Engineering-geological and geotechnical

Cent. Eur. J. Geosci. • 3(3) • 2011 • 260-270 DOI: 10.2478/s13533-011-0028-0

Central European Journal of Geosciences

Engineering-geological and geotechnical investigation for risk assessment Research article

Moufida El May1∗ , Mahmoud Dlala1 , Aouicha Bedday2 1 U.R. Paleoenvironment, Geomaterial and Sismic Risk, Departement of Geology, Faculty of Sciences Tunis, University Tunis El Manar, 2092, Tunisia 2 Military Acdademy of Foundouk Jedid, Tunisia

Received 15 May 2011; accepted 8 August 2011

Abstract: Before construction activities could begin, engineering geological and geotechnical investigations had to be approved in order to establish a map with suitable areas for safe construction. The example used in this study is Tunis City which has complex geology and geomorphology. The risk analysis was based on a large-scale land-suitability map that was prepared using Geographic Information Systems (GIS). The approach used in this study combined physical data with the geotechnical properties of Tunis City. The adopted methodology and analyses were performed to assess the risk of urban expansion and landscape management. Results are presented as a zoning map that shows the suitable area for safe extension of the urban area. The data used and multicriterion analysis of geotechnical and geological data seems to be useful for similar case studies and the adopted methodology can be used successfully for identifying similar cities for risk assessment. Keywords: geological engineering • geotechnical investigations • GIS • urban extension • Tunisia © Versita sp. z o.o.

1.

Introduction

An essential part of meeting the basic needs of people is addressing the housing backlog. Therefore, vast areas of land are needed for the construction of houses. Due to rapid urban expansion, most of the land suitable for development has been exhausted mainly in close proximity to urban centres. Prior to the development and the management, future urban land is required and must be identified and approved by the decisions makers [1, 2]. The investigation includes environmental impact assessments, using ∗

E-mail: [email protected]

geotechnical and geological urban data. The geotechnical and geological engineering investigation for safe management of urban expansion is divided into two parts: (1) physical investigation of urban environmental parameters and (2) geotechnical test interpretations. The purpose of the geotechnical studies is to establish a zoning map according to the risk for safe construction and harmless urban extension [3]. Geotechnical risk assessment aims to evaluate and propose measures to be improved in these areas by planners and decision makers. The ample geotechnical mapping in urban areas is therefore an essential part of rational development planning. The methodological approach coupling geological engineering and geotechnical tests interpretations tries to supply solutions for these problems.

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Moufida El May, Mahmoud Dlala, Aouicha Bedday

Geotechnical and geological engineering investigations are divided into three categories:

In the study area, soil is heterogeneous and has engineering properties that vary markedly with location and causes damage to urban buildings.

1. planning, providing information for decision-makers and planners involved in urban development;

Preliminary planning for the geotechnical investigations at the study site is required to outline the technical and managerial tasks and to plan the detailed scope of the study. This involves data collection, urban geological investigation and geotechnical study (in situ and in the laboratory).

2. development, as well as information to developers of urban areas [4]; 3. specialized investigations of complex sites for specific and geotechnical engineering design parameters need to be determined. Among the available methodology for the geological and geotechnical data analysis is GIS which are mainly used for complex case studies which cover large geographic areas and multiple data layers [4–7]. The current study focuses on the engineering-geological and geotechnical investigations, such as those regarding the lithology classification, the dynamic resistance of the rocks and the swelling potential for the risk assessment of the study site. The objective of this study is to produce a risk assessment map using GIS in the Tunis City area. The final map highlighting the risk for urban extension is presented.

2.

Methodology

2.1.

Study area

Tunis City is characterized by recent sedimentary formation outcrops and heterogeneous deposits. These sedimentary features define shallow aquifers. The sediment properties and hydrogeological conditions make the site favorable for seismic wave amplification. As a result the soil is prone to liquefaction and increased damage [8–10]. Tunis City is a complex geological structure [9, 10]. During the geological history of the area, sedimentary events have influenced deposit characteristics, as well as morphology of the site. Sedimentation of thick detritus materials in the Tunis lagoon shore evokes an old estuary [9–11]. Slope stability is one of the most severe problems affecting urban planning in the study area. As mentioned in previous studies [11–13], there have been numerous landslides. In Tunis City (Figure 1), flooding is an environmental problem which restricts and influences urban development. The occurrence of flooding depends upon the intensity of rainfall [14]. In this study the stream network is mapped to identify the area endangered by floods.

2.2.

Data collection and approach used

To supply a working base for the geotechnical investigations, all the available literature and data describing the studied city should be collected and reviewed. The review should include discussions with geologists, seismologists, and other experts in related fields who might possess pertinent knowledge of the region. An inspection of the study city and immediate vicinity is essential to gain awareness of the geological and geomorphological characteristics, and to note some of the specific features and problems that will have to be considered during the risk assessment mapping. In order to determine the most suitable zone for safe urban extension, factors such as; lithology, slope, tectonic activity, drainage distribution, depth to groundwater table, liquefaction susceptibility, resistant level and depth of foundation derived from geotechnical laboratory tests are used. To achieve this, thematic maps were prepared relatively for these parameters. All the thematic layers considered in the geotechnical mapping are classified and integrated in a GIS environment [15, 16]. The mapping approach is based on the coupling of the GIS multi-criteria analysis method. Thematic layers are then superimposed two by two to give a final zoning map. The flowchart (Figure 2) explains the methodology used in further detail. The adopted methodology requires the establishment of a GIS project using ARCVIEW 9.1 software to facilitate manipulation, visualization and superimposition of layers.

3.

Results and discussions

3.1.

Geological investigations

Located in the northeast of Tunisia, Tunis City provides a link between North Africa and Sicily (Figure 1). Geologically it belongs to the Eastern Tunisian Atlas. The landscape is dominated by the alternation of reliefs and plains. In this area, collapse caused by the large major

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Engineering-geological and geotechnical investigation for risk assessment

Figure 1.

Location of the study area.

faults have created structures such as high Jebel Ressas, Bou kournin Zaghouan and the lithology consists of limestones of Jurassic and Cretaceous [10]. The landscape has given rise to plains and depressions. The relief is the resultof the location of human activities and biophysical processes. This must be taken into account, especially in mountainous areas, by geomorphologists, hydrologists, climatologists and ecologists. It is an important factor for risk and natural landscape assessment studies. The land level represented in three dimensions is an important component for the establishment of virtual environments (figure 3).

Figure 2.

Flowchart of the adopted methodology.

The geological map is the essential starting point for the preparation of the geotechnical maps. However, to make geotechnical maps, complex methods and techniques are used to collect and interpret engineering geological information. One inherent difficulty in the techniques of geological and geotechnical mapping, lies in the fact that the characteristics of rocks and soil often vary gradually and that these variations can occur both horizontally and vertically.

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it has an important depth near the surrounding relief. In the plains, the depth of the water is low and it is confused with the natural topographic elevation even in areas near lakes and Sebkha (Figure 6). For the aquifer with water table depth between 0 m and 2 m the risk for management and secure urbanization is high. These regions where the water is too shallow are mapped as areas with major risk for future planning projects. Figure 3.

Uniform grid of ground elevation (in meter) in Tunis City.

There was still not a single document which can be based on the geological survey of Tunis City. To do so, we review the previous geological investigations in the study area [17–23]. The seismotectonic map of Tunis City was created by Dlala and Kacem (2007) [24]. The final geological map of Tunis City is given in Figure 4. The morphological structures of Tunis City consist of various lithological units with different geotechnical characteristics. In fact, if we superimpose the ”geological” map and the topographic layer, it shows that the structures constituting the high reliefs are made of limestone and limestone-marl alternations. The reliefs forming the lower hills of the Eastern sector, the hills surrounding Tunis and the Southern hills of Tunis Lake are made up of Oligocene to Pliocene formations mainly composed of sandstone, clay and sand. These areas of high topographic standpoint have resisted the natural degradation due to their good mechanical and physical characteristics. Plains, occupied by less resistant formations of Quaternary are formed mainly by silt. The lithology offers a high capacity for water infiltration to the major aquifers in Tunis City. They constitute the area for safe urban extensions [25]. The study of weather is an important factor not only in agricultural and economic studies but also in the field of civil engineering. Indeed, monitoring and evaluation of climate parameters such as precipitation, temperature, evapotranspiration, sunshine and humidity can reduce the effect of some natural phenomena. The groundwater level fluctuation is also to be carefully studied and reviewed for the risk assessment. Tunis City was subdivided in two major watersheds namely, the catchment of Medjerda and the watershed of Oued Milian (Figure 5). The map of the temporary and the permanent drainage system is derived from scanning the topographic maps at the scale of 1/25000 that cover the study area (Figure 5). This map shows a network of permanent streams and the large plains. The study of water table level in Tunis City shows that

The long-term evaluation of the seismic activity of the region is interesting in estimating the risk for the safe urban extension. The object of the seismic hazard is to assess the level of seismic hazard, but it also works to provide adequate strength with earthquakes. The seismic hazard can be estimated by two different approaches namely:(1) The deterministic approach, which defines the maximum effects of a seismic event in a given site. It is commonly used, especially for the study of high-risk sites; (2) The probabilistic approach [26], which is evaluated taking into account the frequency of earthquakes, the probability that a given earthquake (characterized by an intensity or a parameter of ground motion, such as acceleration, speed or displacement) occurs at a given point, site and during a set time interval. For the study area, the seismic hazard assessment has been made by the National Institute of Meteorology of Tunisia, as part of the evaluation project of regional seismic hazard in Tunis City developed by Kacem (2004) [27]. The results of this study were represented as isoacceleration maps that were made for return periods of 50, 100, 200, 300 and 475 years. Analysis of these maps show that map of iso-acceleration for the return period of 475 years (Figure 7), is considered the reference map as it is used for standard works. From this reference we see that the entire study area has a potentially strong regional seismic hazard = 0.2g.

3.2.

Geotechnical investigation

The practical purpose of the geotechnical study is to enable the specific adaptation of the work site in order to limit the natural and induced risks. It is done to inform the developer and builders of the nature and behavior of the site so they can define and justify the technical solutions they will develop and implement to achieve the development safely and at low cost [28]. The geotechnical study of the surface soils of Tunis City was possible after the examination of 841 boreholes. Geotechnical testing conducted in this study is summarized in this table (table 1). The relief is a key factor in environmental studies [29, 30]. Topographic attributes and geomorphology, represented

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Figure 4.

Simplified geological map of Tunis City.

Figure 5.

Map of the drainage network and watershed boundaries.

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Figure 6.

Iso-depth map of the water table level.

Figure 7.

Map of iso-acceleration for a return period of 475 years (Kacem, 2004).

Table 1.

Geotechnical testing used in this study.

Geotechnical testing Identification tests

Mechanical tests

-

- Compressibility test on the odometer - Shear test rectilinear (Casagrande box) -Triaxial test - Unconfined compression test - Pressuremeter test

Particle Analysis Atterberg Limits Soil classification Water Content

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by Digital Elevation Models (DEM) to determine parameters that describe the environment, can be used in the analysis of surface states. Indeed, Digital Terrain models are carried out using the geomorphology of the ground and we can therefore learn about the watershed area [31]. The overlay of elevation data on the hydrographic network, already presented in a previous figure (Figure 5), allows us to identify flood zones (Figure 8). The area located in the zone with a slope less than 2 ◦ is considered a flooding district [32]. The operating results of geotechnical investigations in the study area allow us to produce maps that characterize the soil; such as the map of the swelling soils and the map of the resistant level depth. The relevant data of 180 tests were examined for mechanical assessment of the swelling soils (Figure 9). Clay swelling occurs when water-base filtrates from drilling and stimulation fluids enter the formation. Clay swelling can be caused by ion exchange or changes in salinity. It occurs when a clay has a liquid limit of more than 50% ? The identification of the swelling clay is commonly based on the measurement of the limit of liquidity by the Casagrande experiment. The interpretation of the geotechnical testing data permits the establishment of a map of the foundation depth (Figure 10). This shows that the depth of foundation varies with lithology and geotechnical character. It shows that the bedrock is mostly deep in some localities (Figure 10).

3.3.

Risk map: Criteria and process

Geotechnical zoning criteria should be compiled to determine the conditions under which a region can be treated as homogeneous [33]. Some of these criteria should include: (a) a degree of dispersion of the data acquired, (b) the maximum number of intervals and (c) a minimum area of the region which can be considered as a homogeneous area. Obviously, increasing the degree of the dispersion of data will result in fewer intervals and increasing the size of the zoned area, but will also increase the dispersion of data within an interval. On the other hand, increasing the number of intervals will cause the decrease in the degree of dispersion of the data and reduce the size of zoned area. When the number of zones is larger and their sizes are smaller, the degree of reality of a model is higher and uncertainty will be less, but it requires more analysis and interaction. Geotechnical zoning is complex and interactive; it consists of a series of activities starting with the initial estimation of homogeneity from preliminary investigations. They are improved and modified by information collected during successive surveys. Geotechnical zoning should

be implemented in a logical order, where the elimination of some steps can decisively influence the quality of final results and the usefulness of modeling of geotechnical engineering. In this process, four main stages can be distinguished: (1) analysis of available data, (2) preliminary zoning, (3) selection of appropriate parameters, specifying the criteria and final zoning. The geotechnical mapping methods adopted for this study involves the establishment of some maps. Each map was represented as a separate layer in the GIS database. GIS is able to incorporate the related spatial data into traditional geotechnical databases in order to present a more comprehensive view of the target region [34]. For geotechnical mapping studies, GIS offers a spatial representation of complex geological systems [32, 35–37]. A number of thematic maps reflecting geological and geotechnical parameters were prepared from the gathered data. Thematic maps established using GIS techniques include: (1) lithology, (2) topography; (3) slope, (4) seismotectonic, (5) liquefaction, and (6) flooding susceptibility. A multi criteria analysis is used to integrate all qualitative thematic layers prepared from the collected data and to assess a map of risk. In this study, all data are processed using the multi criteria analysis by superimposing geo-referenced information layers. As a result, the study area was categorized into four different zones as: (1) hillsides of potential surficial perturbation risks; (2) a low flat zone with flooding risk; (3) a low zone with flooding and sliding risk; and (4) a region with mud levels and settlement risk. The risk map that was generated identifies the suitable area for safe urban extension and the unsuitable area. Compared to previous studies [38–40], the multi criteria and multidisciplinary analysis, using geological and geotechnical engineering investigation employs many thematic maps and careful mapping. The novelty of this work is the adopted methodology of mapping, assisted by Geographic Information Systems, and the various data sources used.

4.

Conclusion

Based on the guidelines proposed by the adopted methodology, this study was developed aiming to provide a risk assessment and a better picture of the geological and geotechnical condition of the studied area for planners. The risk map has four classes that indicate the suitability for landscape and urban extension with regard to geotechnical and geological conditions. These classes depict

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Figure 8.

Flooding zones map.

Figure 9.

Map of swelling soils in Tunis City.

the area’s potentialities and vulnerability regarding simultaneously the geological properties of the City and the geotechnical parameters, and consist of a database that enables the orientation of landscape and urban management in the future. The maps that were generated in this study allow us to indicate the existing geological and geotechnical problems in Tunis City and function as a guide for landscape management and urbanization. These maps do not take away the obligation of more detailed

geotechnical testing, they serve only as an indicator for the decision making. The greatest advantage of the GIS use is the simple manipulation of the information update. It is indispensable to consider such a multidisciplinary study with the presented methodology in the Tunis City prior to any urban extension and landscape management; to ensure that environmental impacts and disasters can be minimized or avoided.

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Figure 10.

Map of the resistant level depth.

Figure 11.

Final geological and geotechnical zoning map of Tunis City.

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Engineering-geological and geotechnical

Engineering-geological and geotechnical

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