Essais d’altération chimique accélérée au laboratoire par le double extracteur Soxhlet. Application à des roches granitiques de Corée du Sud

A double Soxhlet extractor was used in the laboratory to perform accelerated chemical weathering tests in order to simulate the leaching of granitic rocks under tropical climate. Several types of granitic rocks corresponding to various initial weathering grades were sampled on two test sites in South Korea. They were subjected to leaching with hot and pure water in the double soxhlet extractor in order to measure the evolution of some of their physical and mechanical properties. Tests showed a decrease of the physical characteristics, more particularly the P-wave velocity and quick water absorption ratio. This decrease mainly occurred during the first leaching cycles. Uniaxial compressive strength also decreased during the leaching test, and the fresher the granitic rock, the most important the UCS decrease. This kind of test allows the evaluation of the chemical weathering effects and also the estimation of the long-term mechanical properties of the rock.     

Gestion des risques et des crises liés à l'instabilité des carrières souterraines abandonnées

Instability of underground excavations and karsts generally leads to violent events that affect zones which are difficult to locate precisely. For this reason, monitoring and alarm systems are not very efficient and risk management must therefore be essentially focused on prevention. In France, the aim of such prevention is to find a balance. Although the French legal and regulatory framework is rational, it is deficient in certain respects, and particularly faces difficulties concerning implementation as revealed by the development of the Plans of Exposure to Risks (PER) having taken too long. The comments made during the symposium held at Nainville-les-Roches have lead to the proposal of a number of improvements for the present system:

-promote a global approach which integrates risk management into land-use planning, ensuring coherence between national, regional, departmental and local plans,

-develop an operational approach for each “basin” at risk,

-speed up hazard mapping by its separation from regulatory procedures,

-develop plans for crisis management at a municipal level; bring experts, technical services and emergency services closer together and involve populations at risk in the preparation process,

-launch an operational information policy on the risks and develop a true culture of risk prevention,

-set up a local public service responsible for risk management of underground excavations within areas of greatest risk.

     

Évaluation et cartographie des risques géotechniques de dégradation des ouvrages d'assainissement une aide à la gestion des eéseaux

A sewage collector can be considered as a composite structure, comprising the collector itself and its geological envelope both sticking together. Its durability is directly linked with the geotechnical behaviour of the soil, as testified by the analysis of numerous pathological cases observed in structures which it is possible to inspect in the Paris urban area. The author proposes a methodology enabling these parameters of influence to be taken into account without recourses to costly procedures of investigation. Based on the principles of mapping risks of soil movements, the method involves two stages:

-The geotechnical classification of soils in terms of their vulnerability to stresses to which they may be subjected.

-The establishments of maps or profiles predictive of risks. in accordance with qualitative criteria derived from the interpretation of existing geological information.

Tested and applied to the inspectable sewage system of the Val-de-Marne and Seine-Saint-Denis Departments, this procedure can be extended to urban systems where sufficient geotechnical documentation is available to obviate the need for any in situ investigation.     

Importance du facteur lithologique sur le développement des ravins du bassin versant d'Oued Maiez en Tunisie Centrale

L'érosion ravinante réduit de plus en plus les terres cultivables en Tunisie Centrale en les transformant en badland. Le développement des ravins est variable dans l'espace et dans le temps suite à la variabilité des paramètres physiques et naturels qui régissent l'accroissement des ravins. Nous nous sommes proposés dans cette recherche d'étudier l'effet des formations lithologiques sur le développement des ravins dans le bassin versant de 1500 ha d'Oued Maiez situé en Tunisie Centrale. Les cartes du réseau hydrographique de 1963 et 1989 ont étéétablies à partir des photographies aériennes au 1/12,500 non redressées. La prospection du terrain nous a permis de constater que la nature lithologique des différentes unités géologiques et superficielles identifiées comme relativement homogènes dans le bassin versant est un facteur déterminant dans le développement des ravins. Ainsi, les argiles gypseuses de la formation Chérichéra (Lutétien-Priabonien) présentent un chevelu de ravins dense alors que les sables fins de l'Oligocène inférieur sont incisés par des ravins en forme de gorges étroites dont la profondeur dépasse souvent les 5 m (Fig. 7). Nous avons alors établi un levé cartographique détaillé au 1/12,500 de ces différentes unités qui sont au nombre de 8. La détermination de la densité du ravinement, définie comme étant le rapport de la longueur des ravins dans une unité lithologique sur sa superficie, a été faite par la numérisation des cartes du réseau hydrographique et géologique et l'extraction à l'aide du système d'information géographique (SIG) Arc/info des ravins inclus dans chacune de ces unités (Figs. 9, 10). Il ressort de cette quantification que, dans les unités dont la superficie varie entre 33.96 ha et 416.86 ha, la densité du ravinement varie entre 101 m/ha et 285 m/ha en 1989 (Tableau 2). La plus faible densité se trouve dans la formation Chérichéra datée de l'Oligocène inférieur à moyen et constituée par la superposition de plusieurs séquences élémentaires d'argile feuilletée verte, de silt et de grès roux coquillier à ciment calcaire. La plus forte densité se trouve dans l'unité 2 datée du Lutétien supérieur- Priabonien et composée d'argiles verdatres à violacées riches en gypses lamellaires.     

Etude des caractéristiques hydrodynamiques des sols par la méthode TDR (time domain reflectometry)

The object of this laboratory study was to determine any relationship between the soil volumetric water content θ, the dielectric constant (permissivity –ε), and the impedance of the same soil (R₁). Time domain reflectometry (TDR) allows the measurement of material dielectric constant and impedance by determining electromagnetic pulse transit time and pulse attenuation. Using the established relations of the TDR method, it is possible to trace the volumetric water content and electric conductivity (or the impedance) of the soil simultaneously in a continuous manner. It was hoped that, thereafter, it may be possible to calculate the soil suction using soil permissivity and impedance. TDR measurements were carried out for different leachates and soils. Relationships found in previous work by the authors were used to compare water contents given by the TDR method with water contents obtained by drying the soil samples. The test results showed a relationship between water content and soil type and the electrical conductivity of the liquid when the TDR parameter was soil impedance. It was concluded that permissivity could be used to characterise the soil's water content. This relationship is used to study the changes in bentonite water content during air drying. Volumetric water content and impedance vs time curves allows the determination of shrinkage limit.     

Analyse numérique et expérimentale du comportement des ouvrages fondés sur un sol mou renforcé par des colonnes ballastées flottantes

This paper work is in the frame of the study about the reduction of settlements in the case of floating columns. The prediction of a settlement remains a prickly problem to which geotechnical engineer is confronted. Experience has shown that structural pathologies are caused by the effects of excessive differential settlements before reaching soil bearing capacity. To palliate this problems in the case of soft and compressible soils, we advocate, as an alternative solution, piles or the reinforcement of the soils with stone columns. This strengthening method, introduced in Algeria in 1999, is economical. However, it does not inihilate settlements but it speeds up the time factor. Numerical modelings by finite element method (FEM) have been carried out through interest into all mechanical and geometrical aspects that are inherent in the meshing of floating stone columns. An analytical formulation of a correction coefficent (μβ) and an abacus of the settlement reduction factor (β) has been suggested. On the other way, the monitoring instrumentation of experimental site since 2008 to 2017 at the level of the harbour lowland of Bejaia has been carried out. It concerns important storage structures fonded on a very compressible ground reinforced by floating stone columns. The records of the settlements at the scale of the structures, according to variable loads recorded in real time, have revealed excessive settlements under the effects of variable stresses from 1 to 3 bars. An exhaustive geological, geotechnical and geomechanical analysis has been carried out for the characterization of the grounds, and numerical modelling by finite element method (FEM) has been carried out. The experimental and numerical investigation of the concerned structures at issue allowed us to confront the results with regard to settlement prediction of an environment reinforced with floating stone columns. The result is a perfect matching in relation to the suggested formula.

     

Caractéristiques des masses et marnes du gypse et de leurs faciès d'altération dans la région parisienne méthodologie des études géotechniques

The author presents the principal results of a research project concerning a geological formation widely represented in the natural region of Parisis, North of Paris, which is dreaded by geotechnicians: masses and interstratified marls of ludian gypsum. The impossibility of making ground observations essential to a thorough study of the phenomena of weathering of the gypsum, and notably its substitution, led to the work being undertaken along the following lines:

-Modelling of the geological infractructure of the gypseous slopes.

-The quest for indicators of the lithology and distribution of the residual masses and interstratified marls of gypsum, and the valuable role of diagraphs of natural radioactivity.

-The incidence of the weathering of the gypsum on the mechanical (pressiometric) characteristics of the gypseous areas.

-A methodology of geotechnical surveys on gypseous sites.

A progressive approach is proposed, beginning with a feasibility study, followed by a detailed study of the evolutive nature of the site, essential to the establishment of a suitably adapted project, and concluding with a mechanical characterisation necessary for the structural design of foundations.     

Impacts de recouvrements de résidus organiques sur des résidus miniers réactifs

Résumé Les résidus miniers constituent aujourd'hui un problème environmental important. En particulier, les résidus miniers à l'origine du drainage minier acide (DMA) sont les résidus provenant de l'exploitation de minerais tels que l'or, le cuivre, le zinc, le plomb, le nickel, ainsi que le charbon, qui contiennent des minéraux sulfurés généralement associés en formation primaire. L'action intensive et prolongée de l'oxygène atmosphérique et de l'eau de pluie sur les minéraux sulfurés résiduaires contenus dans la zone non saturée des haldes minières conduit à une production d'acide sulfurique. L'une des solutions envisagée pour empêcher la formation de la DMA consiste en un recouvrement des résidus miniers par une couche de matériaux afin de les isoler de l'oxygène atmosphérique. Parmi les matériaux pouvant être utilisés, on trouve ceux composés par des résidus organiques qui peuvent eux-mêmes être composés de différents types. Plusieurs études ont déjà mis en évidence l'efficacité de ces matériaux organiques comme barrière à l'oxygène. Un aspect important qu'il reste à étudier a trait aux interactions entre la couche organique (et ses produits de dégradation) avec les résidus oxydés et non oxydés. Le but de cet article est de présenter les mécanismes théoriques de formation de la DMA et l'utilisation de couches de résidus organiques comme moyen d'y remédier. L'interaction entre les couches de résidus miniers et les couches de résidus organiques est aussi discutée compte tenu de l'importance de ce sujet dans les domaines miniers, civils et environnementaux.

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Diversité des Fougères et leurs alliées le long du gradient altitudinal au sein de l’écosystème forestier des montagnes du Parc National de Kahuzi-Biega (RD CONGO)

Cette étude se focalise sur la vérification de l’hypothèse que l’altitude influence l’occurrence de la flore des Fougères et leurs alliées au sein de l’écosystème forestier des montagnes du Parc National de Kahuzi-Biega. Un échantillonnage de la végétation y a été effectué en considérant 24 parcelles localisées par paires le long de 12 transects installés entre 1278?m et 3123?m d’altitude. Les données obtenues ont été traitées par des méthodes statistiques, principalement la régression et des analyses multivariées. Au total, 157 espèces ont été recensées. Les résultats montrent que la richesse spécifique décroît linéairement avec l’augmentation de l’altitude à partir de 2600?m. Cette tendance prévaut à la fois dans les sites perturbés (R ²?=?0.58; P?R ²?=?0.81; P? This study concerns the hypothesis that altitude influences the occurrence of ferns and their allies in the forest ecosystem of the mountains in Kahuzi-Biega National Park. To obtain an inventory of the ferns and their allies, a sampling of the vegetation was made in 24 squares located in pairs along 12 transects lying between 1278 metres and 3123 metres altitude. The data were statistically analysed, mainly with regression methods and multivariate analysis. A total of 157 species were recorded. Species richness decreases in a linear way with higher altitude from 2600 metres. This tendency is observed in disturbed (R ²?=?0.58; P?R ²?=?0.81; P?相似文献   

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.     

Construction d'habitations dans des zones exposées au vent

Cet article traite du choix de I'emplacement, de la conception et de la construction d'habitations dans trois pays en voie de développement et qui sont, parfois, soumis à des vents violents. II est extrait d'un rapport établi il y a un an environ pour le Service Fédéral de Technologie de la Construction du National Bureau of Standards des Etats-Unis.     

Impacts des aménagements en montagne sur les processus hydrologiques et l’évolution géodynamique des versants (Les Arcs,Savoie, France)

This work focuses on the interaction between human activity in mountainous areas and slope movement hazards. The paper considers the triggering factors and processes of instability in the urbanized area of Les Arcs, an area exploited for mass ski runs and where many debris flows have occurred over the last 40 years. A progressive survey of the geological, geomorphological, hydrological, hydrogeological and climatic contexts is necessary to tackle the complexity of the phenomena. The human impact is considered as well. From the analysis, the geology associated with a specific climate was determined to be the main triggering factor for debris flow hazards. The influence of the glacial and postglacial period on the geomorphological aspects (e.g. steep slopes) and on the mechanical component of the superficial formations is the main factor. Furthermore, an analysis of regional meteorological data over the last 5 decades has emphasized a significant evolution of the climate since the 1960s which resulted in a general increase in temperature, rainfall (with seasonal variations) and the occurrence of extreme weather events, with long term implications for erosion and on water balance. This seems to be exacerbated by recent human activities which have significantly changed the natural environment: deforestation, changes in vegetation, the development of impervious surfaces such as roofs, roads and car parks, the creation of ski runs and the intensive exploitation of the catchment, all of which may have an influence on runoff and erosion affecting the natural surface structure. These changes and their effects on such factors as runoff and erosion have been reviewed and analyzed. In particular, the effects on hydrology have been highlighted by a hydrological modeling created using PCRaster Environmental Software. The hydrology of the Ravoire torrent before the creation of the ski resort was compared with the present configuration of the catchment. The annual water balance model shows that the ski resort generates a significant increase in the river flow compared to that before the change in land use. The parametric analyses undertaken revealed that the deforestation, the development of ski slopes and impervious areas are responsible for the reduction in infiltration inducing runoff. The maximum increase in water flow occurs during periods of natural high water flow (during snow melt) when the risk of debris flows is naturally high. For the last 20 years, this phenomenon has been amplified by the increasing production of artificial snow on the ski runs. Finally, a complementary model was created to consider the effects over a short time scale. The results of this rain event model emphasize the change in the hydrological response of the rivers after rainfall as a consequence of the change in land use. In particular this indicated a rise in the peak flow towards the resort, which may activate erosive processes above the debris flow source areas.      

Modification de la marée par le creusement du chenal d'un estuaire

The present study compares the tide measurements on the left and right banks of the mouth of the river Loire (France) which flows into the Atlantic Ocean and two others points on the French Brittany coast (Fig.?1, 2), for the last twentyfive years. In order to simplify the location of the sites, the points have been numbered from the north to the south as follows: ??Point 1?:?Le Conquet ??Point 2: Port Tudy, île de Groix ??Point 3: St.-Nazaire ??Point 4: Pointe de St.-Gildas The observed tide levels in the estuary of the river Loire show important changes since 1903 (Fig.?3). These modifications can be linked to the dredging works in the navigation channel between Paimboeuf and Nantes (55?km upstream) where the bottom level has been excavated from –2.50?m to –3.00?m in 1929 and to –5.50?m in 1969. Whereas the chart datum (hydrographic zero reading), which in France corresponds to the lowest possible seas for safe navigation, had been set at the end of the last century at 3.03?m under the terrestrial chart datum, on January 1st 1996, the SHOM, Naval Hydrographic and Oceanographic Service, indicated that on all the nautical documents it publishes this chart datum was now being set at 3.43?m under the terrestrial chart datum for the right bank of the mouth of the estuary (corresponding to the northern side of the estuary) while the one for the left bank, although only 15?km away, remained unchanged. The statistical study of surges for a period of twenty five years (1965–90) on both sides of the external estuary shows that the surges are significantly greater on the north-eastern side. It also highlights the incoherence of certain results. The study of level readings on 3 500 high and low tides effected by 4 tide gauges spread out along 300?km of coastline highlighted that a single badly zeroed tide gauge could lead to false results with regards to the average tide level at point?3. The work shows that this discrepancy is neither due to the blocking of fresh river water nor to a set of compartment faults but to the displacement and dredging of the channel along the right bank of the mouth of the estuary. The storm surges have also been studied between 1985 and 1990 using a dynamic model where surge levels depend on the duration of the meteorological disturbances. In this model the surge associated with a stationary meteorology S_∞ depends on the wind (velocity and direction) and the atmospheric pressure. The theoretical law for the surge due to atmospheric pressure is valid in the 4 points studied; for each hectoPascal of lowering of barometric pressure, the sea level increase is 1?cm. For a wind of constant velocity and direction, the model shows that the surges are greater at points 3 and 4 than at points 1 and 2. This fact can be explained by the local morphology of the external part of the estuary; it also gives an explanation for the coastal erosion of this area. This model confirms that the reduction of the extreme low water level observed at point 3 is a local disturbance. In fact, the increase of the mean sea level is approximately 0.10?m per century at points 1 and 2 as in Brest harbour where level measurements have been undertaken since 1808. In contrast, this level appears to be stable at the two points situated on the estuary while the amplitude of the tide increases. This phenomenon is better marked at point 3 in the north of the estuary. Finally, this study shows that the installation of a mobile dam in the Loire estuary (similar to the one existing in the Thames estuary) would permit regulation of the upper water levels above the dam although the amplitude of the tide could be enhanced downstream of the dam.     

Déformations gravitaires post glaciaires en terrains métamorphiques. Comparaison des indices de déformation du versant sud de la Toura (Saint-Christophe-en-Oisans, France) avec le phénomène de rupture interne du versant sud du Mont Sec (Séchilienne, France)

Introduction: The Sechilienne slope movement, near Grenoble, causes a majour risk and is intensively monitored since 1985. An original deformation mechanism has been proposed to explain the data. Some morphologicial similarities have been observed upstream on air photographs of the south slope of la Toura, naer Saint-Christophe-en-Oisans (France). The question is: is the same phenomenon occuring there too? The site of Saint-Chistophe-en-Oisans (France): The site is located in the Vénéon valley, 13?km upstream from Bourg d'Oisans (Fig.?1), under the Pointe de la Toura peak. There is no permanent stream flowing down the slope. The slope is constituted of gneiss of different formations separeted by North-South faults (Fig.?2). The foot of the slope is covered by alluvial deposits. On the slope itself, there are some recent moraines. The average slope angle is 36° (Fig.?6), which is around 10° lower than the mean value of the valley. The slope is much steeper near the crest, which could correspond to the summit scarp of a slope movement. The middle part of the slope is marked by huge transversal cracks with up-facing scarps, alternating with vertical cliffs. The screes are stopped by the up-facing scarps and underline the cracks on air photographs (Fig.?4). The geological structure is dominated by?(Fig.?5): –?north-south fractures corresponding to the main faults and to the foliation; –?hectometric cracks dipping of 65° in direction NE (N60). They show up-facing scarps and determine tilted-looking rock domains, as seen on the side thanks to the cut made by the "Torrent du Diable" stream; –?two less persistent discontinuity families, one dipping 65° toward SE (N 245) and the other one vertical, in direction N 80. The opening of the transversal cracks, their upfacing scarps and their curvature in the central part of the slope suggests a tilting of the slices. This phenomenon could date from the end or from low-level periods of the Würm glacial age (80000 to 10000?yr B.P.) The manual monitoring of five cracks shows that the phenomenon is still slowly active: mean opening 2?mm in two years, maximum 5?mm (Fig.?7 and 8). Tectonic hypothesis (Fig.?9) may explain the genesis of the atypical transversal cracks and will be tested by geochronological studies (fission traces on apatites) and geodetic measurements. The "Internal Rupture" deformation mechanism: The movement of the south slope of Mont-Sec (Séchilienne, France), constituted by micaschists causes a major risk near Grenoble (rockslide, damming of the Romanche valley, floods...), and is monitored since 1985. The main observations are: a settlement near the crest with opening of local cracks, no localised disorders on foot of the slope, the opening of transversal continuous cracks (with up-facing scarps), and the low inclination of the displacement vectors, whose direction is normal to the continuous cracks. A 240?m long investigation gallery revealed the rotation of rigid blocks (up to 80?m large) with opening or shearing of the contact planes (Fig.?10). These data do not fit with classical translationnal mechanisms and indicate a strong structural control. The mean direction of the rock anisotropy is not favourable neither to a generalised toppling mechanism. Data have been modelled with two main continuous joint families issued from the simplified geological model (one is vertical and the other steeply dipping toward the valley). The computation with the distinct elements method (code UDEC) reveals two distinct zones (Fig.?11): the upper zone is settling and horizontally expanding by shearing of the two joints families while the lower zone is retaining it with a "block toppling like" flexion deformation. The simulation reveals a very deep zone of yielding joints (extending down to the valley level), stress accumulations at the foot of each "cliff" of the final topography and important internal stresses inside the blocks in the flexion zone. Combined with the fluctuation of hydraulic conditions, this subcritical state of stress can, at the geological scale of time, induce secondary fracture propagation. The progressive localisation of this time-defered non reversible damaging phenomenon can lead to global failure of the slope on a non predetermined rupture surface. Conclusion: The morphological and structural observation of the gneissic south slope of la Toura (Saint-Christophe-en-Oisans, France) has revealed so many similarities with the Séchilienne case that the proposed mechanism appears to fit here too. Pursuing the studies on this lowly active site where the observation conditions are excellent will help understanding and controlling more dangerous sites like Sechilienne. Moreover, this phenomenon seems to concern many metamorphic slopes of the Alps.