Soil Tensile Strength Research Articles

The effects of organic matter application and intensive tillage and traffic on soil structure formation and stability

Management strategies like organic matter (OM) amendment and mechanical energy inputs are known to influence the soil system. A long-term (13–14-year) field experiment was used to evaluate the effects of these management strategies on soil structural formation, structural stabilization and soil tilth of a sandy loam soil in Denmark. OM was applied as manure and by retention of plant residues (ORG) to be compared with plots dressed only with mineral fertilizer (MIN). The soils were rotovated (ROT), compacted (PAC) or left undisturbed (REF) as split-plot treatments in the main plots with OM management over two years prior to sampling. In two consecutive years, undisturbed soil samples were collected from the 6 to 13cm soil layer in the field grown with winter wheat to assess soil organic carbon (C) fractions (total organic C, polysaccharide C and microbial biomass C), total organic C and polysaccharide C of 1–2mm macro-aggregates, bulk density, hyphal length, aggregate stability, clay dispersibility, aggregate tensile strength, direct tensile strength and shear strength. The ease of fragmentation and the torsional shear strength of soil were measured in the field as well. OM application increased all soil organic C fractions. Response patterns of organic C fractions in aggregates were the same patterns as for whole-soil. Polysaccharide C appeared to be an important agent in the aggregation process. The effect of microbial C and fungal hyphae on the aggregation process was not clear. Extensive tillage and traffic produced unfavourable tilth conditions in terms of a greater degree of clay dispersion, lower aggregate stability, higher soil tensile strength and poorer soil fragmentation. OM affected soil reaction to compressive and tensile stresses applied at differing initial bulk densities. The results also indicated the profitability of supplementing the classical laboratory analysis with in situ measurements to better evaluate management effects on soil structure.

Physical Properties of an Alfisol Under Biofuel Crops in Ohio

There is an increasing need to develop renewable energy sources from biofuel crops to replace fossil fuels. Biofuel crops may also enhance ecosystem functions such as soil quality, water availability, and nutrient reserves. Therefore, the effects of four biofuel crops (corn (Zea mays), switchgrass (Panicum virgatum), indiangrass (Sorghastrum nutans) and willow (Salix spp.) were evaluated on soil quality at three sites in Ohio to assess the effects of crop species on soil bulk density (ρb), soil moisture characteristics (SMC), water stable aggregate distribution (WSA), and aggregate tensile strength (TS) to 40 cm depth. Overall, results were site-specific, with most differences occurring for the clayey soil at the Northwest site. At the Jackson site, soil in the 0-10 cm layer under switchgrass had a higher moisture content (θ) between 0 and 100 kPa than that under indiangrass. At the Western site, θ under corn at 1500 kPa was higher at 30-40 cm depth. At the Northwest site, soils under corn in the 0-10 cm depth tended to have the lowest θ at 0 and 3 kPa, while soils under switchgrass and willow had 50% more large macroaggregates and fewer small microaggregates than that under corn. Soil TS in the 0-10 cm depth under corn was nearly 160% more than that under other perennial crops. These results suggest that management of perennial biofuel crops can improve soil physical quality. Changes over seven years occur first in the surface soil layers, but further differences may evolve in subsoil layers with increase in time.

Parameters Controlling Tensile and Compressive Strength of Fiber-Reinforced Cemented Soil

AbstractEnhancement of local soils with fibers and cement for the construction of stabilized pavement bases, canal lining, and support layers for shallow foundations shows great economical and environmental advantages, avoiding the use of borrow materials from elsewhere, in addition to the need of a spoil area. In previous studies, a unique dosage methodology for cemented soils has been established based on rational criteria in which the porosity-to-cement ratio plays a fundamental role in the assessment of the target unconfined compressive strength (qu). The present paper extends previous work by quantifying the influence of the amount of cement, the porosity, and the porosity-to-cement ratio in an assessment on tensile strength (qt) and compressive strength of a fiber-reinforced artificially cemented sand, in addition to the evaluation of the qt/qu relationship. A program of splitting tensile tests and unconfined compression tests considering four distinct dry densities and five cement contents, varying...

Estimation of Tensile Strength of Soils from Penetration Resistance

AbstractTensile strength of soil plays a decisive role in formation/initiation of cracks and their propagation, and its knowledge is essential to study and analyze the stability of earth-fill dams, slopes, embankments, highways, airfield pavements, deterioration of cricket pitches, and development of ice lenses during freezing. Many researchers have worked on determination of tensile strength of soil, and many methodologies and subsequent correlations between the tensile strength and basic properties of soil have been developed and reported. However, these methodologies are time-consuming and yield results that are soil specific. Further, these studies either neglect the effect of overall soil properties or the water content of soil, which are the most crucial parameters affecting tensile strength. An undermined fact is that there are no easy to adopt methodologies or generalized relationships available in the literature that would yield tensile strength of soil quickly and precisely. As such, to address ...

Theoretical and experimental study of initial cracking mechanism of an expansive soil due to moisture-change

Swelling and shrinkage due to moisture-change is one of the characteristics of the expansive soil, which is similar to the behavior of most materials under thermal effect. If the deformation is restricted, stress in expansive soil is caused by the swell-shrinking. The stress is defined as “moisture-change stress” and is adopted to analyze swell-shrinkage deformation based on the elasticity mechanics theory. The state when the total stress becomes equal to the soil tensile strength is considered as the cracking criterion as moisture-change increases. Then, the initial cracking mechanism due to evaporation is revealed as follows: Different rates of moisture loss at different depths result in greater shrinkage deformation on the surface while there is smaller shrinkage deformation at the underlayer in expansive soil; cracks will grow when the nonuniform shrinkage deformation increases to a certain degree. A theoretical model is established, which may be used to calculate the stress caused by moisture-change. The depth of initial cracks growing is predicted by the proposed model in expansive soil. A series of laboratory tests are carried out by exposing expansive soil samples with different moisture-changes. The process of crack propagation is investigated by resistivity method. The test results show good consistency with the predicted results by the proposed theoretical model.

Stabilisation of soil against wind erosion by six saprotrophic fungi

Soils with biological crusts that consist of fungal hyphae, bacteria and other small organisms usually resist erosion. However, the processes by which soil organisms stabilise air-dry aggregates against wind erosion are not well understood. We used saprotrophic fungi to examine some of these processes in a sandy clay loam (Hypercalcic Calcarosol). Soil aggregates, wetted with distilled water or glucose solution, were inoculated with one of six fungi, and incubated in darkness at 24 °C for 7 d in petri dishes under sterile conditions. Abrasion resistance (taken as resistance to wind erosion), tensile strength of soil, hot-water extractable carbohydrates (HWEC), dispersion index, pH, and hyphal length density (HLD) were each measured across all treatments. In all treatments, stability (abrasion resistance) and tensile strength, were positively related to HLD. Such relationships have not been reported elsewhere. All fungi enlarged the aggregates of the soil by cross-linkage and entanglement of particles, but with different processes, or different intensity of the processes, between species (for the same amount of substrate). The skins seen in scanning electron micrographs of stabilised soil were probably extracellular polysaccharides also produced by the fungi. We propose that the ductile failure of disks of soil, particularly those inoculated with Mucor sp., under tensile stress was due to movement of enmeshed particles, whereas the brittle failure of disks of soil inoculated with the other fungal species was due to metabolites or dispersed clay on the surface of the hyphae which limited deformation.

Clay Dispersibility and Soil Friability—Testing the Soil Clay‐to‐Carbon Saturation Concept

Soil organic carbon (OC) influences clay dispersibility, which affects soil tilth conditions and the risk of vertical migration of clay colloids. No universal lower threshold of OC has been identified for satisfactory stabilization of soil structure. We tested the concept of clay saturation with OC as a predictor of clay dispersibility and soil friability. Soil was sampled 3 yr in a field varying in clay content (∼100 to ∼220 g kg−1 soil) and grown with different crop rotations. Clay dispersibility was measured after end‐over‐end shaking of field‐moist soil and 1‐ to 2‐mm sized aggregates either air‐dried or rewetted to −100 hPa matric potential. Tensile strength of 1‐ to 2‐, 2‐ to 4‐, 4‐ to 8‐, and 8‐ to 16‐mm air‐dried aggregates was calculated from their compressive strength, and soil friability estimated from the strength–volume relation. Crop rotation characteristics gave only minor effects on clay dispersibility and no detectable effects on soil friability. Dispersed clay correlated to soil content of clay, but the correlation increased if subtracting a fraction assumed protected by OC. This trend was less convincing for soil tensile strength and friability. Increased clay dispersibility and reduced soil friability for 1 yr of measurements could be ascribed to wet conditions for potato (Solanum tuberosum L.) harvest and tillage the preceding year. Literature data indicate soils' content of clay and silt (Fines20) to be a better predictor of specific surface area than clay. We conclude that a clay/OC ratio of 10 and a Fines20/OC ratio of 20 may serve as corresponding thresholds for clay dispersibility, the latter probably best reflecting organo‐mineral interactions of importance to the soil physical properties.

Factors Influencing Crack-Induced Tensile Strength of Compacted Soil

Mode I (tensile) fracture is the most commonly observed failure in geostructures resulting from cracks in soil. Direct or indirect tensile tests have been used to evaluate the tensile strength of geomaterials. In this paper, the unconfined penetration device and experimental procedure were modified to reduce measurement errors. It was then used to determine the tensile strength of compacted soil. Factors influencing the tensile strength of the compacted soil, including the plasticity index, rate of loading, and size of specimen were discussed in detail, as well as its reliability. Experimental results indicated that the modified, unconfined penetration technique is sufficiently reliable and operator-friendly for determining the tensile strength of compacted soil.

Voids/Cement Ratio Controlling Tensile Strength of Cement-Treated Soils

The improvement of locally available soils with cement can provide great advantages, including avoiding the need to borrow volumes of appropriate material and disposing of the local soil in deposits. This research aims to quantify the influence of the amount of cement, the porosity, and the voids/cement ratio in the assessment of splitting tensile strength (qt), also known as indirect diametrical tensile (IDT) strength, of three distinct soils from Brazil and Portugal. From Brazil, clayey sand derived from Botucatu sandstone and uniform Osorio sand were considered; from Portugal, silty sand derived from weathered Porto granite was studied. A number of splitting tensile strength tests were carried out. The results show that qt increased with the amount of cement (C) and decreases in porosity (η) for the three soil-cement mixtures. A power function was well-adapted to fit both qt-C and qt-η. Finally, the tensile strength was plotted against the porosity/volumetric cement content relationship (η/Civ), in whi...

Numerical modelling of desiccation cracking

AbstractThe ability to model and predict the formation of desiccation cracks is potentially beneficial in many applications such as clay liner design, earth dam construction, and crop science, etc. However, most studies have focused on statistical analysis of crack patterns and qualitative study of contributing factors to crack development rather than prediction. Because it is exceedingly difficult to capture the nonlinear processes during desiccation in analytical modelling, most such models handle crack formation without considering variation of material properties with time, and are unattractive to use in realistic modelling. The data obtained from laboratory experiments on clay soil desiccating in moulds were used as a basis to develop a more refined model of desiccation cracking. In this study, the properties, such as matric suction, stiffness and tensile strength of soil, and base adhesion, could be expressed approximately as functions of moisture content. The initial conditions and the development of suction due to desiccation and the varying material properties were inputted to UDEC, a distinct element code, using its internal programming language FISH. The model was able to capture some essential physical aspects of crack evolution in soil contained in moulds with varying lengths, heights, and materials of construction. Extension of this methodology is potentially beneficial not only for modelling desiccation cracking in clay, but also in other systems with evolving material properties such as concrete structures and road pavements. Copyright © 2010 John Wiley & Sons, Ltd.

Theoretical Assessment of Increased Tensile Strength of Fibrous Soil Undergoing Desiccation

Soil reinforcement with discrete fibers is a viable technique to reduce desiccation cracking in compacted clay soils. The reduction in cracking is attributed to an increase in the tensile strength of the fiber-soil composite. A theoretical model is developed to describe the mechanism of the increased tensile strength due to fiber inclusion of soil undergoing desiccation. The model includes a distinctive effective stress combination acting on the fiber strings due to the generated matric suction by desiccation. Model formulation makes use of Mohr-Coulomb failure criterion at the interface area between fibers and the surrounding soil. The desiccation process of the soil generates matric suction within the soil mass, under given stress condition. The basic elements used in the model formulation include soil-water characteristic curve, Mohr-Coulomb parameters, and unsaturated soil parameters. Fiber inclusion increases significantly the tensile strength of the fiber-soil composite. This increase in tensile strength is expressed as a function of fiber content and soil-water content in this paper. Comparisons are made to published data regarding changes in tensile strength with variable water content.

Tensile strength, friability and organic carbon in an oxisol under a crop-livestock system

The crop-livestock system can promote soil compaction in surface layers, mainly due to animal trampling. However, plants and their root growth, in interaction with animal trampling, can decrease the deleterious changes in soil structure caused by this system. Up to the present time, the physical soil modifications in crop-livestock systems, including oat and ryegrass crops for winter animal forages are unknown. The objective of this study was to quantify and to relate tensile strength, friability and soil organic carbon in an Oxisol under a crop-livestock system. The study was conducted in Campo Mourão - Paraná, Brazil. Four forage heights were used for the winter forages: 7, 14, 21 and 28 cm. For each forage height, five soil blocks were randomly collected from each layer of 0 - 0.1, 0.1 - 0.2 and 0.2 - 0.3 m of depth. The increase in carbon content promotes an increase in soil tensile strength at the 0.1 - 0.2 m soil depth, this layer having the highest values for tensile strength. The forage height of 21 cm was found to be the best height for soil friability, and the soil was very friable at this height. Despite a decrease in friability in the upper layers of the soil, the crop-livestock system was not found to be a limiting factor for the subsequent cultivation of annual crops.

A Critical Review of the Methodologies Employed for Determination of Tensile Strength of Fine-Grained Soils

Abstract Tensile strength of fine-grained soils plays a significant role in assessing their cracking characteristics, which govern their suitability as a construction material for landfill liners and covers, earthen dams, embankments, and pavements. As such, determination of this property of these soils becomes essential. In this context, various experimental techniques that have been developed by earlier researchers to determine tensile strength of fine-grained soils are worth appreciating. Based on the experimental results obtained from these studies, several empirical relationships have been proposed. However, these relationships relate tensile strength of the soil with a single parameter (i.e., suction, plasticity index, liquid limit, CEC, clay content, or water content). This necessitates: (a) critical evaluation of such relationships, and (b) development of a generalized relationship that employs multiple soil properties. With this in view, investigations were carried out on some fine-grained soils and their tensile strength was determined by conducting undrained triaxial tests and suction measurements. Details of the methodologies adopted are presented in this paper and efforts have been made to achieve the two objectives mentioned above.

Use of Factory-Waste Shingles and Cement Kiln Dust to Enhance the Performance of Soil Used in Road Works

An experimental work was conducted to study the use of factory-waste roof shingles to enhance the properties of fine-grained soil used in road works. Cement kiln dust (CKD), a cogenerated product of Portland cement manufacturing, was used as a stabilizing agent while the processed shingles were added to enhance the soil tensile strength. The effects of shingles on strength and stability were evaluated using the unconfined compressive strength, splitting tensile strength, and California Bearing Ratio (CBR) tests. The results showed that the use of CKD alone resulted in a considerable increase in the unconfined compressive strength but had a small effect on the tensile strength. The addition of shingles substantially improved the tensile strength of the stabilized soil. A significant reduction in the capillary rise and a slight decrease in the permeability were obtained as a result of shingle addition. An optimal shingle content of 10% is recommended to stabilize the soil.

Resistência tênsil e friabilidade de um Latossolo sob diferentes sistemas de uso

Diferentes propriedades físicas têm sido utilizadas para avaliar a qualidade física do solo. Nesse contexto, a resistência tênsil e a friabilidade são indicadores da qualidade estrutural e física de um solo. O objetivo deste estudo foi quantificar a resistência tênsil e a friabilidade de um Latossolo Vermelho distrófico em diferentes sistemas de uso: floresta nativa, pousio e cultivado. A amostragem de solo foi realizada em 2006, na Fazenda Experimental da Universidade Estadual de Maringá, Estado do Paraná, em três áreas contíguas sob mata nativa, pousio e cultivo de culturas anuais. A amostragem de solo consistiu da coleta de 10 blocos de solo (15 x 20 x 10 cm) na profundidade de 0-15 cm. Em cada sistema de uso do solo, determinou-se a resistência tênsil de 400 agregados com diâmetro entre 12,5 e 19,0 mm. Também foram feitas determinações do teor de carbono orgânico do solo (CO) e da densidade deste. A friabilidade foi estimada pelo coeficiente de variação da resistência tênsil. Para o solo estudado, tendo a mesma classe taxonômica e textural, mas sob diferentes sistemas de uso, a resistência tênsil de agregados aumentou proporcionalmente com a redução dos teores de CO. O solo foi classificado como friável independentemente do sistema de uso, e a friabilidade foi maior no solo sob mata, comparado com pousio e cultivado. A resistência tênsil de agregados, a friabilidade, o teor de CO e a densidade do solo caracterizaram a redução da qualidade do solo proporcionalmente à intensidade da sua utilização. A resistência tênsil dos agregados e a friabilidade do solo retrataram os efeitos dos sistemas de uso nas condições físicas e estruturais do solo.

Measurement of Soil Tensile Strength and Factors Affecting its Measurements

ABSTRACT This paper describes the tensile strength measured for three kinds of statically compacted unsaturated soils; mixtures of clay ~ silt ~ sand, Narita-sand and Kanto loam. Specimens were directly prepared either under controlled compaction stress or under controlled dry density by statically compacting them within the tensile mold of the apparatus. Image analysis was done to show the normality of tensile force to the tensile failure plane. Tensile strengths (qt) were compared with the unconfined compressive strengths (qu) for silt ~ sand mixture, clay ~ sand mixture, clay ~ silt mixture and Narita sand, respectively. Increment in tensile strength (also qu/qt ratio) with the increase in the percentage and decrease in the size of finer soils could be seen. Effects of number of compaction layers and tensile pulling rates on the qt were also examined. Increase in the tensile strength with the increase in the number of compaction layers was observed; and it was suggested to prepare the unsaturated compacted specimen by 3 to 4 layers compaction. Increase in tensile strength of 0.3 kPa and 0.003 kPa per one cycle of logarithm of tensile pulling rate was observed for clay ~ sand-4 (1:3) and clay ~ sand-5 (3:1) for the pulling rate of 0.01 to 1.0 mm/min.

Tensile strength of soil cores in relation to aggregate strength, soil fragmentation and pore characteristics

Tensile failure of soil is desirable in tillage. Soil tensile strength estimates most often are obtained from compression tests of dry aggregates. As tillage is performed under moist conditions, it would be highly relevant to measure tensile strength at high water contents. Plough-layer soil was sampled in a compacted soil (PAC) and in a non-compacted reference soil (REF). Tensile strength was measured in a new direct tension test using undisturbed soil cores (4.45 cm in diameter and 5.00 cm in height) adjusted to either −50 or −100 hPa matric potential. The air-filled pore space, ε a, was determined from water retention measurements. Air permeability, K a, was determined at −30, −100 and −300 hPa matric potentials and from these measurements an index of pore organization (PO= K a/ ε a) was calculated. Soil behaviour in the field was evaluated at approximately −300 hPa matric potential by measuring soil strength using the torsional shear box method and soil fragmentation using a simple soil drop test. The direct tensile strength results showed that the PAC soil had significantly higher tensile strength than the REF soil (e.g. 3.2 and 2.0 kPa, respectively at −100 hPa matric potential). This finding was in accordance with the aggregate tensile strength results and also agreed well with soil fragmentation in the field (i.e. geometric mean diameter (GMD) equal to 38.7 and 14.2 mm, respectively, for PAC and REF samples dropped from 75 cm height). The tensile strengths of the soil cores were close to the predicted values determined from the aggregate tensile strength results. The energy input in the soil drop test (i.e. approximately 8.9 J kg −1 dry soil) was low in comparison with the energy input in tillage but high compared with the specific rupture energy of soil aggregates (e.g. 0.4 and 0.3 J kg −1, respectively for PAC and REF aggregates adjusted to −100 hPa matric potential). The relatively poor fragmentation in the soil drop test indicated that a substantial amount of the energy input was stored as volumetric strain energy and/or lost to processes such as plastic deformation. The tensile strength of soil cores was negatively correlated to the macroporosity of the soil, whereas the ease of soil fragmentation was positively correlated to PO.

Einfluss des Porenwasserdruckes auf die Zugfestigkeit von Bodenproben

Anhand von direkten Zugversuchen mit Erfassung der Wasserspannung wird der Frage nachgegangen, welcher Zusammenhang zwischen der Zugfestigkeit und dem Porenwasserdruck besteht. Die Untersuchungen erfolgten an einem homogenisierten Geschiebemergel und einem ebenso behandelten Beckenton. Im Zentrum der Betrachtung steht der Zusammenhang zwischen Materialdehnung und Wasserspannungsentwicklung sowie eine Betrachtung des χ-Faktors der effektiven Spannungsgleichung nach Bishop (1959). Dieser wurde aus der Beziehung zwischen Zugfestigkeit und korrespondierender Wasserspannung errechnet und mit den volu metrischen χ-Faktoren der untersuchten Materialien verglichen. Es konnte gezeigt werden, dass die für den Moment des Zugbruchs relevante Wasserspannung von den initialen Wasserspannungen der getesteten Proben abweicht und die Beziehung zwischen Wasserspannung und Zugfestigkeit eines Bodens nur auf der Grundlage dieser aktuellen Wasserspannung dargestellt werden darf. Darüber hinaus führte die Verwendung des volumetrischen χ in der effektiven Spannungsgleichung, verglichen mit dem experimentell für den Zugbruch bestimmten, zu einer 1,6- bis 2,8-fachen Überschätzung des Beitrages der Wasserspannung zur Zugfestigkeit der getesteten Substrate. Effect of pore water pressure on tensile strength Direct tensile testing with measurements of the pore water suction was used to investigate the relationship between tensile strength and suction. The tests were conducted on a till and a clayey soil, both homogenized. A closer view is focused on the relationship between material strain and the development of suction. Beyond, the factor χ of the effective stress equation for unsaturated soils by Bishop (1959), which was calculated by the data of tensile strength and corresponding matric suction is compared to the volumetric χ of the tested soil specimens. It could be shown, that the pore water pressure changes with strain. Therefore, not the initial suction of a soil is relevant for its failure but the actual one that can be measured in the failure zone at the moment of fracture. In addition the application of the volumetric χ in the effective stress equation compared to the χ derived from tensile testing leads to an 1.6 to 2.8 fold overestimation of the contribution of matric suction to soil tensile strength.

Soil friability: theory, measurement and the effects of management and organic carbon content

SummaryThe weakest link between particles or aggregates determines the strength of soil. We have re‐examined the theory and, as a result, have re‐defined friability, F, as the coefficient of variation of soil tensile strength. The formal relationship between the parameter 1/α of the weakest link theory of strength, which has previously been used as a measure of friability, and the newly defined measure, F, is described by a simple equation which has an accuracy of within 2% over the range of interest. The quantity F is used to show that friability reaches maximum at water contents around the lower plastic limit, that mechanical disturbance of wet soil by tillage reduces the friability, and that friability is strongly positively correlated with the organic carbon content of the soil. These results show the merit of measuring friability for determining the optimum water content for tillage, for quantifying the damage done by different tillage practices, and as a theoretically based index of soil physical quality.

Soil tensile strength as affected by time, water content and bulk density: D. Blazejczak, R. Horn & J. Pytka, International Agrophysics, 9(3), 1995, pp 179–188

Soil tensile strength as affected by time, water content and bulk density: D. Blazejczak, R. Horn & J. Pytka, International Agrophysics, 9(3), 1995, pp 179–188