The use of classic geologic maps, where geological layers are grouped according to their age or origin, makes difficult the interpretation and use for civil engineer design or urban planning to people without deep knowledge in geology. Due to this reason engineering geological mapping has been carried out in the southern part of the Metro- politan Area of San Salvador using the ]]>
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Key words: Engineering geology, stripe method, geology, San Salvador Formation, Balsamo Formation, Tierra Blanca.
Resumen
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El uso de mapas geológicos clásicos que agrupan los estratos por edad u origen, dificulta la interpretación y uso para diseños de ingeniería civil o planificación urbana, para las personas sin conocimientos profundos en geología. Debido a esto se ha llevado a cabo mapeo de ingeniería geológica en sector sur del Área Metropolitana de San Salvador, haciendo uso de la metodología de bandas. El objetivo de la metodología es que la información geológica, peligrosidad ]]>
Palabras clave: Ingeniería geológica, método de bandas, geología, formación San Salvador, formación bálsamo,Tierra Blanca.
Introduction
This project is the beginning of missing systematic engineering geology mapping for civil engineering purposes in El Salvador. One of the goals is to introduce a systematic method to rep- resent the geological materials and potential geohazards; this will aid in urban planning and early stages of civil engineer design. The study area was chosen in the southern part of the Metropolitan Area of San Salvador (MASS) where pressure for urban ]]>
Central America is located between the Pacific and Atlantic Oceans; tectonic and volcanism of the area is related to the Ring of Fire. This region (Gonzalez et al., 2004) is exposed to volcanic eruptions, earthquakes, floods, mass movements, drought and heavy rains that cause human victims, damages in the ]]>
These past years the problematic is more frequent in the countries of the area; for example in El Salvador heavy rains connected with tropical depressions or hurricanes like Mitch (1998), Stan (2005), Ida (2009), Alex (2010), Agatha (2010) and 12-E (2011), as well as earthquakes in 1986 and 2001 forced the ]]>
According to the government the damage costs of the rains in October of 2011 (12-E) were 4 % of the Gross Domestic Budget (El Diario de Hoy, 2011) and for the 2001 earthquakes the economic losses were around 12% the ]]>
et al., 2004).
The lack of proper knowledge prior to build most of the housing projects and infrastructure in the MASS has been one of the reasons of the elevated vulnerability to geological hazards. Others reasons are rapid urbanization, connected with an elevated population density of 2 569 hab/km2 (Ministerio de Economía. 2008). Also the migration to the city and persons of ]]>
et al., 2004) like steep slopes, next to the scarps or invading the floodplains and stream fans, raise the vulnerability to geological hazards.
For land-use planners, information like geology, geomorphology, geotechnical data, seismicity, hydrogeology among others; can give information like the necessary ]]>
One of the tasks of the project of the Czech Republic foreign Development Programme under guarantee of the Czech Ministry of Environment RP/6/2007 was improving land use decisions by proposing a ]]>
Area of study
The Metropolitan Area of San ]]>
Fig. 1) consisting of 14 municipalities and is located between the San Salvador Volcano and the Ilopango Caldera (two active volcanoes) and the major part is inside of the Central Graben, a tectonic depression (Lexa et al., 2001), which is connected to the subduction process in the Ocean. Inside the MASS most of the governmental agencies, industries, economic and financial institutions are concentrated. ]]>
The study area at the present is part of the natural protection zone proposed by the Ordinance of Protection and Conservation of the Natural Resources (Diario Oficial, 1998). Some of the reasons for been chosen for mapping, was the recent improvement of the roads in the sector that could increase the pressure for urbanization in the area; also the area is almost in its natural state and this made the mapping job easier. ]]>
The zone is in the upper part of the Bocana de Toluca basin. The actual land-use of the slopes in the region is coffee plantations, woods and rural environmental with crops in some slopes; the more problematic areas (mass wasting) are mostly located in the artificial road cuts(Fig. 2).
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Some landfills and dumps were placed to increase the area of construction of some of the properties. During the field work it was observed that most of them weren´t compacted properly and as a result cracking, mass movements and collapse was observed (fig. 3).
Geology and Geomorphology
The basic geological map of the whole El Salvador, which is currently in use, was made by the German Geological Survey (bosse et al., 1978) in a 1:100 000 scale; after that, the geological mapping was only limited to some areas, for example the 1:15 000 scale map of a sector of San Salvador (Centro de ]]>
et al. (2011) made a 1:50000 geological map of the southern part of the MASS where the study area is located. Geomorphological assessments of the MASS was performed by Šebesta (2006, 2007a, 2007b) and Šebesta & Chavez (2010, 2011)
According ]]>
et al. (2011), the study area has presence of the Late Miocene–Pliocene Bálsamo Formation and the Late Pleistocene–Holocene San Salvador Formation (Fig. 4)
The Bálsamo Formation (Lexa ]]>
et al., 2011) is conformed by andesite lavas, tuffs and epiclastic volcanic breccias/conglomerates representing remnants of andesite stratovolcanoes. The San Salvador formation includes products of basaltandesite stratovolcanoes associated with the evolution of the Central Graben as well as interstratified silicic tephra/ignimbrites of the Coatepeque and Ilopango calderas.
According to Šebesta ]]>
et al. (2011) some faults connected with the Central Graben cross the area and for this reason part of the study area (fig. 5) is composed by tectonic scarps and descending diastrophic blocks that are distributed very chaotically. Relative tectonic uplift and depression is inferred by the juxtaposition of recent and older deposits. The region has been impacted ]]>
Mass wasting processes (Šebesta, 2006 and Lexa et al., 2011) are connected with slopes covered by tuffs of the San Salvador formation, but also with other regions where rocks of the Balsamo Formation are exposed and there is presence of laterite or weathered ]]>
On the southern part of the Central Graben, remnants of old stratovolcanoes are present, where the thickness of the laterite is important. In finca la labranza there is an area of polygenetic fill within a tectonic depression (Fig. 5) ]]>
Engineering Geological Mapping
Part of the project, was to observe ]]>
Currently in El Salvador the practice of soil and rock mechanics is weak, and that’s one of the reasons why the civil infrastructure like housing projects, roads, slopes and bridges are easily damaged after a major event (rain or earthquake), increasing the costs of rebuilding
When a project is prepared, the geological risk is not fully taken into consideration, and occasionally the project will originate, increase or be affected by a natural hazard. Investors, trying to save money, build in conditions, which sometimes are inadequate for construction; in addition the concentration of buildings in the cities is unreasonably high. ]]>
According to Anon (1972) one of the shortcomings of conventional geological maps, from the point of view of civil engineer, is that rocks with different engineering properties are characterized as a single unit because they are of the same age or origin. ]]>
The aim of the engineering geological mapping (Commission on engineering geological maps, 1976) is the selection of the geological and engineering characteristics of rocks and soils which are more related, and group the layers consequently. The degree of simplification depends on the purpose and scale of the map, the accuracy of the information and the techniques of representation.
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The most important tool during mapping was the correlation of soil and rocks in different places. The stratigraphy according to Hernandez (2008) was very helpful through the field work. Other aspects used were the interpretation of existing geological maps, field work, and evaluation of probable rock behavior from previous knowledge.
The ]]>
The stripe method was chosen to symbolize the engineering geological units that exist in the area. ]]>
In the stripe method ]]>
An engineering geological map illustrating the aptitude for urbanization is presented in figure 7. ]]>
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The map legend represent: dark gray color for inadequate areas because of hazards; light gray color for suitable areas if some conditions are fulfill (more detailed research); light gray color with hatch lines for suitable areas if some conditions are fulfill and where complementary geotechnical measures are necessary (walls, gabions etc.) but the solution has to be based on a detailed research; and gray color for areas where is possible to build from the geotechnical and engineering geological point of view, but always based on a proper research.
The engineering geological map (Fig. 6) evaluates geological aspects of the area (mostly quaternary geology), hardness, properties of soils and rocks, exogenous processes (weathering), morphology and hydrogeology using the stripe method.
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The map is divided between the cover layers and the rock basement. Since most of the surface is covered by Tierra blanca Joven (TbJ), the stripe method is able to present what is underneath it. The outcrops visited during the field work helped to define the areas of stripes in the map, representing simulated windows of layers with different level below TbJ. The surface layers of Plan de la Laguna and San Salvador scoria identified in the field were grouped together. Also the combined set of tuffs between TbJ and the rock basement (Arce and Congo, Tb4, G1, Tb3, TB2, G2, IB and ]]>
The ISRM (1981) rock classification was used for the different rock basement units and is represented as text in the map above the corresponding unit (figure 6 and
table 1). The description of the units is presented next.
Description of rocks and soils
According to Hernandez (2008) the ]]>
Engineering ]]>
Fig. 6):
The bedrock (balsamo formation) is comprised mainly of solid lava rocks with presence of boulders and blocks on the slopes, as well as epiclastic rocks product of debris flows and mud flows of the ancient volcanoes (Fig. 8). Lava and debris ]]>
figure 9). The result is that slightly weathered blocks of lava flow are surrounded by completely weathered lava flow (almost clayey material). The weathered material is exposed in slopes and there is a hazard of mass movements in many cases. According to Lexa et al. ]]>
The surface layers described by Hernandez (2004, 2008) and Lexa et al. (2011) are the group of volcanic tuffs (Tierra Blanca 4 (TB4), G1, Tierra Blanca 3 (TB3), Tierra Blanca (TB2), G2, IB, Plan de la Laguna (PL), Tierra Blanca Joven (TBJ) including Arce and Congo) which are placed above the bed rock (figure 10 and 11). This group is characterized by some weathering on the top (paleosoils) which are interbedded in the volcanic materials, being very important for engineering geology purposes. According to Hernandez (2008) the deposits from the Ilopango Caldera that have paleosoils are Tb4, Tb3, Tb2; and G1, G2 from the San Salvador Volcano. Congo and Arce are the deposits originated from Coatepeque Caldera and are weathered as well. The third unit is a surface of Tierra Blanca Joven (TBJ) cover with unknown thickness (Fig. 12), and is located in areas with no information due to poor accessibility or lack of outcrops; for this reason, extrapolation and interpolation was necessary.
In El Salvador the ]]>
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Tierra Blanca Joven (TBJ) is the most important volcanic tuff and is present in important sectors of the MASS (as is shown in Figure 6). TBJ is quite sensitive to erosion, water content change, natural or artificial vibrations and compressibility (Rolo et al. 2004; hernandez, 2004; Šebesta, 2007b; Šebesta & Chavez, 2010). This material is also quite sensitive for ]]>
2 and 13).
Guzmán & Melara (1996); Amaya & Hayem (2000); Rolo et al. (2004); Hernández (2004); Molina et ]]>
(2009) and Avalos & Castro (2010) studied the geotechnical and lithological aspects of the younger Ilopango tuffs Tierra Blanca Joven (TBJ). All of the authors characterized TBJ as silty sands and sandy silts.
At the present, the soil mechanics and theory for saturated soils is used in El Salvador. There are two distinct seasons throughout the year: summer and rainy season, and ]]>
et al., 2004), remaining almost the same all the year; this means that most soils in the country are unsaturated and there are capillary forces that act on the soil structure making that an “apparent cohesion” (suction) improve the strength of the soil; this situation makes that the slopes are almost vertical and temporally stable, but will collapse when wetted or dried.
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For the present publication, the values of cohesion and friction angle of different authors were compiled and compared, (figure 14) showing scatter in the data. The authors that made the tests concluded that the strength of undisturbed or disturbed samples with natural moisture decreases, if they´re saturated. In spite that most of the tests were made with the shear box, the results show disparity; since for unsaturated soils the combination of total stress σ, pore water pressure uw, pore air ]]>
This brings out the necessity of researching about the behavior and constitutive modeling using the critical state and unsaturated soil mechanics, to avoid the dispersity of the results and have consistency into a more coherent framework. Rolo et al (2004, table 3) compared suction values from TBJ samples and ]]>
There is almost no geotechnical information available for the other geological layers because it´s not usual to identify the name of the layer that is tested ]]>
Conclusions
There is a need to simplify the geological maps in order to help the interpretation for the developers ]]>
The engineering geological mapping can serve as first or final steps in planning an infrastructure and can ]]>
The presence of layers of laterites and paleosoils are very problematic in the sector but is important ]]>
et al, 2004). Also the mass wasting processes acts intensively in the surface and above the ground of TBJ, the anthropic disturbance (for example broken pipes and urban growth) increase the problems. The volume changes ]]>
It is recommended to continue ]]>
Acknowledgements
The work has been carried out in the framework of cooperation between the Czech Geological Survey and the Oficina de Planificación del Área ]]>
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*Correspondencia: José A. Chávez: Oficina de Planificación del Área Metropolitana de San Salvador (OPAMSS), San Salvador, El Salvador. Czech Technical University in Prague, Faculty of Civil Engineering, Department of Geotechnics, Czech Republic. Autor para contacto: jose.alexander.chavez.hernandez@fsv.cvut.cz Jan Valenta: Czech Technical University in Prague, Faculty of Civil Engineering, Department of Geotechnics, Czech Republic Jan Schröfel: Czech Technical University in Prague, Faculty of Civil Engineering, Department of Geotechnics, Czech Republic ]]>
Walter Hernandez: Servicio Nacional de Estudios Territoriales, San Salvador, El Salvador Jiří Šebesta: Czech Geological Survey, Prague, Czech Republic
1 Oficina de Planificación del Área Metropolitana de San Salvador (OPAMSS), San Salvador, El Salvador 2 Czech Technical University in Prague, Faculty of Civil Engineering, Department of Geotechnics, Czech Republic 3 Servicio Nacional de Estudios Territoriales, San Salvador, El Salvador 4 Czech Geological Survey, Prague, Czech Republic *Autor para contacto: jose.alexander.chavez.hernandez@fsv.cvut.cz
Recibido: 17/02/212; aceptado: 29/08/2012
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