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


Soil & Tillage Research 59 (2001) 97±106

Shear strength of surface soil as affected by soil bulk density and soil water content
B. Zhanga,*, Q.G. Zhaoa, R. Hornb,1, T. Bau

mgartlb,2
b

Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, PR China Institute of Plant Nutrition and Soil Science, CAU, University of Kiel, Olshausenstr. 40, 24118 Kiel, Germany Received 6 June 2000; received in revised form 22 November 2000; accepted 3 December 2000

a

Abstract This paper proposes a new method to measure the soil strength parameters at soil surface in order to explain the processes of soil erosion and sealing formation. To simulate the interlocks between aggregates or particles within top 2 mm of the soil, a piece of sandpaper (30 particles cm?2) was stuck on the bottom face of a plastic box of diameter of 6.8 cm with stiffening glue and used as shear media. The soil strength for the soils from sandy loam to clayey loam was measured with penetrometer and the new shear device at soil surface at different bulk density and soil water content. The normal stresses of 2, 5, 8, 10 and 20 hPa were applied for the new shear device. The results indicated that signi?cant effect of bulk density on soil strength was detected in most cases though the difference in bulk density was small, ranging from 0.01 to 0.09 g cm?3. It was also indicated that the measurement with the new shear device at soil surface was reproducible. The changes in soil shear strength parameters due to changes in bulk density and soil moisture were explainable with the Mohr±Coulomb's failure equation and the principles of the effective stress for the unsaturated soils. The implications of the method were later discussed. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Shear test at soil surface; Shear strength parameters; Penetrometer

1. Introduction Soil erosion and surface sealing are among the most deleterious processes to agriculture and environment (Sumner, 1995). During rainfall, raindrop compaction and soil suspension movement by water result in high shear stresses, leading to an intensive local deformation in soil erosion (Ghadiri and Payne, 1986; Rose et al., 1990). As a concomitant process the soil surface
* Corresponding author. Fax: ?86-25-335-3590. E-mail addresses: bzhang@issas.ac.cn (B. Zhang), rhorn@soils.uni-kiel.de (R. Horn), tbaumgartl@soils.uni.kiel.de (T. Baumgartl). 1 Tel.: ?49-431-880-3190; fax: ?49-431-880-2940. 2 Tel.: ?49-431-880-3190; fax: ?49-431-880-2940.

transfers into a layer, ranging from 1 to 10 mm, and results in higher bulk density, lower porosity and lower hydraulic conductivity (Moore, 1981) and in an increase in soil strength (Bradford et al., 1992). Consequently, shear strength of surface soil can be proposed as a measure of soil resistance to water erosion. Soil strength was linked to soil erosion (Torri et al., 1987), soil aggregate detachment (Nearing and Bradford, 1985; Torri et al., 1987) and seal formation (Bradford et al., 1992). Tensile strength of soils has been reported to decrease with decreasing bulk density and increasing water content. Nearing et al. (1991) found that the tensile strength ranged between 0.93 and 3.23 kPa at small bulk density and high water content, which was much higher than typical shear

0167-1987/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 1 6 3 - 5

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B. Zhang et al. / Soil & Tillage Research 59 (2001) 97±106

stress (<5 Pa) applied in rill erosion. Shainberg et al. (1994) suggested that the binding forces between particles at the soil±water surface were much weaker than the tensile forces in bulk soil. Soil particles at the interface are not con?ned, as the soil particles are within the bulk soil. Thus, the clay particles are free to swell and possibly even disperse, resulting in smaller cohesion forces between adjacent particles. Conventional methods of determining soil strength include cone penetrometer, shear vane, torsional shearbox, direct shear method. However, these methods cannot measure the properties at a soil surface with required resolution and the parameters were not suf?cient to explain the mechanical dynamics during soil water erosion. With the ?rst attempt to measure soil strength of surface soil, Collis-George et al. (1993) reported a resin plate method. This method was quick, inexpensive and the results were highly reproducible. However, the failure plane was not easily de?ned as the author indicated making the estimate of the sheared area dif?cult. In addition, it led to a tension crack and the wave-like failure surface at the edges of the square shear plate. The objective of this paper was to provide a new method to measure shear strength at soil surface at a range of low normal stresses and interpret the parameters derived from the shear tests as affected by different initial bulk density, soil water content and soil. Penetration resistance was also measured so as to compare and con?rm the effects on soil strength detected by the new device if any.
Table 1 The soils and their selected physical properties Soils Parent material Gc Gw Pc Pp Qc Qp Qw Sw
a b

2. Materials and methods 2.1. Soil preparation The samples were covering the soil parent materials of quaternary red clay (Q), sandstone (S), granite (G) and purple mudstone (P), making up the dominant parent materials in subtropical China. The soil samples were taken from the top layer (0±15 cm). Complete soil properties determined with routine methods (ISSAS, 1978) been reported elsewhere (Zhang and Horn, 2001), but selected physical properties are given in Table 1. The subscripts represent the land uses, i.e. c for cultivation, p for parent material and w for wasteland with grass and sparse pine tree. Except the cultivated soil from mudstone (Pc), which had dominant swelling clay minerals, all soils dominated with kaolinite. Air-dried samples were crushed and passed through 2 mm mesh. By adding distilled water as ?ne spray while gently stirring the soils they were wetted up to a certain water content (Table 2). To get the equilibrium of soil water content, the wetted soils were kept in plastic bags at least for 2 weeks before being used. A wetted soil was ?lled into a cylinder, 10 cm in diameter and 3 cm in height, and compacted with a ?at-face piston to the desired bulk densities (Table 2). After the preparation of soil cores, penetration resistance and shear stresses at soil surface were immediately measured with the methods described below. These measurements will be mentioned as at the same soil water content. Thereafter,

Soil texture

Classification

Sanda (%)

Siltb (%) 30.5 29.4 48.7 64.8 33.6 37.5 40.0 26.1

Clayc (%) 18.8 10.6 48.2 19.3 45.8 43.2 40.5 17.3

SOCd (g kg?1) pH (H2O) 5.24 6.68 15.14 1.47 9.06 1.68 3.94 2.84 5.53 4.58 7.21 7.61 4.59 4.67 4.62 5.18

Granite Granite Purple mudstone

Gravelly sandy loam Typic Paleudults 50.7 Sandy loam Typic Paleudults 60.0 Silty clay Haplaquepts 3.1 Silt loam Haplaquepts 15.9 Quaternary red clay Clay Typic Plinthodults 20.6 Clay Typic Plinthodults 19.3 Clay Typic Plinthodults 19.5 Sandstone Sandy loam Typic Hapludults 56.6 2±0.05 mm. 0.05±0.002 mm. c <0.002 mm. d Soil organic carbon.

B. Zhang et al. / Soil & Tillage Research 59 (2001) 97±106 Table 2 Soil water content (y), aggregate mean weight diameter and bulk densities Soils Soil water content (g kg?1) (?30 hPa) Initial Gc Gw Pc Pp Qc Qp Qw Sw
a b

99

MWDa (mm) S-dBc 242.3 228.7 425.3 305.2 342.7 376.3 270.2 1.63 1.17 1.45 1.84 0.87 0.80 0.65 1.12 (0.04) (0.05) (0.06) (0.04) (0.03) (0.01) (0.01) (0.03)
d

Bulk density (g cm?3) H-dB 1.26 1.32 1.30 1.30 1.18 1.24 1.18 1.24 (0.02) (0.00) (0.02) (0.00) (0.02) (0.00) (0.02) (0.01)
d

H-dBb 238.1 222.3 416.8 298.8 286.1 332.1 369.9 263.8

S-dB 1.21 1.27 1.21 1.25 (0.02)d (0.00) (0.00) (0.00)

193.4 167.0 225.3 205.2 203.1 219.3 201.8 170.2

1.17 (0.00) 1.14 (0.02) 1.23 (0.01)

Mean weight diameter. High bulk density. c Small bulk density. d Values in brackets are standard deviation.

the soil cores were saturated for 1 day and then placed on a sandbox for 5 days at a suction of 30 hPa (equal to the soil matric potential of ?30 hPa). These soil cores were again used to measure penetration resistance and shear stresses at soil surface. These second measurements will be mentioned as the same at soil matric potential. The initial soil water content was lower than that at the soil matric potential of ?30 hPa (Table 2). 2.2. Measurement of soil strength 2.2.1. Penetrometer Penetration resistance was measured with a needle with a ?at tip in order to avoid destruction of the micro-relief of the soil surface (Zhang et al., 2001). The needle has an end diameter of 1.3 mm and a shaft diameter of 1.0 mm. The penetrometer was mounted on a rack, which allowed an easy movement downwards and upwards. The soil core was placed on an electronic digital balance with a resolution of 0.1 g in order to determine the force needed to penetrate. The maximum reading of the mass component of a force (g) was manually recorded during the penetration distance of 10 mm into a soil core. Each value was converted into a force (N). Maximum penetration resistance, Pmax (kPa) was then estimated by its de?nition, the force divided by the area of probe base. For each soil core two readings of maximum penetration force were recorded.

2.2.2. Shear device at soil surface Fig. 1 shows the new shear device for the measurement of soil strength at small normal stresses. To simulate the interlocks between aggregates or particles and water ?lm with suspension of particles, a piece of sandpaper (30 sand cm?2) was stuck on the bottom face of a plastic box of a diameter of 6.8 cm with stiffening glue. Vertical load was added in the plastic box and the vertical stress, a load divided by the area of the plastic box's bottom, was designed at ?ve levels, i.e. 2, 5, 8, 10 and 20 hPa. A horizontal force was easily applied through a loop of string over two chain wheels by adding water into a bottle, which was connected with the loop. The weight of the empty bottle was equal to that of a weight W (g) on the other side of the chain wheel (Fig. 1). Addition of water was controlled to increase the load slowly by adjusting the height of the water supply tank. When the plastic box under certain vertical load moved for 10 mm, the valve was closed to stop the water supply. Water in the bottle was weighed. The shear stress under the applied vertical load was calculated by the weight of water in the bottle divided by the area of the bottom of the plastic box. A sequence of normal stresses 2, 8 and 20 hPa was applied on one soil core and another sequence of 5 and 10 hPa was applied on another soil cores. At each sequence of normal stresses four soil cores were randomly chosen as replicates. At one condition with given bulk density and soil water

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Fig. 1. Laboratory lay-out for determining shear strength of surface soil by direct surface shear device.

content, eight soil cores were used to measure shear stress at the ?ve levels of normal stresses so as to draw a Mohr±Coulomb's failure line. This was to minimize the number of soil cores to be prepared and the in?uence between the measurements. 3. Theory The accepted shear strength equation for saturated soils in its linear function of effective stress is given as the Mohr±Coulomb's equation t ? cH ? ?s ? uw ? tan fH (1) where t is the shear strength, cH the effective cohesion, s the total stress, uw the pore water pressure and fH the effective angle of shearing resistance. Shear strength equation for unsaturated soils was developed in terms of two independent stress variables (Fredlund et al., 1978): t ? cH ? ?ua ? uw ? tan fb ? ?s ? ua ? tan fH (2)

where s ? ua is the net normal stress, fb the angle of shearing resistance with respect to matric suction and ua, uw the pore air and water pressures, respectively. The cohesion can be thought of as having two parts; one due to physicochemical cohesion, cH and the other due to matric suction, ?ua ? uw ? tan fb . Actually, the matric suction is an isotropic type of stress as is cH . Therefore, shear strength of unsaturated soil can be approximated with Eq. (1). In this study the shear-sliding plane was between the sandpaper and a soil, not as within soil. Different parameters were used. Thus, we applied a modi?ed equation to calculate the shear strength, t: t ? ca ? sn tan d (3) where ca is the adhesion between sandpaper and soil, sn the normal stress applied on the soil surface, d the boundary surface angle of friction. ca and d play the same roles on an interface as do cohesion and angle of internal friction on planes within soil from Eqs. (1) and (2).

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According to effective stress concept, the effective stress, sH is given for saturated soil by s H ? s ? uw (4) where s is the normal stress and uw the pore water pressure. The effective stress, sH is given for unsaturated soil by sH ? ?s ? ua ? ? w?ua ? uw ? (5) where w is a factor which depends on the degree of saturation of soil. w ? 1 at saturation while w ? 0 for a fairly dry soil. Assuming that the soil air is nearly at atmospheric pressure, soil pore water pressure or soil matric potential is minus soil water suction, c. 4. Results 4.1. Penetration resistance The soil cores showed a negligible variation of bulk density in between the samples (Table 2). Fig. 2 showed the results of penetration resistance of the soils at different bulk density at the same matric

potential and at the same water content. The effect of bulk density on penetration resistance was signi?cant for all tested soils, especially at the same water content although the difference of bulk density was small, ranging from 0.01 to 0.09 g cm?3 (Table 2). For a given soil, the higher the bulk density the higher the penetration resistance detected with penetrometer. At the same matric potential (?30 hPa) the signi?cant effect of bulk density on penetration resistance was not detected for Gw and Sw, which had high sand contents and the lowest difference in bulk density. Penetration resistance of the soils of corresponding bulk density was smaller at the same soil matric potential than at the same initial water content. 4.2. Shear strength parameter at soil surface Figs. 3 and 4 shows the shear stress as a function of normal stress measured with the new shear device. The results were well reproducible for all tested soils as indicated by small standard deviation of the means of the shear stresses. Big standard deviation was usually at the biggest normal stress, i.e. at 20 hPa. The signi?cant effect of bulk density on shear stress was not detected at one or two level(s) of normal

Fig. 2. Penetration resistance of the soils at different bulk density and soil water condition: (a) at the same matric potential (?30 hPa); (b) at the same soil water content. The soils are ranked with increasing clay content. H-dB, high bulk density; S-dB, small bulk density.

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Fig. 3. Shear stress as a function of normal stress for the soils at high (-H) and small (-S) bulk densities at the same soil matric potential of ?30 hPa (-30). Bars are standard deviation of the means of four observed data; sd, signi?cant difference (at <5%) at the level of normal stress below.

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Fig. 4. Shear stress as a function of normal stress for the soils at high (-H) and small (-S) bulk densities at the same initial water content (-W). Bars are standard deviation of the means of four observed data; sd, signi?cant difference (at <5%) at the level of normal stress below.

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stresses for the clayey soils Qw, Qp and Pp at the same matric potential (?30 hPa) (Fig. 3). It was not found for the sandy soils, Gc and Sw, and the swelling soil, Pc, at the same initial water content either (Fig. 4). The signi?cant difference was often found at the normal stress level of 20 hPa at the same soil matric potential (Fig. 3) and at the normal stress level of 5 or 10 hPa at the same water content (Fig. 4). At a given bulk density, shear stress at the same normal stress was signi?cantly higher at the soil matric potential of ?30 hPa than at the same water content for all soils except Pp which showed signi?cantly lower shear stress. Fitting to the linear equation (3), shear stress was well related to the applied normal stress over the stress range with correlative square coef?cient ranging from 0.94 to 1.00. Fig. 5 shows the shear strength parameters, the boundary surface angle of friction (d) and adhesion, which were affected by bulk density, soil water content and soil type. The description of the effect of bulk density on the parameters of surface

shear strength is only con?ned to the soils which had signi?cant differences in shear strength on the shear failure lines in Figs. 3 and 4. The values of d ranged from 42.7 to 51.88. The angle values were lower for the soils of higher bulk density at the same matric potential (?30 hPa), while they were higher at the same soil water content. As to adhesion, the soils of higher bulk density had lower adhesion at the same water content except that the soil Pp decreased adhesion, while they had higher adhesion at the same water matric potential (?30 hPa). The soils at smaller bulk density except the soils from the purple mudstone (Pp and Pc) had higher boundary surface angle at the same matric potential (?30 hPa) than at the same water content, while the soils except Gw of small bulk density and Pp of high bulk density had higher adhesion. 5. Discussion Soil strength depends not only on soil and the measuring condition but also on the method of measurement itself (Bradford et al., 1992). Penetrometer is used to determine an overall soil strength within soil to the 10 mm depth in this study. The new shear device is used at surface soil to determine the parameters of soil shear strength. They are not comparable because the physical properties of the related soil volume or area were different and the parameters derived from these methods have different physical meaning. In this study, the penetrometer was used to detect the effect of bulk density on soil strength and later on to testify the measurement by the new device, but not directly to compare the measured strength itself. In this case both methods agreeably de?ned the effect of bulk density, soil water content and soil on soil strength for most of the soils. The new shear device at soil surface has the following advantages in measuring shear strength parameters. The device is very simple, cheap and easy to use. It is reproducible at a range of small normal stresses simulating the situation during a rainfall. At the same suction, the soils of higher bulk density may have a lower saturation degree Sr which will on one hand reduce the w factor in Eq. (5) (Oeberg and Saellfors, 1995; Horn et al., 1995). On the other hand, the decrease in Sr may decrease the contact points between sandpaper and soil aggregates standing out of

Fig. 5. Boundary surface angle of friction (d) and adhesion on soil surface derived with surface shear device for the soils at high (-H) and small (-S) bulk densities and at the same water content (-W) and at the same matric potential of ?30 hPa (-30). nd, signi?cance (at <5%) not detected of the bulk density.

B. Zhang et al. / Soil & Tillage Research 59 (2001) 97±106

105

the soil surface through water ?lm since soil water content was higher for the soils of lower bulk density (Table 2) resulting in a lower surface boundary angle (Fig. 5). The soils of high bulk density have relatively smaller diameter of effective capillary and larger contact angle at the same suction. On contacting with certain sized sands on sandpaper, the smaller the diameter of the capillary, the soil having larger suction can be produced. Therefore, the soils having high bulk density resulted in higher adhesion (Fig. 5). Therefore, soil strength was lower at the range of lower normal stress for the soils of lower bulk density, but higher at the range of higher normal strength at the same suction. Theoretically at the same gravitational water content, the soils of higher bulk density exert higher matric potential or smaller soil water suction, resulting in a decrease in effective stress (Eq. (5)) and a decrease in adhesion as well. However, the high bulk density causes more ?ne pores and then more contact points between sandpaper and the aggregates standing out at soil surface, which increases the boundary surface angle. This explains why the soil strength was higher at higher range of normal stress and smaller at lower range for the soil of higher bulk density at the same water content. At the condition of the same water content, the sands on sandpaper and the aggregates at soil surface could not be connected by water ?lm since the water content was relatively low. Therefore, the water suction between the sandpaper and the aggregates may be very small. However, at the same suction, soil water content was high and there exists free water between aggregates at soil surface. The sands and aggregates can be connected through water ?lm, resulting in higher suction and more contact area, contributing to higher adhesion values and surface boundary angle. The soil Pp had the greatest mean weight diameter (Table 2). Thus, the big pores were also found on the soil surface. The apparent big pores decreased the contact area/points between the sandpaper and the soil aggregates even though at suction of ?30 hPa. The soil Pc was a swelling soil. After saturation and then de-saturation at suction of 30 hPa, the structure of surface soil may have changed. As discussed above it is obvious that the derived parameters of soil shear strength, adhesion and surface boundary angle between sandpaper and surface soil,

depend on the properties of soil and characteristics of sandpaper such as sand size and sand density on the sandpaper. However, the selection of certain sandpaper or other material with strong and unchangeable roughness is beyond the objective of the study which needs further study. To standardize the method for practical use, it needs further work on evaluation and selection of sandpaper or the alternatives, its size and density. Sandpaper of ?ner sand seems to be preferable. Anyhow, this method can be easily used especially if micro-relief effects on soil strength, e.g. due to soil particle movement by erosion or sealing, and organic mineral bonding effects are indeed under consideration. It is the bene?t of the very small size of the sand particles acting as the shear vane that the differences of micro-relief strength can be even detected. However, it might be dif?cult to simultaneously measure soil water suction, which is important to explain the consequences for the parameters of the effective stress equation. 6. Conclusion The new shear device for measurement of soil shear strength at soil surface is very simple, cheap and easy to use. It is reproducible at a range of small normal stresses and the effects of bulk density, soil water content and soil type on soil shear strength were detected in most cases though the difference in bulk density was small, ranging from 0.01 to 0.09 g cm?3. The values of surface boundary angle were lower for the soils of higher bulk density at the same matric potential (?30 hPa), while they were higher at the same soil water content. The soils of higher bulk density had lower adhesion at the same water content except that the soil Pp decreased adhesion, while they had higher adhesion at the same water matric potential (?30 hPa). The soils at smaller bulk density except the soils from the purple mudstone (Pp and Pc) had higher boundary surface angle at the same matric potential (?30 hPa) than at the same water content, while the soils except Gw of small bulk density and Pp of high bulk density had higher adhesion. These results can be explained by the Mohr±Coulomb's failure equation and the principles of the effective stress. It is also indicated that the characters of sandpaper in?uence the results and more work has to do on selection of

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B. Zhang et al. / Soil & Tillage Research 59 (2001) 97±106 Advances in Soil Science: Soil Structure, its Development and Function. CRC Press, Boca Raton, pp. 31±52. ISSAS, 1978. Soil Physical and Chemical Analysis. Shanghai Science and Technology Press, Shanghai, 532 pp. Moore, I.D., 1981. Effect of surface sealing on in?ltration. Trans. ASAE 24, 1546±1552. Nearing, M.A., Bradford, J.M., 1985. Single waterdrop splash detachment and mechanical properties of soils. Soil Sci. Soc. Am. J. 49, 547±552. Nearing, M.A., Bradford, J.M., Parker, S.C., 1991. Soil detachment by shallow ?ow at low slope. Soil Sci. Soc. Am. J. 55, 339±344. Oeberg, A.-L., Saellfors, G., 1995. A rational approach to the determination of the shear strength parameters of unsaturated soils. In: Alonso, E.E., Delage, P. (Eds.), Unsaturated Soils. A.A. Balkema, Rotterdam, Netherlands, pp. 151±158. Rose, C.W., Hairsine, P.B., Prof?tt, A.P.B., Misra, R.K., 1990. Interpreting the role of soil strength in erosion process. Catena 17 (suppl.), 153±165. Shainberg, I., La?en, J.M., Bradford, J.M., Norton, L.D., 1994. Hydraulic ?ow and water quality characteristics in rill erosion. Soil Sci. Soc. Am. J. 58, 1007±1012. Sumner, M.E., 1995. Soil crusting: chemical and physical processes. The view forward from Georgia, 1991. In: So, H.B., Smith, G.D., Raine, S.R., Schafer, B.M., Loch, R.J. (Eds.), Sealing, Crusting and Hardsetting Soils: Productivity and Conservation. ASSSI Queensland Branch, Queensland, pp. 1±14. Torri, D., Sfalanga, M., Chisci, G., 1987. Threshold conditions for incipient rilling. Catena 8 (Suppl.), 97±105. Zhang, B., Horn, R., 2001. Mechanisms of aggregate stabilization of ultisols from subtropical China. Geoderma 99, 123±145. Zhang, B., Horn, R., Baumgartl, T., 2001. Changes in penetration resistance of ultisols from southern China as affected by shearing. Soil Till. Res. 59, 193±202.

proper sandpaper or alternatives before the method is to be a standard method. Acknowledgements We thank the Alexander von Humboldt Foundation for the fellowship provided to Dr. Zhang Bin and the National Foundation of Sciences in China (NSFC) (Grant Nos. 49701008 and 40071044) for the funded research project. We thank Mr. J. Lohse for building the penetrometer and the shear device. References
Bradford, J.M., Truman, C.C., Huang, C., 1992. Comparison of three measures of resistance of soil surface seals to raindrop splash. Soil Technol. 5, 47±56. Collis-George, N., Philippa, E., Tolmie, E., Moahansyah, H., 1993. Preliminary report on a new method for determining the shear strength of a soil surface: the resin plate method. Aust. J. Soil Res. 31, 539±548. Fredlund, D.G., Morgenstern, N.R., Widger, R.A., 1978. The shear strength of unsaturated soils. Can. Geotech. J. 16, 121±139. Ghadiri, H., Payne, D., 1986. The risk of leaving the soil surface unprotected against falling rain. Soil Till. Res. 8, 119±130. Horn, R., Baumgartl, T., Kayser, R., Baasch, S., 1995. Effect of aggregate strength on strength and stress distribution in structured soils. In: Hartge, K.H., Stewart, B.A. (Eds.),


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