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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..).
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