Les sols sableux peuvent constituer une réserve potentielle de sols cultivables mais leur mise en valeur entraîne toujours des dégradations de la structure et une baisse du potentiel agricole. Une dégradation très fréquente est l’affaissement des couches labourées sous l’action de la pluie, ou slumping. Notre objectif était d’identifier les mécanismes et les facteurs déterminants du slumping afin de proposer des techniques de prévention. L’étude a été conduite sous pluies naturelles (au champ) et simulées (au laboratoire). Au champ, la cinétique d’évolution des caractéristiques hydriques et physiques du sol a été suivie après un labour à 20 ou à 40 cm de profondeur, dans un sol sableux tropical Thaïlande. Au laboratoire, un sol cultivé modèle a été élaboré dans un cylindre (h et Ø = 20 cm) à partir i) du sol prélevé dans le champ expérimental, ii) de la fraction sable (> 50 μm) extraite de ce même sol. Les sols modèles ont subi des pluies d’intensité variable (de 20 à 120 mm/h) après mise en place sur une table à succion (20 à 90 hPa). Le slumping se produit lorsque deux conditions sont réunies: (1) l’eau doit atteindre un potentiel proche de 0 hPa ce qui réduit la cohésion et aboutit à un équilibre fragile du sol comme décrit par la physique des milieux granulaires; (2) ce potentiel doit être maintenu assez longtemps pour qu’une instabilité se produise et entraine une brusque et importante diminution de la cohésion, donc l’effondrement du matériau. Si la fraction fine (<50 μm) est retirée du sol, aucun effondrement n’est observé, démontrant l’importance de cette fraction limonoargileuse, pourtant minoritaire (<10-15 % en masse), dans l’instabilisation et le slumping. La prévention du slumping et la gestion des sols sableux sont discutées en guise de conclusion. / The sandy soil can serve as a potential reserve for cultivation. However, they are often considered as marginal because prone to have low productivity and problems of structure degradation. One of the degradations is compaction caused by rainfall or irrigation, which is called slumping. Our objective is to identify the processes and factors that affect slumping. The study had been done under natural conditions (field) and simulated conditions (laboratory) respectively. In the field, a tropical sandy soil in Thailand was selected. After 20 cm and 40 cm depth tillage, the dynamic of hydraulic and physical characteristics were measured. In laboratory, a cultivated soil model was build in a cylinder (height and diameter was 20 cm). We used two materials: 1) the original soil from the experiment field site, and 2) the sand fraction (> 50 μm) extracted from this soil. These model soils were submitted to rainfalls with different intensity (20 to 120 mm h-1), combined with different suction (20 to 90 hPa) on a suction table which can support the cylinder. The slumping for soil happens when two conditions were met: 1) the water potential reached a threshold value near 0 hPa. In this value, the soil cohesion decreased and a fragile equilibrium conditions were reached according to granular material theory; 2) the potential value was kept long enough and the cohesion continue decrease until they can no more support the equilibrium, so slumping happens. Surprisingly, in our treatment, when the fine particles (<50 μm) were taken out from the soil, no slumping was observed. This demonstrates the particles of clay and silt are very important to soil stability, even their content were very low as less than 10 percent. The possible ways to prevent slumping and management of sandy soils are also discussed.

No description available.

Le remaniement sédimentaire sous forme de glissements synsédimentaires est un témoin de l'évolution géodynamique à long terme d'un bassin sédimentaire. Cette étude, réalisée dans le Bassin vocontien (SE France), dans le contexte paléogéographique d'une vallée sous-marine, a cherché à mettre en évidence le rôle des propriétés initiales du sédiment dans le phénomène de glissement. L'essentiel des travaux réalisés concerne les alternances marno-calcaires et leurs remaniements (slumps) pendant l'Hauterivien. La biostratigraphie des slumps a été réalisée grâce à l'abondance des ammonites. Pour caractériser la lithologie originelle de la tranche affectée par le glissement, une coupe composite représentative a été construite à partir de différentes coupes exemptes de slumps. La corrélation entre cette coupe composite et la coupe de Serres-Montclus, suivie d'une interprétation en temps, a permis de distinguer d'une part deux types du matériel glissé (alternance très marneuse et alternance riche en calcaire), et d'autre part de suggérer que les changements climatiques provoquent une condition (indirecte) favorable à la génération des glissements (cycles de Milankovitch). L'hétérogénéité de la masse glissée et des structures internes caractéristiques dans les slumps ont mis en évidence un mécanisme de désintégration (incomplète) par cisaillement dans les intervalles marneux. Une configuration initiale de la tranche affectée a été restaurée par une décompaction sommaire. Les éléments de base pour une modélisation géomécanique du comportement d'un massif hétérogène à alternance marno-calcaire, en cours de lithification sur la pente, ont été mis en place.

Le travail qui m'a été confié comportait deux principaux axes de recherche. Il s'agissait tout d'abord de mettre au point des méthodes d'analyse quantitatives, pour étudier les processus de mise en piace des sédiments. Ensuite, d'utiliser toutes les informations enregistrées dans le sédiment pour reconstituer la paléogéographie du bassin . Le terrain servant de support à cette étude est localisé dans les chaînes subalpines méridionales, entre Saillans et la Tinée, et de Gap à Castellane . La période étudiée comporte deux étages, le Valanginien et l'Hauterivien Situés entre les deux pôles calcaires symétriques du Tithonique et du Barrémien, ils sont le domaine des alternances marno-calcaires, le pôle marneux correspondant au Valanginien moyen . Les alternances sont interrompues par des faisceaux contournés dont l'abondance et la diversité justifiera une étude particulière. Les sédiments détritiques (calcarénite, brèche) sont relativement rares. Ils seront ici essentiellement considérés par le biais de leurs relations avec le phénomène de l'alternance et du glissement synsédimentaire . Leur importance paléogéographique, non négligeable, sera soulignée.

On the lower continental slope off Vancouver Island near scientific ocean drilling IODP Site U1326, traveltime modeling along several ocean bottom seismograph (OBS) profiles shows anomalous high velocities of about 2.0 km/s at 70 - 100 m depth (compared to a no-hydrate reference of about 1.6 km/s). These velocities are consistent with the Site U1326 downhole sonic logs that show velocities up to 2.8 km/s near these depths. The drillhole high velocities are interpreted as caused by nearly massive hydrate with concentrations as large as 60-80% of the pore space. The OBS seismic velocities show that high hydrate concentrations of at least 20-30% are laterally extensive out to distances of at least 6 km on either side of the drillhole. A grid of migrated single-channel data shows a sequence of 15- to 75-m-high seafloor scarps, cutting across the ridge perpendicular to the deformation front. These are interpreted as normal faults. Two of the largest fault scarps bound a prominent ~2.5-km-wide slump feature on the steep seaward slope of the frontal ridge. This provides strong evidence that the slump is fault-controlled, and the base of the slump is near the base of hydrate stability suggesting that the slumping is also related to the presence of gas hydrate. At IODP drill Site U1327 on the mid-continental slope, seismic data were recorded along a 1-km-long profile of 10 OBSs. Traveltimes from wide-angle and vertical-incidence arrivals were inverted simultaneously for velocity structure. Corresponding hydrate concentrations increase with depth with an average of about 15% in the 100-m-thick layer above the base of hydrate stability . The seismic structure shows that this local hydrate distribution extends on the kilometer-scale away from the drillhole, as also suggested by multichannel interval velocities in the region. At Site U1328 (Bullseye Vent), seismic images derived from the very high resolution deep-towed DTAGS reflection data show that the top of a zone of high reflectivity, 10-25 m in thickness, extends from the seafloor to a depth of ~30 m. This zone likely corresponds to the shallow region of massive methane hydrate detected in the upper 40 m in the drillhole, and may represent a system of fractures through which fluids and gas pass from the main vent to the seafloor.

No description available.

In the southern Appalachians, the dominant soil order, Ultisols, is highly susceptible to erosion. If left unmanaged these soils can develop into gully systems resulting in land degradation. This study examines gully development through sidewall mass-wasting events at a high temporal resolution using 30 minute time-lapse photography. Prior research at this site found significant mass wasting events occurring between weekly monitoring periods. By shortening the interval of observation to 30-minutes, a more accurate understanding of the frequency and intensity of these mass-wasting events, and their relation to meteorological factors, can be determined. Photographs of a gully (approximately 1.5 m deep by 3 m wide at the top) were captured every 30 minutes from 11/29/17 - 2/18/2018 with a WingScape outdoor time-lapse camera mounted on a plastic stake 3.16 m from the gully facing northwest and upstream into the gully channel. A total of n=1648 images were coded using presence/absence indices for six observed geomorphic processes: creep on NE facing sidewall, creep on SW facing sidewall, slump on NE facing sidewall, slump on SW facing sidewall, channel aggradation, and channel development. Precipitation and temperature data were collected every 5 minutes using a Davis Vantage Pro 2 weather station located 240 m from the gully, and were aggregated to various time intervals. Precipitation received in previous 0.5, 1, 1.5, 2, 3, 6, 12, 24, 36, 48, and 72 hours were calculated for each image. Two binary temperature variables were generated with values of “1” if temperature dropped below 0 °C (32 °F) during the prior 30 minutes or 24 hours, respectively, and “0” otherwise. Logistic regression models (forward conditional method) for the six geomorphic index variables were generated using the precipitation and temperature data. For creep on the NE facing sidewall, the significant independent variables are 3 hour and 72 hour prior rain, and freeze conditions in the previous 0.5 and 24 hours. On the SW facing sidewall, rain and temperature variables were also important for creep; rain in the previous 12 and 24 hours, and freeze conditions within the previous 24 hours were retained in the model. For slumping on both the NE and SW facing sidewall, recent and prolonged rain were important. Specifically, 1, 6, 12, and 24 hour rainfall were retained in both models, with the addition of 3 hour rainfall in the NE facing sidewall slump model. No temperature variables were retained. For channel aggradation (deposition of material in the channel), rain in the prior 12 and 72 hours, and freezing in the prior 24 hours were important, suggesting that freeze-thaw processes loosen the soil, and subsequent rain events carry material into the channel. When rain stops, the material is then deposited in the channel. Interestingly, no viable model could be developed for channel development (erosion) using these parameters. These results will be useful to quantify meteorological controls on gully erosion at short temporal scales.