			Home About us Contact

Various Soils (various + soil)

Distribution by Scientific Domains

Engineering	50%

Selected Abstracts

Syngenetic permafrost growth: cryostratigraphic observations from the CRREL tunnel near Fairbanks, Alaska

PERMAFROST AND PERIGLACIAL PROCESSES, Issue 4 2004
Y. Shur
Abstract Syngenetic permafrost forms when alluvial, aeolian and/or colluvial sediment accumulates under cold-climate conditions. Observations from within the CRREL permafrost tunnel near Fairbanks, Alaska, indicate that layered, lenticular-layered and micro-lenticular cryogenic structures are characteristic of this type of permafrost. In contrast, reticulate cryogenic structures indicate local thaw modification. During the growth of syngenetic permafrost, episodes of thermokarst erosion may operate preferentially along ice wedges leading to the development of gullies and tunnels in the near-surface sediments. The local thaw unconformities that result are inferred by the recognition of thermokarst-cave ice (,pool' ice), and various soil and ice pseudomorphs. These may be regarded as further characteristics of syngenetic permafrost growth. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Pressure plate studies to determine how moisture affects access of bacterial-feeding nematodes to food in soil

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2002
G.W. Yeates
Summary Nematode activity in the soil depends on the presence of free water. We conducted pressure plate experiments to understand better how soil matric potential and structural degradation affect the population growth of three bacterial-feeding nematodes (Cephalobus, Pristionchus, Rhabditis). We took undisturbed cores from six soils (sand, silt loam and silty clay loam with four management regimes), and removed all fauna from them. Ten or 30 nematodes were added, and pressures corresponding to soil matric potentials of ,10, ,33, ,50, ,100 or ,1500 kPa were applied for 35 days. The nematodes were then counted. Significant reproduction of all bacterial-feeding nematodes occurred when the diameters of water-filled pores were approximately 1 ,m. This confirms observations using repacked soils and field manipulations. Only for Pristionchus did declining populations match the reduction in total soil porosity related to intensification of land use on the silty clay loam. We had not expected Cephalobus to have the fastest increase in population of the three nematodes in intact soil cores, and our evidence questions the relative importance given to the three nematode families in soil processes. The differing rates of population increase of the three nematodes in the various soils reflect both habitable pore space and trophic interactions. This suggests that the very diversity of nematode assemblages is crucial in the resilience of biological soil processes. That water-filled pores as small as 1 ,m provide suitable spaces for sizeable populations of bacterial-feeding nematodes accords with the observed migration of infective juveniles of trichostrongylid nematodes and mermithids in water films on herbage. Our results imply that assessment of the role of nematodes in soil processes may be a key to the understanding of biological interactions in water films, and the selection pressures on nematode morphology. [source]

Analysis of Transient Data from Infiltrometer Tests in Fine-Grained Soils

GROUND WATER, Issue 3 2000
Dominique Guyonnet
Data collected during ring infiltrometer tests are often analyzed while assuming either that the effect of gravity is negligible (early-time, transient data) or that it is dominant (late-time, steady-state data). In this paper, an equation is proposed for inter-preting both early-time and late-time data measured during infiltration tests under falling head conditions. It is shown that the method used by previous authors for interpreting both early-time data is a special case of the proposed equation. The equation is applied to data collected during tests performed in fine-grained soils, and results are discussed. The analysis suggests that to assume a priori values of the soil sorptive number, as indicated in the literature for various soils, may in some cases lead to severely underes-timated values of the saturated hydraulic conductivity. Conversely, in low permeability soils, to assume steady-state gravity drainage may lead to order of magnitude overestimates of the saturated hydraulic conductivity. A dimensionless analysis provides characteristic times that correspond either to the duration of the log-log half slope displayed by early-time data or to the log-log unit slope characteristic of late-time data. [source]

A viscoelastic model for the dynamic response of soils to periodical surface water disturbance

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2006
P. C. Hsieh
Abstract In many instances soils can be assumed to behave like viscoelastic materials during loading/unloading cycles, and this study is aimed at setting up a viscoelastic model to investigate the dynamic response of a porous soil layer of finite thickness under the effect of periodically linear water waves. The waves and homogeneous water are described by potential theory and the porous material is described by a viscoelastic model, which is modified from Biot's poroelastic theory (1956). The distributions of pore water pressures and effective stresses of various soils such as silt, sand, and gravel are demonstrated by employing the proposed viscoelastic model. The discrepancies of the dynamic response between the simulations of viscoelastic model and elastic model are found to be strongly dependent on the wave frequency. Copyright © 2006 John Wiley & Sons, Ltd. [source]