Stress relaxation of five different soil samples when uniaxially compactedat different water contents

The average size of rainfed and irrigated agricultural farms in Spain has g rown steadily over the past two decades. This has called for the use of mac hinery of higher field capacity and greater weight that in turn requires a high drawbar power. All this has resulted in soil changes such as an increa sed compaction and compactibility. The confined uniaxial compression test w as used to assess compaction and viscoelastic behavior of five soil samples from different agricultural areas of Spain. The bulk density-compression s tress line was fitted to a three-parameter multiplicative compaction model and viscoelastic behavior was evaluated by means of stress-relaxation tests . The objectives were to determine to what extent the parameter coefficient s of the compaction model equation and the relaxation of the stress induced in the compacted soil were influenced by the type of soil, its water conte nt and the compression stress applied. Gravimetric crater contents of 5, 10 , 15, 20 and 25% were considered, and maximum normal stresses of 50, 100, 2 00 and 400 kPa were applied to the soils in a universal testing machine. Th e soil samples considered differed in texture, sandy loam (SL), sandy clay loam (SCL), loam (L), clay (C) and silt-loam (SiL), and organic matter cont ent. The slope of the bull: density-compression stress line at zero normal stres s was strongly dependent on soil water content and plasticity index; wherea s the slope of the curve at high applied normal stresses was influenced by soil moisture but not by soil plasticity. The viscoelastic behavior of the soils compared was dictated by their water content and plasticity index, as well as by the compression stress applied. The stress relaxation rate at t ime t = 0 was scarcely influenced by water content. In fact, the rate remai ned constant over the water content range from 10 to 20% (w/w) at values th at were higher than those obtained at 5 and 25% (w/w), which in turn were i dentical to each other. The stress-relaxation rate was also found to increa se linearly with the logarithm of the compression stress. On the other hand , the residual stress decreased linearly with increasing water content. How ever, the: latter increased linearly with compression stress. Increasing so il plasticity resulted in decreasing relaxation rate and increasing residua l stress. Therefore, the more plastic the soil was the lower was the rate a t which stress relaxation started and the smaller was the amount of stress dissipated. (C) 2001 Elsevier Science B.V. All rights reserved.