U.S. patent number 3,673,861 [Application Number 04/837,216] was granted by the patent office on 1972-07-04 for method and apparatus for in situ measurement of soil creep strength.
This patent grant is currently assigned to Iowa State University Research Foundation, Inc.. Invention is credited to Richard L. Handy.
United States Patent |
3,673,861 |
Handy |
July 4, 1972 |
METHOD AND APPARATUS FOR IN SITU MEASUREMENT OF SOIL CREEP
STRENGTH
Abstract
Soil creep rate is measured in situ by lowering a shear testing
device into a bore hole and expanding it so that opposing pressure
surfaces of the device engage facing surfaces of the bore hole with
a constant normal pressure. A force less than that which would
exceed the soil shearing strength under the applied normal pressure
is then exerted on the device axially of the bore hole to induce a
predetermined and constant shearing stress in the soil. A record is
made of the deformation of the soil as a function of time for
different shearing stresses to generate a family of characteristic
deformation curves. The first derivative of each curve with respect
to time is the shear rate, which tends to decrease to a
substantially constant value with increasing time. This constant
shear rate is referred to as the creep rate. A plot of the creep
rate against shearing stress yields an empirical value for the
creep threshold stress; and a plot of different creep threshold
stresses for different values of normal stress defines a stable
zone in the shearing stress-normal stress plot.
Inventors: |
Handy; Richard L. (Des Moines,
IA) |
Assignee: |
Iowa State University Research
Foundation, Inc. (Ames, IA)
|
Family
ID: |
25273839 |
Appl.
No.: |
04/837,216 |
Filed: |
June 27, 1969 |
Current U.S.
Class: |
73/841; 73/84;
73/784; 73/787; 73/794; 92/98D |
Current CPC
Class: |
E02D
1/022 (20130101) |
Current International
Class: |
E02D
1/02 (20060101); E02D 1/00 (20060101); G01n
003/24 () |
Field of
Search: |
;73/88E,84,151,101,146
;33/125 ;92/98D,99 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
540,806 |
|
Dec 1931 |
|
DD |
|
1,449,921 |
|
Jul 1966 |
|
FR |
|
Other References
Sail Mechanics in Engineering Practice, K. Terzaghi & R. B.
Peck, J. Wiley & Sons, N.Y., 1948, pp. 79-93 .
Sail Mechanics, Foundations, Earth Structures, by G. P.
Tschebotarioff, McGraw-Hill Book Comp., N.Y., 1952, pp. 122-130
& 143-145.
|
Primary Examiner: Queisser; Richard C.
Assistant Examiner: Smollar; Marvin
Claims
I claim:
1. In a method of in situ measuring soil creep rate comprising the
steps: forming a hole in the soil, forcing a contact surface
against a side wall of said hole to exert a predetermined force
thereon, inducing for a period of time in said contact surface
engaging said side wall a predetermined substantially constant
shear force less than the maximum or quick shear force for said
soil; and measuring the deformation of said soil for said period of
time after said shear force has been induced in said contact
surface.
2. The method of claim 1 further comprising: then increasing said
shearing force to a second value less than the maximum or quick
shearing force, and again measuring the deformation of said soil
for a period of time after said increased shearing force has been
induced in said contact surface, whereby a family of curves may be
generated relating deformation of the soil to time for discrete
values of shearing stress.
3. The method of claim 2 further comprising plotting values of
stabilized creep rates for said family of deformation curves as a
function of said discrete values of shear force to define the creep
threshold stress for a constant normal stress.
4. The method of claim 1 wherein said step of forcing said contact
surface against said side wall comprises lowering an expandable
device in said hole in a contracted state and expanding said device
at a predetermined depth in said hole to engage said side wall with
opposing contact surfaces.
5. The method of claim 3 wherein said step of inducing said
shearing force comprises pulling said device longitudinally of said
hole after said device has been expanded.
6. A method of empirically determining the creep failure envelope
for soil comprising inducing a shearing force on the soil under
test while maintaining a constant normal force on said soil,
recording the displacement of said soil under said shearing force
over a period of time until said displacement per unit time becomes
generally constant, thereby defining a creep rate for a known
shearing stress, then incrementally changing said shearing stress
and recording the displacement of said soil under said new shearing
force over a period of time for a number of constant but
incrementally changed shearing stresses to generate a relationship
between creep rate and shear stress thereby to define a first value
of creep threshold stress, then changing said normal stress on said
soil to a second value and repeating the above-named steps to
define a second value of creep threshold stress for said second
normal stress, said values of creep threshold stress defining a
creep failure envelope based on empirical data.
7. In a method of in situ determination of soil creep, the steps
of: forming a hole in the soil under test, forcing a contact
surface against soil surrounding said hole to induce a
predetermined normal force in said soil; then urging said contact
surface in a direction perpendicular to said normal force to induce
a shearing force of substantially constant value less than the
maximum or quick shear force in the soil; recording the
displacement of said contact surface in the direction of shear
force until the soil deformation becomes substantially linear with
time, the slope of said linear relationship between deformation and
time defining a creep rate for the applied normal force; increasing
said shear force to a second, substantially constant, predetermined
value less than the maximum or quick shear value; recording the
displacement of said contact surface for said second shear force
for a period of time until the deformation of said soil becomes
substantially linear with time, thereby defining a second value of
creep rate for a different normal force; and plotting said first
and second values of creep rate against the two values of shear
stress, the extrapolation of said plot to zero creep rate providing
one data point on a creep envelope defining a stable zone in a
shearing stress--normal stress plot.
8. The method of claim 7 further comprising the steps of increasing
the normal force on the soil surrounding the hole to a second value
and repeating the above-named steps to provide a second data point
which, with said first data point, defines a stable zone in the
normal stress/shear stress plot.
9. Apparatus for in situ testing of soil by forming a bore hole in
said soil comprising: support means adapted for placement on said
soil above said bore hole, frame means carried by said support
means for vertical movement, means for exerting a force on said
frame means to urge said frame relative to said force means, an
expandable device connected to said frame means for being lowered
into said bore hole and including first and second contact surfaces
movable relative to each other for engaging opposing surfaces of
said bore hole, said device including first and second flexible
side straps, one of said contact surfaces mounted to each of said
straps and further including a housing defining a double-acting
piston rod and cylinder unit for selectively moving said contact
surfaces apart, said housing further including first and second
flexible bellows connected to said piston rod and cooperating with
said housing to define first and second chambers for forcing said
rod to expand or contract said contact surfaces, source means for
selectively forcing pressurized fluid into one of said first and
second chambers of said housing, and means for measuring the
displacement of said contact surfaces when a shear force is applied
thereto.
Description
BACKGROUND
The present invention relates to in situ testing of soils; more
particularly, it pertains to a method of and apparatus for
empirically determining soil creep in situ.
The economic importance and destructiveness of landslides, for
example in construction of the Panama Canal, eventually led to the
widespread involvement of soils engineers and resulted in
development of analytic slope design methods now firmly established
in engineering practice. These analyses are usually based on field
data or laboratory observations of the processes involved. For
example, the familiar Swedish circle method is based on extensive
field studies which indicated that a cross section of failure
surfaces in clays closely approximates a circular arc. Other
failure surfaces commonly assumed in analytic treatment are planes
or sections of a logarithmic spiral.
In addition to reasonable potential failure surfaces, slope design
and analysis requires an evaluation of soil shear strength. Various
strength tests have been successful for determining the soil shear
strength based upon laboratory or in situ determination of the
quick shear strength and applying a "safety factor." That is to
say, based upon an actual determination of shear strength,
engineers have applied a safety factor to account for slower
shearing reactions which were not actually measured. Attempts to
correlate laboratory-determined soil strength to actual observed
slope stability have not been very successful. In some cases the
test data may indicate considerable stability where none in fact
exists. Laboratory test data may be used with confidence within
certain geographic areas or single formations if based upon
previous empirical correlations to performance.
Prevailing approaches to soil shear testing may be classified into
sampling techniques and in situ testing. Conventional techniques
used in the laboratory include triaxial tests and direct shear
tests conducted on undisturbed samples of the soil.
One commonly used method of obtaining a sample is to insert a
thin-wall cylindrical tube called a "Shelby tube" into the soil.
Several types of laboratory tests may be performed in which the
soil is subjected to a pressure and than a load is applied to cause
shearing. In the direct shear test, a short cylinder of soil is
loaded axially with a vertical load and sheared on a plane normal
to the load in a shear box. That is, the top half of a shear box is
made to slide laterally with respect to the bottom half; and the
necessary force to accomplish this is measured.
A more accurate laboratory test is the triaxial test in which a
cylinder of soil is sealed in a membrane and confined laterally
with fluid pressure. A vertical load is applied until the sample
fails. In this test the orientation of the shear plane must be
determined; and cohesion and internal friction are calculated using
Mohr theory.
In either case of laboratory testing, normal procedure requires
taking two or three samples and determining shear strength at
different values of confining pressure in order to define the
linear Coulomb relationship from which cohesion and internal
friction may be determined.
Another problem in applying laboratory test results to field
situations is the effect of the rate of shearing. Even when the
shear rate is slow enough to assure a uniform distribution of pore
pressures within a test sample, the rate is usually much higher
than that which will occur in the field. The effect is most
pronounced with clays where a higher shear rate tends to produce a
higher strength.
Based on extremely slow triaxial tests, one researcher states that
creep strength may be as low as 60 percent of the maximum shear
strength (i.e., the quick shearing strength determined by most
laboratory tests). Other researchers report that the creep strength
of undisturbed clay varies from 40 to 80 percent of the laboratory
compression strength. Still others report that from direct shear
tests the creep strength of a clay is 60 percent of its ultimate
strength, a figure which correlates with excessive settlement
observations in the field. Still others advocate the use of
residual (ultimate or remolded) strength rather than peak strength
for landslide calculations in fractured clays, because in one
sense, failure has already occurred and destroyed cohesion.
Normally, laboratory tests to evaluate susceptibility to slow
shearing or creep may take from 2 to 3 weeks before results are
achieved, and they are relatively expensive. Further, since the
samples are tested in an artificial environment and the actual
mechanism involved in soil creep is not known precisely, it is
difficult to assess the validity of a laboratory test for
determining creep rates.
Pore water pressures were recognized as an important factor in
determining soil shear strength; and laboratory shear tests are
commonly adapted to evaluate the pore pressure during shearing.
Pore pressures are then subtracted from the applied normal stress
to give shear strength parameters on an "effective stress basis."
The major difficulty in applying the results of this method is
anticipating the field pore pressure. The effective stress method
of analysis is most applicable to slow field loading conditions
where pore pressures are either monitored or are nearly zero.
For granular soils such as sand or gravel, a standard penetration
test has been developed in which a probe or sampler is driven into
the soil and the blow count (i.e., the number of blows of
predetermined force) determines the angle of internal friction
based upon previously obtained empirical data. Alternatively, the
energy required to drive a predetermined distance is recorded and a
similar correlation with empirical data is required.
For clay soils, the vane shear test has come into widespread use.
In this case, a cross-bladed vane is inserted vertically into the
soil, and a torque is applied to cause shearing about a cylindrical
surface. The vane shear test gives a total shear strength, but it
does not separate the two commonly recognized components of
cohesion and internal friction.
A U. S. Pat. No. 3,427,871 of Handy and Fox for BORE-HOLE SOIL
TESTING APPARATUS, discloses apparatus for in situ testing of soils
to determine shear strength. The apparatus disclosed therein
includes a device which is lowered into a preformed bore hole. The
device includes two pressure surfaces which are movable relative to
each other for engaging opposing sides of the bore hole.
With a constant radial pressure exerted on the pressure surfaces, a
shearing force is applied by pulling the device axially of the bore
hole. After a first maximum shear force is measured, the hydraulic
pressure expanding the pressure surface is increased so that a
second shearing force may be applied and measured. The data points
relating shearing stress at failure to the normal pressure upon the
shear plane defines the classical linear Coulomb relationship from
which cohesion and internal friction may be determined by
inspection. The teaching of that patent was principally directed
toward the determination of the two parameters of cohesion and
internal friction in determining the maximum shear strength of a
given soil on an in situ basis.
SUMMARY
The present invention contemplates applying a constant pressure
normal to the shearing plane and a constant, predetermined shear
stress less than that which would cause the soil around the bore
hole to fall in quick shear. The deformation of the soil as a
function of time is measured for different shear stresses (all such
shear stresses being less than the quick shear stress) thus
generating a family of characteristic deformation curves. The first
derivative of each curve for constant shear stress defines the rate
of shear for that stress. This rate tends to decrease and approach
a constant value with time; and it is referred to as the soil creep
rate or creep rate in shear. A plot of the creep rate against shear
stress for constant normal stress defines an empirical value for
the creep threshold stress--that is, the lowest shearing stress
which will produce deformation. The values for creep threshold
stress in relation to normal pressure may also be obtained by
following the same procedure but increasing the normal stress to a
second constant value to generate a set of data points used to
define a creep threshold envelope analogous to the Mohr-Coulomb
shear failure envelope. Thus, a more accurate and reliable
empirical value of minimum creep stress is derived from an in situ
test which eliminates the delay and expense of prior sampling and
laboratory testing methods as well as provides a more reliable
basis on which to predict long-term stability of earthern slopes
and embankments, soil bearing capacity against external loadings,
and other engineering problems involving long-term soil shear
strength.
A further advantage of the in situ soil creep strength test
described herein is a greatly reduced time requirement for testing.
About 30 minutes are ordinarily required to establish a constant
creep rate for a given shear and normal stress. Repetition at
several shearing stresses to establish the creep threshold stress
for a given normal stress requires about 2 hours. Repetition of the
above complete sequence to establish a relationship between creep
threshold stress and normal stress usually requires 4 to 6 hours. A
similar investigation by way of conventional laboratory testing
requires a minimum of several weeks.
Other features and advantages of the instant instant invention will
be apparent to persons skilled in the art from the following
detailed description of a preferred method and apparatus
accompanied by the attached drawing.
THE DRAWING
FIG. 1 is an elevation view of a tripod for supporting the testing
device;
FIGS. 2, 3 and 3A illustrate the testing device in detail;
FIGS. 4-5 illustrate the insertion and expansion of the testing
device in a bore hole;
FIG. 6 is a plot of a family of characteristic deformation curves
according to the present invention;
FIG. 7 is a plot of creep rate vs. shearing stress for determining
the creep threshold stress; and
FIG. 8 is a plot of shearing stress against normal stress
illustrating the classical Coulomb relationship.
DETAILED DESCRIPTION
Turning now to FIGS. 1-3A, the illustrated embodiment comprises a
tripod seen in FIG. 1 and generally designated 10 for supporting
the testing element, generally designated 11 and seen in FIG. 2.
The device 11 is capable of assuming a contracted or an expanded
condition, as will be explained in detail presently. The tripod
supporting element includes legs 10a, two of which are illustrated
in the drawing and supported on a base member 10b adapted to be
placed on the surface of the soil to be tested. The base 10b
defines a central aperture 10c through which the testing device 11
is lowered (in its contracted condition) into the pre-formed bore
hole, as illustrated.
At the top of the tripod 10 there is a mounting plate 10d on which
is mounted a pneumatic cylinder and piston rod unit 13 including a
cylinder 13a which receives a piston, 13b to which is attached a
cylinder 13a which receives a piston, 13b to which is attached a
rod 13c. Gas under pressure is coupled into the cylinder 13a
through a conduit 13d from a source (not shown) to force the piston
head 13b and its associated rod 13c upwardly. The pressure of the
gas (which may preferably be CO.sub.2) is regulated by any suitable
means so that a constant upward force is generated independent of
small displacements of the piston. To the underside of the mounting
plate 10d there is attached a member 15 which serves as a reference
measurement point.
The device 11 is supported on the tripod 10 by means of a yoke
generally designated 12 and including two vertical side rods 12a
and 12b connected together with upper and lower transverse end
pieces 12c and 12d to form a rigid frame. The device 11 is attached
to the lower end piece 12d of the yoke 12 by means of a pulling rod
16 and a flexible frame 17. The flexible frame 17 includes a cross
bar 17a which threadably receives the lower end of the pulling rod
16 and first and second flexible straps 17b and 17c which may be
made of strap steel.
The upper end piece 12c is provided with a threaded aperture
through which an elongated externally threaded screw 20 is
received. The screw member 20 is provided with a turning handle 20a
and its lower end portion 20b engages the upper portion of the
piston rod 13c. Thus, as pressurized gas is forced into the
cylinder 13, the piston 13b and its associated rod 13c will be
forced upward bearing against the end portion 20b of the screw 20
to exert an upward force on the yoke 12 and device 11.
Strain gauge means generally designated by reference numeral 21 is
secured by means of a bracket 21a and thumb screw 21b to the side
rod 12b of the yoke 12; and it includes a cantilever member 21c on
which is mounted a strain gauge 21d. The distal end of the
cantilever member 21c engages the reference surface 15 of the
tripod 10. The strain gauge may be used to measure vertical
displacement of the yoke 12 and device 11 relative to the tripod
10. Similarly, a direct-read dial gauge 22 may be secured by means
of a bracket 22a and thumb screw 22b to the other side rod 12a of
the yoke 12. The gauge 22 includes a movable arm 22c which engages
the reference point 15 to measure displacement of the yoke 12
relative to the tripod 10.
Turning now to the device 11 and particularly to FIG. 3, the device
includes first and second shoes 25 and 26 which provide pressure
surfaces for engaging the soil at opposing sides of the bore hole
for exerting a lateral pressure on that soil.
The soil-engaging exterior surfaces of the shoes 25 and 26 are
serrated as seen at 27 and 28 to provide alternate ridges and
grooves extending in a generally horizontal direction for firmly
engaging the interior surface of the bore hole; and in horizontal
cross section, the shoes 25 and 26 may be curved to conform to the
interior shape of the cylindrical bore hole. The shoes 25 and 26
are attached respectively to the straps 17b and 17c; so they are
freely movable relative to each other. A housing generally
designated by reference numeral 30 in FIG. 2 includes a first side
wall 31 and a second side wall 32 as seen in FIG. 3. The side walls
31 and 32 are rigidly secured together by means of four corner
bolts 35 which extend through the side plate 32 and are threadably
received in the second side plate 31.
The side plate 32 defines a central aperture 36 for receiving the
piston 37 which is secured to the inside of the strap 17c at one
end and fastened to a piston head 38 at the other end.
A first flexible bellows member 39 having the shape of a cup with a
peripheral lip or flange is secured to the left side of the piston
38 as seen in FIG. 3 by means of a bolt 40 and washer 40a. The
member 39 is commercially available under the tradename Bellofram.
The flange 39a of the bellows member 39 is sealingly secured to the
side plate 31 of the housing 30 to form a fluid cavity or chamber
41. A conduit 42 is formed through the side plate 31 and
communicates with the cavity 41 for coupling a second source of gas
under constant pressure into the chamber to expand the bellows 39.
Preferably, the second source of gas is a regulated source of
CO.sub.2. When pressurized fluid is forced into the chamber 41, the
piston 38 will be moved to the right as viewed in FIG. 3 thus
causing the shoe 28 to expand relative to the shoe 27 so that they
will engage opposing sides of the bore hole.
A second cup-shaped bellows member 43 is secured to the right side
of the piston 38 by means of a washer 44 and has a peripheral
flange 43a secured to the side plate 32 to form a sealed chamber or
cavity 45. A conduit 46 couples pressurized fluid through the side
plate 32 to the cavity 45. When pressurized fluid is forced into
the cavity 45, the piston 38 moves to the left thus retracting the
shoe 28 under positive pressure; and this has been found to provide
an advantage in moving the device 11 from a bore hole after tests.
Use of the bellows members 39 and 43 has been found to
significantly reduce the frictional forces in the device and thus
enhance its accuracy and sensitivity; however, persons skilled in
the art will be able to substitute other sealing structures (for
example O-rings) having low friction and obtain similar
results.
The devices seen in its contracted state in partially schematic
form in FIG. 2; and in its expanded form, it is illustrated in FIG.
3. If desired, one or the other of the shoes 27 and 28 may be
provided with a transducer for measuring pore pressure, as
disclosed in the above-identified patent.
OPERATION
In operation, a bore hole of predetermined diameter is formed in
the soil to be tested; and the tripod 10 is placed on the surface
of the soil with the aperture 10c above the bore hole as
illustrated in FIG. 1. With the device in its contracted stage (by
forcing pressurized CO.sub.2 into the cavity 45 via line 46, if
necessary), the device is lowered to a predetermined depth. The
strain gauge 21 and dial gauge 22 are adjusted to the reference
surface 16 form making subsequent readings. The screw 20 is lowered
to a point at which the base 20b engages the rod 13c so that the
entire weight of the yoke 12 and device is supported thereon. The
device is then expanded by forcing pressurized CO.sub.2 from its
regulated source into the cavity 41 via line 42; and the shoes 27
and 28 are forced apart and engage opposing inner surfaces of the
bore hole to exert a predetermined force normal to these bore hole
surfaces.
Having thus established a constant normal stress, a shearing force
less than the quick shear force (for the existing normal stress) is
induced in the soil surrounding the bore hole at the location of
the device by forcing pressurized CO.sub.2 from the first regulated
source to actuate the pneumatic cylinder and piston rod unit 13.
This tends to lift the yoke 12 and exerts an upward force on the
device 11. The vertical displacement of the device 11 as a function
of time is then measured by either the strain gauge 21 or the dial
gauge 22. The same measurement is recorded for an increased shear
stress, still less than the quick shear value.
Equivalent results may be achieved by pushing the device 11 in a
direction further into the bore hole since a similar shearing force
is then induced in the soil.
Turning now to FIGS. 4-5, an alternative device, based on the
disclosure of the above-identified U. S. patent, is schematically
illustrated; and it includes a winch schematically illustrated at
50 and provided with a flexible cable 51 at the bottom of which
there is located an expandable pressure-inducing device 52. The
device 52 includes a wrist pin 53 which slidably receives first and
second side elements 54 and 55 which are respectively provided with
shoes 56 and 57 for engaging opposing sides of a bore hole 59.
Coil springs 60 are fitted about the wrist pin 53 for urging the
side halves 54 and 55 respectively into a contracted state; and the
left side 54 of the device 52 is provided with upper and lower
cylinder and piston rod units generally designated by reference
numerals 62 and 63. The rod portion of each of the cylinder and
piston rod units 62 and 63 is attached to the right hand side 55 of
the device 52; and pressurized fluid introduced into the cavities
of these cylinder and piston rod units force the sides of the
device apart. Operation of this embodiment is otherwise similar to
that which has already been described. The shear force may be
measured by a dynamometer (not shown).
THEORY
As mentioned above, with a constant normal stress (hereinafter
designated by the symbol .sigma.) exerted against the walls of the
bore hole, a predetermined constant shearing stress (.tau.) is
induced in the soil surrounding the bore hole by means of the
cylinder and piston rod unit 13; and the deformation of the soil is
measured by means of the strain gauge 21 or the dial gauge 22 as a
function of time. The shearing stress is then increased to another
constant value (while keeping the normal stress constant); and
again, the deformation of the soil is measured as a function of
time. In this manner, a family of characteristic deformation curves
is generated. The results from one example have been plotted in the
graph of FIG. 6 for shearing stresses of 3.0 psi, 4.8 psi, 5.5 psi,
and 6.2 psi. For this single graph, the normal stress was held
constant.
Referring to the family of characteristic deformation curves in
FIG. 6, it will be observed that after an initial consolidation of
the soil (represented in the graph by the portion closest to the
origin), each of the deformation curves defines a "knee" and
thereafter is essentially linear. The first derivative of each
curve defines a deformation per unit time, and this is herein
referred to as creep rate. For purposes of defining a creep
threshold stress, the creep rates used are those calculated by the
linear portion of the characteristic deformation curves; in other
words, the creep rates used have a constant value which is a
function of a predetermined applied shearing stress and a
predetermined applied normal stress. It will be observed that as
the shearing stress increases, the creep rate also increases for
the same normal stress.
Turning now to FIG. 7, the creep rate (ordinate) is plotted against
the shear stress values determined as in FIG. 6 by the hollow
circles. It is seen that a generally linear relationship is defined
as illustrated by the straight line 65 in FIG. 7. As the creep rate
increases, the curve 65 curves upwardly as illustrated by the
portion 66 and asymptotically approaches the vertical line 67 which
defines the quick shearing stress (i.e., the maximum shearing
strength of the soil) for the applied normal stress. By extending
the line 65 until it intersects the abscissa (as at A') there is
defined a shear stress corresponding to a zero creep rate. This is
identified in FIG. 7 as the creep threshold stress; and it is
denoted .tau..sub.o '. In practice, the .tau..sub.o ' will mark a
safety level below which there will be no appreciable creep for any
value of shear stress.
In order to define a stable zone in the plot of normal stress
(.sigma.) against shearing stress (.tau.), a second deformation
record as shown in FIG. 6 and described above is made for a
different (preferably higher) normal stress. This may simply be
accomplished with the same apparatus by increasing the pressure in
the chamber 41 of the device 11 to generate a higher normal stress;
and then inducing a small shearing stress and measuring the
resultant deformation as a function of time. The shearing stress is
increased in incremental steps while holding the normal stress
constant; and for each increase, deformation is measured to
generate a second family of deformation curves of the kind
illustrated in FIG. 6.
From this second record of deformation curves, the creep rates
(i.e., derivatives) of constant value are determined and plotted as
the hollow triangles in FIG. 7 as a function of the incremental
values of shearing stress .tau.. This defines the straight line 68
which is extended to intersect the abscissa (as at A") to define a
shear stress .tau..sub.o " corresponding to a zero creep rate for
the then applied normal stress.
The creep envelope is the interrelationship between the normal
sress and the shear stress for which no shearing of the soil will
take place. That is to say, the creep envelope defines a zone in
which all values of shearing stress are permissible without having
to fear that shearing of the soil will take place--even over an
extended period of time.
Referring to FIG. 8, a creep envelope is shown based on the
previous example. As will be recalled, two values of the creep
threshold stress (A.sub.o ' and A.sub.o " ) were there
developed--each value of creep threshold stress being for a
different value of applied normal stress. These two values of creep
threshold stress are then plotted in the shearing stress-normal
stress plane, or .sigma. and .tau. plot (FIG. 8). In FIG. 8, the
two values A.sub.o ' and A.sub.o " define a line 68 which is
labeled the CREEP ENVELOPE (or creep failure envelope). Beneath the
CREEP ENVELOPE is the stable zone for the soil.
It will be appreciated that the CREEP ENVELOPE in the present
example was determined from only two sets of deformation data; and
in the interest of reliability and accuracy, one may wish to
generate a number of creep threshold stresses (by changing the
normal shearing force on the soil surrounding the bore hole).
However, in either case, the inventive method permits an empirical
determination of the CREEP ENVELOPE on an in situ basis for the
design of stable slopes.
In distinguishing this empirical approach to the determination of a
minimum creep stress from the prevailing prior approach, the
inventive method may be compared to the conventional method by
referring to FIG. 8. In FIG. 8, the solid line 69 represents the
linear Coulomb relation (i.e., .tau. = C + .sigma. tan .phi. where
.tau. is shearing strength, C is cohesion, .sigma. is the normal
stress, and tan .phi. is the angle of internal friction) for quick
shear.
In this prior method, a maximum allowable shearing stress is
calculated based on a calculated value of normal stress; and the
result is divided by a "factor of safety" to give an allowable
value. Since the normal stress usually varies along the potential
shear surface depending on the geometry of a problem, equations and
charts have been drawn by expert members of the profession, which
equations utilize the parameters C and .phi. to describe the
shearing resistance of the soil, and .gamma. to describe its bulk
density.
For example, the well-known Terzaghi equation for the maximum
bearing capacity of a long footing is:
where q.sub.o represents the bearing capacity, .gamma. and C are
soil parameters as previously described, b is the width of the
footing, D.sub.f is the depth of the footing below the adjacent
ground surface, and N , N.sub.c and N.sub.q are dimensionless
parameters which depend on the soil friction angle .phi.. In the
graph of FIG. 7, .phi. is approximately 25.degree., which gives
according to the Terzaghi calculation N =10, N.sub.c =25, and
N.sub.q =14. Using these values and C = 1,000 pounds per square
foot we may for example calculate the bearing capacity of a 4-foot
wide footing located at 5 feet depth in soil with .gamma. =120
pounds per cubic foot:
By the prior method this would be divided by an appropriate "factor
of safety" which is usually of the order of 2 to 5, depending on
the cost of the building, presumed reliability of the soil data,
and willingness of the owner to pay for apparent over-design or
accept a risk of failure. A common compromise is a factor of safety
of 3.0, which would give an allowable bearing capacity of
a = 35,800 .div. 3 = 11,933 pounds per foot.
The same mathematical solutions may be used with the parameters for
creep strength C.sub.o and .phi..sub.o substituted for the normal
shear strength parameters C and .phi., and applying a relatively
low factor of safety because of the greater assurance of the soil
data. From the data of FIG. 7, C.sub.o = 0 and .phi..sub.o =
25.degree., which gives for the bearing capacity by the Terzaghi
equations:
which is somewhat higher than the prior method solutions with a
presumed "factor of safety" of 3.0. Thus the true safety factor
with conventional design would be much less than 3.0, and in fact
in this case would have been 12,800 .div. 11,933 = 1.07, indicating
that the prior method design was on the verge of failure, and if
the used "factor of safety" had been 2.5, as is often the case, the
footing and building would have failed. The use of the term "factor
of safety" to describe the divisive factor used to overcome
deficiencies and inadequacies in the soil strength data has been
widely criticized within the soil engineering profession. By use of
more accurate soil data as made possible by the instant method, the
"factor of safety," may be safely reduced, a value of 1.2 to 2.0
being tentatively suggested. Whereas in some cases as in the
example illustrated this will result in a more conservative and
safer design, in other cases it will allow recognition of an overly
conservative design and institution of design economies.
It will be appreciated that the present invention provides a more
accurate method of empirically determining the creep failure
envelope while maintaining all the advantages of in situ
testing.
Having thus described a preferred embodiment of the inventive
method, it will be obvious that other apparatus may be employed
while continuing to practice the inventive principle; further, that
certain variations in the method are possible without departing
from the inventive principle. It is, therefore, intended that all
such modifications and equivalents be covered as they are embraced
within the spirit and scope of the invention.
* * * * *