This article was previously published in NZ Construction in June/July 2019 and is reproduced by permission.
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 30,000m2 plus site has been an exciting yet rewarding project for our team to be part of. 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, the seismic challenges of the site.
Being a global expert in geotechnical contracting, Menard Oceania, and local civil contractor and sister company March Construction, were delighted to be appointed as March Menard Joint Venture (MMJV) to the project by Ōtākaro Limited in August 2018. Ōtākaro is tasked with delivering all Crown-led anchor projects in central Christchurch and 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 metre, 10-lane competition swimming pool, a separate diving pool, five hydro slides, fitness spaces and nine indoor sports courts.
In consultation with Ōtākaro, MMJV’s responsibility has been to develop a robust and sustainable ground improvement solution that would stabilise the ground, making it ready for construction of the new Metro Sports Facility. MMJV responded to the challenge at hand, bringing in experts and equipment from around the world to come up with the best solution available to create a solid foundation for Christchurch’s new landmark sporting facilities.
Laying the groundwork
Centrally located, the site for the new sports centre had an interesting previous life as the former home of 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. We did discover asbestos piping but once it was 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 this remedial work had altered the sites 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 (meter Reduced Level). The bottom (or toe) of stone columns that we needed to install varies in different areas and it is based on elevation (mRL). As a result, some columns should be longer than others. Knowing the existing ground level, and the toe level from the designer, provides the site team the length of stone columns that need to be installed.
Prior to start of the stone column installation, the MMJV conducted a pre-level survey to determine the topography of the jobsite to accurately identify the correct length of the stone columns 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 recent significant earthquake events. Dealing with land in an area prone to seismic activity brought new challenges to consider. It is well-known the city has been prone to liquefaction, and after extensive geotechnical investigations were carried out on site, we knew the area designated for the Metro Sports Facility needed future proofing.
To achieve this, the stone columns ground improvement that we designed were 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 is greater than 1.2 in clean sands and greater than 0.7 in siltier soil.
Liquefaction mitigation by vibro-replacement stone columns has shown itself to perform well during major earthquake events. In earthquake conditions, stone columns create a base that are effective at overcoming liquefaction because they increase the drainage of 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 Ōtākaro.
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 robust shallow foundation and slab to bear on.
This method accepts that liquefaction may occur, and with the construction of a non-liquefiable surface, with a sturdy, shallow foundation system, will reduce the potential of damage happening to the building’s superstructure in the future.
Trialling stone columns to assess if modifications were needed
As part of the Metro Sports project requirement, 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 7,200 stone columns needed to be installed to support the sports centre. But before work began, the teams started with a trial to make sure their method worked. This meant installing 96 stone columns to a depth of up to 15.5 metres.
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 chances of earthquake-induced liquefaction was reduced in the lower sand levels and on the upper silty, sand crust. This means the sports centre can reply on a shallow foundation to support the venue. Once the trial proved the required depth and diameter was achievable, the rest of the stone columns were given the green light by Ōtākaro.
Challenges faced along the way
The stone column technique utilised for the groundwork 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 this 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 7,200 columns would be completed within six months.
The project also had a slow start as 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 9m without too much effort, extending past this depth to 15.5m became difficult as the earth became harder. To navigate this project 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 and without impacting on the program.
Making the project work
The Menard March’s contract duration was 180 calendar days and the stone column installations, including post testing, were 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 this project worked smoothly was because the different groups involved work effectively with each other. The MMJV worked closely with Ōtākaro to make sure they were delivering the results Ōtākaro were expecting in a safe and timely manner.
This relationship wasn’t hard to achieve because we share the same values and are committed to a safe workplace where everyone onsite returns home safely at the end of each day. This was recognised earlier this year when Ōtākaro named MMJV its contractor of the month, acknowledging our commitment to health and safety.
Creating a stable foundation for Canterbury sports
Since the 2011 earthquake, the number of facilities 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 a home for 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 column foundations 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.