mechanisms of slope failure in volcanic soils during earthquakes

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distance between source and site is a poor indicator of the ability of an earthquake to inducelandslides, except in a general manner. This observation does not improve substantially if thedistance between landslide and projected fault rupture is used in stead of epicentral distance.TERRAIN EFFECTSDue to the nature of the terrain in the Balsamo Cordillera, topographic amplification couldhave been the cause of site specific effects. Murphy [6] discusses the importance of slopeangle and slope length in the initiation of landslides during the Chi Chi, Taiwan earthquake in1999. It was demonstrated that the majority of landslides were initiated at breaks of slopewhen the slope facet was similar to the wavelength of the incident wave.In order to investigate the hypothesis a random selection of slopes was chosen from the areaunder investigation. These were then divided into a number of categories based ongeomorphology (i.e. the morphogenetic classification of a terrain facet), geology (was theground dominated by Tierra Blanca, the Balsamo Formation or the Cuscatalan Formation)and the geometry of the slope unit (length and orientation). The presence or absence oflandslides on each of these slopes was then noted.The use of the χ 2 test revealed that there was no statistical difference between the terrain unitswhich showed landslide activity and those which did not. This observation, combined withthe poor correlation between landslide size and epicentral distance, tends to suggest that thedevelopment of slope instability arose from problems related to soil behaviour as opposed tosite specific topographic effects.THE LAS COLINAS LANDSLIDEThe landslide at Las Colinas was examined in the field. Additionally, samples of the soil thatwas involved in this landslide were collected for laboratory analysis. Three block sampleswere taken from a trial pit off the site of the landslide. Due to the abundant evidence ofinstability at the site, sampling from around the landslide itself was considered hazardous.Figure 6 shows an outline geomorphological map of the landslide. The main observationsderived from field examination were:1. The rocks exposed in the scarp of the landslide were part of the Balsamo Formation.Examination of fragments of the landslide debris showed fragments of buff-white tephradeposits that were believed to be the Tierra Blanca. Failure appears to have occurred atthe junction between the Balsamo Formation and the mantling Tierra Blanca.2. Several components of the main sliding mass could be observed. Movement appears tohave been in two phases. An investigation of the geomorphology suggested that thelandslide was regressive in nature. The movement was dominantly translational without arotational component.3. Abundant evidence existed for slope instability developed along the ridges and close tothe main slide body. Tension cracks could be observed up to 23 m behind the crown ofthe landslide and near the flanks. Smaller scale translational (slumps) landslides could beobserved at the crest of the slope as well as adjacent to the main landslide track.4. No evidence of liquefaction was observed on low-lying ground. Sand volcanoes and sandboils were not evident on the crest or the top of the slope or at the flat ground at the footof the slope.5. Once initiated, the landslide moved on low slope angles. The trim line on buildingsadjacent to landslide suggested a partially fluidised material. The absence of splash marks
however suggested that the soils were probably not fully fluidised. Based on thisobservation, and the content of the material, the slope failure was categorised as a debrisflow (Dikau [2]).6. The debris flow extended approximately 730m downslope with approximately 300-350 mof movement occurring on slope angles of c 4-5 o . The landslide was estimated to beapproximately 240 m wide.7. The debris was eroded by a spring emerging from the Balsamo Formation (the presenceof a spring line was marked on topographic maps). The emergence of this seepage linewas above the landslide debris.8308256 o 3 o7 o10 o95026 o24 o 22 o1000105028 o90028 o33 o10 oNLandslide ToeLandslide TrackSlump BlockScarpTension CrackGullyContour (m)Dip of slope1085 0 500metres15 o69021 oFigure 6. Geomorphological map of the landslide at Las Colinas.These observations suggested that the landslide was initiated in the Tierra Blanca. Thisfailure is believed to have begun as a debris slide or slump that became a flow withincreasing strain. A rise in pore water pressures in these partially saturated soils resulted inthe sliding mass being able to move on low angle surface in a partially fluidised conditions.SOIL BEHAVIOUR AND LANDSLIDE MOVEMENTOther landslides examined elsewhere in the Balsamo Cordillera showed similarcharacteristics. Debris flows were associated with the presence of pyroclastic ashfall deposits.While the majority of these landslides moved on slopes that were steeper than the slide at LasColinas, the length of runout associated with these large failures were significant. In somecases, these displacements were in excess of a kilometre in length.Investigation of slopes formed in the Tierra Blanca throughout the El Salvador area indicatedan absence of landslide scars on slopes below 20-21 o . While the slopes around the Las
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mechanisms of slope failure in volcanic soils during earthquakes

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