DEEP FOUNDATIONS - Items for consideration
What material is the pile sitting
on and how thick is it ?
The thickness of the material needs to be about 5 times
the diameter or width of the pier to found effectively,
usually about 2.0m. The material on the coast is
generally sand, clay, silt, a combination or rocky
stuff. The height of the water table is critical to the
load bearing capacity of the soil.
Soil report. Does it go deep enough ?
It should go down at least 2m below the planned founding
level. Has it identified how thick and what is below the
first hard layer it encounters. The hard layer may be
only a 100mm thick layer of cemented shells sitting on
highly compressible marine clay. This profile is common
on the coastal plain.
CLICK HERE for article "Inadequate
Soil Reports and Deep Foundations".
Bored piers. Will the hole collapse? Can you get the
depth ?
In sandy and high water table areas, the soil often
collapses before the concrete is poured. Also,
conditions across a site will vary and the bearing
capacity of piers varies considerably. With bored piers,
our experience is that they often do not found on
suitable material. With screw piers, the installation
torque is an excellent measure of the pile capacity,
although other factors do need consideration.
Will the founding soil change its load bearing capacity
?
Our experience shows that silt content can cause the
founding layer to be either rock hard in dry conditions
or slush in wet. Also high activity by earthmoving
equipment can change the bearing capacity of the soils.
It is easy to get it wrong in coastal areas.
A soil
report done before and after heavy activity will show
different results in certain soils.
“Piles ain’t piles”…
A pile has two main components, the
shaft and the helix. Basically, the helix carries the
building load and the shaft connects the load to the
helix. The shaft size needs to be strong enough to allow
the installer to penetrate thin hard layers as well as
obstacles such as roots and pipes and rocks.
The shaft
and helix need to be able to withstand corrosion and
still deliver their rated capacity over time. This shaft
size is a major cost component of the job. Screw piles
are from 1.5m to 30m in depth.
As they go deeper, the
shaft needs to be stronger to resist lateral bending,
and to overcome the friction of the soil against the
pipe. This skin friction is very significant at depths
below 5.0m; and is often many times the size of the
building load.
Differential settlement in foundations.
This needs consideration in some building designs, and
commonly where the pool and house levels need to be held
constant. Generally, the pool and house piers need to be
at the same level or in identifiable material to
minimise differential settlement.
Do Grouping Factors
come into play ?
Pile design and installers.
Deep foundations require a
clever combination of soil mechanics, engineering design
and proper judgement derived from past experience and
testing. The piling company needs a Qld BSA Contractors
licence and the installer should have accurate and
sensitive monitoring equipment as well as a Qld BSA
Supervisors licence.
Local Installers.
Your local installer, if experienced, will have
geo-technical knowledge that is specific to your area,
and will be able to provide on-site advice regarding
logistics, sequence of events, potential problems and
able to carry out test piling to accurately determine
costing.
Your local installer can more cheaply
accommodate delays in the sequence of events; such as
cutting-off piles after footings have been dug or other
variations.
Testing.
If in doubt about the soil test and expect to go deep,
or if the building is costing $1mill+, then ask for a
“CPT Test, and find the bottom”. The Cone Penetrometer
Test is a truck mounted unit that will electronically
probe the depths and identify the materials. A CPT
printout is at the end of this article.
Instrumentation.
Generally the safe working load of a
pile is 2.0 - 2.5 times its working load. A calibrated
Digital Torque gauge is +/- 5%, whereas a hydraulic
gauge can often be out by 40%. Make sure that your
installer has a digital gauge.
Phone NOW for
assistance with your deep foundations.
0409 133 455
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