Physical processes during freeze-thaw cycles in clayey silts

Cold Regions Science and Technology, 16 ( 1989 ) 291-303 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

PHYSICAL PROCESSES DURING FREEZE-THAW

291

C Y C L E S IN C L A Y E Y SILTS

J . - M . Konrad Department of Earth Sciences, University of Water/oo, Water/oo, Ont. N2L 3G 1 (Canada)

(ReceivedJuly25, 1988; revisedand acceptedOctober31, 1988)

ABSTRACT Repeated freezing and thawing affect the structure of clayey silts over a wide range of overconsolidation ratios. While the overall void ratio of the thawed soil may either increase, as in lightly overconsolidated soils, or decrease, as in heavily overconsolidated samples, freezing and thawing caused an increase in effective void ratio in all cases. This, in turn, led to a reduction in segregation potential after each freeze-thaw event and to an increase of vertical hydraulic conductivity of the thawed soil. All the changes occurred during the first three cycles. It was also inferred that no structural changes occurred in the frozen fringe but rather somewhere in the colder zone at a temperature between - 0.40 and - O.57 ° C, dependent upon the overconsolidation ratio. A mechanistic model based on differential freezing in macro- and micro-pores of a clayey silt accounts for the experimental results.

INTRODUCTION It has been reported in the literature that freezing and thawing affects the structure of soils and thus their engineering properties. Several reports by agronomists and soil physicists are available in the literature on the effect which frost has on soil structure. Bayer (1956) found that freezing and thawing caused a granulating action on soil clods that is usually more effective than drying and wetting. Gardner (1945) studied the effect of freezing and thawing on permeability of soils. Ice

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crystallization had a dehydrating and densifying effect in semi-dispersed soils. It must be stressed, however, that the soils studied by these scientists consisted usually of unconsolidated slurries. Chamberlain and Gow ( 1979 ) conducted a comprehensive study on the effect of freezing and thawing on the permeability and structure of four finegrained soils. The soils were obtained from a slurry and tested in a normally consolidated state. Both permeability and structure were observed to be changed by freezing and thawing. In all cases freezethaw cycles caused a reduction in void ratio and an increase in vertical permeability. The increase in vertical permeability was attributed to the formation of polygonal shrinkage cracks for soils where clay particles dominated. For coarser-grained soils (clayey silts), no shrinkage cracks were observed and the authors suggest that the increase in permeability is caused by a reduction in the volume of clay aggregates in the pore spaces. This paper is concerned with the effects of freezing and thawing on the structure of a saturated clayey silt slurry, consolidated to various overconsolidation ratios (i.e. a wide range of initial porosities). The change in soil structure is substantiated by direct measurements of vertical hydraulic conductivity and from the analysis of laboratory onedimensional freeze-thaw tests. Changes in the segregation potential (or the heaving response of the samples) during successive runs can be used to infer the location of structural changes during freezing. These findings, in turn, serve as a basis for a mechanistic model describing the physical processes operative during repeated freezing and thawing of saturated clayey silts.

© 1989 Elsevier Science Publishers B.V.

292

MATERIAL A N D TEST PROCEDURES The soil type used in the freezing experiments was a frost-susceptible clayey silt obtained from a test site in Calgary, Alberta, selected to study the behaviour of full-scale chilled pipelines buried in unfrozen, saturated soil (Slusarchuck et al., 1978 ). The properties of Calgary silt (series CS) are as follows: liquid limit=28%; plastic limit=15%; specific gravity=2.78; passing # 2 0 0 = 9 5 % ; and % clay size = 26%. The samples were consolidated from a slurry at an initial moisture content of about 45% to various void ratios. After primary consolidation was complete, the samples were allowed to rebound under a vertical pressure of 50 kPa, which produced overconsolidated samples of various overconsolidation ratios (OCR). Thermal equilibrium was attained before freezing. The freezing equipment and instrumentation used in these experiments were fully described by Penner ( 1986 ). Freezing conditions were similar in all tests and consisted of ramping the cold and the warm plate temperatures by - 0 . 6 9 °C day-1 under a relatively constant temperature gradient of 0 . 3 4 + 0 . 0 2 ° C cm -~ (Konrad, 1989a). These conditions resulted in a fairly constant rate of cooling of the frozen fringe of about 0.43°C day -~. The samples were frozen from the bottom up with free access to water at the top. At the end of some freezing runs, a portable 200 kV X-ray generator was used to obtain X-ray photographs showing the ice structure within the samples. These images, together with a temperature profile recorded at the same time, yield a close estimate of temperature at the growing face of the warmest ice lens. At the end of the last freeze-thaw consolidation cycle, hydraulic conductivity tests were conducted in a triaxial cell on both the unfrozen and the thawed-consolidated soil for each sample.

TESTING PROGRAM The testing program was designed to investigate the effects of alternate freezing and thawing on the characteristics of a clay.ey silt at four different o v -

erconsolidation ratios (OCR) of 1 (i.e. normally consolidated), 2, 4, and 8. A complete test on each sample consisted of three to four cycles of freezing, thawing and reconsolidation at 50 kPa. In the first freezing run the samples were frozen to about half their initial height while in the second freezing run the frost front penetrated to the same depth or a different one in order to create zones of various freezethaw histories in the same sample. In the final run the frost front was allowed to penetrate into unfrozen soil that was never subjected to previous freezing. Detailed description of the laboratory program and of frost-heave data is given in Konrad (1989b). It was also demonstrated that the experimental procedure minimizes void-ratio changes in the unfrozen soil during the first freeze-thaw cycle. For normally consolidated clayey silt, the void ratio decreased slightly while for overconsolidated samples it increased somewhat. Because the swelling and the recompression curves are fairly flat these changes were extremely small, and are regarded as negligible in the latter case.

TEST RESULTS The segregation potential (SP) during transient freezing of any saturated soil is readily obtained from a laboratory test in which the water-intake rate or frost-heave rate and the temperature profile are determined with time (Konrad, 1987b). SP is defined as the ratio of water-intake rate and temperature gradient in the frozen soil near the frost front. As pointed out by Konrad and Morgenstern (1980), the water-intake rate is directly proportional to the temperature gradient near the frost front, all other factors such as the applied load, the suction at the frozen-unfrozen interface, and the rate of cooling being the same. In the present case, where all tests were conducted at the same rate of cooling and a constant temperature gradient, segregation potential reflects directly segregational heave rate. In order to facilitate the interpretation and the discussion of the results, the following presentation was adopted for each value of OCR: ( 1 ) change in SP with time for each freezing run; (2) selected temperature profiles; and (3) X-ray photographs il-

293

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the warmest ice lens determined from the X-ray pictures. In all cases, freezing and thawing caused a reduction in segregation potential. All tests also confirm that the characteristics of the unfrozen soil (i.e., soil that was never subjected to any freezing) was not significantly affected by the successive runs. This is best illustrated by the results from test CS-1 shown in Fig. 4. At the end of run 1 the thickness of unfrozen soil was 6 cm and the value of SP corresponded to point A. The second freezing run showed that when the frozen fringe reached soil unaffected by the previous run, point C, the segregation potential was consistent with that obtained in run 1. Furthermore, the rate of change between points C and D is also consistent with that of run 1. Run 3 was designed to produce a frost penetration within the soil that underwent two freeze-thaw cycles. Run 4 showed again that there is a change in SP each time the current frozen fringe reaches soil unaffected by previous freezing. For instance, at point F the soil was not affected by run 3 but by run 2, and both SP and its rate of change agree with the data obtained from run 2. At point H, the current frozen fringe reaches soil unaffected by run 2 and the SP displays values that are again consistent with those obtained

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lustrating the changes in ice structure after a given number of freeze-thaw cycles. Thus, the results for tests CS-4, CS-1, CS-2, and CS-3 are summarized in Figs. 1-3, 4-6, 7-9 and 10-1 l, respectively. The temperature profile plots indicate the frost penetration for each run as well as the position of 24

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in run 1 and 2 when the freezing occurred in unfrozen soil. The SP plots indicate also the time corresponding to a change in observed ice-lens structure. The position of the boundary between two distinct ice-lens structures was also shown on the temperature profiles by an arrow labelled S. In all cases, the change in ice-lens structure occurred when a portion of the current frozen fringe began to enter soil unaffected by previous freezing. More details will be provided in the discussion section.

DISCUSSION Hydraulic conductivity of thawed soil The previous findings can be used to discuss structural changes in clayey silt after freezing and thawing and their influence on the hydraulic conductivity of the thawed reconsolidated soil. In a recent paper by Konrad (1989a) presenting the analysis of the results of phase 1 of this research, i.e. the influence of OCR on the freezing characteristics of the same clayey silt, it was established that SP in-

creased with decreasing void ratio, all other factors being constant. It was also suggested that lower initial void ratios yield higher unfrozen water contents, thus affecting the soil's frost susceptibility. Since freeze-thaw cycles reduce SP, all other factors being constant (Konrad, 1989b), it is postulated that the effective void ratio of the thawed, reconsolidated soil increased, even though the overall void ratio decreased slightly in tests CS-4 and CS- 1. The vertical hydraulic conductivity of the thawed soil in all samples increased significantly after the freeze-thaw cycles (Fig. 12 ), confirming thereby the increase in effective void ratio in the soil affected by freeze-thaw. The magnitudes of the increases in hydraulic conductivity are illustrated in Fig. 13. It appears that there is a tendency for larger changes in hydraulic conductivity with increasing OCR, i.e. decreasing void ratio of the initial unfrozen soil. For instance, the ratio of thawed to unfrozen hydraulic conductivity was 2.4 with a possible range between 2 and 5, while it was 9 with possible limits between 4 and 10 for the normally consolidated case and the most overconsolidated sample, respectively. The X-ray photographs can be used to detect any ice structure that is not a horizontal, segregated ice lens. There was no evidence of vertical crack fea-

299

silt used herein. Furthermore, for void ratios comparable to the samples used in this study, the above researchers found increases in hydraulic conductivity by factors of 1.5-8.

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Figures 7 and 10 reveal that, for a period of approximately 1000-1500 min in tests CS-2 and CS3 there is a marked difference in segregation potential between any subsequent freezing and that of the first freezing run. In the other two tests, this feature is not as marked. The author is led to believe that this behaviour is associated with the change in void ratio in the thawed soil and the lower hydraulic conductivity of the unfrozen, highly overconsolidated soils. Table 1 presents the calculated suction at the frost front after 500 and 1000 min of freezing for the last run in all tests. The suction at the frost front was obtained from Darcy's Law applied to a two-layered system and is given by:

Pu = v(lu/Ku + lt/Kt) I0-

where: v is the observed water intake rate; lu and It are the thickness of unfrozen and thawed soils; and Ku and K, represent hydraulic conductivities. The back-calculated suction after 500 rain is respectively - 46 and - 49 kPa in tests CS-2 and CS3. Since the hydraulic conductivity of the thawed

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tures in any of the frozen soil samples. This observation is consistent with data reported by Chamberlain and Gow (1979) showing that shrinkage cracks did not occur at all in Hanover silt and in CRREL clay, which are soils similar to the clayey

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300 soil was approximately 10 - 6 m m s-1, which corresponds to that of the unfrozen soil in test CS-4, it is reasonable to expect that the suction at the frost front was sufficiently large to cause cavitation in the thawed soil. Results of the first phase of this study reported in Konrad (1989a) suggest indeed that cavitation is likely to occur in sample CS-4 for a suction around - 6 0 to - 8 0 kPa. It is noted that cavitation produces also a change in degree of saturation, hence in hydraulic conductivity. The actual suction at the frost front during the first 500 rain of freezing is thus larger than the calculated one, and is likely to induce cavitation. After 1500 min, steady-state flow is reached and the backcalculated suctions at the frost front are representative of the actual situation. The steady increase in SP thereafter indicates no further change in degree of saturation in the vicinity of the frost front.

Location of structural changes The increased hydraulic conductivity in the thawed state of the clayey silt samples suggests that structural changes are occurring during freezing. Forces available to affect the soil structure may arise from the slightly less than nine-percent phase expansion of water contained in the pores, depending on unfrozen water content effects. Some of this expansion pressure is relieved by heaving of the soil in the direction of least resistance. Since the samples are confined laterally, it may be plausible that expansion pressures act more in the horizontal plane (for a vertical sample) causing particles to be rearranged. It is thus also logical to expect the structural changes to occur at the frost front where pore ice forms. A close examination of Fig. 4, however, indicates that the structural changes after the first freezing run as recorded by a drastic increase in SP occurred before point C. According to Fig. 5, this suggests that there were no structural changes in the frozen fringe during run 1 since the location of the frozen fringe at point C of run 2 was the same as that at the end of run 1. Similar conclusions can be drawn from the study of the changes in SP during the fourth freezing run. Since the frozen fringe was pushed deeper into the unfrozen soil during run 2, but did not penetrate to the same depth in run 3, point F of run 4

should reflect only the effect of one freezing, which is verified since it is located on the SP curve corresponding to run 2. At point H, the fringe reached the same location as at the end of run 2 and the SP is back on the curve obtained during the first freezing run. The results of test CS-3 shown in Figs. 10 and 11 give additional support to the fact that structural changes occur in the frozen soil at a temperature lower than that of the warmest ice lens. After three repetitive freeze-thaw cycles, where the frost front was not allowed to penetrate beyond its position after the first freezing, one should expect to see the change in SP in run 4 when the frozen fringe reaches the position it had during run 1. This is the case as illustrated by the temperature profile at point F on Fig. 11. Similar observations can be made for tests CS-4 and CS-2, although the changes in SP are not as abrupt as for tests CS- 1 and CS-3. This rather surprising finding warrants further explanation. The author believes that because there is water flow to the active ice lens, the excess water resulting from phase change flows to the ice lens as ice fills up the available pore space in the frozen fringe. This process occurs continuously as the frost front advances into unfrozen soil. The pore geometry is not affected since the volume of ice and that of the associated unfrozen water for given conditions of temperature and pressure is equal to the pore volume. No change in the soil structure should therefore occur in the frozen fringe. As discussed earlier, if the suction at the frozen-unfrozen interface is less than the overconsolidation pressure, little or no additional consolidation of the unfrozen soil near the frost front is to be expected. The fact that the results suggest that no structural changes occur in the frozen fringe of a clayey silt is not only surprising at first sight, but also has other implications in freezing soils similar to the soil tested herein. For instance, if the pore ice in the fringe does not occupy an increased volume, what can be said about models which assume that one component of total heave is the nine-percent expansion of the pore water? Since it seems reasonable to assume that the excess water flows to the active ice lens where it is freezing and thereby expanding by nine percent, nothing is actually lost and the predictive models still track the overall picture, although they are unable to account for the above described mechanisms.

301

More importantly, no change in the soil structure in the frozen fringe also means that there is no overconsolidation in the frozen fringe despite large negative pore water pressures generated in the unfrozen water. What this finding means in terms of pressure in the pore ice and effective stress at grain to grain contacts is not clear at present and needs further study. Two further questions come to mind: what causes the change in structure of the thawed soil and where do these changes occur? So far, we have inferred that they must occur at a temperature lower than that of the warmest ice lens. In the frozen zone behind the fringe there is negligible or no water migration owing to extremely low hydraulic conductivities, despite the existence of a thermal gradient and associated suction gradient. When a fine-grained soil is frozen, not all the water within the pores freezes at 0 ° C. The water remains unfrozen mainly as a result of capillary and surface adsorption effects (Anderson and Morgenstern, 1973). The quantity of the unfrozen interracial water depends principally on the temperature (Tice et al., 1978; Williams, 1964). For clayey silts the unfrozen water content changes rapidly from 100 to about 30% of the initial water content when the temperature is lowered from 0 to approximately - I ° C . For temperature below - 1 ° C, the rate of reduction in unfrozen water content is much smaller. The unfrozen water content in the fringe is thus larger than that of the frozen soil immediately behind the warmest ice lens. Additional freezing occurs and since the water flow behind the cold side of the active ice lens is strongly reduced in salt-free saturated soils, ice pressure will be generated within the pores. This pressure may be relieved by heaving, thus distorting the soil skeleton in a vertical direction but it may also affect the soil structure in horizontal planes. Figures 2, 5 and 8 indicate by an arrow labelled S the position of a visible change in ice structure, suggesting a change in the soil properties. These changes occurred when the temperatures in the frozen soil were respectively - 0 . 4 0 , - 0 . 5 7 and - 0 . 5 7 ° C in samples CS-4, CS-1 and CS-2. It is of value to compare these temperatures to the temperatures at the base of the corresponding warmest ice lens which were - 0.27, - 0.40 and - 0.43 ° C, respectively. In

most clayey silts, this temperature drop may cause about 10-30% of the unfrozen water existing on the warm side of the ice lens to freeze and to expand by nine percent, creating some 3-dimensional structural changes. This mechanism is consistent with the previous observation that the magnitude of the change in hydraulic conductivity of thawed soil appears to increase with increasing OCR. As established by Konrad (1989a), the unfrozen water content increases with increasing OCR, and the additional volumetric expansion on the cold side of the actively growing ice lens will be larger with increasing values of OCR. The soil structure will thus undergo larger changes in heavily overconsolidated soils.

A MECHANISTIC MODEL OF FREEZET H A W ACTION IN CLAYEY SILTS This section summarizes the main findings of this study and proposes physical processes that are thought to be operative during freezing and thawing of saturated clayey silts. It must be emphasized that the main characteristics of these soils are: ( 1 ) no formation of shrinkage cracks during freezing; (2) development of ice lenses in order to ensure water flow through a frozen fringe (sandier soils may not always fall in the above category); and (3) relatively low compressibility of the unfrozen soil. Figure 14a shows a schematic diagram of idealized types of particles and their arrangements for a clayey silt. In general, the coarser silt grains control the packing and the macro-pore space, while the clay-size silt and clay minerals control the micropore distribution within the soil skeleton. When water freezes at the frost front (i.e. 0 ° C for pure water), ice fills an increasing portion of the macro-pores as temperature decreases in the frozen fringe without, however, disturbing the existing soil structure. As shown on Fig. 14b the unfrozen water content in the frozen fringe is then composed of capillary free water in the wedges between particles and of water in the micro-pores that ice cannot penetrate if the pore size is smaller than a critical value depending on temperature and pressure conditions at a given location. The unfrozen water in the micro-pores consists of free water and adsorbed water

302 NO ICE IN MICROPORES

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films around clay minerals as depicted in Fig. 15a. Continuous frost penetration results in continuous temperature decrease in the frozen fringe, eventually leading to the initiation of a new ice lens. If the frost front continues to advance, the temperatures of the frozen soil between the current and the former ice lens drop as well as additional ice is formed in its pores (Fig. 14c). As illustrated in Fig. 15b, the existing ice in the macropores penetrates deeper into the pore necks, thus reducing the capillary free water. Furthermore, temperature and pressure conditions allow ice to penetrate into the micro-pores which causes eventually a reduction of the thickness of adsorbed films around clay minerals. Because the freezing occurs in a relatively confined manner, at least in the horizontal plane, volume change due to the nine-percent expansion during phase transformation of this additional amount of

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unfrozen water causes a structural rearrangement in both the macro- and micro-pores. After thawing, the smallest particles do not move back to their initial positions, thus leading to permanent changes in the pore-size distribution characteristics. In these experiments the reduction in normalized

303 SP with repeated freezing and thawing indicated that three to four cycles were sufficient for all the change in void ratio or hydraulic conductivity to occur ( K o n r a d , 1989b). T h e same was observed by Chamberlain and Gow, 1979. This relationship is consistent with the above-described processes. As shown on Fig. 16, the unfrozen water content o f a given soil depends on its void ratio. The relationship between unfrozen water and temperature moves closer towards the axes as the void ratio increases. The rate o f water-content reduction near 0 ° C also decreases with increasing void ratio. Thus, it should be expected that less additional pore ice will form between ice lenses as the void ratio increases after each freezing. After a sufficient number o f freezing episodes the v o i d ratio is such that no further m a j o r structural changes will occur in the pores controlling the freezing mechanism.

CONCLUSIONS Freezing and thawing cause significant changes in the structure o f a saturated clayey silt consolidated to various overconsolidation ratios. Direct measurements o f the hydraulic conductivity o f the thawed soil and interpretation o f frost-heave data f r o m one-dimensional freezing tests suggest an increase of effective v o i d ratio after each f r e e z e - t h a w event. The frost-heave results indicated that three to four cycles are sufficient to p r o d u c e all the structural change. In all cases, the hydraulic conductivity o f the thawed soil increased by factors o f 2-10. The increases appeared to be larger at the highest overconsolidation ratios. This research d e m o n s t r a t e d that in clayey silts where the coarser-grained particles control the packing, structural changes are negligible since the suction at the frost front did not exceed the maxim u m past consolidation stress. N o n e o f the samples showed vertical shrinkage cracks. The interpretation o f the test results also led to the finding that the structural changes occur in the frozen part o f clayey silts at temperatures below that o f the warmest ice lens. These temperatures were dependent on the values o f O C R and were respectively - 0.40 and - 0.57 ° C for O C R equal to 1 and 4.

These results also suggest that there are no structural changes in the frozen fringe. A freeze-thaw m e c h a n i s m based on differential freezing in macro-pores and micro-pores o f clayey silts explains the observed changes as well as their magnitudes.

ACKNOWLEDGEMENTS The author gratefully acknowledges the conscientious efforts o f Mr. D. Eldred, technical officer o f the Institute o f Research in Construction, in conducting the freezing experiments. The soil was provided by Dr. Nixon, Hardy-BBT, Calgary.

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