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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Erosion Mechanics of Soils with an Impermeable Subsurface Layer

^a Dep. of Natural Resource Sciences and Landscape Architecture, H.J. Patterson Hall, Univ. of Maryland, College Park, MD 20742 USA
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA

froese{at}wam.umd.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
More than 50% of annual soil loss in a number of temperate regions of the world occurs when frozen soils are thawing. Such losses occur as a consequence of relatively high surface soil water contents due to the presence of a subsurface impermeable layer. This laboratory study addressed the soil mechanical principles governing erosion of soils with an impermeable subsurface layer (frozen or compacted, etc.). We examined (i) the effect of a freeze–thaw cycle on soil cohesion and (ii) the effect of a subsurface impermeable layer on soil detachment during raindrop impact for two soils: a Galva loess (silty clay loam, mixed, mesic Typic Hapludoll) and a Nicollet glacial till (loam mixed, superactive mesic Aquic Hapludoll). A Mohr diagram was constructed, based on a series of triaxial tests at three matric potentials. Soil cohesion for each treatment was determined from the Mohr diagrams. Soil water content treatments of 0.15 and 0.25 g g^-1 were imposed before freezing and thawing. The freeze treatment was 90 min at -12°C, followed by a 30-min thaw period. Immediately thereafter, shear strength and detachment were measured. Soil cohesion values were <50 kg m^-2 and were not affected by freezing or by water content-at-freezing. The single-water-drop detachment values for soils with an impermeable layer vs. without an impermeable layer increased from 0.016 ± 0.0065 g to 0.054 ± 0.0265 g for loess and 0.036 ± 0.0071 g to 0.145 ± 0.0635 g for till. With an impermeable subsurface layer, soil matric potential is driven towards 0 Pa, resulting in extremely weakened soils prone to high soil-detachment values and quite likely high soil-erosion losses.

Abbreviations: A, cross-sectional area of the soil sample at the shear plane • C, cohesion • M_s, oven-dry mass of soil above the shear plane • s, load applied by the Instron at failure • s, mechanically added load • t, shear strength • X, f (degree of soil saturation) • ₁, major principal stress • ₂, intermediate principal stress • ₃, minor principal stress • _n, applied load normal to the shear plane • , shear strength • , angle of internal friction • _m, soil water matric potential

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
SOIL EROSION RATES vary throughout the season, with the most erodible soil conditions occurring when frozen soils are thawing. More than 50% of annual soil loss in a number of temperate regions of the world takes place during this relatively brief period (Kirby and Mehuys, 1987). Although extensive research has addressed the erosion processes occurring during the summer and winter months, little attention has been directed to the disproportionately high soil-loss rate exhibited by thawing soils.

Shear strength () is a quantitative measure of a soil's internal resistance to externally applied forces before the soil fails. When raindrop-impact forces applied to the soil exceed the soil's and cause a shear plane to develop, soil detachment occurs and the potential arises for soil erosion. Shear strength is calculated in Eq. [1] as

(1)

where = shear stress along the plane of failure (kg m^-2), C = cohesion (kg m^-2), _n = applied load normal to the shear plane (kg m^-2), and = angle of internal friction. The _n is the sum of the external loads (compression and mass of soil) and internal loads (matric potential forces) (Eq. [2]):

(2)

where s = mechanically added load (kg m^-2), M_s = oven-dry mass of soil above the shear plane (kg), A = cross-sectional area of the soil sample at the shear plane (m²), X = f (degree of soil saturation: 1 at saturation, 0 at a volumetric water content of 0 g g^-1), and _m = soil water matric potential at the shear plane (Pa, converted to kg m^-2). At the surface of a field soil, mechanically added load (s) and mass of soil above a shear plane (M_s) approximates 0 kg m^-2. Therefore, the strength of the surface soil (Eq. [1] and [2]) is solely a function of the cohesion parameter (which varies among soils and soil conditions), the angle of internal friction, and _m.

Soil detachment is the initial phase of the water-erosion process (Farmer, 1973). Detachment due to raindrop impact increases as _m increases (Al-Durrah and Bradford, 1981 and 1982; Cruse and Larson, 1977) and as bulk density decreases (Al-Durrah and Bradford, 1981; Cruse and Larson, 1977), both resulting in lower . In saturated or nearly saturated soils, is inversely related to _m (Towner, 1961; Towner and Childs, 1972). The role of _m on soil strength and soil detachability is well documented; however, no study identifies the role of a subsurface impermeable layer on surface _m, , and erodibility.

A soil-surface seal develops as aggregates are broken into fragments or primary particles by raindrop impact. Bradford et al. (1987) noted that detachment varies greatly among soils and depends largely upon the degree of surface sealing. At least two reasons exist for this observation, both of them related to the surface-seal effect on soil strength. First, the surface-seal bulk density is higher than that for unsealed soil. Increasing bulk density increases soil and concurrently reduces soil detachment from raindrop impact (Cruse and Larson, 1977). The second reason is less well documented, but evidence suggests it may play a significant role as well. Water flow through the high bulk-density layer is restricted. The hydraulic conductivity of this layer is reduced by seal development, whereas that of the subseal layer is not altered. Thus, water flow through the seal is impeded, whereas flow away from the seal base is unimpeded (Segeren and Trout, 1991). This combination of conditions results in a _m less than 0 at the base of the seal, but at a few millimeters above this depth the surface _m may be very close, or equal, to 0 during rainfall (Edwards and Larson, 1969). A _m gradient through the seal results in the _m very near the surface, i.e., within the depth of the crater formed by raindrop impact, of less than 0 (Sharma et al., 1981) and, therefore, is influential in determining detachment zone (Eq. [1]) and, in turn, soil detachment. Simulating surface crust formation in the laboratory, Sharma et al. (1981) found decreases in the _m gradient across the seal of up to 1.4 kPa during simulated rainfall. This is critical because _m changes between -0.5 and 0 kPa seem to influence detachment (at least for individual aggregates) much more than comparable changes at lower _m (Francis and Cruse, 1983). In the Francis and Cruse (1983) study, detachment differences between _m of 0 and -0.5 kPa were 31 times greater than those observed for _m changes between -0.5 and -1.0 kPa. Cohesion has also been found to increase 3.5-fold from _m values of 0 to -0.5 kPa (Formanek et al., 1984). Thus, even a slight change in _m near the surface where _m is very close to 0 may significantly affect detachment.

Based on detachment rates, Moore and Singer (1990) suggested that the surface seal is more cohesive than the nonsealed soil and hence less erodible. Froese and Cruse (1997) hypothesized that surface-seal formation is impeded by the presence of an impermeable, frozen layer beneath the surface. The resulting restricted infiltration, high _m, and low soil , a consequence of the subsurface impermeable layers, could conceivably increase detachment from impacting raindrops.

This study focused on the soil mechanical principles that cause soils with an impermeable subsurface layer (or slowly permeable subsurface layer so as to influence surface layer matric potential during a rain event, such as is found under frozen or compacted conditions) to be highly erodible. The objectives were twofold: (i) identify the effect of one freeze–thaw cycle on soil cohesion (a parameter) for two soil phases (loess and till) differing in texture and organic matter content and two soil water contents-at-freezing (0.15 and 0.25 g g^-1), and (ii) determine the effect of a subsurface impermeable layer on soil detachment during simulated raindrop impact.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils
Two Iowa soils were examined in this laboratory study. Glacial-till and loess-derived soil materials were chosen for examination because of their differing parent materials and resultant contrasting physical characteristics (Table 1) .

Matric Potential Establishment
Matric potential was established at the base of the cores using a tension table. (A schematic diagram of the tension table may be found in Cruse and Larson, 1977.) The tension table was constructed from a 0.4- by 0.5-m acrylic sheet with two outlet ports. Tygon¹ tubing connected these ports to a 0.46-m length of glass tubing marked in millimeters. The glass tubing (i.e., the end of the hanging water column) was positioned horizontally. Adjusting the height of the glass tubing with respect to the soil cores on the tension table changed the _m in the soil cores. The _m at a particular level in the cores was determined by Eq. [3]:

(3)

where _m = matric potential acting at a particular level in the core (m, converted to Pa), H_tt-c = the height difference between the tension table and a particular level in the core (m of water), H_t-tt = the height difference between the middle of the glass tubing and tension table (m of water), and H_c = the potential resulting from capillary forces in the glass tubing (m of water). A T-connection and shut-off valve were placed in the Tygon tubing, allowing water to enter the system from a reservoir.

Filter paper was placed directly over the two ports and overlaid with blotter paper (the wetting medium). Six cores were placed 6 cm from each port in a circular pattern. To minimize evaporation from the system, a polyethylene sheet (with holes slightly greater than the diameter of the soil cores) covered the blotter paper.

Water was added to the system so that the blotter supported a continuous water column to the glass tubing. The water contained 0.001 L/L formaldehyde and methanol, which retards fungal growth in the soil cores and has a negligible effect on the surface tension of water at these low concentrations (Green, 1962). Equilibrium was reached when the water meniscus in the horizontal glass tubing ceased to move.

To better simulate infiltration under rainfall, water equal to 20% of the soil core pore volume was gently added to the top of each core with a syringe 10 min before initiating either the or detachment test. (Ten minutes was the time required for 20% of the pore volume, or 0.5 mL of water, to percolate through a core as determined by meniscus movement in the hanging water column.)

Mohr Diagram
Effective normal stress, _n, has three components: the major (₁), intermediate (₂), and minor (₃) principal stresses. The normal stress acting on a potential plane of failure may be determined by plotting a series of Mohr circles (Fig. 1) . The abscissa and ordinate represent _n and , respectively. Each circle is centered at the coordinates , where ₁ = applied load normal to the surface of the core and ₃ = the absolute value of _m (Das, 1994). Towner (1961) and Childs (1955) have shown that under saturated conditions, negative _m in a soil specimen is equivalent to an external load (that is, the minor principal stress, ₃) on the soil as it affects . Thus ₂ (the intermediate principal stress) and ₃ (the minor principal stress) are assumed to be equal to each other and to the absolute value of soil _m. The effective intermediate principal stress, ₂, has no influence on the of the soil (Craig, 1983).

View larger version (17K): [in this window] [in a new window]   Fig. 1 Sample Mohr diagram

Triaxial Test
An Instron Universal Testing Instrument (Model 1125, Instron Universal Testing Instrument, Canton, MA) was used to perform the unconfined, drained compression tests (Bishop and Henkel, 1962). The tension table (which ensured a constant ₃) was placed directly underneath the load cell and a slow rate of vertical strain, 3.3 x 10^-6 m s^-1, was applied to the cores. This slow rate of strain eliminated, or at least minimized, _m changes in the core from those imposed by the hanging water column. During load application, stress was plotted as a function of time on a strip chart recorder.

Major principal stress at failure (or applied load), ₁, is given in kg m^-2 and was determined by Eq. [4]:

(4)

where s = load applied by the Instron at failure (kg m^-2), 2/3M_s = mass of the soil in the soil core above the failure plane (kg), and A = cross-sectional area of the soil core (m²). The _m was assumed to be equal to the potential at the top of the soil core (Pa, converted to kg m^-2). The X was assumed to be equal to 1. Compression tests for each _m were performed on three cores, thus obtaining three replicate measurements at each _m.

Detachment Measurements
A raindrop tower of 1.98 m was constructed so that single drops of 0.0037-m diam. (2.56 x 10^-5 kg) fell through a 0.075-m diam. acrylic cylinder. Based on drop diameter and height of fall, the velocity at impact was estimated at 5.2 m s^-1 (Meyer, 1965). During the detachment test, the tension table was slid into position underneath the tower and one drop allowed to hit the target core. Modified aluminum cupcake tins (with holes slightly larger than the diameter of the cores cut into the base of the cupcake tin and the top edge of the tin curved inward) caught the raindrop splash. A tin was placed over each core before waterdrop impact.

Water Content-at-Freezing
The soil water contents in the and detachment cores were lowered before they were frozen. Following their initial wetting, the cores were removed from the tension table and dried under a heat lamp to the desired water contents (i.e., either 0.15 or 0.25 g g^-1). The cores were weighed periodically to determine when the desired water content had been reached.

Soil State
After their soil water contents were lowered, those cores randomly designated to be frozen were then placed in a -12°C freezer for 90 min. Immediately thereafter, the cores were withdrawn from the freezer and allowed to thaw at room temperature for 30 min. The cores appeared to be completely frozen after 90 min and completely thawed after 30 min. Cores not exposed to the freezing treatment remained in an enclosed chamber at room temperature.

A key element of this experiment was examining the effect of an impermeable subsurface layer on soil detachment. Impermeable conditions were created by placing thawed cores in a small, metal container. As a result, when the cores were placed on the tension table, matric equilibration was impeded by the impermeable layer (e.g., an ice lens in the field, compacted layer, or the metal container in this study).

Matric Potential
Once the cores had undergone the freeze and thaw treatment, they were placed on the tension table and allowed to absorb water. Cores were wet to -65 Pa and equilibrium was maintained for 30 min. Following this, the _m was adjusted to the desired level for the particular (or detachment test) and once reached, allowed to equilibrate there for 45 min.

In the portion of the experiment, cores at three levels of _m (-225, -520, and -1010 Pa) were used to construct the Mohr-Coulomb failure envelope. The _m in the core at the level of shear plane development (the plane evidenced by a bulge located approximately one-third of the distance from the bottom of the core to the top) was assumed to be ₃. Detachment cores, however, were exposed to four _m (measured at the top of the core, where detachment occurs): 0, -65, -560, and -1045 Pa.

Considering that under field conditions, the water content of a soil above a subsurface impermeable layer (such as an ice lens) may reach saturation, special procedures were used for the detachment test in applying _m to the impermeable layer treatment (also referred to as the partly thawed treatment). To simulate this condition in the laboratory, after the core was frozen and thawed, it was placed inside a metal container on the tension table and the water level within the container was gently raised to the surface of the core. (In the absence of the metal container to confine the water, the core's surface _m would equal the height of the soil core and would be lower than that in the field with free water perched to the soil surface.)

Statistical Design
The experiment followed a completely randomized factorial design with three replications. The experimental unit was a soil core; the response variable was ; and the treatments were soil phase (loess and till), soil state (never frozen, and frozen and thawed), and water content-at-freezing (0.15 and 0.25 g g^-1). The detachment experiment was a completely randomized split-plot factorial design with three replications. The experimental unit was again a soil core, but the response variable was soil detachment, and the treatments were soil phase (loess and till), soil state (never frozen, frozen, and thawed, as well as frozen and partly thawed), and water content-at-freezing (0.15 and 0.25 g g^-1). The factorial combination of phases, soil states, and water content-at-freezing were randomly placed on the tension table. The tension table was maintained at a given _m (0, –65, –560, and –1045 Pa) and the _m treatments (main plots) were randomized in time. Data were analyzed using an analysis of variance. Means for those factors identified as being significant (P 0.05) sources of variation were separated using an LSD_0.05.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Shear Strength
Significant differences in cohesion were detected only between soil-phase treatment means (loess and till), but not between soil water content-at-freezing (0.15 and 0.25 g g^-1) or soil state (never frozen vs. frozen and thawed) treatments (Table 2) . No interactions between main effects were significant. Loess soil's higher cohesion is likely due to its greater clay fraction (Staricka and Benoit, 1995; Lehrsch, et al., 1991) and higher organic matter content (Lehrsch et al., 1991) than the till soil. Given the limited role that cohesion plays in the strength of sands (Yong and Warkentin, 1966) and the large percentage of sand in the till (Table 1), one would expect lower cohesion for the till than for the loess.

There is disagreement in the literature regarding the effect of a small number of freeze–thaw cycles on aggregate stability. Lehrsch (1998) and Lehrsch et al. (1991) also found that stability of field-moist aggregates increased as the number of freeze–thaw cycles (from zero to three) increased. Dagesse et al. (1997), however, observed no significant difference between one and two freeze–thaw cycles. In the present study, cohesion was statistically unchanged after one freeze–thaw cycle. Differences in soils and technique (soil type, aggregate size, sample handling, pretreatment water content, etc.) likely account for these inconsistent results.

Shear strength was linearly related to _m between -1010 and -225 Pa (Fig. 2) . Statistical analysis revealed that loess soils were more sensitive to changes in _m than till soils; however, no difference was observed between the never-frozen and frozen-and-thawed treatments or the 0.15 and 0.25 g g^-1 water contents-at-freezing.

View larger version (23K): [in this window] [in a new window]   Fig. 2 Effect of matric potential on shear strength

View larger version (27K): [in this window] [in a new window]   Fig. 3 Effect of matric potential on detachment

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
An impermeable layer (or slowly permeable layer as is found under frozen or compacted conditions) serves as a barrier to water movement during rainstorms, driving _m towards 0 Pa. In contrast, water infiltration is less impeded under permeable (unfrozen) conditions. Examining the equation, tan (Eq. [1] and [2]), one sees that it is the _m component that causes such an impact on . Considering the soil surface during a rainstorm in which a seal has developed and under conditions of a subsurface impermeable layer, components s and M_s are 0, and the _m rises to 0 Pa. Based on this study, typical cohesion values for soils under conditions of a surface seal are <50 kg m^-2 and do not change because of freeze treatment or water content-at-freezing (Table 2). Surface _m, however, changes dramatically and can easily rise to 0 Pa or possibly change to positive potential during raindrop impact. Under these circumstances, becomes a function solely of cohesion. As a result, detachment rates remain high and do not decline as they do when, for example, the soil is permeable and the surface layer bulk density can increase.

An important observation in this study was the rapid escalation of detachment as _m approaches 0 Pa (Fig. 3). The relationship between detachment and _m near 0 Pa needs to be defined. Furthermore, this elevated detachment is not explained by the relationships defined in the Mohr-Coulomb equation (Eq. [1] and [2]). Therefore, some other factor must be affecting detachment. One possible explanation considers the role of a raindrop as both a wetting source and a source of impact energy. When a drop impacts the soil surface, part of its energy goes to wetting the soil (increasing _m) and part to detaching soil particles. If the existing soil _m is 0 Pa, then the raindrop's energy may be directed only toward soil detachment. If true, this may explain the escalation in detachment from soils at _m approaching 0 Pa and, therefore, the highly erodible conditions that precipitate the disproportionate losses of soil during the thawing period. Another possible explanation of the radically high detachment values between 0 and -65 Pa in this study involves _m dynamics in the impact zone. With negative _m and permeable soil conditions, the impacting waterdrop may have a relatively small effect on _m in the impact zone. That is, equilibration with water potential in the surrounding soil may be very rapid. As _m increases to 0 Pa and impermeable conditions hold _m at or near 0 Pa in the impact zone, a potential that exceeds 0 Pa in the impact zone may result from waterdrop impact pressures. The resulting impact potentials greater than those imposed as treatments would lead to higher-than-expected detachment.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Journal paper No. J-17654 of the Iowa Agriculture and Home Economics Experimental Station, Ames, Iowa. Project No. 3325.

¹ Mention of a trademark or a product name is for information purposes only and does not imply an endorsement by Iowa State University or the University of Maryland.

Received for publication February 18, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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