GEOTEKNIK-Geotechnical Characterization of the Quaternary Alluvial Deposits in Tunis City Tunisia

7/26/2019 GEOTEKNIK-Geotechnical Characterization of the Quaternary Alluvial Deposits in Tunis City Tunisia

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Geotechnical characterization of the quaternary alluvial deposits

in Tunis City (Tunisia)

Moufida El May , Dhekra Souissi, Hela Ben Said, Mahmoud Dlala

UR13ES26, Department of Geology, Faculty of Sciences, Mathematics, Physics and Naturals of Tunis, 2092 El Manar, Tunisia

a r t i c l e i n f o

Article history:

Received 12 January 2015Received in revised form 2 May 2015Accepted 5 May 2015Available online 14 May 2015

Keywords:

Geotechnical mappingBearing capacityDepth of foundationQuaternary depositsPhysical parametersTunis City

a b s t r a c t

Tunis City is always in perpetual extension. The urban extension takes place in areas covered by quater-nary alluvium. To evaluate the spatial variability of geotechnical parameters, 190 boreholes and in situtests, across the quaternary deposits, were selected with a series of physical properties and mechanicalparameters. To accomplish this research, the adopted approach was divided into four steps: (1) data col-lection; (2) geotechnical data georeferencing; (3) interpolation methods and (4) establishment of mapsshowing the spatial variability of the geotechnical parameters. Results of this study show that the qua-ternary deposits provide high water infiltration capacity and form the main aquifer of the study area.The low topography in Tunis City emphasizes the problem of flooding. The presence of sensitive claycauses a swellingshrinkage behavior of the soil if the clay is dry. The disorders and damages causedby the swelling clay are especially located in the Ksar-Said BardoMannouba, in Bab Saadoun, in the east-ern part of Sebket Essijoumi and in the southwestern side of Tunis lagoon. On the other hand, in situ testsshow that quaternary deposits contain strong shallow foundation level and vary from 0 to 5 m of thick-ness. These levels are made up by sand, tuff and hard clay. Muddy soils, in the Tunis City and its surround-ing area, have a poor geotechnical characteristics and cause differential and absolute risk of settlement.The good bedrock in these locations is defined by muddy and sand levels and it exceed 60 m of depth. Theexistence of a higher concentration of gypsum causes local disorders such as the Henchir El Yahoudia site.

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1. Introduction

Quaternary alluvial deposits are the most extended outcrops inTunis City with an area covering all the plains and relief in thestudy area (Fig. 1). Since the 1960s, the largest urban expansionis located in these area. The urbanization area had greatlyexpanded from 1960 to 2014. Unfortunately, this prominent urbanextension is not well controlled by decision makers. To avoid theimpact of urbanization, the first solution is to explore the periph-eric area of Tunis City. Therefore, the urban development ofTunis City needs a new proposal. The urban extension of the stud-ied City depends on the development of the metropolitan area thatincludes Bizerte, Zaghouan, Hammamet-Nabeul and Medjez El Bab.These small cities are now threatened by the development of spon-taneous residential cities (Chabbi and Abid, 2008).

The development of urbanization in Tunis City will continue toincrease in the coming years. The urban dynamics express the eco-nomic interest of the study City. The damage of uncontrolled

building is not noticed before the disaster. In fact, the constructionsare not always in harmony with the physical proprieties of the site.

Indeed, urban planificators underestimate the effect of the tem-porary water network. During the flooding event that occurs in2003 and 2007 in Tunis City, the exceptional rainfall caused manydisasters such as physical damages and casualties. The topographicmap and the stream network map shows that all watercourseswere connected to sebkha and lagoon naturally before the urban-ization. The impact of urban extension modifies the runoff andcauses flooding in cities.

In addition, the urban extension causes under-exploitation ofaquifer and subsequently generates a rise in groundwater levels.The groundwater level fluctuation causes geotechnical problemsresulting from the modification of the physical and the mechanicalproperties of the soil. The strength decreases while the porewaterpressure increase. In addition, the water causes the dissolution ofthe intergranular cement. The water reduces the bearing capacityof the soil and generates the swellings clay. Floods may occur inareas with low topography. If the groundwater level reaches thesoil, it induced seismic events such as liquefaction phenomenonthat is directly related to the static level (Evangelista et al., 2011).

http://dx.doi.org/10.1016/j.jafrearsci.2015.05.003

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Corresponding author. Tel.: +216 52674191; fax: +216 71885408.E-mail address:[email protected](M. El May).

Journal of African Earth Sciences 108 (2015) 89100

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In Tunis City, the arid climate is characterized by a high evapo-ration rate. This climate condition generates a large gradient ofwater content between built area and cleared zones. The gradientof water is at the origin of the differential settlement of soil. The

generated gradient of water causes the damage of superficialbuildings.

In addition, in slope land, buildings cause instability and anyintervention generates significant disorders (Besson, 1996;Collins and Sitar, 2009). The landslides phenomena occur in builtslope lands. It is caused by overload and high porewater pressure.Finally, the earthworks, the production of embankments, the back-fills and the roads contribute to the activation of the slip phe-nomenon. The advanced problems are the results of adisharmonious urban extension (Schweitzer and Johnson, 1997;Davies and Chaplow, 1998; Bilotta et al., 2005; Cattoni et al.,2007; De Rienzo et al., 2008; Fragaszy et al., 2011; Bhukte andChaddha, 2014; Athapaththu et al., 2015).

To define clearly the geotechnical problems and to better eval-

uate and characterize the subsurface soil in the study area filled byquaternary alluvium, several geotechnical investigations have beenconducted. In Tunis City, some research work focused on thegeotechnical characterization of the subsurface. This attention isdirectly related to the socio-economic interest of the region.Results of these different studies are summarized in the next sec-tion. En 1976, Guilloux and Nakkouri have established the firstgeotechnical map of Tunis City. This research showed that the qua-ternary formation, made up by the complex of mud, sand andsandy loam, is considered as a compressive soil.

In 1989, the research work made byKaanichefound that plainshave poor geotechnical characteristics. These plains are formed bya complex of mud with a thickness reaching 60 m and are sur-mounted by a clay-silty complex forming a good bed-rock. The

western part of Tunis City is surrounded by plains and slopes area.The tuff is a lithological horizon with bad geotechnical parameters.

The sandy clay complex is a good horizon for semi-deep founda-tions. The hilltops formed by a sandy clay horizon having homoge-neous geotechnical characteristics, are considered as bedrock.

Researchers and geotechnical engineers have established the

first geotechnical map of Tunis City in 1998 (Amar and Jezequel,1998). In the adopted methodology, authors have subdivided thestudy area in eight lithological formations to facilitate the evalua-tion of their geotechnical characteristics. The geotechnical zoningmaps established based on the lithological formations propertiesshows the following entities: (1) embankments, (2) mud, (3)sandy-clayey sandstone spits, (4) clay-sandy tuffs with a crust,(5) soil and salted sebkhas, (6) scree, (7) the Mio-Plio-Quaternaryalluvium and (8) Cretaceous-lower Eocene deposits.

The main objectives of this research are to: (1) contribute to thecomplete understanding of the geotechnical parameters of quater-nary alluvium in Tunis City, (2) to establish a zoning map showingthe variability of geotechnical parameters, (3) to make a sustain-able strategy to avoid damage in future urban extension by consid-

ering the geotechnical specificity and properties of the soil.The methodology adopted in this study is based on: (1) theinterpretation of the geological map to elaborate the lithologicalmap, (2) the collection of data related to the core boreholes, (3)the collection and the compilation of laboratory tests and in situtests, (4) the analysis and interpretation of results, and (5) theestablishment of maps showing the spatial variability of thegeotechnical parameters using a Geographical InformationsSystems software (ArcGis9.3).

2. Geological setting and geotechnical properties of alluvial soil

Tunis City is located in north-eastern part of Tunisia. It includes

the area located between Lambert coordinates 375,000 m and405,000 m in latitude and 505,000 m and 545,000 m in longitude.

Fig. 1. Urban extension of Tunis City.

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The study area is limited to the north by Ariana region and to thesouth by the Cap Bon peninsula. The Mediterranean Sea boundedTunis City on the eastern part. To the west, the study area isbounded by the plain of Wadi Mejerda, the plain of Mornaguia,Djebel Ain Krima and Djebel Sidi Salah. The study area covers thefollowing topographic maps at the scale 1:50,000: Ariana, LaMarsa, La Goulette and Tunis.

Tunis City area is characterized by a complex topography whereelevations are separated by plains, lakes and Sebkhas. Lithologicalmap of the study area was established based on the available geo-logical maps of Ariana, El Marsa, La Goulette and Tunis at the scale1/50,000. This map shows the age and the lithology of geologicalformations in the study area. The lithological map shows that,excluding the Cretaceous carbonate formations, Jurassic andEocene outcrops make up mountainous areas and hills. The otherpart of the study area is formed by loose deposits (Fig. 2).

In Tunis City area, soil was classified by El May et al. (2010),according to the morphology, in three zones: (1) alluvial plainsarea occupied by a variety of recent quaternary alluvial depositsformed by clay, sand, silts and tuff; (2) area surrounding plainsand slopes of hills are characterized by a Pleistocene to Holoceneoutcrops formed by clay, sand and silt, and (3) Hills areas that con-sist of Pliocene to Oligocene deposits formed by compact sand,clay, sandstone and conglomerate. Areas surrounding Sebkhasand lagoon are formed by evaporate and mud. Holocene formationis formed by the fine shell-filled sand of Soukra area. The depositsof Jurassic to Eocene are constituted by a very compact formationsformed by sandstone, marl and limestone alternations (Masoud,2015).

2.1. Subsurface lithology of alluvial plains

To have a clear illustration of the structure of the soil horizons,three cross section was established in the plain of Mannouba (A), inthe plain of Soukra (B) and through Tunis Basin (C). The cross sec-tion A shows that in the plain of Mannouba the soil structure con-sists essentially of clays with thin levels of tuff and coarse

elements. The second cross section B reveals that the subsurfacesoil structure in Soukra plain is mainly formed by sand in the northside near Sebkhet Ariana and by muddy levels to the south partnear Tunis lagoon. The third cross section B which is establishedthrough Tunis basin shows that from the north eastern side tosouth western part, the subsurface horizons are made up by alter-nations of clay, sand and mud. Near Esijoumi sebkha, the soil hori-zons are essentially formed by clay formations (seeFig. 3)

2.2. Used data and methods

Laboratory identification tests and in situ tests such as dynamicand static penetrometer are used in this study to evaluate geotech-nical properties of quaternary alluvial deposits in Tunis City. Soilprofiles and lithological horizons from boreholes are used to per-form the lithological correlation and cross section. In this study,we use also the mechanical tests of intact samples to establishgeotechnical parameters of lithological horizons (Avsar et al.,2014). All the geotechnical parameters were located in thematicmaps using ArcGIS software. To establish geotechnical maps, theinterpolations approach are applied by means of the spatial analystextension of the ArcGIS. The established maps such as bearing

Fig. 2. Map of the spatial location of soil units.

M. El May et al. / Journal of African Earth Sciences 108 (2015) 89100 91
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capacity map and depth of foundation of the quaternary levels andthe spatial variability of the geotechnical parameters help decisionmakers to secure urban extension (Fig. 4).

To simplify the study of geotechnical characteristics of the qua-ternary alluvium, we subdivided the lithological formations in the

study area into six zones according to their location on the map.The recent quaternary deposits are composed of alluviums andhave outcrops located in different plains situated in the study area.The zone A is located in the Soukra-Choutrana plain between thelagoon of Tunis and Ariana Sebkha. The zone B is situated betweenthe hills of El Manar and the lagoon of Tunis. The zone C and D aresituated in the Tunis basin. The plain of MannoubaMornaguia isconsidered as the zone E. The zone F is located in the plain ofMornag. (Esposito and Guadagno, 1998; Panda et al., 2014)(Fig. 5)

2.3. Physical features of the quaternary deposits

Physical features of the quaternary deposits are given in Fig. 6.In the plasticity chart, the plasticity index and the liquidity limitparameters are placed to evaluate the plasticity condition of thequaternary alluvium (Casagrande, 1932)(Fig. 7). The Casagrandediagram shows that most plastic clay are located in the easternpart of the Sebkhat Essijoumi (zone D), between Tunis lagoonand Belvedere hills (zone B), in the south part of the Tunis lagoon(zone F) and in some places such as Bardo-Mannouba (zone E)(Das, 1999).

2.4. The activity chart

The activity chart published bySung et al., 2012(Fig. 8) showsthat the major of the samples have not a high swelling potential.The samples from the eastern part of Essijoumi Sebkha located in

zone D and from Bardo and Ksar Said located in zone E are charac-terized by a high swelling potential.

2.5. Groundwater level

The groundwater level is one of the main factors that controlthe stability of foundation in the execution of the excavation work-ings. In liquefaction assessment, the position of the water table

level within non-cohesive sediments should also be known. Thelithological units in the study area define an unconfined aquifersmade up by the alluvium horizons. Groundwater level variationwas evaluated using the groundwater static water levels availablefrom piezometer in the study area. In 2012, the depth to watertable level of the Ariana-Soukra aquifer doses not exceeds 6.6 m.The most superficial static level is situated at 1 m of depth. Inthe aquifer of Mannouba, the deepest groundwater static level isabout 29 m and the most superficial one is about 1.8 m of depth.

2.6. Consistency index

The variation of the Consistency Index CI as a function of the

depth is illustrated in Fig. 6. For zones A, B and C, the CI is lessthan1, while for the zones B, F and E, the CI is greater than 1.Zones with low CI are characterized by soft to mid-consistent litho-logical levels, while zones with high CI, located in the south part ofTunis lagoon and in sebkhat Essijoumi, are made up by swellingclays and form compact levels.

3. Results

3.1. Geotechnical investigation of zone A

The drilling core carried out in the quaternary deposits locatedin the southern part of Soukra and Charguia shows a lithological

horizons formed by embankment, clay and sand levels, thin tuffand consolidated sands and silts.

Fig. 3. Cross sections.



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The results of laboratory tests of soils from zone A are summa-rized inTable 1. Clays have a varied plasticity. The soil has highwater content. The average value of the consistency index(CI = 0.55) shows that the subsurface soils are soft with a low swel-ling potential (d0g= 0.27 bars) (Amar and Jezequel, 1998). TheCompressibility index, which varies between 0.11 and 0.17, per-mits the classification of subsurface soils as moderately compress-ible. The depth of the foundation is around 2 m. In the zone A, soilslevels are characterized by a medium geotechnical properties. Theswelling clay phenomenon does not occur in the zone A, but soilsare not consisting with a tendency to the compression.

3.2. Geotechnical investigation of zone B

This zone is located between Tunis lagoon and Belvedere hills. Itconsists of embankments, sand, clay, tuff, coarse elements andmud. In this zone, subsurface soils are classified as a high plasticbased on the plasticity chart. Subsurface soils contain gypsum witha percentage up to 63% with high water content. The compressibil-ity index is sometimes lesser than 0.2 for compressible soil, andgreater than 0.05 for a low compressible soil. The swelling stress(d0g) is evaluated as 0.523 bars. Although the plasticity chart showsthat some soils are very plastic due to the high concentration ofgypsum.

In the sandy soil, the static resistance is between 20 and160 bars. It is classified as a consistent soil suitable for shallow

foundations (Celik and Canakci, 2014). The dynamic resistance var-ies between 23 and 320 bars and the soil has a high compactness.

Generally, in the zone B, soils are characterized by a good compact-ness with a depth of 3.6 m.

The sandy levels located in zone B are characterized by dynamicresistances that vary between 23 and 320 bars. The sand levels areconsidered as high compact horizons with a depth of about 3.6 m.In this zone, except the clayey levels that have a problem of consis-tency, sands and tuffs are considered as a good foundation soil.

Charguia city, located in zone B, is characterized by muddy soilsat a depth from 7.5 m to 15 m and with a thickness of 1.5 m.

3.3. Geotechnical investigation of zone C

In this zone, the drilling core shows a lithological successionthat consists of clays, sandy clays, sandy mud, coarse elementsand tuffs. The clayey soil of this zone is soft. The average value ofthe consistence index is equal to 0.58. Muddy levels are very com-pressible. The sandy levels are distinguished by a low thicknessand are laterally discontinuous. A differential settlement risk mayoccur in this zone. Generally, the zone C is characterized by a poorgeotechnical parameters where foundation depth exceeds 23 m.

3.4. Geotechnical investigation of zone D

In the zone D, subsurface soils are formed by clays, sands, tuffsand marls. The projection of clay samples analyzed in this zone inthe plasticity chart shows that clayey levels belong to the range of

moderate to high plastic clays. The Liquidity Limit (LL) value isevaluated as 73%. The average of the unconfined compressive

Fig. 4. Flow chart of the adopted methodology.

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stress is around 2.69 bars and reaches in some localities 6.265 bars.The values of the unconfined compressive stress confirm that claylevels are extremely consistent. In this zone, the ancient humanmade cavities in the El Kasba locality causes geotechnical prob-lems. The sand and tuffs levels have dynamic resistance that canreach 260 bars. The bearing capacity problems are not noticed in

this zone.

3.5. Geotechnical investigation of zone E

In this zone, the soil consists mainly of sand and clay withcoarse elements and tuffs. The clay horizons are consistent withan average of the CI equal to 1.03. The unconfined compressivestress is equal to 1.927 bars. The major problem in this zone is

the swelling clay potential that is relatively high. The swelling

Fig. 5. Geotechnical zone location.

Fig. 6. Physical parameters of quaternary deposits in Tunis City.



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strain is about 5 bars in Bab Saadoun and 1 bar in Bardo locality. Inthe zone E, the other lithological levels are formed by tuff and sandwithout any geotechnical problems (Saadeldin and Siddiqua, 2004;Skipper et al., 2005).

3.6. Geotechnical investigation of zone F

The dominant lithology in this zone are sand, mud and anembankments layer characterized by a thickness reaching from 1to 2.3 m. The southwestern part of this zone which extends fromBen Arous to El Mourouj Cities is the subsurface levels are madeup by sandy silts. The clay levels are very plastic, and the swellingpotential is very high. In Ben Arous, the swelling stress reaches16.7 bars.

The sand horizons have variable geotechnical characteristicwith a dynamic resistance that varies between 15 and 98 bars.The static resistance varies between 2 and 80 bars. Because ofthe presence of the compressible clays interposed between sandylayers, a high safety coefficient was used (Fabbrocino et al., 2014;Yal and Akgun, 2013).

In this zone, muddy levels are of poor geotechnical characteris-

tics with a Liquidity Limit that varies between 42% and 73%. Thefriction angle is always equal to zero and the cohesion varies

between 0.12 and 0.18 bar. Dynamic resistance varies between0.3 and 2.9 bars.

3.7. Geotechnical characterization of muddy levels

The position of the muddy levels in the plasticity chart showsthat the majority of points are situated in the field of highly plasticclays. The liquidity limit varies between 62% and 73% ( Table 2). Thewater content varies between 43% and 77% whereas, the consis-tency index is always lesser than 1. The maximum value of the

CI is about 0.52 thats why muddy levels are considered as a liquidto soft deposits. The friction angle u is equal to 0and the maxi-mum value of cohesion is equal to 0.18 bars. The compressionindex varies between 0.208 and 0.646 defining highly compressiblelevels. The void ratio is greater than 1. The dynamic resistance isbetween 0 and 40 bars. The static resistance is still less than10 bars. Hence, the muddy levels are compressible with a very poorgeotechnical characteristic and not acceptable to be considered asfoundation levels.

Sandy levels are characterized by a dynamic resistance that var-ies between 200 bars and 240 bars and the water content rangesbetween 16% and 48%. In this case, there is a superposition of com-pact and soft levels.

4. Geotechnical parameter mapping

The spatial variability of geotechnical parameters was mappedusing Geobased Information Systems (Bayisa et al., 2015). Theengineering geology issues are discussed using the basic geologicalmap, which is a thematic map that considers the information hav-ing an effect on the urban extension processes. The main geotech-nical parameters that have been mapped are the depth offoundation and the bearing capacity (Ansal et al., 2001; Phoonand Kulhawy, 1999). The evaluation of the depth of foundationand the bearing capacity of soil was carried out by the TunisianTesting Center and Construction Techniques. The map showingthe spatial variability of the depth of foundation and the bearingcapacity map were established using collected data at punctual

sites. The interpolation method was used to convert punctual datainto maps to facilitate the geotechnical interpretation. In fact, the

Fig. 7. Plasticilty chart.

Fig. 8. Activity chart.

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Table 1

Geotechnical parameter of alluvial soil.

Zone Identification tests Mechanical tests

Liquid LimitLL

Plasticityindex PI

WatercontentWn (%)

Consistencyindex CI

Gypsumcontent(%)

Soilorganicmattercontent(%)

Absolute drydensitydd(t/m3)

Unconfinedcompressiondmax

(kg/cm2)

Voidratioeo

Compressibilityindex CI

Compresstressd 0c

Zone A Maximum 54 33 38 0.92 0.709 0.174 Minimum 26 13 21 0 0.583 0.116 Average 40 22 27 0.55 0.666 0.137

Zone B Maximum 58 34 34 1.5 0.63 0.5 4.076 0.862 0.249 2.2 Minimum 20 6 11 0.08 0.269 0.508 0.078 0.52 Average 39 20 22 0.76 1.958 0.652 0.160 1.00

Zone C Maximum 67 43 53 0.76 Minimum 36 21 20 0.33Average 55 34 37 0.58

Zone D Maximum 75 44 30 1.3 1.85 6.265 0.751 0.16 0.73 Minimum 33 17 14 0.5 1.72 0.281 0.453 0.074 Average 50 29 21 0.9 1.77 2.690 0.545 0.104

Zone E Maximum 63 39 27 1.5 1.96 2.36 0.517 0.15 2.16 Minimum 27 15 11 0.8 1.66 1.416 0.36 0.063 1.125 Average 41 23 17 1.00 1.81 1.927 0.44 0.092 1.578

Zone F Maximum 72 45 29 1.2 15.65 0.29 1.52 2.006 0.646 1 Minimum 18 4 12 0.00 0.56 0.626 0.158 0.79 Average 47 27 21 0.92 0.875 0.984 0.293 0.90

Table 2

Geotechnical parameter of mud.

Zone Nombre demesures

Identification tests Mechanical tests

LiquidLimit LL

Plasticityindex PI

Watercontent Wn(%)

Consistencyindex CI

Unconfined compressiondmax(kg/cm2)

Voidratio eo

Compressibilityindex CI

Comstres

Mud AroundTunis lagoon

6 Maximum 73 64 77 0.2 2.006 0.646

Minimum 40 20 36 0.16 1.081 0.272 Average 61 36.7 58 0.1 1.544 0.459

Ezzahra city 6 Maximum 71 44 61 0.52 Minimum 62 39 43 0.15 Average 61 37 44 0.4


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natural Neighbor interpolation method used in this study hasmany positive features. It can be used for both interpolation andextrapolation. Generally, this interpolation approach is used withclustered scatter point. The basic equation used in NaturalNeighbor Interpolation is the same as the equation used inInverse Distance Weighting interpolation. This method used forthe interpolation processes handle efficiently a large amount ofinput point datasets. For the natural neighbor method, the local

coordinates define the amount of influence for each point andany scatter point will have an output cell (Childs, 2004).

4.1. Foundation depth

The depth of foundation seems to be deeper in three locations inTunis City (fig. 9): (1) the first location is observed around the lake,it is related to the presence of muddy levels; (2) the second loca-tion is identified in the Carthage area where lithological levelsare characterized by a low dynamic resistance caused by the thicklayers of fill and sandy levels; and (3) the third location is the

southern side of Rades where the muddy levels have a considerableinfluence on the foundation depth.

Fig. 9. Plasticity chart of mud.

Fig. 10. Map showing the depth of foundation.

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4.2. Bearing capacity

The lowest value of the bearing capacity is observed aroundthe lakes and in the Soukra plain. By contrast, the highest val-ues where located in area with a soil composed of sand andsandstone in the region of La Goulette and Rades (see

Fig. 11). The higher values of the bearing capacity are alsonoticed in the oligocene sands and in El Manar area where soils

are made up by coarse element and conglomerates (Fakheret al., 2007).

4.3. Depth of the first compact level

A depth map of the first lithological compact level whose

dynamic resistance is greater or equal to 100 bars was established(Waschkowski, 1979; Meardi, 1971)(Fig. 10).

Fig. 11. Map of the bearing capacity.

Fig. 12. Map of the depth of compact level.



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The established map shows a deepening of the compact level inTunis City and its surroundings as well as in the south of the lagoonand in isolated outcrops of the quaternary formations. The deepen-ing phenomenon becomes more important in the west side of thelagoon of Tunis where the depth of resistant level is between 10 mand 30 m. The presence of muddy layers around the lagoonexplains the important depth of the first compact level. The

Muddy levels are also considered as the consequence of the deepresistant level detected to the south-eastern side and southern partof Rades City.

The quaternary formations without muddy levels form in somelocalities several shallow resistant levels. The lithological variationproves the geotechnical heterogeneity of the quaternary alluviumdeposits in the study area. A detailed geotechnical investigationat an exhaustive scale is required for each urban project (seeFig. 12).

5. Discussion and conclusion

The achievement of this study was based on the collection andthe interpretation of the lithological and the geotechnical data inthe study area. Because of the heterogeneity of the collected dataand the spatial variability of geotechnical parameters, all geotech-nical maps are established using an interpolation method. The lackof geotechnical parameters influences the realization of the depthof foundation and the bearing capacity maps. The establishment ofthe geotechnical maps is based on the laboratory tests and on thein situ tests results. Despite the heterogeneity of the geotechnicalparameters, the obtained results are helpful for decision makersand planners to promise a secured urban extension.

The quaternary alluviums in Tunis City are located in the mostconcentrated urban zones. The geotechnical characteristics of theheterogeneous quaternary alluviums are considered as mediocregiven that they are formed by clays, sands, silts, tuff, vases and seb-

khas deposits. The subsurface soil levels made up by mud and bysebkhas deposits are compressibles. Besides, the clays are some-times highly plastic and have a high swelling potential. A high gyp-sum content is found in the southern part of the Tunis lagoon in J.

Jeloud, in El Kabbaria and in El Mourouj area with a maximum ofconcentration of about 15.65%.

The coarse elements are detected in a NESW direction fromMannouba-Bardo to Charguia area. The coarse deposits are locatedalong the old bed of the Mejerda watercourse as was advanced byPiemienta (1959). The surface flow of the watercourse was fromthe Casbah to the Belvedere Hills.

The analysis of the quaternary formations, which cover a largearea in Tunis City, demonstrates the heterogeneity of the geotech-nical parameters. Additionally, some subsurface soils are known by

their swelling sensitivity. These zones are located in the KsarSaid-Bardo-Mannouba area. The eastern side of Sebket Essijoumiand the south-eastern part of Tunis lagoon are also characterizedby a swelling sensitivity.

The subsurface soils with muddy horizons are characterized bytheir special geotechnical characteristics with a differential settle-ment risk.

In urban geology, the geotechnical mapping is considered as animportant tool for providing the spatial variability of geotechnicalparameters and is used to establish a suitable map for a safe urbanextension. The geotechnical mapping is a helpful tool for decisionsmakers and planners given that all areas with geotechnical risk areidentified and specified. The Geobased Information System usedduring this study is a helpful tool for the data management and

the mapping process. It offers to users the facility of the actualiza-tion and the establishment of all geotechnical maps.

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GEOTEKNIK-Geotechnical Characterization of the Quaternary Alluvial Deposits in Tunis City Tunisia

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