Centrifuge tests on suction anchors
The results from a number of centrifuge model tests on suction anchors in soft clay performed by LCPC in Nantes in 2005 have been interpreted. The tests were performed under 100g in order to model prototype anchors with a diameter of 8 m and a penetration depth of about 24 m.
The anchors were with and without internal ring stiffeners. The interpretation was mainly performed as part of a project for Total which has been reported in 20071857-1. However, due to complex soil conditions and some rather unexpected results additional FE analyses have been performed as part of this research project.
It was first of all the following main results that were difficult to understand:
| 1. |
The measured net rapid vertical pull-out capacities of the anchors with ring stiffeners are significantly larger than without ring stiffeners. |
| 2. |
The measured under pressures (suctions) at skirt tip level are more negative than the pressure corresponding to a realistic inverse bearing capacity of the soil plug. |
Based on Plaxis analyses, it is supposed that the increased pull-out capacities of the anchors with ring stiffeners are that more soil is displaced outside the anchor during installation for the anchors with stiffeners which increases the subsequent set-up factor along the outer skirt wall. Furthermore, the unrealistic suction at skirt tip level is expected to be due to relative displacements between the pore pressure probe and the soil during pull-out.
Figure: Measured net vertical pull-out capacity normalised by the measured penetration resistance for anchors with and without stiffeners (upper left), measured and calculated excess pore pressure versus applied pull-out force (lower left), calculated vertical stress around skirt tip for an anchor with (upper right) and without ring (lower right) stiffeners.