- Email: [email protected]
Shear strength of an unsaturated loam soil as affected by vetiver and polyacrylamide
Soil & Tillage Research 194 (2019) 104331
Contents lists available at ScienceDirect
Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Shear strength of an unsaturated loam soil as affected by vetiver and polyacrylamide
T
⁎
Elham Amiri, Hojat Emami
Ferdowsi University of Mashhad, Soil Science dept. Faculty of Agriculture, 9177948978, Mashhad, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Angle of internal friction Angle of internal friction related to matric suction Cohesion Direct shear test Soil rehabilitation
Shear strength is an important soil mechanical property that its improvement is one of the rehabilitation program for the reduction of soil erosion and degradation. The objective of this study was to investigate the effect of polyacrylamide (PAM) and the vetiver system as a cheap and long-term bioengineering method and their combination on unsaturated shear strength parameters (effective cohesion, c' (kPa), angle of effective internal friction, φ' (°) and angle of internal friction related to matric suction, φb (°)) of a loam soil. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water [0.2% (V0P2) and 0.4% (V0P4)], and simultaneous presence of vetiver and two concentrations of PAM (VP2 and VP4). 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 (12 tests per each treatment) to determine the shear strength parameters. It was found that vetiver and PAM decreased apparent angle of internal friction (φ) and φ', and increased c', total cohesion (c), φb, and as a result increased unsaturated shear strength. However, the positive effect of vetiver on shear strength was greater than that of PAM. It seems that PAM and vetiver enhanced the contact area, inter-particles bonds, aggregation, and inter-aggregate porosity, and as a consequence, increased the soil effective saturation, effective stress, cohesion and shear strength. Also, when matric suction increased the c and shear strength increased although the φ increased slightly. Simultaneous application of vetiver and 0.4% of PAM resulted in maximum shear strength, indicated although vetiver can increase the cohesive strength, it would be more pronounced when PAM was simultaneously applied. Simultaneous application of vetiver and PAM through increasing soil shear strength can be suggested as the rehabilitation program to reduce soil erosion and degradation.
1. Introduction Soil strength is one of the most dynamic soil mechanical properties that is important to tilth, plant growth, and control erosion. The strength of soil is the greatest stress that it can sustain. The shear strength of a soil is its resistance against shearing stresses. Soil shear strength is the internal resistance per unit area that the soil mass can offer against sliding along any plane inside it (Das, 1990). It can be proposed as a measure of the soil resistance to deformation by continuous displacement of its individual soil particles. Shear strength in soils depends primary on interactions between particles, so that shear strength within a soil matrix is the result of resistance to movement at
inter-particle contacts, due to particle interlocking through physicochemical bonds (cohesion, c (kPa)) and inter-particle (internal) friction (as quantified by angle of internal friction, φ (°)) (Craig, 2004). So, soil drives its shear strength from two sources; cohesion between particles (stress independent component) and frictional resistance between particles (stress dependent component). It is as a useful dynamic index for evaluating soil erodibility (Zhang et al., 2001; Knapen et al., 2007a; Torri et al., 2013) and resistance against seedling emergence and root growth (Marsh and Dixon, 1991). Blanco-Canqui et al. (2005) pointed out that knowledge about shear strength dynamics is crucial to explain the mechanical behavior and structural sustainability of agricultural soils. Improvement of the soil mechanical properties such as shear
Abbreviations: CCE, calcium carbonate equivalent (kg 100 kg−1); c, total cohesion (kPa); c', effective cohesion (kPa); OM, organic matter (kg100 kg−1); PAM, polyacrylamide; SWCC, soil water characteristic curve; ua, pore air pressure (kPa); uw, pore water pressure (kPa); (ua-uw), matric suction (kPa); V0P0, control treatment; V0P2, 0.2dissolved PAM treatment; V0P4, 0.4% dissolved PAM treatment; VP0, vetiver plants; VP2, combination of vetiver and PAM level 0.2; VP4, combination of vetiver and PAM level 0.4%; σn, normal stress (kPa); (σn-ua), net normal stress (kPa); σ', effective stress (kPa); τ, Shear strength (kPa); τf, unsaturated shear strength (kPa); φ, apparent angle of internal friction (°); φ', effective angle of internal friction (°); φb, angle of internal friction related to matric suction (°) ⁎ Corresponding author. E-mail address: [email protected] (H. Emami). https://doi.org/10.1016/j.still.2019.104331 Received 29 October 2018; Received in revised form 3 July 2019; Accepted 6 July 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
strength is one of the rehabilitation program in order to reduce soil erosion and degradation. Appropriate amendments and vegetation may improve soil shear strength through physical bonds formed across the contact areas of particles and chemical bonds or cementation between the soil particles. Considering the importance of vegetation, the bioengineering methods have been proposed to increase land stability and to control soil degradation and erosion. In this regard, vetiver grass technology (VGT) i.e. using of vetiver grass (Vetiveria zizanioides L. Nash) was first developed by World Bank for soil and water conservation in India in the 1980s (CTA, 1989). Vetiver is a tropical perennial plant and naturally adapted to low and high lands, and a wide range of soil types and climatic conditions (Truong, 2002). Also, the massive and deep roots of vetiver with high mean tensile strength of 75 MPa (Diti, 1999) has been caused to be suitable for soil stabilization. It has been found that vetiver is successful to improve soil physical properties (Materechera, 2010; Oku et al., 2011), control soil erosion and improve the land stability (Amiri et al., 2017; Donjadee et al., 2010). In addition, soil amendments such as polymers have been shown to be beneficial for soil quality and to resist against degradation (Levy and Warrington, 2015) and to improve soil physical properties (Lentz, 2015; de Melo et al., 2016). Anionic polyacrylamide (PAM) is water-soluble, non-toxic polymer and one of the most effective polymers commonly used to increase structural stability, to prevent crust formation and soil erosion, and consequently to reduce soil degradation (Green and Stott, 1999). The PAM has been widely applied since the 1990s due to its easy preparation (Levy et al., 1992). Kumar and Saha (2011) reported the various aspects of PAM effectiveness on increasing soil stability, and reducing soil erosion. Fan and Su (2008) investigated the role of roots in the shear strength of root-reinforced soils at high water contents and found that root efficiency, defined as the ratio of increment in shear strength due to presence of roots over the initial shear strength of soils, may be more than 1.0 at high degrees of saturation. In addition, their results indicated the approximately linear relationship between the increment in shear strength due to the presence of roots and the tensile strength of roots per unit area of soil. Furthermore, Fattet et al. (2011) studied the effects of plant vegetation on soil resistance against erosion and relationship between aggregate stability and shear strength and obtained a significant positive linear relationship between mean weight diameter (MWD) of water-stable aggregates and soil cohesion, while no relationship was found with internal angle of friction. Ma’ruf (2012) also investigated shear strength of Apus bamboo root-reinforced soil and concluded that peak shear strength of the root-reinforced soil increased when soil-root volume ratio increased. Though the soil-root volume ratio was only up to 5%, the increment in the peak shear strength was up to 55%. Isa et al. (2017) determined the shear strength of carbonaceous shale as affected by vegetation and pointed out that presence of vegetation roots decreased the c and enhanced the φ, and consequently would result in an overall increment in the shear strength. Akbarzadeh et al. (2009) used soil binders (PAM and gypsum) and found that addition of the soil amendments to soil surface led to shear strength increase due to improvement of the soil physical properties and production of stable aggregates. It has been found that vetiver and PAM have good efficiency on improving the soil physical properties and to control erosion (e.g., Amiri et al., 2017). The unsaturated shear strength is one of the important parameters affecting soil erosion, therefore, evaluating the influence of vetiver and PAM on unsaturated shear strength of soil is necessary. In addition, the effect of PAM and vetiver on unsaturated shear strength parameters (i.e., c', φ' and φb) have not been studied so far. Thus, the objective of this study was to investigate the effect of PAM and vetiver system (as a cheap and long-term bioengineering method) and their combination on parameters of unsaturated shear strength (i.e., c', φ' and φb) of a loam soil.
Fig. 1. The soil failure envelope; c', effective cohesion; φ, angle of internal friction.
2. Theory Soil shear strength is the maximum shear stress that a soil can sustain before shear failure happens. The shear strength of saturated soils is determined using the Mohr–Coulomb failure (Johnson et al., 1987):
τ = c + σntanφ
(1)
Where, 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 (Fig. 1). Thus, the shear strength of a soil can be described by two parameters i.e. cohesion and internal friction angle (McKyes, 1985). Cohesion is due to chemical bonds between soil particles and surface tension within the water films around soil particles (Knapen et al., 2007a; Morgan, 2005). Frictional shear strength (σn tan φ) is a result of internal friction between soil particles, that depends on the normal stress acting on the failure surface. The shear strength of unsaturated soil can be described using the extended Mohr–Coulomb failure criterion which is expressed as (e.g. Fredlund et al., 1978):
τf = c′ + (σ − ua )f tan φ′ + (ua − u w )f tan φb
(2)
where τf (kPa) is the unsaturated shear strength, c' (kPa) is the effective cohesion, φ' (°) is the effective angle of internal friction, (σ − ua )f (kPa) is the net normal stress on the failure plane, (ua − u w )f (kPa) is the matric suction at failure, σ is the total normal stress on the failure plane, ua and u w are the pore-air and pore-water pressures, respectively, φb (°) is an friction angle indicating the rate of change in shear strength relative to changes in matric suction (Fredlund et al., 1996). Hence, total cohesion (c) in unsaturated soils consists of two components; effective or true cohesion (c') due to physicochemical cohesion, and the apparent cohesion ((ua − u w )tanφb ) due to matric suction (Zhang et al., 2001), which could be written as:
c = c′ + (ua − u w )tanφb
(3)
When an unsaturated soil becomes saturated, matric suction is equal to zero, and the total cohesion approaches to the effective cohesion (Zhang et al., 2014). 3. Materials and methods 3.1. Experimental site and treatments The study was conducted in a loam soil in the Agriculture Campus, Ferdowsi University of Mashhad, Iran (36°18′23′'N, 59°31′29′'E). The 2
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Table 1 Physical and chemical properties of the studied soil. Bulk density Mg cm 1.60
Texture
−3
Sand
Silt
Clay
OM
CCE
−1
Loam
———————————————————kg 100 kg ———————————————————— 48 41 11 0.21
12.6
EC dS m 0.90
pH −1
7.70
*OC: Organic carbon; CCE: Calcium carbonate equivalent; EC: Electrical conductivity.
500, 1000, and 1500 kPa (Dane and Jan, 2002).
average rainfall and temperature in the studied area are 257 mm and 15°C, respectively. The minimum and maximum temperatures were 14 and 37 °C, respectively. The soil of the study area was classified as Xeric Torriorthents according to Soil Taxonomy (Soil Survey staff, 2006). Some properties of the Ap horizon of the studied soil, as determined by standard analytical methods (Klute, 1986; Page et al., 1986), are shown in Table 1. Organic matter content (OM) and calcium carbonate equivalent (CCE) were measured using the Walkley-Black wet-oxidation (Nelson, 1982) and back-titration methods (Nelson and Sommers, 1986), respectively. Soil texture was determined using the pipette method (Gee and Bauder, 1986). Experimental plots (1 m × 1 m) were prepared in the given area. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water (0.2% (V0P2) and 0.4% (V0P4) which are approximately equal to 20 and 40 kg ha−1 by considering a soil layer with 20-cm thickness, respectively), and combination of vetiver and above PAM concentrations (i.e., VP2 and VP4). The properties of the used PAM (provided by Chemicals Tianrun Company, Benghu City, Anhui Province, China) are given in Table 2. In addition, V0P0 (no PAM and vetiver) was regarded as control treatment. Vetiver (Vetiveria zizanioides L. Nash) seedlings were planted at a distance of 35 cm × 35 cm in each plot on 1 May 2015. They were irrigated during the first one month, every two weeks and after then every month. Water content for irrigation was equal to field capacity at the roots depth. Vetiver plants were fully grown after 3 months. PAM solution was sprayed on the surface of plots using a sprinkler about 45 days after vetiver planted i.e. on 15 Jun 2015. Finally, the soil samples were collected on 1 August 2015.
3.4. Soil shear strength measurement Direct Shear apparatus (Fig. 2a) was used to measure the soil shear strength and its parameters. The unconsolidated undrained (UU) direct shear tests were performed using a shear box (Fig. 2b) on the undisturbed soil specimens at different matric suctions of 0, 10, 30, and 50 kPa (Fredlund and Vanapalli, 2002). The shear box was used with an inner cross section of 60 × 60 mm and height of 20 mm. Undisturbed samples with cross section of 60 × 60 mm and height of 20 mm were inserted in the shear box. The data of SWCC were used to determine the soil water contents at pre-defined matric suctions. For adjusting the water status of soil samples at pre-defined matric suctions (ua–uw), the required amount of water (by weighing) was added to the soil specimens in a desiccator and they were equilibrated for a few days in plastic bags. The weight of samples were about 90–100 grams. Normal stresses of 25, 50, and 100 kPa were applied on the soil samples with pre-adjusted matric suction. Then by applying the shear load the specimens were sheared at a constant rate of 1.001 mm min−1. The shear strength was considered equal to the shear stress at relative horizontal displacement of 10% because all of the failure patterns were compressive or ductile. Three replications were considered for each combination of normal stress and matric suction. Totally, 216 direct shear tests (6 treatments × 3 normal stresses × 4 matric suctions × 3 replications) were performed. The Mohr–Coulomb failure criterion was applied to calculate the apparent and effective shear strength parameters. The parameters c and φ were obtained from the intercept of the Mohr-Coulomb failure envelope (Eq. 1) by plotting the shear strength vs. net normal stress pairs at a specific matric suction. The unsaturated soil shear strength parameters (c', φ' and φb) of the model proposed by Fredlund et al. (1978) (see Eq. 2) were estimated using optimization technique of Excel® Solver (Microsoft Office, 2013) by minimizing the sum of square errors (SSE):
3.2. Soil sampling For each treatment, 36 (3 normal stress × 4 matric suction × 3 replication) undisturbed samples were collected from the surface layer (0–15 cm) using special metal square molds with an inner cross section of 60 × 60 mm and height of 20 mm. In total, 216 square undisturbed samples (3 replication for each treatment) were collected. The inner wall of the sampling molds was lubricated by oil to minimize the friction between the soil and mold. Also 18 undisturbed core samples (5 cm in diameter and 5 cm in height) were collected (3 cylinders for each treatment) from the surface layer to determine the soil water characteristic curve (SWCC).
SSE =
∑ (τ measured − τpredicted )2
(4)
where τmeasured and τpredicted are measured and predicted (by Eq. 2) values of shear strength, respectively. Therefore, unsaturated soil shear strength parameters were derived from 12 corresponding values of τf, σn–ua and ua–uw (3 normal stresses × 4 matric suctions) for each treatment and replicate.
3.3. Measurement of soil water characteristic curve The SWCC was determined on the undisturbed samples using pressure plate apparatus at matric suctions of 5, 10, 30, 50, 100, 300,
3.5. Statistical analyses Table 2 Some properties of the studied anionic polyacrylamide (PAM). Properties of PAM
Specification
Appearance Hydrolysis degree (Ionicity) (%) Molecular weight (Mg mol−1) Density (Mg m−3) pH value Water insolubility (%) Solid content (%)
White granular powder 30–35 15–20 ≥0.67 6–8 ≤ 0.08 95
Data analyses were performed based on the randomized complete block design in a factorial arrangement with three replications using SPSS software ver. 23 (SPSS 23 Inc.). Analysis of variance was used to difference between treatments. Mean’s comparsions were made using Duncan’s multiple range test at P < 0.05 to compare the parameters of unsaturated shear strength (c', φ ', and φb) between the studied treatments.
3
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Fig. 2. The picture of direct shear apparatus (a), the shear box (b), and the schematic of the shear box (c).
4. Results and discussion
particles, would enhance the aggregate stability and increased the interaggregate meso- and macro-pores (Amiri et al., 2017). Therefore, greater effective saturation, effective stress and total cohesion between soil particles as a consequence of root effects on soil pore size distribution, could ultimately increase the soil shear strength. The results indicated that the positive effect of vetiver on shear strength was greater than that of PAM. Isa et al. (2017) found that increasing water uptake by roots and evapotranspiration are reasons for reducing pore water pressure thereby, increasing effective stress and subsequently enhancing effective shear strength. Akbarzadeh et al. (2009) reported that addition of soil amendment i.e. PAM, gypsum, and mixture of them to soil surface, increased shear strength due to improvement of soil physical properties and production of stable aggregates. Also, shear strength increased with an increment of PAM concentration, yet the influence of gypsum on shear strength was greater than that of PAM. Ma’ruf (2012) also expressed that the bamboo root system could increase the soil shear strength.
4.1. Shear failure envelopes at different matric suctions The Mohr-Coulomb shear failure envelopes (means of replicates) at different matric suction values for the studied treatments are presented in Fig. 3. It was observed that the studied treatments increased shear strength compared to the control at all three normal stresses of 25, 50, and 100 kPa and matric suctions of 0, 10, 30, and 50 kPa. Moreover, it was found that all the treatments enhanced total cohesion (c, intercept of the Mohr-Coulomb failure envelope), and shear strength at normal stress of 25 kPa, compared to control (Table 3). However, different trends were obtained for the studied treatments, so that the maximum and minimum increments of c were obtained for VP4 and V0P0 (control) treatments, respectively. In addition, it was found that at matric suctions of 0 and 10 kPa, shear strength in all the treatments slightly increased with increasing normal stress compared to the control, indicating that the slope of shear envelope or internal friction angle (φ) decreased (Table 3). This finding indicates greater inter-particle forces (i.e., c) and lesser inter-particle friction (i.e., φ) due to presence of vetiver roots and PAM. The reduction of φ for different treatments did not show similar trend at all studied matric suctions. It indicated the different effects of treatments on φ, so that maximum decrease of φ was found for the VP4 treatment. In addition, similar to Amiri et al. (2018) it was found that the c and φ, and consequently shear strength increased when matric suction increased in all the treatments; but in vetiver and PAM treatments the φ increased lower than the c. According to Eq. (3), total cohesion in the unsaturated soil depends on effective cohesion and capillary (apparent) cohesion (Spoor, 1975). The former is due to inter-particle bonds (physicochemical attractions) (Koolen and Kuipers, 1983). The latter is due to capillary and suction forces as film cohesion that relates to net inter-particle forces generated within a matrix of unsaturated soil due to the combined effects of negative pore water pressure and surface tension (Aluko and Koolen, 2000; Scholtès et al., 2009). The PAM application and vetiver cultivation increased total cohesion. Although vetiver increased c greater than the PAM. PAM increased c through increasing the contact area of soil particles and chemical bonds between them. Moreover, hydrophilic molecules of PAM decreased the surface tension between soil particles, creating water bridges, and therefore increased the effective stress and consequently increased the total cohesion. The roots of vetiver by binding the soil
4.2. Unsaturated shear strength parameters The results of ANOVA for the parameters of unsaturated shear strength (c', φ' and φb) showed that vetiver and PAM treatments had significant effects on c', φ' and φb (P < ≤ 0.05) (Table 4). Means’ comparisons showed that the PAM application and vetiver cultivation increased c' significantly (Fig. 4a). The minimum (3.0 kPa) and maximum (7.8 kPa) values of c' were obtained in control and VP4 treatments, respectively. The c' significantly increased with an increment of PAM concentration without vetiver cultivation (i.e., V0P2 and V0P4 treatments). Effective cohesion is due to physicochemical bonds between soil particles. Adsorption of long PAM polymer chains on the external surfaces of soil particles would bind the particles and could enhance the cohesion through chemical bonds and creating bridges between particles. In addition, vetiver cultivation significantly increased the c' compared to control and PAM treatments. These findings support the important role of vetiver roots in increasing contact area and binding of particles, and subsequently effective cohesion. The effects of roots on contact area and cohesion of soil particles, and consequently shear strength can be explained by two mechanisms: (i) chemical bonds and inter-particles’ contacts due to plant roots exudates and (ii) physical forces i.e. either loosening (Yoo et al., 2011) or radial compression of surrounding soil during root penetration (Bengough et al., 2006). It seems that the extensive networks of vetiver roots in the 4
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Fig. 3. Shear failure envelopes at different matric suctions as affected by the studied treatments. V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM, respectively; VP0, vetiver plants; and VP2 and VP4, combination of vetiver and PAM levels 0.2 and 0.4%, respectively. Table 3 Means’ comparisons of soil apparent cohesion (c) and angle of internal friction (φ) in the studied treatments at matric suctions of 0, 10, 30, and 50 kPa. Treatment
Matric suction (kPa) 0
V0P0 V0P2 V0P4 VP0 VP2 VP4
Table 4 Results of ANOVA for parameters of unsaturated soil shear strength as affected by vetiver and PAM treatments.
c (kPa) 3.9e 5.1d 6.2c 8.5b 10.5a 11.1a
Treatment
10 φ (°) 29.7a 29.7a 28.8b 27.9c 27.0d 27.0d
c (kPa) 4.5f 5.7e 6.7d 9.3c 10.8b 11.7a
30 φ (°) 31.0b 30.6c 29.7d 23.4f 32.2a 27.9e
c (kPa) 5.4f 6.8e 7.5d 9.9c 11.2b 12.5a
df
50 φ (°) 31.4c 32.0b 31.4c 33.4a 29.7d 31.8d
c (kPa) 6.5f 7.7e 8.4d 11.3c 12.6b 13.5a
φ (°) 31.4d 33.0b 32.2c 32.2c 33.4a 32.2c
Vetiver PAM Vetiver × PAM Errors CV (%)
1 2 2 12
Mean square c' (kPa)
φ' (°)
φb (°)
58.9*** 3.9*** 1.2*** 0.002 1.0
2.6*** 0.3*** 0.1*** 0.001 0.1
38.3*** 4.5*** 5.0*** 0.003 0.6
* c': effective cohesion; φ': effective angle of internal friction; φb: matric suction angle of internal friction. † ***stand for significance at 99.9% probability levels.
* Means with same letter in each column are not significantly different (P > 0.05). V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM respectively; VP0, vetiver plant; and VP2 and VP2, combination of vetiver and PAM levels P2 and P4 respectively.
effect will be more pronounced when PAM (chemical factors) is simultaneously applied. In addition, the studied treatments significantly decreased the φ' parameter (Fig. 4b), so that φ' for the control was 30.6° and when the concentration of PAM increased its value reduced to 30.2° (V0P4). Effective frictional angle is a result of frictional forces met by soil particles when they are forced to slide over one another or to move out of interlocking positions (Morgan, 1986). Particle roughness is another source of inter-particles friction. The PAM application caused the soil particles to be closer together and inter-particles cohesion increased. However, it seems that the PAM located between soil particles,
soil matrix (i.e., physical forces) and compression of the root-surrounding soil mainly increased the c'. However, organic compounds released by vetiver roots into the soil might act as binding agents of particles and subsequently increased the c'. Moreover, significant difference was found between VP0 and combination of vetiver and PAM (VP2 and VP4). However, the differences between VP2 and VP4 in terms of soil cohesion were not significant. These results indicated although vetiver (biologic factors) can increase the cohesive strength, the 5
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
(Amiri et al., 2018). Fan and Su (2008) also concluded that the effective cohesion between particles in the root-reinforced soils was greater than root-free soils. They found that greater shear strength of root-reinforced soils compared to root-free soils was almost equal to the increase in apparent cohesion, and roots had little influence on the angle of internal friction. Endo and Tsuruta (1969) also indicated that roots increased soil cohesion, while they had no effect on the angle of internal friction. Operstein and Frydman (2000) also found that for chalky and clay soils that stabilized with roots, angle of internal friction essentially remained constant, while the apparent cohesion increased in the rooted soil. Therefore, the increase of shear strength in the root-reinforced soil may be equal to the increase in apparent cohesion. Vetiver and PAM significantly increased the φb, indicating that the increase of shear strength with an increment in the matric suction in the studied treatments was greater than the control (Fig. 4c). PAM significantly increased φb in comparison with control. However, φb decreased for V0P4 treatment compared to V0P2. It seems that 0.2% of PAM has greater positive influence on φb. As mentioned before, shear strength increased with increasing matric suction; whereas PAM is hydrophilic, it resulted in greater contact area of particles, therefore PAM would increase the effective stress and as a result increased the shear strength. Moreover, adsorption of the long chains of PAM polymer on soil particles and aggregates, increased the aggregate stability and created meso- and macro-pores due to flocculation and binding of the soil particles (Amiri et al., 2017). These changes enhanced the effective saturation and effective stress originated from internal forces (i.e., matric suction) and ultimately increased the φb. It seems that higher concentration of PAM can clog the micropores and some mesopores; as a result, the effective saturation and effective stress, and shear strength were decreased. Therefore, the role of high concentration of PAM on shear strength at higher matric suction decreases. Thus the V0P4 had little effect on φb compared to V0P2 and subsequently V0P4 decreased φb. Vetiver also significantly increased φb, and its influence was greater than that of PAM (Fig. 4c). In fact, the vetiver roots by enhancing the inter-particles bonds and creating inter-aggregate pores increased the degree of saturation and increased effective stress, consequently shear strength increased. In addition, simultaneous application of vetiver and PAM increased φb, so that VP2 increased φb compared to the control, while it reduced φb compared to VP0. It seems that at low concentration of PAM (0.2%), some agglomerated soil particles due to PAM may be migrated to micropores that created by vetiver roots and clogged them. Therefore, VP2 decreased effective saturation, effective stress, and consequently φb compared to VP0. On the other hand, VP4 led to maximum increase in φb (11.1°) compared to the control (5.2°). These results indicated that high concentration of PAM would strengthen the influence of vetiver on aggregation, inter-aggregate porosity, and the φb, and thus on soil shear strength.
Fig. 4. Means’ comparisons of parameters of unsaturated soil shear strength as affected by vetiver and PAM treatments: (a) effective cohesion (c'), (b) effective angle of internal friction (φ'), (c) angle of internal friction related to matric suction, φb; Means with the same letters are not significantly different (P > 0.05); V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM, respectively; VP0, vetiver plants; and VP2 and VP4, combination of vetiver and PAM levels 0.2 and 0.4%, respectively.
although increased the contact points and inter-connection of particles, but decreased the internal roughness due to lubricating effect and consequently decreased the internal friction forces and φ. Moreover, significant difference was observed between two concentrations of PAM in without vetiver cultivation (i.e., V0P2 and V0P4). However, the decreasing effect of vetiver on φ' (29.7°) was considerably more than that of PAM. Vetiver roots probably has released organic exudates which enhanced soil particles to be closer together, interlocked and stabilized. Moreover, organic compounds released by vetiver roots into the soil might have lubricating effect and reduced the internal roughness and consequently, decreased the internal friction. The VP0 had no significant difference with VP2, but its difference with VP4 was significant. The results indicated that the effects of PAM and vetiver on binding soil particles and enhancing cohesion were much more than those on the internal friction between particles, and consequently cohesive forces between particles were greater than the frictional forces. In fact, as the soil particles become closer together and connection between particles strengthen (i.e., inter-particles cohesion increases), the φ decreased due to reduced inter-aggregates friction and roughness
5. Conclusions 1) This study was conducted to investigate the effect of PAM and vetiver on soil shear failure envelope and unsaturated shear strength parameters. It was found that vetiver and PAM decreased angle of internal friction, φ and effective angle of internal friction, φ', but increased total cohesion, c, effective cohesion, c' and angle of internal friction related to matric suction, φb, and as a result increased unsaturated soil shear strength. However, the positive effect of vetiver on soil shear strength was greater than that of PAM. Moreover, soil shear strength increased with an increment in the matric suction. Simultaneous application of vetiver and 0.4 percent of PAM led to the highest increase in the shear strength, indicated although vetiver (biologic factors) can increase the cohesive strength, it will be more pronounced when PAM (chemical factors) is simultaneously applied. 2) Adsorption of long PAM polymer chains on the external surfaces of soil particles would bind the particles and could enhance the 6
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
cohesion through chemical bonds and creating bridges between particles, and consequently the c' increase. The extensive networks of vetiver roots in the soil matrix and compression of the root-surrounding soil mainly increased the c'. Organic compounds released by vetiver roots into the soil also might act as binding agents of particles and subsequently increased the c' and unsaturated soil shear strength. However, It seems that the PAM located between soil particles and organic compounds released by vetiver roots into the soil although increased the contact points and inter-connection of particles, but decreased the internal roughness due to lubricating effect and consequently decreased the internal friction forces and the φ. PAM and vetiver roots by enhancing the effective saturation and effective stress originated from internal forces (i.e., matric suction) and ultimately increased the φb. Simultaneous application of vetiver and PAM considerably increased the soil shear strength therefore they can be suggested as the rehabilitation program to reduce soil erosion and degradation.
America, Madison, WI, pp. 329–361. Fredlund, D.G., Xing, A., Fredlund, M.D., Barbour, S.L., 1996. The relationship of the unsaturated soil shear to the soil-water characteristic curve. Can. Geotech. J. 33 (3), 440–448. Green, V.S., Stott, D.E., 1999. Polyacrylamide: a review of the use, effectiveness, and cost of a soil erosion control amendment. In: Stott, D.E., Mohtor, R.H., Sreinhardt, G.C. (Eds.), 2001. Sustaining the Global Farm. Selected Papers from the 10th International Soil Conservation Organization Meeting Held, May 24–29, 1999 at Purdue University and the USDA_ARS National Soil Erosion Research Laboratory. Isa, I., Shamshuddin, J., Khairuddin, M.N., 2017. Effects of mix vegetation and root shear strength grown on carbonaceous shale. AJAS. 10, 66–78. Johnson, C.E., Grisso, R.D., Nichols, T.A., Bailey, A.C., 1987. Shear measurement for agricultural soils: a review. Trans. ASAE 30, 935–938. Klute, A. (Ed.), 1986. Methods of Soil Analysis. Part 1, 2nd ed. Agronomy Monograph 9. ASA and SSSA, Madison, WI. Knapen, A., Poesen, J., De Baets, S., 2007a. Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices. Soil Tillage Res. 94, 425–440. Koolen, A.J., Kuipers, H., 1983. Agricultural Soil Mechanics. Advanced Series in Agricultural Sciences, vol. 13 Springer-Verlag, Berlin. Kumar, A., Saha, A., 2011. Effect of polyacrylamide and gypsum on surface runoff, sediment yield and nutrient losses from steep slopes. Agr. Water Manage. 98, 999–1004 WATER MANAGE. Lentz, R.D., 2015. Polyacrylamide and biopolymer effects on flocculation, aggregate stability, and water seepage in a silt loam. Geoderma 241–242, 289–294. Levy, G.J., Levin, J., Gal, M., Ben-Hur, M., Shainberg, I., 1992. Polymers’ effects on infiltration and soil erosion during consecutive simulated sprinkler irrigations. Soil Sci. Soc. Am. J. 56, 902–907. Levy, G.J., Warrington, D.N., 2015. Polyacrylamide addition to soils: impacts on soil structure and stability. In: In: G, Cirillo, U.G, Spizzirri, Iemma, I. (Eds.), Functional Polymers in Food Science: From Technology to Biology, vol. 2. John Wiley and Sons, Inc., Hoboken, NJ, USA, pp. 9–32 food processing. Marsh, J.D., Dixon, A.J., 1991. Effect of heavy addition of organic residues on physical characters of three soil types in Queensland. Australia. Soil Tillage Res. 20, 109–122. McKyes, E., 1985. Soil Cutting and Tillage. In Development in Agricultural Engineering Vol. 7 Elsevier Science Publishers, Amsterdam, the Netherlands. Ma’ruf, M.F., 2012. Shear strength of Apus bamboo root reinforced soil. Ecol. Eng. 41, 84–86. Materechera, S., 2010. Soil Physical and Biological Properties As Influenced by Growth of Vetiver Grass (Vetiveria zizanioidesL.) in a Semi-arid Environment of South Africa Simeon. 19th World Congress of Soil Science, Soil Solutions for a Changing World. August 1–6. Morgan, R.P.C., 2005. Soil Erosion and Conservation, 3rd ed. Blackwell Publishing Ltd, UK. Oku, E., Fagbola, O., Troung, P., 2011. Evaluation of vetiver grass buffer strips and organomineral fertilization for the improvement of soil physical properties. Kasetsart J. (Natural Sci.). 45, 824–831. Operstein, V., Frydman, S., 2000. The influence of vegetation on soil strength. Proc. Inst. Civ. Eng. Ground Improv. 4, 81–89. Page, A.L., Miller, R.H., Keeney, D.R., 1986. Methods of Soil Analysis. Part 2, 2nd edn. Agronomy Monograph 9. ASA and SSSA, Madison, WI. Soil Survey Staff, 2006. Keys to Soil Taxonomy. Department of Agriculture, Natural Resources Conservation Service, Washington, DC. Scholtès, L., Chareyre, B., Nicot, F., Darve, F., 2009. Micromechanics of granular materials with capillary effects. Int. J. Eng. Sci. 47, 64–75. Spoor, G., 1975. Fundamental Aspects of Cultivations. Technical Bulletin. Ministry of Agriculture, Fisheries and Food., UK. Torri, D., Santi, E., Marignani, M., Rossi, M., Borselli, L., Maccherini, S., 2013. The recurring cycles of biancana badlands: erosion, vegetation and human impact. Catena 106, 22–30. Truong, P.N., 2002. Vetiver grass technology. In: Maffei, M. (Ed.), Vetiveria. Taylor & Francis, London and New York, pp. 114–132. Yoo, G., Yang, X., Wander, M.M., 2011. Influence of soil aggregation on SOC sequestration: a preliminary model of SOC protection by aggregate dynamics. Ecol. Eng. 37, 487–495. Zhang, Y., Ishikawa, T., Tokoro, T., Nishimura, T., 2014. Influences of degree of saturation and strain rate on strength characteristics of unsaturated granular subbase course material. Trans. Geotech. 1, 74–89. Zhang, B., Zhao, Q.G., Horn, R., Baumgartl, T., 2001. Shear strength of surface soil as affected by soil bulk density and soil water content. Soil Tillage Res. 59, 97–106.
References Akbarzadeh, A., Taghizadeh Mehrjardi, R., Refahi, G.H., Rouhipour, H., Gorji, M., 2009. Using soil binders to control runoff and soil loss in steep slopes under simulated rainfall. Int. Agrophys. 23, 99–109. Aluko, O.B., Koolen, A.J., 2000. The essential mechanics of capillary crumbling of structured agricultural soils. Soil Tillage Res. 55, 117–126. Amiri, E., Emami, H., Mosaddeghi, M.R., Astaraei, A.R., 2017. Investigating the effect of vetiver and polyacrylamide on runoff, sediment load and cumulative water infiltration. Arid. Soil Res. Rehabil. 55, 769–777. Amiri, E., Emami, H., Mosaddeghi, M.R., Astaraei, A.R., 2018. Estimation of unsaturated shear strength parameters using easily-available soil properties. Soil Tillage Res. 184, 118–127. Blanco-Canqui, H., Lal, R., Owens, L.B., Post, W.M., Izaurralde, R.C., 2005. Strength properties and organic carbon of soils in the north Appalachian region. Soil Sci. Soc. Am. J. 69, 663–673. Bengough, A.G., Bransby, M.F., Hans, J., McKenna, S.J., Roberts, T.J., Valentine, T.A., 2006. Root responses to soil physical conditions: growth dynamics from field to cell. J. Exp. Bot. 57, 437–447. Craig, R.F., 2004. Craig’s Soil Mechanics, 7th ed. Spon Press, London. CTA, 1989. Vetiver Grass (Vetiveria Zizanioides) – a Method of Vegetative Soil and Moisture Conservation. Spore 23. CTA, Wageningen, The Netherlands. Dane, J.H., Jan, W.H., 2002. Water retention and storage. In: Dane, J.H., Topp, G.G. (Eds.), Methods of Soil Analysis. Part 4. Physical Methods. Soil Sci. Soc. Am. J. Madison, Wisconsin USA, pp. 671–717. Das, B.M., 1990. Principles of Geotechnical Engineering, 2nd ed. PWS-KENT Publishing Company, Boston. de Melo, D.V.M., de Almeida, B.G., Andrade, K.R., Souza, E.R., da Silva Souza, W.R., de Almeida, C.D.G.C., 2016. Pore size distribution and hydro-physical properties of cohesive horizons treated with anionic polymer. Afr. J. Agric. Res. 11 (44), 4444–4453. Diti, H., 1999. 15 years of bio-engineering in the wet tropics. First Asia-Pacific Conference on Ground and Water Bio-Engineering 1999. Donjadee, S., Clemente, R.S., Tingsanchali, T., Chinnarasri, C., 2010. Effects of vertical hedge interval of vetiver grass on erosion on steep agricultural lands. Land Degrad. Dev. 21 (3), 219–227. Endo, T., Tsuruta, T., 1969. The Effect of the Tree’s Roots Upon the Shear Strength of Soil. Annual Report No 18, Hokkaido Branch. Tokyo Forest Experiment Station, Japan. Fan, C.C., Su, C.F., 2008. Role of roots in the shear strength of root reinforced soils with high moisture content. Ecol. Eng. 33, 157–166. Fattet, M., Fu, Y., Ghestem, M., Ma, W., Foulonneau, M., Nespoulous, J., Le Bissonnais, Y., Stokes, A., 2011. Effects of vegetation type on soil resistance to erosion: relationship between aggregate stability and shear strength. Catena 87, 60–69. Fredlund, D.G., Morgenstern, N.R., Widger, R.A., 1978. The shear strength of unsaturated soils. Can. Geotech. J. 15 (3), 313–321. Fredlund, D.G., Vanapalli, S.K., 2002. Shear strength of unsaturated soils. In: Klute, A. (Ed.), Methods of Soil Analysis: Part 4 Physical Methods. No 5. Soil Science Society of
7
Contents lists available at ScienceDirect
Soil & Tillage Research journal homepage: www.elsevier.com/locate/still
Shear strength of an unsaturated loam soil as affected by vetiver and polyacrylamide
T
⁎
Elham Amiri, Hojat Emami
Ferdowsi University of Mashhad, Soil Science dept. Faculty of Agriculture, 9177948978, Mashhad, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Angle of internal friction Angle of internal friction related to matric suction Cohesion Direct shear test Soil rehabilitation
Shear strength is an important soil mechanical property that its improvement is one of the rehabilitation program for the reduction of soil erosion and degradation. The objective of this study was to investigate the effect of polyacrylamide (PAM) and the vetiver system as a cheap and long-term bioengineering method and their combination on unsaturated shear strength parameters (effective cohesion, c' (kPa), angle of effective internal friction, φ' (°) and angle of internal friction related to matric suction, φb (°)) of a loam soil. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water [0.2% (V0P2) and 0.4% (V0P4)], and simultaneous presence of vetiver and two concentrations of PAM (VP2 and VP4). 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 (12 tests per each treatment) to determine the shear strength parameters. It was found that vetiver and PAM decreased apparent angle of internal friction (φ) and φ', and increased c', total cohesion (c), φb, and as a result increased unsaturated shear strength. However, the positive effect of vetiver on shear strength was greater than that of PAM. It seems that PAM and vetiver enhanced the contact area, inter-particles bonds, aggregation, and inter-aggregate porosity, and as a consequence, increased the soil effective saturation, effective stress, cohesion and shear strength. Also, when matric suction increased the c and shear strength increased although the φ increased slightly. Simultaneous application of vetiver and 0.4% of PAM resulted in maximum shear strength, indicated although vetiver can increase the cohesive strength, it would be more pronounced when PAM was simultaneously applied. Simultaneous application of vetiver and PAM through increasing soil shear strength can be suggested as the rehabilitation program to reduce soil erosion and degradation.
1. Introduction Soil strength is one of the most dynamic soil mechanical properties that is important to tilth, plant growth, and control erosion. The strength of soil is the greatest stress that it can sustain. The shear strength of a soil is its resistance against shearing stresses. Soil shear strength is the internal resistance per unit area that the soil mass can offer against sliding along any plane inside it (Das, 1990). It can be proposed as a measure of the soil resistance to deformation by continuous displacement of its individual soil particles. Shear strength in soils depends primary on interactions between particles, so that shear strength within a soil matrix is the result of resistance to movement at
inter-particle contacts, due to particle interlocking through physicochemical bonds (cohesion, c (kPa)) and inter-particle (internal) friction (as quantified by angle of internal friction, φ (°)) (Craig, 2004). So, soil drives its shear strength from two sources; cohesion between particles (stress independent component) and frictional resistance between particles (stress dependent component). It is as a useful dynamic index for evaluating soil erodibility (Zhang et al., 2001; Knapen et al., 2007a; Torri et al., 2013) and resistance against seedling emergence and root growth (Marsh and Dixon, 1991). Blanco-Canqui et al. (2005) pointed out that knowledge about shear strength dynamics is crucial to explain the mechanical behavior and structural sustainability of agricultural soils. Improvement of the soil mechanical properties such as shear
Abbreviations: CCE, calcium carbonate equivalent (kg 100 kg−1); c, total cohesion (kPa); c', effective cohesion (kPa); OM, organic matter (kg100 kg−1); PAM, polyacrylamide; SWCC, soil water characteristic curve; ua, pore air pressure (kPa); uw, pore water pressure (kPa); (ua-uw), matric suction (kPa); V0P0, control treatment; V0P2, 0.2dissolved PAM treatment; V0P4, 0.4% dissolved PAM treatment; VP0, vetiver plants; VP2, combination of vetiver and PAM level 0.2; VP4, combination of vetiver and PAM level 0.4%; σn, normal stress (kPa); (σn-ua), net normal stress (kPa); σ', effective stress (kPa); τ, Shear strength (kPa); τf, unsaturated shear strength (kPa); φ, apparent angle of internal friction (°); φ', effective angle of internal friction (°); φb, angle of internal friction related to matric suction (°) ⁎ Corresponding author. E-mail address: [email protected] (H. Emami). https://doi.org/10.1016/j.still.2019.104331 Received 29 October 2018; Received in revised form 3 July 2019; Accepted 6 July 2019 0167-1987/ © 2019 Elsevier B.V. All rights reserved.
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
strength is one of the rehabilitation program in order to reduce soil erosion and degradation. Appropriate amendments and vegetation may improve soil shear strength through physical bonds formed across the contact areas of particles and chemical bonds or cementation between the soil particles. Considering the importance of vegetation, the bioengineering methods have been proposed to increase land stability and to control soil degradation and erosion. In this regard, vetiver grass technology (VGT) i.e. using of vetiver grass (Vetiveria zizanioides L. Nash) was first developed by World Bank for soil and water conservation in India in the 1980s (CTA, 1989). Vetiver is a tropical perennial plant and naturally adapted to low and high lands, and a wide range of soil types and climatic conditions (Truong, 2002). Also, the massive and deep roots of vetiver with high mean tensile strength of 75 MPa (Diti, 1999) has been caused to be suitable for soil stabilization. It has been found that vetiver is successful to improve soil physical properties (Materechera, 2010; Oku et al., 2011), control soil erosion and improve the land stability (Amiri et al., 2017; Donjadee et al., 2010). In addition, soil amendments such as polymers have been shown to be beneficial for soil quality and to resist against degradation (Levy and Warrington, 2015) and to improve soil physical properties (Lentz, 2015; de Melo et al., 2016). Anionic polyacrylamide (PAM) is water-soluble, non-toxic polymer and one of the most effective polymers commonly used to increase structural stability, to prevent crust formation and soil erosion, and consequently to reduce soil degradation (Green and Stott, 1999). The PAM has been widely applied since the 1990s due to its easy preparation (Levy et al., 1992). Kumar and Saha (2011) reported the various aspects of PAM effectiveness on increasing soil stability, and reducing soil erosion. Fan and Su (2008) investigated the role of roots in the shear strength of root-reinforced soils at high water contents and found that root efficiency, defined as the ratio of increment in shear strength due to presence of roots over the initial shear strength of soils, may be more than 1.0 at high degrees of saturation. In addition, their results indicated the approximately linear relationship between the increment in shear strength due to the presence of roots and the tensile strength of roots per unit area of soil. Furthermore, Fattet et al. (2011) studied the effects of plant vegetation on soil resistance against erosion and relationship between aggregate stability and shear strength and obtained a significant positive linear relationship between mean weight diameter (MWD) of water-stable aggregates and soil cohesion, while no relationship was found with internal angle of friction. Ma’ruf (2012) also investigated shear strength of Apus bamboo root-reinforced soil and concluded that peak shear strength of the root-reinforced soil increased when soil-root volume ratio increased. Though the soil-root volume ratio was only up to 5%, the increment in the peak shear strength was up to 55%. Isa et al. (2017) determined the shear strength of carbonaceous shale as affected by vegetation and pointed out that presence of vegetation roots decreased the c and enhanced the φ, and consequently would result in an overall increment in the shear strength. Akbarzadeh et al. (2009) used soil binders (PAM and gypsum) and found that addition of the soil amendments to soil surface led to shear strength increase due to improvement of the soil physical properties and production of stable aggregates. It has been found that vetiver and PAM have good efficiency on improving the soil physical properties and to control erosion (e.g., Amiri et al., 2017). The unsaturated shear strength is one of the important parameters affecting soil erosion, therefore, evaluating the influence of vetiver and PAM on unsaturated shear strength of soil is necessary. In addition, the effect of PAM and vetiver on unsaturated shear strength parameters (i.e., c', φ' and φb) have not been studied so far. Thus, the objective of this study was to investigate the effect of PAM and vetiver system (as a cheap and long-term bioengineering method) and their combination on parameters of unsaturated shear strength (i.e., c', φ' and φb) of a loam soil.
Fig. 1. The soil failure envelope; c', effective cohesion; φ, angle of internal friction.
2. Theory Soil shear strength is the maximum shear stress that a soil can sustain before shear failure happens. The shear strength of saturated soils is determined using the Mohr–Coulomb failure (Johnson et al., 1987):
τ = c + σntanφ
(1)
Where, 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 (Fig. 1). Thus, the shear strength of a soil can be described by two parameters i.e. cohesion and internal friction angle (McKyes, 1985). Cohesion is due to chemical bonds between soil particles and surface tension within the water films around soil particles (Knapen et al., 2007a; Morgan, 2005). Frictional shear strength (σn tan φ) is a result of internal friction between soil particles, that depends on the normal stress acting on the failure surface. The shear strength of unsaturated soil can be described using the extended Mohr–Coulomb failure criterion which is expressed as (e.g. Fredlund et al., 1978):
τf = c′ + (σ − ua )f tan φ′ + (ua − u w )f tan φb
(2)
where τf (kPa) is the unsaturated shear strength, c' (kPa) is the effective cohesion, φ' (°) is the effective angle of internal friction, (σ − ua )f (kPa) is the net normal stress on the failure plane, (ua − u w )f (kPa) is the matric suction at failure, σ is the total normal stress on the failure plane, ua and u w are the pore-air and pore-water pressures, respectively, φb (°) is an friction angle indicating the rate of change in shear strength relative to changes in matric suction (Fredlund et al., 1996). Hence, total cohesion (c) in unsaturated soils consists of two components; effective or true cohesion (c') due to physicochemical cohesion, and the apparent cohesion ((ua − u w )tanφb ) due to matric suction (Zhang et al., 2001), which could be written as:
c = c′ + (ua − u w )tanφb
(3)
When an unsaturated soil becomes saturated, matric suction is equal to zero, and the total cohesion approaches to the effective cohesion (Zhang et al., 2014). 3. Materials and methods 3.1. Experimental site and treatments The study was conducted in a loam soil in the Agriculture Campus, Ferdowsi University of Mashhad, Iran (36°18′23′'N, 59°31′29′'E). The 2
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Table 1 Physical and chemical properties of the studied soil. Bulk density Mg cm 1.60
Texture
−3
Sand
Silt
Clay
OM
CCE
−1
Loam
———————————————————kg 100 kg ———————————————————— 48 41 11 0.21
12.6
EC dS m 0.90
pH −1
7.70
*OC: Organic carbon; CCE: Calcium carbonate equivalent; EC: Electrical conductivity.
500, 1000, and 1500 kPa (Dane and Jan, 2002).
average rainfall and temperature in the studied area are 257 mm and 15°C, respectively. The minimum and maximum temperatures were 14 and 37 °C, respectively. The soil of the study area was classified as Xeric Torriorthents according to Soil Taxonomy (Soil Survey staff, 2006). Some properties of the Ap horizon of the studied soil, as determined by standard analytical methods (Klute, 1986; Page et al., 1986), are shown in Table 1. Organic matter content (OM) and calcium carbonate equivalent (CCE) were measured using the Walkley-Black wet-oxidation (Nelson, 1982) and back-titration methods (Nelson and Sommers, 1986), respectively. Soil texture was determined using the pipette method (Gee and Bauder, 1986). Experimental plots (1 m × 1 m) were prepared in the given area. The experimental treatments included vetiver plant (VP0), two concentrations of PAM dissolved in water (0.2% (V0P2) and 0.4% (V0P4) which are approximately equal to 20 and 40 kg ha−1 by considering a soil layer with 20-cm thickness, respectively), and combination of vetiver and above PAM concentrations (i.e., VP2 and VP4). The properties of the used PAM (provided by Chemicals Tianrun Company, Benghu City, Anhui Province, China) are given in Table 2. In addition, V0P0 (no PAM and vetiver) was regarded as control treatment. Vetiver (Vetiveria zizanioides L. Nash) seedlings were planted at a distance of 35 cm × 35 cm in each plot on 1 May 2015. They were irrigated during the first one month, every two weeks and after then every month. Water content for irrigation was equal to field capacity at the roots depth. Vetiver plants were fully grown after 3 months. PAM solution was sprayed on the surface of plots using a sprinkler about 45 days after vetiver planted i.e. on 15 Jun 2015. Finally, the soil samples were collected on 1 August 2015.
3.4. Soil shear strength measurement Direct Shear apparatus (Fig. 2a) was used to measure the soil shear strength and its parameters. The unconsolidated undrained (UU) direct shear tests were performed using a shear box (Fig. 2b) on the undisturbed soil specimens at different matric suctions of 0, 10, 30, and 50 kPa (Fredlund and Vanapalli, 2002). The shear box was used with an inner cross section of 60 × 60 mm and height of 20 mm. Undisturbed samples with cross section of 60 × 60 mm and height of 20 mm were inserted in the shear box. The data of SWCC were used to determine the soil water contents at pre-defined matric suctions. For adjusting the water status of soil samples at pre-defined matric suctions (ua–uw), the required amount of water (by weighing) was added to the soil specimens in a desiccator and they were equilibrated for a few days in plastic bags. The weight of samples were about 90–100 grams. Normal stresses of 25, 50, and 100 kPa were applied on the soil samples with pre-adjusted matric suction. Then by applying the shear load the specimens were sheared at a constant rate of 1.001 mm min−1. The shear strength was considered equal to the shear stress at relative horizontal displacement of 10% because all of the failure patterns were compressive or ductile. Three replications were considered for each combination of normal stress and matric suction. Totally, 216 direct shear tests (6 treatments × 3 normal stresses × 4 matric suctions × 3 replications) were performed. The Mohr–Coulomb failure criterion was applied to calculate the apparent and effective shear strength parameters. The parameters c and φ were obtained from the intercept of the Mohr-Coulomb failure envelope (Eq. 1) by plotting the shear strength vs. net normal stress pairs at a specific matric suction. The unsaturated soil shear strength parameters (c', φ' and φb) of the model proposed by Fredlund et al. (1978) (see Eq. 2) were estimated using optimization technique of Excel® Solver (Microsoft Office, 2013) by minimizing the sum of square errors (SSE):
3.2. Soil sampling For each treatment, 36 (3 normal stress × 4 matric suction × 3 replication) undisturbed samples were collected from the surface layer (0–15 cm) using special metal square molds with an inner cross section of 60 × 60 mm and height of 20 mm. In total, 216 square undisturbed samples (3 replication for each treatment) were collected. The inner wall of the sampling molds was lubricated by oil to minimize the friction between the soil and mold. Also 18 undisturbed core samples (5 cm in diameter and 5 cm in height) were collected (3 cylinders for each treatment) from the surface layer to determine the soil water characteristic curve (SWCC).
SSE =
∑ (τ measured − τpredicted )2
(4)
where τmeasured and τpredicted are measured and predicted (by Eq. 2) values of shear strength, respectively. Therefore, unsaturated soil shear strength parameters were derived from 12 corresponding values of τf, σn–ua and ua–uw (3 normal stresses × 4 matric suctions) for each treatment and replicate.
3.3. Measurement of soil water characteristic curve The SWCC was determined on the undisturbed samples using pressure plate apparatus at matric suctions of 5, 10, 30, 50, 100, 300,
3.5. Statistical analyses Table 2 Some properties of the studied anionic polyacrylamide (PAM). Properties of PAM
Specification
Appearance Hydrolysis degree (Ionicity) (%) Molecular weight (Mg mol−1) Density (Mg m−3) pH value Water insolubility (%) Solid content (%)
White granular powder 30–35 15–20 ≥0.67 6–8 ≤ 0.08 95
Data analyses were performed based on the randomized complete block design in a factorial arrangement with three replications using SPSS software ver. 23 (SPSS 23 Inc.). Analysis of variance was used to difference between treatments. Mean’s comparsions were made using Duncan’s multiple range test at P < 0.05 to compare the parameters of unsaturated shear strength (c', φ ', and φb) between the studied treatments.
3
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Fig. 2. The picture of direct shear apparatus (a), the shear box (b), and the schematic of the shear box (c).
4. Results and discussion
particles, would enhance the aggregate stability and increased the interaggregate meso- and macro-pores (Amiri et al., 2017). Therefore, greater effective saturation, effective stress and total cohesion between soil particles as a consequence of root effects on soil pore size distribution, could ultimately increase the soil shear strength. The results indicated that the positive effect of vetiver on shear strength was greater than that of PAM. Isa et al. (2017) found that increasing water uptake by roots and evapotranspiration are reasons for reducing pore water pressure thereby, increasing effective stress and subsequently enhancing effective shear strength. Akbarzadeh et al. (2009) reported that addition of soil amendment i.e. PAM, gypsum, and mixture of them to soil surface, increased shear strength due to improvement of soil physical properties and production of stable aggregates. Also, shear strength increased with an increment of PAM concentration, yet the influence of gypsum on shear strength was greater than that of PAM. Ma’ruf (2012) also expressed that the bamboo root system could increase the soil shear strength.
4.1. Shear failure envelopes at different matric suctions The Mohr-Coulomb shear failure envelopes (means of replicates) at different matric suction values for the studied treatments are presented in Fig. 3. It was observed that the studied treatments increased shear strength compared to the control at all three normal stresses of 25, 50, and 100 kPa and matric suctions of 0, 10, 30, and 50 kPa. Moreover, it was found that all the treatments enhanced total cohesion (c, intercept of the Mohr-Coulomb failure envelope), and shear strength at normal stress of 25 kPa, compared to control (Table 3). However, different trends were obtained for the studied treatments, so that the maximum and minimum increments of c were obtained for VP4 and V0P0 (control) treatments, respectively. In addition, it was found that at matric suctions of 0 and 10 kPa, shear strength in all the treatments slightly increased with increasing normal stress compared to the control, indicating that the slope of shear envelope or internal friction angle (φ) decreased (Table 3). This finding indicates greater inter-particle forces (i.e., c) and lesser inter-particle friction (i.e., φ) due to presence of vetiver roots and PAM. The reduction of φ for different treatments did not show similar trend at all studied matric suctions. It indicated the different effects of treatments on φ, so that maximum decrease of φ was found for the VP4 treatment. In addition, similar to Amiri et al. (2018) it was found that the c and φ, and consequently shear strength increased when matric suction increased in all the treatments; but in vetiver and PAM treatments the φ increased lower than the c. According to Eq. (3), total cohesion in the unsaturated soil depends on effective cohesion and capillary (apparent) cohesion (Spoor, 1975). The former is due to inter-particle bonds (physicochemical attractions) (Koolen and Kuipers, 1983). The latter is due to capillary and suction forces as film cohesion that relates to net inter-particle forces generated within a matrix of unsaturated soil due to the combined effects of negative pore water pressure and surface tension (Aluko and Koolen, 2000; Scholtès et al., 2009). The PAM application and vetiver cultivation increased total cohesion. Although vetiver increased c greater than the PAM. PAM increased c through increasing the contact area of soil particles and chemical bonds between them. Moreover, hydrophilic molecules of PAM decreased the surface tension between soil particles, creating water bridges, and therefore increased the effective stress and consequently increased the total cohesion. The roots of vetiver by binding the soil
4.2. Unsaturated shear strength parameters The results of ANOVA for the parameters of unsaturated shear strength (c', φ' and φb) showed that vetiver and PAM treatments had significant effects on c', φ' and φb (P < ≤ 0.05) (Table 4). Means’ comparisons showed that the PAM application and vetiver cultivation increased c' significantly (Fig. 4a). The minimum (3.0 kPa) and maximum (7.8 kPa) values of c' were obtained in control and VP4 treatments, respectively. The c' significantly increased with an increment of PAM concentration without vetiver cultivation (i.e., V0P2 and V0P4 treatments). Effective cohesion is due to physicochemical bonds between soil particles. Adsorption of long PAM polymer chains on the external surfaces of soil particles would bind the particles and could enhance the cohesion through chemical bonds and creating bridges between particles. In addition, vetiver cultivation significantly increased the c' compared to control and PAM treatments. These findings support the important role of vetiver roots in increasing contact area and binding of particles, and subsequently effective cohesion. The effects of roots on contact area and cohesion of soil particles, and consequently shear strength can be explained by two mechanisms: (i) chemical bonds and inter-particles’ contacts due to plant roots exudates and (ii) physical forces i.e. either loosening (Yoo et al., 2011) or radial compression of surrounding soil during root penetration (Bengough et al., 2006). It seems that the extensive networks of vetiver roots in the 4
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
Fig. 3. Shear failure envelopes at different matric suctions as affected by the studied treatments. V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM, respectively; VP0, vetiver plants; and VP2 and VP4, combination of vetiver and PAM levels 0.2 and 0.4%, respectively. Table 3 Means’ comparisons of soil apparent cohesion (c) and angle of internal friction (φ) in the studied treatments at matric suctions of 0, 10, 30, and 50 kPa. Treatment
Matric suction (kPa) 0
V0P0 V0P2 V0P4 VP0 VP2 VP4
Table 4 Results of ANOVA for parameters of unsaturated soil shear strength as affected by vetiver and PAM treatments.
c (kPa) 3.9e 5.1d 6.2c 8.5b 10.5a 11.1a
Treatment
10 φ (°) 29.7a 29.7a 28.8b 27.9c 27.0d 27.0d
c (kPa) 4.5f 5.7e 6.7d 9.3c 10.8b 11.7a
30 φ (°) 31.0b 30.6c 29.7d 23.4f 32.2a 27.9e
c (kPa) 5.4f 6.8e 7.5d 9.9c 11.2b 12.5a
df
50 φ (°) 31.4c 32.0b 31.4c 33.4a 29.7d 31.8d
c (kPa) 6.5f 7.7e 8.4d 11.3c 12.6b 13.5a
φ (°) 31.4d 33.0b 32.2c 32.2c 33.4a 32.2c
Vetiver PAM Vetiver × PAM Errors CV (%)
1 2 2 12
Mean square c' (kPa)
φ' (°)
φb (°)
58.9*** 3.9*** 1.2*** 0.002 1.0
2.6*** 0.3*** 0.1*** 0.001 0.1
38.3*** 4.5*** 5.0*** 0.003 0.6
* c': effective cohesion; φ': effective angle of internal friction; φb: matric suction angle of internal friction. † ***stand for significance at 99.9% probability levels.
* Means with same letter in each column are not significantly different (P > 0.05). V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM respectively; VP0, vetiver plant; and VP2 and VP2, combination of vetiver and PAM levels P2 and P4 respectively.
effect will be more pronounced when PAM (chemical factors) is simultaneously applied. In addition, the studied treatments significantly decreased the φ' parameter (Fig. 4b), so that φ' for the control was 30.6° and when the concentration of PAM increased its value reduced to 30.2° (V0P4). Effective frictional angle is a result of frictional forces met by soil particles when they are forced to slide over one another or to move out of interlocking positions (Morgan, 1986). Particle roughness is another source of inter-particles friction. The PAM application caused the soil particles to be closer together and inter-particles cohesion increased. However, it seems that the PAM located between soil particles,
soil matrix (i.e., physical forces) and compression of the root-surrounding soil mainly increased the c'. However, organic compounds released by vetiver roots into the soil might act as binding agents of particles and subsequently increased the c'. Moreover, significant difference was found between VP0 and combination of vetiver and PAM (VP2 and VP4). However, the differences between VP2 and VP4 in terms of soil cohesion were not significant. These results indicated although vetiver (biologic factors) can increase the cohesive strength, the 5
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
(Amiri et al., 2018). Fan and Su (2008) also concluded that the effective cohesion between particles in the root-reinforced soils was greater than root-free soils. They found that greater shear strength of root-reinforced soils compared to root-free soils was almost equal to the increase in apparent cohesion, and roots had little influence on the angle of internal friction. Endo and Tsuruta (1969) also indicated that roots increased soil cohesion, while they had no effect on the angle of internal friction. Operstein and Frydman (2000) also found that for chalky and clay soils that stabilized with roots, angle of internal friction essentially remained constant, while the apparent cohesion increased in the rooted soil. Therefore, the increase of shear strength in the root-reinforced soil may be equal to the increase in apparent cohesion. Vetiver and PAM significantly increased the φb, indicating that the increase of shear strength with an increment in the matric suction in the studied treatments was greater than the control (Fig. 4c). PAM significantly increased φb in comparison with control. However, φb decreased for V0P4 treatment compared to V0P2. It seems that 0.2% of PAM has greater positive influence on φb. As mentioned before, shear strength increased with increasing matric suction; whereas PAM is hydrophilic, it resulted in greater contact area of particles, therefore PAM would increase the effective stress and as a result increased the shear strength. Moreover, adsorption of the long chains of PAM polymer on soil particles and aggregates, increased the aggregate stability and created meso- and macro-pores due to flocculation and binding of the soil particles (Amiri et al., 2017). These changes enhanced the effective saturation and effective stress originated from internal forces (i.e., matric suction) and ultimately increased the φb. It seems that higher concentration of PAM can clog the micropores and some mesopores; as a result, the effective saturation and effective stress, and shear strength were decreased. Therefore, the role of high concentration of PAM on shear strength at higher matric suction decreases. Thus the V0P4 had little effect on φb compared to V0P2 and subsequently V0P4 decreased φb. Vetiver also significantly increased φb, and its influence was greater than that of PAM (Fig. 4c). In fact, the vetiver roots by enhancing the inter-particles bonds and creating inter-aggregate pores increased the degree of saturation and increased effective stress, consequently shear strength increased. In addition, simultaneous application of vetiver and PAM increased φb, so that VP2 increased φb compared to the control, while it reduced φb compared to VP0. It seems that at low concentration of PAM (0.2%), some agglomerated soil particles due to PAM may be migrated to micropores that created by vetiver roots and clogged them. Therefore, VP2 decreased effective saturation, effective stress, and consequently φb compared to VP0. On the other hand, VP4 led to maximum increase in φb (11.1°) compared to the control (5.2°). These results indicated that high concentration of PAM would strengthen the influence of vetiver on aggregation, inter-aggregate porosity, and the φb, and thus on soil shear strength.
Fig. 4. Means’ comparisons of parameters of unsaturated soil shear strength as affected by vetiver and PAM treatments: (a) effective cohesion (c'), (b) effective angle of internal friction (φ'), (c) angle of internal friction related to matric suction, φb; Means with the same letters are not significantly different (P > 0.05); V0P0, the control; V0P2 and V0P4, 0.2 and 0.4% dissolved PAM, respectively; VP0, vetiver plants; and VP2 and VP4, combination of vetiver and PAM levels 0.2 and 0.4%, respectively.
although increased the contact points and inter-connection of particles, but decreased the internal roughness due to lubricating effect and consequently decreased the internal friction forces and φ. Moreover, significant difference was observed between two concentrations of PAM in without vetiver cultivation (i.e., V0P2 and V0P4). However, the decreasing effect of vetiver on φ' (29.7°) was considerably more than that of PAM. Vetiver roots probably has released organic exudates which enhanced soil particles to be closer together, interlocked and stabilized. Moreover, organic compounds released by vetiver roots into the soil might have lubricating effect and reduced the internal roughness and consequently, decreased the internal friction. The VP0 had no significant difference with VP2, but its difference with VP4 was significant. The results indicated that the effects of PAM and vetiver on binding soil particles and enhancing cohesion were much more than those on the internal friction between particles, and consequently cohesive forces between particles were greater than the frictional forces. In fact, as the soil particles become closer together and connection between particles strengthen (i.e., inter-particles cohesion increases), the φ decreased due to reduced inter-aggregates friction and roughness
5. Conclusions 1) This study was conducted to investigate the effect of PAM and vetiver on soil shear failure envelope and unsaturated shear strength parameters. It was found that vetiver and PAM decreased angle of internal friction, φ and effective angle of internal friction, φ', but increased total cohesion, c, effective cohesion, c' and angle of internal friction related to matric suction, φb, and as a result increased unsaturated soil shear strength. However, the positive effect of vetiver on soil shear strength was greater than that of PAM. Moreover, soil shear strength increased with an increment in the matric suction. Simultaneous application of vetiver and 0.4 percent of PAM led to the highest increase in the shear strength, indicated although vetiver (biologic factors) can increase the cohesive strength, it will be more pronounced when PAM (chemical factors) is simultaneously applied. 2) Adsorption of long PAM polymer chains on the external surfaces of soil particles would bind the particles and could enhance the 6
Soil & Tillage Research 194 (2019) 104331
E. Amiri and H. Emami
cohesion through chemical bonds and creating bridges between particles, and consequently the c' increase. The extensive networks of vetiver roots in the soil matrix and compression of the root-surrounding soil mainly increased the c'. Organic compounds released by vetiver roots into the soil also might act as binding agents of particles and subsequently increased the c' and unsaturated soil shear strength. However, It seems that the PAM located between soil particles and organic compounds released by vetiver roots into the soil although increased the contact points and inter-connection of particles, but decreased the internal roughness due to lubricating effect and consequently decreased the internal friction forces and the φ. PAM and vetiver roots by enhancing the effective saturation and effective stress originated from internal forces (i.e., matric suction) and ultimately increased the φb. Simultaneous application of vetiver and PAM considerably increased the soil shear strength therefore they can be suggested as the rehabilitation program to reduce soil erosion and degradation.
America, Madison, WI, pp. 329–361. Fredlund, D.G., Xing, A., Fredlund, M.D., Barbour, S.L., 1996. The relationship of the unsaturated soil shear to the soil-water characteristic curve. Can. Geotech. J. 33 (3), 440–448. Green, V.S., Stott, D.E., 1999. Polyacrylamide: a review of the use, effectiveness, and cost of a soil erosion control amendment. In: Stott, D.E., Mohtor, R.H., Sreinhardt, G.C. (Eds.), 2001. Sustaining the Global Farm. Selected Papers from the 10th International Soil Conservation Organization Meeting Held, May 24–29, 1999 at Purdue University and the USDA_ARS National Soil Erosion Research Laboratory. Isa, I., Shamshuddin, J., Khairuddin, M.N., 2017. Effects of mix vegetation and root shear strength grown on carbonaceous shale. AJAS. 10, 66–78. Johnson, C.E., Grisso, R.D., Nichols, T.A., Bailey, A.C., 1987. Shear measurement for agricultural soils: a review. Trans. ASAE 30, 935–938. Klute, A. (Ed.), 1986. Methods of Soil Analysis. Part 1, 2nd ed. Agronomy Monograph 9. ASA and SSSA, Madison, WI. Knapen, A., Poesen, J., De Baets, S., 2007a. Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices. Soil Tillage Res. 94, 425–440. Koolen, A.J., Kuipers, H., 1983. Agricultural Soil Mechanics. Advanced Series in Agricultural Sciences, vol. 13 Springer-Verlag, Berlin. Kumar, A., Saha, A., 2011. Effect of polyacrylamide and gypsum on surface runoff, sediment yield and nutrient losses from steep slopes. Agr. Water Manage. 98, 999–1004 WATER MANAGE. Lentz, R.D., 2015. Polyacrylamide and biopolymer effects on flocculation, aggregate stability, and water seepage in a silt loam. Geoderma 241–242, 289–294. Levy, G.J., Levin, J., Gal, M., Ben-Hur, M., Shainberg, I., 1992. Polymers’ effects on infiltration and soil erosion during consecutive simulated sprinkler irrigations. Soil Sci. Soc. Am. J. 56, 902–907. Levy, G.J., Warrington, D.N., 2015. Polyacrylamide addition to soils: impacts on soil structure and stability. In: In: G, Cirillo, U.G, Spizzirri, Iemma, I. (Eds.), Functional Polymers in Food Science: From Technology to Biology, vol. 2. John Wiley and Sons, Inc., Hoboken, NJ, USA, pp. 9–32 food processing. Marsh, J.D., Dixon, A.J., 1991. Effect of heavy addition of organic residues on physical characters of three soil types in Queensland. Australia. Soil Tillage Res. 20, 109–122. McKyes, E., 1985. Soil Cutting and Tillage. In Development in Agricultural Engineering Vol. 7 Elsevier Science Publishers, Amsterdam, the Netherlands. Ma’ruf, M.F., 2012. Shear strength of Apus bamboo root reinforced soil. Ecol. Eng. 41, 84–86. Materechera, S., 2010. Soil Physical and Biological Properties As Influenced by Growth of Vetiver Grass (Vetiveria zizanioidesL.) in a Semi-arid Environment of South Africa Simeon. 19th World Congress of Soil Science, Soil Solutions for a Changing World. August 1–6. Morgan, R.P.C., 2005. Soil Erosion and Conservation, 3rd ed. Blackwell Publishing Ltd, UK. Oku, E., Fagbola, O., Troung, P., 2011. Evaluation of vetiver grass buffer strips and organomineral fertilization for the improvement of soil physical properties. Kasetsart J. (Natural Sci.). 45, 824–831. Operstein, V., Frydman, S., 2000. The influence of vegetation on soil strength. Proc. Inst. Civ. Eng. Ground Improv. 4, 81–89. Page, A.L., Miller, R.H., Keeney, D.R., 1986. Methods of Soil Analysis. Part 2, 2nd edn. Agronomy Monograph 9. ASA and SSSA, Madison, WI. Soil Survey Staff, 2006. Keys to Soil Taxonomy. Department of Agriculture, Natural Resources Conservation Service, Washington, DC. Scholtès, L., Chareyre, B., Nicot, F., Darve, F., 2009. Micromechanics of granular materials with capillary effects. Int. J. Eng. Sci. 47, 64–75. Spoor, G., 1975. Fundamental Aspects of Cultivations. Technical Bulletin. Ministry of Agriculture, Fisheries and Food., UK. Torri, D., Santi, E., Marignani, M., Rossi, M., Borselli, L., Maccherini, S., 2013. The recurring cycles of biancana badlands: erosion, vegetation and human impact. Catena 106, 22–30. Truong, P.N., 2002. Vetiver grass technology. In: Maffei, M. (Ed.), Vetiveria. Taylor & Francis, London and New York, pp. 114–132. Yoo, G., Yang, X., Wander, M.M., 2011. Influence of soil aggregation on SOC sequestration: a preliminary model of SOC protection by aggregate dynamics. Ecol. Eng. 37, 487–495. Zhang, Y., Ishikawa, T., Tokoro, T., Nishimura, T., 2014. Influences of degree of saturation and strain rate on strength characteristics of unsaturated granular subbase course material. Trans. Geotech. 1, 74–89. Zhang, B., Zhao, Q.G., Horn, R., Baumgartl, T., 2001. Shear strength of surface soil as affected by soil bulk density and soil water content. Soil Tillage Res. 59, 97–106.
References Akbarzadeh, A., Taghizadeh Mehrjardi, R., Refahi, G.H., Rouhipour, H., Gorji, M., 2009. Using soil binders to control runoff and soil loss in steep slopes under simulated rainfall. Int. Agrophys. 23, 99–109. Aluko, O.B., Koolen, A.J., 2000. The essential mechanics of capillary crumbling of structured agricultural soils. Soil Tillage Res. 55, 117–126. Amiri, E., Emami, H., Mosaddeghi, M.R., Astaraei, A.R., 2017. Investigating the effect of vetiver and polyacrylamide on runoff, sediment load and cumulative water infiltration. Arid. Soil Res. Rehabil. 55, 769–777. Amiri, E., Emami, H., Mosaddeghi, M.R., Astaraei, A.R., 2018. Estimation of unsaturated shear strength parameters using easily-available soil properties. Soil Tillage Res. 184, 118–127. Blanco-Canqui, H., Lal, R., Owens, L.B., Post, W.M., Izaurralde, R.C., 2005. Strength properties and organic carbon of soils in the north Appalachian region. Soil Sci. Soc. Am. J. 69, 663–673. Bengough, A.G., Bransby, M.F., Hans, J., McKenna, S.J., Roberts, T.J., Valentine, T.A., 2006. Root responses to soil physical conditions: growth dynamics from field to cell. J. Exp. Bot. 57, 437–447. Craig, R.F., 2004. Craig’s Soil Mechanics, 7th ed. Spon Press, London. CTA, 1989. Vetiver Grass (Vetiveria Zizanioides) – a Method of Vegetative Soil and Moisture Conservation. Spore 23. CTA, Wageningen, The Netherlands. Dane, J.H., Jan, W.H., 2002. Water retention and storage. In: Dane, J.H., Topp, G.G. (Eds.), Methods of Soil Analysis. Part 4. Physical Methods. Soil Sci. Soc. Am. J. Madison, Wisconsin USA, pp. 671–717. Das, B.M., 1990. Principles of Geotechnical Engineering, 2nd ed. PWS-KENT Publishing Company, Boston. de Melo, D.V.M., de Almeida, B.G., Andrade, K.R., Souza, E.R., da Silva Souza, W.R., de Almeida, C.D.G.C., 2016. Pore size distribution and hydro-physical properties of cohesive horizons treated with anionic polymer. Afr. J. Agric. Res. 11 (44), 4444–4453. Diti, H., 1999. 15 years of bio-engineering in the wet tropics. First Asia-Pacific Conference on Ground and Water Bio-Engineering 1999. Donjadee, S., Clemente, R.S., Tingsanchali, T., Chinnarasri, C., 2010. Effects of vertical hedge interval of vetiver grass on erosion on steep agricultural lands. Land Degrad. Dev. 21 (3), 219–227. Endo, T., Tsuruta, T., 1969. The Effect of the Tree’s Roots Upon the Shear Strength of Soil. Annual Report No 18, Hokkaido Branch. Tokyo Forest Experiment Station, Japan. Fan, C.C., Su, C.F., 2008. Role of roots in the shear strength of root reinforced soils with high moisture content. Ecol. Eng. 33, 157–166. Fattet, M., Fu, Y., Ghestem, M., Ma, W., Foulonneau, M., Nespoulous, J., Le Bissonnais, Y., Stokes, A., 2011. Effects of vegetation type on soil resistance to erosion: relationship between aggregate stability and shear strength. Catena 87, 60–69. Fredlund, D.G., Morgenstern, N.R., Widger, R.A., 1978. The shear strength of unsaturated soils. Can. Geotech. J. 15 (3), 313–321. Fredlund, D.G., Vanapalli, S.K., 2002. Shear strength of unsaturated soils. In: Klute, A. (Ed.), Methods of Soil Analysis: Part 4 Physical Methods. No 5. Soil Science Society of
7