Foundation correction works on soft soil: insights from practice

Peat/organic soils are characterised by low undrained shear strength, high compressibility and are susceptible to significant ground settlements, creating challenges for engineers and developers. Since construction in 2015, a housing subdivision in Auckland, has been impacted by ground settlement and distortions to foundations and superstructures. The pattern of distortions are indicative of differential settlements resulting from mixed foundation types, being part-piled and part-ground bearing over soft compressible peat.

Mixed foundations are sometimes used where dwellings are built adjacent to or over services, with piles being used to transfer loads to below the level of the services. A number of dwellings within the subdivision, which were constructed with mixed foundation types, have required regular intervention works to maintain dwelling serviceability. Foundation distortion correction works were trialled on one damaged property with the purpose of proving the adopted remedial methodology and substantiating future foundation performance.

The remedial works involved disconnecting piles and implementing staged and cycled surcharge loading with water tanks on the foundation slab. This method was used to verify the performance of the slab and to measure the dwelling response to the change of support. Water tanks were positioned near the locations of the most significant distortions and subjected to loading cycles over several weeks while monitoring changes in floor distortion. The trial results successfully demonstrated compliance of the corrective works with the performance expectations of the New Zealand Building Code.

This paper presents the challenges and solutions implemented on the trial property during the foundation distortion correction works.