Look Before You Leap

William Brown - New Business Manager

In New Zealand we’re lucky to enjoy a wide variety of outdoor environments – mountains of rock forced up by earth movement, ancient forests, rocky rivers carrying stones and sand onto open plains, wetland areas, sandstone cliffs and beautiful sandy beaches. As much as we like to get out in these environments, we also like to build in them, and the ground beneath our feet can be as varied as the view from above.

If you’re building, you will probably be expected to get a geotechnical report, but how do you know that your report will be enough to design the foundation that holds up your building, and minimise the chance of nasty surprises (and hidden costs) when you start building?

Something that we often see when a customer first makes contact is that their current geotechnical investigation isn’t detailed enough to design deep foundations, and more investigation is needed. Often this is a valid approach – geotechnical investigation can be quite an iterative process and the first round of investigation often focuses on shallow foundations – why would you look deep if you don’t need to? However, often the need for additional investigation comes as an unplanned expense and time delay, which people could do without. Choosing a geotechnical engineer who is familiar with local conditions can be helpful to reduce surprises in this area, as they may have an idea of what foundation types have worked in your area in the past.

So why bother with the additional investigation?

The simple answer is so that you know what you’re dealing with before you start building. Is that ‘hard’ layer 7 metres below the surface strong enough? Is it thick enough? Does it vary in depth and thickness across the building site, or does it disappear altogether? Will the ground liquefy in an earthquake? Just as rivers meander across the ground, conditions underground can vary from one side of a building to the other. Other times the ground beneath can be quite consistent. Although a geotechnical investigation is only ever an indication of what lies beneath, the more information you have the clearer this picture becomes.

At Piletech, we’re happy to work with your geotechnical engineer to make sure the right information is gathered, saving you time and money, and helping you to identify what lies beneath so that you can plan this into your build.

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.