Strain rate effect on the compressive strength of frozen sand

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Engineering Geology, 13, pp. 223-231, 1979

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Strain rate effect on the compressive strength of frozen sand

Baker, T. H. W.

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Council Canada de recherches Canada

STRAIN RATE EFFECT ON THE COMPRESSIVE

STRENGTH OF PROZEN SAND

by T.H.W. Baker Reprinted from E n g i n e k g Geology VoL 13, 1979 p. 223 231 DBR Paper No. 849 Division of Building Research

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SO M U I R E

Des Cchantillons cylindriques de sable fin dtOttawa (dksignation C-109 de llASTM), compact6 'a l a teneur optimale en humidit6 e t s a t u r 6 avant le stade du gel ont 6t6 soumis 'a des e s s a i s de compression uniaxiale 'a une temp6rature d e -5. 5 ° C e t 'a d e s taux de deformation variant e n t r e 1 0 - ~ e t 1 0 - ~ s - l . Les rdsultats correspondent aux valeurs obtenues par extrapolation d e s don- n6es de Sayles et Epanchin ( l ) , mais sont beaucoup plus dlevks que l e s r6sultats de Goughnour e t Andersland (2) e t d e P e r k i n g e t Ruedrich ( 3 ) , 'a des t a u de dkformation infbrieurs 5 S - I . Certains facteurs prouvent que c e s r6sultats peuvent 8 t r e impu- tables 'a l a teneur totale en humidit6 contenue dans l e s 6chan- tillons, a w conditions d e gel (syst'eme ouvert ou f e r m 6 ) et aux effets mesur6s 'a l l e n d r o i t de l'ensemble plateau de , e 6chantillon.

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Engineering Geology, 1 3 (1979) 223-231

I @ Elsevier Scientific Publishing Company, Amsterdam - I

223 Printed in The Netherlands

I _{STRAIN RATE EFFECT ON THE COMPRESSIVE STRENGTH OF}

I FROZEN SAND I I I T.H.W. BAKER

Division o f Building Research, National Research Council o f Canada, Ottawa (Canada)

I

(Received June 15, 1978)

I

I I

I ABSTRACT

Baker, T.H.W., 1979. Strain rate effect on the compressive strength of frozen sand. Eng. Geol., 1 3 : 223-231.

Cylindrical specimens of fine Ottawa sand (A.S.T.M. designation C-log), compacted at the optimum moisture content and saturated before unidirectional freezing, have been tested in uniaxial compression at a cold room temperature of -5.5"C and strain rates between

lo-'

and 10-2 s-'. The results agree with an extrapolation of data obtained by Sayles and Epanchin [ I

1 ,

but are much higher than those obtained by both Goughnour and Andersland [ 2 ] and Perkins and Ruedrich [3] at strain rates below

lo-'

s-'. There is evidence that this may be due t o variation in total moisture (ice) content, the conditions under which the specimens were frozen (closed system or an open system) and to the end effects a t the platenspecimen interface.

INTRODUCTION

Several investigators have studied the strength and deformation behavior of naturally and artificially frozen sands under various conditions of temperature, pressure and loading rate. In comparing the results, the author noted a change in the strain-rate dependence of the unconfined compressive strength at a strain rate of about 3 X

lo-'

s-' (Fig.1). This is of concern when trying to establish the long-term stressstrain behavior required for the design of foundations in frozen soil by extrapolation from short-term tests. It should be recognized that Sayles and Epanchin's data [ I ] in Fig.1 extend t o higher strain rates and that only part of the data are shown here.

This paper gives observations of the strength and deformation behavior of artificially frozen sand-ice specimens tested in the same range of strain rates as those presented in Fig.1.

EXPERIMENTAL PROCEDURE

Sample preparation

Fine Ottawa sand (A.S.T.M. designation C-109) was compacted in layers in a Plexiglass split mould at optimum moisture content (14% by dry weight)

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I 1 1 1 1 1 ~ 1 ~ 1 1 1 1 1 1 1 1 ~ 1 1 1 1 1 1 1 1 ( 1 1 1 1 1 1 1- ' -

-

_----

-

- SAYLES A N D E P A N C H I N Ill AT -6.5"C

-

I

GOUGHNOUR A N D ANDERSLAND 121 AT -7.5, -12°C

-

_{. . . .}

-

.

. . . .

. _{PERKINS AND RUEDRICH I31 AT -3.9"C}

-

L PERKINS A N D RUEDRICH I31 AT -6.7"C

-

SAYLES 141 AT -3.9"C - L.

-

- -

_----

=.

-

_-

-

_-

-

O T T A W A S A N D

-

- - A S T M C - 1 0 9

-

1 1 1 1 1 1 1 1 [ I I 1 1 1 1 1 1 1 I I 1 1 1 1 1 1 1 I I 1 1 1 1 1 1 1 I I l 1 l l l L S T R A I N R A T E , s - I

Fig. 1. Strain rate data for Ottawa fine sand.

as determined by a standard Proctor compaction test. The mould was

connected t o a vacuum pump, after compaction, and evacuated and saturated with de-aired distilled water prior t o freezing. Loose insulation was placed around the mould t o ensure uniaxial freezing at a cold room temperature of -5.5"C. Following complete freezing, which took about three days, the sample was taken from the mould and machined and faced in a lathe t o obtain a specimen 7 5 mm in diameter and 150 mm in length. Specimen characteristics are listed in Table I. A more detailed description of the procedures followed in the preparation of these specimens can be found in refs. [5] and [ 6 ] .

During the freezing phase, four specimens (of a total of 55) expanded in

the mould by as much as 1 0 mm. No ice lenses were visible on the surface of these specimens, but their densities were slightly lower than those of the remainder. The specimen moulds were designed t o allow water t o be expelled from the bottom during uniaxial freezing. A heater wire used t o prevent freezing of expelled water malfunctioned in four cases so that these specimens were frozen in an essentially closed system. Their volume expan-

sion was due t o in-situ freezing of the pore water [7]

.

Testing program

All specimens were tested in compression using an Instrom universal testing machine (Model 1127, capacity 25,000 kg). The limiting strain rates in this program were due t o the minimum speed of the testing machine and its

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TABLE I

Specimen characteristics

No. of spec. Mean avg. Stand. dev.

Moisture 5 5 19.3 1.3 content (%) Dry density 5 5 1.68 0.03 (glee) Void ratio 5 5 0.577 0.024 Moisture 55 0.97 0.03 saturation Ice 5 5 1.07 0.03 saturation

maximum load capacity. Some tests were performed with the specimen immersed in a kerosene bath, others in air with the specimen covered by a membrane t o reduce sublimation. The kerosene bath did not significantly

affect the results except in very long-term tests (approximately

lo-'

s-I).

Some of the long-term tests using the bath had to be discarded because the bath temperature increased as a result of heat given off by the gears in the loadframe. This could have been overcome by using a refrigerated bath.

Four platen types were used t o investigate the influence of various end conditions (Fig.2). The disk and end-cap platens are most commonly used in testing brittle materials [8]. Flexane compliant platens were developed [ 9 , 1 0 ] to reduce the effects of rough specimen ends and t o provide a uniform normal stress. Maraset compliant platens [ l l , 121 were designed to provide an elastic match between specimen and platen in order t o reduce interface radial shear stresses. They were the only type that did not affect the unconfined compressive strength of frozen sand specimens of different

slenderness (lengthtdiameter) ratio [ l l ]

.

Load was measured by a load transducer mounted between the specimen and the loadframe cross-head. Load was converted to stress by dividing it by the cross-sectional area of the specimen. The maximum (failure) stress in each test was taken t o be the unconfined compressive strength. Axial deformation of the specimen was determined from displacement of the cross-

head. Linear displacement transducers (DCDT) were mounted on some

specimens tested with each platen type. By comparing cross-head movement and actual specimen deformation measured by the transducers a correction factor was obtained for each platen type. This factor was used to obtain the specimen deformation from the cross-head displacement and to correct for deformation in the platens. Output from all transducers was recorded at regular time intervals on a data acquisition system.

TEST RESULTS

Fig.3 shows the relation of unconfined compressive strength a,, t o the

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A L U M I N U M D I S K P L A T E N

A L U M I N U M E N D C A P

F L E X A N E C O M P L I A N T P L A T E N

7 6 m m

M A R A S E T C O M P L I A N T P L A T E N Fig. 2. Platen types.

was found t o be related t o strain rate by:

urnax = A t b (1)

Values for the regression coefficients A and b , the correlation coefficient R and the number of tests N , are given in Table 11.

Fig. 4 shows the relation of axial strain at failure E ~ , to time t o failure tf,

for all of the tests performed for this study. The axial failure strain was found t o be related t o the time t o failure by:

Values for the regression coefficients C and d, the correlation coefficient R and the number of tests N , are given in Table 111.

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END CAP PLATEN

A A L U M I N U M D I S K PLATEN 1 ALL PLATENS

o FLEXANE COMPLIANT PLATEN 2 HEAVED SPECIMENS

v MARASET COMPLIANT PLATEN 3 SAYLES AND EPANCHIN 111 SOLID SYMBOL MEANS TEST (EXTRAPOLATED)

PERFORMED WITH S P t C I M E N

O T T A W A S A N D ASTM C-109 TEMPERATURE -5.5"C

S T R A I N R A T E , 5.'

Fig.3. Strain rate versus unconfined compressive strength.

(ice) content between about 1 7 and 24% is shown in Fig.5 for a strain rate of

lo-'

s-'. The moisture content dependence of the unconfined compressive

strength of frozen fine sand for the range 5-100% is shown in Fig.6 for a

strain rate of 2.2 X

lou6

s-' and a temperature of -12°C.

DISCUSSION

It may be seen in Fig.3 that the author's data are in good agreement with the extrapolation of the data from Sayles and Epanchin [ I ] for a similar temperature. Change in strain-rate dependence reported by other authors

(Fig.1) at about 3 X

lo-'

s-' does not appear in the present data. Strength

showed no tendency t o plateau for strain rates above 3 X

lo-'

s-', as shown

TABLE I1

Consfants for eq.1: a,, = A E ~ where a,, is in kPa, h is in s-'

Platen A b R N Aluminum 28960 0.090 0.52 19 disks Aluminum 24099 0.051 0.72 14 end caps Flexane 29397 0.088 0.58 11 compliant Maraset 7577 -0.057 0.54 11 compliant All platens 24796 0.066 0.41 5 5 Heaved 27610 0.131 0.96 4 specimens

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1

-

0 ~ ~ ~

-

o END C A P PLATEN

-

_-

-

A A L U M I N U M D I S K PLATEN

-

0 FLEXANE COMPLIANT PLATEN

-

V MARASET COMPLIANT PLATEN -

SOLID SYMBOL MEANS TEST PERFORMED

l o 1 =_

_-

WITH SPECIMEN I N A BATH

-

- oO ob

-

m

-

I -1 l o 0 A A A

-

- '

m

-

1 ALL PLATENS O T T A W A S A N D

-

2 HEAVED SPECIMENS _{TEMPERATURE -5.5"C}ASTM C-109

-

T I M E T O F A I L U R E , s

Fig.4. Time to failure versus axial strain to failure.

by Perkins and Ruedrich [3] (Fig.1). Lower unconfined compressive strengths were found in samples that had expanded during the freezing process. The expansion of pore water during freezing lowers the relative compaction of the sand grains, thereby reducing interparticle friction of sand grains. The correlation coefficient determined for the combined data from all platen types is very low. As will be discussed, this is mainly due to the variation in total moisture content of the specimens. The results indicate that unconfined compressive strength increases uniformly with increasing strain rate in the range

lo-'

~ - ' - 1 0 - ~ s-I.

TABLE I11

Constants for eq.2: E * = ~ t f where E * is in %, t f is in seconds

Platen C d R Aluminum 1.203 0.134 0.48 disks Aluminum 0.933 0.176 0.88 end caps Flexane 7.011 -0.030 0.04 compliant Maraset 1.402 0.132 0.87 compliant All platens 1.143 0.154 0.58 Heaved 0.34 0.262 0.71 specimens

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OTTAWA SAND

1 6 0 0 0 ASTM C-109

STRAIN RATE I o - ~ ,-I

b TEMPERATURE -5 S°C 1 4 0 0 0 ~ n ~ ~ A v 8 0 0 0

-

v 6 0 0 0

-

v 4 0 0 0

-

PLATENS

-

A ALUMINUM DISKS END CAPS 2 0 0 0

-

0 FLEXANE

-

V MARASET 0 I I I 1 I I T O T A L M O I S T U R E C O N T E N T , %

Fig. 5. Total moisture content and unconfined compressive strength.

I

1

I I

I

F I N E S A N D

-

STRAIN RATE 2.2 x 1 0 ' ~ s" TEMPERATURE -12°C

*

7

I

*

----=={

0

-

GOUGHNOUR AND ANDERSIAND I21

-

0 BAKER 1131

I I I I I I 1 I

T O T A L M O I S T U R E C O N T E N T , %

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Data presented in Fig.4 show that axial strain t o failure increases as the time to failure increases in the range 1-lo4 s. Axial strain to failure was also dependent on moisture content. Specimens frozen without permitting water exudation had a lower strain t o failure than expected.

Neither the unconfined compressive strength nor the axial strain t o failure appear t o depend significantly on the type of platen. The modes of failure at low strain rates were different for each platen type. End-cap platens and disk platens induced bulging at mid-height on the specimen. Flexane compliant platens induced bulging only after the specimens came in contact with the ring. Maraset compliant platens did not produce any noticeable bulging. All the platens produced conjugate shear failure planes at high strain rates. In a previous study [ l l ]

,

use of friction reducers was found to induce tensile splitting at low strength values.

Considerable care was taken t o control the total moisture content of the test specimens. Slight variation from sublimation influenced unconfined compressive strength (Fig.5). Strength of frozen soil is dependent on moisture content, as shown in Fig.6; strength increases until the soil is fully saturated with ice, and then decreases until the soil particles no longer influence it [13]. All specimens in this study were oversaturated with ice. CONCLUSIONS

Observations show that unconfined compressive strength of saturated frozen sand at a temperature of -5.5"C increases uniformly with strain rate in the range

lo-'

s-' t o s-'. Axial strain to4failure increases from 1 to 5% as the time t o failure increases from 1 to 10 s. The strength results agree with an extrapolation of data obtained by Sayles and Epanchin [ I ] , but they are much higher than those reported by both Goughnour and Andersland [2] and Perkins and Ruedrich [3] at strain rates below

lo-'

s-'. Low values reported by other investigators may be related to the total moisture (ice) content of the specimens, expansion of the specimen during freezing, or the use of inserted friction reducers at the specimen-platen interface. Reasons for the systematic change in strain rate dependence found by Perkins and Ruedrich [3] at 3 X

lo-'

s-' were not determined in the present study.

Variation in the total moisture (ice) content influenced unconfined compressive strength. The results from several specimens tested at the same strain rate S-') indicated that specimens with the highest moisture content had the lowest strength. This was typical of frozen sands slightly oversaturated with ice.

Specimens that expanded during freezing showed a reduction in unconfined compressive strength and strain t o failure. They had strength values similar to those reported by Goughnour and Andersland [2] and Perkins and Ruedrich [ 3 ] .

The four platen types investigated did not significantly affect the value of the unconfined compressive strength or the strain to failure. The mode of

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failure at the lower strain rates was different for each platen type. Maraset compliant platens produced the most uniform deformation, with no specimen bulging; A previous investigation

[Ill

has shown that these platens are most useful in distributing load uniformly and in reducing friction between platen and specimen. In the same investigation, friction-reducing inserts produced vertical tensile failures at much lower compressive strengths than other platen types.

ACKNOWLEDGEMENTS

The author wishes to thank Dr. V.R. Parameswaran, G . Mould and C. Hubbs, D.B.R./N.R.C., for their help in carrying out these tests. This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the permission of the Director of the Division.

REFERENCES

Sayles, F.H. and Epanchin, V.N., 1966. Rate of Strain Compression Tests on Frozen Ottawa Sand and Ice. U.S. Army CRREL, Tech. Note, 54 pp.

Goughnour, R.R. and Andersland, O.B., 1968. Mechanical properties of a sand-ice system. A.S.C.E. J. S.M.F.E., 94(SM4): 923--950.

Perkins, T.K. and Ruedrich, R.A., 1973. The mechanical behaviour of synthetic permafrost. Soc. Petrol Eng. J.(Aug.): 211-220.

Sayles, F.H., 1974. Triaxial constant strain rate tests and triaxial creep tests on frozen Ottawa sand. U.S. Army CRREL, Tech. Rep. 253, 29 pp.

Baker, T.H.W., 1976. Preparation of artificially frozen sand specimens. N.R.C. of Canada, Div. Build. Res., N.R.C.C. 15349, 1 6 pp.

Baker, T.H.W., 1976. Transportation, preparation and storage of frozen soil samples for laboratory testing. A.S.T.M. Spec. Tech. Publ. No.599, pp.88-112.

McRoberts, E.C. and Morgenstem, N.R., 1975. Pore water expulsion during freezing. Can. Geotech. J., 12(1): 130-141.

Hawkes, I. and Mellor, M., 1970. Uniaxial testing in rock mechanics laboratories. Eng. Geol., 4: 177-285.

Kartashov, Y.M. e t al., 1970. Determination of the uniaxial compressive strength of rocks. Sov. Min. Sci., No.3 (May-June): 339-341.

Haynes, F.D. and Mellor, M., 1976. Measuring the uniaxial compressive strength of ice. Symp. Appl. Glacial., Cambridge, 25 pp.

Baker, T.H.W., 1978. Effect of end conditions on the uniaxial compressive strength of frozen sand. Proc., Int. Conf. Permafrost, 3rd, Edmonton, 1: 608-614.

Law, K.T., 1977. Design of a loading platen for testing ice and frozen soil. Can. Geotech. J., 14(2): 266-271.

Baker, T.H.W., 1976. Compressive Strength of Some Frozen Soils. M.Sc. Thesis, Dept. Civ. Eng., Queen's Univ., Kingston, Ontario, 245 pp.

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