When it comes to ground engineering, topography and geological conditions will often impact the built environment. The ground conditions will also inform the remediation approach Mainmark uses for different projects. With a global footprint spanning from Australia and New Zealand to Japan and the United Kingdom, Mainmark has considerable experience working in different regions, many with complex issues specific to local areas.
In the first of our regional spotlight series, Mike Baker, Area Manager in NZ, takes a closer look at ground conditions in New Zealand, in particular those in the three major hubs of Auckland, Wellington and Christchurch. We delve into the unique ground engineering challenges in each city, and some of the solutions that Mainmark frequently uses to address the varying issues, often impacted by local soil conditions.
While each region has unique challenges, Mainmark’s approach is not simply about quick fixes. Instead, we look closely at the site and apply tailored ground improvement solutions that can address the underlying cause of the problem.
In New Zealand, the complex weather patterns range from subtropical conditions in the North Island to a cooler alpine climate in the South Island. Often referred to as the ‘Shaky Isles’, New Zealand is also an earthquake-prone region due to the number of fault lines running almost the entire length of the country.
Here, we take a closer look at the geotechnical considerations in New Zealand’s three biggest cities.
Auckland
Auckland’s climate has two distinct ‘wet’ and ‘dry’ seasons. In summer, New Zealand’s largest city experiences subtropical conditions, compared to the rest of the country, which enjoys a more temperate climate. With the prevalence of reactive clay soils, weather and seasonal shifts can play havoc on Auckland’s buildings due to the ‘shrink and swell’ effect that sees soils expand when it gets wet in the winter and shrink back in the summer when it dries out. Visible cracking may appear in the ground’s parched surfaces, and these weakened soils may cause a home’s foundations to ‘sink’.
This ground reaction is something that property owners in Auckland need to consider when cracks appear in the walls of their home. The Mainmark team has seen a number of structural problems stemming from subsidence – the propensity for upward, lateral, or downward ground movement which can result in structures moving – including cracks forming in walls, and windows and doors failing to open or close properly. If subsidence worsens, larger cracks in the home’s brickwork can form, visible both inside and outside the home, and the sinking foundations might also lead to floors becoming unlevel. These signs of subsidence can become serious and very costly if left untreated.
Recent drought conditions in the Auckland area have led to drier than usual ground conditions. Mainmark’s Auckland team has received an increasing number of calls from concerned homeowners wanting to rectify problems relating to subsidence. In many of these cases, Teretek® engineered resin injection solution is used to help to raise and re-level sunken homes, both cost-effectively and non-invasively.
Wellington
New Zealand’s capital, Wellington, sits on the southernmost point of the North Island. The city is regularly shaken by small and medium-sized earthquakes due to the Wellington region being crossed by a number of major faults, including the Wairarapa, Wellington and Ohariu faults. Wellington currently experiences minor seismic activity every few months, but these tremors have not been a significant threat to homes or lives. However, the ongoing study of these fault lines has prompted concern from scientists that a large earthquake may occur in the future, like the 7.5 magnitude 2016 Kaikōura earthquake which left Wellington with cracked buildings, windows smashed and without power in several areas. This seismic activity has prompted an increased awareness of the hazard and Mainmark has undertaken several ground resilience and liquefaction mitigation projects in the area since.
As the country’s administrative centre, Wellington is home to many government agencies and departments that have received investment for building upgrades and structural reinforcement to help protect these buildings from the effects of a future earthquake. For example, in 2019, the Mainmark team undertook a very large ground strengthening project for the Seaview Wastewater Treatment Plant, using Mainmark’s proprietary Terefirm® Resin Injection technique to densify the soils and increase its New Building Standard (NBS) rating to ensure the facility can continue operating following an earthquake.
While ground improvement projects are increasing in Wellington, the area’s ground conditions consist largely of soft sand and silt with clays and gravels sometimes present. One of the key challenges for new home builds is that many buyers are unaware of the potential long term foundational implications that can affect homes built on soft ground. The ground must be adequately prepared which may include a site excavation to remove the soft soils before laying the foundation. This type of foundation, where a more stable layer of material is placed beneath the foundation, is known as a gravel raft and provides greater foundation support prior to pouring the slab for a new home. Mainmark’s Terefil® is also a great alternative to gravel rafts as the lightweight cementitious fill creates a stable slab that helps protect structures against liquefaction and future subsidence. The solution was used for a four-unit complex in Christchurch and an electricity substation in Auckland.
Wellington’s topography can also present a challenge to building stability, with many of the city’s houses located on steep hills with critical retaining walls and often no vehicle access. Many older homes, some more than a century, are in desperate need of repair including ground remediation work.
Christchurch
Located on the east coast of New Zealand’s South Island, Christchurch has been an important focus for the Mainmark business following the 2010-2011 earthquakes which caused significant damage to infrastructure and buildings due to liquefaction. Homes, businesses, public buildings and historical monuments either sunk, shifted, or crumbled and many are still awaiting remediation.
Mainmark has played an important role in helping the Christchurch community remediate their properties, including the re-levelling of significant large-scale structures using JOG Computer-Controlled Grouting. In 2013, the company initiated a series of liquefaction mitigation trials using expanding polyurethane resin which was found to be a viable ground strengthening and re-levelling solution for use underneath existing buildings. While it has been 10 years since the earthquakes, Mainmark continues its work in Christchurch by remediating structures affected by earthquake damage and liquefaction. Many building owners are seeking to future proof their buildings to ensure they can withstand the impact of a potential future earthquake. Commercial properties are also required to comply with the Building (Earthquake-prone Buildings) Amendment Act 2016, which identifies high risk buildings that must be structurally remediated.
While much of Mainmark’s work in the South Island has focused on Christchurch, the organisation also services the entire region including Queenstown, a growing trade and tourist hub. Mainmark was recently contracted to remediate O’Connells, the city’s largest indoor shopping centre, helping the building achieve an increased NBS rating, a seismic standard indicator that rates a building’s ability to withstand an earthquake.
New Zealand may be a relatively small geographic area, however each of these three regions presents unique ground engineering and remediation challenges, which the Mainmark team tackles using a range of solutions tailored to the individual site requirements. As a solutions-based business, Mainmark goes to great lengths to understand the individual ground problems of each project, and creates bespoke solutions by working through the process with its customers. With more than 25 years’ experience, Mainmark is continually broadening its capabilities, working across the different geotechnical nuances from one end of the country to the other.
By Mike Baker
Mike Baker is Mainmark New Zealand’s Area Manager in Wellington. After joining Mainmark in 2013, Mike supervised remediation of the modern Christchurch Art Gallery which was shaken by the 2010 and 2011 earthquakes that hit New Zealand’s South Island. In 2019 he led the first of its kind Seaview Wastewater Treatment Plant liquefaction mitigation project in Wellington.
