Engineering geology mapping in the southern part of the metropolitan area of San Salvador

Mapeo de ingeniería geológica en parte sur del área metropolitana de San Salvador

José A. Chávez¹*,²*, Jan Valenta², Jan Schröfel², Walter Hernandez³* & Jiří Šebesta⁴*

*^{Dirección para correspondencia:}

Abstract

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 stripe method. The objective of the methodology is that geological information, geological hazards and geotechnical recommendations as well, can be represented and grouped depending on the intrinsic characteristics of each zone. This information can be easily interpreted for urban planners, private builders and government agencies. The weakness in the compilation and research of geological and geotechnical information in El Salvador, are some of the reasons for the current problems that experiment the region, indicating the importance of improving risk management, as well as soil and rocks mechanics.

Key words:Engineering geology, stripe method, geology, San Salvador Formation, Balsamo Formation, Tierra Blanca.

Resumen

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 geológica y recomendaciones geotécnicas puedan representarse y agruparse dependiendo de las características intrín- secas de cada zona. Esta información puede ser fácilmente interpretada por los planificadores urbanos, constructores privados y agencias gubernamentales. La debilidad en la recopilación e investigación de información geológica y geo- técnica en El Salvador, son unas de las razones de la problemática que experimenta la región, indicando la importancia de mejorar el manejo del riesgo, así como la mecánica de suelos y de rocas.

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 growth was expected at the time.

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 (Gonzalezet al., 2004) is exposed to volcanic eruptions, earthquakes, floods, mass movements, drought and heavy rains that cause human victims, damages in the infrastructure and economical losses that prevent sustainable development of the countries.

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 local authorities to adjust the national and municipal budgets and ask for international loans to rebuild the infrastructure and assist the affected people. (MARn, 2009 and 2010; la Prensa Grafica, 2011; El Diario de hoy, 2011)

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 GDB of 2002 (Gonzalezet 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 scarce economic resources that live in risky areas (Gonzalezet 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 type of foundation, mass wasting processes, water resources and flood hazard. Recognition of these factors (Dearman, 1987) will improve planning and development of the city allowing rational decisions to be taken in the territory.

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 methodology of engineering geology mapping. The results of the field work and mapping completed during the year 2008 are presented next.

Area of study

The Metropolitan Area of San Salvador (MASS) has an area of 609,9 km2 (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 (Lexaet 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).

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 (bosseet 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 Investigaciones Geotécnicas, 1987) and the geological map 1:50 000 made for a seismic zoning after the 1986 earthquake (Consorcio, 1988). lexaet 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 to the geological map of Lexaet 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 (Lexaet 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 (2006) and lexaet 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 historically by earthquakes (Larde, 2000 and Šebesta, 2007a).

Mass wasting processes (Šebesta, 2006 and Lexaet 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 tuffs. This situation happens principally in the steep slopes of the erosion hillsides in the upper part of the basin.

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) which is composed by accumulated material of volcanic eruptions and eroded material.

Engineering Geological Mapping

Part of the project, was to observe how the geotechnical work and risk management is made in El Salvador including the usual lab and field tests.

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.

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 engineering geology mapping was done without any drilling boreholes or geophysical measurements and it was based on the existent topographical cartography 1:25 000 (CnR, 1980, 1981).
 
The stripe method was chosen to symbolize the engineering geological units that exist in the area. Dearman (1987) explains that the stripe method was originated in Czechoslovakia in the 60´s and is analogous to trenching down through the surface layer of soil to the next layer below (rock base). The stripe method is the representation of the character of the existing layers and the thickness of the top layer. The basement or intermediate layers can be shown by stripes (simulating a window) underneath the surface layer.

In the stripe method a single color represents a single unit and the map unit patterns describe the lithological character (Dearman, 1987); for example lava, pyroclastic flows, epiclastic breccia, weathered lava and ashes.

An engineering geological map illustrating the aptitude for urbanization is presented infigure 7. To build this map information like mass wasting processes, slope inclination, human disturbance, flooding, lithology, weathering and permeability were identified in the field. Then the areas with the same level of aptitude for urbanization were grouped according to the information compiled above, and general recommendations were proposed for the more problematic. Some hazards like the seismicity, extreme rains or volcanism were not evaluated though, because of the lack of complete information at the time of mapping.

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.

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 PL) were grouped into one unit and represented with stripes in the map. Roman numerals in the map indicate the thickness of the engineering geological units.

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 (f^{igure 6} Fig. 6 f Fig. 6 6 Figure 6 and table 1). The description of the units is presented next.

Description of rocks and soils

According to Hernandez (2008) the volcanic tuffs predominates in the surface, belonging mostly to the San Salvador volcano and to the Ilopango Caldera, decreasing the thickness and changing the grain particle size as they move away from their source center. The location of the tuffs was controlled depending on the direction of winds, erosion processes and explosive force during the eruption; for these reasons in the outcrops there´s no presence of all the layers.

Engineering geological units presented in the map are (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 flows are located in different places and the depth decrease and disappear indicating how the original morphology depressions was used for their transport. This rock base is usually massive and it’s affected by selective weathering (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 Lexaet al.(2011) the weathered rocks (laterites) have presence of illite, Kaolinite/ halloysite, smectite and goethite. The montmorillonite (smectite) presence is important since swelling can occur.

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 (figure¹⁰ 10 and ¹¹ 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.

Landfills were identified also, in order to avoid future problems if urban growth or constructions are established on them (Figs³ fig. 3 3 and⁶ Fig. 6 f Fig. 6 6 Figure 6 ).

Geotechnical Knowledge

In El Salvador the moisture content, particle size analysis, Atterberg limits, cohesion, angle of internal friction, compressibility, permeability and compressive strength are the parameters usually obtained. Standard penetration tests (SPT) and Modified Proctor are the tests that normally are done. The triaxial and shear box tests, for saturated soils, using the ASTM norms are done also. For the rock mechanics drill-hole bores, geophysics and compressibility strength are done sometimes, depending on the importance of the project.

Tierra Blanca Joven (TBJ) is the most important volcanic tuff and is present in important sectors of the MASS (as is shown inFigure 6). TBJ is quite sensitive to erosion, water content change, natural or artificial vibrations and compressibility (Roloet al.2004; hernandez, 2004; Šebesta, 2007b; Šebesta & Chavez, 2010). This material is also quite sensitive for slope stability problems (Figure ² Fig. 2 2 and ¹³ 13 ).

Guzmán & Melara (1996); Amaya & Hayem (2000); Roloet al.(2004); Hernández (2004); Molinaet al.(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 usually the groundwater level is deep, (35 m in urban areas according to Roloet 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.

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 pressure ua and degree of saturation are needed. For unsaturated soils characterizing (Fredlund & Rahardjo, 1993), the use of two independent stress variables: net stress σ-ua and suction uauw are needed. Murray & Sivakumar (2010) say that the most recent stage in the understanding of unsaturated soils is the analysis of the behavior in terms of constitutive relations linking the volume change, shear stress and deformation due to shear stress in elasto-plastic models. Nowadays considerable progress has been achieved through this approach, because it’s possible to explain some of the characteristics of soil behavior.

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. Roloet al (2004, table 3) compared suction values from TBJ samples and concluded that the negative pore pressure, weak interparticle bonding or cementation have a contribution to the shear strength, creating an apparent cohesion that is lost when is saturated or during seismic events. Guzman & Melara (1996) and Hernandez (2004) conclude the same.

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 during a project.
 
Conclusions

There is a need to simplify the geological maps in order to help the interpretation for the developers and urban-planners. The experience has shown the necessity to introduce in each project a proper geological risk assessment to avoid future problems; taking into account the mass movements, seismic effects, floods, groundwater situation, geological material properties and erosion principally.

The engineering geological mapping can serve as first or final steps in planning an infrastructure and can have an emphasis according to the tent or interests to evaluate. In the rocks and soils (Anon, 1972) the description of color, grain size, texture, structure, fracture state, weathered state, strength properties and permeability can indicate the properties and characteristics of them.

The presence of layers of laterites and paleosoils are very problematic in the sector but is important to note that in areas where there is significant presence and thickness of Tierra Blanca Joven (TBJ) is necessary to be careful with the loss of apparent cohesion, experienced when the suction or cementing decrease or when an earthquake affects the area (Roloet 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 (collapse or swelling) and the dynamic properties in the geological layers can cause damage to the structures; this situation makes imperative the introduction of the unsaturated soils mechanics in the design of the projects that will reduce the problems that the MASS is experimenting at the present.

It is recommended to continue systematic engineering geological mapping and data collection. The scientific research, probing, field and lab testing will improve the knowledge of the risk and properties of all the geological layers. The built of a public archive like the “Geofond” (http://www.geofond.cz/en/homepage) in the Czech Republic; with geotechnical and geological hazard data; which will be open to all (designers, engineering geologists, geotechnicians, state and town authorities, agricultural engineers, environmentalist etc.) would help improving the knowledge applied during the conception of the projects.

Acknowledgements

The work has been carried out in the framework of cooperation between the Czech Geological Survey and the Oficina de Planificación del Área Metropolitana de San Salvador (OPAMSS) that was a part of the Project of the Czech Republic Foreign Development Programme under auspices of the Czech Ministry of Environment RP/6/2007. Authors acknowledge support of the Czech Geological Survey, the Planning Office of the Metropolitan Area of San Salvador de San Salvador (OPAMSS) and the Geological Survey of the Environmental and Natural Resources Ministry of El Salvador. We are grateful to the reviewers and editors whose remarks improved the quality of the paper.

References

AMAYA, C. & HAYEM, E., 2000: Introducción al estudio de suelos parcialmente saturados e inicio de la caracterización de la Tierra Blanca del AMSS.-147 pp. Universidad Centroamericana José Simeón Cañas,

References

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