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

Highlights:

• Soil deformation due to wheeling depended on the applied loading stresses.

• Compaction led to less reduction in $\epsilon$, $\theta$, and $k_a$ on structured soils.

• The structured soils had more sensitive $u_w$ values than a packed soil in shearing.

• Soil shearing following compaction further deteriorated soil pore functions.

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 (Δ$H$), air/water-filled pores ($\epsilon$/$\theta$), air permeability ($k_a$), and pore water pressure ($u_w$) 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 (Δ$H$: 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 (Δ$H$: 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 $\epsilon$/$\theta$ and maintained 10 times higher $k_a$ 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 $u_w$ (${\mathrm{\Delta }u}_w$) 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 $u_w$.

Introduction

Soil compaction by heavy machinery is one of the major causes of soil degradation in farmland (Pagliai et al., 2003, Hamza and Anderson, 2005). Harmful effects of compaction on soil porosity and pore functions (e.g., air permeability) have been reported broadly (BRUAND and COUSIN, 1995, Pagliai et al., 2000, Richard et al., 2001, Alaoui et al., 2011, Zhai and Horn, 2018, Zhai and Horn, 2019a). During stress-induced deformation, the loss of total porosity (especially of large pores) and the reduction of pore continuity occur simultaneously (Schäffer et al., 2008, Zhai and Horn, 2019a, Chen et al., 2014), which ultimately affect the distribution and mobility of soil air and water (Hartge and Horn, 2016). When the frequency of wheeling increases on the same soil, the effects of compaction are strengthened and may even propagate into deeper layers (Horn et al., 2003, Pulido-Moncada et al., 2019, Krümmelbein et al., 2008, Schjønning et al., 2016).

The compaction-induced deformation is stress dependent and can be divided into elastic and plastic deformations (Hartge and Horn, 1984). Elastic deformation occurs when the external loading stresses (normal stress) are lower than the soil internal strength (e.g., precompression strength). When the loading stress is removed, soil recovers to its initial state with a slightly changed or constant volume and pore functions. In contrast, plastic deformation is irreversible and primarily occurs when external loading stress exceeds the soil precompression strength. Plastic deformation is a common but undesirable phenomenon in farmland because it may permanently decrease pore volume and adversely affect pore functions (Horn, 2021). Within the same (or even shorter) duration, if a certain stress level from static loading is applied repeatedly (known as cyclic loading) the compaction effect would be more obvious. In the plastic deformation zone, cyclic loading further rearranges soil particles with every reloading cycle resulting in more prominent changes in soil structure and pore function than static loading (Peth and Horn, 2006). In some cases, even though the precompression strength is not exceeded, repeated loading still compacts the soil slightly, leading to a cumulative deformation (known as quasi elastic behavior) (O’Sullivan et al., 1999b; Peth and Horn, 2004). Therefore, soil deformation and pore systems respond differently to compaction, depending largely on the loading types (static/cyclic loading) and stress levels.

During the wheeling process, shearing simultaneously occurs with compaction. In contrast to volume-changed deformation by compaction, shearing results in a relatively small or volume-constant deformation with changes in soil shape. Shearing-induced deformation may result in a complete homogenization and decreased soil strength especially if pore water cannot be drained off adequately (Horn, 1976; Wiermann, 1998; Horn and Peth, 2011). Thus, soil becomes weaker if more intense shear occurs in samples with low hydraulic conductivities (Alaoui et al., 2011, Hartge and Horn, 2016). Even if the soil becomes stable after static loading, the following shear process again causes additional compaction and extra changes of the soil pore system (Horn and Peth, 2011). After compaction, some remaining pores (e.g., intra aggregate pores and vertical connected pores) may still exist and function, but the shearing would further affect the size, continuity, and functions of these pores (Jim, 1990, Kirby, 1991, Li and Zhang, 2009, Gregory et al., 2010, Hartge and Horn, 2016).

Similar to compaction, the shearing-induced deformation is also a stress dependent behavior which can be affected by soil precompression strength (Horn, 2003). At low loading stress ratios (the ratio of applied normal stress to precompression strength) the soil expands when sheared while it compresses at high loading stress ratios (Kirby, 1991). Under moist or wet conditions, loading and wheeling (or shearing) cause soil deformation that can be principally defined in 3 stages: Firstly, at low loading stresses, only a slight soil movement and shearing occurs with minor changes in pore functions; secondly, high loading stresses and/or in combination with shear speed increase result in an intense stress/shear induced homogenization and puddling; thirdly, very high loading stresses and shearing finally cause a deep rutting and mostly homogenized muddy soil material within rut formation even deeper down (Horn, 1976). At the last two stages, soil air filled pores vanish and the pore connectivity as well as function will be completely destroyed (BRUAND and COUSIN, 1995, Richard et al., 2001, Cui et al., 2010, Riggert et al., 2019).

Structured and homogenized (or disturbed) arable soils have different deformation behaviors in response to compaction and shearing (Wiermann et al., 2000, Arvidsson et al., 2001, Keller et al., 2012). Under natural conditions, farmland soils undergo a sequence of structure formation processes such as mechanical compression, aggregation due to swelling and shrinkage, and biological gluing. These processes increasingly stabilize soil structure (Huang et al., 2021b), resulting in an over-compaction or recompression state (Hartge and Horn, 2016). The formation of a more rigid structure enables the soil to withstand external stresses. Even under cyclic loading, structured soils show less pore volume reduction and decrease in air permeability than homogenized soils (Peth and Horn, 2006). Besides, structured soils usually have dual pore system which consists of large inter-aggregate pores and small intra-aggregate pores (Barden and Sides, 1970, Alonso et al., 1987, Koliji, 2008, Li and Zhang, 2009). These pores react differently to compaction and shearing depending on loading levels. It is generally true that loading stresses below the precompression strength result in a recoverable deformation and the large inter-aggregate pores still function (Hartge and Horn, 2016). If, however, the applied normal stress exceeds the precompression strength, soil inter-aggregate pores collapse while the originally formed intra-aggregate pores are less affected (Coulon and Bruand, 1989, Li and Zhang, 2009). More importantly, due to biological activities (e.g., root growth and earthworm movement), structured soils have more vertically aligned connected pores (Pagliai et al., 2004, Tavares Filho and Tessier, 2009), which have been proved to be less affected by normal stress (Zhai and Horn, 2018, Zhai and Horn, 2019a).

Soil deformation due to stress applications is a coupled procedure of soil mechanical and hydraulic processes (Hartge and Horn, 2016). It depends not only on externally applied stresses (e.g., compression or shearing stress) but also on soil internal properties (i.e., soil structure, strength, texture, and water/air phases). Considering all strength components, soil pore water pressure affects the total soil strength in both saturated and unsaturated soils (Bishop, 1959, Cui et al., 2010, de Lima et al., 2018) because it determines not only the water saturation degree but also enhances soil swelling in dependence of the altered accessibility of particle surfaces. Thereby the dynamic changes of the soil pore water pressure also reflect the variations of soil pore functions (i.e., air/water-filled pores) induced by applied stresses. More importantly, soil pore water pressure also manifests the dynamical interaction between mechanical and hydraulic processes as a function of applied stress (Fredlund and Rahardjo, 1993, McCarthy, 2007, Horn, 2021). The changes of pore water pressure during a single compaction event (Tang et al., 2009; Peth and Horn, 2010; Zhai and Horn, 2019b) or shearing (Burland, 1990, Tarantino and Tombolato, 2005) have been widely reported. However, due to the difficulty to measure the changes in shear dependent matric potential, the knowledge on the shear-induced soil deformation and the changes of pore water pressure are still limited (Thu et al., 2006, Huang et al., 2021a), especially for structured soils. This is furthermore affected by the various pore systems and their continuity, which influence the pathways of water and air under stress application on the small scale. Thus, it is expected that structured soils and homogenized soils react differently to compaction and shearing, and their deformation behaviors are rarely compared until now.

In a previous study (Huang et al., 2021a), we explored the combined effects of compaction and shearing on soil deformation and changes of pore water pressure of homogenized soils. The findings gave, however, only a texture related hint to these interactions, while soil structure as well as the internal strength that dominates natural farmland soils were not considered. Because both compaction and shearing are stress dependent processes and are related to soil structure, we may expect different soil deformation behaviors and changes in pore system on structured and homogenized soils. Therefore, the objectives of this study were to compare i) soil deformation and the changes in air/water-filled pores and pore water pressure, and ii) their related consequences for air permeability on homogenized and structured soils during compaction and subsequent shearing.