Constitutive modeling of ice rubble in first-year ridge keel

Abstract

In-situ full scale loading tests were conducted in the Northern Gulf of Bothnia in order to measure the ridge keel mechanical properties. Altogether 33 loading tests in full scale were conducted during five winters (1998-2003). 12 of them were punch shear tests, in which a circular plate of the consolidated layer was punched downwards to break the rubble underneath. In all ridge loading tests, the sail was first removed and the consolidated layer was cut free from the surrounding solid ice field to allow well defined boundary conditions. Maximum loads in the punch shear tests varied from 74 kN to 1.1 MN. The diameter of the platen varied between 2.5 and 4.7 m. The average keel depth varied from 3.3 to 6.4 m while the corresponding effective thickness of rubble under the platen varied from 2.2 to 5.0 m.

A continuum material model for ice rubble was developed and implemented into commercial finite element software ABAQUS/Standard. The constitutive law was written in similar form to that used in the plasticity theory based on the strain decomposition into elastic and plastic parts. The shear cap yield surface with evolution laws both for cap hardening and cohesive softening describe also the compaction phenomenon in addition to shear failure.

An axisymmetric finite element model was created to simulate punch shear tests. Time history analysis in finite element method observes progressive failure through the keel occurring non-simultaneous global keel failure. Good agreement in the load-displacement relationship was achieved by calibrating the material parameters to fit the full-scale measurements. The admissible combination of cohesion and the friction angle was evaluated by parametric studies to simulate the measured maximum force correctly. The failure progression in the keel and the relation between the failure modes (compaction and shear) depended strongly on the friction angle. Increased friction resulted in more dilatation at the region of shear failure and more compaction at the region of cap failure. Due to the volumetric expansion during shear failure, a slightly inclined shear failure zone created finally a conical plug punched through the keel.