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Soil Strength (soil + strength)

Distribution by Scientific Domains

Earth and Environmental Science	41%
Engineering	25%

Selected Abstracts

Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration

AUSTRAL ECOLOGY, Issue 8 2005
I. E. BASSETT
Abstract Soil compaction can affect seedling root development by decreasing oxygen availability and increasing soil strength. However, little quantitative information is available on the compaction tolerances of non-crop native species. We investigated the effects of soil compaction on establishment and development of two New Zealand native species commonly used in restoration programmes; Cordyline australis (Agavaceae) (cabbage tree) a fleshy rooted species, and Leptospermum scoparium (Myrtaceae) (manuka) a very finely rooted species. Seedlings were grown in a range of soil compaction levels in growth cabinet experiments. Low levels of soil compaction (0.6 MPa) reduced both the number and speed of C. australis seedlings penetrating the soil surface. In contrast, L. scoparium seedlings showed improved establishment at an intermediate compaction level. Root and shoot growth of both species decreased with increasing soil strength, with L. scoparium seedlings tolerating higher soil strengths than did C. australis. Despite these results, soil strength accounted for only a small amount of variation in root length (R2 < 0.25), due to greater variability in growth at low soil strengths. Soil strengths of 0.6 MPa are likely to pose a barrier to C. australis regeneration. This is consistent with adaptation to organic and/or soft, waterlogged soils. Active intervention may be necessary to establish C. australis from seed on many sites previously in farmland. [source]

Modelling increased soil cohesion due to roots with EUROSEM

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 13 2008
S. De Baets
Abstract As organic root exudates cause soil particles to adhere firmly to root surfaces, roots significantly increase soil strength and therefore also increase the resistance of the topsoil to erosion by concentrated flow. This paper aims at contributing to a better prediction of the root effects on soil erosion rates in the EUROSEM model, as the input values accounting for roots, presented in the user manual, do not account for differences in root density or root architecture. Recent research indicates that small changes in root density or differences in root architecture considerably influence soil erosion rates during concentrated flow. The approach for incorporating the root effects into this model is based on a comparison of measured soil detachment rates for bare and for root-permeated topsoil samples with predicted erosion rates under the same flow conditions using the erosion equation of EUROSEM. Through backwards calculation, transport capacity efficiencies and corresponding soil cohesion values can be assessed for bare and root-permeated topsoils respectively. The results are promising and present soil cohesion values that are in accordance with reported values in the literature for the same soil type (silt loam). The results show that grass roots provide a larger increase in soil cohesion as compared with tap-rooted species and that the increase in soil cohesion is not significantly different under wet and dry soil conditions, either for fibrous root systems or for tap root systems. Power and exponential relationships are established between measured root density values and the corresponding calculated soil cohesion values, reflecting the effects of roots on the resistance of the topsoil to concentrated flow incision. These relationships enable one to incorporate the root effect into the soil erosion model EUROSEM, through adapting the soil cohesion input value. A scenario analysis shows that the contribution of roots to soil cohesion is very important for preventing soil loss and reducing runoff volume. The increase in soil shear strength due to the binding effect of roots on soil particles is two orders of magnitude lower as compared with soil reinforcement achieved when roots mobilize their tensile strength during soil shearing and root breakage. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Mineral soil surface crusts and wind and water erosion

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2004
Michael J. Singer
Abstract The ,rst few millimetres of soil largely control the soil's response to the eroding forces of wind and water. The tendency of soils to form surface seals and crusts in,uences the processes of wind and water erosion differently. For wind, dry particle size distribution and particle organization determine the shear strength and threshold wind velocity necessary to initiate particle movement. In loams and clay loams, seals and crusts decrease roughness but increase surface soil strength, generally decreasing wind erosion. Conversely, in sand and sandy loams, loose erodible sandy material may either deposit on the crust and is subject to erosion or it may disrupt the crust, accelerating the erosion process. For water erosion, particle size distribution and structure determine in,ltration rate, time to ponding, and energy required for soil particle detachment. Seals and crusts tend to decrease in,ltration rate and time to ponding thus increasing overland ,ow and soil erosion. This paper brie,y reviews how permanent and time-dependent soil properties in,uence surface seals and crusts and how these affect soil erosion by wind and water. The tendency of a soil to form a seal and crust depends to some degree on the time-dependent property of soil structural stability, which tends to increase with increasing clay content and smectitic mineralogy which are permanent properties. These permanent properties and their effect on structure are variable depending on dynamic properties of exchangeable sodium percentage and soil solution electrical conductivity. Antecedent water content prior to irrigation or rainfall, rate of wetting before an erosive event and aging, the time between wetting and an erosive event, greatly in,uence the response of soil structure to raindrop impact. The effect of these dynamic processes is further in,uenced by the static and dynamic properties of the soil. Weak structure will be less in,uenced by wetting rate than will a soil with strong structure. Process-based models of wind and water erosion need to consider the details of the interactions between soil static and dynamic properties and the dynamic processes that occur prior to erosive events. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Material stiffness, branching pattern and soil matric potential affect the pullout resistance of model root systems

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 6 2007
S. B. Mickovski
Summary Understanding of the detailed mechanisms of how roots anchor in and reinforce soil is complicated by the variability and complexity of both materials. This study controlled material stiffness and architecture of root analogues, by using rubber and wood, and also employed real willow root segments, to investigate the effect on pullout resistance in wet and air-dry sand. The architecture of model roots included either no laterals (tap-root) or a single pair at two different locations (herringbone and dichotomous). During pullout tests, data on load and displacement were recorded. These studies were combined with Particle Image Velocimetry (PIV) image analysis of the model root-soil system at a transparent interface during pullout to increase understanding of mechanical interactions along the root. Model rubber roots with small stiffness had increasing pullout resistance as the branching and the depth of the lateral roots increased. Similarly, with the stiff wooden root models, the models with lateral roots embedded deeper showed greatest resistance. PIV showed that rubber model roots mobilized their interface shear strength progressively whilst rigid roots mobilized it equally and more rapidly over the whole root length. Soil water suction increased the pullout resistance of the roots by increasing the effective stress and soil strength. Separate pullout tests conducted on willow root samples embedded in sand showed similar behaviour to the rigid model roots. These tests also demonstrated the effect of the root curvature and rough interface on the maximum pullout resistance. [source]

Nonlinear FE-analysis of tunnel excavation , comparison of EC7 design approaches.

GEOMECHANICS AND TUNNELLING, Issue 1 2010
Nichtlineare FE-Analysen im Tunnelbau, Vergleich der Nachweisverfahren des EC
Structural analysis; Support; Statik; Stützmittel Abstract Although Eurocode 7 does not mention tunnelling explicitly it is under discussion amongst practitioners to what extent the principles of EC7 can be applied to the design of tunnels. In EC7 three different design approaches, which differ in the application of partial factors on actions, soil strength and resistance, have been established. A characteristic feature of tunnelling is that the surrounding soil or rock represents loading and support for tunnel, similar to deep excavation problems. In addition, the highly nonlinear behaviour of shotcrete linings needs special attention and therefore care must be taken when choosing partial factors of safety. In this contribution the applicability of the different design approaches for tunnelling employing numerical methods are investigated and the differences obtained are discussed. Two examples are presented, namely a station tunnel in soil and a tunnel in rock, both constructed based on NATM principles. Obwohl Tunnelbau im Eurocode 7 nicht explizit erwähnt wird, steht zur Diskussion, ob der EC7 prinzipiell auch für die Bemessung im Tunnelbau anwendbar ist. Im EC7 sind drei Nachweisverfahren definiert, die sich in der Anwendung der Teilsicherheitsfaktoren auf Einwirkungen, Widerstände und Bodenkennwerte unterscheiden. Ein wesentlicher Aspekt im Tunnelbau ist, dass der Baugrund sowohl Einwirkung als auch Widerstand sein kann, und somit sind Parallelen zur Bemessung von Baugruben gegeben. Zusätzlich kommt hinzu, dass das mechanische Verhalten von Spritzbeton hochgradig nichtlinear ist, was bei der Wahl der Teilsicherheitsbeiwerte beachtet werden muss. In diesem Beitrag wird die Anwendbarkeit der unterschiedlichen Nachweisverfahren des EC7 für den Tunnelbau bei Anwendung numerischer Methoden untersucht und die Unterschiede in den Ergebnissen aufgezeigt. Zwei Beispiele wurden gewählt, ein Stationstunnel im Lockergestein und ein Tunnel im Fels, beide nach der Neuen Österreichischen Tunnelbauweise aufgefahren. [source]

Indentation of a free-falling lance penetrometer into a poroelastic seabed

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 2 2005
Derek Elsworth
Abstract A solution is developed for the build-up, steady and post-arrest dissipative pore fluid pressure fields that develop around a blunt penetrometer that self-embeds from freefall into the seabed. Arrest from freefall considers deceleration under undrained conditions in a purely cohesive soil, with constant shear strength with depth. The resulting decelerating velocity field is controlled by soil strength, geometric bearing capacity factors, and inertial components. At low impact velocities the embedment process is controlled by soil strength, and at high velocities by inertia. With the deceleration defined, a solution is evaluated for a point normal dislocation penetrating in a poroelastic medium with a prescribed decelerating velocity. Dynamic steady pressures, PD, develop relative to the penetrating tip geometry with their distribution conditioned by the non-dimensional penetration rate, UD, incorporating impacting penetration rate, consolidation coefficient and penetrometer radius, and the non-dimensional strength, ND, additionally incorporating undrained shear strength of the sediment. Pore pressures develop to a steady peak magnitude at the penetrometer tip, and drop as PD=1/xD with distance xD behind the tip and along the shaft. Peak induced pressure magnitudes may be correlated with sediment permeabilities, post-arrest dissipation rates may be correlated with consolidation coefficients, and depths of penetration may be correlated with shear strengths. Together, these records enable strength and transport parameters to be recovered from lance penetrometer data. Penetrometer data recorded off La Palma in the Canary Islands (J. Volcanol. Geotherm. Res. 2000; 101:253) are used to recover permeabilities and consolidation coefficients from peak pressure and dissipation response, respectively. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Microstructural deformation mechanisms of unsaturated granular soils

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2002
J. A. Gili
Abstract A discrete model for unsaturated granular soils has been developed. Three discrete entities have been defined: particles, water menisci and pores. Local interaction forces and water transfer mechanisms have been integrated into a model through the appropriate equilibrium and balance equations. The results of several numerical tests using this model have been described and discussed. Simulations include wetting and drying under load tests, the application of suction cycles and the effect of a deviatoric stress ratio on wetting-induced collapse. The model reacts just as true granular soil samples behave in laboratory tests. The model provides a new insight into the internal mechanisms leading to large-scale features of behaviour such as wetting-induced collapse or the increase in soil strength provided by suction. The paper also stresses that matric suction changes acting on a granular structure are capable of explaining most of the macroscopic features of stress,strain behaviour. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Comparison of critical limits for crop plant growth based on different indicators for the state of soil compaction

JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 4 2010
Manfred Kaufmann
Abstract Soil compaction affects physical soil condition, in particular aeration, soil strength, and water availability and has adverse effects on plant growth. Bulk density is the most frequently used indicator to describe the state of compaction of a soil. However, this parameter lacks a direct functional relationship with plant growth. Various indicators have been proposed to simultaneously characterize the state of compaction of agricultural soil and its suitability for plant growth. This paper examines and compares the critical limits for crop plant growth based on three of these indicators: packing density, least limiting water range, and S parameter (the latter is the slope of the soil water-retention curve in the inflexion point). In a first step, we reviewed the literature for published optimum and limiting values of bulk density and found that these values were highly dependent on clay and silt content. Converting them into corresponding values of packing density (composite index of bulk density and clay content), a value of 1.70 was found to effectively distinguish between optimum and limiting soil conditions for plant growth. In a second step, the packing density of 59 soil horizons sampled in N Switzerland was compared with the least limiting water range and the S parameter of these soil horizons (both determined by means of pedotransfer functions taken from the literature). A linear relationship between the three parameters was found, which allowed for a comparison of the published critical limits for plant growth based on these parameters. The critical limits of the three indicators, which had been postulated independently of each other in the literature, were found to agree well with each other. This means that all of them could equally be used to describe the compaction state of a soil and its physical suitability for plant growth. However, the proposed critical limits of packing density, least limiting water range, and S parameter still need further validation by field studies relating plant growth to soil compaction. [source]

Quantification of soil structural changes induced by cereal anchorage failure: Image analysis of thin sections

JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 3 2007
Sacha J. Mooney
Abstract Cereal anchorage failure, or lodging, is the permanent displacement of a crop from the vertical and results in significant annual yield losses globally. Several factors have been identified as contributors to this phenomenon but the precise mechanisms of failure are still largely unknown because of difficulties in observing these processes as they occur in situ. To identify potential soil management practices to minimize losses associated with cereal root failure, an understanding of the nature of root-soil interactions attributed to lodging is needed. An experiment was conducted that involved field impregnation and subsequent thin sectioning of lodged and unlodged root-soil complexes from contrasting soils, cereal crops, and management practices to elucidate the effects of lodging on soil structure and porous architecture. Using image analysis, size and distribution of pores in soils were quantified at both meso- (100,30 ,m) and microscales (<30 ,m). A significant effect of lodging on porosity was recorded whereby lodging reduced total porosity through compaction created by movement of the stem base, although this was variable among soil types. Pore-size distributions comprehensively supported these trends since alteration in the relative frequency of pores within specific size classes was clearly observed. The effects of lodging were more pronounced at the mesoscale because the data were more susceptible to variations created by natural soil heterogeneity at the microscale. These data suggested that sideways movement of the subterranean stem within the soil is a significant factor which is likely to affect the propensity for a cereal plant to lodge, indicating soil strength in the upper part of the soil profile is crucial. [source]