher viscosity result in a helpful alteration of the pressure gradient thus reduced penetration depth and capillary rise of the suspension, therefore a reliable tunnel face support and a high flexibility for managing geological and hydrological challenges.

The contractor lowered the alignment by 5m under the airport runways to keep it in the more stable Osborne Formation, and mitigate the risks of unforeseen settlements.

Considering the project requirement to tunnel under an active runway, a conservative approach is understandable.

The TBMs are 130m long and are split into nine backup gantries.

Further up the machine, the cutterheads were fitted with 18 no. 17” monobloc cutter discs.

The contractor has alternated the arrangement of cutter discs and 160x100mm rippers based on ground conditions, to manage wear.

Further to that, the TBMs were upgraded with additional slurry pumps to increase the flushing flow rates at the excavation chamber and slurryfier box when in clay formation.

The tools of the sizers (rotary crushers) were replaced midproject to a carbide-tipped crusher picks to deal with clogging ground conditions. These new picks are forcing the clay to ‘flow’ in between of the two rollers instead of blocking the sizer.

The STP was also upgraded with two additional centrifuges to help the filter presses treating used bentonite and the frequently very high quantity of fines.

The Salini-NRW JV has managed to get a good rate of production out of the machines despite the varied conditions and at the time of Tunnels and Tunnelling’s visit, daily production was around 9-12 rings with a peak of 21 rings and a weekly record of 103 rings.

The chemicals used (apart of those normally used in bi-component grout, waste water treatment plant, RO plants and STP) have been:

  • Viscosifier agent to increase the viscosity of the bentonite when required. Used in gravel and sand formation as a contingency in case of drop of face pressure or leak of slurry through the face.
  • Anti-Clay polymer: Used to clean the cutting wheel and excavation chamber from clogged clay, especially before a cutter head intervention.

Site setup

The TBM auxiliary plant installations at Forrestfield covers about 50,000m2. From here, nine pipelines run into each tunnel to supply slurry, compressed air, grout, industrial water, cooling water and provide dewatering (a total of approximately 140km of pipes by the time two TBMs reach Bayswater), plus cables for high voltage and low voltage power, telecommunication and fibre optic cables.

Spoil is transported 5km by trucks to a temporary stockpiling treatment area that is 155,000m2 in size. A number of initiatives to beneficially reuse excess fill from the Forrestfield-Airport Link are being investigated. Transfer of soil from one site to another is routinely undertaken on land development and infrastructure projects in Perth and around Australia. To date, approximately 25 per cent of the project’s excess fill has been earmarked for reuse as engineered backfill on transport projects across Perth, including within the Forrestfield-Airport Link site, within two road interchanges on the NorthLink project and at the Kenwick Rail Freight Facility.

A segment casting factory, with a carousel system with 54 moulds (nine sets of six moulds), provided by Euroform was set up by the contractor near Forrestfield. Rings are steam-cured for six to eight hours at a temperature of between 50- 70°C. Segments are demoulded when they reach 20MPa, before being coated with two layers of epoxy on the extrados for waterproofing reasons and corrosion resistance against some potential harsh ground conditions. Once casted, the segments underwent an extensive curing process in 100% humidity to obtain the highest possible durability.

Each of the approximately 54,000 segments for the 9,000 rings is identified by a barcode sticker and stockpiled across the project. The barcode carries the whole story of the segment from its construction to the location of storing to that of actual installation in the tunnels.

Some 88,000m3 of concrete was used in total and the mix-design was developed based on triple-blend cementitious materials. This mix and the very high quality standard precast factory were recognised with an award from the Concrete Institute of Australia.

Lining

The segmental rings are of a 5+key configuration, are universally tapered and are 300mm thick and 1,600mm long for an OD of 6,770mm and an ID of 6,170mm. Two reinforcement categories are in use on the project. Type 1 is reinforced with 35kg/m3 of Dramix steel fibres and has a light perimeter rebar cage. It is in use for the standard running tunnel lining and accounts for 95% of the segments. Type 2, which is reinforced with 130kg/ m3 conventional rebar cages and 35kg/m3 of fibres, is for cross-passages and any tunnel connections with dive or station structures where the lining loading is higher. Polypropylene fibres in dosage of 2kg/m3 were added to ensure fire resistance compliance for both segment types. These were tested, on full scale segments, in MFPA Leipzig laboratories.

The segments have double convex radial joints, dowels and are double-gasketed with soft EPDM gaskets with compressible corners from Fama, for both radial and circumferential joints. The segments are also sealed with a foam joint tape. This is to help prevent the grout (or groundwater) entering the tail skin brushes and to avoid grout or slurry leakages into the TBM during the ring build. There is no impact on the finished ring.

Hard wood packers, which were 2 to 3mm thick after compression, were applied to both radial and circumferential segment faces.

Bolts were used to build the rings, but these will be removed prior to commissioning. While the reason is not stated explicitly in the documentation, but this is typical for modern fast railways, as the air bubble pressure can loosen bolts over time which could potentially cause problems for the end user of the asset. Bolts either had to be stainless steel (durable) or fully removable.

Cracking of the segments has been negligible, and apart from a few broken corners, owing to the early gasket problems, the segment build on the project is particularly neat and clean, with no visible cracks, gaps or spalling.

All underground structures, particularly station boxes, needed to be waterproofed with a composite waterproofing system that involved a PVC membrane and protective geotextile layers with custom made high durability details for all terminations. Groundwater also needed to be controlled with wells. However, the cross passages required extra attention.

Cross Passage types

According to the contractor, the cross passages are the riskiest part of the project (apart from the low-probability, high-impact risk of settlement under the airport runway). The challenges relate to groundwater management, ensuring the strength and stability of the soil (especially in wet, unconsolidated ground), and the risks to project schedule, when you consider that works can only start after the completion of the tunnel and the onward movement of the TBM to some distance.

The 12 cross passages along the alignment have required several ground support techniques.

Six have been jet-grouted, requiring a rig on the surface that is appropriate for sandy clays to fine gravels, producing a fluid jet to break up and loosen the ground, before backfilling with grout. The design has opted for interlocked columns of 2m diameter in sands and gravels or 1.5m in clayish soils. Jet grouting was carried out by highly specialised Italian subcontractor Trevi with a Soilmec SM-28 rig.

Three located in harder rock with low permeability, in this case the silty sandstone with a UCS of 2-4MPa, relied on forepoling. An umbrella arch consisting of nine 24mm pipes grouted into place and supported by lattice girders is enough to ensure ground stability.

The three final cross passages are located between the airport runways and have required ground freezing, owing to the lack of surface access above the passages and the ground conditions, which would otherwise have led to jet grouting. This was carried out by Trevi with CDM Smith as a freezing designer, and the principle is to extract heat until the groundwater freezes, creating a ring of ice around the cross passage excavation area. On the Forrestfield- Airport Link project, the ‘slow brine heat exchange’ method was chosen, which involves a closed circuit that requires the use of an industrial refrigeration plant connected to a coolant system. The ice wall is achieved by circulating brine through freezing pipes made of two coaxial tubes: the outer tube has a closed end, while the inner tube is open. This method was considered being the safest for both people and the environment of tunnel.

Airport Monitoring

The project made use of a web-based Monitoring Information Management System (MIMS) which incorporated all monitoring from the project from over 8,000 instruments along the tunnel alignment. This covered settlement, excavation, groundwater and structural monitoring.

The airport itself had a more unusual monitoring arrangement. In what the contractor claims as a global first, a system was set up using Class 3R lasers aimed at reflector-less targets from a network of 18 automated robotic total stations, observing at incident angles greater than 85°. This required automated instruments able to detect and validate (in near real time) displacement trends and necessitated compliance with the stringent regulations and safety controls of an airport. During nearly two years of discussions with the aviation authorities, it was even necessary to look at the possibility of a laser reflecting into a pilot’s eye as a plane approached the runway to land.

It was also necessary to take manual readings at times when no planes were landing (between 2am and 4am) to double-check and validate the continuous automatic monitoring that had to take place around the clock.

Tunnelling challenges

Local

Unlike the eastern cities of Australia, Perth has not undertaken tunnelling on such a scale before, so the contractor needed to bring in specialist people and train others from different trade and industry backgrounds. As with all tunnelling works, each person needs to be situationally aware of what other team members are always doing and the risk assorted with both their work and that of their team members. This is more prominent than in the largely open-cast mining industry of Western Australia, which much of the local workforce is more familiar with. A tunnel is a linked system that requires consideration of what is 3km up or down the pipe, and this calls for a different mindset, rather than just focusing on the safety of your immediate area.

This requires continuous training and drilling and while staff retention has fortunately been good so far, this might become more of a challenge with the amount of tunnelling work picking up on the east coast (see Tunnels and Tunnelling International, January 2020 for an article on the Sydney Metro).

Another challenge has been the security that goes along with working in the airport environs. There is a safety and security committee that meets regularly. More than 20 people from the emergency and security services gather to discuss any potential issues. An airport is a federal installation and there needs to be constant co-ordination between federal and state authorities.

The project also places a number of restrictions on site operations to reduce the impact of the works on adjacent neighbourhoods. These restrictions include:

  • Noise restrictions between 7pm and 7am and all day on Sundays. This impacts the project’s ability to work at a full capacity 24 hours/day. Overnight works are restricted to reduce noise impact.
  • Vibration restrictions to avoid damage to nearby properties.

Works next to the ecologically sensitive Munday Swamp and Swan River also have restrictions around dewatering and other water management. This has necessitated a number of licenses and a strict monitoring programme to reduce the risk to nearby ground and surface water. Finally there are restrictions on the height of equipment on site, due to the airport.

Sinkhole

On 22 September 2018, a leak developed at the first cross passage, located approximately 200m north of the Forrestfield Station site. The leak grew to 100mm in size resulting in water and silt entering one of the tunnels and leading to the formation of a 1m-diameter, 700mm-deep sinkhole alongside Dundas Road. The pressure of the groundwater, and the associated loading, also caused localised distortion to the tunnel shape and movement of the segments which make up the tunnel lining, ultimately damaging a 26m-long section, or 16 rings. The water inflow was stopped and the situation stabilised a few days later following an intensive campaign from inside the tunnel.

The designer for the solution of the retrofit is Mott MacDonald, who proposed to partially and locally remove approximately 120-150mm of concrete off the most affected rings in order to comply with the space-proofing (i.e., the survey and study done to verify that the train can pass in the damaged section of the tunnel at the correct speed) and install in its place a spheroid graphite iron (SGI) lining composed of 800mm-wide rings. The annulus between SGI and remaining concrete lining is then grouted.

For subsequent cross passages, to prevent possible reoccurrence, the following measures were adopted:

  • Applying additional contact grout to replace annulus grout around the exterior of the tunnel
  • Applying additional grouting in the block where required
  • Installing additional bolts in the tunnel rings adjacent to cross passage construction
  • Installing a dewatering system

Clogging

Aside from the Airport Central Station, the main challenge has been clogging. The encountered geological formations proved to be complex. The slurry pumps have had a lot of variations in pressure on the lines – a pump could be running quite normally and experiencing a normal load, but then all of a sudden there is a spike in the pressure and it jolts the system. Machines and electrical systems do not like jolts, so efforts had to be made to flatten these out by making modifications to the PLC to try to synchronise all the pumps, and all of the other variables of a slurry system.

As mentioned previously, additional flushing was also required – flushing to the excavation chamber has been increased (after TBM slurry system upgrade) from 250m3/h to 800m3/h per ring – to help reduce the clogging.

To reduce the viscosity of the slurry (when in clay) was required to also add 20-40m3 of water to the excavation chamber as well.

During the whole project the TBMs are being operated in slurry mode, allowing the face pressure to be maintained by slurry. There are different slurry KPI ranges (Yield point, filter loss, viscosity, working ranges are different depending on the ground formation) to be used depending on the ground condition.

In some of the clay formations the slurry properties are higher than the upper KPI range (very high viscosity), therefore the slurry requires dilution with water in order to return to the normal KPI range. This requires large volumes of slurry disposal at slurry treatment plant.

To manage the increased volumes, the plant was upgraded with two centrifuges, in addition to the four filter presses, to increase the treatment of disposed slurry (used bentonite).

Without these, the plant would have become clogged and overflowed, and been unable to keep pace with any reasonable TBM production rate.

Lessons learnt

During at a presentation in Sydney in late 2019 the contractor reminded the audience of a number of obvious, but often overlooked, aspects:

  1. Theory vs. actual – It doesn’t matter how good and accurate your Geotechnical information is, you will only fully understand how your assumed TBM / Plant / Slurry performs once the entire excavation ‘system’ is exposed to the actual geotechnical conditions.
  2. Slurry – slurry characteristics, which must be defined after extensive testing on the foreseen geological conditions, must constantly be under review and adjustment in order to respond to sudden changes in ground conditions.
  3. Process plant – The entire excavation process is effectively a process plant, and thus must be designed and operated as such. The TBM, slurry plant, water and waste water treatment plants, grouting plant, process piping and pumping systems must all be fully integrated and synchronised to work efficiently and safely.
  4. Flexibility of plant – The assumed TBM / plant / slurry ‘system’ should be designed with inbuilt potential flexibility (if not redundancy) to enable modification to manage actually encountered conditions. In the case of Forrestfield increased capacities of water and wastewater treatment plants, two added centrifuges for increased fines, etc.