The central city site for Christchurch’s Metro Sports Facility – the Menard March Joint Venture completed installation of 7200 stone columns for the facility’s foundations ahead of schedule
Creating solid ground for Christchurch’s Metro Sports Facility – By Andrew Vencer
It would be an understatement to say that the Christchurch earthquake created a series of major headaches for those tasked with the monumental challenge of rebuilding the city. But it also presented an opportunity to utilise new technology and systems and build iconic structures like the new Metro Sports Facility.
From August 2018 to May 2019, the 30,000 sq m plus site has been an exciting yet rewarding project for our team. It’s one that has caused a lot of interest among the international construction community, all curious to know more about how we would rise to the geotechnical and, more importantly, seismic challenges of the site.
Menard Oceania, global experts in geotechnical contracting, and local civil contractor and sister company March Construction were appointed to the project as Menard March Joint Venture (MMJV) by Otakaro in August 2018. Otakaro is tasked with delivering all Crown-led anchor projects in central Christchurch and is responsible for managing the design and construction of the Metro Sports Facility project.
When completed, the Metro Sports Facility will be the largest aquatic, indoor recreation and leisure venue in New Zealand with a 50 m, 10-lane competition swimming pool, a separate diving pool, five hydro slides, fitness spaces and nine indoor sports courts.
In consultation with Otakaro, MMJV’s responsibility was to develop a robust and sustainable ground improvement solution that would stabilise the ground, creating a solid foundation ready for construction of the new sports facility. MMJV responded to the challenge, bringing in experts and equipment from around the world to come up with the best solution available.
Laying the groundwork
Centrally located, the site for the new sports centre had an interesting previous life as the former home of the Canterbury Brewery, along with several other commercial properties. It was also once a motor vehicle workshop. After what was left of these buildings following the earthquakes was demolished and materials cleared, work began on fully assessing the site. Asbestos piping was discovered, but once identified we were able to manage its safe removal in accordance with our accidental discovery plan.
Engineering and infrastructure specialists Aurecon surveyed the site to ensure we had an up-to-date topographic map of the land, including identifying any ground level changes as the remedial work had altered the site’s topography.
Although the topography of the site was generally flat, the elevation of the existing ground surface did show variance between 15.2 and 16.4 mRL (metre reduced level). The bottom (or toe) of the stone columns that we needed to install would therefore vary in different areas based on elevation – some columns would be longer than others. Knowing the existing ground level and the toe level from the designer provided the site team with the length of stone columns that needed to be installed.
Prior to starting the stone column installation, MMJV conducted a pre-level survey to determine the topography of the site to accurately identify the correct length of each stone column to be installed.
Choosing a robust method
A geotechnical-developed design report prepared by Aurecon identified that the site lies close to the epicentres of the significant earthquake events. Dealing with land in an area prone to seismic activity brought new challenges to consider. It is well known that the city is prone to liquefaction, and after extensive geotechnical investigations were carried out onsite, we knew the area designated for the Metro Sports Facility would require future-proofing.
To achieve this, the stone column ground improvement solution that we designed was in accordance with Aurecon’s design specification. Central to this was stiffening and strengthening the upper plastic soils, providing adequate bearing capacity and stiffness for a shallow foundation and minimising long-term settlement.
Also important was the need to suppress liquefaction and ground softening effects so that the factor of safety against liquefaction triggering would be greater than 1.2 in clean sands and greater than 0.7 in siltier soil.
Liquefaction mitigation by vibro-replacement stone columns has been proven to perform well during major earthquake events. In earthquake conditions, stone columns create a base that is effective at overcoming liquefaction because the columns increase drainage within the soil. When an earthquake hits, underground water is displaced and works its way through the soil. The stone columns hold their place, stabilising the ground, unlike the original sand layers that would move under the pressure.
After much consideration, stone columns were determined to be the best technique for this ground improvement project and signed off by Otakaro.
For the new sports facility, ground conditions were improved through both densifying and reducing earthquake-induced liquefaction in lower sand layers, and strengthening and stiffening the upper silty sand layers, with the intent to provide a stiff, non-liquefiable crust raft for the building’s shallow foundation slab to bear on.
This method accepts that liquefaction may occur, but the construction of a non-liquefiable surface, combined with a sturdy shallow foundation system, will reduce the potential of damage happening to the building’s superstructure in the future.
Learning to coordinate the four stone column cranes and other associated equipment across the site was a challenge for the team, but one that was successfully mastered
Trialling the stone columns
As part of the project’s requirements, the stone columns designed by MMJV increase soil-bearing capacity on the underside of the shallow foundation and control both total and differential settlement under static loading conditions.
A total of 7200 stone columns needed to be installed to support the sports centre. But before work began, the team started with a trial to make sure their method would work. This meant installing 96 stone columns to a depth of up to 15.5 m.
The trial was designed to prove our technique could reach the required depth, desired diameter of the stone column, and verify improvements of the ground. It showed the ground conditions improved dramatically. The earth’s surface became denser, and the chance of earthquake-induced liquefaction was reduced in the lower sand levels and on the upper silty sand crust. This meant the sports centre would be able to rely on a shallow foundation to support the venue.
Once the trial proved the required depth and diameter were achievable, the rest of the stone columns were given the green light by Otakaro.
Challenges faced along the way
The stone column technique we utilised is a well- established ground improvement solution and used throughout New Zealand. However, it did not come without its challenges. Due to the project size, specialist equipment had to be sourced from around the world to supplement the equipment available in New Zealand, specifically Australia, Germany and the Middle East.
As well as equipment, Menard ground improvement experts from around the world travelled to Christchurch to play a part in the project. Emphasis was placed on building a positive working relationship with all significant parties involved to ensure everyone was working effectively together. With the combined New Zealand and international expertise, an impressive installation pace was set to ensure all 7200 columns would be completed within six months.
The project had a slow start while the team learned to coordinate four stone column cranes and other associated equipment. The importance of putting in place correct set-out and validation processes was also established, and this contributed to a successfully implemented project.
The consistency of the ground became an obstacle that needed to be worked through as harder pockets of ground resulted in difficult installation. For much of the project, the vibroflot reached 9 m without too much effort, but extending past this depth to 15.5 m became difficult as the earth became harder. To navigate this risk, a pre-drilling machine had to be utilised to loosen the localised soil and allow easier penetration of the vibroflot. This process ensured all columns were installed to the design depth without impacting on the programme.
Making the project work
MMJV’s contract duration was 180 calendar days, and installation of the stone columns, including post-testing, was completed four days ahead of schedule. This was largely achieved through excellent planning by the site team to keep all cranes working productively until the last day.
A key reason why this project ran so smoothly was because the different groups worked effectively together while liaising closely with Otakaro to make sure they were delivering the expected results in a safe and timely manner. This commitment to a safe workplace was recognised earlier this year when Otakaro named MMJV its contractor of the month, acknowledging our commitment to health and safety.
Creating a stable foundation
Since the 2011 earthquake, the number of facilities that locals have access to has been limited, making the centre’s completion an essential part of Christchurch’s regeneration. With the stone columns firmly in place, a stable ground has been laid, making way for the completion of the Metro Sports Facility. Once construction is finished in late 2021, the centre will be home to school sports, regional and national competitions, and a place for families to take their children to enjoy all that is on offer.
Menard and March are proud to have been part of this landmark project, and to have demonstrated the use of stone columns as an affordable and robust foundation solution for major infrastructure projects. The lessons learnt will form a valuable input for future projects in addressing the challenging geotechnical environment for Christchurch and the wider New Zealand construction industry.