Challenges in construction monitoring – adapting to unforseen conditions: a case study in Riccarton, Christchurch, New Zealand

Geotechnical investigations are limited by the data available through subsurface investigations, which are typically isolated borings, providing snapshots of the subsurface profile. Difficulties can arise were the encountered profiles fail to identify isolated subsurface features. We present a case study of this situation during construction of a Multi-Unit Residential Building (MUB) in Riccarton, Christchurch, New Zealand. Initial investigations encountered interbedded silt and sand with gravel below 5.3 m and one location with peat soils, which were deemed an isolated feature. A gravel raft and concrete slab foundation was therefore designed. While excavating for construction, a linear peat feature was encountered, extending across the building footprint. The remedial solution for this required careful consideration and consultation with the owners and stakeholders. It was deemed appropriate to excavate and replace with ballast to minimise construction delays, however this presented challenges associated with dewatering, liquefaction and creation of sand boils at subgrade level caused by operating machinery. Concurrent to construction of the MUB gravel raft, consideration was given to the suitability of the foundations of the adjacent garage. The peat deposit trended towards the garage footprint and would likely contribute to the same construction challenges. To provide better understanding prior to construction commencing, additional investigation was undertaken in the garage footprint, which raised further uncertainty and ultimately triggered a foundation re-design to a pile supported solution. This project highlighted the difficulties faced within geotechnical investigations, and the importance of construction monitoring in conjunction with additional investigation to minimise project cost and time delays.