Red steel poppies lining Arras Tunnel remind travellers on State Highway 1 through Wellington that they are in a memorial space below Pukeahu National War Memorial Park, the Government’s centrepiece WWI centenary project.
Both tunnel and park opened well in advance of their respective deadlines and hosted Anzac Day centenary commemorations on 25 April 2015 when 60,000 people gathered at dawn in the park to honour those who have served at war.
Arras Tunnel and Pukeahu Park in central Wellington were designed and constructed by the Memorial Park Alliance of NZ Transport Agency, Downer NZ, HEB Construction, Tonkin & Taylor and Aecom (formerly URS). The park concept was won through competition in 2008 by Wraight Athfield Landscape and Architecture and adapted in 2012 to include the tunnel - removing the state highway from the surface as it was in the original concept and integrating the tunnel into the park design.
November 2012 Panorama: Clearing the site, SH1 Buckle St still operating directly in front of the National War Memorial. Photo Stephen Patience
TUNNEL DESIGNED TO WITHSTAND A 2500-YEAR EARTHQUAKE
The geology of the central Wellington site was such that unique engineering solutions were devised so the tunnel could withstand a 1-in-2500 year earthquake. The high seismicity of the site less than 2km from the active Wellington Fault and evidence of past lateral spread of the soils required a robust design for the tunnel which has 800mm thick walls and floor slab and a roof slab varying in thickness from 1200mm to 450mm.
A 300m long, 18m wide, 12m deep cut-and-cover trench was excavated to allow formation of the 130m tunnel - essentially a reinforced concrete box cast fully in situ with the floor, walls and roof cast in separate pours. Construction was facilitated by temporary retaining walls so the tunnel could be cast within.
Liquefaction buoyancy was addressed with ductile tension piles beneath the floor slab. Significant pile testing was undertaken to determine the most appropriate design after initial testing on straight shaft piles failed at 20% of the required design load. A belled-base pile was constructed using a custom-made belling tool and exceeded the design load requirements, allowing a reduction in number and length of piles.
August 2013 Panorama: Excavation of the trench, SH1 temporarily running along the north side of the site. Photo Stephen Patience
STATE HIGHWAY REROUTED
The state highway through this area of Wellington west of the Basin Reserve and passing in front of the National War Memorial was formerly known as Buckle Street. That name now applies only to the access lane on the southern side of the park.
Before construction, the bend west from Sussex Street into Buckle Street was narrow with 2 lanes in one direction, a slight incline upwards and a substandard centreline radius of 22.5m. Traffic flows were 22,000 (2.8% heavy vehicles) and frequently queued back around the bend from traffic signals at the Tory Street intersection about 100m away.
Today the highway turning west from Sussex Street heads down a trench into the one-way Arras Tunnel, surfaces at Taranaki Street and carries on out of the capital city through the Terrace Tunnel. Tory Street no longer intersects the state highway and is a link road through the park. Traffic flows along SH1 through Arras Tunnel are predicted to be 30,500 by 2025 (2.8% heavy vehicles) equating to 25.2MESA.
Following construction, the 2-lane entry has improved in terms of curvature, super elevation and lane layout. It was a challenging design because of geometry restrictions, including retention of existing twin 33kV cables. The 3.5m lanes plus 1.0m margins have a super elevation at 5.0% around a 35.4m centreline radius.
The 2 lanes filter into 3 lanes through the tunnel and 4 lanes at the Taranaki Street intersection. The design speed throughout is 60kph with a posted limit of 50kph.
The pavement developed by MPA has a 40-year design life and used Polymer Modified Binders (PMB), resulting in a reduction in the overall pavement thickness from 240mm to 180mm. The surfacing layer was AC14 using Melter Slag aggregate for durability.
For the future, the geometric design for the road into the tunnel can accommodate a future Basin connection, should a project proceed.
August 2014: Inside Arras Tunnel, spraying bitumen emulsion in preparation for asphalting.
Photo Colin McLellan
ALIGNMENT AMENDED TO CLEAR HISTORIC SEWER
The tunnel specimen guide design included 4% grades into and out of the tunnel. Through accurate survey of the 1895-vintage 1.5m-high brick Tory Street sewer and careful design of the Tory Street minor road connection above the tunnel, the alignment was amended by lifting it 1.1m to clear the sewer.
This minor movement of the sag point and change to the entry grade of 2.8% provided a smoother alignment. Careful design enabled a reduction in roof slab thickness to 450mm - 708mm, which at the tightest point is only 200mm below the connecting road and footpath above.
March 2015: Early morning traffic heading west through the tunnel. Photo Colin McLellan
Pukeahu National War Memorial Park above the tunnel consists of a series of boulevards and terraces - 300m-long and up to 70m-wide (21,000m²) - that extends the presence of the existing National War Memorial into the capital city.
The Australian War Memorial of red sandstone pillars opposite the New Zealand memorial is a tribute to the shared experiences of both our countries and is the first of the international memorials that will be integrated into the park.
The national significance of the park was recognised by the 60,000 who attended the Anzac Day dawn ceremony and the many thousands who visited throughout the day for subsequent ceremonies. At 5.00pm every evening for the next three years to 11 November 2018 - the end of the centenary of WWI – the Last Post bugle call will be heard across the park from the Tomb of the Unknown Warrior.