title: The Response of Fibre Composite Reinforced Micropiles Under Uplift Loads
Creators: name: Newell, Darren
Publisher: name: Griffith University
Year: 2022
Subjects: subject: micropiles
subject: fibre reinforced polymer
subject: design standards
subject: civil engineering

description: Micropiles have evolved over many decades since their inception in the 1950s through continuous advances in material and construction technologies. They were initially designed as low-capacity elements interacting as a network, but now, micropiles are commonly designed to act individually as high-capacity elements. Their high capacity is achieved through the use of a high proportion of reinforcing steel that is encapsulated in a cementitious grout. The use of fibre reinforced polymer (FRP) reinforcing materials in civil engineering structures such as piles is relatively new with a relatively low uptake by industry. Micropile design standards and guidelines are based on the use of steel reinforcing, but the use of FRP in lieu of steel may be feasible in certain situations. A key characteristic of FRP reinforcing material is its low modulus of elasticity compared to steel (around four times lower). Due to their small diameter ( 300 mm), micropiles will often behave as slender structures, which inherently results in low lateral capacity. For this reason, they are often designed to act as a group, meaning they are often required to act as tension piles. Typically, the stiffness of tension micropiles is based on the assumption that the grout is cracked and the reinforcing material alone provides the structure’s stiffness. This research investigated the use of FRP reinforcing bars in micropiles and examined their load-response behaviour compared to equivalent steel reinforced micropiles. Full-scale tension load tests were undertaken on both FRP and steel micropiles installed in single and multilayered, partially saturated cohesive soils. All test micropiles were instrumented with resistance strain gauges which were calibrated under laboratory conditions prior to installation. The results from the load tests show that the displacement of FRP reinforced micropiles is substantially higher than that for equivalent steel reinforced micropiles. Some methods used for assessing the uplift capacity of micropiles are based on the rate of displacement (mm/kN) measured at the pile head. This research identified that some methods that use a displacement rate for the failure criteria may underestimate an FRP reinforced pile’s uplift capacity due to their higher displacement rate under tension loads. The lower stiffness of an FRP reinforced micropile is somewhat offset by the tension stiffening effect of the grout. This research also investigated the extent to which this stiffening contributed to the stiffness of FRP reinforced lean cement grout structural elements. Laboratory testing was performed on both FRP and steel reinforced lean cement grout specimens. When the specimens were loaded in uniaxial tension, it was found that the lean cement grout increased the overall stiffness of the FRP reinforced specimens by around 90 percent. However, this increased stiffness is only present up to cracking of the grout, which occurred at relatively low loads (5 percent of the ultimate tensile capacity). Therefore, this research confirmed that the widely adopted assumption that the stiffness of a tension micropile be based on a fully cracked state is also valid for FRP reinforced micropiles.