Traitements et analyse des mesures de diagraphies différées pour la reconnaissance géotechnique

Résumé Les diagraphies différées, essentiellement employées, à l'origine, dans le domaine pétrolier, sont de plus en plus utilisées en hydrogéologie pour la recherche d'aquifère et en reconnaissance géotechnique. En effet, les informations qu'elles apportent sur les caractéristiques physiques des sols et des roches, in situ, sont particulièrement riches. Cet apport est à deux échelles: l'échelle de l'unité lithologique et l'échelle de l'intercalation ou de la discontinuité dans cette unité. Toutefois, pour un apport d'information optimal, il convient de traiter les mesures diagraphiques. Nous montrons qu'une transformation de la structure des supports de mesure (volumes investis) des diagraphies différées permet d'obtenir, par rapport à une interprétation classique de ces mesures, une détermination plus précise des caractéristiques mesurées, un meilleur positionnement des limites de couches en profondeur et une meilleure estimation de la variabilité de la caractéristique mesurée. Nous montrons également que la définition verticale d'un outil de diagraphie peut être directement définie par le pas d'échantillonnage de la mesure le long du forage. La recherche d'un transfert d'information entre les mesures diagraphiques et les paramètres géotechniques de calculs permettrait aux diagraphies différées de devenir un complément indispensable aux forages destructifs dans toute reconnaissance géotechnique. Received: 11 May 1999 · Accepted: 27 November 1999     

Prévision de la résistance mécanique du ciment en 30 heures

Le développement considérable de I'Arabie saoudite a obligé les laboratoires d'essai des matériaux à faire de grands progrès en peu de temps. Dans cet article, les chercheurs du Laboratoire d'essai des matériaux de Riyad expliquent comment ils ont utilisé un procédé accéléré pour prévoir la résistance à la compression de différents ciments Portland après 30 heures. Les résultats obtenus concordent, en moyenne, à ± 5% près avec les résistances que donnent les méthodes traditionnelles de conservation dans I'eau.     

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 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.     

Eco‐biogéographie du genre Peperomia en Afrique: cas de la région Guinéo‐Congolaise

Le principal but de cette étude était d’établir une caractérisation écologique fondamentale du genre. Peperomia pellucida est une espèce terrestre parmi les représentants du genre africain. P. molleri est fortuitement terrestre dans le sous bois. La majorité des espèces sont des épiphytes et constituées essentiellement des herbacées. L’abondance des individus dans les régions basses et moyennes sur le phorophyte montre que la vie est étroitement liée à l’humidité. Les interactions biotiques des espèces épiphytes avec des mousses améliorent leur nutrition hydrique et minérale. L’absence des espèces dans les zones chaudes et leur disparition dans les écosystèmes dégradés approuvent le statut bio‐indicateur écologique du genre. En dehors de Peperomia pellucida rudérale et absente au‐delà de 500 m, les autres espèces sont submontagnardes et montagnardes. Elles sont abondantes entre 1000–2500 m d’altitude. Les espèces sont réparties du Sénégal en Tanzanie avec une abondance remarquable dans les régions de la basse Guinée et du bassin du Congo. Certaines espèces sont plurirégionales tandis que d’autres sont restreintes dans certaines phytochories.

Mots clés: Altitude; bassin du Congo; bio‐indicateur; habitat; phytochories et région Guinéo‐Congolaise

The goal was to establish an ecological characterisation of the genus. P. pellucida is a terrestrial species among the African genus of Peperomia. P. molleri is often found terrestrial in the understorey. Most of the species are epiphytes and especially herbaceous. The abundance of the individuals in the low and medium levels of phorophytes shows that life is closely related to humidity. The biotic interactions of the epiphytic species with the mosses have improved their mineral and water uptake. The absence of the members in the hot areas and degraded ecosystems gives reason for the genus to be a bio‐indicator. P. pellucida is the only ruderal species while the others are common in sub‐mountain and mountain forests. They are abundant between 1000–2500 m. The distribution extends from Senegal to Tanzania with a remarkable abundance in the low Guinean region and Congo basin. Some species are widely distributed; others are restricted to certain phytochoria.     

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.     

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.     

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...     

Exploitation de bambous (Sinarundinaria alpina (K. Schum.) C.S. Chao & Renvoize), cause des conflits entre le Parc National de Kahuzi-Biega et la population environnante: stratégie de conservation et de résolution de conflit

Sinarundinaria alpina is a species of mountain bamboo which is a source of various conflicts between the managers of the Kahuzi-Biega National Park (KBNP) and surrounding communities. This mountainous species is threatened by these communities to meet their need for various uses. Six permanent plots of 3 hectares were set between 2000–3000 metres above sea level and floristic inventories were made. In addition, information related to anthropogenic threats related to bamboo exploitation was recorded. All data were analysed using quantitative statistical parameters. Results suggest the existence of 196 species in these six permanent plots of the group Bamboo. The position of this Sinarundinaria alpina vegetation in the dynamic rainforest recovery dynamics is confirmed by the presence of trees and tree-nurseries of these trees. Fifty-three species, 27% of the plants observed, including Sinarundinaria alpina seedlings, contribute to the daily feeding of great apes, mainly Grauer’s Gorilla (Gorilla beringei graueri). This forest appears to be one of their critical habitats. The decrease in the area covered by bamboos is due primarily to anthropogenic activities but also some natural hazards (expansion of Sericostachys scandens, natural fading of Bamboo and attacks by beetles). Following these findings, conservation strategies were proposed. Several strengths were noted, including multiplication of bamboo nurseries and distribution of seedlings to local communities. This can be seen as a strong development of this sector in the future. This study is a preliminary assessment for further investigation.