On New Zealand's largest wastewater project, a liner delivery delay threatened a 76-metre deep shaft. Blick designed a fluid system that kept it stable for 91 days beyond design specifications.
The Challenge
New Zealand's largest bored wastewater tunnel — the $1.2 billion Central Interceptor, 14.7km and 4.5m diameter running 15–110m deep — required large-diameter shaft support at Keith Hay Park. When the shaft liner's delivery was delayed, CLL's 76-metre deep shaft needed to stay open for 62 days beyond its 90-day design specification. Collapse was not an option on a project of this scale.
The Approach
Blick designed a unique bentonite-based shaft support fluid system using Wyo-Ben MAX GEL and M-I SWACO UNITROL filtration control polymer. The system created a thin, stable filter cake that prevented fluid infiltration while providing full hydrostatic support. Critically, viscosity had to stay below 38 seconds Marsh to allow SHAPE sonar equipment to accurately assess excavation dimensions — a constraint that required careful reformulation.
The Outcome
The shaft held for 152 days total — 90 days design life plus 62 days beyond specification — with no deterioration. The thin filter cake prevented fluid migration throughout. 'Thanks to Blick and the excellent support of their technical support team, we had a successful outcome from start to end of the shaft drilling at Keith Hay Park.' — Sivam Thivaharan, Site Engineer, CLL Service and Solutions.
The Full Story
Innovative solutions overcome challenging large-scale piling project
A significant engineering feat has been achieved in the drilling of large shafts on New Zealand's largest wastewater infrastructure project. Watercare's 14.7km Central Interceptor tunnel represents a major challenge in deep foundation work.
Blick, an Australasian fluid and drilling tools supplier based in New Zealand, was called upon to design a shaft support fluid system that kept a 76-metre-deep shaft open for nearly twice as long as scheduled due to a delay in the arrival of the shaft liner.
The Central Interceptor is a $1.2 billion super-sized tunnel that will reduce wastewater overflows into central Auckland waterways. It is New Zealand's largest bored wastewater tunnel at 14.7 kilometres in length, 4.5 metres in diameter and runs at depths from 15-110 metres from Grey Lynn under central Auckland and the Manukau Harbour to Mangere Wastewater Treatment Plant.
Piling specialists CLL Service and Solutions is using New Zealand's largest piling rig with specially designed piloting tools and sonar technology to constantly monitor progress to drill three shafts at Keith Hay Park of 76 metres deep, followed by a 28m shaft in Hillsborough and a 63m shaft in Mt Roskill.
CLL used a SHAFT AREA PROFILE EVALUATOR (SHAPE), a cost-effective quality assurance testing device for deep foundations such as drilled shafts, slurry walls, barrettes, and more to determine the excavation dimensions and verticality. It provides a fast, economical visual representation of the foundation excavation and verticality before placing concrete in wet conditions.
SHAPE required a viscosity in the support fluid of less than 38 seconds to read the shaft dimensions correctly, and our team redesigned the fluid system to a viscosity of less than 38 seconds while still meeting the design life of 90 days.
The shaft was handed over to the Ghella Abergeldie joint venture 91 days after commencing the pilot. Due to the delay in the shaft liner's arrival, Blick was asked to design a unique bentonite-based shaft support fluid system to protect the shaft's integrity until the liner arrived.
The support fluid system was designed to allow the hydrated bentonite to flow into and seal the fractures. To maximise filtration control, M-I SWACO UNITROL, a filtration control polymer, was added to the Wyo-Ben MAX GEL.
A thin filter cake generated by the Wyo-Ben MAX GEL and M-I SWACO UNITROL prevented fluid from moving into the formation from the shaft and allowing hydrostatic pressure to push against it, supporting the 76m deep shaft for an additional 62 days beyond the design specifications with no deterioration.
The shaft support fluid prevented swelling and provided effective support to the shaft for 152 days, which is an incredible feat.
Blick's high-quality sodium bentonite Wyo-Ben MAX GEL creates a highly effective filter cake with precisely the qualities needed in a piling setting. It is also very effective at holding cuttings in suspension, which is considered a negative side effect during piling.
Blick also specified an additive that, when added to the shaft, caused suspended cuttings to drop out of suspension without impacting the filter cake.
Thanks to Blick and the excellent support of their technical support team, we had a successful outcome from the start to the end of the shaft drilling at Keith Hay Park. We are currently drilling another deep shaft with Blick providing amazing support again.
Sivam Thivaharan, Site Engineer, CLL Service and Solutions
CLL is completing two more shafts for the Central Interceptor tunnel, a 5.4m diameter, 28m deep shaft at the PS23 pump station, Fredrick Street in Hillsborough, followed by a 4.2m diameter and 63m deep shaft at Walmsley Park in Mt Roskill.