What Do We Need To Design a Pile??
The answer lies in the soil….
To design a pile, we need to know about the soil profile under your building .
There are usually many different layers or combinations of clays, silts, sands, weathered rock and water. All have greatly different characteristics, both desirable and un-desirable.
We work out their load bearing capacity and their settlement characteristics and the effect that a varying water table level will have on them.
We then design the pile to transfer the building loads down to the most appropriate soil layer.
Most important to us is the quality of the geo-technical report in classifying the soils and the measurement of soil strength.
The information we require comes from the borehole samples and the strength of the various soils which is calculated from the DCP or SPT or CPT tests which are reported on the borehole log as counts of blows per 100 or 150mm of depth which are then converted to “N60” results.
Other laboratory information such as Atterberg limits, compressive, shear and pH tests are reported and used.
(SPT = Standard Penetrometer Test. CPT = Cone Penetrometer Test. DCP = Dynamic Cone Penetrometer).
DCP’s are usually only useful for the first couple of meters in depth. DCP’s are a small hand held sliding hammer on probe rods.
Better results are from SPT’s which are truck mounted slide hammers and can penetrate 30m or more.
CPT’s are electronic probes that are pushed down and give continuous readings, and from which many other characteristics can be calculated. These are my preferred data.
How deep do we need to test ??
Basically, the load from the building is transferred through the screw pile into the ground beneath the pile and is dissipated in a depth about 5 times the pier or helix diameter. For a helix diameter of 600mm that is about 3.0m deep. (Also, some load is transferred through the sides of the pile.)
For domestic buildings, we need a minimum consistent SPT reading for 3.0m of N=27+ in sand and N=55+ in clay.
If you advise your Geotechnical consultant or driller of that requirement, you will probably get the required information in the first soil report.
Sometimes the report can show up anomalies that require further investigation.
The latest Piling Code AS2159-2009 has an assessment of the factors associated with the piled foundations and soils, and allocates a numerical risk assessment which relates to a design safety factor. The better the soil information means that the safety factor is lower and the pile will be cheaper (less over-engineering..).