A large two-storey home situated in Papanui, Christchurch, suffered extensive damage during the 2010 and 2011 Christchurch earthquakes.
The 275m2 property featured
a concrete slab-on-grade foundation with interconnecting deep piles that were supporting the home’s internal and external perimeter.
Prior to the earthquakes, it was considered unusual to find voids under large structures such as this home. However, rounded river gravel tailings had been used as a sub-base under the home’s slab and, due to the severity of the seismic forces, these particles had been shaken to such an extent that the piles became exposed and large voids formed beneath the home.
The voids had subsequently impacted the performance of the slab, which was now only supported on the piles and had a noticeable hollow sound. While an initial geotechnical report undertaken by the homeowner had estimated the voids to be approximately 250mm deep, a pre-works scan using ground penetrating radar (GPR) technology later confirmed the voids were between 300mm to 500mm deep, almost double in size than originally indicated in the geotechnical report.
The homeowner’s building contractor engaged Mainmark to deliver a cost-effective and non-invasive void filling solution to reinstate the foundations without damaging the floor tiles. The tiles, which covered 50 percent of the home’s floor area, were very important to the homeowner as they were imported from overseas and had been discontinued.
The client required Mainmark to re-support the home by filling ground voids that had formed beneath the concrete floor slab, restoring the structural integrity of the foundations to their pre-earthquake state. It was also important that the solution was non-invasive to ensure the home’s floor tiles could be retained.
Mainmark recommended a combination of their proprietary Teretek® engineered resin injection solution and Terefil® lightweight cementitious grout to reinstate the foundations and fill the voids beneath the floor slab.
Mainmark first injected Teretek around the home’s ring foundation to seal gaps and minimise the risk of the Terefil grout seeping beyond the perimeter of the home. Teretek is an advanced polyurethane resin that is injected through very small tubes into the ground beneath foundations. On entering the ground, the components mix and expand by chemical reaction, compacting soils and creating an impenetrable and impermeable seal.
Once the home’s foundations had been sealed using Teretek, Mainmark proceeded to fill the voids beneath the internal floor slab using Terefil, a lightweight cementitious grout. While Teretek resins can also be used as a void fill solution, Terefil was considered a more appropriate and economical solution for this project due to the extensive amount of voiding and volume of product required. Terefil was also able to provide a superior level of structural support to the exposed foundation piles.
Mainmark had to carefully balance the amount of Terefil product used on site to ensure the project achieved the required engineered outcomes, while minimising additional cost to the homeowner.
Despite the increased scope of work, Mainmark was able to successfully complete the project in 10 days using proprietary non-invasive solutions that also allowed the floor tiles to be retained.
Project Resources & Downloads
Terefil® and Teretek® Fill Voids Beneath Earthquake Damaged Home
Print At Home Crack Gauge
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Teretek® Re-Levels External Brick Staircase and Remediates Cracks
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Mainmark Ground Engineering Pty Ltd
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