One of two residential homes in a duplex property, occupying a 20m high sandstone and siltstone clifftop in Auckland, was suffering from severe cracking due to differential ground settlement.
The affected rear dwelling was situated 11m from the coastal cliff crest and built on concrete foundations measuring approximately 20 lineal metres. The lower level of the home was constructed from masonry blocks, with a timber upper level and a timber deck attached.
Prior to selling the property, the owner was seeking a solution to address the ground settlement which had resulted in substantial cracking in the ground floor masonry walls, as well as cracking in the driveway and lawn area above the cliff crest. The front wall of the home had settled up to 76mm, and the timber deck had also moved horizontally away from the building, exposing nails used to secure the deck to the upper level wall.
Geotechnical investigations were undertaken beneath the driveway and the slopes descending from the property boundary to help identify the cause of settlement. The inspection revealed uncertified, non-engineered fill had been placed directly onto the silty clay topsoil when the home was constructed, and this was the primary contributing factor for the differential settlement. If left unaddressed, ground settlement would continue to damage the property which would eventually require expensive repairs.
The homeowner had initially considered traditional underpinning methods, but found that it would be an expensive and time intensive process, potentially taking weeks to complete.
As the structure was part of a cross-lease duplex, approval from the adjoining property owner was required before works could be undertaken. Fortunately, the other property owner was happy to collaborate having used Mainmark previously, and had recommended their ground engineering solutions as an alternative to costly underpinning.
The priority was to return the building as close to level as possible by re-supporting the concrete strip footing that had been affected by the extensive ground settlement. It was determined that the building should be re-levelled by 38mm to meet current building code tolerances, prevent further damage to the home, and allow the homeowner to progress with the sale of the property.
Mainmark’s engineered resin injection solution, Teretek® was ideal for treating the settlement and improving ground bearing capacity of the non-engineered fill. The residence needed to be re-levelled in careful increments to avoid placing further stress on the two level building.
Liaising closely with the other duplex owner prior to and during works, Mainmark was granted permission to position its self-contained Rig on the neighbouring property in order to safely access and treat the affected ground.
In a process likened to keyhole surgery, Teretek® resin is injected into the ground through tiny tubes, and expands within minutes, filling any voids and compressing the surrounding soil or fill. Once in the ground, the resin remains inert and does not break down, providing a long-term solution against the risk of further subsidence.
Teretek® was injected into the non-engineered fill to a depth of 1.5m below ground level. The deep-lift application method was used to inject the Teretek® deeper beneath the affected sections to maximise ground bearing capacity under the home, gradually lifting the structure with controlled force as the resin expanded.
Technicians constantly managed the process using laser levels to monitor the foundation lift as 380kgs of Teretek® was injected. The target of lift of 38mm was achieved, being the maximum amount of lift required to meet building code tolerances, while also avoiding any damage to the home. The entire process was completed within a single day, without the need for excavation works that may have caused further damage to the property or grounds, and allowed the owner to proceed with the sale.
Project Resources & Downloads
Teretek® Re-levels Clifftop Home Impacted by Differential Ground Settlement
Print At Home Crack Gauge
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Grout injections Improve Earthquake Resilience For Christchurch Apartment Building
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