Can piles founded in the intermediate gravel layer in Christchurch perform to your specification? Don’t punch above your weight!

By Mike Abbott

For those unfamiliar with Christchurch geology, Christchurch is underlain with relatively recent alluvium deposits with substantial variability of the layered strata.  Among these, there is often a dense, competent and non-liquefiable gravel layer, which is often viewed as an attractive option for piling.  Clear commercial advantages exist by founding in an intermediate gravel layer as this will be a cheaper piling option over founding piles in the Riccarton Gravel layer that are considerably deeper.  But will this layer offer the appropriate pile response required by the structure and the specification?  What considerations need addressing to ensure a shallow piling option is appropriate?  

Diagram 1: sketch of typical geology of an intermediate layer in the Christchurch region.

Firstly, the specification should identify design loads for all loading combinations as well as providing acceptable deflection criteria.  NZS1170:2002 provides information on serviceability and ultimate loading combinations.  A load case that is often overlooked is the post seismic static load case 1.2G + YQ + Su, where Su is 1.2 times the potential negative skin friction that may act as a result of settling ground following a seismic event.

Acceptable deflections will vary from structure to structure, potentially even varying within different parts of a structure.  Generally, it is the Structural Engineer who will determine the deflection criteria required to protect the superstructure.  In the absence of specific criteria, AS2159:2009 Piling Code provides a set of default deflection criteria based on pile type and size.

Once these criteria are established, it must be determined whether the intermediate gravel layer can provide the necessary strength and resistance to pile deflection.  Factors contributing to pile deflection may include:

• Elastic shortening of the pile shaft
• Structural deformation of the helix
•  Geotechnical deformation of bearing strata
•  Liquefaction induced geotechnical settlement of underlying layer

It is the punching of piles into the lower strength underlying layer (as shown in Diagram 1) that is often not considered.  It is also this factor that is most likely to determine the sufficiency of the intermediate layer to provide the required bearing and deflection performance.  Punching into a liquefiable material cannot be determined by load testing as the weaker material underlying the intermediate layer will not be in its liquefied (weaker) state during testing.  Therefore, numerical analysis and modelling is the only way to justify using piles in intermediate layers. 

We generally consider an elastic stress analysis (Boussinesq 1885) ensuring that the thickness of competent material is sufficient to ensure stress at the interface with any weaker layers is less than the weaker layers capacity.  Determination of the ground strength of liquefied sand can be estimated using equations developed by Stark and Olsen (2002) detailed in their paper ‘Liquefied strength ratio from liquefaction flow failure case histories’.

The following links may be of interest relating to this article:

•  Available on our website is a video showing the effects of load bearing piles on a dense layer overlying a weaker sub-layer HERE. 
•  Geotechnical interpretive report for the Christchurch CBD area HERE.