Soil precompression stress: II a comparison of different compaction tests and stress-displacement behaviour of the soil during wheeling

Precompression stress is often used as a criterion for soil susceptibility to compaction. The objective of this study was to compare precompression stresses derived from different compression test methods and relate these values to measured stress and displacement during wheeling in the field. Precompression stress was measured at three depths at two sites on a Eutric Cambisol in Sweden using in situ plate sinkage test, and by compressing soil cores in the laboratory with sequential loading (oedometer) and at constant displacement speed, respectively. At the same sites, wheeling experiments were carried out where vertical soil displacement and vertical soil stress were measured simultaneously at three depths. No clear relationship between precompression stress and compression method was found. Precompression stress values derived from the oedometer and the in situ plate sinkage test did generally not differ from each other. Precompression stress derived from the constant speed test was either higher (site A, silty clay loam) or lower (site B, silty clay) compared to the other methods. Precompression stress did not work as a threshold value between reversible and irreversible deformation when precompression stress values derived from the different methods were compared with stress and displacement measured in the field during the wheeling experiment. Even when the observed stress was lower than the precompression stress, a residual displacement was observed. The study demonstrates that the precompression stress is not a sharp value but depends on the compression test method and its analysis, and that compression tests might not represent the soil behaviour in the field sufficiently. There is an urgent need to study soil behaviour in the field and link these experiments to soil mechanical (laboratory) tests

Comportamento compressivo de um nitossolo: efeito do tamanho de agregados, do teor de água e da pressão aplicada Compression behaviour of a clay soil: effects of aggregate size, water content and applied pressure

A compactação do solo tem sido assunto de intensivas pesquisas nos últimos anos; no entanto, os mecanismos que implicam o processo de compactação dos solos agrícolas, ainda permanecem pouco conhecidos. A contribuição do tamanho de agregados do solo, bem como o efeito do teor de água e da pressão normal aplicada na compactação e pressão de pré-compactação do solo, foi investigada em um Nitossolo Vermelho eutrófico. Amostras de solo deformado, constituídas por agregados menores que 2,5 mm e de 9,3 a 19,4 mm, foram submetidas a ensaio de compressão uniaxial drenado. O índice de vazios e a pressão de pré-compactação foram avaliados. Os resultados obtidos mostram que o tamanho de agregados teve efeito no processo de compactação do solo. A mudança da compactação do solo pode ser prevista em função do estado inicial do solo, da pressão aplicada e do teor de água.<br>Soil compaction has been the subject of intensive researches in the last ten years, but the mechanisms involved in the soil compaction process remain a little known. The contribution of aggregate size of the soil as well as the effect of water content and normal pressure applied on soil compaction and pressure of pre-compaction were investigated in a Clay soil during this trial. Disturbed soil samples constituted by aggregates less than 2.5 mm and 9.3 to 19.4 mm were submitted to a drained compression uniaxial test. The void ratio and the pre-compaction pressure were also evaluated. The results obtained show that the size of the aggregates had effect on the process of soil compaction. The change of soil compaction can be predicted according to the initial state of soil, the applied pressure and the water content of the soil