Contractors and structural engineers went to great lengths to save an earthquake-damaged house in the Christchurch suburb of St Martins, with foundation piles 17 m long.
For any structure, piled foundations need to be extended down until they reach soil of suitable strength to hold the structure’s weight. Generally, the weight of a residential house and the soil type below means sufficient bearing strength is available not far below the ground surface.
This wasn’t the case in St Martins, a Christchurch suburb suffering damage in the February earthquake. The quake had shaken several timber-framed houses off their shallow foundations. These neighbouring properties were propped on temporary piles awaiting a permanent fix.
Contractors and structural engineers were amazed when foundation piles 17 m long were necessary to support one house, a length considerably greater than the 1 m piles usually used on residential properties.
Drilling for the good soil
In St Martins, a thick layer of weak clay meant sufficiently strong soil was a long way below ground. Builder Russell Perrett has over 23 years’ experience building across Christchurch, and hadn’t seen anything like it before. When drilling test holes to gauge the depth of the soft clay layer and understand how long the piles needed to be, the result far exceeded his guestimate. His expectation was 6.5–8 m, not 17 m.
Thinking outside the square
With the house sitting on shallow temporary foundations, the work was urgent. The structure was at risk of greatly magnified shaking and damage in the event of further earthquakes.
Seismic expert Bernard Toh of Wellington-based Skytec was engaged to design suitable piles. Bernard recognised that the house needed to get back onto permanent foundations as soon as possible to minimise further risks. ‘We needed to think outside the square and make use of materials that were on hand.’
His design used telegraph poles as the pile structure. Two poles were connected end to end by welding together steel brackets placed on the tips of each. This provided the necessary length. Before installation, a prototype was created to ensure all design and performance standards were met. Telegraph poles have been used as piles for structures in the past, but not on a residential property.
Residential piles may have to be much longer
The unexpected depth of weak soil remains a puzzle. The two possible scenarios are:
- an isolated deep pocket of this weak clay soil type, with surrounding areas much shallower
- the movement caused by successive earthquakes has further weakened already weak soil.
In this case, it appears the existing house piles were never up to the task.
Upcoming residential repiling works – of which there will be a large number across the city – may result in previously existing short piles being replaced with much longer structures.
Articles are correct at the time of publication but may have since become outdated.