The cohesive Naranjo debris-flow deposit (10 km 3):
Mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams. A good example of this occurred 18.5 ka ago when the eastern flank of the Nevado de Colima Volcano collapsed producing a debris-avalanche deposit that was previously described as one of the largest in the world. The deposit extended from the volcanic summit as far as the Pacific coast, 120 km away. New stratigraphic, sedimentological, and componentry data suggest that the volcanic collapse of Nevado de Colima resulted in a debris avalanche that traveled 20 km southeast to the Naranjo River. There it crashed against a topographic barrier consisting of Cretaceous limestones (Cerro la Carbonera) and the flow direction was diverted to the south down the Naranjo River channel for another 25 km before the avalanche came to a halt. The obstruction of the drainage produced a temporary dam that stored ca. 1 km3 of water and deposited fluvial and slack-water sediments. Some time after the damming, the accumulated water-sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3.5 million m3/s. The resulting flood (cohesive debris flow) followed the channel of the Naranjo River and, due to the high erodibility of the channel and introduction of substrate material, the debris flow progressively increased its volume up to 10 km3, six times its initial volume. This study highlights the relevance of evaluating the potential remobilization of debris-avalanche deposits to initiate large magnitude cohesive debris flows. Therefore, the hazard and risk analysis of future potential events of this nature must consider the pre-eruption conditions and the topography surrounding a volcano. 7 2002 Elsevier Science B.V. All rights reserved.
Volcanic Natural Dams Associated with Sector Collapses: Textural and Sedimentological Constraints on Their Stability
The collapse of a volcanic edifice can enhance several secondary effects, including the formation of one or more natural dams. Their duration depends on the volume of the obstructing mass and on its textural characteristics. A block facies of a debris avalanche produces “durable” dams, whereas a mixed facies is easily eroded after overflowing. Analysis of the sedimentological characteristics of different volcaniclastic deposits that formed natural dams indicates that low sorting and coarse grain-size (but significant clay content) can promote a longer duration of a natural dam. It appear that a –1 phi mean grain-size represents the limit between volcanic natural dams, which last just few days (Md >–1 phi) from those lasting more that 1 year or are still existing (Md <–1 phi). By considering the relation between M/G (M: matrix, G: gravel) and S+C/C (S: silt; C: clay) it seems that for a M/G lower than 1, there is an inverse relation between mud content and dam duration. For dams formed exclusively of fine material (phi less than –1, with comparable fine fraction), the stability decreases as matrix content increases.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.