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 Harris 1, Michael C.R. Davies 2 1. Department of Earth Sciences, Cardiff University, P.O. Box 914, Cardiff CF1 3YE UK e-mail: HarrisC@cardiff.ac.uk 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 semiconductor temperature sensors. A pronounced Òzero curtainó was observed during both soil freezing and thawing, 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 Since the early work of Taber (1929) and Beskow (1935), a considerable body of theory has developed to explain frost heaving of fine-grained soils. Underpinning thermodynamic treatments of the problem of ice segregation is the Clausius-Clapeyron equation (Edlefson and Anderson, 1943; Williams, 1988; Smith and Onysko, 1990), which considers the difference in pressure between ice and unfrozen water in a freezing or thawing soil, and may be written as: (P i - P w ) = -(T-T o )L f V i /V w T o [1] where (P i - P w ) is the difference in pore ice pressure and pore water pressure (the matric potential), T is the temperature (K), T o is the freezing point of pure water, L f is the latent heat of fusion and V i and V w are the specific volumes of ice and water respectively. Williams (1988) and Smith and Onysko (1990) show that in order for frost heaving to occur, pressure within the growing ice lenses must not only exceed the overburden pressure (relatively small in natural near-surface soils), but also the tensile strength of the freezing soil, since the soil matrix must expand to accommodate ice lens growth. Changes in ice pressure must be balanced by changes in water pressure and/or by changes in the freezing temperature (T). The matric potential is equivalent to porewater suction, and accounts for water migration towards the freezing front to nurture the growth of segregation ice. Thus, in a freezing soil, ice pressure, water pressure and freezing temperature are mutually dependent, but the ice pressure must exceed the stresses resisting soil expansion (overburden pressure plus internal strength (cohesion) of the frozen soil) if frost heaving is to occur. 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 silty soil. The experimental procedure was primarily designed to investigate solifluction processes on a thawing laboratory model slope (see Harris et al., 1996a, b). However, the model was monitored through the freezing stage, and the pressure records from three complete freeze-thaw cycles are presented here, since they offer further evidence concerning the status of both ice and water in freezing and thawing soils. Experimental design Experimental design and instrumentation are described in detail by Harris et al. (1996b), and only a brief description will be given here. The experimental 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 natural soils formed adjacent strips 2.0 m wide, 5 m long and 0.3 m thick over a basal sand drainage layer. One soil, from a fresh quarry face at Vire, in Normandy (France), consisted of a sandy silt (3% clay, 39% silt, 42% sand, 16% gravel) derived from Precambrian slate, and data from this test soil are presented here. Particle size distribution indicated D 10 of 0.007 mm, D 50 0.1 mm and D 60 /D 10 (uniformity coefficient) of 28.57. Thus the soil has a larger grain-size range than many soils used in laboratory soil freezing tests. The second test soil comprised a gravelly silty sand, but pore pressure transducers in this soil worked intermittently, and data will not be presented. The soil was initially allowed to wet up slowly as water was introduced via the basal sand drainage layer, and an open hydraulic system was maintained through all freezing phases. Some variation in inlet water pressure was recorded by the Druck transducers early in each freezing phase. Air temperatures above the test slope were lowered to -10 C until the experimental soils were frozen to their base. The slope was then allowed to thaw from the surface downwards under ambient laboratory temperatures which ranged from +5 C in winter to +15 C in summer. Seven cycles of freezing and thawing were completed, lasting between 30 and 60 days and results from cycles 2, 3 and 5 are presented below. Semiconductor temperature sensors with accuracy ±0.1 C were installed adjacent to porewater pressure sensors at depths of 50, 150 and 250 mm (Figure 1). Porewater pressures were measured using Druck miniature pore pressure transducers, each comprising a 6.4 mm diameter, 11.4 mm long, stainless steel cylinder with a ceramic filter tip. Transducers were filled with antifreeze and de-aired in a vacuum desiccator prior to installation. Variations in soil water pressures were transmitted via the transducer fluid to an electrical pressure element behind the ceramic tip. Each transducer measured to a maximum of 350 kpa with combined non-linearity and hysteresis of ±0.2 % best straight line, and thermal sensitivity of ±0.2% of reading per C. Due to an intermittent fault in the nearsurface transducer, only data from the 150 mm and 250 mm depths are presented below. Soil surface movements (frost heave and downslope displacements) were monitored using a pair of LVDTs, mounted on slotted steel tracks supported by a horizontal beam above the slope surface (Figure 1). The LVDTs formed a fixed-base triangle, with the apex connected to a perspex footplate equipped with four 20mm deep anchor points. The footplate was embedded in the surface of each experimental soil, and progressive surface displacements due to frost heave and solifluction were detected by changes in the geometry of the LVDT triangles with an accuracy of ± 1.5 mm. All instrumentation was scanned at halfhourly intervals using a PC-based logging system. Thus, readings of soil temperature, porewater pressure and soil surface displacement were all recorded on a common time-base, allowing direct comparisons to be made. Results Variations in pore pressure transducer readings through each cycle of freezing and thawing showed a consistent trend (Figure 2), with an initial fall in pore pressure being recorded during the so-called Òzero curtainó period and immediately following it. As temperatures 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, but pressure generally continued to rise until the beginning of the soil warming phase, when a rapid fall occurred. As rising soil temperatures approached the Òzero curtain rangeó, pore pressure transducer readings continued to fall, and became strongly negative, but rose once again towards the end of the Òzero curtain periodó, to become positive during consoli-dation of the supersaturated thawed soil. The main period of latent heat flux (the Òzero curtain) occurred at temperatures of between -0.1 C and -0.25 C. Soil freezing and thawing was accompanied by frost heaving and thaw Charles Harris, Michael C.R. Davies 435

Figure 3. Frost heave recorded during cycles 2, 3 and 5. settlement at the surface (Figure 3), so that the depth labels Ò150 mmó and Ò250 mmó applied to the pressure and temperature sensors are correct only for the thawed consolidated soil condition. It is estimated from the frost heave data and soil temperatures, that the frozen soil thickness above the Ò150 mmó sensors was 208 mm, 195 mm and 200 mm and above the Ò250 mmó sensors, 315 mm, 312 mm and 320 mm for cycles 2, 3 and 5 respectively. Further detail of the pressure-temperature relationships is apparent from Figure 4 where temperature is plotted against recorded pressures. During initial freezing, the transition from negative to positive pressures occurred at soil temperatures ranging from -1 C to -1.7 C at the 150 mm depth and between - 0.25 C and -0.5 C at the 250 mm depth, that is, after the passage of the freezing front. The reverse transition from positive to negative pressures recorded during soil thawing occurred between -0.5 C and -0.8 C for the 150 mm transducer and at around -0.25 C for the 250 mm transducer, that is at the onset of the Òzero curtainó period. 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 heaving pressures in soils. Previous studies have utilised total load cells (Williams and Wood, 1985; Smith and Onysoko, 1990), which are not in hydraulic continuity with the soil water. Pressure increases during soil freezing recorded in earlier experiments are interpreted as resulting from the growth of segregation ice, and transmission of ice pressures generated during frost heave to the load cells. In the present case it is argued that the rapid transition from negative to positive readings during freezing is in response to the sealing of the pressure transducer within an effectively closed frozen soil system. The transducer became isolated from pore fluid films and dominated by the positive ice pressures which develop during heave. Since there is considerable uncertainty as to the mechanism of pressure transfer from the heaving soil to the transducer, it cannot be assumed that pressure readings necessarily accurately reflect ice pressures. Within small-scale laboratory samples, Williams and Wood (1985) recorded pressures of between 100 and 250 kpa at temperatures of -0.5 C, while in slightly larger scale laboratory tests using a soil column 230 mm in diameter, Smith and Onysko (1990) recorded pressures 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 may have increased the resistance of the frozen soil to heave. In a larger ÒfieldÓ scale pipeline experiment, where such edge effects would have been minimal, heave pressures at a depth of 800 mm, beneath a buried pipeline, were around 100 kpa at temperatures a few degrees below zero. Thus, the frozen pressures recorded here are of a similar order of magnitude, if slightly lower, than those reported elsewhere. Much higher pressures may be developed, however, when measurements are made against a reaction frame that effectively prevents heaving (e.g., Sutherland and Gaskin, 1973). Turning to the thaw stage, it is envisaged that soil warming led to a progressive increase in thickness of unfrozen water films, and a reduction in ice pressures as the phase change temperature (T in equation (1)) rose. During the Òzero curtainó period, continued phase change from ice to water led to re-establishment of hydraulic continuity between the unfrozen water and the transducer tip, and recorded pressures became strongly negative, reflecting suction within the unfrozen water in the partially frozen soil. As ice lenses decreased in size and water films thickened towards the end of the Òzero curtainó period, soil water tension fell, and when hydrostatic conditions, coupled with transfer of stress from the soil grains to the pore fluid occurred during thaw consolidation, pore pressures became positive once again. Discussion Data presented here provides direct evidence for the development of positive ice pressures and high water suction values within frozen soils. The development of ice pressures in this experiment occurred rapidly, following the main period of phase change (Òzero curtainó). This may reflect the functioning of the hydraulically open pressure transducers, since pressure could not be registered until they became encapsulated in the frozen soil. However, the large increase in tensile strength of the soil between unfrozen and frozen states would not have become established until an ice-bonded matrix had developed, and ice pressures in excess of the overburden pressure were therefore unlikely until this stage. It is widely accepted that ice segregation does not generally take place at the freezing front (0 C isotherm), but at some distance behind it, at a lower temperature (Harlan, 1973; Loch and Kay, 1978; Miller, 1978, 1980; Konrad and Morgenstern, 1980, 1981, 1982). The Òfrozen fringeó, lying between the level of ice segregation and the 0 C isotherm, contains ice and unfrozen, mobile, water. As the freezing front advances downwards through a soil, water enters the frozen fringe from the unfrozen soil below and moves through the fringe to the growing ice lenses on its upper boundary in response to a steep hydraulic gradient across it. As predicted by the Clausius-Clapeyron equation, a corresponding increase in ice pressure occurs across the fringe, towards the level of ice lens growth (Miller, 1978). According to Williams and Smith (1989) this cold-side boundary where ice segregation commences is typically at a temperature of -0.1 C to -0.2 C. Due to a progressive increase in the ratio of ice to unfrozen water as the temperature of the fringe falls, permeability decreases towards the cold-side boundary, and falls sharply within the frozen soil above, where, in the context of the experimental time scale, water is no-longer mobile (e.g., F rland et al., 1988). The experimental data presented in this paper may be interpreted in the context of the above model. It is proposed that the development of negative pore pressure readings at each transducer during soil freezing marked the arrival of the frozen fringe, and the transition to positive readings occurred when the transducer became isolated from the mobile unfrozen water of the fringe and fully encapsulated in the frozen soil behind the fringe. The transducers were then responding to heaving pressures generated to overcome the tensile strength of the frozen soil during frost heaving (Williams, 1988; Smith and Onysko, 1990). During thaw it is assumed that phase change begins initially in the finer pores, and progressed into larger pores as the soil temperature approached the freezing point of the soil ice. At a certain stage, hydraulic continuity between the transducers and the mobile unfrozen soil water was reestablished, and a rapid response occurred, with the transducers recording suction within the unfrozen water films. In an ice-bonded soil matrix, the reduction in volume during phase change from ice to water would be expected to result in suction within the melt water. As ice bonding of the soil matrix was broken ice pressures fell, water pressures rose, and as overburden stress was transferred from soil/ice contacts to the soil water during thaw consolidation, porewater pressures in excess of hydrostatic were induced. It was at this stage that downslope displacement by slow viscous flow was observed on the experimental slope. Although the experiment discussed here was designed principally to monitor the behaviour of soil immediately following thaw, it provides a useful insight into the physical characteristics of fine-grained soils through a complete cycle of freezing and thawing. Since the transducers were in hydraulic continuity with the soil, the ethylene glycol antifreeze may have influenced the freezing behaviour of the soil immediately adjacent to the transducer tips, but in subsequent tests using silicon oil rather than antifreeze in the transducers, pressure variations followed the same pattern as is 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 contribution of J.-P. Coutard to this work. References Beskow G. (1935). TjŠbildningen och tjšllyfningen med sšrskild hšnsyn till všgar och jarnvšgar. Sveriges Geol Undersškning, Arsbok 26, No 375 Edlefson N.E. and Anderson A.B.C. (1943). Thermodynamics of Soil Moisture. Hilgardia, 298pp. F rland, K.S., F rland, T. and Ratkje S.K. (1988). Frost Heave. Proceedings of the Fifth International Conference on Permafrost, Trondheim, Tapir, 344-348. Harlan, R.L. (1973). Analysis of coupled heat-fluid transport in partially frozen soil. Water Resources Research, 9, 1314-1323. Harris C., Davies, M.C.R and Coutard, J.-P. (1996a). Laboratory simulation of periglacial solifluction: significance of porewater pressure, moisture contents and undrained shear strength during thawing. Permafrost and Periglacial Processes, 7, 293-312. Harris C., Davies, M.C.R and Coutard, J.-P. (1996b). An experimental design for large-scale modelling of solifluction processes. Earth Surface Processes and Landforms, 21, 67-76. Konrad, J.M. and Morgenstern, N.R., (1980). A mechanistic theory of ice lens formation in fine-grained soils. Canadian Geotechnical Journal, 17, 473-486. Konrad, J.M. and Morgenstern, N.R., (1981). The segregation potential of a freezing soil. Canadian Geotechnical Journal, 18, 482-491. Konrad, J.M. and Morgenstern, N.R., (1982). Prediction of frost heave in the laboratory during transient freezing. Canadian Geotechnical Journal, 19, 250-259. Loch, J.P.G. and Kay, B.D. (1978). Water redistribution in partially saturated silt under several temperature gradients and overburden loads. Journal of the Soil Science Society of America, 42, 400-406. Miller, R.D. (1978). Frost heaving in non-colloidal soils. In Proceedings Third International Conference on Permafrost, Edmonton, National Research Council of Canada, Ottawa, pp. 708-713. Miller R.D. (1980). Freezing phenonomena in soils. In Applications of Soil Physics, Hillel, D. (ed.) Acadamic Press. Smith, M.W. and Onysko, D. (1990). Observations and significance of internal pressures in freezing soils. In Proceedings of the Fifth Canadian Permafrost Conference, Laval, Nordicana No.54, pp. 75-82. Sutherland, H.. B. and Gaskin, P.N. (1973). Pore water and heaving pressures developed in partially frozen soils, In North American Contribution to the 2nd International Conference on Permafrost, Yakutsk, National Academy of Sciences, Washington, DC, pp. 409-419. Taber S. (1930). The mechanics of frost heaving. Journal of Geology, 38, 303-317. Williams P.J. (1988). Thermodynamic and mechanical conditions within frozen soils, and their effects. In Proceedings of the Fifth International Conference on Permafrost, Trondheim, Tapir, Trondheim, pp. 493-498. Williams P.J. and Smith M.W. (1989). The Frozen Earth. Cambridge University Press, 306 pp. Wood, J.A. and Williams, P.J. (1985). Stress distribution in freezing soil. Proceedings of the Fourth International Symposium on Ground Freezing, Sapporo, Japan, vol. 1, 165-171. Charles Harris, Michael C.R. Davies 439