Geotechnical characteristics and slope stability in the Gulf of Cadiz

Sedimentological and geotechnical analyses of thirty-seven core samples fro m the Gulf of Cadiz continental margin were used to define the regional var iability of sediment properties and to assess slope stability. Considering the sediment property data set as a whole, there is an association between grain size, plasticity and water content. Any one of these properties can b e mapped regionally to provide an indication of the dominant surface sedime nt lithology. Based on static sediment strength, a simplified slope stabili ty analysis showed that only steep slopes (>16 degrees for even the most vu lnerable sediment) can fail under static loading conditions. Accordingly, t ransient loads, such as earthquakes or storms, are needed to cause failure on more moderate slopes. A regional seismic slope stability analysis of the Cadiz margin was performed based on detailed geotechnical testing of four gravity core samples. The results showed that the stability of these slopes under seismic loading conditions depends upon sediment density, the cyclic loading shear strength, the slope steepness, and the regional seismicity. Sediment density and cyclic loading shear strength are dependent upon water content, which can act as a proxy for plasticity and texture effects. Spec ifically, sediment in the water content range of 50-56% is most vulnerable to failure under cyclic loading within the Cadiz margin. As a result, for a uniform seismicity over the region, susceptibility to failure during seism ic loading conditions increases with increasing slope steepness and is high er if the sediment water content is in the 50-56% range than if it is not. The only sampled zone of failure on the continental slope contains sediment with water content in this critical range. Storm-wave-induced instability was evaluated for the continental shelf. The evaluation showed that a storm having hundreds of waves with a height in the range of 16 m might be capab le of causing failure on the shelf. However, no sediment failures were obse rved on the shelf that might have been caused by this mechanism. (C) 1999 E lsevier Science B.V. All rights reserved.