Interpretation of failure load tests on micropiles in heterogeneous Alpine soils

This paper is concerned with the execution and interpretation of load tests on small diameter piles, commonly referred to as micropiles, drilled and grouted under gravity only into highly heterogeneous soils. These soils, forming the slopes of many areas in the Italian Alpine Region, are composed of a chaotic and erratic mixture of gravel, sand with silt and clay including cobbles and boulders and can be often poorly characterized, due to the difficulty in performing laboratory tests or in-situ tests, with the exception of the classical dynamic penetration test. Nevertheless, these soils show significantly high particle interlocking and dilative mechanical response under shear, thus providing both high base resistance and shaft friction at the relatively low overburden stress surrounding the micropile, even if the latter is grouted without any additional grouting pressure. As a consequence of that, the micropile design using the customarily used approaches leads often to much conservative estimate of the vertical limit load. To improve the design of micropiles in such soil deposits, an experimental test site, located in the Northeastern Italian Alps, has been selected, where pile tensile and compressive load tests up to failure have been performed under controlled conditions. On the basis of the results of these tests, the reliability of the most common micropile calculation methods is discussed.