Injection Micropile Bar Fatigue Resistance at Loads Lower and Greater than the Yield Strength of Steel

One of the techniques commonly applied today for deep foundation construction is based on self-drilling injection micropiles. Micropiles are structural elements intended primarily for reinforcing foundations and buildings, particularly under difficult terrain conditions. The goal of the tests presented herein is to inspect the fatigue resistance, strength and ductility of injection micropiles formed from 28Mn6 steel at loads significantly exceeding the values defined for the fatigue test in the requirements of the relevant European Assessment Document (EAD). The test results and the micropile bar strain model εM presented in this paper are primarily of interest to designers for the purposes of determining the fatigue resistance of steel micropiles, which find particular application in land degraded by mining activity that is characterised by frequent terrain vibration and mining-induced tremors. None of the R25N injection micropile bars failed during the fatigue resistance testing at 2·106 cycles at a load Fu = 0.7·FRe0.2 (under the yield strength of the 28Mn6 steel) as well as at Fu = 1.0·FRe0.2 and Fu = 1.2·FRe0.2, where the bars operated at the limit of and significantly above the load FRe0.2 which results in stress at the yield point of the 28Mn6 steel. Furthermore, the bar tests conducted at static and cyclic loading demonstrated the high strength and good ductility of the 28Mn6 steel.