Zyklische torsion eines Rohrs in sand

The cyclic torsion of a tube in sand is investigated experimentally and numerically in order to allow a better understanding of the mechanisms controlling the development of settlements and the reduction of penetration resistance when driving a tube into the soil with the aid of cyclic torsion. In the experiment a tube was installed in the centre of a loose sand cylinder and rotated cyclically with constant amplitude, while the surface settlements were measured. In the numerical simulation the FE-method was applied. The behaviour of the sand was modelled using a hypoplastic constitutive law. The results show that shear cycles induced by the cyclic rotation of the tube lead to densification of the soil around the tube. Since densification decreases with depth and distance from the tube a settlement funnel develops. At the same time a cycle-wise reduction of the effective stresses takes place which leads to a decay of the pressure normal to the wall and thus, following the Mohr-Coulomb friction law, a cycle-wise decay of the friction force acting on the tube wall.