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Geological Society of America Bulletin, 72, 10, pp. 1575-1577, 1961-12-01
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Ice-grain structure and crystal orientation in an ice lens from leda clay
Penner, E.
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NATIONAL
RESEARCH
COUNCIL
4 N ~ ~ ~ Z E D
C A N A D A
DIVISION OF BUILDING RESEARCH
ICE -GRAIN STRUCTURE AND CRYSTAL ORIENTATION IN AN
ICE LENS FROM LEDA CLAY
BY BUILDING R E S E A R C H
E. PENNER
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LIBRARY
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REPRINTED FROM
GEOLOGICAL SOCIETY O F AMERICA BULLETIN. VOL. 72, N O . 10.
OCTOBER 1961, P. 1575 - 1577
RESEARCH PAPER NO. 145 O F T H E
DIVISION OF BUILDING RESEARCH
OTTAWA
DECEMBER 1961
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A N A L Y Z E D
I
ICE-GRAIN STRUCTURE AND CRYSTAL ORIENTATION
IIN AN ICE LENS FROM LEDA CLAY
Abstract: The long axes of ice grains n.crc oriented the c-axes were as much as 45' apart. This disorder parallel to the dircctio~l of hcat flow. A random seems to be consistent wit11 the disorderly distribu- c-axis oricntation, determined by an etching tcch- tion of clay particles in the ice lens.
n i q ~ ~ c , appearcd to crist. 111 some acljacent crystals Marked differences have lrequently been observed in the character of frozen soil and the disposition of ice and soil resulting lrom the ice-segregation process in different soil types (Czeratzki and Frese, 1958). T h e soil structure and the related leatures that consistently occur 1v11en renlolded 1,eda clay is lrozen (Pl. 1) are in sharp contrast to the r e g ~ ~ l a r soil strata, well-ordered ice lenses, and flat lreezing fronts so common in coarser frost-susceptible soils.
T h e main purpose o l this preliminary investi- gation mas t o determine whether there is some well-defined reg~llarity in the ice s t r ~ l c t u r e both with resDect t o the crvstal orientation and the form and orientation o l the ice grains despite the apparently random occlusions of clay in the ice and the undulating soil-ice interface. Such regularity is common for ice produced fro111 bull< water in a container bv one-dimensional cooling.
A block sample of Leda clay was obtained lrom a depth o l 2 0 feet in a freshly dug excava- tion. I t was tl~oroughly remolded a t its n a t ~ ~ r a l moisture content o l about 60 per cent and consolidated in a 6-inch-diameter mold t o a dry density o l 1.58 gm/cm3 (101 lb/lt3). T h e resultant moisture content o l this sat~lrated specimen was about 26 per cent. Grain size was determined ~1si11g tlle hydrometer metllod. T h e clay content (<0.002 mm) and silt con- tent (0.002 t o 0.2 mm) were 69 and 25 per cent respectively. Sonle details o n the origin and geotechnical properties of this soil are given by Eden and Crawlord (1957).
A cylindrical specimen 6 inclles in diameter and 2 inches high was lrozen ~~nidirectionally in the frost-cell apparatus described by Penner (1960). T h e pre-freezing temperature gradient was l.lG°F/inch with the cold side of the specimen close t o 32OF. Alter a constant heat Aom was achieved the cold end nras supercooled t o about 31°F, and crystallization of the soil
water was easily induced by rubbing the top o l the soil with a previously inserted steel wire. T h e cold-side temperature of the conditioning plate was set, and freezing was allowed to pro- ceed without any changes in the temperature of the conditioning plates. A moderate rate of heat removal from the specimen insured that only a part o l the specimen ~ v o u l d be frozen in a period o l a weel<. At the same time a slolvly penetrating frost line was needed t o build up an ice lens with some soil inclusions. A heave o l 0.59 incll was achieved over a period of 174 hours giving a n average heave of about 0.08 inch/day. T h e rate of heaving at the beginning was nearly 0.1 inch/day and after 7 days slowed down t o 0.0605 incll/day.
Alter the freezing period the speci~nen was re~noved from the frost cell and taken into a cold ( l j ° F ) room. Ilalf of the s a m ~ l e \ , \vas C L I ~ in about 1/8-inch slices with a band saw to permit examination of the vertical section of the ice lens. T h e other half was cut l~orizontally l o r e s a ~ n i ~ l a t i o ~ l of the horizontal section.
'The etching procedure used t o d e t e r n l i ~ ~ e the c-axes orientation was essentially that described by I-Iiguchi (1958) and was carried out a t a cold-room temperature of l j ° F . Sarnples were prepared by polishing the surlace of the ice lens in the cut slices. A 1 per cent solution of polyvinyl lormal in ethylene dichloride mas applied t o the surface of the ice lens directly alter polishing. Alter evaporation o l the solvent the surface was examined n~icroscopically (mag. 40 X) under transmitted light lor the apfearance of etch pits. I t tool< LIP t o 30 m ~ n u t e s lor mature pits t o develop.
A vertical slice of the specimen is shown in Plate 1. T h e heat flow during freezing was uni- directional from t h e bottom t o the top. All the soil below the ice lens was ~ ~ n f r o z e n when re- moved from the host cell. T o preserve the sample it was rapidly lrozen and kept a t a Geological Soclety oEX~llerica Bulletin, r. 72, p. 1575-1578, 1 fig., 4 pls., Octobcr 1961
E. PENNER-ICE L E N S 1:ROICi I.ED.1 CI.1Y
regular bo~undaries. The orientation of the long axes of the ice grains seems t o be the same as that reported by Gold (1960) when water i n a tank is frozen by one-din~ensional cooling in the direction normal to the water surface.
T h e etch pits developed o n the surface of horizontal and vertical sections of the ice lens are shown i n Plates 3 and 4. T h e diagram by
CIII]
Higuchi (1958) showing the raxis orientation of an ice crystal in relation t o the type of etch pit is reproduced in Figure 1. A ri.sum6 of all these results is given in Table 1.T h e disorder in the optic axes of the ice grains appears to be consistent with the dis-
4 3 orderly nature of the clay and ice distribution
F ; 1. ~~ ~~ ldiagram ~ ~ ~ of possible types ~ ~ ~ t i in the frozen layer and the uneven character of ~ etch pits on an ice crystal (alter H i g u c h i , 1958) the ice-soil interface at thc freezing front. I-Iow
.
-Plane of icc surface mith Orientation of c-ares rcfcrcnce to dirccrion o l \ v i ~ h rcfcrcncc to Illustration lieat flo\v Typc of etch pit' direction of heat flow PI. 3, fig. 1 Parallel Uct~vcen type 1 and Ncnrlv perpendicular . - -
type 2
PI. 3, fig. 2 Parallel Betnreen typc 1 and Nc;u-ly pcrpcndicular
Ivnr -lr- 7
-
1'1. 4, fig. 1 lower half Pcrpendicular Typc 6 I'arallc! 1'1. 4, fig. I uppcr half I'erpendicular T Y P ~ 1 :\bout -ljO
PI. 4, fig. 2 lower half Pcrpendicular T Y P ~ 1 A b o u ~ -15'
PI. 4, fig. 2 uppcr half I'crpendicular Becwccn typc 4 and Ncarly p;~rallcl LYPe 6
A - rypc o l ctch pit refers to thc various Lypes sho\vn in F ~ g u r e 1.
room temperature of 15OF. T h e r;uldom fissures visible in the soil below the ice lens developed as a result of the quicli freeze.
Plate 1 clearly shows the concoidal shape of the soil fragments, the undulating or uneven soil-ice interface, and the discontinuous ice- lens formation. Some soil fragments have been rotated during the ice lensing.
Vertical sections (+1 mm thick) of the same specimen, placed between crossed polar- oids and viewed in transmitted light, show that the long axes of most ice grains are in the direction of the heat flow (PI. 1, fig. 2b). N o t all ice-grain boundaries, however, are exactly vertical. In many cases the grains extend the full distance across the ice lens and terminate in soil a t both ends.
Horizontal sections show the short-axes ice- grain pattern which is normal to the heat flow (PI.
2).
T h e grains range i n size and have ir-much the two arc relatcd is still ;L matter for conjecture.
The disorderly nature of the clay distribution may be related to nonlinear heat flow and un- even moisture supply a t local sites at the freezing front ~vllere the icc is formed. Uneven shrinkage a t the interface is allnost certainly involved in brcalcing away tlle clay fragments a t the lreezing front.
The rotation of the soil particles could be caused by the differcntinl rates of growth of two adjacent ice crystals with dillerent optical orientations since the rate of crystal growth depends o n its orientation (Hillig, 1958). T h e random orientation in the c-axes of ice crystals found in the ice lens studied may result from random nucleation. In ice produced from bulk water the orientation changed nit11 depth (Perey and Pounder, 1958). In these studies the ice lens was too thin t o permit study of the
TOP
6"
)L
BOTTOM
Figure 1. Normal photograph of specimen show~ng soil structure, ice lens, shape of freezing front, andoccluded clay. Note rotation of large soil fragment to left of center and near bottom of Ice lens.
Figure 2. Ice lens showing ice-grain boundaries in vertical section. (a) Norrnal photograph of specimen; (b) specimen photographed from same locat~on as in (a) but with transmitted l ~ g h t between crossed polaroids. Note grain boundaries are not exactly vertical and mostly extend full distance between soil particles.
VERTICAL SLICE O F S P E C I M E N A F T E R F R E E Z I N G
P E N N E R , P L A T E 1
PENNER, PLATE 2
HEAT
F L O W
Figure 1HEAT
FLOW
Figure 2E T C H PITS DEVELOPED O N VERTICAL FACE O F ICE LENS
I N T W O LOCATIONS (MAG. 180 X)
Shape of pits are between type 1 and 2 (Fig. 1 in text), i.e., the c-axis is nearly perpendicular todirection of heat flow; consequently, growth direction of crystals was normal to c-axes.
P E N N E R , PLATE 3
Figure 1. Note crystal boundary running diagonally from bottom right to top left. In lower crystal c-axis nearly parallel to heat flow, and is type 6 (Fig. 1 in text). Upper crystal is oriented at about 45" to lower crystal and is type 1.
Figure 2. Crystal in bottom half belonging to type I indicatingc-axes at about 45" to direction of heat flow. Crystal upper half is oriented between types 4 and 6 ; thus c-axes nearly parallel to heat flow.
E T C H PITS DEVELOPED O N HORIZONTAL FACE O F I C E L E N S Heat flow and long axes of grains normal to plane of paper
P E N N E R , PLATE 4
SHORT NOTES
1577
change of orientation w i t h d e p t h ; this should his assistance a n d suggestions. This paper is a be checked in f u t u r e studies. contribution from the Division o f Building Research, National Research C o ~ u ~ ~ c i l , Canada, Ac~notuledgn~ents a n d is published w i t h the approval o f the Di-
T h e a u t h o r is indebted t o L. W. Gold for rector of t h e Division.
References Cited
Czeratzki, W., and Frese, H., 1958, Importancc o l n.atcr in for~nation of soil structure: I-Iighnaay Re- search Bd. Special Rcpt. 40, p. 200-212
Eden, W. J., and Crawford, C. B., 1957, Geotechnical propcrtics of Leda clay in the Ottawa arca: 4th Conf. Intcrnat. Soc. Soil mechanics and Foundation Engineering Proc., v. I? Div. 1, 11. 22-27 Gold, L. W., 1960, The cracking activity ill ice during creep: Canad. Jour. Physics, v. 38, p. 1137-1148 Higuchi, K., 1955, The etching or ice crystals: Acts Metallurgica, v. 6, p. 636-642
Penner, E., 1960, The iinportance of frcczing rate on frost action in soils: Am. Soc. Testing Materials Proc., v. 60, p. 1151-1165
Perey, F. G. J., and Pounder, E. R., 1958, Crystal orientation in ice sheets: Canacl. Jour. Physics, v. 36,
