Buckling Behavior
Buckling is typically ignored for conventional piling made of steel, concrete, and timber. Because of its low modulus, polymeric piling might buckle under extreme loading conditions or during driving. Han and Frost evaluated the buckling of vertically loaded fiber reinforced polymer piling, including the effects of shear deformations. They concluded that “buckling of FRP piling may occur only when the surrounding soils are very soft or when a large portion of the pile extends above the ground.”
Interface Friction
Polymers exhibit a lower surface hardness and a higher surface roughness than conventional piling materials. Experimental studies have been performed to characterize the interface behavior of FRP and soils. These studies suggest that t δ(FIBERGLASS) =δSTEEL can be used in the design of piling made of FRP. The interface friction of geosynthetics made of HDPE is typically in the range of 8–15°. Therefore, the interface friction angle of RPP made of HDPE δ(HDPE) might be significantly lower than that of steel, on the order of 8° to 15°.
End Bearing
Virtually no research has been performed on the bearing capacity of polymeric piling. Nevertheless, it is believed that the bearing capacity of polymeric piling is similar to that of conventional materials, because bearing capacity is controlled by the properties of the soil, not of the pile.
Structural Design
Concrete, steel, and timber are much stronger than sand and clay. Therefore, soil properties dominate the design considerations of conventional piles. As a result, conventional design practice is mostly involved with determining a suitable factor of safety against geotechnical failure. Polymeric materials exhibit a non-linear elastoplastic-viscoplastic behavior, which might influence the structural design of polymeric piling in a number of ways, as follows:
- Polymers respond differently according to the type, duration, and rate of loading. Therefore, polymeric piling systems are expected to resist rapid and short-term loads, such as driving loads, with less deformation than they would long-term loads such as dead loads. Different ultimate strengths and moduli of elasticity are needed for analysis of different loading conditions.
- Under long-term loading, the allowable creep stress is typically smaller than the ultimate capacity of many polymers and might thus control the allowable structural load capacity of the pile. Viscoelastic/viscoplastic creep of polymeric material might also influence soil structure interaction and the load transfer mechanism, but this effect has not been investigated.
- Depending on the reinforcement ratio of RPP, strain compatibility considerations might cause the ultimate load capacity of a composite member to be dominated by the response of the FRP particularly at small strains. The presence of the weaker material (HDPE) is, however, essential in order to prevent buckling of the entire cross section.
Primary References
- Iskander, M. (2012). “Sustainable piling made of recycled polymers, state of the art review,” Journal of ASTM International, Vol. 9, No. 2, doi: 10.1520/JAI103677, ASTM.
- Iskander, M., and Bozorg-Haddad, A., (2011). “Spatial Distribution of the Compressive Stress-Strain of Recycled Polymeric Piling,” Journal of Testing and Evaluation, Vol. 39, No. 4, 103198. [link]
- Iskander, M., A. Mohamed and S. Sadek (2003). “Compressive strength of foamed polymeric piling,” Proceedings, Transportation Research Board Meeting, CD, Paper No: 03-2448, National Academy Press. [link]
- Iskander, M., A. Mohamed and S. Sadek (2003). “Compressive strength of foamed polymeric piling,” Proceedings, Transportation Research Board Meeting, CD, Paper No: 03-2448, National Academy Press. [link]
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