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1 英语原文及翻译：
EARTHQUAKE-INDUCED LANDSLIDE STABILITY ANALYSIS OF THE LAS COLINAS LANDSLIDE IN EL SALVADOR
H.Y. LUO, W. ZHOU, S.L. HUANG, G. CHEN
Abstract: The January 13, 2001 earthquake (M7.6) off the coast of El Salvador triggered widespread damaging landslides in many parts of the country. The Las Colinas landslide, located in Balsamo Ridge west of San Salvador, caused the greatest loss of life in a single location from the earthquake. The preearthquake and post-earthquake seismic stability analysis of the Las Colinas slope is studied using the limit equilibrium and finite element methods in this paper. The safety factor of the pre-earthquake slope was likely over 1.50 based on the low groundwater level, which was noted by local residents prior to the January 13 earthquake. The slope failed as the horizontal earthquake coefficient, kc, reached between 0.18g and 0.31g depending on the failure analysis methods (i.e. circle and trial surface methods) and the low soil strength assumed. The safety factor of the post-earthquake slope falls in a range from 1.851 at low water saturation to 1.337 when fully saturated. As compared with the pre-earthquake slope, the stability is about 8% higher for the post-earthquake slope.
Keywords: Earthquake, landslide, seismic, stability, limit equilibrium, finite element.
1、INTRODUCTION
El Salvador locates on the Pacific coast of theisthmus and bordered by Guatemala to the west and Honduras to the north and the east. The country has experienced, on average, one destructive earthquake every decade during the last hundred years. The earthquake of January 13, 2001 that struck El Salvador was the first major seismic
disaster of the third millennium and the fifth destructive earthquake to affect this small Central America republic in 50 years (Bommer and Benito, 2002).
The January 13 earthquake triggered the Las Colinas landslide off the steep northern flank of Balsamo Ridge. The landslide originated at an elevation of about 1,040m to 1,070m and traveled northward about 700m to 800m into the Las Colinas neighborhood of Santa Tecla, west of San Salvador. The vertical drop from the ridge to the neighborhood is about 160 m. The volume of the landslide material is estimated at about 250,000 m3 (Jibson and Crone, 2001).
According to a prior investigation (Jibson and Crone, 2001), the landslide material exposed in the headwall scarp appeared somewhat moist but not saturated, but once this material mobilized it behaved as a semi-liquid mass with a soupy consistency; this allowed the mass to travel anunusually long distance from the base of the slope. The transformation of the landslide material from behaving as solid to fluid in the absence of large quantities of water (such as in a heavy storm) and the long runout distance indicate an abnormal material behavior that requires detailed investigation and analysis.
Landslide hazards and stability study is carried out by several organizations in the earthquakerelated areas. The pre-earthquake and postearthquake seismic stability analysis and sensitivity analysis of the Las Colinas landslide are studied using both the limit equilibrium and the finite element methods in this paper. Safety analysis and
suggestions are given in the following sections of the paper.
2. GEOLOGY AND VOLCANIC SOIL
The landslide scarp of the Las Colinas slope exposes the upper 25m - 30m of deposits involved in the landslide. The stratigraphy of these deposits is typical of the volcanic deposits for Balsamo Ridge and much of the surrounding Cordillera Balsamo. Based on the reconnaissance work by USGS, the locations of abundant earthquake induced landslides in the Cordillera Balsamo seem to be influenced by the presence of relatively thick deposits of TB (Tierra Blanca tephra), which erupted from a volcanic source near Lago de Ilopango about 10,000 years ago, and the overlying fresh and weathered tephra.
Tierra Blanca is a dalacitic pumice ash composed of acidic and epiclastic deposits, which covers most of the upper part of San Salvador, and is poorly consolidated and originated from multiple volcanic eruptions that can reach up to 50 m thick. Study demonstrated that one of the most important factors in terms of seismic slope stability is the relatively weak cementation that is broken when the soil is subjected to even small strain (Bommeret al., 2001). It is most probably that the soils may experience a drastic reduction of shear strength during earthquake and hence as soon as the slope failure begins the material undergoes almost adebris flow.
The strong ground shaking, along with the presence of thick, loose to poorly consolidated,young volcanic deposits, the steep topography on the northern flank of Balsamo Ridge, and possibly the presence of a relatively impermeable ancient soil at the top of the Balsamo Formation were contributing factors to the Las Colinas landslide. The cross-section of the Las Colinas slope is shown in Figure 1. The relatively thin layer of paleo-soil might have played an important role in initiating the movement. There are four main geological strata in the slope section: 1) pyroclasts; 2) brown ashes; 3) 1.0m~1.5m paleo-soils; and 4) consolidated tuffs and pyroclastic flows. Figure 2 is a profile at the top of the Las Colinas landslide where landslide mitigation is under construction. The light color formation near the bottom of the profile is paleo-soil.
Figure 1. Cross-section of the Las Colinas slopeshowing the geological stratum Figure 2. A profile at the top of the Las Colinas landslide
One of the most critical steps in slope stability analysis is to determine the shear strength parameters (c and f) along the sliding surface. Laboratory and field soil testing (e.g. Standard penetration test, downhole seismic velocity, unconsolidated triaxial test, etc.) were performed by C. Lotti and Associati (2001). The physical and mechanical properties of the soils at the site are summarized in Table 1.
The available strong ground motion recordings (UCA 2001) during the earthquake are shown in Figure 3. The largest peak ground velocity (PGV) was observed at the Santa Tecla Station (Te) with a value of approximately 57 cm/s and a peak ground acceleration (PGA) of 485 cm/s2 (around 0.5g). This ground motion was sufficiently large to trigger large landslide in the area.
Table 1. Soil properties of the Las Colinas slope(C. Lotti & Associati, 2001)
Figure 3. Acceleration history from TE station
3. SLOPE STABILITY ANALYSIS
The pseudo-static limit equilibrium analysis and the finite element method were employed in slope stability analysis. The primary method of evaluation was based on the Modified Bishop methods for circular and trial surface failures. For comparison, Janbu circle and random surface and Sarma methods were also used to analyze the safety factor of the landslide. The potential sliding surface, circle or polygonal, can be pre-specified or randomly generated. The programs, PCSTABL5 (Achilleos, 1988) developed at Purdue University and Reinforced Slope Stability (RSS) by Geocomp Corporation (1996), were used in the static and pseudo-static limit equilibrium analyses. ABAQUS/Standard was used in the finite element analysis.
3.1 Static analysis
3.1.1 Pre-earthquake
The failure plane was likely restricted by the consolidated pyroclastic flows, which have much higher strength properties than the other formations above. The most probable sliding plane would be along (or through) the third stratum, paleo-soils that is relatively thin and has the lowest strength. Both Bishop and Janbu circle methods employ automatic search for a failure surface. However, the trial surface method requires a pre-determined sliding surface. In the study, a slip surface following the weak paleo-soil layer was treated as a potential sliding plane. The effect of groundwater on the slope stability was assessed at different water saturation conditions. The locations of groundwater table are shown in Figure 4.
Three strength groups (i.e. low, medium, and high) were considered for the sensitivity analysis of slope stability (Table 2) that includes ranges of c and f found from the field and lab testing. Table 3 summarizes the analysis results based on different methods and failure modes with the assumed water table more or less following the paleo-soil layer.
Table 3. The safety factor of pre-earthquakestability analysis with water table following the paleo-soil layer.
Table 2. Strength parameters groups used instability analysis.
The results of pre-earthquake static stability analysis indicate that the slope with the lowest strength properties and under the saturation condition noted in the field would have been stable if not because of the earthquake. Even if the slope was fully saturated, it would still be stable with safety factor larger than 1. The most probable circular failure surface of a fully saturated slope is a circle cutting through the top three strata, following the relatively weak third stratum, and then emerging out near the boundary between thefirst and second strata. The same circular failure surface was analyzed using the Sarma method, and it gives a safety factor of 1.52.
The most critical surface, however, is determined based on the trial surface that follows the actual slip surface observed in the field (Figure 4). The minimum safety factor for a fully saturated slope with the lowest strength properties is 1.235 based on the Janbu trial surface method. The Bishop Trial surface method yielded a similar Result.
3.1.2 Post–earthquake
During earthquake, the potentially unstable soil mass detaches from the strata, and results in a more stable slope. The results of post-earthquake slope analysis (i.e. the current slope at the site) are listed in Table 4. The ranges of safety factor are higher than that of the pre-earthquake slope under all conditions of water saturation.
Similar to the pre-earthquake slope, the most critical surface is a slip surface along the paleo-soil stratum. As shown in Figure 5, the safety factor falls in a range from 1.851 at the lowest degree of water saturation to 1.337 at full saturation based on Janbu trial surface method. As compared with the pre-earthquake slope, the stability is about 8% higher for the current slope.
Table 4. The safety factor of the post-earthquake slope with the lowest degree of water saturation.
Figure 4 Figure 5
3.2 Pseudo-static approach
In pseudo-static methods, the cyclic earthquake motion is replaced with a constant horizontal acceleration equal to kc(g), where kc is the seismic coefficient, and g is the acceleration of gravity. A force is applied to the soil mass equal to the product of the acceleration and the weight of thesoil mass. The seismic sensibility analysis of the Las Colinas landslide is illustrated in Figure 6. The safety factors calculated by Bishop circle, Janbu random surface, and Bishop trial surface methods have some variations, however, the trends are similar.
The Bishop trial surface method gives the lowest safety factor. The Bishop circle method, however, has the highest safety factor. Based on the Bishop trial surface method for the slope with the lower strength parameters, the safety factor decreases from 1.522 to 0.565 as k c increases from 0.0g to 0.5g. When kc approaches 0.18g, the slope becomes critical with a safety factor of 1.0. Using the Bishop circle method, the critical horizontal earthquake coefficient, which triggers landslide, is about 0.31g. Although these two methods give different critical k c values, both triggering horizontal earthquake coefficients are below the 0.5g peak ground acceleration recorded by a nearby seismic station (Te) during earthquake.
Figure 6 Figure 7
The seismic safety factor of the post-earthquake slope with the lower strength and the lower groundwater condition is shown in Table 5. The safety factors calculated based on the Bishop circle and Janbu circle methods are higher than that of the pre-earthquake slope and they are above 1.0 until kc reaches a value between 0.4g and 0.5g. The safety factor analyzed by the Janbu trial surface method is the lowest and it shows that the slope would fail if k c reaches about 0.3g.
The analysis of the groundwater and kc effects on the slope with lower strength is shown in Figure 7. Under a fully saturated condition, the postearthquake slope has a safety factor of 1.337. It becomes marginal as kc increases to about 0.12g. 3.3 Finite element analysis Slope displacement and yield failure were analysed using elastic and elastic-plastic models with horizontal earthquake force. The deformation parameters used in analysis are shown in Table 1 and Table 6. The Drucker-Prager hardening materal model was used for the slope rock and paleo-soils.
When horizontal earthquake acceleration reached 0.1g, the yield failure developed 30m behind the crown of the slope. As the acceleration increased to 0.2g, the deeper paleo-soil layer started to yield. The failure surface extended to the layer above paleo-soil as the earthquake acceleration further incresed to 0.3g (Figure 8). The slope elastic-plastic displacement near the crown of the slope was over 7.5 cm when k c was
0.3g (Figures 9 ). This result is comparable with the analysis from the limit equilibrium methods.
Table 6. Soil deformation properties of the LasColinas slope
Figure 8. Yield areas of slope
Figure 9. Horizontal displacement distribution
4. CONCLUSIONS
Strong ground motion can drastically reduce the stability of slopes. For the Las Colinas landslide, the safety factor of the pre-earthquake slope was likely over 1.50 based on the low groundwater level, which was noted by local residents prior to the January 13 earthquake. The slope failed as the horizontal earthquake coefficient reached a value between 0.18g and 0.31g based on the failure
analysis methods (i.e. circle and trial surface methods) and the lower soil strength used. The post–earthquake slope (i.e. the current slope) is slightly more stable than the initial slope before earthquake. The stability factor of the current slope is in a range between 1.85 based on the trial surface method and 2.41 based on the circle method, when groundwater is low. It decreases to 1.34 based on the trial surface method and 2.03 based on the circle method, as the slope becomes fully saturated. The slope become unstable if it is fully saturated and kc reaches about 0.12g during earthquake. During dry season and the water table is at its low condition, the slope becomes unstable as k c reaches about 0.3g. The slope displacement is over 7.5 cm when k c is 0.3g and yield failure occured at the top and along the paleo-soil layer.
5. REFERENCES
[1]Bommer, J. J., Rolo, R., Mitroulia, A., and Berdousis,
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