Effects of soil structure on soil behaviour: Illustrated with loess, glacially loaded clay and simulated flaser bedding examples     

Gerard A.M. Kruse  Tom A. Dijkstra 《Engineering Geology》2007,91(1):34-45

Soil structure is considered to be a systematic homogeneously repetitive heterogeneity of properties and conditions of soil. For upscaling and homogenisation of properties and conditions it is necessary to understand the ways in which soil structures occur, and to obtain understanding of the ways in which it influences soil behaviour. Using three case studies, different aspects of the effects of soil structure in soil mechanics are discussed. The very loose grain scale packing of loess deposits, held together by weak structural bonds between particles, results in dramatic propagating failure when the strength of the bonds is exceeded by deformations, as illustrated in the first case. Nonhorizontal stresses induced in the subsurface below moving continental ice sheets result in rotated stress fields within the affected soil. The rotated stress field is stored in the structure of the soil, as is shown in the second case, and influences present day properties and behaviour of these clays. It is shown in the third case that there is a large difference in the behaviour of clayey sand depending on whether the clay occurs in the form of the common clay flasers or whether the clay is homogeneously dispersed in the sand. The deformation and stress propagation in the case of sand with clay flasers results in local failures and shear planes due to the large contrast in the properties of the materials making up the sedimentary structure. The result is that many of the properties of sand with flasers are rather more like the clay of the flasers than the sand. Incorporation in the engineering practice of effects of common types of soil structure is made possible with the widespread availability of techniques such as numerical modelling. These techniques enable the development of appropriate tools for practical application.  相似文献   

Specific sediment yield in Tigray-Northern Ethiopia: Assessment and semi-quantitative modelling   总被引：2，自引：0，他引：2  

Nigussie Haregeweyn  Jean Poesen  Jan Nyssen  Gert Verstraeten  Joris de Vente  Gerard Govers  Seppe Deckers  Jan Moeyersons 《Geomorphology》2005,69(1-4):315-331

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River channel response to short-term human-induced change in landscape connectivity in Andean ecosystems   总被引：4，自引：2，他引：4  

Veerle Vanacker  Armando Molina  Gerard Govers  Jean Poesen  Gerd Dercon  Seppe Deckers 《Geomorphology》2005,72(1-4):340-353

The drainage basin of the Deleg River (88 km²), located in the southern Ecuadorian Andes, was studied to assess the geomorphic and hydrologic response of a fluvial system to human-induced environmental change in its contributing area. Historical data on land use, channel morphology and sedimentology were collected, based on a spatial analysis of aerial photographs (1963–1995) and a field survey (2002). Analysis of channel cross-sectional profiles and sedimentological data revealed a major change in morphology and sedimentology of the Deleg River during the past four decades: (i) the active river channel narrowed by over 45%, (ii) the riverbed incised on average by over 1.0 m and (iii) the median grain size of the bed surface decreased from 13.2 cm to 4.7 cm. The spatial pattern of land cover within the Deleg catchment also changed considerably: highly degraded agricultural land in the low-lying areas was abandoned and partially afforested for timber and wood production, whereas secondary upland forest was increasingly cleared for expansion of cropland and pastures. Notwithstanding large changes in the spatial organization of land use within the catchment, the overall land use did not change significantly during the past four decades. This suggests that the response of the Deleg River to land-use change not only depends on the overall land-use change, but also on the spatial pattern of land-use/cover change within the catchment. Although forestation and regeneration of bare gully slopes and floors throughout the catchment only represented a minor part of the total land-use change, these land-use/cover changes had a major impact on the hydrological and sediment connectivity in the landscape.  相似文献   

A slowness approach to the reflectivity method of seismogram synthesis   总被引：4，自引：0，他引：4  

Gerard J. Fryer 《Geophysical Journal International》1980,63(3):747-758

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Of the many schemes available for computing synthetic seismograms, the reflectivity method is probably the most widely used because of its ability to provide complete solutions. The method does, however, suffer the disadvantage that intermediate results are quite difficult to interpret. A new reflectivity technique, here called reflectivity-slowness, results if the original method is reformulated using a slowness rather than a spectral approach. The new procedure bears a strong similarity to the WKBJ method, but retains the ability to give complete solutions. The reflectivity-slowness and WKBJ methods share the property that intermediate results are readily interpreted; this feature may eventually be exploited in the solution of the inverse problem.  相似文献   

A diatomaceous origin for long-chain diols and mid-chain hydroxy methyl alkanoates widely occurring in quaternary marine sediments: indicators for high-nutrient conditions     

Jaap S. Sinninghe Damsté  Sebastiaan Rampen  C. Rijpstra  Gerard Muyzer  Stefan Schouten 《Geochimica et cosmochimica acta》2003,67(7):1339-1348

For the first time a biological source for the long-chain alkyl 1,14-diols and 12-hydroxy methyl alkanoates, lipids widely occurring in the marine water column and sediments, has been identified. Cultures of Proboscia indica and Proboscia alata, rhizosolenoid diatoms belonging to the widespread diatom genus Proboscia, contain C₂₈, C_28:1, C₃₀, and C_30:1 alkyl 1,14-diols, and C₂₇ and C₂₉ 12-hydroxy methyl alkanoates as major neutral lipids. These components form a substantial fraction of lipid fractions from sediment traps or sediments, especially in areas with an elevated primary production such as upwelling regions. Examination of literature data reveals that as much as 20 to 35% of the total lipid flux in the Arabian Sea is derived from Proboscia diatoms during the start of the upwelling season. Their rapid transfer to the water-sediment interface may explain why corresponding 1,14-keto-ols, inferred oxidation products of diols, are hardly formed. These interpretations are supported by compound-specific carbon isotopic analysis of long-chain keto-ols and diols in surface sediments of the Arabian Sea. The data indicate that long-chain alkyl 1,14-diols and 12-hydroxy methyl alkanoates can be applied as indicators for high-nutrient conditions in the photic zone.  相似文献   

2-D seismic trenching of colluvial wedges and faults     

David Sheley  Travis Crosby  Min Zhou  Jamie Giacoma  Jianhua Yu  Ruiqing He  Gerard T. Schuster   《Tectonophysics》2003,368(1-4):51

Images of the depth and shape of colluvial wedges by 3-D travel time tomography can be valuable in estimating the past history of ancient earthquakes and assessing the earthquake hazard of a fault. Unfortunately, 3-D seismic surveys can be both costly and time consuming. In this paper, we report our first successful results of detecting the shapes and depths of colluvial wedges with 2-D travel time tomography. The locations of the colluvial wedges are along the Oquirrh fault and the Salt Lake City segment of the Wasatch fault. We also report that the tomogram from one of our 2-D surveys suggests the possibility of detecting multiple colluvial wedges at depth. Using both reflection and tomographic images at another site clearly reveals the presence of a known shallow fault and the contact between native soil and recent fill. Our results suggest the synergistic use of both seismic tomography and reflection imaging, we denote as seismic trenching, as a means of detecting shallow colluvial wedges and faults. Seismic trenching has the potential to significantly expand the lateral extent and depth of investigation of paleoseismology.  相似文献   

Impact of road building on gully erosion risk: a case study from the Northern Ethiopian Highlands   总被引：1，自引：0，他引：1  

Jan Nyssen  Jean Poesen  Jan Moeyersons  Edith Luyten  Maude Veyret‐Picot  Jozef Deckers  Mitiku Haile  Gerard Govers 《地球表面变化过程与地形》2002,27(12):1267-1283

Although obvious in the field, the impact of road building on hydrology and gullying in Ethiopia has rarely been analysed. This study investigates how road building in the Ethiopian Highlands affects the gully erosion risk. The road between Makalle and Adwa in the highlands of Tigray (northern Ethiopia), built in 1993–1994, caused gullying at most of the culverts and other road drains. While damage by runoff to the road itself remains limited, off‐site effects are very important. Since the building of the road, nine new gullies were created immediately downslope of the studied road segment (6·5 km long) and seven other gullies at a distance between 100 and 500 m more downslope. The road induces a concentration of surface runoff, a diversion of concentrated runoff to other catchments, and an increase in catchment size, which are the main causes for gully development after road building. Topographic thresholds for gully formation are determined in terms of slope gradient of the soil surface at the gully head and catchment area. The influence of road building on both the variation of these thresholds and the modification of the drainage pattern is analysed. The slope gradient of the soil surface at the gully heads which were induced by the road varies between 0·06 and 0·42 m m^?1 (average 0·15 m m^?1), whereas gully heads without influence of the road have slope gradients between 0·09 and 0·52 m m^?1 (average 0·25 m m^?1). Road building disturbed the equilibrium in the study area but the lowering of topographic threshold values for gullying is not statistically significant. Increased gully erosion after road building has caused the loss of fertile soil and crop yield, a decrease of land holding size, and the creation of obstacles for tillage operations. Hence roads should be designed in a way that keeps runoff interception, concentration and deviation minimal. Techniques must be used to spread concentrated runoff in space and time and to increase its infiltration instead of directing it straight onto unprotected slopes. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Ecosystem responses in the Rhine-Meuse delta during two decades after enclosure and steps toward estuary restoration     

Henk Smit  Gerard van der Velde  Rob Smits  Hugo Coops 《Estuaries and Coasts》1997,20(3):504-520

Changes in hydrology, and developments in morphology, water quality, and ecology of the Rhine-Meuse estuary after its enclosure in 1970 are analyzed on the basis of existing monitoring data. Annual averages of ammonium, nitrate plus nitrite, total phosphate, total lead, and chlorophyll-a concentrations as well as transparency of the water are presented. Abundances of some water bird species are given for the period 1970–1993, and the relative fish biomass for the period 1971–1988 is discussed. The gradual evolution led toward the dominance of bream. The area has developed toward a system with generally low natural ecological values. Ecological impacts of present sluice management are discussed and include the accumulation of contaminated sediments, disappearance of intertidal areas and nursery grounds for fish, disturbance of fish migration, and less mixing of river and seawater. Recent policy developments have brought the present management of the Haringvliet sluices back in to discussion. A recent policy document has presented several management alternatives, including partial and complete reopening of the sluices to permit saltwater intrusion. Three management options are compared in terms of costs and ecological benefits. It is concluded that a complete reopening, and thus a partial restoration of the estuarine characteristics, is most beneficial for the ecosystems of the area itselt, for the upstream Rhine and Meuse rivers, and for the adjacent North Sea, but costs amount to about 600 million US $.  相似文献   

Coalification stages from confined pyrolysis of an immature humic coal     

P. Landais  L. Gerard 《International Journal of Coal Geology》1996,30(4):285-301

An immature humic coal (subbituminous rank) from the Mahakam delta (Kutei basin, Kalimantan, Indonesia) was isothermally pyrolyzed in confined conditions at temperatures ranging from 250 to 400°C (10°C steps) at 700 bar pressure for 72 h. Solid, liquid and thermodesorbable phases originating from the pyrolyzates have been analyzed by different analytical techniques. Results indicate that a 10°C pyrolysis step is necessary to determine the timing and the sequence of the different transformations affecting the kerogen as well as the effluents. Four maturation/coalification stages are distinguished. Stage 1 (75–80 wt.% C) occurs when modifications mainly concern the oxygen-bearing functions of the kerogen. Stage 2 (82–85 wt.%o C) is characterized by the decrease of the aliphaticity and the primary cracking of the coal. Stage 3 (86–89 wt.% C) corresponds to the production of methane and the condensation of aromatic rings in the solid residue.  相似文献   

Spatial p-q Resonant Orbits of the RTBP     

Esther Barrabés  Gerard Gómez 《Celestial Mechanics and Dynamical Astronomy》2002,84(4):387-407

The purpose of this paper is to extend the study of the so called p-q resonant orbits of the planar restricted three-body problem to the spatial case. The p-q resonant orbits are solutions of the restricted three-body problem which have consecutive close encounters with the smaller primary. If E, M and P denote the larger primary, the smaller one and the infinitesimal body, respectively, then p and q are the number of revolutions that P gives around M and M around E, respectively, between two consecutive close approaches. For fixed values of p and q and suitable initial conditions on a sphere of radius around the smaller primary, we will derive expressions for the final position and velocity on this sphere for the orbits under consideration.  相似文献