This paper reports on an experimental investigation on the behaviour of small-diameter piles, commonly referred
to as micropiles. This particular type of deep foundation is frequently used in many barely accessible Italian
mountainous areas, often characterised by complex ground profiles composed of mixtures of coarse soils with some
fine matrix elements including cobbles and large-diameter boulders. In such ground conditions, the lack of reliable
site and laboratory geotechnical investigations providing an accurate soil mechanical description and conservative
approaches for micropile design often lead to significant underestimation of the vertical ultimate load. In order
to improve micropile design in such geological contexts, a new field trial investigation involving tension and
compression load tests on micropiles up to failure was set up in a selected test site located in the Italian alpine region.
From interpretation of the load tests carried out so far, the reliability of commonly used calculation methods for
estimating bearing capacity is discussed. As a result, a new approach for a more suitable calculation of the mobilised
shaft and base resistance of micropiles bored in highly coarse soils is tentatively propos
