M. Tahmasbi, A. Hemmat, M. Vafaian, M. R. Masaddeghi,
Volume 12, Issue 44 (7-2008)
Abstract
For prevention of soil compaction, knowledge of allowable compression stress limit (compaction strength) in soil is important. Pre-compaction stress ( σpc) was introduced as soil compaction strength and often used as a criterion for evaluation of soil susceptibility to compaction. In this research, pre-compaction stress was measured for a sandy loam soil with plate sinkage (PST) and confined compression (CCT) tests. To prepare soil samples with different initial compactness, two soil water contents (17 and 19%db) and six pre-loading stresses (0, 25, 50, 100, 150 and 200 kPa) were used. The effects of soil water content and pre-loading stress on estimated pre-compaction stress were studied using a factorial experiment in a completely randomized design with three replications. The σpcvalues were significantly influenced by loading combination and soil water content. For PST, pre-load increase and higher soil water content resulted in higher and lower values of σpc , respectively. However, predicted σpc value increased with higher soil water content for CCT. The results also showed that the σpc predicted with PST was accurate, whereas the values obtained with CCT were 4.5 (at 17 %db) and 8.5 (at 19 %db) times higher than the applied pre-loads. Overall, the findings indicated that σpc prediction depends on the compression test, and PST could be a suitable method for soil pre-compaction stress (compaction strength) determination in sustainable soil management, i.e., soil trafficability and tillage. The PST method is also suitable to assess the effect of managing factors on pre-compaction stress.
N. Aghilinategh, A. Hemmat, M. Sadeghi,
Volume 15, Issue 55 (4-2011)
Abstract
One important parameter in determining the allowable stress to prevent soil compaction is precompaction stress (σpc). If the stress induced into the soil due to agricultural machinery traffic is lower than the σpc, the possibility of the irreversible (plastic) deformation is low. In this study, plate sinkage test (PST) and confined compression test (CCT) were used to determine the σpc of the disturbed soil samples obtained from the topsoil of a long-term organic amendment experiment. In the organic amendment experiment, organic manures (sewage sludgeو compost, farmyard manure) at three rates (25, 50 and 100 t ha-1) and one inorganic fertilizer combination (250 kg urea ha-1 and 250 kg ammonium phosphate ha-1) were added to a silty clay loam soil for seven years under wheat-corn rotation. The effect of organic and inorganic fertilizers and gravimetrical water contents (17.1 and 20.9% db) on the σpc values was determined using two statistical designs: 1) factorial design and 2) orthogonal contrasts in a completely randomized design with three replications. The σpc values were estimated by the following methods: 1) Casagrande, 2) maximum curvature and 3) intercept of virgin compression line (VCL) with the x-axis at zero strain. The results showed that in both tests, the σpc values for treatments amended with organic manures had over-estimations and the values of the over-estimation were increased as the manure application rate increased. The values of the over-estimations for CCT were higher than for PST. Therefore, it can be concluded that the results of the PST are more reliable. The σpc values estimated by any of the three procedures depend on the manure treatment as well as the initial water content of the soil. For determining the stress at the threshold of the compaction for the un-manure soils (control and inorganic fertilizer) or the treatments amended with low application rate (25 t ha-1) of the organic manures, the PST and the Casagrande procedure can be recommended. In contrast, for the soils amended with high application rates (50 and 100 t ha-1), the PST and the maximum curvature procedure can be used.
