Soil structure effects on deformation, pore water pressure, and consequences for air permeability during compaction and subsequent shearing

Abstract

Soils may react to the impact of wheeling with volume-changed deformation by compaction and volume-constant deformation by shearing. The different deformation mechanisms with their deteriorated effects on soil pore functions have been studied frequently, but less is known about the compaction/shearing-induced deformation on structured soils as compared with homogenized substrates. In order to document both the effect of aggregation on soil deformation behaviors as well as the sensitivity due to texture, compaction and shearing measurements were performed on soil samples from the A- horizon of a homogenized Luvisol (a silt loam with a bulk density of 1.37 g cm-3) and two structured Gleysols (clay loam soils with bulk densities of 1.34 g cm-3 and 1.11 g cm-3). The changes of vertical settlement (ΔH), air/water-filled pores (ε/θ), air permeability (ka), and pore water pressure (uw) during compaction and subsequent shearing were studied. Results showed that the soil deformation behaviors due to compaction and shearing depended highly on the level of applied normal stress, especially on structured soils. At a normal stress of 50 kPa, which was smaller than the precompression strength, the structured Gleysols had only minor compaction-induced deformation (ΔH: 0∼1 mm) and a contractive shear behavior. At a high normal stress of 200 kPa, which exceeded the precompression strength, both homogenized and structured soils displayed greater compaction-induced deformation (ΔH: 3.5∼6.0 mm) and a dilative shear behavior. Compared with static loading, cyclic loading resulted in further deformation and dilative shear behaviors in both structured and homogenized soils. In addition, the structured soils showed a smaller decrease in ε/θ and maintained 10 times higher ka value than homogenized soils. However, shearing reduced the inter-aggregate pore continuity and enhanced the relative functionality of the sheared intra-aggregate pores, as was proofed by the more pronounced changes of uw (Δuw) in the structured soils (from -93.0 hPa to +335.6 hPa) compared with that in the homogenized silt loam (from +2.1 hPa to +140.2 hPa). In conclusion, the well-structured clayey soils exhibited less deformation during compaction compared with the homogenized (tilled) soil with coherent structure and more silty texture. The dynamic stress application and shearing resulted in more intense weakening of soil structure because the accessibility of particle surfaces for mobilized water coincides with an enhanced stress dependent swelling and sliding due to the rapidly increased uw.