By Mike Abbott
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
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.