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.
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.
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.
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.
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