Role of Compaction and Drying-Wetting Cycles on the Capability of Precompression Stress to Indicate Load History of Heavily Disturbed Soils

Wetting and drying are common processes in agricultural soils, which change particle rearrangement, cementation and related properties. Some recent studies have noted that soil precompression stress (sigma(p)) - the soil capacity to support stress - can be affected by wetting and drying cycles and should be seen as state variable instead of a soil mechanical characteristic. In this study, we investigated how wetting and drying cycles change the Cip of remolded samples, i.e. soil without distinct loading history. Soil material from three highly weathered soils with different textures (62, 35 and 15% of clay) of the most common soil classes of Brazil were used. The soils were milled and then used to create two set of samples with different compactness. In one set, named not compacted (NC), the milled material was just poured (loose material) into the cylinders, while in the other set the milled material inside the cylinders was compacted (COMP). In the NC samples, zero and three drying (under 33 kPa matric suction) and wetting (saturation) cycles (0D-S and 3D-S) were applied. The COMP samples were subjected to 0, 2, 4, and 6 D-S cycles. In the NC samples, the sigma(p) (35-49 kPa) in 0D-S was similar to the matric stress soil samples had experienced prior to the compression test (33 kPa), but sigma(p) increased to 60-92 kPa after 3D-S. In the COMP samples, sigma(p) increases ranged between 65 and 53 kPa from 0 D-S to 2 D-S and then decreased to a value similar to that of 0 D-S again after 6 D-S. Changes of Cip tended to be more remarkable in the soils with 62 and 35% of clay, but only a weak association between sigma(p) and texture was noticed. These results indicate that internal forces a soil experiences during shrinkage and swelling are key drivers of the sigma(p) parameter of the topsoil of agricultural areas. Thus, one Cip value cannot characterize a site over time as it might be altered thoroughly during the year and thus fail in reflecting documented load history by means of traffic.