Key considerations when building a stable structure on a difficult site

Australia and New Zealand are lands of diverse topography, with thousands of communities situated in areas where hills, slopes, embankments and elevations are prevalent. And while that may mean stunning views and natural light for those wishing to live on an elevated site, it raises significant challenges for builders and engineers who must consider the ground terrain and other geotechnical issues.

As a ground engineering specialist, we have seen many examples of challenging building sites, including an elevated property in Parramatta, NSW. A retaining wall between a home and an adjacent public park collapsed shortly after construction. The residence was situated on a very steep slope with a high retaining wall on the property boundary. Mainmark was responsible for backfilling approximately 40m3 behind the new wall with Terefil®, a structural, lightweight, cementitious based fill.

As traditional concrete or fill materials were unfeasible for this project, due to the heavy load they would have placed on the wall, Terefil provided a unique lightweight yet structural solution that allowed the engineers to reduce the wall’s thickness. Terefil’s fully permeable formulation also ensured the wall complied with the engineered specifications for full water drainage.

While sites that feature slopes, cliffs and other scenarios where there is a steep drop in ground levels can make the construction process more complex, they are not impossible for engineers and builders to overcome. The key is to identify the site’s soil and ground bearing conditions to help select the right solution to manage the issue. Here are some common considerations when building on sloping or elevated building sites.

  • Degree of site slope. The steepness of the slope will inform the necessary engineering requirements, site safety considerations, cut and fill work, and ground remediation solutions needed to ensure stable foundations and footings for the structure. Mainmark has successfully worked to remediate many challenging sites, including a duplex property situated on a 20m high sandstone and siltstone clifftop in Auckland, which was experiencing structural damage resulting from unstable ground. In the duplex’s case, Mainmark was required to inject Teretek® engineered resin into ground voids to consolidate the surrounding soil.
  • Ground conditions. The geotechnical properties of soils beneath the surface of a sloped building site can range from hard rock or reactive clay to softer conditions such as sand that may be prone to shifting. These conditions may impact the stability of a sloped site’s foundations. In uncertain ground conditions, a geotechnical report is required to identify the ground’s properties including bearing capacity (or strength) and the presence of any slip planes, which can have a significant impact on the way a building is constructed or remediated. There are ground engineering solutions available that may help to address unstable ground conditions. For example, loose sand that is prone to shifting can be strengthened and consolidated using Permeation Grouting, a solution which agglomerates sandy or loose soils into a cemented mass to support structures and fill voids in the ground. Mainmark used Permeation Grouting to remediate a house in the beachside suburb of Bondi which was affected by subsided footings and a failed retaining wall due to loose sands below the structure.
  • Environmental conditions. Depending on the site’s location, the environmental conditions may need to be addressed. For example, if the sloped site is situated in an area prone to heavy rainfall, a suitable solution to manage overland flows, mitigate saturation of soils and address drainage issues may be required to help stabilise a structure and prevent excess water or runoff from eroding the building’s foundations.

What are the solutions?

Mainmark has a range of products and solutions to help overcome the challenges of building or remediating structures on sloping or elevated sites. In many cases, they can also help avoid the need to use invasive ground remediation processes, such as traditional underpinning and piling, and avert costly repairs down the track should structural damage occur due to poor ground conditions.

Teretek®, Terefil®, and Permeation Grouting are just a few solutions that Mainmark uses to help overcome the complexities of remediating buildings on sloping sites with unfavourable soil conditions.

Teretek is an expanding polyurethane resin mix which is injected through small tubes into the ground beneath the footings. It is ideal for strengthening and re-levelling settled and uneven ground using a process that is likened to keyhole surgery. The application is completed in a fraction of the time compared to conventional re-levelling methods and causes minimal disruption to the site.

Terefil is a lightweight, specialist cementitious fill with a uniformly distributed matrix of air bubbles allowing it to flow into places where other cementitious fills may be unable to reach. Terefil can be supplied as either a permeable or an impermeable cementitous mix, to meet specific project requirements, allowing project engineers to manage ground water flows. It is typically used to backfill load bearing retaining walls, fill voids and sinkholes, and abandon pipes and shafts. Terefil is environmentally inert, cost-effective and uses much less water than traditional options such as cellular lightweight concrete or foam concrete methods.

Permeation Grouting is a commonly used technique for turning granular soils into a solid mass while filling cracks, joints, and other ground defects in non-cohesive conditions. The solution is ideal for resolving ground settlement and subsidence or rectifying other ground problems, such as voids, in locations with soft granular soil or sandy ground conditions.

The secret to building on difficult sites is to design and construct with the appropriate materials for the site conditions, to ensure that a buyer’s ‘house on the hill’ dream is a safe and long-term reality.  Mainmark’s experience and product range provides the designing engineer with targeted solutions to address the demands of the site.


By James O’Grady

James O’Grady is a Sales and Business Development Manager at Mainmark. He is a civil engineer with 25 years’ experience in structural engineering, construction materials and ground treatment.