Pressures Recorded During Laboratory Freezing and Thawing of a Natural Silt-Rich Soil

PRESSURES RECORDED DURING LABORATORY FREEZING AND THAWING OF A NATURAL SILT-RICH SOIL

Charles Harris1, Michael C.R. Davies2

1. Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF1 3YE UK e-mail: [email protected]

2. Department of Civil Engineering, University of Dundee, Dundee DH1 4HN, UK

Abstract

Porewater pressures in a natural silty soil were measured during seven cycles of downward soil freezing and thawing using Druck electronic pore pressure transducers placed at 50,150 and 250 mm below the surface. Surface frost heave/thaw settlement was monitored using LVDTs, and soil temperatures recorded using semi- conductor temperature sensors. A pronounced Òzero curtainÓ was observed during both soil freezing and thaw- ing, and pore pressure change followed a consistent pattern through each freeze/thaw cycle. Arrival of the freezing front led to a gradual fall in pressure to between -5 kPa and -15kPa. Just before the end of the Òzero curtainÓ period pressures rose rapidly to between 15 kPa and 40 kPa, with higher pressures at greater depths. These high pressures were maintained as the soil cooled but fell when soil warming began. Warming ahead of the thaw front progressed rapidly through the frozen soil, leading to an accelerating fall in pressure. With the arrival of the thaw Òzero curtainÓ, pore pressures became strongly negative, then rose, and became positive again when soil thawing adjacent to the transducer was complete. Freezing processes responsible for these observed pressure changes are discussed in the context of the mechanisms of soil phase change and associated frost heaving and thaw consolidation.

Introduction freezing temperature (T). The matric potential is equi- valent to porewater suction, and accounts for water Since the early work of Taber (1929) and Beskow migration towards the freezing front to nurture the (1935), a considerable body of theory has developed to growth of segregation ice. Thus, in a freezing soil, ice explain frost heaving of fine-grained soils. pressure, water pressure and freezing temperature are Underpinning thermodynamic treatments of the pro- mutually dependent, but the ice pressure must exceed blem of ice segregation is the Clausius-Clapeyron equa- the stresses resisting soil expansion (overburden pres- tion (Edlefson and Anderson, 1943; Williams, 1988; sure plus internal strength (cohesion) of the frozen soil) Smith and Onysko, 1990), which considers the differ- if frost heaving is to occur. ence in pressure between ice and unfrozen water in a freezing or thawing soil, and may be written as: In this paper, we present measurements of both soil water tension (negative pressures), and ice pressure developed during the freezing and thawing of a natural (Pi - Pw) = -(T-To)LfVi/VwTo [1] silty soil. The experimental procedure was primarily designed to investigate solifluction processes on a where (P - P ) is the difference in pore ice pressure i w thawing laboratory model slope (see Harris et al., and pore water pressure (the matric potential), T is the 1996a, b). However, the model was monitored through temperature (K), To is the freezing point of pure water, the freezing stage, and the pressure records from three Lf is the latent heat of fusion and Vi and Vw are the spe- complete freeze-thaw cycles are presented here, since cific volumes of ice and water respectively. Williams they offer further evidence concerning the status of (1988) and Smith and Onysko (1990) show that in order both ice and water in freezing and thawing soils. for frost heaving to occur, pressure within the growing ice lenses must not only exceed the overburden pres- Experimental design sure (relatively small in natural near-surface soils), but also the tensile strength of the freezing soil, since the Experimental design and instrumentation are soil matrix must expand to accommodate ice lens described in detail by Harris et al. (1996b), and only a growth. Changes in ice pressure must be balanced by brief description will be given here. The experimental changes in water pressure and/or by changes in the slope, of gradient 12¡, was constructed within a 5 m

Charles Harris, Michael C.R. Davies 433 Figure 1. Experimental design. TM thermistors, T semiconductor temperature sensors; PWP pore pressure transducers. square by 1.5 m high refrigerated container. Two natu- with a ceramic filter tip. Transducers were filled with ral soils formed adjacent strips 2.0 m wide, 5 m long antifreeze and de-aired in a vacuum desiccator prior to and 0.3 m thick over a basal sand drainage layer. One installation. Variations in soil water pressures were soil, from a fresh quarry face at Vire, in Normandy transmitted via the transducer fluid to an electrical (France), consisted of a sandy silt (3% clay, 39% silt, 42% pressure element behind the ceramic tip. Each trans- sand, 16% gravel) derived from Precambrian slate, and ducer measured to a maximum of 350 kPa with com- data from this test soil are presented here. Particle size bined non-linearity and hysteresis of ±0.2 % best distribution indicated D10 of 0.007 mm, D50 0.1 mm straight line, and thermal sensitivity of ±0.2% of rea- ding per ¡C. Due to an intermittent fault in the near- and D60/D10 (uniformity coefficient) of 28.57. Thus the surface transducer, only data from the 150 mm and 250 soil has a larger grain-size range than many soils used mm depths are presented below. Soil surface move- in laboratory soil freezing tests. The second test soil ments (frost heave and downslope displacements) were comprised a gravelly silty sand, but pore pressure monitored using a pair of LVDTs, mounted on slotted transducers in this soil worked intermittently, and data steel tracks supported by a horizontal beam above the will not be presented. slope surface (Figure 1). The LVDTs formed a fixed-base triangle, with the apex connected to a perspex footplate The soil was initially allowed to wet up slowly as equipped with four 20mm deep anchor points. The water was introduced via the basal sand drainage layer, footplate was embedded in the surface of each experi- and an open hydraulic system was maintained through mental soil, and progressive surface displacements due all freezing phases. Some variation in inlet water pres- to frost heave and solifluction were detected by changes sure was recorded by the Druck transducers early in in the geometry of the LVDT triangles with an accuracy each freezing phase. Air temperatures above the test of ± 1.5 mm. All instrumentation was scanned at half- slope were lowered to -10¡C until the experimental soils hourly intervals using a PC-based logging system. were frozen to their base. The slope was then allowed Thus, readings of soil temperature, porewater pressure to thaw from the surface downwards under ambient and soil surface displacement were all recorded on a laboratory temperatures which ranged from +5¡C in common time-base, allowing direct comparisons to be winter to +15¡C in summer. Seven cycles of freezing made. and thawing were completed, lasting between 30 and 60 days and results from cycles 2, 3 and 5 are presented below. Results

Semiconductor temperature sensors with accuracy Variations in pore pressure transducer readings ±0.1¡C were installed adjacent to porewater pressure through each cycle of freezing and thawing showed a sensors at depths of 50, 150 and 250 mm (Figure 1). consistent trend (Figure 2), with an initial fall in pore Porewater pressures were measured using Druck pressure being recorded during the so-called Òzero cur- miniature pore pressure transducers, each comprising a tainÓ period and immediately following it. As tempera- 6.4 mm diameter, 11.4 mm long, stainless steel cylinder tures fell, following the main period of phase change,

434 The 7th International Permafrost Conference Figure 2. Temperature and pressure recorded during soil freezing and thawing: 150 mm, cycle 2; (b) 250 mm, cycle 2; ( c) 150 mm cycle 3; (d) 250 mm cycle 3; (e) 150 mm cycle 5; (f) 250 mm cycle 5. pressures rose rapidly. The rise in pressure then slowed, rose once again towards the end of the Òzero curtain but pressure generally continued to rise until the begin- periodÓ, to become positive during consoli-dation of ning of the soil warming phase, when a rapid fall the supersaturated thawed soil. The main period of occurred. As rising soil temperatures approached the latent heat flux (the Òzero curtain) occurred at tempera- Òzero curtain rangeÓ, pore pressure transducer readings tures of between -0.1¡C and -0.25¡C. Soil freezing and continued to fall, and became strongly negative, but thawing was accompanied by frost heaving and thaw

Charles Harris, Michael C.R. Davies 435 Maximum and minimum pressures are summarised in Table 1. Maximum pressure recorded within the frozen soil was consistently higher at the Ò250 mmÓ transducer the Ò150 mmÓ transducer. This difference is not simply due to overburden pressure, since for the Ò150 mmÓ transducer this was only around 3 kPa and for the Ò250 mmÓ transducer around 5 kPa when the soil was frozen. The minimum pressures during the thaw phase were generally lower at Ò150 mmÓ than Ò250 mmÓ, though similar values were recorded in Cycle 5.

Significance of pressure readings

The significance of these readings requires careful consideration, since the sensors differ fundamentally from those used in earlier experiments to record hea- ving pressures in soils. Previous studies have utilised Figure 3. Frost heave recorded during cycles 2, 3 and 5. total load cells (Williams and Wood, 1985; Smith and settlement at the surface (Figure 3), so that the depth Onysoko, 1990), which are not in hydraulic continuity labels Ò150 mmÓ and Ò250 mmÓ applied to the pressure with the soil water. Pressure increases during soil freez- and temperature sensors are correct only for the thawed ing recorded in earlier experiments are interpreted as consolidated soil condition. It is estimated from the resulting from the growth of segregation ice, and trans- frost heave data and soil temperatures, that the frozen mission of ice pressures generated during frost heave to soil thickness above the Ò150 mmÓ sensors was 208 mm, the load cells. 195 mm and 200 mm and above the Ò250 mmÓ sensors, 315 mm, 312 mm and 320 mm for In the present case it is argued that the rapid transi- cycles 2, 3 and 5 respectively. tion from negative to positive readings during freezing is in response to the sealing of the pressure transducer Further detail of the pressure-temperature relation- within an effectively closed frozen soil system. The ships is apparent from Figure 4 where temperature is transducer became isolated from pore fluid films and plotted against recorded pressures. During initial free- dominated by the positive ice pressures which develop zing, the transition from negative to positive pressures during heave. Since there is considerable uncertainty as occurred at soil temperatures ranging from -1¡C to to the mechanism of pressure transfer from the heaving -1.7¡C at the 150 mm depth and between - 0.25¡C and soil to the transducer, it cannot be assumed that pres- -0.5¡C at the 250 mm depth, that is, after the passage of sure readings necessarily accurately reflect ice pres- the freezing front. The reverse transition from positive sures. Within small-scale laboratory samples, Williams to negative pressures recorded during soil thawing and Wood (1985) recorded pressures of between 100 occurred between -0.5¡C and -0.8¡C for the 150 mm and 250 kPa at temperatures of -0.5¡C, while in slightly transducer and at around -0.25¡C for the 250 mm trans- larger scale laboratory tests using a soil column 230 mm ducer, that is at the onset of the Òzero curtainÓ period. in diameter, Smith and Onysko (1990) recorded pres- sures of up to 260 kPa at temperatures between -1¡C Table 1. Maximum and minimum pressures recorded during freeze-thaw cycles 2, 3, and 5

436 The 7th International Permafrost Conference Figure 4. Plots of soil temperature against pressure: 150 mm, cycle 2; (b) 250 mm, cycle 2; ( c) 150 mm cycle 3; (d) 250 mm cycle 3; (e) 150 mm cycle 5; (f) 250 mm cycle 5.

Charles Harris, Michael C.R. Davies 437 and -2¡C. In both these examples, however, edge effects fringe to the growing ice lenses on its upper boundary may have increased the resistance of the frozen soil to in response to a steep hydraulic gradient across it. As heave. In a larger ÒfieldÓ scale pipeline experiment, predicted by the Clausius-Clapeyron equation, a corre- where such edge effects would have been minimal, sponding increase in ice pressure occurs across the heave pressures at a depth of 800 mm, beneath a buried fringe, towards the level of ice lens growth (Miller, pipeline, were around 100 kPa at temperatures a few 1978). According to Williams and Smith (1989) this degrees below zero. Thus, the frozen pressures record- cold-side boundary where ice segregation commences ed here are of a similar order of magnitude, if slightly is typically at a temperature of -0.1¡C to -0.2¡C. Due to a lower, than those reported elsewhere. Much higher progressive increase in the ratio of ice to unfrozen water pressures may be developed, however, when measure- as the temperature of the fringe falls, permeability ments are made against a reaction frame that effectively decreases towards the cold-side boundary, and falls prevents heaving (e.g., Sutherland and Gaskin, 1973). sharply within the frozen soil above, where, in the con- text of the experimental time scale, water is no-longer Turning to the thaw stage, it is envisaged that soil mobile (e.g., F¿rland et al., 1988). warming led to a progressive increase in thickness of unfrozen water films, and a reduction in ice pressures The experimental data presented in this paper may be as the phase change temperature (T in equation (1)) interpreted in the context of the above model. It is pro- rose. During the Òzero curtainÓ period, continued phase posed that the development of negative pore pressure change from ice to water led to re-establishment of readings at each transducer during soil freezing marked hydraulic continuity between the unfrozen water and the arrival of the frozen fringe, and the transition to the transducer tip, and recorded pressures became positive readings occurred when the transducer became strongly negative, reflecting suction within the isolated from the mobile unfrozen water of the fringe unfrozen water in the partially frozen soil. As ice lenses and fully encapsulated in the frozen soil behind the decreased in size and water films thickened towards fringe. The transducers were then responding to hea- the end of the Òzero curtainÓ period, soil water tension ving pressures generated to overcome the tensile fell, and when hydrostatic conditions, coupled with strength of the frozen soil during frost heaving transfer of stress from the soil grains to the pore fluid (Williams, 1988; Smith and Onysko, 1990). During thaw occurred during thaw consolidation, pore pressures it is assumed that phase change begins initially in the became positive once again. finer pores, and progressed into larger pores as the soil temperature approached the freezing point of the soil Discussion ice. At a certain stage, hydraulic continuity between the transducers and the mobile unfrozen soil water was re- Data presented here provides direct evidence for the established, and a rapid response occurred, with the development of positive ice pressures and high water transducers recording suction within the unfrozen suction values within frozen soils. The development of water films. In an ice-bonded soil matrix, the reduction ice pressures in this experiment occurred rapidly, fol- in volume during phase change from ice to water lowing the main period of phase change (Òzero cur- would be expected to result in suction within the melt tainÓ). This may reflect the functioning of the hydrauli- water. As ice bonding of the soil matrix was broken ice cally open pressure transducers, since pressure could pressures fell, water pressures rose, and as overburden not be registered until they became encapsulated in the stress was transferred from soil/ice contacts to the soil frozen soil. However, the large increase in tensile water during thaw consolidation, porewater pressures strength of the soil between unfrozen and frozen states in excess of hydrostatic were induced. It was at this would not have become established until an ice-bonded stage that downslope displacement by slow viscous matrix had developed, and ice pressures in excess of the flow was observed on the experimental slope. overburden pressure were therefore unlikely until this stage. Although the experiment discussed here was designed principally to monitor the behaviour of soil It is widely accepted that ice segregation does not immediately following thaw, it provides a useful generally take place at the freezing front (0¡C isotherm), insight into the physical characteristics of fine-grained but at some distance behind it, at a lower temperature soils through a complete cycle of freezing and thawing. (Harlan, 1973; Loch and Kay, 1978; Miller, 1978, 1980; Since the transducers were in hydraulic continuity with Konrad and Morgenstern, 1980, 1981, 1982). The the soil, the ethylene glycol antifreeze may have influ- Òfrozen fringeÓ, lying between the level of ice segrega- enced the freezing behaviour of the soil immediately tion and the 0¡C isotherm, contains ice and unfrozen, adjacent to the transducer tips, but in subsequent tests mobile, water. As the freezing front advances down- using silicon oil rather than antifreeze in the transduc- wards through a soil, water enters the frozen fringe ers, pressure variations followed the same pattern as is from the unfrozen soil below and moves through the described above. It is considered, therefore, that these

438 The 7th International Permafrost Conference observations correctly model porewater suction within the partially frozen fringe, heaving pressures within the ice-rich frozen soil, and highlight the complex changes that occur during thaw.

Acknowledgments

This research was supported by the British Natural Environment Research Council (Grant GR9/1089), the British Council, CNRS France, and the University of Wales, Cardiff. The author acknowledges the contribu- tion of J.-P. Coutard to this work.

References

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