Screw Piling – Sheet Piling

Factors in a Screw Pile Design

When 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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