Geotechnical characteristics of the Plaisancian marls of Algiers: Fid-Bau Portal

Geotechnical characteristics of the Plaisancian marls of Algiers

Derriche, Z. / Cheikh-Lounis, G.

Abstract The Plaisancian Marl covers an extensive area to the south west of the Port of Algiers and forms the bedrock on which much of the future urban development will take place, in the area known as the Sahel of Algiers. Previous attempts to build on this formation were inadequately planned and as a result, soil instabilities occurred with significant financial consequences. This paper discusses the geomorphological setting and the environmental context in which the marls were formed before presenting the principal geotechnical characteristics of this problematic formation and the types of instabilities which are likely to occur and affect development in this area. Geormophologically, the Sahel is a succession of hills formed of Plaisancian Marls with gradients varying from 5 to 30% and a general slope to the south towards the Mitidja plain. The topography is dominated by post-Astian tectonics, which caused the uplift of the Atlas Mountains and formed the anticline of the Sahel and the depression of the Mitidja. The geomorphological setting of the Marls is therefore a dip-slope where the dip direction is relatively consistent with the direction of the slope. Algiers is characterised by a Mediterranean climate with warm, often wet winters and long dry summers when there is a very marked water deficit. The main Marl unit is more than 200 m thick and generally forms a homogeneous massive stratum. This is overlain by weathered/fissured material. The thickness of this upper horizon varies according to the angle of slope and the topographic location. The geotechnical study has indicated the Plaisancian Marl is a fine grained soil with very high plasticity and low permeability. These properties are consistent with the mineralogy of the sediment. A significant amount of montmorillonite was recorded in the X-ray diffraction analysis with an increase in montmorillonite in the near surface horizon. Interstratified illite-montmorillonite is also present, indicating the likelihood of depotassification. These very fine sediments containing varying proportions of smectite have a low shear strength and high shrink/swell capacity. As a consequence, the instabilities are frequently associated with volumetric change due to swelling and desiccation. In addition, cracks which have developed in the ground naturally over hundreds and thousands of years may be exacerbated by engineering construction due to changes in stress in the near surface horizons. The paper draws attention to many of these problems and it is hoped will assist planners to take due cognisance of the impact of the lithology, morophology and climate on the viability of any proposed works.