TECHNICAL PAPER Geotechnical characterisation JOURNAL OF THE SOUTH AFRICAN INSTITUTION OF CIVIL ENGINEERING and back analysis of a ISSN 1021-2019 Vol 62 No 4, December 2020, Pages 2–10, Paper 0948 landslide in marl deposit: MIRA FILALI is presently a PhD student in A case study of Algiers the Civil Engineering Faculty and in the Environment, Water, Geomechanics and Structures Laboratory at the University of Sahel (coast), Algeria Sciences and Technology Houari Boumediene, Algiers. She is also an engineer at the National Polytechnic M Filali, A Nechnech, J de Rosa, H Gadouri, B Meziani School of Algiers (ENPA), and a teacher and researcher in the Earth Sciences Department at the Djilali Bounaama University, Khemis Miliana. Her research interests focus mainly on landslides in clayey soil. The purpose of this study is to present the results of geotechnical investigations and landslide Contact details analysis in a marl deposit at the Sahel (coast) of Algiers in northern Algeria, where many landslides Water Environment and Geomechanics Laboratory take place in the Plaisancian marls, particularly following rainfall periods each year, causing Civil Engineering Faculty severe damage to infrastructures and buildings. The physico-mechanical characteristics of the University of Sciences and Technology Houari Boumediene soils obtained from three different sites (El-Achour, Daly-Brahim and Ouled-Fayet) were analysed Algiers, Algeria to identify the mechanism of these landslides. In the study, the laboratory test results providing T: +213 772 47 48 34 grain-size distribution, Atterberg limits, water content, shear strength, and compressibility were E: [email protected] analysed. The findings showed that, although the soils were characterised by slightly higher plasticity at Ouled-Fayet, they were generally homogeneous in the studied sites. The upper PROF AMMAR NECHNECH, who holds a doctorate from the National Polytechnic soils, generally weathered, exhibited low shear strength parameters, which are lower than the Institute of Loraine, Nancy, France, is an undisturbed formation beneath. The stability analysis based on limit equilibrium methods (LEM) engineer at the National Polytechnic School showed the significant influence of pore water pressures on slope stability, suggesting that the of Algiers (ENPA), Algeria, and Director of weathered soils are prone to instability processes due to the effect of long rainy periods. the Environment, Water, Geomechanics and Structures (LEEGO) Laboratory at the Houari Boumediene University of Science and Technology (USTHB) in Algiers. He is also Dean of the Faculty of Civil INTRODUCTION where they are triggered by a combination Engineering at USTHB. Landslides are considered one of the most of several factors linked to the site geology, Contact details destructive natural hazards in the world, land morphology, hydrology and climate, as Civil Engineering Faculty University of Science and Technology accounting for approximately 9% of the well as antropic activities (Bahar & Djerbal Houari Boumediene natural disasters that occurred worldwide 2016). Studies have revealed that several Algiers, Algeria during the 1990s (Yilmaz 2009), caus- landslides occurred in many regions, such as T: +213 21 24 72 24 E: [email protected] / [email protected] ing large numbers of casualties and huge Constantine (Benaissa et al 1989; Lafifi et al economic losses in the mountainous areas 2009; Bougdal et al 2013), Medea (Medjnoun DR JACOPO DE ROSA is a post-doctoral of the world (Dai et al 2002). Landslides 2014), and Kabylie (Machane et al 2009; research fellow at the University of are very common in northern Algeria, Bouaziz & Melbouci 2015; Meziani et al Basilicata (UniBas), Italy. He received his PhD in “Engineering for Innovation and Sustainable Development” at UniBas in 2019, with a dissertation titled Ion DR HAMID GADOURI works in the Department of DR BRAHIM MEZIANI works in the Department transport from KCl wells and K+ effects on Earth Sciences at Khemis Miliana University, of Earth Sciences at Khemis Miliana University, the shear strength of a clayey earthflow. His Algeria. He obtained his Master’s degree in Algeria. He received the PhD degree in Imaging current research interests focus on the chemo-mechanical processes Geotechnical Engineering from Khemis-Miliana and Digital Mapping in Geosciences from the affecting clay behaviour, the influence of such processes on the University in 2010, and his PhD in Construction University of Houari Boumediene in 2018. He evolution of landslides in clayey soils, and the hydro-chemo mechanics Engineering and Geotechnical from Medea worked at the National Laboratory of Habitat of earthflows. University in 2017. Currently he lectures and Construction, and at the Centre for Research Contact details geotechnical engineering at Khemis-Miliana in Astronomy, Astrophysics and Geophysics for School of Engineering University and is a researcher in the Geomaterials Laboratory at Chlef six and two years respectively. His research interests are in geology, University of Basilicata University. His research interests are in the field of chemical soil stabilisation. geotechnical engineering and geophysical methods. He is the author and Potenza, Italy He also reviews papers for several journals. co-author of seven scientific articles. At present, he is a lecturer at Khemis-Miliana University. T: +097 141 1044 Contact details E: [email protected] / [email protected] Earth Sciences Department Contact details Djillali Bounaama University Earth Sciences Department Khemis Miliana Djillali Bounaama University T: +213 27 72 17 94 Khemis Miliana E: [email protected] / Ain Defla, Algeria [email protected] T: +213 675 323 207 Keywords: Plaisancian marls, landslides, weathering, physico- E: [email protected] mechanical properties, limit equilibrium methods Filali M, Nechnech A, De Rosa J, Gadouri H, Meziani B. Geotechnical characterisation and back analysis of a landslide in marl deposit: a case study 2 of Algiers Sahel (coast), Algeria. J. S. Afr. Inst. Civ. Eng. 2020:62(4), Art. #0948, 9 pages. http://dx.doi.org/10.17159/2309-8775/2020/v62n4a1 495000 496000 497000 498000 499000 2°61E 2°54E 2°57E 3°E 3°3E 3°6E 3°9E Kms 4068000 Dely Brahim 0 2 4 8 12 16 N Landslide W E 4067000 Ain Benian S Bouzareah Bab El Oued Beni Messous 4066000 Ouled Fayet El Biar Staoueli Dély Brahim Hydra Ouled Fayet 4065000 0 0.5 1 2 El Achour El Harrach Km Legend Zeralda Souidania Saoula Oued Smar Quaternary indefferentiated Alluvial terraces 36°42N Douaouda Marine terraces Douera Baraki Upper Pliocene (Astien) Les Eucalyptus Lower Pliocene (Plaisancien) Birtouta Miocene 36°39N Kabylian basement Sahel Fault Figure 1 Geological map of study sites (Royer et al 1961) 2018). Algiers is one of the cities affected by distribution, index properties, direct shear, (Meghraoui 1988). The Sahel is a succes- this geological hazard which causes severe and compressibility tests) obtained from sion of hills formed by Plaisancian marls damage to structures and infrastructure. inspections of an existing database of soil with a slope inclination varying from 5% After the independence of Algeria (in laboratories and companies. In order to to 30% (Derriche & Cheikh-Lounis 2004). 1962), rapid urbanisation took place, which conduct a deep investigation, the study The topography resulted from the post- extended to marginal lands southwest of examined several samples from the subsoil Astien tectonics, which caused the uplift the Sahel (coast) of Algiers once the areas of El-Achour (site A), Daly-Brahim (site B) of the Atlas Mountains and the formation became more favourable for construction. and Ouled-Fayet (site C). Other tests were of the Sahel anticline and the depression El-Achour, Daly-Brahim, Ouled-Fayet and also carried out to evaluate both the residual of Mitidja (Royer et al 1961; Aymé 1956). It Sidi-Abdellah are among the new urbanisa- strength parameters and the susceptibility should be noted that the landslides, char- tion areas which are affected by landslides. to progressive failure. Finally, a back-analysis acterised by wavy morphology (Bougdal The superficial or deep instability pro- based on limit equilibrium methods (LEM) 2007), occur in the weathered marl hori- cesses (rotational or complex) were mainly was performed to examine the role of soil zons of which the thickness varies with the observed in the Plaisancian marls and sandy mechanical deterioration due to weathering, degree of weathering, reaching about 8 m clays providing the transition between and the role of pore water pressures on the in depth, even at a slope inclination greater the Plaisancian and molassic Astien facies slope stability of the study area. than 10%. The degree of weathering affects (Aymé 1965). Some studies showed that the the strength parameters and therefore the landslide triggers in most cases were linked Study area slope stability of the Plaisancian marls. to rainfall or to antropic actions which The study area is located on thick deposits cause modification of the soil stress state of Plaisancian marls in the southwest of the (Bahar & Djerbal 2016). The study of Bièvre Algiers Sahel (coast) (Figure 1). The geo- METHODOLOGY et al (2016), on the other hand, purported logical formation of the Plaisancian marls The methodological approach adopted by that the variations in the soil strength outcropping in the Algiers Sahel, with a the study included the different phases as parameters due to weathering may