Screw Piling – Sheet PilingPiling since 1994 – Sunshine Coast Based – Travel Anywhere
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Steel Screw Piling Benefits
- Load bearing capacity is related to the installation torque, and this often reveals local soft spots in the site that would not be detected when boring for concrete piers.
- Bored holes for concrete that collapse due to dry sand or a high water table are easily replaced with steel screw piers.
- Load testing is easily carried out.
- Steel screw piers can be used for underpinning, bridges; jetties; house, commercial & industrial building foundations; along easements; electricity and communications towers; boardwalks; pipelines; conveyor belt frames in mining.
- Fast installation time during the vulnerable groundwork period gets your foundations out of the ground.
- Less heavy machinery activity on site.
- Piles are designed for the loads and ground conditions at your site.
- Piles range from 50 kN (5 t) to 2000 kN (200 t).
- Lower set-up cost than deep bored & grouted piers.
- No soil to remove from site
- No noise from hammered piles
- No vibration from hammered piles
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Factors in a Screw Pile DesignWhen we are asked to design a pile, the first information we require is the load on the pile. For example, compression , tension and any lateral loads; identified as SWL, ultimate or limit state terms. Next we need a soil report that gives us enough information to be able to calculate the load bearing capacity of the various soil layers and the settlement that we can expect. This re-quires that the soil report goes about 3.0m below the base of the pier. How do we know in advance how deep the pier will go? We ask that the driller take soil density readings using a SPT (Standard Penetrometer Test) hammer. This is a slide hammer that comes in various sizes. DCP is a small hand held one and the SPT is a truck mounted hammer. We need SPT readings in the order of N=27 in sand and N=57 in clay for a layer of 3.0m thick. Other test methods include the CPT (Cone Penetrometer Test) which is an electronic probe that is pushed into the ground and measures tip pressure, sleeve friction and water pressure. This is my preferred analysis. We also need to know the soil/water pH, chlorides and soil resistivity so that we can work out the corrosion factor for the steel pile and its helix. Once we have this data, we can design the screw pile. We are bound by parameters set in the Australian Standard Piling Code AS2159-2009. The soil report and the pile design are analysed and categorized according to the code and safety factors applied to the bearing capacity of the soils and structural capacity of the pile. We take into account the potential for “down-drag” on the pile by sticky clays. This nega-tive skin friction can be quite significant with larger diameter piles and is usually associated with pile failure. The torque associated with installing the pile is also calculated, and is monitored during in-stallation as a quality control measure to verify that the pile is in the expected material. The soil layers can vary tremendously over a short distance. I have seen instances where two piles were 2.4 m apart and can be 5.0 m difference in their depths. Usually the soil re-port will not show this, and the torque is the indicator that something is wrong. Load testing of the pile is sometimes carried out on site to verify the pile performance.
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. One reason for this is the GDF (safety factor) multiplier of 0.4 for piles without expensive testing. 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 safety factor).
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PILE NOTES – Small tube diameter
I am often asked “Is that pile as small as you can get it ?”; meaning “can it be done cheaper?” Which is a fair enough question.
Generally , the smallest load we see on plans is 50 kN SWL which works out to about 67.5 kN in limit state load.
CORROSION. The piling code generally requires the pile to have a 50 year life and still hold its specified load. We use the generally accepted corrosion factor of 0.03mm/year if we don’t have tests to prove otherwise; so at the end of 50 years with corrosion attacking both sides of the pile tube , the pile thickness will lose 3.0mm.
ECCENTRIC LOAD. The pile load capacity is also affected by the eccentricity of the load from the axial line of the pile. AS2159-2009 says that we have to allow for the greater of 0.05 tube diameter or 75mm off centre.
PILE SHAFT SIZE. Putting the above two together and plotting the effect on a small tube pile of 76mm diameter and thickness of 4.7mm; we can see from the chart that the maxi-mum load with an open ended pile and no grout filling is 22 kN. The same pile with the base capped with no grout is 50 kN and with grout added is 70 kN.
Summary. This analysis is applied to our pile designs and the sizes are controlled by
parameters in the piling code AS2159-2009; which means that the piles are as small as we can safely get them.
100 Link Crescent
Coolum Beach Qld 4573
BSA Trade Contractor # 1044678. (Foundations Piling & Anchors)
ABN 32 107 041 387 QMBA Trade member