Design and construction of multi storey building foundation piles in liquefiable ground in Christchurch, New Zealand

A five storey office building is being constructed at St Andrews Square in Christchurch. The building is founded on soft and liquefiable ground and is located 60 m from the Avon River. The ground conditions are variable both throughout the soil column and across the building. Due to the size of the building and the structural system, column loads are very high and vary considerably across short distances. Ground improvement, shallow foundations and deep foundations were considered. Due to the physical and environmental constraints of the site and construction advantages a bored pile foundation was chosen. The bored piles are designed to carry large vertical loads enabling wide column spacing which is favoured in office style buildings. The contractor also founded a tower crane on the piles to help with super structure construction.

The construction methodology involved open-hole technology which is relatively new in the NZ market. A partial length of temporary casing was used with the remainder of the pile open-hole augered using a combination of bentonite and synthetic polymer fluids for side wall support. It required international support on the fluid technology and pro-active risk management from the contractor and onsite staff. This type of pile is quick to construct which allowed part of the superstructure construction to begin before piling was completed. Design considerations including learnings from the Canterbury Earthquakes, pile capacity, negative skin friction and total and differential settlement are presented. The construction supervision required from the consultant in order to achieve Construction Review PS4 sign off is discussed.