SEDIMENT STABILITY ON THE WESTERN FLANKS OF THE CANARY-ISLANDS

Sediment failures on the flanks of the Canary Islands are well documen ted but few geotechnical studies have been conducted. In this study th e sedimentological and geotechnical properties (water content, grain s pecific gravity, bulk density, grain-size, consolidation and triaxial test data) of sediment recovered in cores from the submarine slopes of the island of La Palma, Canary Islands, NE Atlantic indicate that the sediment is heavily to lightly overconsolidated; the overconsolidatio n is considered to be apparent, in the absence of any evidence suggest ing sediment erosion. Normalised strengths range from 0.53-0.77 for th e carbonate-rich sediment. The sediment stability was semiquantitative ly assessed. The sediment off La Palma is stable under drained and und rained static loading conditions at present, however, if affected by h orizontal ground accelerations in excess of 15% of gravity the sedimen t will become unstable. Locally, on sediment waves for instance, slope angles may be greater thus enhancing the potential for sediment failu re. Triggering can potentially be explained in terms of the likely mag nitude and frequency of seismic accelerations, although other trigger mechanisms may have enhanced the failure potential of these sediments in the past. Steady state analysis indicates that all the sediments te sted are state II type and are unlikely to result in disintegrative mo rphologies (e.g. debris flows) after initial failure. The sediment in the source region of the Canary Debris Flow must, therefore, have unde rgone transition from state II to state I type sediment perhaps brough t about by a progressive style of failure during or after initial fail ure. This may be caused by a debris avalanche from the volcanic island flanks.