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A comparison of the creep behavior of saline ice and frozen saline Ottawa sand at −8°C
ColdRegions Scienceand Technology, 14 ( 1987) 273-279 Elsevier SciencePublishers B.V., Amsterdam-- Printed in the Netherlands
273
A C O M P A R I S O N OF THE CREEP BEHAVIOR OF SALINE ICE A N D FROZEN SALINE O T T A W A S A N D AT - 8 ° C G.M.
Pharr
Dept. of Materials Science, Rice University, P.O. Box 1892, Houston, TX 77251 (U.S.A.)
and P.S. Godavarti Dept. o f Nuclear Engineering, North Carolina State University, Raleigh, NC 2 7650 (U. S. A .)
(Received February 23, 1987; accepted in revised form June 3, 1987)
ABSTRACT The creep behavior of saline ice (prepared in the laboratory by liquid phase sintering) and frozen saline Ottawa sand has been examined in tests performed at constant load in unconfined compression. The range of stress over which the tests were conducted was 0.345 to 1.034 MPa. In both materials, the salinity of the ice-brine mixture was 32 ppt NaCl. The saline ice was found to be quite plastic and to deform to strains in excess of 25% without failing. The Ottawa sand, on the other hand, was observed to enter into tertiary creep at strains of about 2% and then abruptly fail. Despite these differences, the creep behavior at small strains (< 2%) is remarkably similar. This is evidenced by a comparison of the strain rates at 2% strain and the times to reach 2% strains, both of which are very nearly the same for two materials. The results are discussed with respect to simple ideas about the mechanisms of deformation and the role of the sand in the deformation process.
and Pharr, 1984; Sego and Chernenko, 1984; Pharr and Merwin, 1985). From a microstructural standpoint, these soils consist of three physically distinct components: (1) a plastic solid (ice), (2) a relatively non-deformable solid (soil particles), and (3) a liquid (brine) (Ting, 1981; Ting et al., 1983; Godavarti and Pharr, 1985). Their deformation involves a complicated interaction of all three components, and in order to begin to understand the relative importance of each, we have recently performed a series of unconfined compression creep tests on laboratory prepared saline ice and frozen saline Ottawa sand, both of the same salinity. The results, which are presented here, shed some light on the role of non-cohesive soil particles on the unconfined compression creep behavior of frozen saline soils.
2. EXPERIMENTAL PROCEDURE Specimen preparation
1. INTRODUCTION The mechanical properties of frozen saline soils have received considerable attention in the literature in the past few years, primarily because of their importance in the construction of artificial islands for the exploration and production of oil in the Beaufort Sea (Tsytovich et al., 1973; Jessberger, 1982; Sego et al., 1982; Chamberlain, 1983; Domaschuck et al., 1983; Ogata et al., 1983; Nixon
0165-232X/87/$03.50
© 1987 ElsevierSciencePublishers B.V.
Cylindrical specimens of saline ice and frozen saline Ottawa sand suitable for compression creep testing were prepared using techniques described elsewhere (Godavarti and Pharr, 1985; Pharr and Merwin, 1985). Briefly, the saline ice was made by liquid phase sintering of green compacts of powders of pure ice and reagent grade anhydrous NaC1. The sintering was performed at - 1 9 ° C for 86,400 sec (24 hours) in an evacuated cylindrical mold to which pressure could be applied through sliding end
274 plugs. The ice grain size was in the 1-2 m m range, and the sintering pressure was 1 MPa. The a m o u n t of NaC1 was chosen so that the net salinity of the ice-brine mixture was 32 ppt (parts per thousand by weight). This salinity was used because its freezing behavior is very similar to that of c o m m o n sea water, with freezing commencing at - 1 . 9 ° C . The green density of the specimens was about 90%, and the density of all the fully sintered specimens was in excess of 99.5% of theoretical. This method of specimen preparation was chosen because it produces an ice structure which is similar to that in frozen saline soils; i.e. randomly oriented, equixed grains of pure ice surrounded by an ice-brine mixture (Ting, 1981; Godavarti and Pharr, 1985). Making the saline ice in this way then allowed for a direct comparison of its creep behavior to that of frozen saline Ottawa sand with the n u m b e r of structural differences kept to a m i n i m u m . The structure is significantly different from that of natural sea ice, the bulk of which consists of long columnar grains of ice exhibiting strong crystallographic fabric with brine interspersed in intergranular channels (Weeks and Ackley, 1982; Schwarz and Weeks, 1977). For this reason, care should be exercised in comparing the results presented in this paper with those for natural sea ice (for reviews of the mechanical behavior of sea ice, see Mellor, 1983; Weeks and Ackley, 1982; Schwarz and Weeks, 1977; and Weeks and Assur, 1967). Precise specimen dimensions and densities are given in Table 1. The Ottawa sand specimens were prepared from standard Ottawa sand (ASTM standard C-190) and 32 ppt NaC1 brine. The salinity of the frozen ice-brine mixture was thus exactly the same as that of the saline ice. The Ottawa sand was sieved prior
to specimen preparation to assure that virtually 100% of it passed through a U.S. 20 mesh sieve but was retained on a U.S. 30 mesh sieve, thus producing a fairly coarse, non-cohesive soil. A split arcylic mold was packed with the sand, saturated with NaC1 brine, and frozen unidirectionally from the top down. Specimens were virtually 100% saturated and had a water content of about 20% of dry weight. Data for each Ottawa sand specimen are given in Table 2.
Creep testing All creep testing was performed in unconfined compression at - 8 . 0 + 0 . 1 ° C in a constant load apparatus. Prior to testing, specimen ends were lubricated with Exxon Beacon 290 low temperature lubricant to reduce friction and barrelling. Displacement was monitored and continuously recorded with a LVDT displacement transducer. Tests were performed in the range of stress from 0.345 to 1.034 MPa. Because it is known that the mechanical properties of saline ice can be influenced by thermal history (Weeks, 1961), efforts were made to standardize the thermal conditions experienced by each saline ice specimen prior to testing. The thermal conditions were as follows: first, sintering was performed at - 1 9 ° C for 86,400 sec (24 hours), after which the temperature was reduced to - 26 °C to prevent brine drainage, and the specimen was removed from the sintering mold. The specimen was then stored in a sealed polyethylene bag at - 2 6 °C for about 86,400 (24 hours) and mounted into the creep testing apparatus. The temperature was increased to - 8.0 _+0.1 °C in a period of about 3600
TABLE 1 Specimen and test parameters for 32 ppt saline ice creep tests* Test Specimen Specimen number l e n g t h diameter (m) (m)
Density (kg/m 3)
% Theoretical density
Creep stress (MPa)
~2o~ ( sec- ~)
t2o~ (sec)
Brine lost (% frozen wt)
Salinity after test (ppt)
SI1 SI2 S13
933.2 932.9 933.0
99.59 99.56 99.57
0.345 0.931 1.034
1.46>(10 6 7.08× 10-s
7735 167 58
10.36 10.42 10.44
14.2 15.2 14.0
0.1087 0.1107 0.1118
0.07699 0.07601 0.07700
*All tests conducted at - 8.0 + 0.1 °C.
1 . 9 3 × 10 - 4
275
TABLE 2 Specimen and test parameters for 32 ppt Ottawa sand creep tests* Test Specimen number length (m)
OSI OS2 OS3 OS4 OS5 OS6
0.1619 0.1369 0.1406 0.1489 0.1571 0.1572
Specimen diameter (m)
Frozen density (kg/m 3)
Dry density (kg/m 3)
Net water content
0.07660 0.07657 0.07658 0.07678 0.07676 0.07676
1996 2018 2075 2001 1993 2018
1637 1681 1730 1670 1664 1656
21.4 19.1 19.2 21.0 20.8 21.9
Volume % sand
(%dry wt.)
Creep stress
E2%
[2%
(sec-')
(sec)
Failure time (sec)
1.25X 10 6 3.36X10 6 1.39X 10 5 3.05X10 - s 6.41X I0 5 1.21×10 4
7623 2362 721 320 167 87
54,600 12,758 2,811 866 417 199
(MPa) 60.9 63.3 62.4 63.9 64.1 60.5
0.345 0.469 0.586 0.828 0.931 1.034
*All tests conducted at - 8.0 _+0.1 ° C.
sec (l hour) and held at this temperature for about 72,000 sec (20 hours), after which the creep test was begun.
10-2
i
32 ppt SALINE ICE -B.O -+ O. l°C
10-3
3. RESULTS Results of the creep tests are presented in Figs. 1 and 2, where the creep curves are plotted in semilogarithmic form as engineering strain rate, ~ vs. engineering strain, e. This plotting method allows for a direct comparison of results, even though the range of creep rates and failure times vary over several orders of magnitude. In addition, Mellor (1983) and Mellor and Cole (1982, 1983) have noted that plotting the data in this way often leads to significant insights in data interpretation, insights which are not possible by plotting the data in the conventional manner of strain vs. time.
Stress (MPo)
T
d
10-4
v W
Z
10-5
n~ FO'3
10-6
x•5
10-7 0.00
4. OBSERVATIONS A close inspection of the creep curves in Figs. 1 and 2 reveals that there are both similarities and differences in the creep behavior of the two materials. Perhaps the most notable difference is in the creep ductility. While the saline ice specimens crept to strains as large as 25% with no signs of failure, the Ottawa sand specimens entered into tertiary creep at about 2% strain and failed shortly thereafter. Another difference is in the steady state creep behavior. There is clearly no steady state region for
vk I
O. 20
O. I0
O. 30
STRAIN Fig. 1. Creep curves for 32 ppt saline ice tested unconfined compression at - 8.0 _+0.1 ° C.
the Ottawa sand, while for the saline ice the question of whether or not a steady state exists is not as easy to answer. This is because the tests were conducted at constant load rather than constant stress, and the linearly decelerating creep rates observed in Fig. 1 could conceivably be due to the decreases in stress which occur naturally during a constant load
276 10-2
i
i
32 ppt OTTAWA SAND - 8 . 0 ± O.l°C 10-3
Stress (MPa)
T 10-4
z
\
/
i0-5
i0-5
10-7
O. O0
,
O. 02
,
O. 04
,
,
O. 08
,
,
O. 08
,
O. 10
STRAIN
Fig. 2. Creep curves for 32 ppt frozen saline Ottawa sand tested in unconfined compression at - 8.0 + 0.1 °C. test conducted in compression. This issue can be resolved only by performing constant stress tests. Physically, the saline ice specimens were observed to deform fairly uniformly with very little barreling. There was no evidence of cracking, even at 25% strain, and the only really noticeable change • -imparted to a specimen by deformation was a slight roughening of its surface thereby producing an "orange peel" appearance. In all the saline ice tests a significant amount of brine was observed to drain from the specimens. This occurred both during the temperature equilibration period before the creep test and during the creep test itself. Following testing, there was then a pool of brine around the base of the specimens. Weight loss measurements revealed that the amount of lost brine was quite substantial - - about 10% by weight for each test (see Table 1 for exact figures). Apparently, the liquid brine phase, which constitutes about 24.0% of the specimen volume at - 8 °C (see Pharr and Merwin, 1985, for a derivation of this figure), is interconnected in a way which allows a significant fraction
of it to migrate to and drain from the surface of the specimen in a manner similar to the drainage of brine from other forms of saline ice (Weeks and Ackley, 1982; Cox and Weeks, 1975; Eide and Martin, 1975; Lake and Lewis, 1970). Because of this drainage, the net salinity of the ice was reduced, and salinity measurements made after creep showed that the specimen salinity had fallen to about 15 ppt (see Table 1 for exact figures). The physical observations for the Ottawa sand were quite different. Virtually no drainage was observed, either prior to or during creep, and a significant amount of cracking occurred. In general, the cracks ran in a direction parallel to the long axis of the specimen, i.e., in a direction parallel to the axis of compression. Apparently, it is the growth and coalescence of these cracks which leads to the premature failure and low creep ductility of the Ottawa sand. The fact that no drainage was observed is actually not surprising, since a large fraction of the volume of these specimens is occupied by sand (about 62.3%), and on average, only about 9% of the specimen volume is liquid at - 8.0°C. Thus, the ease with which the liquid can migrate out of the material is undoubtedly less than that in the saline ice. In addition, it is conceivable that surface tension effects between the brine and sand particles act to further retard drainage. Although the differences in the creep behavior of saline ice and frozen Ottawa sand are numerous, there are some notable similarities, too. In particular, the deformation behavior at small strains, strains less than those at which tertiary creep begins in the Ottawa sand (typically about 2%), is strikingly similar. This is not obvious in Figs. 1 and 2, but can be demonstrated by plotting the stress dependence of two parameters: (1) the strain rate at 2% strain, ~2%, and (2) the time to reach 2% strain, t2%, as has been done in Figs. 3 and 4. An inspection of the figures reveals that both these parameters, which are descriptive of the early stages of creep, are remarkably similar for the two materials, thereby suggesting that the small strain deformation of the saline ice and frozen saline Ottawa sand is very nearly the same. Since the data in both plots are linear on log-log axes, the stress dependencies of ~2,o and t2o~ can be written in power law form. Least squares fits of the data show that the relations are
277
10-2
T. i0-~ 0
I II IIII
I
I
I
10 5
I
0 saline ice []Ottawa sand
d o
0 v
v
Z
Z
Od HtO ~d
/~'
Z Od HO3
W ~E
10-6
10-7 0.2
\ []
,.0, Xo
CO
seo ice
I I I Illl
I
STRESS
I
I 5.0
1.0
~2%----1.02X 10 -4 0"4"15 (sec - l )
(1)
(2)
where 0- is in units o f MPa. The stress exponent in eqn.. 1, n=4.15, is higher than the value most frequently reported for the steady state creep of pure ice, n = 3 ( G o o d m a n et al., 1981 ), but is close to the value n = 4 used by Mellor (1983) to describe the deformation of both natural sea ice and fresh water ice when tested in the higher strain rate regime ( 10- 5 s e c - ' ~ ~< 10 -3 s e c - ' ) . For the sake of comparison, also shown in Fig. 3 is a band proposed by Mellor (1983) to be representative of the deformation behavior of natural sea ice. From this data it is seen that the liquid phase sintered saline ice used in this study is considerably weaker than natural sea ice. A large part o f this difference can be attributed to the fact that natural sea
[]\
[]
o\
101 0.2
I
1
I
STRESS
and
t2%= 1 . 0 4 X 10 2 0--4.04 ( s e c ) ,
i 10 2
(MPo)
Fig. 3. The stress dependence of the strain rate at 2% strain, ~2%, for saline ice and frozen saline Ottawa sand. Also shown is a band representing the strength of natural sea ice as surmised by Mellor (1983).
ice sand
[]
L~J Or
//~/
0 saline []Ottawa
\
"liD
H n or l
I I I II
[]
10 3
/
Od
I
\
HtO
4.15
10-5 W
I
nl
/°
10-4
10 4
I
I I I II 1.0
2.0
(MPo)
Fig. 4. The stress dependence of the time to reach 2% strain, t.,%, for saline ice and frozen saline Ottawa sand.
ice, in its most common form, is considerably less saline than the ice studied here and is thus stronger because it contains smaller quantities of brine. Typical salinities for natural sea ice are in the 5-10 ppt range (see, for example, Weeks and Ackley, 1982). It is also conceivable that some of the strength difference is due to the structural differences referred to earlier in this paper.
5. DISCUSSION Before undertaking this study, it was our general belief that the addition of sand to the saline ice would improve its creep resistance. While we did expect the sand to have a somewhat detrimental effect on the creep ductility, we also expected that the rate of creep during the initial stages of deformation would be greatly reduced. However, the experimental results clearly show that this is not the case; the deformation rates during primary creep of saline Ottawa sand are virtually the same as those
278 o f saline ice. Apparently, the sand has no effect whatsoever on the rate o f creep at small strains, and its only effect on d e f o r m a t i o n is to p r o m o t e tertiary creep and failure in a material which is otherwise quite ductile. F r o m an engineering standpoint, the sand then plays a very undesirable role. An exact explanation for this b e h a v i o r and a complete understanding o f the d e f o r m a t i o n mechanisms involved cannot be obtained without the aid o f careful microstructural work and, unfortunately, this is b e y o n d our current capabilities. Nevertheless, it appears that creep d e f o r m a t i o n in saline ice and frozen saline Ottawa sand is ultimately controlled by plasticity in the ice-brine mixture. This is a process in which both the solid (ice) and liquid phases are probably important, with the liquid causing the solid to creep at m u c h faster rates than it would if no liquid were present. Liquid e n h a n c e d creep is now recognized as an i m p o r t a n t m o d e o f d e f o r m a t i o n in m a n y materials, including high temperature structural ceramics containing amorphous grain b o u n d a r y phases and alkali halide salts containing aqueous brine solutions (Raj, 1982; Raj and Chyung, 1981; Lange et al., 1980; Pharr and Ashby, 1983; Sheikh and Pharr, 1985). As for the role o f the sand, evidently the sand grains act to produce tensile stress concentrations in the ice-brine mixture, most probably in a direction roughly perpendicular to the compression axis, and this then results in the internal cavitation and cracking which eventually lead to failure. Careful microstructural work would greatly enhance our understanding o f these p h e n o m e n a . In closing, it should be pointed out that the experimental results and conclusions discussed here are based on data acquired in unconfined tests. Since a confining pressure would act to inhibit the opening o f internal cracks and voids, the confined creep behavior o f frozen saline Ottawa sand is probably very different. Thus, it would not be surprising to discover that, when tested u n d e r confined conditions, frozen saline Ottawa sand is considerably more creep resistant than was indicated in this study.
ACKNOWLEDGEMENTS The authors wish to express their gratitude to Paul Bryant and Martin Wiedenmeyer, both o f w h o m
helped with experimental aspects o f the program. One o f the authors ( G M P ) would like to gratefully acknowledge the IBM Corporation, which p r o v i d e d financial assistance during the course of much o f this work through a Faculty D e v e l o p m e n t Grant.
REFERENCES Chamberlain, E.J. (1983). Frost heave of saline soils. Proc. 4th Int. Conf. on Permafrost, Fairbanks, AK, July 1983, pp. 121-126. Cox, G.F.N. and Weeks, W.F. (1975). Brine drainage and initial salt entrapment in sodium chloride ice. CRREL Res. Rep. 354, 1975. Domashuk, L., Man, C.-S., Shields, D.H. and Young, E. (1983). Creep behavior of frozen saline silt under isotropic compression. Proc. 4th Int. Conf. on Permafrost, Fairbanks, AK, July 1983, pp. 238-243. Eide, L. and Martin, S. (1975). The formation of brine drainage features in young sea ice. J. Glaciol., 14(70): 137-154. Godavarti, P.S. and Pharr, G.M. (1985). A technique for producing ice from NaC1 brine for studying fundamental deformation behavior. J. Energy Resour. Technol., 107: 173-176. Goodman, D.J., Frost, H.J. and Ashby, M.F. (1981). The plasticity of polycrystalline ice. Phil. Mag. A, 43(3): 665-695. Jessberger, H.L. (1982). Frozen saline sands subjected to dynamic loads. Proc. 3rd Int. Symp. on Ground Freezing, Hanover, NH, June 1982, pp. 19-25. Lake, R.A. and Lewis, E.L. (1970). Salt rejection by sea ice during growth. J. Geophys. Res., 75(3): 583-597. Lange, F.F., Davis, B.I. and Clarke, D.R. (1980). Compressive creep of Si3NJMgO alloys. J. Materials Science, 15: 601-610. Mellor, M. (1983). Mechanical behavior of sea ice. CRREL Monograph 83- l, June 1983. Mellor, M. and Cole, D.M. (1982). Deformation and failure of ice under constant stress or constant strain-rate. Cold Reg. Sci. Technol., 5: 201-219. Mellor, M. and Cole, D.M. (1983). Stress/strain/time relations for ice under uniaxial compression. Cold Reg. Sci. Technol., 6: 207-230. Nixon, M.S. and Pharr, G.M. (1984). The effects of temperature, stress, and salinity on the creep of frozen saline soil. J. Energy Resour. Technol., 106 (3): 344-348. Ogata, N., Yasuda, M. and Kataoka, T. (1983). Effects of salt concentration on strength and creep behavior of artificially frozen soils. Cold Reg. Sci. Technol., 8:139-153. Pharr, G.M. and Ashby, M.F. (1983). On creep enhanced by a liquid phase. Acta Metall., 31:129-138. Pharr, G.M. and Merwin, J.E. (1985). Effects of brine content on the strength of frozen Ottawa sand. Cold Reg. Sci. Technol., 11: 205-212. Raj, R. (1982 ). Creep of polycrystalline aggregates by matter
279 transport through a liquid phase. J. Geophys. Res., 87 (B6): 4731-4739. Raj, R. and Chyung, C.K. (1981). Solution-precipitation creep in glass ceramics. Acta Metall., 29:159-166. Schwarz, J. and Weeks, W.F. (1977). Engineering properties of sea ice. J. Glaciol., 19(81 ): 499-531. Sego, D.C., Schultz, T. and Banasch, R. (1982). Strength and deformation behavior of frozen saline sand. Proc. 3rd Symp. on Ground Freezing. Hanover, NH, June 1982, pp. 11-17. Sego, D.C. and Chernenko, D. (1984). Confining pressure influence on the strength of frozen saline sand. Proc. 3rd Int. Specialty Conf. on Cold Regions Engineering, Montreal, April 1984, pp. 565-578. Sheikh, Gh.R. and Pharr, G.M. (1985). Further observations on creep enhanced by a liquid phase in porous potassium chloride. Acta Metall., 33(2): 231-238. Ting, J.M. ( 1981 ). The creep of frozen sands: qualitative and
quantitative models - - Final report II. U.S. Army Research Office, Res. Rep. N. R81-5. Ting, J.M., Martin, R.T. and Ladd, C.L. (1983). Mechanics of strength of frozen sand. J. Geotech. Eng., 109(10): 1286-1302. Tsytovich, N.A., Kronik, Y.A., Markin, K.F., Aksenov, V.I. and Samuelson, M.V. (1973). Physical and mechancial properties of saline soils. 2nd Int. Conf. on Permafrost, Yakutsk, USSR, July 1973, pp. 238-246. Weeks, W.F. ( 1961 ). Studies of salt ice, 1: The tensile strength of NaCl ice. CRREL Res. Rep. 80, August 1961. Weeks, W.F. and Ackley, S.E. (1982). The growth, structure, and properties of sea ice. CRREL Monograph 82-1, November 1982. Weeks, W.F. and Assur, A. (1967). The mechanical properties of sea ice. In: Cold Regions Science and Engineering, Part II: Physical Science, Section C: Physics and Mechancis of Ice. CRREL, September 1967.
273
A C O M P A R I S O N OF THE CREEP BEHAVIOR OF SALINE ICE A N D FROZEN SALINE O T T A W A S A N D AT - 8 ° C G.M.
Pharr
Dept. of Materials Science, Rice University, P.O. Box 1892, Houston, TX 77251 (U.S.A.)
and P.S. Godavarti Dept. o f Nuclear Engineering, North Carolina State University, Raleigh, NC 2 7650 (U. S. A .)
(Received February 23, 1987; accepted in revised form June 3, 1987)
ABSTRACT The creep behavior of saline ice (prepared in the laboratory by liquid phase sintering) and frozen saline Ottawa sand has been examined in tests performed at constant load in unconfined compression. The range of stress over which the tests were conducted was 0.345 to 1.034 MPa. In both materials, the salinity of the ice-brine mixture was 32 ppt NaCl. The saline ice was found to be quite plastic and to deform to strains in excess of 25% without failing. The Ottawa sand, on the other hand, was observed to enter into tertiary creep at strains of about 2% and then abruptly fail. Despite these differences, the creep behavior at small strains (< 2%) is remarkably similar. This is evidenced by a comparison of the strain rates at 2% strain and the times to reach 2% strains, both of which are very nearly the same for two materials. The results are discussed with respect to simple ideas about the mechanisms of deformation and the role of the sand in the deformation process.
and Pharr, 1984; Sego and Chernenko, 1984; Pharr and Merwin, 1985). From a microstructural standpoint, these soils consist of three physically distinct components: (1) a plastic solid (ice), (2) a relatively non-deformable solid (soil particles), and (3) a liquid (brine) (Ting, 1981; Ting et al., 1983; Godavarti and Pharr, 1985). Their deformation involves a complicated interaction of all three components, and in order to begin to understand the relative importance of each, we have recently performed a series of unconfined compression creep tests on laboratory prepared saline ice and frozen saline Ottawa sand, both of the same salinity. The results, which are presented here, shed some light on the role of non-cohesive soil particles on the unconfined compression creep behavior of frozen saline soils.
2. EXPERIMENTAL PROCEDURE Specimen preparation
1. INTRODUCTION The mechanical properties of frozen saline soils have received considerable attention in the literature in the past few years, primarily because of their importance in the construction of artificial islands for the exploration and production of oil in the Beaufort Sea (Tsytovich et al., 1973; Jessberger, 1982; Sego et al., 1982; Chamberlain, 1983; Domaschuck et al., 1983; Ogata et al., 1983; Nixon
0165-232X/87/$03.50
© 1987 ElsevierSciencePublishers B.V.
Cylindrical specimens of saline ice and frozen saline Ottawa sand suitable for compression creep testing were prepared using techniques described elsewhere (Godavarti and Pharr, 1985; Pharr and Merwin, 1985). Briefly, the saline ice was made by liquid phase sintering of green compacts of powders of pure ice and reagent grade anhydrous NaC1. The sintering was performed at - 1 9 ° C for 86,400 sec (24 hours) in an evacuated cylindrical mold to which pressure could be applied through sliding end
274 plugs. The ice grain size was in the 1-2 m m range, and the sintering pressure was 1 MPa. The a m o u n t of NaC1 was chosen so that the net salinity of the ice-brine mixture was 32 ppt (parts per thousand by weight). This salinity was used because its freezing behavior is very similar to that of c o m m o n sea water, with freezing commencing at - 1 . 9 ° C . The green density of the specimens was about 90%, and the density of all the fully sintered specimens was in excess of 99.5% of theoretical. This method of specimen preparation was chosen because it produces an ice structure which is similar to that in frozen saline soils; i.e. randomly oriented, equixed grains of pure ice surrounded by an ice-brine mixture (Ting, 1981; Godavarti and Pharr, 1985). Making the saline ice in this way then allowed for a direct comparison of its creep behavior to that of frozen saline Ottawa sand with the n u m b e r of structural differences kept to a m i n i m u m . The structure is significantly different from that of natural sea ice, the bulk of which consists of long columnar grains of ice exhibiting strong crystallographic fabric with brine interspersed in intergranular channels (Weeks and Ackley, 1982; Schwarz and Weeks, 1977). For this reason, care should be exercised in comparing the results presented in this paper with those for natural sea ice (for reviews of the mechanical behavior of sea ice, see Mellor, 1983; Weeks and Ackley, 1982; Schwarz and Weeks, 1977; and Weeks and Assur, 1967). Precise specimen dimensions and densities are given in Table 1. The Ottawa sand specimens were prepared from standard Ottawa sand (ASTM standard C-190) and 32 ppt NaC1 brine. The salinity of the frozen ice-brine mixture was thus exactly the same as that of the saline ice. The Ottawa sand was sieved prior
to specimen preparation to assure that virtually 100% of it passed through a U.S. 20 mesh sieve but was retained on a U.S. 30 mesh sieve, thus producing a fairly coarse, non-cohesive soil. A split arcylic mold was packed with the sand, saturated with NaC1 brine, and frozen unidirectionally from the top down. Specimens were virtually 100% saturated and had a water content of about 20% of dry weight. Data for each Ottawa sand specimen are given in Table 2.
Creep testing All creep testing was performed in unconfined compression at - 8 . 0 + 0 . 1 ° C in a constant load apparatus. Prior to testing, specimen ends were lubricated with Exxon Beacon 290 low temperature lubricant to reduce friction and barrelling. Displacement was monitored and continuously recorded with a LVDT displacement transducer. Tests were performed in the range of stress from 0.345 to 1.034 MPa. Because it is known that the mechanical properties of saline ice can be influenced by thermal history (Weeks, 1961), efforts were made to standardize the thermal conditions experienced by each saline ice specimen prior to testing. The thermal conditions were as follows: first, sintering was performed at - 1 9 ° C for 86,400 sec (24 hours), after which the temperature was reduced to - 26 °C to prevent brine drainage, and the specimen was removed from the sintering mold. The specimen was then stored in a sealed polyethylene bag at - 2 6 °C for about 86,400 (24 hours) and mounted into the creep testing apparatus. The temperature was increased to - 8.0 _+0.1 °C in a period of about 3600
TABLE 1 Specimen and test parameters for 32 ppt saline ice creep tests* Test Specimen Specimen number l e n g t h diameter (m) (m)
Density (kg/m 3)
% Theoretical density
Creep stress (MPa)
~2o~ ( sec- ~)
t2o~ (sec)
Brine lost (% frozen wt)
Salinity after test (ppt)
SI1 SI2 S13
933.2 932.9 933.0
99.59 99.56 99.57
0.345 0.931 1.034
1.46>(10 6 7.08× 10-s
7735 167 58
10.36 10.42 10.44
14.2 15.2 14.0
0.1087 0.1107 0.1118
0.07699 0.07601 0.07700
*All tests conducted at - 8.0 + 0.1 °C.
1 . 9 3 × 10 - 4
275
TABLE 2 Specimen and test parameters for 32 ppt Ottawa sand creep tests* Test Specimen number length (m)
OSI OS2 OS3 OS4 OS5 OS6
0.1619 0.1369 0.1406 0.1489 0.1571 0.1572
Specimen diameter (m)
Frozen density (kg/m 3)
Dry density (kg/m 3)
Net water content
0.07660 0.07657 0.07658 0.07678 0.07676 0.07676
1996 2018 2075 2001 1993 2018
1637 1681 1730 1670 1664 1656
21.4 19.1 19.2 21.0 20.8 21.9
Volume % sand
(%dry wt.)
Creep stress
E2%
[2%
(sec-')
(sec)
Failure time (sec)
1.25X 10 6 3.36X10 6 1.39X 10 5 3.05X10 - s 6.41X I0 5 1.21×10 4
7623 2362 721 320 167 87
54,600 12,758 2,811 866 417 199
(MPa) 60.9 63.3 62.4 63.9 64.1 60.5
0.345 0.469 0.586 0.828 0.931 1.034
*All tests conducted at - 8.0 _+0.1 ° C.
sec (l hour) and held at this temperature for about 72,000 sec (20 hours), after which the creep test was begun.
10-2
i
32 ppt SALINE ICE -B.O -+ O. l°C
10-3
3. RESULTS Results of the creep tests are presented in Figs. 1 and 2, where the creep curves are plotted in semilogarithmic form as engineering strain rate, ~ vs. engineering strain, e. This plotting method allows for a direct comparison of results, even though the range of creep rates and failure times vary over several orders of magnitude. In addition, Mellor (1983) and Mellor and Cole (1982, 1983) have noted that plotting the data in this way often leads to significant insights in data interpretation, insights which are not possible by plotting the data in the conventional manner of strain vs. time.
Stress (MPo)
T
d
10-4
v W
Z
10-5
n~ FO'3
10-6
x•5
10-7 0.00
4. OBSERVATIONS A close inspection of the creep curves in Figs. 1 and 2 reveals that there are both similarities and differences in the creep behavior of the two materials. Perhaps the most notable difference is in the creep ductility. While the saline ice specimens crept to strains as large as 25% with no signs of failure, the Ottawa sand specimens entered into tertiary creep at about 2% strain and failed shortly thereafter. Another difference is in the steady state creep behavior. There is clearly no steady state region for
vk I
O. 20
O. I0
O. 30
STRAIN Fig. 1. Creep curves for 32 ppt saline ice tested unconfined compression at - 8.0 _+0.1 ° C.
the Ottawa sand, while for the saline ice the question of whether or not a steady state exists is not as easy to answer. This is because the tests were conducted at constant load rather than constant stress, and the linearly decelerating creep rates observed in Fig. 1 could conceivably be due to the decreases in stress which occur naturally during a constant load
276 10-2
i
i
32 ppt OTTAWA SAND - 8 . 0 ± O.l°C 10-3
Stress (MPa)
T 10-4
z
\
/
i0-5
i0-5
10-7
O. O0
,
O. 02
,
O. 04
,
,
O. 08
,
,
O. 08
,
O. 10
STRAIN
Fig. 2. Creep curves for 32 ppt frozen saline Ottawa sand tested in unconfined compression at - 8.0 + 0.1 °C. test conducted in compression. This issue can be resolved only by performing constant stress tests. Physically, the saline ice specimens were observed to deform fairly uniformly with very little barreling. There was no evidence of cracking, even at 25% strain, and the only really noticeable change • -imparted to a specimen by deformation was a slight roughening of its surface thereby producing an "orange peel" appearance. In all the saline ice tests a significant amount of brine was observed to drain from the specimens. This occurred both during the temperature equilibration period before the creep test and during the creep test itself. Following testing, there was then a pool of brine around the base of the specimens. Weight loss measurements revealed that the amount of lost brine was quite substantial - - about 10% by weight for each test (see Table 1 for exact figures). Apparently, the liquid brine phase, which constitutes about 24.0% of the specimen volume at - 8 °C (see Pharr and Merwin, 1985, for a derivation of this figure), is interconnected in a way which allows a significant fraction
of it to migrate to and drain from the surface of the specimen in a manner similar to the drainage of brine from other forms of saline ice (Weeks and Ackley, 1982; Cox and Weeks, 1975; Eide and Martin, 1975; Lake and Lewis, 1970). Because of this drainage, the net salinity of the ice was reduced, and salinity measurements made after creep showed that the specimen salinity had fallen to about 15 ppt (see Table 1 for exact figures). The physical observations for the Ottawa sand were quite different. Virtually no drainage was observed, either prior to or during creep, and a significant amount of cracking occurred. In general, the cracks ran in a direction parallel to the long axis of the specimen, i.e., in a direction parallel to the axis of compression. Apparently, it is the growth and coalescence of these cracks which leads to the premature failure and low creep ductility of the Ottawa sand. The fact that no drainage was observed is actually not surprising, since a large fraction of the volume of these specimens is occupied by sand (about 62.3%), and on average, only about 9% of the specimen volume is liquid at - 8.0°C. Thus, the ease with which the liquid can migrate out of the material is undoubtedly less than that in the saline ice. In addition, it is conceivable that surface tension effects between the brine and sand particles act to further retard drainage. Although the differences in the creep behavior of saline ice and frozen Ottawa sand are numerous, there are some notable similarities, too. In particular, the deformation behavior at small strains, strains less than those at which tertiary creep begins in the Ottawa sand (typically about 2%), is strikingly similar. This is not obvious in Figs. 1 and 2, but can be demonstrated by plotting the stress dependence of two parameters: (1) the strain rate at 2% strain, ~2%, and (2) the time to reach 2% strain, t2%, as has been done in Figs. 3 and 4. An inspection of the figures reveals that both these parameters, which are descriptive of the early stages of creep, are remarkably similar for the two materials, thereby suggesting that the small strain deformation of the saline ice and frozen saline Ottawa sand is very nearly the same. Since the data in both plots are linear on log-log axes, the stress dependencies of ~2,o and t2o~ can be written in power law form. Least squares fits of the data show that the relations are
277
10-2
T. i0-~ 0
I II IIII
I
I
I
10 5
I
0 saline ice []Ottawa sand
d o
0 v
v
Z
Z
Od HtO ~d
/~'
Z Od HO3
W ~E
10-6
10-7 0.2
\ []
,.0, Xo
CO
seo ice
I I I Illl
I
STRESS
I
I 5.0
1.0
~2%----1.02X 10 -4 0"4"15 (sec - l )
(1)
(2)
where 0- is in units o f MPa. The stress exponent in eqn.. 1, n=4.15, is higher than the value most frequently reported for the steady state creep of pure ice, n = 3 ( G o o d m a n et al., 1981 ), but is close to the value n = 4 used by Mellor (1983) to describe the deformation of both natural sea ice and fresh water ice when tested in the higher strain rate regime ( 10- 5 s e c - ' ~ ~< 10 -3 s e c - ' ) . For the sake of comparison, also shown in Fig. 3 is a band proposed by Mellor (1983) to be representative of the deformation behavior of natural sea ice. From this data it is seen that the liquid phase sintered saline ice used in this study is considerably weaker than natural sea ice. A large part o f this difference can be attributed to the fact that natural sea
[]\
[]
o\
101 0.2
I
1
I
STRESS
and
t2%= 1 . 0 4 X 10 2 0--4.04 ( s e c ) ,
i 10 2
(MPo)
Fig. 3. The stress dependence of the strain rate at 2% strain, ~2%, for saline ice and frozen saline Ottawa sand. Also shown is a band representing the strength of natural sea ice as surmised by Mellor (1983).
ice sand
[]
L~J Or
//~/
0 saline []Ottawa
\
"liD
H n or l
I I I II
[]
10 3
/
Od
I
\
HtO
4.15
10-5 W
I
nl
/°
10-4
10 4
I
I I I II 1.0
2.0
(MPo)
Fig. 4. The stress dependence of the time to reach 2% strain, t.,%, for saline ice and frozen saline Ottawa sand.
ice, in its most common form, is considerably less saline than the ice studied here and is thus stronger because it contains smaller quantities of brine. Typical salinities for natural sea ice are in the 5-10 ppt range (see, for example, Weeks and Ackley, 1982). It is also conceivable that some of the strength difference is due to the structural differences referred to earlier in this paper.
5. DISCUSSION Before undertaking this study, it was our general belief that the addition of sand to the saline ice would improve its creep resistance. While we did expect the sand to have a somewhat detrimental effect on the creep ductility, we also expected that the rate of creep during the initial stages of deformation would be greatly reduced. However, the experimental results clearly show that this is not the case; the deformation rates during primary creep of saline Ottawa sand are virtually the same as those
278 o f saline ice. Apparently, the sand has no effect whatsoever on the rate o f creep at small strains, and its only effect on d e f o r m a t i o n is to p r o m o t e tertiary creep and failure in a material which is otherwise quite ductile. F r o m an engineering standpoint, the sand then plays a very undesirable role. An exact explanation for this b e h a v i o r and a complete understanding o f the d e f o r m a t i o n mechanisms involved cannot be obtained without the aid o f careful microstructural work and, unfortunately, this is b e y o n d our current capabilities. Nevertheless, it appears that creep d e f o r m a t i o n in saline ice and frozen saline Ottawa sand is ultimately controlled by plasticity in the ice-brine mixture. This is a process in which both the solid (ice) and liquid phases are probably important, with the liquid causing the solid to creep at m u c h faster rates than it would if no liquid were present. Liquid e n h a n c e d creep is now recognized as an i m p o r t a n t m o d e o f d e f o r m a t i o n in m a n y materials, including high temperature structural ceramics containing amorphous grain b o u n d a r y phases and alkali halide salts containing aqueous brine solutions (Raj, 1982; Raj and Chyung, 1981; Lange et al., 1980; Pharr and Ashby, 1983; Sheikh and Pharr, 1985). As for the role o f the sand, evidently the sand grains act to produce tensile stress concentrations in the ice-brine mixture, most probably in a direction roughly perpendicular to the compression axis, and this then results in the internal cavitation and cracking which eventually lead to failure. Careful microstructural work would greatly enhance our understanding o f these p h e n o m e n a . In closing, it should be pointed out that the experimental results and conclusions discussed here are based on data acquired in unconfined tests. Since a confining pressure would act to inhibit the opening o f internal cracks and voids, the confined creep behavior o f frozen saline Ottawa sand is probably very different. Thus, it would not be surprising to discover that, when tested u n d e r confined conditions, frozen saline Ottawa sand is considerably more creep resistant than was indicated in this study.
ACKNOWLEDGEMENTS The authors wish to express their gratitude to Paul Bryant and Martin Wiedenmeyer, both o f w h o m
helped with experimental aspects o f the program. One o f the authors ( G M P ) would like to gratefully acknowledge the IBM Corporation, which p r o v i d e d financial assistance during the course of much o f this work through a Faculty D e v e l o p m e n t Grant.
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279 transport through a liquid phase. J. Geophys. Res., 87 (B6): 4731-4739. Raj, R. and Chyung, C.K. (1981). Solution-precipitation creep in glass ceramics. Acta Metall., 29:159-166. Schwarz, J. and Weeks, W.F. (1977). Engineering properties of sea ice. J. Glaciol., 19(81 ): 499-531. Sego, D.C., Schultz, T. and Banasch, R. (1982). Strength and deformation behavior of frozen saline sand. Proc. 3rd Symp. on Ground Freezing. Hanover, NH, June 1982, pp. 11-17. Sego, D.C. and Chernenko, D. (1984). Confining pressure influence on the strength of frozen saline sand. Proc. 3rd Int. Specialty Conf. on Cold Regions Engineering, Montreal, April 1984, pp. 565-578. Sheikh, Gh.R. and Pharr, G.M. (1985). Further observations on creep enhanced by a liquid phase in porous potassium chloride. Acta Metall., 33(2): 231-238. Ting, J.M. ( 1981 ). The creep of frozen sands: qualitative and
quantitative models - - Final report II. U.S. Army Research Office, Res. Rep. N. R81-5. Ting, J.M., Martin, R.T. and Ladd, C.L. (1983). Mechanics of strength of frozen sand. J. Geotech. Eng., 109(10): 1286-1302. Tsytovich, N.A., Kronik, Y.A., Markin, K.F., Aksenov, V.I. and Samuelson, M.V. (1973). Physical and mechancial properties of saline soils. 2nd Int. Conf. on Permafrost, Yakutsk, USSR, July 1973, pp. 238-246. Weeks, W.F. ( 1961 ). Studies of salt ice, 1: The tensile strength of NaCl ice. CRREL Res. Rep. 80, August 1961. Weeks, W.F. and Ackley, S.E. (1982). The growth, structure, and properties of sea ice. CRREL Monograph 82-1, November 1982. Weeks, W.F. and Assur, A. (1967). The mechanical properties of sea ice. In: Cold Regions Science and Engineering, Part II: Physical Science, Section C: Physics and Mechancis of Ice. CRREL, September 1967.