The main objective of the present paper is the application of a design method, traditionally used for medium- and large-diameter piles, to nongrouted hollow steel bar micropiles with very small diameters. The compressive resistance of this piling system, which is generally used for underpinning works, can be controlled during installation. The adopted analytical design method is based on the results of cone penetration tests to calculate the resistance of the micropiles under compression. Thanks to the availability of field data from micropiles installed at different sites, it was possible to fit the analytical method and to compute the characteristic value of micropile resistance. A Monte Carlo analysis has been used to test the reliability of the method and determine the reduction correlation factor for the calculation of the characteristic resistance. The obtained results show how the analytical method can be effectively adopted for micropiles installed in prevalently coarse-grained soils. An appropriate value of the correlation factor that fulfills the reliability requirements has been derived. On the one hand, this correlation factor is only slightly lower than that prescribed by European regulations (Eurocode 7); on the other hand, it appears much less conservative than that prescribed by the Italian Code. The adopted statistical approach appears rather promising to develop appropriate design methods for similar micropile systems in different soils.
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