Experimental Investigation of Installation and Pullout of Dynamically Penetrating Anchors in Clay and Silt

Abstract

This paper reports the results from a series of model tests undertaken to provide insight into the behavior of torpedo anchors during dynamic installation and pullout in lightly overconsolidated kaolin clay and calcareous silt. The tests were carried out in a drum centrifuge at 200g, varying the drop height (hence the impact velocity) and the time delay for consolidation before pullout. The pullout angle at the mudline was also varied to encompass various mooring systems, including catenary (0°), taut leg (45°), and tension leg (∼80°). Two geometries of torpedo anchors were explored, varying the fin and tip geometry. The results demonstrated that the anchor embedment depth increased as the drop height (and hence the impact velocity) increased and the soil undrained shear strength decreased. In stronger silt, the cavity above the installing anchor remained open, whereas in soft clay, it was fully backfilled and replenished. The corresponding anchor embedment depth was also about 0.63 times compared with that in clay. The anchor holding capacity was found to increase with increasing postinstallation consolidation time, depth of embedment, and soil undrained shear strength and with reducing pullout angle at the mudline. The anchor rotation during pullout, and hence the pullout distance for attaining the maximum capacity, reduced as the load inclination at the mudline increased. The fin geometry (rectangular or elliptical) and tip geometry (conical or ellipsoid) were shown to have remarkable influence on the holding capacity, with rectangular fins and conical tip proving to be more effective. The negligible adherence of silt along the anchor sides and the presence of semiliquid material around the extracting anchor resulted in a significantly (34–47%) lower holding capacity in the calcareous silt, with significantly higher intact undrained shear strength.