Shear Measurement for Agricultural Soils — A Review


Jul 18, 2012 (6 years and 2 days ago)


Shear Measurement for Agricultural Soils — A Review
C. E. Johnson, R. D. Grisso, T. A. Nichols, A. C. Bailey
HEAR measurement methods and devices are
reviewed and compared. Problems associated with
shear measurement are identified and recommendations
are given.
In production agriculture, tillage and tractive force
systems are applied to soil to prepare seedbeds and
accomplish other cultural practices including harvesting
and transporting crops. These force systems can cause
the soil to yield by shear, compression, tension, and/or
plastic flow. Yield or failure conditions in soil are much
more complex than in many engineering materials
because soils may vary from a near liquid state to a
brittle state. Shear failure and fracture for brittle
materials have a clear meaning. Fracture by shear is also
observed in soil. In some cases, however, fracture is not
apparent in soils which may exhibit plastic flow and
permanent deformation. The stress state that causes soil
fracture or plastic flow is a measure of the soil's shear
strength. So shear failure is some function of the stress
state than just causes failure.
A theory of shear failure was proposed by Coulomb.
He postulated that failure occurs when the maximum
shear stress reaches some critical value (Gill and Vanden
Berg, 1967). Gill and Vanden Berg (1967) trace the
history of the original Coulomb theory from its
postulation to the straight line failure envelope of the
Mohr theory.
^ m a x"C + a t a n 0
which is often referred to as the Mohr-Coulomb equation
in classical soil mechanics where C = cohesion, <}) =
angle of internal friction, and o and T^^^ X are the normal
and maximum shearing stress, respectively, acting at a
point on the failure surface. Equation [1] represents shear
failure at one single point within a soil mass; therefore,
the criterion of shear failure must be carefully
distinguished from the distribution of shear point
failures — the shear plane or surface.
Article has been reviewed and approved for publication by the Power
and Machinery Div. of ASAE. Presented as ASAE Paper No. 83-1548.
Alabama Agricultural Experiment Station Journal No. 2-83553.
Presented at a special session sponsored by ASAE Subcommittee
PM-45/3 Soil Classification and Soil Strength Measurement, S. J.
Clark, Chairman.
The authors are: C. E. JOHNSON, Professor, R. D. GRISSO and T.
A. NICHOLS, Former Graduate Research Associates, Agricultural
Engineering Dept., Alabama Agricultural Experiment Station,
Auburn University, AL; and A. C. BAILEY, Agricultural Engineer,
National Soil Dynamics Laboratory, USDA-ARS, Auburn, AL.
In reference to equation [1], Gill and Vanden Berg
(1967) state that:
"While the straight line envelope of the Mohr theory
does not rigorously represent shear yield in all soil
conditions, the theory has been close enough to
experimentally observed behavior so that equation
''[1]'' has been almost universally accepted as a law.
One confusing factor is that C and <(> are so firmly
entrenched that they are often referred to as real
physical properties of the soil. In reality they are
only parameters of the assumed yield equation.
Their logical existence can be explained only by an
interpretation of the equation and not from the
physical nature of the soil itself."
Therefore, one must realize that equation [1] is just one
theory of failure, and may not adequately describe actual
soil behavior in all conditions. In some cases a linear
failure envelope may not adequately describe the actual
shear behavior of soil, or the normal stress may not be
adequate to describe the stress state at failure.
Much apparatus and many techniques have been
contrived to quantify soil shear strength. But all the
apparatus and techniques are primarily based on one of
two methods to measure failure by shear in soil. Both
methods attempt to quantify C and ^ of equation [1] or
the shear failure envelope for the soil.
One method—direct shear—involves controlling the
shear plane of a small area of soil and measuring the
stresses on that plane. Maximum shear stress, T^ax' is
measured for a number of normal stresses, o. These data
can be plotted, T^^ax vs o, then C and <|) determined from a
straight line fitted to the data. In the second
method—the triaxial test—known stresses are applied to
a soil mass and failure is allowed to occur unrestrained.
Two or more soil specimens are sheared at different
levels of confining pressure (03) and a series of Mohr's
circles that represent the states of stress at failure are
determined. C and c(> are determined from a straight line
drawn tangent to the circles (Fig. 1).
( 7
Fig. 1—Schematic of Mohr-Coulomb failure theory.
Vol. 30(4):July-August, 1987
© 1987 American Society of Agricultural Engineers 0001-2351/87/3004-0935$02.00
Fig. 2—Shear stress-strain behavior in three soil conditions; A
cemented, B — loose, C — dense. NSDL Photo No. P10-363d.
The straight line failure envelope of the Mohr-
Coulomb theory, equation [1], does not contain any
information about soil behavior prior to shear failure and
disregards the role of soil displacement caused by stress.
Shearing displacement has been recognized as important
in traction mechanics (Wills, 1963; Taylor and Vanden
Berg, 1966; Yong and Youssef, 1978; and Turner, 1982).
The role of displacement in shearing strengt h
measurements is largely dependent on the state of
compactness of the soil. This dependence is exhibited for
three soil conditions—cemented, dense and loose—in
Fig. 2. Agricultural soils may exist throughout a wide
range of compactness from cemented to loose. Shearing
displacement may be important when soils are not
Based on observation of loose soil conditions, an
empirical expression of shearing stress, x, as a function
of shearing displacement, j, along the controlled failure
surface is
r = ^ m a x ( l - e - j/^ )
where k is a coefficient. Equation [2] has been useful for
initially loose soils (Wills, 1963 and Turner 1982), but
may be misleading in compacted soil conditions when
excessive displacement may result from ^tension' cracks
beneath the grousers of the measuring device.
Equation [2] may be combined with equation [1] to
r =( C + at an0) (l-e-J/^^) [3]
For simplicity, k is assumed not to be a function of o
although there may exist some relationship between the
two. Physically, C, (j) and k are not separate and unique
entities. Thus they are not true physical properties; but
rather, they are parameters of a mathematical model,
equation [3], representing shear. Shearing strength
characteristics are also influenced by moisture content
(Spoor and Goodwin, 1979).
Strain rate effects or loading rate effects have been the
subject of much debate. In 1948, Casagrande and
Shannon studied the effect of loading rate on soils
common to the Panama Canal zone. They found that the
least strain-rate effect was exhibited by the strongest and
driest clays and the greatest strain-rate effect was
exhibited by the weakest and wettest clays. So the role of
strain-rate depends on the soil's state of compactness
and moisture content—whether or not it exhibits plastic
behavior. For certain soil conditions, some researchers
(Larew and Atakol, 1967; Wilkins, 1966; and Cheng,
1976) have found that soil shear strength for the dynamic
case to be higher than that for the static case. Stafford
and tanner (1983) found that soil cohesion, C, increased
linearly with the logarithm of deformation rate. They
found that <t> was independent of deformation rate.
Many devices tend to utilize the direct shear method of
measurement. The ultimate use of any particular device
or technique depends on its convenience and practical
utility with respect to the soil condition being
investigated, the soil-machine relationships studied and
the desired purpose of the shear strength data.
The principle direct shear devices that have been used
to measure shear strength of agricultural soils are the
grouser plate, the translational shear box, the NIAE
torsional shear box, the sheargraph and an annular
torsional shear apparatus.
Grouser Plate
A rectangular grouser plate may be placed on a soil
surface and pulled to cause shear of the soil (Gill and
Vanden Berg, 1967). The maximum shearing force
required to shear the soil area under the grouser plate is
measured for a given applied normal load. Thus, a
number of tests must be conducted with various normal
forces for enough data to define the soil shear failure
Translational Shear Box
The translational shear box is a direct shear device in
which a two-piece rectangular box is the soil container
and loading device (Lambe, 1951; Osman, 1964). In the
direct shear test, the soil is stressed to failure by moving
one part of the shear box relative to the other. This
causes the soil to fail along a plane predetermined by the
apparatus. The maximum shearing force required to
shear the soil sample is measured for different normal
loads. Thus, a number of tests must be conducted to
define the soil shear failure envelope.
NIAE Shear Box
The NIAE shear box was developed to measure
strength of agricultural topsoils in the field (Payne and
Fountaine, 1952). It is a torsional shear device in which a
solid cylindrical sample of soil is sheared on a plane
perpendicular to the axis of the cylinder. The shearing
stress, T, for a given normal load is calculated as.
where M is the moment required to create shear and r is
the radius of the shear box or cylindrical soil sample.
Equation [4] is based on the assumptions: (a) a pure
couple is applied to the confined plane shear; (b) a
uniform normal stress is applied over the plane of shear;
(c) the shear stress is uniformly distributed from the
cylindrical center to the outer edge and is independent of
strain; (d) the rate of strain has negligible influence.
Cohron (1962) developed a soil sheargraph—a
torsional device that shears a column of confined soil at
the outer surface of a grouser shear head. Its usage is
based on the same assumptions as the NIAE shear box
and consequently equation [4] applies to the sheargraph.
The sheargraph displays results directly on a built-in
plotter using pressure sensitive paper. Compared to the
NIAE shearbox, the sheargraph is smaller. It is
convenient to use in the field, especially in compacted
Annular Torsional Shear Apparatus
Various annular torsional shear devices have been
created and studied for various purposes (Taylor, 1967;
Hvorslev, 1970; Dunlap et al., 1966; Wills, 1963; Harris
et al., 1968; Bailey and Weber, 1965; Janasi and
Karafiath, 1981). These devices consist of a grousered
ring with some inside radius, rj, and outside radius, YQ.
The shearing stress, T, for a given normal load is given by
the expression
27r ( r,3_,.3 )
Assumptions a and b for the NIAE Shear box are also
assumed to apply for annular shear devices. Both the
shear stress and normal stress are assumed to be
uniformly distributed over the grousered ring and plane
of shear failure. The soil is assumed to behave as a
plastic material. Often the shearing displacement is
measured so that shearing stress as a function of both
normal stress and shearing displacement (j in Equation
[3]) may be determined. Sinkage may also be measured.
Hvorslev (1970) investigated the soil behavior beneath
the annulus and concluded that the actual shearing
displacement in the soil is probably not equal to the
annular displacement of the annulus. Problems were
experienced when the normal load exceeded the bearing
capacity of the soil and excessive sinkage resulted.
Dunlap et al. (1966) varied the dimensions of the
annulus on a torsional shear device and found an inverse
relationship between the shear annulus size and the
measured maximum shearing stress.
Other Direct Shear Devices
Schafer et al. (1963) conducted tests in agricultural
soils with a shear vane. Both C and ^ cannot be
determined from tests conducted with a shear vane.
However, Yong and Youssef (1978) have developed a
vane-cone device which gives data for deformation and
shear-slip energy expenditures. The vane-cone device
appears useful in soft soil conditions in which both
penetration and shear strength are important and may
provide a means of assessing both the flotation and
traction thrust capability of a material. Yong and
Youssef (1978) were able to predict the measured
drawbar pull developed by a traction member in soft soil
with data from the vane-cone device better than with
data from some other measuring devices.
The conventional triaxial cell has been used by civil
engineers for soil shear measurement under saturated
and dense or cemented soil conditions at high stresses.
These conditions are not normally observed in
agricultural soils during most field operations. Other
devices similar to the design and concept of conventional
triaxial shear apparatus are the unconfined compression
test apparatus and the three-dimensional shear
apparatus. The main advantage of these triaxial devices
over direct shear methods is the stress state can be more
precisely controlled and measured.
Conventional Triaxial Shear Apparatus
A conventional triaxial apparatus, in which one can
control the stress state (a, > 02 = 03) on a cylindrical soil
sample until shear occurs, is widely used for obtaining
Mohr's circles at failure. Usually the test is began by
increasing the confining pressure on the soil sample to a
prescribed stress level (02). Then an axial force is applied
to cause a shear component (oj, on the Mohr's circle).
This shear component is increased until shear failure
occurs. Several soil samples tested at various confining
pressures o^) must be used to determine the shear failure
The role of shearing displacement cannot be easily
evaluated by this method. Thus the coefficient k
(equations [2] and [3]) cannot be determined.
The unconfined compressive strength of soil may also
be determined with a triaxial test in which Oj and 03
remain at zero gage pressure. This test condition can be
used for oen of the two or more soil specimens needed to
develop a series of Mohr's circles that represent the shear
failure envelope.
Three-Dimensional Shearing Apparatus
According to the Mohr strength theory, the shear
strength is independent of Oj. Likewise, the conventional
triaxial device does not recognize any influence of this
intermediate stress. There is evidence that the stress
path, including 02, does influence the behavior of soil
(Osman, 1964; Dunlap and Weber, 1971; Kumar and
Weber, 1974). These influences were studied with a
three-dimensional triaxial apparatus which loaded a
square prismatic shaped soil sample by independently
controlling the three principal stresses (o,, Oj, O;^). Thus,
the advantage of this device is that independent control
of the principal stresses may be arranged to simulate
actual field conditions.
It is apparent from the literature that much effort has
been expended to invent, design and develop apparatus
capable of soil shear measurement. Some apparatus and
techniques are more reliable in soft soil conditions while
others are more reliable in hard soil conditions.
While the triaxial apparatus yields results most
acceptabl e from a theoretical analysis, it has
deficiencies. It is a laboratory instrument that is
incapable of providing in-situ shear measurement in the
field. It also applies relatively high stresses during shear;
thus, the soil mass may end much more dense than it was
at the start of the test. However, it is often considered the
st andar d by which other methods and shear
measurement techniques are compared.
Many of the various devices that are based on the
direct shear method have been designed and intended for
in-situ soil shear measurement. Probably the most
portable and convenient device to use in the field is the
Vol. 30(4):July-August, 1987
sheargraph. However, shearing displacement is not
measured by the sheargraph so its utility may be
primarily for rather dense to cemented soil conditions.
Osman (1964) found that a translational shear box,
triaxial test, NIAE shear box and an annulus device give
comparable values of C; but his data were limited to a
wet sand, a dry sand and a clay. Comparisons of various
devices and methods (Bailey and Weber, 1965; and
Dunlap et al., 1966) indicate that the sheargraph,
grousered annulus and NIAE shear box give different
values of C and <t>. The sheargraph gave the highest C
values and the shear box gave the lowest. No consistent
variation of <j) with type of device was noted.
The purpose, need and use of shear measurement data
must be kept in mind when choosing a method and
apparatus for shear measurement in agricultural soil. All
direct shear apparatus involve soil-machine interaction
and so the shear response measured is at best a soil-
machine relationship that is dependent on both the soil
and the apparatus. Therefore, rather than worry about
which device is more theoretically correct, it may be more
important to consider which device creates soil failure
behaviors in approximately the same proportion as the
soil-machine system under study. The best strength
measurement apparatus may be an analog of the soil-
machine system studied (Johnson, et al., 1980).
Since many agricultural soil-machine systems create
both a soil shear failure and penetration of the soil, the
vane-cone device developed by Yong and Youssef (1978)
may be very applicable to these systems. Many
agricultural soil-machine systems, particularly tillage
machines, cause soil displacement and response through
the failure condition and not just to a condition of
impending failure. So a measurement quantifying shear
energy to failure (Schafer et al., 1963) may be more
appropriate when one considers the wide range of
compactness of agricultural soils and the influence of
compactness on the role of shear displacement in soil
shear measurement. The jury is still out on the best
method, apparatus and technique of shear measurement
for agricultural soils.
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