mechanisms of slope failure in volcanic soils during earthquakes

The collapse of this structure has a number of implications for the behaviour of the TierraBlanca when subjected to shaking. It is likely that the collapse of the metastable soil structureoccurred during the movement of the debris flows in the Balsamo Cordillera. It is suggestedthat movement initially occurred as a rigid block (the presence of the slump blocks at thehead of the landslide indicates some form of 'brittle' failure). As a result of strain, either fromsliding or earthquake ground accelerations, the weak cement bonds in the Tierra Blancabroke, and the structure began to collapse. As collapse occurred pore water pressuresincreased and the landslide changed from a debris slide to a debris flow. It is difficult todetermine the role of ground motions in the generation of pore water pressures. While severalstudies (e.g. Holzer [9]) indicate that large increases in pore water pressure can be associatedwith shaking, many of these analyses were carried out in saturated soils. There is no doubtthat a significant proportion of the strength of these pyroclastic ashfall deposits stems fromthe pore tensions developed due to partial saturation. It is therefore, difficult to assess theimpact that a reduction of such pressures on the strength of the slope.Regardless of how pore water pressure increases occurred there is a clear effect in thefrictional strength of the soil. It can be observed that there is a significant drop in angle ofinternal friction of the Tierra Blanca in the laboratory between peak (φ p = 34-39 o ) andresidual (φ r c. 23 o ) conditions. However, based on analysis of landslide movement patterns, asignificantly lower apparent friction is observed (φ = 6-9 o ). This suggests that the effects ofpore water pressures were substantial.DEBRIS FLOWS TRIGGERED BY EARTHQUAKESWhile the landslide at Las Colinas was the most closely examined of the debris flowstriggered by January 13 earthquake, there were many more such failures. It can be seen thatmany of these landslides had a long runout. There are a number of common factors involvedin all of the slope failures that are worth noting.Firstly, all the observed debris flows were initiated as some other form of landslide. Thelandslide at Las Colinas was clearly a debris slide (slump). Other debris flows appear to haveoriginated as rock and debris falls. This clearly suggests that some form of strain is necessaryto lead to collapse and flow of the soil. In the case of debris flows which were initiated asrock or debris falls, there is field evidence to suggest the entrainment of unstable materialfrom further downslope. This latter mechanism does not appear to be true for failures thatoccurred as 'slide' type of movements. The difference between the initiation of rock/debrisfalls and translational / rotational slides is one of original slope angle. The former requiredslopes of greater than 40-42 o to occur, while the latter could develop on lower angle slopes.Secondly, the majority of debris flows are associated with the geological association of theTierra Blanca and the Balsamo Formation. Many of the landslides observed on figure 3which occurred north of the Pan American highway show a H/L ratio which does not suggest'flow' type landslides. Field examination of slope failures in the Tierra Blanca that did notdevelop into debris flows shows that these slides happened entirely within pyroclastic ashfalldeposits. It is suggested that the presence of the impermeable horizons within the BalsamoFormation (such as rhyolitic and dacitic lavas and palaeosols) provide an essentialhydrogeological pathway for water to reach, and be held in, the Tierra Blanca.CONCLUSIONSNumerous debris flow landslides were triggered by the M W = 7.6 earthquake of 13 January2001. These failures were initiated as either rockfalls, debris falls or debris slides and