AD-A258 263 URllI

Comparison of

Thawing Soil Strength Measurements

for Predicting Vehicle Performance

Sally A. Shoop September 1992

DTIC

ELECTE DEC .1.7 19921

aWI-

"a • •i•"'"•
Abstrct

The CRREL Instrumented Vehicle (CIV), shear annulus, direct shear, and

triaxial compression devices were used to characterize the strength of

thawed and thawing soil. These strength values can be used in simple

traction models to predict the tractive performance of vehicles. Strength

was evaluated in terms of the parameters c' and V' based on the MohrCoulomb failure criterion. It is proposed here that an instrumented vehicle

is best suited for terrain characterization for mobility studies because the

conditions created by a tire slipping on a soil surface are exactly duplicated. The cd and O' values from the shear annulus were found to overpredict

traction because of the low normal stress applied by the annulus and the

curved nature of the failure envelope. Of all the tests, the direct shear test

yielded the highest 0' value, most likely because the test was run at a slow

deformation rate under drained conditions. The triaxial test results were the

most similar to those from the vehicle. All test methods show 0' increasing

with soil moisture up to the liquid limit of the soil and then decreasing. As

measured With the vehicle, O' was also found to be strongly influenced by

the thaw depth.

Cover: The instrumented wheels of the CRREL Instrumented Vehicle can be

used to measure the soil-Uire shear strength required for vehicle

traction.

For conversion of SI metric units to U.S./Britlsh customary units of measurement consult ASTM Standard E380, Metric Proctice Guide, published by the

American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa.

19103.
"RREL Report 92-17

US Army Corps

of Engineers

Cold Regions Research &

Engineering Laboratory

;omparison of

hawing Soil Strength Measurements

xr Predicting Vehicle Performance

illy A. Shoop September 1992

Ac....... PO

NTIS

Justtif lat I•,

Distribstion/

Availability Codes

S Aai ad/or 1

Dist Special

DTIC QUALITY INSPECTED 1

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pared for

ICE OF THE CHIEF OF ENGINEERS 92 12 16 077

Droved for public releas; distribution Is unlimited.
PREFACE

This report was prepared by Sally A. Shoop, Research Civil Engineer, of the Applied Research Branch, Experimental Engineering Division, U.S. Army Cold Regions Research and

Engineering Laboratory (CRREL). The project was funded by DA Project 4A762784AT42,

Design, Construction, and Operations Technology for Cold Regions; Task CS, Work Unit

007, Off-Road Mobility in Thawing Soils.

This report was originally presented in 1989 and published at the North American ISTVS

Conference in Victoria, British Columbia.

The author thanks the following people who contributed to this work: Glenn Durell performed all the triaxial testing; Richard Roberts performed the direct shear tests; Stephen Decato,

Rosanne Stoops, and Gunars Abele assisted with the shear annulus and the vehicle tests; and

George Blaisdell, Gunars Abele, and Maria Bergstad provided many helpful comments regarding this paper.

The contents of this report are not to be used for advertising or promotional purposes. Citation of brand names does not constitute an official endorsement or approval of the use of such

commercial products.
CONTENTS Page

Introduction ........................................................................................................................... I

Objective .......................................................................................................................... I

Background ...................................................................................................................... 1

Strength m easurem ent techniques ........................................................................................ 2

Triaxial com pression ...................................................................................................... 3

Direct shear ................................................................................................................. 3

Shear annulus device ...................................................................................................... 4

CRREL Instrum ented Vehicle ........................................................................................ 5

Discussion ............................................................................................................................. 6

Com parison of m ethods .................................................................................................. 6

Influence of soil conditions ............................................................................................. 8

Traction prediction ........................................................................................................... 9

Conclusions ........................................................................................................................... 10

References ............................................................................................................................. 10

Abstract ................................................................................................................................. 11

ILLUSTRATIONS

Figure

1. Poor correlation between cone index or cone gradient and tractive coefficient ............ 1

2. Grain-size distribution for Lebanon sand ....................................................................... 2

3. The Mohr-Coulomb failure criterion is a straight-line approximation of the curved

failure envelope ................................................................................................. 3

4. Triaxial com pression test and analysis of results ........................................................... 3

5. D irect shear test m ethod and data analysis ..................................................................... 4

6. Shear annulus device and data analysis ......................................................................... 5

7. Net traction can be resolved into gross traction and motion resistance ......................... 6

8. Vehicle traction test and data analysis ........................................................................... 6

9. Com parison of failure envelopes for each test m ethod ................................................. 7

10. Failure envelopes from the shear annulus and the CIV for each set of soil conditions. 8

11. ' increases with water content until the plastic limit is approached and then drops

rapidly ............................................................................................................... 8

12. Traction increases with water content and then drops sharply when water content

reaches the plastic lim it ..................................................................................... 9

13. As calculated from the CIV, ý' decreases with thaw depth up to approximately

15cm .............................................................................................................. 9

14. Traction predicted from the shear annulus overpredicts traction measured with

the CIV because of the low normal stress applied with the shear annulus .......... Q

15. Comparison of the CRREL Instrumented Vehicle and the Hard Mobile Launcher ...... i0

TABLES

Table

1. Triaxial test data ............................................................................................ ....... 3

2. Data from direct shear tests .......................................................................................... 4

3. c' and V" calculated for each test sequence using the shear annulus, based on peak

shear stress ........................................................................................................... 5

4. c' and V' calculated from CIV tests for each soil condition .......................................... 6

5. Comparison of the characteristics of the strength test methods used ............................ 7

6. Tire loading and contact area of the CIV and the HML and corresponding traction

and resistance measurements made in the same soil conditions ...................... 10

iii
Comparison of Thawing Soil Strength Measurements

for Predicting Vehicle Performance

SALLY A. SHOOP

INTRODUCTION readings for a thawing silty sand do not correlate with

the observed vehicle traction (Fig. 1). This lack of

Objective correlation has also been observed for other soil condiThe objective of this work is to compare terrain tions by Wulfsohn et al. (1988).

strength measurements made using an instrumented Other methods of measuring soil strength have been

vehicle with the more traditional soil strength measure- applied to vehicle mobility. Chamberlain et al. (1988)

ments. In addition, the validity of using strength charac- and Blaisdell et al. (1987) tested cored soil samples in

terizations based on measurements with an instrument- triaxial compression to characterize the soil conditions

ed vehicle to predict the tractive performance of other for mobility. Kogure et al. (1988) compared different

vehicles is assessed. soil strength tests (unconfined compression, vane shear,

Because of technological advancements in tire rub- and direct shear) and determined that each test can give

ber and tread design, the weak link in the soil-vehicle considerably different results. They note that the direct

system is nearly always the soil strength. To predict off- shear test resembles most closely the failure mechanism

road vehicle performance, the soil must be character- of soil beneath a tire. In the 1950s a field instrument

ized. Here, a very simplistic model, based on the well called the Bevameter was developed (Bekker 1969).

known Mohr-Coulomb yield criterion, was used to This apparatus was designed to load and fail the soil in

predict vehicle performance. The soil strength parame- a manner similar to that caused by a vehicle. The soil

ters, cohesion (c') and internal angle of friction (0'), and properties measured would then be more applicable for

the vehicle weight and tire contact area were used as use in predicting vehicle performance.

input to the model. Several different test methods for The constitutive equations describing unsaturated

measuring soil strength were used to determine the most soil behavior are very complicated. Soil strength is deappropriate method. All tests were performed on a pendent on many things, such as soil density and degree

thawing silty sand as part of a larger study of vehicle of saturation, not to mention the natural inhomogeneity

m obility on thaw ing soils. 0 _ _0_1_1_1_1

0.70

Background C-- indeex

In the past, a large number of vehicle mobility pre- 0.65 0 o corrmeltion M 042 ID

diction schemes have been based on Cone Index values Z

and empirical correlations generated from large data I oE 

no

sets. Generally, a hand-held cone penetrometer is used

to obtain a Cone Index of the soil. The penetrometer

has the advantage of being portable, lightweight, and 0.55

easy to use. However, the application of the Cone o 0 o et 0

Index to research on vehicle mobility on thawing soils 0.00

is problematic. The cone penetrometer was intended 0

for gross estimates in "go" or"no-go" situations; it was 0.450 I I

not in-tended to differentiate between more subtle 60 120 Io 240 300

differences in terrain conditions. This makes it diffi- Con Index nd cone Gradient

cult to apply the method to research studies. In addi- Figure 1. Poor correlation between cone index or cone gration, as reported in Shoop (1989), cone penetrometer dient and tractive coefficient (after Shoop 1989).
and anisotropy of the material. Even for a uniform soil STRENGTH MEASUREMENT TECHNIQUES

sample, the failure mode and strength depend on the

stress distribution. Based on the complicated behavior The standard laboratory tests-triaxial compression

of soils and the need for a reasonably simple description and direct shear-were performed along with in situ

ofsoilbehaviorforpracticalmobilitypredictionschemes, tests using a shear annulus device and the CRREL

it makes sense to duplicate the soil loading conditions Instrumented Vehicle (CIV). Both of the in situ methcreated by a vehicle in order to simplify the soil behav- ods aim to duplicate the type of failure initiated by a tire

ior equations. The Bevameter attempted this, although slipping on a soil surface and are therefore more closely

imperfectly. related to vehicle mobility but less similar to the strength

It seems only logical to measure the soil strength of the soil as measured by laboratory tests.

properties using a vehicle, thereby duplicating the load- All in situ testing occurred in the vehicle mobility

ing conditions exactly. The mobility strength parame- test basin in the Frost Effects Research Facility at

ters can then be used to predict the performance of other CRREL. The test area is 36.6 x 13.1 m and can be frozen

vehicles. A method fordoing this using an instrumented using freezing panels and thawed with elevated convehicle was developed for this purpose. In addition, a stant air temperature. The test soil used for this study

comparison between traditional soil strength measure- was a frost-susceptible silty sand. The grain-size distriments and those obtained using a vehicle is helpful in bution curve is shown in Figure 2.

determining which technique best represents soil strength All the test data were analyzed in terms of cohesion,

for mobility studies if a test vehicle is unavailable or c', and internal angle of friction, 0', as determined by

impractical. using the Mohr-Coulomb yield criterion:

This report presents a comparison of soil strength

measurements obtained using an instrumented vehicle 't = c' + an tan ' (1)

with traditional soil strength measurements. The instrumented vehicle, the direct shear and triaxial compres- The parameters c'and 0', rather than any of the other soil

sion devices, and a shear annulus device (part of the mechanics or mobility parameters, were chosen to charBevameter) are used to characterize the shear strength acterize soil strength for two reasons. First, they can be

of a silty sand under several conditions of moisture and determined using simple and well-known testing and

thawing. The strength values are compared with mea- analysis techniques, assuming linearity orpartial linearsured values of vehicle traction. To indicate the poten- ity of the Mohr failure envelope. Second, the parameters

tial of such measurements, an example is presented c' and 0' have some physical significance, 0' being rewhere the strength determined using the instrumented lated to friction and dependent on applied normal stress

vehicle is used to predict the traction of another, much and c' being related to material cohesion. All of the

larger vehicle. parameters in eq 1 can be measured in both soil mechanU.S. Standard Sieve Numbers Hydrometer

6" 4" 2" 1V W 10 40 200

o I I i I

80C 20

I I-I -- I II __I

t.0

C.

_0 1 1100

! I I I C0

' I [ I1

100 10 1 0.1 0.01 0.001

Grain Size (mm)

Cobbles Gravel Sand Sil or Clay

Coarse I Fine IC.,. Medium I ine

Figure 2. Grain-size distribution for Lebanon sand.

2
C

itt t

Figure 4. Schematic ofthe triaxial com-

/ pression test and analysis of results. /

On, Normal Stress Triaxial test specimens were machined from frozen

ure 3. The Mohr-Coulomb failure (or yield) criteri- cores taken from the vehicle mobility test basin. Cores

were taken during different freeze cycles and soil conesoa straight-ine approximation ofthe curvedfailure ditions so that the water content and density of each

'elope (shown in dashed line), specimen varied. All specimens were 5.1 cm in diameter and approximately 13 cm in height. The frozen samand terramechanics. Although more complicated ples were prepared for testing and then allowed to thaw

ns of yield criteria exist, the simplest form was at room temperature overnight. Tests were performed at

,sen here as an initial approach. In addition, simpler room temperature. Four confining pressures were used:

thods are more likely to be adopted for practical 0, 34.5, 68.9, and 137.8 kPa. The sample was loaded at

dication. a constant axial strain rate of 10% true strain per second.

Since much of the work described here is based on For a sample length of 12.7 cm, this corresponds to a

Mohr-Coulomb model, I will begin with a brief maximum deformation rate of 1.27 cm/s. Previous work

cussion of the model. The Mohr failure envelope is by Blaisdell et al. (1987) showed that the effect of higher

terated by drawing a tangent to a series of Mohr's strain rates on strength was small compared with the

:les plotted in shear stress-normal stress space. The effects of other sample variations. The results of the

ulting tangent is not necessarily linear;, in fact, it triaxial confining test are summarized in Table 1.

ially takes on a concave downward shape (Fig. 3).

ulomb, in studying the shear resistance of soils, ob- Direct shear

ved a stress-dependent component of shear strength The direct shear test is performed by mounting a

I a stress-independent component. He noted that the cylindrical sample in a pair of rings and applying a shear

,ss-dependent component is similar to sliding fric- force across the rings to shear the sample. This is done

a; he named that component "the angle of internal for a range of normal loads, and the peak (or residual)

;tion" and gave it the symbol *'. The other component shearing force for each test is plotted against the applied

med to be related to the intrinsic cohesion of the normal load to determine c' and 0' values, as shown in

terial, and he called it c'. Thus the Mohr failure en- Figure 5.

ope could be approximated by a straight line with a

pe of tano" and an intercept of c'. The combination of

ulomb's observations and Mohr's failure theory re- Table 1. Triaxial test data.

ted in the Mohr-Coulomb failure criterion, as ex- Confining Failure Water Dry Total

ssed in eq 1. Using this model, the effect of the soil Sample pressure stress content density density

trix suction is included in the cohesion term as ap- no. (kPa) (kPa) (%) (glcm..) (glcra)

-ent cohesion. S-9 0 70.97 16.0 1.592 1.846

s-6 0 163.98 16.5 1.672 1.948

iaxial compression s-I 0 175.01 18.7 1.654 1.963

s-3 0 207.39 18.8 1.659 1.971 In the triaxial compression test, a cylindrical sample s-4 34.4 55.81 11.6 1.539 1.718

)laced in a loading frame and loaded vertically while s-3 34.4 122.64 7.4 1.704 1.830

onfining pressure is applied in the horizontal direc- s-2 34.4 115.75 9.7 1.701 1.866

n. By testing the soil at various confining pressures s-5 68.9 228.75 21.7 1.558 1.897

s-12 68.9 CQ.38 18.4 1.630 1.931 1 plotting the corresponding Mohr's circles, the fail- s-7 137.8 319.70 15.1 1.574 1.811

envelope of the material is determined, as indicated s-I1 137.8 266.64 12.5 1.629 1.833

Figure 4. s-4 137.8 320.39 18.6 1.650 1.957

3
N chosen to correspond to the load applied by the vehicle.

The normal pressures ranged from 80 to 234 kPa. A

F different test specimen was used for each test.

A For each shear test, the shear force vs shear displacement was recorded. From such a graph, the peak shear

N stress and a residual shear stress can be chosen. The data

A obtained from all of the direct shear tests are shown in

Figure 5. Direct shear test method and I able 2.

data analysis. Shear annulus device

The test specimens fordirect sheartests were obtained The shear annulus device is an instrument for meain several ways to get a range of soil conditions. Some suring the shear strength of the terrain. A similar instrusamples were machined from frozen cores (similar to the ment, called a Bevameter, has been used in mobility

triaxial test sample preparation) and then thawed over- work for many years. The shear annulus device used in

night before testing at room temperature. Other samples these tests is shown in Figure 6. The basis of the apwere taken from the mobility test section as it thawed. paratus is an annulus that rotates while a vertical load is

These samples were taken by drive cylinders and then applied. The annulus area is 127 cm2, and it is coated

trimmed to the correct dimensions (6.35 cm diameter) or, with rubber. The torque caused by the shear resistance

when the samples did not survive transport, they were of the soil is measured with a torque cell located on tl'e

rebuilt to the correct sample size at the same density and top of the annulus. A test sequence for determining the

moisture content as the undisturbed samples. shear strength of the soil consists of performing three

To duplicate more closely the conditions caused by a sets of tests where each set consists of a measurement at

tire slipping on a soil surface, the samples were sheared five different normal loads. The resulting normal stress

atthe fastest rate allowed on the test apparatus. This was applied to the soil surface ranges from 5.5 to 35.8 kPa.

0.08 cm/s. which is still three orders of magnitude less The annulus rotates at approximately 10 rpm, which is

than thetypical shear rate caused by the slip of the wheel equivalent to shearing the soil at 4.2 cm/s at the midat peak traction (between 10 and 40 cm/s). Four differ- point of the annular ring. Although this is faster than the

ent normal loads were applied to the sample. They were direct shear test, it is still an order of magnitude less than

Table 2. Data from direct shear tests. Data can be grouped by

water content to calculate corresponding c' and ý'.

Normal Peak Residual Water Dry Total

Sample load load load Area content density density

no. (N) (N) (N) (cm2) (%) (8/cmn) (glcm3 )

12 243.48 146.78 93.41 30.53 21.5 1.501 1.823

8 243.48 179.25 160.13 30.61 12.6 1.573 1.772

tcl2r 243.48 224.18 164.58 31.41 12.5 1.696 1.909

tc12 243.48 231.30 157.90 31.41 12.3 1.696 1.905

tcll 243.48 232.63 151.23 31.41 12.6 1.696 1.910

2 243.48 236.19 179.25 30.64 10.6 1.702 1.883

7 360.29 293.57 265.10 30.51 12.4 1.675 1.882

tc72 360.29 320.26 271.33 31.41 11.6 1.696 1.894

5 360.29 324.70 240.19 30.66 18.1 1.695 2.002

t5ll 360.29 326.04 233.52 31.41 12.5 1.696 1.909

tcl2r 360.29 346.94 263.32 31.41 12.3 1.696 1.905

4 360.29 373.63 272.22 30.35 16.9 1.735 2.028

6 556.00 389.20 345.61 30.75 20.5 1.470 1.772

11 556.00 411.44 350.06 30.51 10.6 1.631 1.804

9 556.00 422.56 341.16 30.53 19.1 1.614 1.922

tcl2r 556.00 428.34 311.36 31.41 13.5 1.696 1.925

tci2 556.00 437.02 311.36 31.41 11.9 1.696 1.898

tell 556.00 444.80 400.32 31.41 12.6 1.696 1.911

10 711.68 494.84 422.56 30.41 21.6 1.451 1.765

tc12 711.68 533.76 355.84 31.41 15.8 1.696 1.964

13 711.68 550.44 442.58 30.25 19.7 1.627 1.947

tc12r 711.68 551.55 389.20 31.41 11.5 1.696 1.892

tcl2r 711.68 578.24 378.08 31.41 12.7 1.696 1.911

1 711.68 633.84 456.81 30.57 17.0 1.719 2.011

4
NJ T.,t T, - R

N -R

C, S.N

Motion

•S Resistance TractionWa 1

C

1o Y2 3 C4

Normal Stress Figure 7. Net traction can be resolved into gross

traction and motion resistance. Gross traction is

ýgure 6. Shear annulus device and data analysis. the shearing force applied to the soil surface.

;hear/slip caused by the tire. The data recorded with a fifth wheel. Engine speed is gradually increased using

;hear annulus device are analyzed in a fashion simi- the throttle, while the vehicle speed is held constant with

.o those obtained from the direct shear test in the the rear brakes. The resulting slip of the front wheels is

oratory. The peak shear stress is determined for each recorded as the wheel-to-ground differential interface

nal stress. The shear stress and normal stress are velocity (DIV). Traction is generally reported as the

ted to determine the c' aad ý' of the soil (Fig. 6). tractive coefficient: longitudinal force divided by verti-

"he shear annulus device was used to measure soil cal force. Motion resistance is measured by driving the

rigth in the test basin immediately prior to the vehicle at a constant speed and measuring the resisting

)ility tests with the CIV. The c' and 4' values calcu- force on the front, undriven wheels.

I for each soil condition are listed in Table 3. The soil strength parameters characterizing the failure caused by a slipping pneumatic tire were calculated

REL Instrumented Vehicle from the vehicle traction and motion resistance data.

lehicle mobility tests were performed in the Frost The longitudinal force measured with the CIV during a

vcts Research Facility using the CRREL Instrument- traction test is equivalent to the net traction. The shear

lehicle (CIV) described in Shoop et al. (1991) and stress applied to the soil, however, is actually caused by

op (1989). Briefly, the CIV is instrumented to mea- the gross traction. Gross traction is obtained by sum-

: the forces at the front wheels in three perpendicular ming the net traction from a traction test and the motion

ctions. It also measures the speed of each of the front resistance from a resistance test (Fig. 7):

els and the true vehicle speed. Additional measureits and instrumentation are included as desired. Both Tg = Tnet + R (2)

tion and motion resistance tests are performed. To

tsure traction, a braking force is applied to the rear where Tg = gross traction

els of the CIV while the front wheels are driven. The Tnet = net traction

rator holds a constant vehicle speed as measured by R = motion resistance.

Table 3. c' and 0' calculated for each test sequence using the shear

annulus, based on peak shear stress.

Thaw Water Dry Total

c * depth content density density Saturation

Date (kPa) (degrees) (cm) (%) (glcm3) (glcm3 ) (%)

Nov 9 4.44 33.5 3.81 20.0 1.509 1.811 67.9

Nov 10 0.88 36.5 10.16 23.4 1.546 1.907 83.9

Nov 10 0.30 37.9 10.16 23.4 1.546 1.907 83.9

Nov 10 0.92 36.6 10.16 23.4 1.546 1.907 83.9

Dec 20 0.90 38.4 * 13.0 1.699 1.920 58.9

Dec 20 1.54 37.9 * 13.0 1.699 1.920 58.9

Dec 27 2.82 36.8 * 9.0 1.611 1.756 35.6

Dec 27 4.13 34.8 * 9.0 1.611 1.756 35.6

Dec 27 3.17 33.5 9.0 1.611 1.756 35.6

Dec 28 2.27 33.1 3.81 24.2 1.516 1.883 82.9

Dec 30 3.72 29.9 5.08 27.0 1.547 1.965 97.0

Dec 30 0.25 30.7 5.08 27.0 1.547 1.965 97.0

Soil totally thawed.

5
Table 4. c' and 4' calculated from CIV tests for each soil condition.

Thaw Water Dry Total

c" depth content density density Saturation

Date (kPa) (degrees) (cm) (%) (glcm3) (glcm3 ) (%)

Nov 9 16.39 28.0 3.81 20.0 1.5088 1.811 67.9

Nov 10 15.36 20.0 10.16 23.4 1.5456 1.907 83.9

Dec 20 37.34 21.6 60.96 13.0 1.6992 1.920 58.9

Dec 27 46.64 17.0 60.96 9.0 1.6112 1.757 35.6

Dec 28 12.88 23.3 3.81 24.2 1.5152 1.883 83.0

Dec 30 40.37 11.0 8.89 27.0 1.5472 1.965 97.0

As long as the peak traction occurs when slip sinkage tained from vehicle tests would not necessarily be exis small and DIV is low (as was observed for our case), pected to correspond to the c' and O' parameters used in

gross traction can be accurately obtained from the two classical soil mechanics. But the stress conditions and

separate tests using eq 2. failure mechanisms of a pneumatic tire on soil are exThe maximum shear stress applied to the soil is the actly duplicated, therefore the c' and ' obtained should

peak gross tractive force (measured at the tire/soil inter- be more representative of the strength parameters needface) divided by the contact area of the tire. The normal ed for mobility calculations and should yield more acstress is the measured normal load on the tire divided by curate predictions.

the tire contact area. Using the CIV mobility test results, c' and 0' were

calculated for each set of soil conditions generated in

"t= TgA (3) the test basin. The results are listed in Table 4. (These

Gn N/A are the same soil conditions tested using the shear annulus device, as mentioned earlier.) where r = shear stress applied to soil

Tg = gross traction.

A = contact area of tire DISCUSSION

O= normal stress applied to soil

N = measured normal load. The results of the strength measurements are analyzed in three ways:

Tire contact area was measured by making a print of 1) comparison of test methods and the c' and 0' valthe contact patch of.the tire on a hard surface. As with ues obtained from each;

Tg, A remains reasonably accurate as long as sinkage is 2) the effect of soil conditions on calculated strength

small. parameters c' and *'; and

Traction tests were performed at two or more tire in- 3) the usefulness of c' and (V from the different test

flation pressures and with various normal loads to vary methods for predicting traction.

the contact area and normal stress. Similar to the other

strength measurement test methods mentioned earlier, Comparison of test methods

shear stress and normal stress were plotted, and c' and To compare the results of the different tests it is

0' were calculated based on the Mzhr-Coulomb failure useful to recall that soil strength is nonunique. First, the

criterion (Fig. 8). Such a plot was generated for each soil term "strength" must be defined. For our case, the

condition tested. strength discussed is the peak shear strength. Second,

Because the tractive force developed is the result of and equally important, the measured strength depends

interaction between the vehicle and the soil, c' and(' ob- on the conditions of the test, specifically on the loading

conditions and stress distribution and the strain or de00 • formation rate. (For the type of soil used in this study,

0,,. a nonplastic silty sand, the strain rate has less effect on

T •ostrength than the stress distribution.) The engineer gen-

= a .erally specifies the type of strength test to be used so that

real-world loading conditions are matched as closely as

e possible. Accordingly, it follows that, for vehicle mobility purposes, a strength test using a vehicle is sensi- SN ble.

(n = e A comparison of the loading and deformation rate of

Figure 8. Vehicle traction test and data analysis. the various soil strength test methods is shown in Table 5.

6
ke S. Comparison of the loading characteristics of the IIS O .

mgth test methods used. -

Q.

Normal Deformation Area of applied 100 --- -

stress rate normal load Direct Shear - Veh•c6 .o -

(kPa) (Cm/s) (cm-')

Mxial 34-227 1.27 20.6 o

:ct shear 80-234 0.08 31.6 5

ar annulus 5-36 4.2 126 .. ."

rumented vehicle 124-200 10-40 387-581 •--"Shear Annulus

ieel 0

0 50 100 150 200

[lowing the design guidelines given above, since we Normal Stress, kPa

using this information to predict vehicle perfor- Figure 9. Comparison of failure envelopes for

nce, the test method chosen should match the loading each test method. Solid lines mark the range of

iditions caused by a vehicle. The deformation rates normal stress for each test.

)wn in Table 5 are measured with reference to the test

3aratus and are not those present on the actual soil tions for the vehicle tests (because -)f high strain rate)

i ure surface. The deformation rate listed for the tri- and triaxial tests. This figure also clearly shows that the

al test, for example, is the true axial strain on the sam- shear annulus device operates at a much lower and more

and not the deformation directly on the shear plane. limited range of normal stress than the other tests, which

e deformation rate imparted by the vehicle is the may also lead to disagreement with the other test data.

ferential interface velocity (the velocity of the wheel On the whole, the failure envelope from the triaxial test

nus the velocity of the vehicle) and so indicates the data most closely matches the failure envelope calculate of the shear load application at the soil/tire inter- ed from the vehicle tests.

:e. This is not necessarily the deformation rate on the When considering these various strength measureual shear failure plane in the soil. ment methods, it is appropriate to mention some of the

Table 5 calls attention to notable differences in ap- disadvantages associated with each. All of these tests

ed normal stress, deformation rate, and load area for are based on the assumption of a uniform stress distri-

: test methods used in this study. The range of normal bution within the sample, at least within the zone of failiss applied by the shear annulus is the lowest of all the ure. In reality, this assumption is seldom satisfied. It is

t techniques and is considerably lower than the range generally felt that the direct shear test is the worst offenplied by the test vehicle. The shear annulus device der. However, the triaxial test, the workhorse of a soils

A is a portable instrument; to apply higher stress the laboratory, suffers from stress concentrations at the end

vice must be heavier and thus becomes quite cumber- platens and variations in confining pressure from the

ne. In addition, the deformation rate of the direct flexible membrane around the soil. The shear annulus

.ar test was three orders of magnitude lower than the device tests actual field conditions but fails to duplicate

Formation rate caused by the tire. This was the fastest the motion and loading of a tire. Although these features

e possible, however, using standard test equipment. are duplicated exactly in the instrumented vehicle tests,

Since all of the tests were performed on Lebanon the load applied by the tire and therefore the stresses in

id under various thawing conditions, the tests from the soil, are nonuniform.

-h method were used to calculate an average c' and 0' Because the shear annulus device and the vehicle

the soil (for a range of moisture content and density). tests were performed in exactly the same soil condiese average failure envelopes are shown graphically tions, these two data sets can be examined more closely.

Figure 9. The solid lines indicate the range of the The failure envelopes for the shear annulus and the

rmal stress where the data were actually obtained, vehicle for each of the soil conditions tested are shown

shed lines denote extrapolation of the failure enve- in Figure 10. (The soil conditions corresponding to the

)e beyond individual test limits, dates labeled on the failure envelopes are given in

It is easy to see from Figure 9 that both of the shear Tables 3 and 4.) The ' measured with the shear annulus

t methods (direct shear and shear annulus) yield a is consistently higher than the corresponding vehicle

,her 0' and lower c' than the vehicle and triaxial test failure 0'. This is quite reasonable when we recall that a

ta. This may be because the soil was tested under typical failure envelope for earth materials is actually

;entially drained conditions for both of the shear tests curved rather than linear and flattens at the higher stress

=cause of the relatively slow deformation rate and levels (as shown in Fig. 3). The shear annulus data may

:k of fluid confinement) and under undrained condi- be used to obtain a linear approximation of the failure

7
i50 I I i I

9 Nov'88

10 Nov .

100 28 D

50 Shear~ U

Annulus I-for CIV Operation

0 50 100 i5o 200 250

O-n lkPa)

Figure 10. Failure envelopes from the shear annulus and the CIVfor each set of soil

conditions. Conditions correspond to those given in Tables 3 and 4.

curve at low normal stresses and the vehicle data to ob- moisture content and thaw depth, on the strength paramtain a linear approximation that is representative of the eters. For all the soil conditions, c' remains very low

higher normal stress. Together, then, these test results while *" varies. The soil condition with the most influproduce a failure curve of the shape typical for earth ence on strength was the moisture content. Figure II

materials. When viewed in this way, the test results are shows the effect of moisture content on 0' measured

compatible with each other and with the original Mohr using the direct shear, shear annulus, and instrumented

failure theory. vehicle test methods. There were not enough data from

Altho:-gh the shear annulus has a much lower ap- the triaxial tests for a similar analysis. For each test

plied stress range than wheeled vehicles, this is not true method, the trend shows "' increasing until the water

for tracked vehicles. Thus the portable shear annulus content of the soil is near the liquid limit of the soil, and

may be quite useful in mobility studies of tracked ve- then dropping. (For the soil used in this study, liquid

hicles, limit = 20%.) Similar results have been reported by

others (Ayers 1987, Harrison 1966).

Influence of soil conditions The same trend can also be seen in the vehicle tracIn working with variably saturated thawing soils (or tiondata when tractive force is plotted against soil moisany layered soil of varying wetness), it is important to ture content (Fig. 12). Traction increases or remains

understand the effect of the soil conditions, such as constant with increasing soil moisture until the moisture

44 1 1 1 1 1 1 1 1 t 1 1 1

40 2 pts o

36- Shear Annulus..-

32- Direct Shear0

28- 0

2 Vehicle,7 .

24- o

20 M

160 I I I [ I t IA I I I I Figure 11. *" increases with water content

0 4 8 12 16 20 24 28 until the liquid limit is approached and then

Water Content (0/%) drops rapidly.

8
2 6 2 430 1 1 1

Waterd Conent %"..e ThwDpt

Z~ '4.5 15W

S4.0- 186 We

60

0 310

3.0 15

Water Content, % Thaw Depth, cm

Figure 12. Traction increases with water content Figure 13. As calculated from the CIV, 0' de-

(as afunction of contact pressure) and then drops creases with thaw depth up to approximately 15

sharply when water content reaches the liquid cm (the depth of influence depends on the soil

limit. and the vehicle).

content reaches the liquid limit, and then traction de- traction. Rewriting the Mohr-Coulomb equation (eq 1)

creases rapidly. This trend exists for each of the tire in terms of tractive force (using eq 3) we get

contact pressures; however, the slopes and magnitudes

vary notably at water contents below the liquid limit. At T = c'A + N tan€" (4)

moisture levels greater than the liquid limit, tractive

response is insensitive to contact pressure. where T is the predicted maximum gross traction that a

Changes in thaw depth also influence soil strength particular tire can produce based on soil strength. If we

measurements. In the frozen and thawing soil, the know A andN for a vehicle and measure c'and ý'for the

strength tests are essentially a measure of the composite soil with one of the test methods, peak traction can be

strength of the thawed layer and the underlying frozen predicted.

ground, as detected by the measurement technique. Since side-by-side measurements were made using

Because of the nature of the laboratory tests and the test the shear annulus device (forc' and ý') and the CIV (for

specimens, the effect of thaw depth cannot be deter- traction), these traction values can be compared with

mined in the laboratory. Of the field tests (shear annulus traction values predicted (eq 4) using c' and ' obtained

and instrumented vehicle), the effect of thaw depth is from the shear annulus device. Figure 14 shows the remost apparent in the data from the vehicle. This is be- suiting measured vs predicted traction values. The solid

cause the applied stress distribution caused by the line indicates where the measured values equal the prevehicle load extends much deeper than the stress applied by the shear annulus.

The variation in 0'ehcice with thaw depth can be seen 6 I I

in Figure 13. Clearly, 0' is most strongly influenced by

thaw depth for shallow thaws. As thaw depth increases, Z _ * _

0' calculated from a tire shearing the soil surface soon 5

becomes insensitive to changes in thaw depth. The % ,

depth at which the frozen layer no longer influences i'

is dependent on both soil type and the stress applied by • 4

the vehicle. Although soil density also affects strength, 18

0

the density variation in this study was small and there- X fore the results are inconclusive.

a I -

Traction prediction 3 4 5 6

The knowledge of how the different soil strength Predicted Traction, kN

tests compare with each other and how the soil condi- Figure 14. Traction predicted from the shear annulus

tions influence the strength is important if soil strength overpredicts traction measured with the CIV because of

parameters are to be used in a model to predict vehicle the low normal stress applied with the shear annulus.

9
a. Hard Mobile Launc her.

a. H M lb. CRREL Instrumented Vehicle.

Figure 15. Comparison of the Hard Mobile Launcher (HML) and the CRREL Instrumented Vehicle (CIV) roughly

to scale.

dicted values. It is immediately obvious that using eq 4 low. Although only one example, this case indicates

with c' and 0' from the shear annulus overpredicts peak that an instrumented vehicle can be used to obtain

vehicle traction. This was expected since the shear an- accurate soil-vehicle strength parameters for predictnulus measures c' and 0' in a lower range of normal ing the traction of other vehicles.

stress than that applied by the vehicle (Table 5 and Fig.

9, 10), and thus is in a region where the slope of the fail- CONCLUSIONS

ure curve is steeper, as discussed earlier. The predictions may have been better for a vehicle with lower The conclusions of this study can be summarized as

ground contact pressure (i.e., a tracked vehicle), follows:

To use the c' and 0' calculated from the CIV, inde- 1. An instrumented vehicle can be used to measure

pendent traction measurements from another vehicle terrain strength parameters based on the Mohr-Coulomb

were necessary. No other vehicles were tested in the soil yield criterion. The c' and ý"values obtained are compabasin during this study, but Blaisdell et al. (1987) report rable to those obtained using other soil (and soil/rubber

side-by-side tests of the CIV and the Hard Mobile interface) strength measurement techniques. Using an

Launcher (HML), where both vehicles were tested on instrumented vehicle to characterize the terrain is the

the same soil conditions and the tire pressure of the CIV preferred technique for mobility studies, because the

was varied so that c' and 0' could be calculated. The conditions created by a tire slipping on a soil surface are

HML weighs 47.2 kN and is 35 m long as compared exactly duplicated.

with the CIV at 1.3 kN and approximately 5 m long. A 2. Of the strength tests used, the failure envelope

schematic of the vehicles, roughly to scale, is shown in from the triaxial test was most similar to the failure

Figure 15. Vehicle-soil loading parameters and mea- envelope calculated from the CRREL Instrumented

sured traction and resistance are given in Table 6. Vehicle.

Using the traction and resistance measured with the 3. The direct shear tests yielded the highest values of

CIV, a c' and 0" of 69 kPa and 22.50, respectively, is 4", much higher than *" based on vehicle tests. This was

calculated. These terrain strength values can then be probably because of the drained conditions and the very

used in eq 4 to predict a peak gross traction provided by slow deformation rate of the direct shear test.

each of the driven wheels of the HML of 33.217 kN: 4. The O'calculated using a portable shear annulus is

since the HML has eight driven wheels, the total pre- also higher than 4" calculated from the instrumented

dicted gross traction for the vehicle is 265.733 kN. The vehicle. The higher " is a result of the low normal stress

measured gross traction calculated from the net traction applied during the shearing tests (well below typical

plus the motion resistance (eq 4) is 240.392 kN. The wheeled vehicle contact pressures) and the curved nadifference between the predicted and measured gross ture of the failure envelope at low normal stress (i.e., the

traction is approximately 10%, which is remarkably failure envelope cannot be linearly extrapolated to a

Table 6. Tire loading and contact area of the CIV and the HML and

corresponding traction and resistance measurements made in the

same soil conditions (after Blaisdell et al. 1987).

Tire inflation Normal load Tire contact Measured Measured

pressure on wheels area net traction resistance

Vehicle (kPa) (kN) (cm2 ) (kN) (kN)

CIV 179 6.027 279.4 4.231 0.190

civ 103 6.027 345.2 4.654 0.216

HML 49.133 1864.5 204.363 36.029

10
wheeled vehicle's normal stress range). Although the Blaisdell, G.L.,E.J. Chamberlainand M.Mellor(1987)

parameters calculated from the shear annulus overpre- Evaluation of the cold regions aspects of mobility and

dict the traction of the instrumented vehicle, they may hardening of the mobile test bed at Malmstrom Air

be useful forpredicting traction for low ground pressure Force Base. Final Report for U.S. Air Force Ballistic

or tracked vehicles. Missile Office. (Also USA Cold Regions Research and

5. All test results indicate that ý' is strongly influ- Engineering Laboratory, Internal Report 1004 [unpubenced by soil moisture; it increases with moisture con- lished].)

tent up to the liquid limit of the soil, and then decreases Chamberlain, E.J., M. Mellor, and G. Abele (1988)

rapidly with additional moisture. Frozen ground strength characterization and grouser

6. Peak tractive force follows the same general trend cutter design for hardening transition, and mobility over

as V with increasing moisture content, Below the liquid snow-covered terrain for'the mobile test bed. Draft Relimit, reduced contact pressure and increasing moisture port to U.S. Air Force, Ballistic Missile Office.

content result in improved traction. Above the liquid Harrison, W. L. Jr. (1966) Soil strength prediction by

limit, traction decreases rapidly with increased water use of soil analogs. In Proceedings of the Institute of

content, and the effect of contact pressure is negligible. Environmental Sciences 1966 Annual Technical Meet7. The 0' as measured by the vehicle is strongly in- ing, San Diego, California, p. 577-581.

fluenced by the thaw depth of the soil. 0' rapidly de- Kogure, K., H. Yamaguchi and Y. Ohira (1988) Comcreases with thaw depth for shallow thaws, but becomes parison of strength and soil thrust characteristics among

insensitive to thaw depth at deep thaws. different soil shear tests. Journal ofTerramechanics, 25

8. The CIV can be used to characterize the terrain for (3): 201-221.

input into traction models such as those based on the Shoop, S.A. (1989) Vehicle mobility in thawing soils:

Mohr-Coulombfailurecriterion. With this information, Interim report on CRREL's test program. USA Cold

the upper limit of traction for other vehicles such as the Regions Research and Engineering Laboratory, Special

Hard Mobile Launcher can be predicted. Report 89-31.

Shoop, S.A., E. Berliner and S. Decato (1991) An exREFERENCES perimental method for vehicle mobility testing on freezing/thawing soil. In Proceedings of the Engineering

Ayers, P.D. (1987) Moisture and density effects on soil Foundation Winter Mobility Conference, Santa Barshear strength parameters forcoarse grained soils. Trans- bara, California, June 1991.

actions of the American Society of Agricultural Engi- Wulfsohn, D., S.K. Upadhyaya, and WJ. Chancellor

neers, 30(5): 1282-1287. (1988) Tractive characteristics of radial ply and bias ply

Bekker, M.G. (1969) Introduction to Terrain-Vehicle tyres in a California soil. Journal of Terramechanics,

Systems. Ann Arbor: University of Michigan Press. 25(2): 111-134.

11
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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Comparison of Thawing Soil Strength Measurements PR: 4A762784AT42.

for Predicting Vehicle Performance TA: CS

6. AUTHORS WU: 007

Sally A. Shoop

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 92-17

Hanover, New Hampshire 03755-1290

9. SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

Office of the Chief of Engineers

Washington, D.C. 20314-1000

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

13. ABSTRACT (Maximum 200 words)

The CRREL Instrumented Vehicle (CIV), shear annulus, direct shear, and triaxial compression devices were used to characterize the strength of thawed and thawing soil. These strength values can be used in simple traction models to predict the

tractive pet formance of vehicles. Strength was evaluated in terms of the parameters c' and l$' based on the Mohr-Coulomb

failure criterion. It is proposed here that an instrumented vehicle is best suited for terrain characterization for mobility studies because the conditions created by a tire slipping on a soil surface are exactly duplicated. The c' and l$' values from the

shear annulus were found to overpredict traction because of the low normal stress applied by the annulus and the curved

nature of the failure envelope. Of all the tests, the direct shear test yielded the highest 0$' value, most likely because the test

was run at a slow deformation rate under drained conditions. The triaxial test results were the most similar to those from the

vehicle. All test methods show l' increasing with soil moisture up to the liquid limit of the soil and then decreasing. As

measured with the vehicle, 0' was also found to be strongly influenced by the thaw depth.

14. SUBJECT TERMS 15. NUMBER OF PAGES

Freeze/thaw Thaw depth Vehicle mobility 17

Shear Traction prediction comparison Water moisture content is. PRICE COOE

Soil strength Unsaturated soil

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF REPORT OF THIS PAGE OF ABSTRACT

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

NSN 7540-01-280-5500 Standtard Form 298 (Rev. 2-89) SU.S. GOVERNMENT PRINTING OFFICE: 1902,W-W-3180025 Presote by AN SId. Z18 296-102