Somme Parade drop-out repair

Project typeEarthworks / construction of a retaining wall
Project value$6.6M (Waka Kotahi $3.96M / Council $2.64M)
Contractor nameDowner
Completion Date15 March 2024

The Somme Parade drop-out oopposite Axiam Plastics

Significant erosion to about 136 metres of the Whanganui River bank opposite 131 Somme Parade (Axiam Plastics) has made the riverbank footpath unsafe to use and threatens the integrity of the road and gas pipeline under the footpath.

Repairs to this site will cover:

Stage 1 – Secure the gas pipeline. This will require the pipeline to be anchored to prevent damage from further potential bank erosion

Stage 2 – The construction of an inner and outer sheet pile wall to stabilise and protect the area from further erosion. The two walls are driven deep into the river bank and tied together to form a well-anchored and stable structure.

Whanganui District Council’s Roading Alliance partner Downer is doing the repair work. As the riverbank was damaged during Cyclone Dovi in February 2022, the work is jointly funded by Waka Kotahi NZ Transport Agency and Whanganui District Council.

Check out the timelapse video of the awesome progress being made at the Somme Parade drop-out repair site!

Stage 1 – gas pipeline / footpath stabilisation

The footpath area above the eroded section of river bank on Somme Parade

The footpath area above the eroded section of river bank on Somme Parade

Work to stabilise the gas pipeline is scheduled to begin in late June 2023. Heavy machinery will drive a number of 9-metre length steel columns into the ground, and the footpath area will be excavated. The exposed gas pipeline will then be secured to the steel columns.

This work is expected to take two weeks. Both traffic lanes of Somme Parade alongside the worksite will operate during the stage one work period but the roadside car parking lane outside 131 Somme Parade will not be available until completion of the work. The excavated footpath area will also be closed during the work period.

Stage 2 – river bank stabilisation

Indicative cross-section of the Somme Parade river bank repair

The riverbank erosion will be stabilised by driving several 15- to 18-metre length steel sheet piles* into the bank to form the retaining wall. The steel sheet piling is expected to arrive in New Zealand via ship in late July 2023.

Construction of the retaining wall will begin when the steel sheet piling arrives onsite in Whanganui, and Somme Parade will then be reduced to a single traffic lane near the worksite to allow a large crane to operate safely. A special 160-tonne crane will be used to drive the steel sheet piles into the river bank.

Pedestrians have access via two crossing points – near Spier Street and Kaikokopu Road.

(*Sheet piles – most commonly made of steel – are sections of sheet materials with interlocking edges that are driven into the ground to provide earth retention and excavation support.)

Update - 29 January 2024

The project timeline and subsequent Somme Parade road closure have now extended into around mid-March 2024. It was originally expected the work would be completed by the end of January 2024.

The primary reasons for the extension of time are:

Almost 3 weeks of pile driving time was lost in October and November when the road started to sink under the weight of the crane. This required additional work to stabilise the ground which involved driving 12 metre long steel piles under the crane tracks as the crane moved along the site.
The site continued to slip in depth and length as work progressed. This required changes in installation methodology and extensions to the sheet pile walls in both directions.
The taper wall at the upstream end of the site has been added to improve the river flow past the site during high river level events.

Completed work includes the installation of sheet pile inner and outer walls. Backfilling is underway and the walls are being tied together with steel rods.

Once backfilling is complete, the kerb and footpath will be reinstated and the filled area established with native plants.

Update - 1 March 2024

Somme Parade has reopened to two-way traffic at the drop-out repair site!

The Whanganui Alliance and other work crews have put in a huge effort to get this section of road open between Barrack Street and Kaikokopu Road earlier than expected and detours are no longer in place.

We know this will be welcome news for residents and businesses not only along Somme Parade, but also everyone who lives on or has needed to use the detour route.

As the road is rough in places and still needs resurfacing, a 30km/h speed restriction is in place – at times there may be stop-go traffic management during the road resealing work.

Please slow down through the site so contractors can complete the job safely.

It’s possible we may need to reinstate the detour temporarily while the finishing touches are done but rest assured we’ll keep this to the shortest duration possible.

A big thank you to the Whanganui Alliance and our other contractors for your awesome mahi!

Whanganui Alliance is a partnership between the council and Downer to deliver our roading programme.

Update - 14 June 2024

The issue

During post-construction monitoring by the Whanganui Alliance, it was discovered there had been some movement in a short section of the retaining wall, immediately next to a stormwater outlet (the extent of the movement is within the wall design tolerances determined by the design engineers).

Investigations showed that several pipe sections of the main had separated, resulting in stormwater discharging into the ground. The Alliance mobilised quickly to undertake further geotechnical testing of the area around the pipeline and area of wall movement.

This work is outside the control of the contractor and was unforeseen when the designs for the drop-out repairs were established.

Test results

Initial results of the testing involved constructing a test anchor to provide details of an anchor design to ensure there is no further movement within the area. The test anchor has now been tested with good results. The design engineers have determined the number and location of anchors.

What is being done

The installation of anchors has begun. The work is expected to take around a month to complete.

Following the installation of anchors the Alliance will undertake repairs to the stormwater pipeline. The stormwater pipe repairs will require a full road closure. During this time (approx. three weeks) the Seddon Street detour route will be used for traffic.

Fencing and site reinstatement will be completed following the anchor and stormwater works. All works are expected to be completed by September.

Funding

Funding is currently at an advance Funding Assistance Rate (FAR) rate of 80% from Waka Kotahi NZ Transport Agency. This level of funding will revert to a standard FAR (62%) on 1 July 2024.