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  • 1
    Electronic Resource
    Electronic Resource
    Strength and deformation of agricultural soil: measurement and practical significance (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Soil use and management 7 (1991), S. 0 
    Details
    ISSN: 1475-2743
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract. Soil has a finite strength to resist permanent volume change and permanent shear deformation. When the stresses imposed on the soil are of a sufficient magnitude to overcome the strength, then the deformation falls into one of two regimes. At low stress ratios (ratio of normal stress to maximum past stress) the soil expands when sheared and at high stress ratios it compresses. The maximum past stress in a field soil is the pre-consolidation stress. The pre-consolidation stress is the compressive stress greater than which compression is considerable and permanent.These regimes of soil deformation behaviour are consistent and predictable under a wide range of conditions. They are described by the critical state concept, which can usefully be applied to soil management. Management decisions can be based on whether the stresses imposed by a particular operation result in high or low stress ratios. This governs whether the soil will deform permanently or not (for compaction damage), expand on shear (tillage) or compress on shear (preparation of rice paddy soils). The change in permeability and structure can also be predicted from the deformation regime.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1475-2743.1991.tb00878.x
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  • 2
    Electronic Resource
    Electronic Resource
    Estimating critical state soil mechanics parameters from constant cell volume triaxial tests (1998)
    Oxford, UK : Blackwell Science, Ltd
    European journal of soil science 49 (1998), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Compaction, tillage, stresses around growing roots and other soil deformation events may be predicted by the critical state model of soil mechanics, but estimating the parameters is time consuming and expensive. We develop a back analysis of the constant cell volume triaxial test, in which the critical state parameters are derived from the results of a single test. This both saves much labour and provides more information than traditional analyses, which require several triaxial compression tests and an isotropic compression test to yield the same information. The method finds, using a minimization algorithm and a quasi-analytical solution to the stress–strain equations, the simulated soil deformation (and hence the properties used in that simulation) that best fits the test data. The minimization is a form of regression analysis.For normally consolidated samples the method provides stable estimates of the slope of the critical state line (M), the slope of the virgin compression line (λ) and elastic modulus (E). The standard errors of the estimates are small in relation to the means of these parameters. The estimates appear to be more reliable than those of more commonly used estimation procedures. The slope of the rebound line (κ) is estimated, but a measure of the accuracy of the estimate cannot be calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2389.1998.00142.x
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  • 3
    Electronic Resource
    Electronic Resource
    Estimating critical state soil mechanics parameters from shear box tests (1998)
    Oxford, UK : Blackwell Science Ltd
    European journal of soil science 49 (1998), S. 0 
    Details
    ISSN: 1365-2389
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Prediction of compaction, tillage, root growth or other soil deformation events requires a description of the stress–strain properties of the soil such as the critical state model, but estimating the parameters is time consuming and expensive. I have developed a method of estimating critical state properties from a single shear box test, both saving much labour and providing more information than traditional analyses. The method is based on critical state analyses of the constant stress and constant volume shear box tests using the total stresses applied at the boundary. It derives the critical state property parameters from test data by minimizing the difference between test data and the simulated soil deformation (and hence the properties used in that simulation). The minimization is a form of regression analysis. The analyses resulted in good simulations of the history of states in space defined by τxyγxy and e space (constant stress test) or τxyγxy and σy space (constant volume test).For normally consolidated samples, the analysis of a single constant stress test provided estimates of the slope of the critical state line (M), the slope of the virgin compression line (λ), the slope of the rebound line (K), and the elastic modulus (E). The standard deviation of the estimate of kcould not be found. By contrast, the analysis of the constant volume test resulted in poor estimates, particularly of λ. This is because the test yields no information on changes of volume during deformation: hence volume change parameters are not successfully estimated. The constant volume test is therefore not suitable for back analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-2389.1998.4930503.x
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  • 4
    Electronic Resource
    Electronic Resource
    Growth of seedling roots in response to external osmotic stress by polyethylene glycol 20,000 (1992)
    Springer
    Plant and soil 143 (1992), S. 85-91 
    Details
    ISSN: 1573-5036
    Keywords: plant species ; polyethylene glycol (PEG) ; osmotic potential ; osmotic stress ; root elongation ; root diameter ; seedling roots
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Seedling roots of ten plant species were grown in siliceous sand wetted with solutions of polyethylene glycol (PEG) of MW=20,000 with osmotic potentials of 0.0, − 0.25, − 0.5 and − 1.0 MPa. After 48 h growth under controlled lighting, root elongation and root diameter were measured. Root elongation of all species was reduced by increasing levels of external osmotic stress. Dicotyledonous species were affected more than monocotyledons at potentials of − 0.25 and − 0.5 MPa but less at − 1.0 MPa. Root diameters of all the species were thicker than those of the unstressed at potentials of − 0.25 and − 0.5 MPa. At a potential of − 1.0 MPa the dicotyledons were still thicker, though not by as much as they were at − 0.25 and − 0.5 MPa. The monocotyledons, in contrast, were thinner at − 1.0 MPa. There was a significant positive correlation (r=0.81, p 〈0.01) between root diameter and root elongation at − 1.0 MPa potential. Species were ranked according to the relative root elongation (RRE) and relative root thickness (RRT) at the highest level of stress (− 1.0 MPa). In both rankings dicotyledonous species were in the top ranks and monocotyledous species were in lower positions. The results are compared with those for the elongation and thickening of roots growing against external mechanical stress obtained in a previous study. There were good correlations between the responses observed for the two types of external stress. The implications of these findings are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00009132
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  • 5
    Electronic Resource
    Electronic Resource
    Field evaluation of laboratory techniques for predicting the ability of roots to penetrate strong soil and of the influence of roots on water sorptivity (1993)
    Springer
    Plant and soil 149 (1993), S. 149-158 
    Details
    ISSN: 1573-5036
    Keywords: compaction ; field evaluation ; plant species ; root penetration ; sorptivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The ability of two laboratory screening techniques to predict the abilities of roots of eight crop species to penetrate a compacted soil were evaluated and compared in a field experiment. A soil tilled to remove the effects of mechanical resistance was planted with the same species to serve as a control. Depth of root penetration, root density and the influence of the roots on the sorptivity of water were measured. Roots of all species penetrated deeper in the deep tilled than compacted soils. There were differences in the ability of roots of the species to penetrate the compacted soil. Generally dicotyledonous species had more roots penetrating to depth in both the compact and deep tilled soils. Within the main species classifications, lupin and safflower (dicotyledons) and oats and barley (monocotyledons) had the highest penetration into the compacted soil. Water sorptivities in the deep tilled soils were higher than those of the compact soil. Soil from planted treatments had higher sorptivities than soil which had not been planted. This is attributed to biopores left by the roots. Sorptivities of soils which had dicotyledonous species were generally higher than those of monocotyledons. The soil planted with safflower produced the highest sorptivity in the compacted layer (0.1–0.3 m). A comparison of the accuracy of the two laboratory screening methods in predicting the field penetration of roots suggest that the method involving mechanical stress was better than that involving osmotic stress. Relative root diameter was found to be a better indicator of the penetration ability of roots than relative root elongation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00016604
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  • 6
    Electronic Resource
    Electronic Resource
    Influence of root diameter on the penetration of seminal roots into a compacted subsoil (1992)
    Springer
    Plant and soil 144 (1992), S. 297-303 
    Details
    ISSN: 1573-5036
    Keywords: penetration resistance ; plant species ; root diameter ; soil compaction ; soil strength ; tillage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg-1, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress. Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage. The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00012888
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  • 7
    Electronic Resource
    Electronic Resource
    Modification of soil aggregation by watering regime and roots growing through beds of large aggregates (1994)
    Springer
    Plant and soil 160 (1994), S. 57-66 
    Details
    ISSN: 1573-5036
    Keywords: aggregation ; large aggregates ; plant species ; root growth ; soil watering regimes ; wetting and drying
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract The influence of root growth and soil watering regime on aggregation was studied under controlled conditions. The study examined the influence of pea (Pisum sativum cv Greenfeast), ryegrass (Lolium rigidum cv Wimmera) and wheat (Triticum aestivum cv Kite) roots on changes in aggregation and on the properties of the aggregates. The soil was a non swelling red-brown earth which was either kept wet or was allowed to wet and dry during the experiment. Root growth increased the percentage of small sized aggregates (〈18 mm diameter), organic carbon, tensile strength and stability of aggregates in comparison with a non planted soil. Changes in aggregate size distribution and properties of the aggregates were related to root length density of the species and also to the soil watering regime. Root length density was in the order ryegrass〉pea〉wheat. Wetting and drying of soil increased the strength and stability of aggregates. Incubating aggregates allowed some roots to decompose but did not increase the strength or stability of aggregates compared with unincubated soil. The results of this experiment are of practical significance in soil structural management, and in studies of soil aggregation dynamics. It may be possible to use plant roots to alter the size and properties of aggregates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00150346
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  • 8
    Electronic Resource
    Electronic Resource
    Rheological characteristics of sewage sludge: A granuloviscous material (1988)
    Springer
    Rheologica acta 27 (1988), S. 326-334 
    Details
    ISSN: 1435-1528
    Keywords: Granuloviscous material ; sewage sludge ; constitutive relation ; annularshear cell
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Problems have been experienced in the screw conveying of a centrifuge dewatered sewage sludge, at about 66% by weight water content. The rheology of the sludge was therefore tested in a viscometer in which the normal stress between the platens could be controlled. Rough platens were used to measure internal shear strengths while smooth steel platens were used to study wall slip. The sludge displayed a number of the characteristics of a granuloviscous material. It was found to possess high initial shear strength, but rapidly softened with shear to lower steady-state strengths. The initial shear strength increased with increasing normal stress, indicating that the sludge was frictional. The initial strength was lower against the smooth platen. The steady-state shear strengths were both rate and stress dependent using the rough platen, and this internal shear strength could be described by a frictional pseudoplastic equation. Using the smooth platen the material was nearly rate independent, and was best described by a Coulomb friction equation. At the higher rates the wall shear strengths were lower than the internal shear strengths. The strengths (both initial and steady-state) were characterised by a high degree of variability from sample to sample, and also showed marked hysteresis with the rough platen. They did not show any consistent variation with moisture content. In terms of screw conveying, the major conclusion is that smooth walls to the flights of the screw and barrel of the conveyor are desirable.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01329749
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