Behaviour of screw micropiles subjected to axial tensile and compressive loading

Screw micropiles or “ground screws” (diameter less than 200 mm, length less than 5 m) consist of a steel tubular shaft with continuous spiral threads, and a threaded tapered segment; the installation is achieved by rotary driving. Although ground screws are useful in many applications as well as environment-friendly, research on their behaviour is limited. The paper presents an experimental study on the response of screw micropiles subjected to tensile and compressive axial loading. Twenty full-scale load tests were conducted on four types of screws, with diameter 66-114 mm and length 0.8-1.6 m, at a test site in Italy, characterized by the presence of predominantly clayey soils. The micropiles achieved tensile and compressive capacities of 23-60 kN and 20-75 kN respectively, with pile movements in the range 1-9 mm. The results were interpreted in the light of a recently developed method to compute bearing capacity of screw micropiles.