Elsevier

Soil and Tillage Research

Volume 184, December 2018, Pages 118-127
Soil and Tillage Research

Estimation of unsaturated shear strength parameters using easily-available soil properties

https://doi.org/10.1016/j.still.2018.07.006Get rights and content

Highlights

  • Relationship between unsaturated shear strength and soil properties was investigated.

  • Multiple-linear regression was applied to derive pedo-transfer functions (PTFs).

  • PTFs using easily soil properties could estimate the unsaturated shear strength accurately.

Abstract

Unsaturated soil shear strength is an important parameter in soil erosion and management. Measurement of unsaturated shear strength at field scale is difficult, time-consuming, and very costly. This study was conducted to investigate the relationship between unsaturated shear strength parameters and soil properties, and to predict the unsaturated shear strength parameters (effective cohesion, c', angle of effective internal friction, φ' and angle of internal friction related to matric suction, φb) using multiple-linear regression (MLR). Direct shear tests were performed at combinations of three normal stresses of 25, 50 and 100 kPa, and four matric suctions of 0, 10, 30 and 50 kPa (i.e., 12 tests per each soil) to determine the shear strength parameters in 14 soils. Soil properties including particle size distribution (sand, silt, and clay percentages or geometric mean diameter, dg and geometric standard deviation, σg), organic matter content (OM), calcium carbonate content (CaCO3), compactness indices (bulk density, ρb and, relative bulk density, ρb-rel), and mean weight diameter of aggregates (MWDdry, MWDwet), structural stability indices (aggregate stability, AS, stability index, SI and index of crusting, Ic) were determined and used as predictors in MLR models. Strong negative correlations were found between c' and φ'. The c' positively correlated with clay content. Significant negative correlation was observed between c' and sand fractions and dg. Significant positive correlation was obtained between φ' and fractions of sand and dg. The φ' negatively correlated with clay, fine silt content (FSi), MWDdry, and AS. The φb had no significant correlation with soil properties, indicating that φb is independent of soil properties and basically is affected by matric suction. Clay, coarse sand (CS) and very fine sand (VFS) were applied in the model for predicting c'. Clay had a positive and, CS and VFS had negative effects on c'. Pedotransfer functions (PTFs) using the fine sand (FS) and VFS as predictor could estimate the φ' accurately, so that they entered to PTFs with positive signs. In addition, the φb was predicted by parameter Ic only, so that it had negative effect on φb. Overall, better prediction models were developed for φ' than for c' and φb.

Introduction

Soil shear strength is a useful dynamic measure for evaluating soil erodibility especially in water erosion, and an important input parameter in the process-based soil erosion models (Knapen et al., 2007a, b; Léonard and Richard, 2004; Torri et al., 2013). Splash detachment processes (Nearing and Bradford, 1985; Brunori et al., 1989) and soil resistance to concentrated flow erosion (Foster et al., 1995; Knapen et al., 2007a, b) are closely linked to soil shear strength. Shear strength also affects the water movement, tilth, plant growth, biological activity of the soil (Blanco-Canqui et al., 2005; Eudoxie et al., 2012). Shear strength would also affect the load support capacity (Imhoff et al., 2004) and traction required to pull farm implements (McKyes, 1985).

Soil shear strength is the maximum shear stress that a soil can sustain before shear failure happens. Mohr–Coulomb equation is commonly used to quantify the shear strength of saturated soils (Johnson et al., 1987):τ=c+σntanφwhere τ (kPa) is the shear strength, c (kPa) is cohesion, σn (kPa) is the normal stress acting on the failure surface and φ (°) is the angle of internal friction. Cohesion, cohesive shear strength is due to chemical bonding between soil particles and surface tension within the water films (Knapen et al., 2007a, b; Morgan, 1986). Frictional shear strength (σn tan φ) is owing to internal friction between soil particles, that depends on the normal stress acting on the failure surface. However, determination of the shear strength of unsaturated soil is very difficult and complicated. It can be described using the extended Mohr–Coulomb failure criterion proposed by Fredlund et al. (1978), which is written as follows:τf=c'+(σ-ua)ftanφ'+(ua-uw)ftanφbwhere τf (kPa) is the unsaturated shear strength, c' (kPa) and φ' (°) are the effective cohesion and effective friction angle, respectively, σ-uaf (kPa) is the net normal stress on the failure plane at failure, ua-uwf (kPa) is the matric suction at failure, σ is the total normal stress, ua and uw are the pore-air and pore-water pressures, respectively, φb (°) is an angle indicating the rate of change in shear strength relative to changes in matric suction (Fredlund et al., 1996). Hence, total or apparent cohesion (c) in unsaturated soil consists of two components; i.e., one due to physicochemical cohesion, c' and the other due to matric suction (Zhang et al., 2001), which could be written as:c=c'+ua-uwtanϕb

When an unsaturated soil becomes saturated, matric suction is equal to zero, so the total cohesion approaches to the effective cohesion (Zhang et al., 2014).

Soil shear strength is affected by several soil and environmental properties such as particle size distribution (soil texture) (Gilley et al., 1993; Horn and Fleige, 2003; Knapen et al., 2007b), organic matter content (Wuddivira et al., 2013), bulk density (Gilley et al., 1993; Zhang et al., 2001), aggregation, water content/matric suction (Al Aqtash and Bandini, 2015; Gilley et al., 1993; Zhang et al., 2001; Zhou et al., 2016), network of plant roots and vegetation cover (Fattet et al., 2011; Fan and Su, 2008), and tillage systems (Knapen et al., 2007a). Soil texture is mostly used as primary indicator of soil resistance against erosion. It can affect soil shear strength either through frictional forces in coarse-texture soils or through cohesive forces in fine-textured soils (Knapen et al., 2007b; Shainberg et al., 1994). Havaee et al. (2015) reported that shear strength parameters (c and φ) strongly depended on soil particle size distribution and gravel content. They found a positive correlation between c and fine clay content. However, c was negatively correlated with sand and gravel contents. Meanwhile, significant positive correlation between φ and gravel content indicates the roughness effect of coarse particles on frictional shear strength.

The influence of soil organic matter (OM) on shear strength could be explained through mechanism of modifying the cohesiveness of soil particles and aggregate stability (Blanco-Canqui et al., 2005). Inconsistent effects of OM on soil mechanical behavior have been reported. For example, some researches showed an increase in soil shear strength with an increment in OM due to enhanced cohesive forces between the soil particles (Rachman et al., 2003; Wuddivira et al., 2013). On the contrary, OM may reduce soil shear strength because of increasing soil porosity (or decreasing bulk density) (Horn and Fleige, 2003; Kay and Angers, 1999). Rachman et al. (2003) found that the higher OM and aggregate stability resulted in greater soil shear strength. Cruse and Larson (1977) reported that c' and φ', and as a result shear strength increased with increase of bulk density and matric suction. They stated that changes in bulk density and matric suction would alter solid particle-to-particle contact, and the contact relationships between solid particles and liquid films, respectively. Soil wetting (i.e., decrease of matric suction) reduces contact relationships between solid particles and liquid films, and as a consequence reduces soil shear strength (Cruse and Larson, 1977). Horn and Fleige (2003) pointed out the shear strength parameters were also influenced by matric suction. They indicated that with an increase in matric suction, c and φ increased which is dependent to soil texture and structure. Zhou et al. (2016) determined unsaturated shear strength parameters of a silty sand soil using direct shear test and found that with increment of matric suction, total cohesion increased, φ did not change, and φb decreased non-linearly. They showed that shear strength significantly increased with an increase in net normal stress or matric suction.

Several techniques such as torsional shear apparatus (Johnson et al., 1987; Koolen and Kuipers, 1983), shear vane (Bradford and Grossman, 1982), drop-cone penetrometer (Bradford et al., 1992; Rachman et al., 2003; Wuddivira et al., 2013), direct shear box (Zhou et al., 2016), Zhang’s method (Zhang et al., 2001), in situ direct shear box (Havaee et al., 2015), and tri-axial test (Nearing and Bradford, 1985; Khalili et al., 2004) are used to measure soil shear strength. However, majority of these techniques are rather complicated and time-consuming, and are difficult to use in large scale. On the other hand, some methods are not practical to determine unsaturated shear strength parameters accurately. Therefore, indirect methods, pedotransfer functions (PTFs), such as multiple-linear regression (MLR) are used to estimate the shear strength. The PTFs predict difficult-to-measure soil properties (e.g., hydraulic properties, unsaturated shear strength) using routinely-available properties as predictors (Bouma, 1989; Wösten et al., 2001).

Some PTFs have been derived for predicting shear strength parameters of saturated soil (e.g., Khalilimoghadam et al., 2009; Havaee et al., 2015). Also there are some studies on unsaturated soil shear strength in civil engineering applications (Habibagahi and Bamdad, 2003; Zhou et al., 2016). Khanlari et al. (2012), Mousavi et al. (2011) and Sudha Rani et al. (2013) predicted shear strength and engineering properties of soil using regression methods and artificial networks. However, there was no published research on prediction of the unsaturated shear strength parameters (i.e., c', φ' and φb) in agricultural soils. Therefore, the objectives of this study were to: i) investigate the relationship between unsaturated shear strength parameters (c', φ' and φb) and soil properties, ii) estimate c', φ' and φb using MLR, and iii) determine factors that are most effective on c', φ' and φb in agricultural soils.

Section snippets

Soil sampling

Fourteen soil samples with different intrinsic properties were selected from the Razavi Khorasan province, Northeast of Iran (between 33°19′ to 37°45′N longitudes and 56°19′ to 61°16′E latitudes). Rangeland, farmland, and orchard were the dominant land uses of the studied region. The selected soils had wide ranges of particle size distribution, organic matter content (OM) and calcium carbonate content (CaCO3) as seen in Table 1. Soil samples were selected based on different intrinsic properties

Shear failure envelopes at different matric suctions

The Mohr–Coulomb shear failure envelopes corresponding to different matric suction values are shown in Fig. 1 for two silty loam soils containing the low (Fig. 1a) and high OM values (Fig. 1b). It was found that the shear strength values for the OM-rich soil were greater than for the low-OM soil at normal stress of 25 kPa for all matric suctions. The graph of shear strength envelopes vs. net normal stress showed an approximately linear behavior. This finding is supported by the results of Zhou

Conclusions

  • 1

    This study was conducted to investigate the relationship between unsaturated shear strength parameters and some soil properties, and to predict the unsaturated shear strength parameters (c', φ' and φb) using multiple-linear regression (MLR) in soils. It was found that shear failure envelopes for OM-rich and clayey soils had greater c and smaller φ' compared to those for low-OM and coarse-texture soils, respectively. Parameter φb in OM-rich soils and coarse-texture soils were greater than in

References (71)

  • A. Hemmat et al.

    Shear strength of repacked remoulded samples of a calcareous soil as affected by long-term incorporation of three organic manures in central Iran

    Biosyst. Eng.

    (2010)
  • R. Horn et al.

    A method for assessing the impact of load on mechanical stability and on physical properties of soils

    Soil Tillage Res.

    (2003)
  • R. Horn et al.

    Prediction of mechanical strength of arable soils and its effects on physical properties at various map scales

    Soil Tillage Res.

    (2005)
  • T. Keller et al.

    Estimation of reference bulk density from soil particle size distribution and soil organic matter content

    Geoderma

    (2010)
  • A. Knapen et al.

    Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

    Soil Tillage Res.

    (2007)
  • A. Knapen et al.

    Resistance of soils to concentrated flow erosion: a review

    Earth-Sci. Rev.

    (2007)
  • M. Lebert et al.

    A method to predict the mechanical strength of agricultural soils

    Soil Tillage Res.

    (1991)
  • J. Léonard et al.

    Estimation of runoff critical shear stress for soil erosion from soil shear strength

    Catena

    (2004)
  • M.R. Mosaddeghi et al.

    Soil compactibility as affected by soil moisture content and farmyard manure in central Iran

    Soil Tillage Res.

    (2000)
  • J. Niewczas et al.

    Index of soil aggregate stability as linear function value of transition matrix elements

    Soil Till. Res.

    (2003)
  • D. Torri et al.

    The recurring cycles of biancana badlands: erosion, vegetation and human impact

    CATENA

    (2013)
  • J.H.M. Wösten et al.

    Pedotransfer functions: bridging the gap between available basic soil data and missing soil hydraulic characteristics

    J. Hydrol.

    (2001)
  • M.N. Wuddivira et al.

    Influence of cohesive and disruptive forces on strength and erodibility of tropical soils

    Soil Tillage Res.

    (2013)
  • H.Q. Zhang et al.

    Cohesion in unsaturated sandy soil and the influence of organic matter

    Soil Technol.

    (1990)
  • B. Zhang et al.

    Shear strength of surface soil as affected by soil bulk density and soil water content

    Soil Tillage Res.

    (2001)
  • Y. Zhang et al.

    Influences of degree of saturation and strain rate on strength characteristics of unsaturated granular subbase course material

    Trans. Geotech.

    (2014)
  • W.H. Zhou et al.

    Measurement of unsaturated shear strength parameters of silty sand and its correlation with unconfined compressive strength

    Measurement.

    (2016)
  • ASTM D3080 / D3080M-11

    Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions

    (2011)
  • H. Blanco-Canqui et al.

    Strength properties and organic carbon of soils in the north appalachian region

    Soil Sci. Soc. Am. J.

    (2005)
  • H.L. Bohn et al.

    Soil Chemistry

    (2001)
  • J. Bouma

    Using soil survey data for quantitative land evaluation

    Adv. Soil Sci.

    (1989)
  • J.M. Bradford et al.

    In-situ measurement of near-surface soil strength by the fall-cone device

    Soil Sci. Soc. Am. J.

    (1982)
  • D. Chakraborty et al.

    Triaxial testing to determine the effect of soil type and organic carbon content on soil consolidation and shear deformation characteristics

    Soil Sci. Soc. Am. J.

    (2014)
  • R.M. Cruse et al.

    Effect of soil shear strength on soil detachment due to raindrop impact

    Soil Sci. Soc. Am. J.

    (1977)
  • J.H. Dane et al.

    Water retention and storage

  • Cited by (0)

    View full text