Triggering mechanisms of slope instability processes and sediment failures on continental margins: a geotechnical approach
Introduction
A triggering mechanism is an external stimulus that initiates the slope instability process. According to Locat and Lee (2002), in marine domain, examples of triggering mechanisms include (1) oversteepening, (2) seismic loading, (3) storm-wave loading, (4) rapid sediment accumulation and under-consolidation, (5) gas charging, (6) gas hydrate dissociation, (7) low tides, (8) seepage, (9) glacial loading and (10) volcanic island processes. These mechanisms can be considered as short-term triggers. In addition to the trigger mechanisms, causal factors can contribute to instability but may not initiate failure. Those causal factors are considered as long-term triggers. In marine domain, the list of causal factors can include the slope angle, mass-movement history and unloading. Unloading can be very critical in the case of gassy sediment. Human activities such as slope loading can either be triggering mechanisms or causal factors depending on other slope conditions.
The primary focus of this study is to identify the relation between triggering mechanisms and causal factors on one hand and the stress state and geotechnical parameters on the other hand. In the costa project, from south to north, the working areas cover three sectors of the European continental slope:
- 1
Adriatic margin;
- 2
Western Mediterranean margin;
- 3
NE Atlantic margin.
Table 1 presents all the possible trigger mechanisms for the origin of the observed slope failures in the Costa target areas.
Section snippets
Sediment behaviour in response to a cyclic loading (earthquake)
Under cyclic loading, the sediment dynamic behaviour is influenced by the intensity and duration of the cyclic loading and the state of the sediment (the grain size distribution, the presence or absence of a clay fraction, and the degree of saturation). Cyclic loading may induce degradation of soft clays (see, for instance, Pestana et al., 2000) such as those found in many marine deposits. Indeed, the cyclic loading is associated with (1) degradation of the sediment stiffness, (2) degradation
Geotechnical data from different sliding areas
The aim of this study is to identify the possible trigger mechanisms at the origin of the observed slope failures in the Costa target areas. Thus, different geotechnical parameters were identified from the different areas by carrying out in situ and laboratory measurements (Table 3).
Three types of laboratory tests were used in order to identify the relevant geotechnical parameters (Table 3): (1) Saturated sediment submitted to static loading: to identify the basic classical geotechnical data;
Conclusion
In this work, we have analysed the different slope failures events from the Costa target areas, which reflect diverse triggering mechanisms. Among the three external mechanisms studied to identify the geotechnical response of the sediment and to evaluate the consequence on the slope stability (earthquakes, rapid sediment accumulation and gas hydrate melting), we can mention that the excess pore pressure was recognized as a key parameter for the assessment of slope stability. Laboratory tests
Acknowledgements
This work has been developed within the European Commission (EC) project COSTA (EVK3-1999-00028). Authors have also benefited from the support of the EC funded projects EURODOM (RTN2-2001-00281), EUROSTRATAFORM (EVK3-2001-00200), EURODELTA (EVK3-2001-00033) and AMASON (EVK3-2001-00067), the European Science Foundation WEST-MED (REN2002-11216-E MAR/01-LEC-EMA22F) and SPACOMA (REN2002-11217-E MAR/01-LEC-EMA14F) projects and the Spanish PRODELTA project (REN2002-02323). Generalitat de Catalunya
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