Shear strength of surface soil as affected by soil bulk density and soil water content
Introduction
Soil erosion and surface sealing are among the most deleterious processes to agriculture and environment (Sumner, 1995). During rainfall, raindrop compaction and soil suspension movement by water result in high shear stresses, leading to an intensive local deformation in soil erosion (Ghadiri and Payne, 1986, Rose et al., 1990). As a concomitant process the soil surface transfers into a layer, ranging from 1 to 10 mm, and results in higher bulk density, lower porosity and lower hydraulic conductivity (Moore, 1981) and in an increase in soil strength (Bradford et al., 1992). Consequently, shear strength of surface soil can be proposed as a measure of soil resistance to water erosion.
Soil strength was linked to soil erosion (Torri et al., 1987), soil aggregate detachment (Nearing and Bradford, 1985, Torri et al., 1987) and seal formation (Bradford et al., 1992). Tensile strength of soils has been reported to decrease with decreasing bulk density and increasing water content. Nearing et al. (1991) found that the tensile strength ranged between 0.93 and 3.23 kPa at small bulk density and high water content, which was much higher than typical shear stress (<5 Pa) applied in rill erosion. Shainberg et al. (1994) suggested that the binding forces between particles at the soil–water surface were much weaker than the tensile forces in bulk soil. Soil particles at the interface are not confined, as the soil particles are within the bulk soil. Thus, the clay particles are free to swell and possibly even disperse, resulting in smaller cohesion forces between adjacent particles. Conventional methods of determining soil strength include cone penetrometer, shear vane, torsional shearbox, direct shear method. However, these methods cannot measure the properties at a soil surface with required resolution and the parameters were not sufficient to explain the mechanical dynamics during soil water erosion.
With the first attempt to measure soil strength of surface soil, Collis-George et al. (1993) reported a resin plate method. This method was quick, inexpensive and the results were highly reproducible. However, the failure plane was not easily defined as the author indicated making the estimate of the sheared area difficult. In addition, it led to a tension crack and the wave-like failure surface at the edges of the square shear plate.
The objective of this paper was to provide a new method to measure shear strength at soil surface at a range of low normal stresses and interpret the parameters derived from the shear tests as affected by different initial bulk density, soil water content and soil. Penetration resistance was also measured so as to compare and confirm the effects on soil strength detected by the new device if any.
Section snippets
Soil preparation
The samples were covering the soil parent materials of quaternary red clay (Q), sandstone (S), granite (G) and purple mudstone (P), making up the dominant parent materials in subtropical China. The soil samples were taken from the top layer (0–15 cm). Complete soil properties determined with routine methods (ISSAS, 1978) been reported elsewhere (Zhang and Horn, 2001), but selected physical properties are given in Table 1. The subscripts represent the land uses, i.e. c for cultivation, p for
Theory
The accepted shear strength equation for saturated soils in its linear function of effective stress is given as the Mohr–Coulomb’s equationwhere τ is the shear strength, c′ the effective cohesion, σ the total stress, uw the pore water pressure and φ′ the effective angle of shearing resistance.
Shear strength equation for unsaturated soils was developed in terms of two independent stress variables (Fredlund et al., 1978):where σ−ua is the net normal
Penetration resistance
The soil cores showed a negligible variation of bulk density in between the samples (Table 2). Fig. 2 showed the results of penetration resistance of the soils at different bulk density at the same matric potential and at the same water content. The effect of bulk density on penetration resistance was significant for all tested soils, especially at the same water content although the difference of bulk density was small, ranging from 0.01 to 0.09 g cm−3 (Table 2).
For a given soil, the higher the
Discussion
Soil strength depends not only on soil and the measuring condition but also on the method of measurement itself (Bradford et al., 1992). Penetrometer is used to determine an overall soil strength within soil to the 10 mm depth in this study. The new shear device is used at surface soil to determine the parameters of soil shear strength. They are not comparable because the physical properties of the related soil volume or area were different and the parameters derived from these methods have
Conclusion
The new shear device for measurement of soil shear strength at soil surface is very simple, cheap and easy to use. It is reproducible at a range of small normal stresses and the effects of bulk density, soil water content and soil type on soil shear strength were detected in most cases though the difference in bulk density was small, ranging from 0.01 to 0.09 g cm−3. The values of surface boundary angle were lower for the soils of higher bulk density at the same matric potential (−30 hPa), while
Acknowledgements
We thank the Alexander von Humboldt Foundation for the fellowship provided to Dr. Zhang Bin and the National Foundation of Sciences in China (NSFC) (Grant Nos. 49701008 and 40071044) for the funded research project. We thank Mr. J. Lohse for building the penetrometer and the shear device.
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