Différentes déclinaisons de l'aléa liquéfaction: de la cartographie préventive au projet

As part of its Public Service, Applied Research, and Client Service activities, BRGM's Thematic Centre for Natural Geological Risks (NGR) has recently undertaken studies in order to characterize "liquefaction hazard". The paper considers definitions and presents several approaches to liquefaction hazard assessment, demonstrated by several examples. The first approach is a naturalistic one: Hazard assessment is considered in a structured manner by undertaking: – a regional inventory of historic events – mapping of formations susceptible to liquefaction in homogeneous risk areas subject to seimic activity – preventive mapping of the phenomena at local scale The second approach is a semi-deterministic one: It allows to produce a seismic microzonation, considering parametric calculations and charts (Seed's reverse method). This approach can be used also to recommend soil improvements when liquefaction hazard does exist. The third approach is a deterministic approach: It takes into account a 3D geotechnical model of the analysed area, as well as specific charts to produce a liquefaction hazard assessment of soils and an advanced seismic microzonation. This method was used in Guadeloupe and Martinique districts (French Lesser Antilles) ad for the design of a new TGV railway track in the south of France. The proposed methodology relies on the French Association for Earthquake Engineering recommendations and brings some innovative aspects: combined naturalistic and geotechnical analyses for liquefaction hazard assessment, combined chart use and 3D geotechnical modelling for liquefaction hazard microzonation. Apart from the complete analysis of the liquefaction hazard not always being useful, it also demands major resources, and is consequently sometimes broken down into several less-detailed stages. The various examples considered thus form a continuum and are simply variations of a same definition of the liquefaction hazard: where: X, Y and Z=coordinates t=time Δu=interstitial over-pressure σ'_vo=initial effective stress I_L=Iwasaki's liquefaction index     

Caractérisation minéralogique et géotechnique des argiles marneuses gonflantes de la région de Médéa,Algérie

This paper presents the approach followed for the geological, mineralogical, and geotechnical characterization of swelling marly clays in the Médéa region. This investigation is conducted in order to estimate the swelling potential of this marly clay layer. The studied sites, located at about 80 km south of Algiers, Algeria, cover an area of approximatively 400 hectares. Five sites are considered. In the first step, the geological, tectonic, climatic, and hydrological contexts of the region are described. According to the geological map of Médéa, most of the formations encountered in the area are composed of Miocene layers represented by marly clays (Fig. 2). This region is characterized by its high and low temperature in summer and winter, respectively, and variable humidity (Fig. 3). In a second step, the results of geotechnical studies, X-ray diffraction tests, chemical analyses, and scanning electron microscope (SEM) observations are presented (Figs. 3, 4, 5, 6, 7, 8, 9). The soils contain quartz (20–26 %), calcium carbonates (11–55 %), kaolinite (8–13 %), illite (6–14 %) and Montmorillonite (18–26 %). The study of their microstructure by means of SEM indicates that these soils are formed by a compact marly clay matrix that is relatively homogeneous and oriented in the dip direction of bedding. A grain size analysis shows that the clay content varies between 17 and 70 %. The water content of all samples varies between 8 and 30 %. The values of the liquidity limit (LL) and plasticity index (PI) vary between 28–76 % and 16–36 % respectively, indicating a highly plastic soil; this is also confirmed by a specific surface varying between 99 and 179 m²/g. The dry density γ _d varies between 15 and 19 kN/m³. The swelling potential of the marly clay samples is evaluated firstly using various indirect methods. In literature, a number of empirical classifications are proposed by different authors (BRE 1980; Chen 1988; Komornik and David 1969; Seed et al. 1962; Snethen 1984; Vijayvergiya et Ghazzaly 1973 et Williams and Donaldson 1980). The swelling potential is related to certain physical properties of soils, such as consistency limits, clay content, methylene blue value, etc. In general, these methods indicate that all the tested soils have a high swelling potential, which confirms the results of mineralogical analysis. Secondly, direct measurements of swelling parameters are performed. Swelling tests are carried out using a standard slaved one-dimensional odometer using two methods: free swell and constant volume, according to standard ASTM D 4546-90 and AFNOR (1995). The swell pressure, the swell percentage and the swell index are given in Fig. 16. It is noted that the soils develop very significant swell pressures which vary between 25 and 900 kPa. This is in agreement with the results obtained by empirical methods. This investigation clearly shows that the marly clays of the Médéa region have a high swelling potential. Therefore, taking into account the phenomenon of soil swelling in structure design is essential.     

Le Pont Vasco de Gama – la démarche durabilité

From the concrete example of a large construction, this paper aims to show how the results obtained from the research work carried out on transfer in cement pastes were applied both on site during the concrete construction and in the laboratory to predict the durability of the concrete structure. This unique process, carried out in close collaboration with the constructor, is based on a new concept of High Performance Concrete. The construction concerned is the Vasco da Gama Bridge in Lisbon, which is one of the most important bridges ever constructed in Europe: it is more than 17?km long, built in the maritime zone on the Tagus river estuary. It was opened in April 1998. A guarantee of life duration of 120?years, without major maintenance, has been demanded. In addition to the criteria relative to the concrete constituents (non-reactive aggregates with regard to the alkalireaction, the nature of the cement and a formulation adapted to the direct environmental effects on the different construction parts), one of the main life duration criteria selected is a chloride concentration limit value of 0.4%, compared to the mass of the cement, measured directly at the reinforcement steel position. This value determines the initiation threshold of the corrosion process of steel. Specifications are both a diffusivity coefficient of chloride of less than 1×10^–12?m²?s^–1 determined by the TANG Luping method and an oxygen permeability of less than 1×10^–17?m². These basic criteria have been perfected by measures of porosity to water, tests of resistance to chloride penetration, chloride migration tests and also by extensive examinations of the concrete microstructure. All the measurements and tests have been undertaken both on concrete samples made in the laboratory and on samples made on site and concrete cores from parts of the structure built in 1995. Coefficients of chloride diffusivity measured or calculated from profiles of concentration in concrete cores have been used in a predictive model, using a finite element method integrating the interaction ions with cement paste and the evolution of the diffusivity coefficient in time. The various computing simulations, improved progressively with controls and after comparison with the real bridge, tend to confirm, on the one hand, the validity of the model used and, on the other hand, the accuracy of the chosen formulations. These formulations, having mechanical properties close to those of High Performance Concrete, also have special microstructural properties of diffusivity and permeability, answering the requirements of the durability of this bridge.     

Apport d’une base de données d’éboulements rocheux obtenue par scanner laser dans la caractérisation des conditions de rupture et processus associés

Using laser scanner data, an exhaustive rockfall database has been established for a rockwall located near the town of Grenoble (France). The study site is a long double cliff, on the eastern border of the Chartreuse Massif. The two cliffs consist respectively of thinly bedded and massive limestone, which show different structures, morphologies and rockfall activities. The 3D point clouds obtained by laser scanner allow to detect and model the fallen compartments in 3D. Information about cliff surface, and localization, dimensions, failure mechanism for each compartment were obtained and analyzed in order to characterize the morphological evolutions of the cliffs. It appears that the morphology and the slope of the lower cliff is related to fracturing and torrential erosion which occurs in the marls below the cliff, showing a rockfall frequency 22 times higher than for the upper cliff. These results show that the erosion process in the lower cliff is in a transient state, whereas it could be in a steady state in the upper cliff. Rockfalls have been dated by a near-continuous photographic survey (1 photo each 10 mn) and a monthly survey during 2.5 years. The analysis of the two data bases shows that the rockfall frequency is 7 times higher during freeze-thaw episodes than without meteorological event, and 4.5 times higher during rainfall episodes. Moreover, it becomes 26 times higher when the mean rainfall intensity is higher than 5 mm/h. Based on these results, a 3-level hazard scale has been proposed for hazard prediction.

     

Les dépôts morainiques holocènes de la zone axiale pyrénéenne: approche déterministe de leur instabilitéà Verdun sur Ariège, (Pyrénées centrales, France)

This paper reports a study of the historic instability of the till deposits in an area of the central Pyrenees between France and Spain upslope of the town of Verdun. The object of the work was to create a model based on the geomorphological and geological mapping of landslides and a geotechnical survey of the instability of the whole of the mountainous slopes of the Domanial Verdun area. The detailed mapping allows the historic landslides to be distinguished from the more recent/active movements which involve volumes of 100,000 to 400,000?m³ of material. In addition, it was possible to assess the areal percentage of the actively slipping zones (5%) compared to the historic slide zones (12%) and the stable areas (78%). Following the European classification, two types of slides were identified in the Verdun study area: (1) historic rotational slides and (2) active translational slides located in the Verdun area at between 1000 and 1250?m altitude. The stability was calculated using the classic "method of slices", subdividing the slipped zone into vertical slices along a suitable cross section. The so-called factor of safety (F) was determined by dividing the moments of resisting forces (M_R) by the moments of driving forces (M_D). The state of limiting equilibrium has a "factor of safety" of 1. The physical parameters of the till deposits of Verdun were established as: unit weight γ=24.9?kN/m³ (calculated using the percentage of gneissic blocks contained in the tills) and porosity n=0.24. The results of 15 triaxial tests in a gravelly sand matrix were plotted in a Lambe diagram following a linear regression model [x=(σ₁+σ₃)/2 and y=(σ₁–σ₃)/2 with sin?φ′ =tan?θ]. From this the friction angle φ′=33°±3 and c′=45±5?kPa were established. The factor of safety calculated for the moraine deposits in the historic slides was 1.44 without water (Hw=0). With a height of water of 7?m, representing 85% of the till thickness, this was reduced to F=1. To achieve a factor of safety of 1.3, the maximum water level within the till should not exceed 2.5?m, representing 65% of the till thickness. Similarly, the factor of safety was calculated for the active slides of another area (shown as section 4 in Fig.?3 in the paper). Using slice number 9 from the middle of the slide, the factor of safety was 1.08 when the height of the water was taken as 90% of the till thickness. This high calculated factor of safety for the height of water is consistent with the slow movement of the actual slides. However, a lower internal cohesion of the till deposits or the presence of a weathered zone would decrease the factor of safety from 1 to 0.8. It is also possible that other parameters, such as the regional seismic activity, could have been sufficient to initiate movement (F<1) during the last 50?years. It is of note that the map of seismic activity shows that more than a 100 earthquakes with magnitudes greater than 3 have occurred in the central Pyrenees since 1660. The paper emphasises the importance of high-quality mapping which identifies and classifies areas of historic and recent instability. From this, a single geotechnical model to calculate the stability can be established. The level of the water is shown to be the critical parameter and of more significance than the variations in the effective friction angle, which itself is more important than variations in the effective cohesion. With this information it is possible to determine those areas where some form of stabilisation and/or drainage of the till deposits is necessary.     

Dégradation des eaux de rejets issues d’une raffinerie d’huile de coton par plasma d’arc électrique rampant à pression atmosphérique

Wastewater samples from liquid effluents of cotton oil refinery in Far-Northern Cameroon were exposed to a gliding arc discharge for various exposure times in a cold plasma reactor. The analysed results showed high levels of organic pollution parameters (TOC = 4635 mg/L, COD = 6943, 98 mg/L) before treatment. More than 56% TOC and 75% COD abatement were obtained after 30 min treatment in batch conditions with a laboratory reactor. The resultant pollution abatement is attributed both to strong oxidizing effects of °OH and NO° radicals formed in the plasma and their derivatives (H₂O₂, ONO₂H and NO₃H). The conductivity and TDS increase linearly and the pH, TOC and COD decrease with increasing exposure time to the discharge. The oxidation obeys a pseudo first-order kinetics law (kinetic rate: k_1DCO = 0, 0381 min^?1 and k_1COT = 0, 0245 min^?1).     

Instabilités gravitaires dans la région de Béjaïa (Algérie): Inventaire et appréciation de l’importance relative des différents paramètres conduisant au déclenchement,au maintien ou à l’activation des instabilités

Bulletin of Engineering Geology and the Environment - This article contributes to the analysis of the various factors that interact in triggering and reactivating of landslides in Béjaïa...     

In Praise of Transgression: The Work of Didier Faustino/Bureau de Mésarchitectures

Architect/artist Didier Faustino regards transgression as a means of survival for a service profession, such as architecture, which is effectively up for hire. By illustrating examples of his own work he argues that the lifeblood of practice must be transgression, challenging all established parameters in order to fulfill the brief and explore a disruptive and often disturbing in-between space.     

Impact de la décharge publique de Marrakech (Maroc) sur les ressources en eau

Résumé. Les lixiviats de la décharge publique de Marrakech, qui ne sont ni collectés ni traités, pourraient constituer une source potentielle de pollution des eaux de la nappe phréatique et de celles de l'oued Tensift situé près de la décharge. Le but de cette étude est de caractériser ces lixiviats et de déterminer le degré de pollution des eaux à proximité de la décharge. Les analyses physico-chimiques des lixiviats ont montré qu'ils présentent des concentrations élevées en Cl^–, HCO₃ ^–, Na⁺, K⁺, SO₄ ^2–, Ca²⁺, Mg²⁺, NH₄ ⁺, Cu et Pb. La pollution organique est également importante; la DCO peut atteindre 138.000 mg/l O₂. Les concentrations en certains polluants sont nettement supérieures à celles généralement rencontrées dans d'autres décharges d'ordures ménagères. Le suivi de la qualité des eaux durant l'année 1999, a montré que les eaux souterraines et celles de l'oued Tensift sont de mauvaise qualité pour les besoins domestiques et pour l'irrigation. L'Analyse en Composantes Principales a montré que la pollution des eaux, par les lixiviats, est essentiellement due à la matière organique, HCO₃ ^–, NH₄ ⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^– et SO₄ ^2–. Electronic Publication     

Définition d'une typologie des dépôts de laves torrentielles et identification de critères granulométriques et géotechniques concernant les zones sources

The paper discusses viscous and saturated flows which can propagate at high velocity in torrent channels, referred to here as debris flows. Specific morphological, textural and sedimentological criteria have been identified, on which to distinguish debris flows from other torrent deposits. From a study of numerous debris flows sampled in 11 torrential catchment environments in the French Alps, two types of debris flows have been determined: those with a granular matrix and those with a cohesive matrix. Their typology is based mainly on the fraction of soil smaller than 20?mm although several discriminating particle size criteria have also been considered on the fraction between 20 and 200?μm. The author has selected five criteria for study, that relative to the fractions <50?μm being the most convenient. Geotechnical tests have also been used as a complementary criterion for this typology. The values of the Atterberg limits and blue methylene tests are generally lower for the debris flows with a granular matrix (Fig.?8a and Table?5). In most situations, the plasticity index of debris flow deposits is low. Both the soils sampled in the source areas and those mobilised as debris flows generally have particle size distributions and geotechnical characteristics similar to those of debris flow deposits reported in the literature. The paper concludes that source materials with a granular matrix are more likely to result in debris flows (as defined here) while the source materials with a higher fine fraction are more likely to move as mud flows.     

Dédoublement du tunnel d’Aokas (BEJAIA): geologie et caractérisation géotechnique du massif traversé. Stabilité de l’ouvrage

Underground work’s constructions and design techniques depend closely on the geological environment. The properties of the rock mass influence directly on the various steps of their realization so that geological and geotechnical studies become an absolute necessity to characterize the rock mass and to control the imposed constraints. At the end, we can conclude that the present work aims to define the geological and geotechnical conditions of the drilled rock mass; it consists mainly in determining the nature and the structure of the rock mass to cross, in addition to determining the rock mass stability and the reinforcement works. The route of the tunnel Aokas (Béjaïa) crosses a fractured limestone, where the karstification phenomenon is well developed and the best example being “fairy caves” contiguous on the old tunnel road in the NR 09. The two tunnel heads are located at the foot of two towering cliffs. All these characteristics make the tunnel in precarious potential stability. The detailed geological study allowed to determinate that the tunnel crosses through a karstified dolomitic limestone of the Babor’s chain of the lower Jurassic age, and a geotechnical study allowed a characterization of this rock mass regarding fracturing, mechanical and physical characteristics and led finally to a geomechanical classification. The stability of the tunnel is ensured by a strong support system.     

Les formations plio-quaternaires de la région de Casablanca (Maroc): aspects sédimentologique et géotechnique

The Plio-Quaternary formations of the Casablanca area consist of a sequence of cross-bedded strata. They frequently begin with a lumachelle conglomerate occasionally associated with gravels and marine calcarenite. The marine deposits are overlain by yellow and grey dune sandstones, locally oxidized at depositional horizons. These dunal calcarenites have been extensively exploited and hence their sedimentological and geotechnical parameters have been determined from numerous samples taken from the quarries in the region. The sedimentological study results show that these generally homogeneous materials consist mainly of well-graded fine to coarse carbonate sands (98%) containing both bioclasts and calcareous cements. It is clear that at the time of deposition there was little detrital material, with the marine deposits formed mainly of shell debris. Study shows that the marine deposits underwent early lithification in a shallow marine environment which then continued in a continental environment by the movement of meteoric water. The marine deposits are well cemented. They have a low degree of dissolution but quite a high degree of recrystallization, notably towards the bottom of the individual horizons. The dunal deposits which underwent lithification in a continental environment are characterized by a very low cementation, mainly at point contact, a high dissolution and a weak recrystallization. The difference in diagenesis affects both the physical and mechanical features of these Plio-Quaternary calcarenites. From a geotechnical point of view, the consolidated marine and dunal deposits have a weak to medium compressive strength (502), a very weak resistance to shocks (453. 8.?Density and percentage CaCO₃. 9.?Compression resistance and percentage CaCO₃. In view of the graph relationships, it is possible to estimate the other parameters from the determination of a single physical or mechanical feature of the calcarenite. However, the sedimentological and petrographical analyses show that the following factors can be subject to quantitative variations: (1) the carbonate content; (2) the nature of the bioclastic elements; (3) the grain size of the detrital elements; (4) the cement percentage and its grain size; and (5) the importance of dissolution and the vacuum percentage. These factors control the physical and mechanical features of the Plio-Quaternary formations of the Casablanca area. As a consequence, it is suggested that when considering the sedimentological features and physical and mechanical characteristics of the Plio-Quaternary formations of the Casablanca area, the particular deposits must first be categorized into one of the four following facies: lumachelle; marine calcarenite; yellowish dunal calcarenite; or grey dunal calcarenite.     

Montréal: Towards a post-industrial reinvention

Following the decline of Montréal's manufacturing sector, shipping, and its status as Canada's banking center, a growing portion of the city's economy is tied to the knowledge economy, particularly education, arts, and R&D. With four major universities, comparatively low costs of living, lucrative creative industries, and a vibrant arts scene, in recent years the city government has devised strategies to more formally leverage Montréal's assets in order to expand the knowledge economy. This article first sketches out the urban impacts of some of Montréal's major economic and social transitions, then turns to examine how the city is attempting to build on its assets as part of the official pivot towards fostering a knowledge economy. We examine policies and organizations intended to expand the knowledge economy, how they are impacting the city, and some key tensions and challenges facing the city as it tries to reinvent itself. While the city is experiencing a boom in the knowledge and creative economies, the benefits of this boom are unevenly distributed and they may be undermined by several factors, including gentrification.     

Diversité floristique des mares temporaires de la région d’Annaba (Nord-Est Algérien)

Nine wetlands in Annaba region (North East of Algeria) were the subject of floristic inventories during three years (2014–2016). We have revealed the presence of five types of temporary ponds in which Ranunculus aquatilis L is dominant. The studied flora is rich and diversified. It comprises 190 species and 143 genera belonging to 58 botanical families. These species were either strictly subservient to humid zones or transgressive from terrestrial environments. This flora is characterized by a big proportion of rare and / or threatened taxa (11.05%). The composition of global biological spectra shows the dominance of therophytes, with 99 taxa (52.11%). The majority of the listed species belong to the Mediterranean phytochronic species. Eighteen taxa belonged to the endemic category. The preservation of this exceptional biodiversity, threatened in the short term, urgently requires phytoecological studies and relevant protection measures.