Published by Elsevier Science Ltd. All rights reserved 12th European Conference on Earthquake Engineering

Paper Reference 782

MECHANISMS OF SLOPE FAILURE IN VOLCANIC SOILS DURING EARTHQUAKES

W. Murphy1, J. Bommer2 and J. M. Mankelow3.

1 The School of Earth Sciences, University of Leeds, Leeds, LS2 9JT.

2 Department of Civil & Environmental Engineering Imperial College London SW7 2BU, UK

3British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham. NG12 5GG

ABSTRACT The 13th of January 2001 earthquake resulted in considerable slope instability. The majority of the landslides were either small volume rock and debris falls or large volume debris flows. The majority of Rock and debris falls were in cut slopes in pyroclastic ashfall deposits, especially, a unit known at the Tierra Blanca. The larger volume slope failures occurred as debris flows, again in the Tierra Blanca, but appeared to have been the result of a complex collapse phenomenon related to the properties of the soil. The location of these slides appears to have been dominated by topographic and geological conditions rather than the distance between source and site. Keywords: Landslides, debris flows, volcanic soils INTRODUCTION The El Salvador earthquake of January 2001 resulted in significant loss of life and damage over a large area. The majority of the deaths were the results of two large landslides that were the result of the main shock at 14:33 UTC (table 1). These landslides at Las Colinas (in Santa Tecla) and Las Barrioleras (west of Santa Tecla) resulted in c. 540 of the 870 deaths. This paper discusses the debris flows triggered by the main earthquake, and addresses the implications for the behaviour of pyroclastic ashfall deposits during strong shaking.

TABLE 1. SOURCE PARAMETERS OF THE TWO MAIN EL SALVADOR EARTHQUAKES.

Time Epicentre Depth Magnitude Agency

13 January 2001 17:33:32 13.049 oN 88.660 oW 60 km MW 7.7, MS 7.8, mb 6.4 NEIC 17:33:46 12.97 oN 89.13 oW 56 km MW 7.7, MS 7.8, mb 6.4 HRV 17:33:30 12.868 oN 88.767 oW 60 km MW 7.7 CASC

13 February 2001 14:22:06 13.671 oN 88.938 oW 10 km MW 6.5, MS 7.5, mb 5.5 NEIC 14:22:16 13.980 oN 88.970 oW 15 km MW 6.6, MS 6.5, mb 5.5 HRV

During January and February 2001 the Central American republic of El Salvador was shaken by a succession of strong earthquakes. Two significant shocks occurred which showed markedly different characteristics. The first of these two events had an epicentre within the subducted slab of the Coccos plate at a depth of approximately 60 km, while the second had a focal depth of between 10 and 15 km. The mechanisms of these two events were notably different. The former was an extensional event, the latter was almost pure strike-slip deformation. The main earthquake of 13 January generated a peak horizontal ground acceleration in a North-South direction of 1.1 g in the town of La Libertad approximately 10 km from the projected fault rupture. The second large event of 13 February generated a peak horizontal ground acceleration of 0.41g recorded at Zacatecoluca approximately 9 km from the projected fault rupture. Numerous landslides were triggered by the main earthquake, many of which expanded due to the second large earthquake of 13 February. Numerous large landslides which can be classified as debris flows (Varnes [1], Dikau [2]) affected the Balsamo Cordillera. The majority of these slope failures occurred in natural vegetated slopes in areas underlain by older volcanic rocks. GEOLOGY The geology of San Salvador is entirely volcanic (Schmidt-Thomé [3]). Three broad geological units can be identified which are the Balsamo Formation; the Cuscatalan Formation and the San Salvador Formation. Of these three geological formations, the most important to this study are the Pliocene-Pleistocene rocks of the Balsamo Formation and a Holocene member of the San Salvador formation called the Tierra Blanca.

The Balsamo formation is composed dominantly of felsic igneous rocks. These often form interlayered rhyolites, dacites and pyroclastic deposits and occasional palaeosols. Field

Figure 1. The rocks of the Balsamo Formation exposed in the head of the landslide at Las Colinas

examination indicates that the rock masses formed from rocks of this part of the succession show layers of medium thickness, which are dominantly horizontal. Discontinuities show medium spaced. Making use of Geological Strength Index (GSI) proposed by Hoek [4], these rock masses could be split into two categories of materials. The first category is the competent rocks with GSI between 50 and 60. These are normally rhyolites and dacites and show low degrees of alteration. The second group of rocks is substantially weaker showing GSI values of 20-30 and generally consists of weak pyroclastic ashfall deposits with very closely spaced discontinuities. Material properties of these two groups of materials are generally moderately to extremely STRONG, while the latter group is dominated by materials which are WEAK. Additionally, palaeosols formed from tropical weathering of volcanic materials has resulted in the formation of local aquitards. Figure 1 shows rocks of the Balsamo formation exposed in the rear scarp of the landslide at Las Colinas. Figure 2 shows a typical type of slope failure in the Tierra Blanca.

DEBRIS FLOWS TRIGGERED BY THE EARTHQUAKE The earthquake of 13, January 2001 resulted in the initiation of a significant number of debris flows. The most infamous of these was the landslide at Las Colinas. However, this was not the only such failure; debris flows occurred extensively throughout the Balsamo Cordillera. The question of whether such landslides were the results of strong shaking, topographic amplification or some related site effect was investigated.

Figure 2. The Tierra Blanca exposed in a road cutting showing characteristic rock/

debris fall type failure

Data on the location of landslides were collected using SPOT monospectral satellite imagery collected on 28 January 2001 (figure 3). These data have a spatial resolution of 10m. Additional geographic data on debris flows were collected by field survey. The majority of the debris flows was large, and was easily observed on SPOT data. SPATIAL DISTRIBUTION OF LANDSLIDES The majority of landslides of all types occurred in the Balsamo Cordillera. The ridge itself was strongly affected either by rockfall and debris fall type landslides that resulted in serious blockages due to loss of a section of the road, or, because of landslide debris blocking the carriageway. Due to the metastable nature of the material that failed it, was not uncommon to find the road surface buried in approximately 0.1 to 0.2 m of silty sand. Landslide densities in the order 100-200 failures per kilometre of road were observed along the Balsamo Ridge between Santa Tecla and the town of Comosagua. The majority of these failures were of the rockfall and debris fall varieties, with occasional translational slides. Where rocks of the Balsamo Formation were exposed at crest of the slope larger, rock block falls occurred.

The incidence of debris flows appears to be unrelated to the distance between the epicentre and the site of slope failure. Based on the simple attenuation of seismic energy between the source and a potential landslide, it would seem logical that a decrease in landslide incidence would occur with distance. Such a relationship is in fact supported by Bommer [5] who both observe that to trigger landslides at a larger epicentral distance, a greater energy release is required. Equally therefore, in terms of work done, it would be expected that the size of the landslide would be related to the energy arriving at the site.

Figure 3. SPOT monochromatic image (near infra-red) of the area west of San Salvador. Landslides appear as white areas on the image.

Las ColinasLas Barrioleras

Dendritic drainage patternformed on terrain dominatedby the Balsamo Formation

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Figure 5. Graph showing landslide enlargement after the 13 February 2001 earthquake related to epicentral

Figure 4 shows a plot of the area of landslide area against the epicentral distance. The area of the landslide was measured from SPOT data, and therefore concentrates on the larger failures (smaller landslides, less than about 30 m, being beneath effectively below the resolution of the SPOT data). It can be seen that there is no relationship between the epicentral distance and the size of the landslide. There does however appear to be an upper boundary, therefore while there is no simple relationship between the energy arriving at the site, there is clearly a point where there is insufficient energy to induce further slope deformation.

Because of the second large earthquake of 13th February 2001, many landslides increased in volume. Again, dealing only with large debris flows triggered by the earthquake, it can be seen that there is no correlation between the enlargement of a given landslide and the distance between the epicentre and the landslide (figure 5). This again tends to suggest that the

distance between source and site is a poor indicator of the ability of an earthquake to induce landslides, except in a general manner. This observation does not improve substantially if the distance between landslide and projected fault rupture is used in stead of epicentral distance. TERRAIN EFFECTS Due to the nature of the terrain in the Balsamo Cordillera, topographic amplification could have been the cause of site specific effects. Murphy [6] discusses the importance of slope angle and slope length in the initiation of landslides during the Chi Chi, Taiwan earthquake in 1999. It was demonstrated that the majority of landslides were initiated at breaks of slope when 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 area under investigation. These were then divided into a number of categories based on geomorphology (i.e. the morphogenetic classification of a terrain facet), geology (was the ground 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 of landslides on each of these slopes was then noted. The use of the χ2 test revealed that there was no statistical difference between the terrain units which showed landslide activity and those which did not. This observation, combined with the poor correlation between landslide size and epicentral distance, tends to suggest that the development of slope instability arose from problems related to soil behaviour as opposed to site specific topographic effects. THE LAS COLINAS LANDSLIDE The landslide at Las Colinas was examined in the field. Additionally, samples of the soil that was involved in this landslide were collected for laboratory analysis. Three block samples were taken from a trial pit off the site of the landslide. Due to the abundant evidence of instability at the site, sampling from around the landslide itself was considered hazardous. Figure 6 shows an outline geomorphological map of the landslide. The main observations derived 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 tephra deposits that were believed to be the Tierra Blanca. Failure appears to have occurred at the junction between the Balsamo Formation and the mantling Tierra Blanca.

2. Several components of the main sliding mass could be observed. Movement appears to have been in two phases. An investigation of the geomorphology suggested that the landslide was regressive in nature. The movement was dominantly translational without a rotational component.

3. Abundant evidence existed for slope instability developed along the ridges and close to the main slide body. Tension cracks could be observed up to 23 m behind the crown of the landslide and near the flanks. Smaller scale translational (slumps) landslides could be observed 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 sand boils were not evident on the crest or the top of the slope or at the flat ground at the foot of the slope.

5. Once initiated, the landslide moved on low slope angles. The trim line on buildings adjacent to landslide suggested a partially fluidised material. The absence of splash marks

however suggested that the soils were probably not fully fluidised. Based on this observation, and the content of the material, the slope failure was categorised as a debris flow (Dikau [2]).

6. The debris flow extended approximately 730m downslope with approximately 300-350 m of movement occurring on slope angles of c 4-5o. The landslide was estimated to be approximately 240 m wide.

7. The debris was eroded by a spring emerging from the Balsamo Formation (the presence of a spring line was marked on topographic maps). The emergence of this seepage line was above the landslide debris.

These observations suggested that the landslide was initiated in the Tierra Blanca. This failure is believed to have begun as a debris slide or slump that became a flow with increasing strain. A rise in pore water pressures in these partially saturated soils resulted in the sliding mass being able to move on low angle surface in a partially fluidised conditions. SOIL BEHAVIOUR AND LANDSLIDE MOVEMENT Other landslides examined elsewhere in the Balsamo Cordillera showed similar characteristics. 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 Las Colinas, the length of runout associated with these large failures were significant. In some cases, these displacements were in excess of a kilometre in length. Investigation of slopes formed in the Tierra Blanca throughout the El Salvador area indicated an absence of landslide scars on slopes below 20-21o. While the slopes around the Las

Figure 6. Geomorphological map of the landslide at Las Colinas.

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Colinas site locally exceeded, it should be remembered that there was a significant component of slope strength derived from the underlying Balsamo Formation. An estimate of friction coefficient of a landslide can be derived from a consideration of the height of vertical fall compared with the runout length. For the majority of slide-type failures, this ratio will be relatively high (>0.5). In rock and debris avalanches, this value is normally low (0.2 or lower) as a result of the complex fluidisation processes involved in their movements. Similar ratios calculated for the debris flows observed in the Balsamo Cordillera gave values as low as 0.17 indicating low frictional strength mobilised in the sliding mass. Observations of the Tierra Blanca in the field are not consistent with a material with a low frictional strength, therefore additional laboratory analysis was carried out. Analysis of samples of the Tierra Blanca collected in the field gave values of cohesion and friction of 0-30 kPa and 34-39o respectively (Bommer [7]) However, these data are insufficient to describe the Tierra Blanca, as additional geotechnical data are required. Field and laboratory analysis of these materials indicates porosities of up to 50%. Additionally, it has been noted that these materials are not fully saturated in the field and pore tensions have been developed. Laboratory testing of pore tensions from samples collected at the time of the earthquake indicated pressures of up to 700 kPa (Bommer [7]).

One-dimensional consolidation tests carried out in a oedometer cell are shown in figure 7. A number of distinct sections to this curve can be observed. The first part of the loading curve (A-B) shows the consolidation of the sample subject to applied load. At point B, the cell is flooded and the Tierra Blanca undergoes rapid collapse (B-C). The unloading phase of the curve (C-D) shows that a non-recoverable consolidation of the soil structure has occurred. It can be seen that the magnitude of the collapse is large with up to a 23% decrease in void ratio.

Figure 7. Diagram showing the collapse of the Tierra Blanca when subjected to saturation (after Mavrommati [8])

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The collapse of this structure has a number of implications for the behaviour of the Tierra Blanca when subjected to shaking. It is likely that the collapse of the metastable soil structure occurred during the movement of the debris flows in the Balsamo Cordillera. It is suggested that movement initially occurred as a rigid block (the presence of the slump blocks at the head of the landslide indicates some form of 'brittle' failure). As a result of strain, either from sliding or earthquake ground accelerations, the weak cement bonds in the Tierra Blanca broke, and the structure began to collapse. As collapse occurred pore water pressures increased and the landslide changed from a debris slide to a debris flow. It is difficult to determine the role of ground motions in the generation of pore water pressures. While several studies (e.g. Holzer [9]) indicate that large increases in pore water pressure can be associated with shaking, many of these analyses were carried out in saturated soils. There is no doubt that a significant proportion of the strength of these pyroclastic ashfall deposits stems from the pore tensions developed due to partial saturation. It is therefore, difficult to assess the impact 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 the frictional strength of the soil. It can be observed that there is a significant drop in angle of internal friction of the Tierra Blanca in the laboratory between peak (φp = 34-39o) and residual (φr c. 23o) conditions. However, based on analysis of landslide movement patterns, a significantly lower apparent friction is observed (φ = 6-9o). This suggests that the effects of pore water pressures were substantial. DEBRIS FLOWS TRIGGERED BY EARTHQUAKES While the landslide at Las Colinas was the most closely examined of the debris flows triggered by January 13 earthquake, there were many more such failures. It can be seen that many of these landslides had a long runout. There are a number of common factors involved in all of the slope failures that are worth noting. Firstly, all the observed debris flows were initiated as some other form of landslide. The landslide at Las Colinas was clearly a debris slide (slump). Other debris flows appear to have originated as rock and debris falls. This clearly suggests that some form of strain is necessary to lead to collapse and flow of the soil. In the case of debris flows which were initiated as rock or debris falls, there is field evidence to suggest the entrainment of unstable material from further downslope. This latter mechanism does not appear to be true for failures that occurred as 'slide' type of movements. The difference between the initiation of rock/debris falls and translational / rotational slides is one of original slope angle. The former required slopes of greater than 40-42o to occur, while the latter could develop on lower angle slopes. Secondly, the majority of debris flows are associated with the geological association of the Tierra Blanca and the Balsamo Formation. Many of the landslides observed on figure 3 which 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 not develop into debris flows shows that these slides happened entirely within pyroclastic ashfall deposits. It is suggested that the presence of the impermeable horizons within the Balsamo Formation (such as rhyolitic and dacitic lavas and palaeosols) provide an essential hydrogeological pathway for water to reach, and be held in, the Tierra Blanca. CONCLUSIONS Numerous debris flow landslides were triggered by the MW = 7.6 earthquake of 13 January 2001. These failures were initiated as either rockfalls, debris falls or debris slides and

developed into debris flows. Evidence suggests that the process of 'fluidisation' of the sliding mass was associated with the collapse of the soil structure of the Tierra Blanca observed in the laboratory. The occurrence of debris flows appears to be controlled, at least partly, by the geological conditions. The majority of this type of slope failure had an origin where the Tierra Blanca was deposited on top of the older, stronger and less permeable Balsamo Formation. In the absence of strong motion data, it is impossible to rule out significant differences in the seismic response of the two different geological units. However, it seems likely that the principal control was hydrogeological. The frictional strength of the Tierra Blanca at peak, and even residual, strength conditions is not consistent with the long runout conditions of many of the landslides observed in the field and on satellite imagery. The use of geological and geomorphological observations can be used as a 'first order' method of identifying future hazards. ACKNOWLEDGEMENTS The authors wish to acknowledge the receipt of funding from the Natural Environment Research Council, UK (NER/A/S/00030) and the Royal Academy of Engineering. The assistance of colleagues from PRISMA (especially Herman Rosa) and the University of Central America was invaluable during firledwork. REFERENCES 1. Varnes, D. J. Slope movement type and processes. In: R. L. Schuster & R. J. Krizek,

(eds), Landslides: analysis and control: 11-33, Washington. Transportation Research Board Special Report 176.

2. Dikau, R., Brunsden, D., Schrott, L. & Ibsen, M. L. Introduction. In Dikau, R., Brunsden, D., Schrott, L. & Ibsen, M. L. (Eds), Landslide Recognition: Identification, Movement and Causes. 1996, pp251, Wiley, Chichester, ISBN 0-471-96477-8.

3. Schmidt-Thomé M. The geology in the San Salvador area (El Salvador, Central America), a basis for city development and planning. Geol. Jb. 1975; 13: 207-228.

4. Hoek, E. The strength of rock and rock masses. News Journal, International Society of Rock Mechanics, 1994, 2, 4-16.

5. Bommer JJ, Rodríguez CE. Earthquake- induced landslides in Central America. Engineering Geology 2002, 63(3/4).

6. Murphy, W., Petley, D.N., Bommer, J.J. & Mankelow, J. M.. Uncertainty in ground motion estimates for the evaluation of slope stability during earthquakes. Quarterly Journal of Engineering Geology and Hydrogeology, 2002. In press.

7. Bommer JJ, Rolo R, Mitroulia A, Berdousis P. Geotechnical properties and seismic slope stability of volcanic soils. In: Proceedings of Twelfth European Conference on Earthquake Engineering; UK: London, Paper No. 695, 2002.

8. Mavrommati, Z. C. Seismic behaviour of slopes in an undersaturated volcanic soil. M.Sc. Dissertation, 2000. Imperial College London.

9. Holzer, T. L., Youd, T.L. & Hanks, T.C. Dynamics of Liquefaction During the 1987 Superstition Hills, California, Earthquake. Science, 1989,Vol. 244, pp. 56-59.