Regional spotlight series: How local weather and soil conditions can impact ACT’s ageing residential market

Like in most regions, Mainmark’s operations in Australia’s capital city span residential, commercial and civil infrastructure. While several public infrastructure sites have benefited from Mainmark’s ground remediation expertise, including Lake Burley Griffin and Federation Mall, homeowners in the territory and surrounding areas also keep our team busy.

Understanding the ACT’s ground conditions

The topography surrounding Canberra consists of forested hills and alluvial flats, with much of the urban areas built on gently undulating terrain that features gravelly and sandy soils. However, the broader soil profile in the ACT is primarily clay, ranging from low, medium and high plasticity, and is classified as moderately reactive in accordance with AS2870 “Residential slabs and footings” standard.

The weather also plays a big part in the region’s ground conditions. Depending on the time of year, the ACT can experience temperatures ranging from -8 to 45 degrees, with cyclical weather patterns such as La Nina and El Nino leading to both saturation and dehydration of the ground. This causes reactive clay to shrink and swell much like in many other parts of Australia, creating structural challenges for homeowners.

A typical home in the ACT

While the ACT residential market has become more diverse in styles and architecture over time, it is best known for the colloquial ‘Govie’ style house. Govies are homes that were built quickly and in large volumes to meet high demand. The population growth from politicians and their families moving to the ACT resulted in greater investment and liveability. Govies therefore needed to cater for the influx of residents, built with the most affordable and readily available materials to ensure quick turnarounds.

From the 1920s to 1950s, most houses were built with double brick exterior and a single brick interior, on narrow and shallow concrete strip footings, with little or no articulation in the walls to allow for movement. In the 1960s and 1970s, the government oversaw a large expansion of Canberra, creating the ‘Govie’ design with simple rectangular layouts and wet areas on concrete floors above narrow strip footings. Many of these houses remain standing today and are regularly renovated and extended.

While Govies have always been compliant, they were often constructed quickly, not allowing builders to consider the relationship between the foundations and the reactive clay. During cyclical weather changes, the clay would shrink and swell, affecting the footing and creating a loss in soil volume, resulting in foundational settlement. In many cases, old Govie footings have not been maintained and over the decades, this has resulted in settlement-related damage such as sunken floors and cracks in the interior and exterior walls.

Overcoming the challenges

Fortunately, building codes have improved over the years. They now better consider the ACT’s ground conditions and its impact on residential construction. However, regardless of the age of a building, footings require monitoring over their lifespan and at times, property owners may need to take action to manage ground moisture levels, which may include removing trees or addressing leaking pipes.

While the ACT’s ground engineering sector has historically relied on conventional remediation methods like traditional underpinning, Mainmark’s Teretek® resin injection solution has become a popular alternative as more homeowners recognise the benefits of using a modern technology that is easier to apply, cost effective and less invasive.

In addition to working directly with local homeowners, Mainmark has also formed relationships with local builders and engineers regarding ground remediation solutions for local infrastructure. For example, the Australian Government’s National Capital Authority (NCA) engaged Mainmark to re-support the Lake Burley Griffin foreshore wall by injecting more than 40 tonnes of Teretek into a 4km section of the lake’s concrete and stone wall. The solution successfully filled voids, increased ground bearing capacity and agglomerated the loose soil to mitigate fines being lost into the lake through cracks in the ageing walls.

Mainmark was also engaged by the NCA to address settlement issues affecting the Federation Mall bridge using Teretek® to fill underground voids beneath the bridge’s approach slabs and roadway to rectify ‘dishing’ and minimise the risk of ongoing settlement.

Over the years, Mainmark has established a strong presence in the ACT and is looking forward to continuing to support the region using its innovative range of ground and asset remediation solutions in the residential, commercial and infrastructure sectors.