Compressibility of Oxisol aggregates under no-till in response to soil water potential

The system of no-till sowing stands out as being a technology that suits the objectives of more rational use of the soil and greater protection against the erosion. However, through till, any of it, occurs modifications of the soil's structure. This current work aims to study the influence of the energy state of the water and of the organic matter on the mechanism of compaction of Red Oxisol under no-till management system. Humid and non-deformed sample were collected in horizon AP of two agricultural areas under no-till, with and without rotation of cultures. In the laboratory, these samples were broken into fragments and sifted to obtain aggregates of 4 to 5 mm sized, which were placed in equilibrium under four matrix potentials. Thereafter, they were exposed to uni-dimensional compression with pressures varying from 32 to 1,000 kPa. The results in such a way show that the highest compressibility of aggregates both for the tilling with rotation of cultures as for the tilling without rotation of cultures, occurred for matrix potential -32 kPa (humidity of 0.29-0.32 kg kg-1, respectively), while the minor occurred for the potentials of -1 and -1,000 kPa (humidity of 0.35 and 0.27 kg kg-1, respectively), indicating that this soil should not be worked with humidity ranging around 0.29 to 0.32 kg kg-1 and the highest reduction of volume of aggregates was obtained for the mechanical pressures lower than 600 inferior kPa, indicating that these soils showed to be very influenced by compression, when exposed to mechanical work. Also, the aggregates of soil under no-till and rotation of crops presented higher sensitivity to the compression than the aggregates of soil under no-till and without rotation of crops, possibly for having better structural conditions given to a higher content of organic matter.

Structural changes in latosols of the cerrado region: II - soil compressive behavior and modeling of additional compaction

Currently in Brazil, as in other parts of the world, the concern is great with the increase of degraded agricultural soil, which is mostly related to the occurrence of soil compaction. Although soil texture is recognized as a very important component in the soil compressive behaviors, there are few studies that quantify its influence on the structural changes of Latosols in the Brazilian Cerrado region. This study aimed to evaluate structural changes and the compressive behavior of Latosols in Rio Verde, Goiás, through the modeling of additional soil compaction. The study was carried out using five Latosols with very different textures, under different soil compaction levels. Water retention and soil compression curves, and bearing capacity models were determined from undisturbed samples collected on the B horizons. Results indicated that clayey and very clayey Latosols were more susceptible to compression than medium-textured soils. Soil compression curves at density values associate with edaphic functions were used to determine the beneficial pressure (σ b) , i.e., pressure with optimal water retention, and critical pressure (σcrMAC), i.e., pressure with macroporosity below critical levels. These pressure values were higher than the preconsolidation pressure (σp), and therefore characterized as additional compaction. Based on the compressive behavior of these Latosols, it can be concluded that the combined preconsolidation pressure, beneficial pressure and critical pressure allow a better understanding of compression processes of Latosols.

Soil compressibility and least limiting water range of a constructed soil under cover crops after coal mining in Southern Brazil

The use of cover crops affects the support capacity of soil and least limiting water range to crop growth. The objective of this study was to quantify preconsolidation pressure (sigma(p)), compression index (CI) and least limiting water range (LLWR) of a reclaimed coal mining soil under different cover crops, in Candiota, RS, Brazil. In the experiment, with randomized blocks design and four replicates, the following cover crops (treatments) were evaluated: Hemarthria altissima (Poir.) Stapf & C.E. Hubbard, treatment 1 (T1), Paspalum notatum Flugge, treatment 4 (T4), Cynodon dactilon (L) Pers., treatment 5 (T5), control Brachiaria brizantha (Hochst.) Stapf, treatment 7 (T7) and without cover crop treatment 8 (reference treatment, T8). Soil compression and least limiting water range were evaluated with undisturbed samples at a depth of 0.00-0.05 m. In order to evaluate parameters of soil compressibility, the soil samples were saturated with water and subjected to -10 kPa matric potential and then submitted to a uniaxial compression test under the following pressures: 25, 50, 100, 200, 400, 800 and 1600 kPa. Cover crops decreased the preconsolidation pressure of constructed soils after coal mining and the greatest soil reclamation was obtained with the H. altissima cover crop, where the lowest degree of soil compactness and soil load capacity were observed. Soils cultivated under H. altissima or B. brizantha presented the highest least limiting water range and these two cover crops generated similar soil critical bulk density obtained by least limiting water range and soil load support capacity. (C) 2012 Elsevier B.V. All rights reserved.

Effect of compaction on microbial activity and carbon and nitrogen transformations in two oxisols with different mineralogy

The use of machinery in agricultural and forest management activities frequently increases soil compaction, resulting in greater soil density and microporosity, which in turn reduces hydraulic conductivity and O2 and CO2 diffusion rates, among other negative effects. Thus, soil compaction has the potential to affect soil microbial activity and the processes involved in organic matter decomposition and nutrient cycling. This study was carried out under controlled conditions to evaluate the effect of soil compaction on microbial activity and carbon (C) and nitrogen (N) mineralization. Two Oxisols with different mineralogy were utilized: a clayey oxidic-gibbsitic Typic Acrustox and a clayey kaolinitic Xantic Haplustox (Latossolo Vermelho-Amarelo ácrico - LVA, and Latossolo Amarelo distrófico - LA, respectively, in the Brazil Soil Classification System). Eight treatments (compaction levels) were assessed for each soil type in a complete block design, with six repetitions. The experimental unit consisted of PVC rings (height 6 cm, internal diameter 4.55 cm, volume 97.6 cm³). The PVC rings were filled with enough soil mass to reach a final density of 1.05 and 1.10 kg dm-3, respectively, in the LVA and LA. Then the soil samples were wetted (0.20 kg kg-1 = 80 % of field capacity) and compacted by a hydraulic press at pressures of 0, 60, 120, 240, 360, 540, 720 and 900 kPa. After soil compression the new bulk density was calculated according to the new volume occupied by the soil. Subsequently each PVC ring was placed within a 1 L plastic pot which was then tightly closed. The soils were incubated under aerobic conditions for 35 days and the basal respiration rate (CO2-C production) was estimated in the last two weeks. After the incubation period, the following soil chemical and microbiological properties were detremined: soil microbial biomass C (C MIC), total soil organic C (TOC), total N, and mineral N (NH4+-N and NO3--N). After that, mineral N, organic N and the rate of net N mineralization was calculated. Soil compaction increased NH4+-N and net N mineralization in both, LVA and LA, and NO3--N in the LVA; diminished the rate of TOC loss in both soils and the concentration of NO3--N in the LA and CO2-C in the LVA. It also decreased the C MIC at higher compaction levels in the LA. Thus, soil compaction decreases the TOC turnover probably due to increased physical protection of soil organic matter and lower aerobic microbial activity. Therefore, it is possible to conclude that under controlled conditions, the oxidic-gibbsitic Oxisol (LVA) was more susceptible to the effects of high compaction than the kaolinitic (LA) as far as organic matter cycling is concerned; and compaction pressures above 540 kPa reduced the total and organic nitrogen in the kaolinitic soil (LA), which was attributed to gaseous N losses.

Avaliação da compressibilidade de um Nitossolo Vermelho distroférrico sob sistema plantio direto, preparo convencional e mata nativa

Os efeitos do tráfego de máquinas nos atributos do solo de acordo com o tempo de adoção do sistema plantio direto são ainda pouco pesquisados em ambientes tropicais, e muitas dúvidas ainda persistem sobre a variação dinâmica da estrutura do solo e a sua interação com máquinas e equipamentos. Objetivou-se com este estudo avaliar o efeito do tempo de adoção do sistema plantio direto, comparativamente com área de mata nativa e de preparo convencional, usando os modelos de compressibilidade do solo. O estudo foi realizado em um Nitossolo Vermelho distroférrico, sob mata nativa (MN), preparo convencional (PC), plantio direto com um ano (PD1), plantio direto com quatro anos (PD4), plantio direto com cinco anos (PD5) e plantio direto com 12 anos (PD12). Amostras indeformadas e deformadas foram coletadas em duas profundidades (0-5 e 10-15 cm). O tempo de adoção do sistema plantio direto alterou o comportamento compressivo dos solos em ambas as profundidades, por meio das mudanças na pressão de preconsolidação. A profundidade de 0-5 cm apresentou menor capacidade de suporte de carga do que a profundidade de 10-15 cm. A profundidade de 0-5 cm, em todos os sistemas de manejo, mostrou-se mais susceptível à compactação em relação à profundidade de 10-15 cm. Os sistemas de plantio direto e convencional apresentaram a capacidade de suporte de carga crescente na seguinte ordem: PD5 < PD12 < PD1 < PD4 @ PC, para a profundidade de 0-5 cm e para a profundidade de 10-15 cm: MN @ PD12 < PC @ PD4 < PD5, enquanto o sistema PD1 apresentou comportamento diferenciado.

Consolidômetro: equipamento pneumático-eletrônico para avaliação do estado de consolidação do solo

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

CHARACTERIZATION OF THE MICRO-SITE CONDITIONS FROM RESTORED AREAS WITH DIFFERENT AGES

Regeneration microsites are characterized by diverse combinations of attributes which assure the best conditions for seed germination and seedling establishment. By understanding these attributes, we can contribute to determining better management methodologies for reestablishing ecological process in sites under restoration. Thus, we sought to characterize and differentiate the micro-site conditions of restoration plantings to indentify likely physical-chemical limitations for the establishment of native tree species in the forest understory. This study was carried out in reforestation plantings with different ages (10, 22 and 55 years). The physical-chemical characterization of the micro-site of regeneration of the study areas was carried out by evaluating the soil compression level, porosity, humidity, organic matter and nutrients content and granulometry, as well as litter dry mass and canopy cover. An increase on the canopy cover and soil porosity, humidity, clay and organic matter content were observed in the oldest restored areas, as well as a decrease in soil compression. Thus, these findings demonstrated that the evaluated microsite properties are in process of restoration. Therefore, microsite conditions for seedling establishment become even more similar to reference ecosystems as restoration planting evolve.

Natural fibers for soil reinforcement

The reinforcement of soil is defined as an effective and reliable technique to improve strength and stability. For this propose, the use of natural fibers has been commonly. Over the past years, a series of studies have been performed in order to investigate the influence of randomly oriented fibers, especially for compressible clayey soils. However, less attention has been given to the reinforcing of sandy materials, as well as the use of oriented fibers meshes in order to improve mechanical behaviour. The main aim of this study is to identify the influence that different percentages of fibers, as well as the use of meshes of oriented fibers, has on soil mechanical behaviour. For this purpose, unconfined compression tests with local strain measurements were performed on a silty sand reinforced with Sisal fibers and a comparative study between randomly oriented and 0° and 90° fibers is presented.

Desarrollo sustentable de geoestructuras compuestas por suelo loéssico compactado mejorado con agregado de materiales estabilizantes. Sustainable development of compacted loess soil geo-structures with addition of stabilizing materials.

Los suelos estabilizados mediante compactación, permiten obtener materiales con ventajas ténicas y economicas en diferentes tipos de obras de ingeniería. Ejemplos de su uso se tiene en bases viales de autopistas, rutas o calles urbanas, pistas de aterrizaje, barreras de contención para enterramientos sanitarios o lagunas de estabilización, apoyos de plateas para fundación de edificios, losas industriales, entre otras aplicaciones. Las fallas en este tipo de construcciones pueden resultar en catástrofes ambientales, sociales y elevadas pérdidas económicas, por lo que resulta de gran importancia optimizar el diseño e incrementar la seguridad de este tipo de construcciones. Las obras con estas características involucran grandes volúmenes y/o superficies que requieren controles sistemáticos durante su desarrollo, a los fines de garantizar el cumplimiento de las propiedades de los materiales establecidos en la etapa de diseño. De esta forma, es necesario contar con ensayos de campo sencillos, confiables y eficientes que permitan identificar propiedades físicas, mecánicas e hidráulicas. Las geoestructuras generadas mediante la compactación del suelo próximo al sector de construcción pueden funcionar adecuadamente, con reducidos costos de material y transporte. Su estabilización puede ejecutarse en forma natural, o con la incorporación de agregados minerales como bentonita, cal o cemento. Estas incorporaciones mejoran las propiedades hidráulicas y mecánicas del material, optimizando el comportamiento requerido para la obra. Para establecer la forma en la que estos minerales modifican el comportamiento del suelo local compactado deben realizarse investigaciones especiales con los materiales involucrados. En el ámbito internacional existen numerosas investigaciones sobre comportamiento de suelos compactados, no obstante, si bien aportan antecedentes para la planificación de estudios locales, sus resultados no pueden trasladarse de manera directa. Las características propias del suelo local constituye la principal variable debido a la diversidad en las propiedades geotécnicas de cada Región. Esta investigación, se focaliza en el empleo de suelos limosos de la formación loéssica de la zona central de Argentina. Los suelos de la llanura cordobesa poseen comportamientos particulares, los cuales son contemplados en los diseños presentados como resutado de las investigaciones internacionales. Esta particularidad se relaciona con su inestabilidad, lo que los clasifica como suelos colapsables. Los resultados obtenidos en este trabajo podrán ser extendidos a una gran superficie de la Provincia de Córdoba y a la Región Pampeana en general, a los fines de establecer recomendaciones de diseño y construcción para la confección de Pliegos de Especificaciones Técnicas de diferentes tipos de obras públicas y privadas. El estudio contempla la ejecución de un plan experimental a escala de laboratorio y campo. Los materiales corresponden a suelo limosos puros, y diferentes agregados tales como bentonita, cal y cemento. Se planifican ensayos para evaluar el desempeño del material, a partir de la confección de muestras preparadas con diferentes condiciones de compactación (energía, humedad y método), y en forma de mezcla con los distintos tipos de agregados. Se realizarán ensayos de permeabilidad en celdas de pared rígida y flexible, junto a ensayos mecánicos de compresión confinada, simple y triaxial. Para el trabajo experimental de campo se prevé la ejecución de terraplenes de prueba instrumentados con tensiómetros e infiltrómetros para evaluar el comportamiento hidraúlico en el tiempo, junto con ensayos de penetración y plato de carga para la caracterización mecánica. En forma conjunta se propone el desarrollo de modelos numéricos de caracterización hidromecánica. Stabilized soils by compaction, produce materials technical and economic advantages in different types of engineering works. For example, road bases in highways, roads or city streets, containment barriers for sanitary landfill or stabilization ponds, foundation support of building, industrial flat, and other applications. Failures can result in environmental catastrophes, social, and economic loss, so it is important to optimize the design and increase the safety of such buildings. These works involve large surfaces that require systematic tests during construction, so it is necessary to have simple field tests, reliable and efficient to identify physical, mechanical and hydraulic properties. The geo-structures generated by local soil compaction have reduced material and transportation costs. Stabilization can be naturally, or with the addition of mineral aggregates as bentonite, lime and cement. These additions improve the hydraulic and mechanical properties of the material. So, special investigations should be conducted with the materials involved. There are many international studies on compacted soils behavior but their results can not be transferred directly due to the particularities of regional soils. For this research silty soils of central Argentina are the main focus. The soils of Córdoba plains are instability, so are classified as collapsible soils. The results obtained in this work may be extended to a large area of the Province of Cordoba and the Pampas region in general, in order to establish design and construction recommendations. The study includes laboratory and field tests. The materials are pure silty soil, and different aggregates such as bentonite, lime and cement. Tests are planned to evaluate the performance. Laboratory includes rigid and flexible wall cells, confined, triaxial and simple compression tests. For field experimental instrumented embankments will be constructed. A numerical hydromechanical model will be developed.

Structural changes in latosols of the cerrado region: I - relationships between soil physical properties and least limiting water range

The agricultural potential of Latosols of the Brazilian Cerrado region is high, but when intensively cultivated under inappropriate management systems, the porosity can be seriously reduced, leading to rapid soil degradation. Consequently, accelerated erosion and sedimentation of springs and creeks have been observed. Therefore, the objective of this study was to evaluate structural changes of Latosols in Rio Verde, Goiás, based on the Least Limiting Water Range (LLWR), and relationships between LLWR and other physical properties. Soil samples were collected from the B horizons of five oxidic Latosols representing the textural variability of the Latosols of the Cerrado biome. LLWR and other soil physical properties were determined at various soil compaction degrees induced by uniaxial compression. Soil compaction caused effects varying from enhanced plant growth due to higher water retention, to severe restriction of edaphic functions. Also, inverse relationships were observed between clay content and bulk density values (Bd) under different structural conditions. Bd values corresponding to critical soil macroporosity (BdcMAC) were more restrictive to a sustainable use of the studied Latosols than the critical Bd corresponding to LLWR (BdcLLWR). The high tolerable compression potential of these oxidic Latosols was related to the high aeration porosity associated to the granular structure.

MODELING OF SOIL LOAD-BEARING CAPACITY AS A FUNCTION OF SOIL MECHANICAL RESISTANCE TO PENETRATION

Estimation of soil load-bearing capacity from mathematical models that relate preconsolidation pressure (σp) to mechanical resistance to penetration (PR) and gravimetric soil water content (U) is important for defining strategies to prevent compaction of agricultural soils. Our objective was therefore to model the σp and compression index (CI) according to the PR (with an impact penetrometer in the field and a static penetrometer inserted at a constant rate in the laboratory) and U in a Rhodic Eutrudox. The experiment consisted of six treatments: no-tillage system (NT); NT with chiseling; and NT with additional compaction by combine traffic (passing 4, 8, 10, and 20 times). Soil bulk density, total porosity, PR (in field and laboratory measurements), U, σp, and CI values were determined in the 5.5-10.5 cm and 13.5-18.5 cm layers. Preconsolidation pressure (σp) and CI were modeled according to PR in different U. The σp increased and the CI decreased linearly with increases in the PR values. The correlations between σp and PR and PR and CI are influenced by U. From these correlations, the soil load-bearing capacity and compaction susceptibility can be estimated by PR readings evaluated in different U.

Soil compaction due to forest harvest operations

The objective of this work was to determine, through the use of the bearing capacity model, the traffic effects of the forest harvest operations on the preconsolidation pressure (sigmap), during one cycle of the eucalyptus plantation. The work was conducted using undisturbed soil samples, collected at the surface of the A horizon and in the top of the B horizon of an Udult (PA), Aquox (FX) and Udox (LA) soils. The undisturbed soil samples were used in the uniaxial compression tests. The soil sampling was done before and after the harvest operations. The operations performed with the Forwarder caused greater soil compaction than the ones done with the Feller Büncher and Harvester. The percentage of soil samples, in the region with additional soil compaction, indicated that the Udult was the soil class more susceptible to soil compaction, followed by the Aquox and Udox. Despite Udult is the more susceptible to soil compaction, the regeneration of the soil structure in this soil class was more efficient than in Aquox. The percentage of soil samples with sigmap values in the region with additional soil compaction in 1996, 1998 and 2004, after harvest operations, indicated a sustainable forest exploration in this period.

Characterization of soil parameters of two soils of Rio Grande do Sul in modeling the prediction of tractive effort

The interaction between the soil and tillage tool can be examined using different parameters for the soil and the tool. Among the soil parameters are the shear stress, cohesion, internal friction angle of the soil and the pre-compression stress. The tool parameters are mainly the tool geometry and depth of operation. Regarding to the soils of Rio Grande do Sul there are hardly any studies and evaluations of the parameters that have importance in the use of mathematical models to predict tensile loads. The objective was to obtain parameters related to the soils of Rio Grande do Sul, which are used in soil-tool analysis, more specifically on mathematical models that allow the calculation of tractive effort for symmetric and narrow tools. Two of the main soils of Rio Grande do Sul, an Albaqualf and a Paleudult were studied. Equations that relate the cohesion, internal friction angle of the soil, adhesion, soil-tool friction angle and pre-compression stress as a function of water content in the soil were obtained, leading to important information for use of mathematical models for tractive effort calculation.

Behaviour of displacement, cast-in-place piles on collapsible soil

The paper presents the results of instrumented compression load tests carried out in three uncased cast-in-place piles (pounded piles, called apiloadas in Brazil) which is largely used in interior part of the state of São Paulo. The tests were conducted with the soil in the natural condition and after soaking with water, before loading, in order to evaluate the influence of the soil collapsibility in the results. The values of the ultimate loads (total, point and lateral loads) obtained in the tests are presented and compared with the values predicted by empirical methods that utilize SPT and CPT results.

Application of the modified Cam-Clay model to a residual soil

The modified Cam - Clay model was used to model experimental results of a saturated residual sandy soil from Sao Carlos - SP. Triaxial compression tests were performed using Bishop - Wesley cell with internal transducers to measure axial and radial strains. It was observed that the model fairly fitted experimental results, specially the principal stress difference at critical state. In general it was observed a good qualitative agreement between experimental and predicted strain values, considering compression or expansion of the samples. However, in all the stress path used, but 100 degrees and 140 degrees, the model yielded strains larger than that measured in the tests.