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

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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.
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