The Experimental Simulation of Ice-Wedge Casting

The experimental simulation of ice-wedge casting

J.B. Murton Department of Geography, University of Sussex, Brighton, UK C. Harris Department of Earth Sciences, University of Cardiff, UK

ABSTRACT: This paper describes a new methodology to simulate ice-wedge casting by scaled physical modelling experiments in a geotechnical centrifuge. The purpose of the experiments is to determine the effect of host sediment granulometry and ice content on (a) the mechanisms of casting and (b) the size and structure of casts. Six models were constructed with host sediments comprising medium sand, loessic silt and silt-clay mixtures, and gravimetric moisture contents ranging from ϳ15% to 60%. The models thawed from the surface downwards. Model ice wedges were 150 mm high and tapered uniformly downward from a maximum true width of 50 mm, equivalent to prototype- scale wedges 4.5 m high and 1.5 m wide. Model 6 data on temperature, pore-water pressure and drainage during thaw, and on cast geometry after thaw demonstrate the potential of this approach to the study of ice-wedge casting.

1 INTRODUCTION described in an unpublished study by T. Rudzinska (1972, cited in Jahn 1975, pp. 72–73). Alternating layers Ice-wedge casts are key stratigraphic indicators of for- of gravel, sand and silt (ϳ10–40 mm thick) were placed mer permafrost and are often used to reconstruct in a small vessel and the sediment frozen. A wedge- palaeotemperatures (Huijzer and Vandenberghe 1998, shaped hole was cut in the middle of the frozen sedi- Murton & Kolstrup 2003). However, the mechanisms ment, filled with water and the vessel again chilled, of casting and their controlling factors have rarely producing an artificial ice wedge ϳ80 mm high and been studied and are difficult to monitor in the field with a maximum width of ϳ40 mm. Above the wedge (Dylik 1966, Jahn 1975, Harry & Gozdzik 1988, and host sediment was laid a sand layer ϳ10 mm thick. Murton & French 1993). Two key factors that that are The sediment was thawed and then refrozen, before cut- believed to influence casting are the particle size and ting the sediment block in half to compare the resultant ice content of the host sediment (Black 1976). A new ice-wedge cast with the size and shape of the initial ice approach to verify the influence of these factors is to wedge. The experiment was repeated using two freeze- simulate casting by laboratory modelling experiments thaw cycles to determine the effect of repetitive freeze- and to compare the artificial casts with natural ones. thaw on the cast structure. This approach permits (1) control of particle size and The experiments caused downturning of sediment ice content; (2) monitoring of temperature, pore pres- layers into the cast due to subsidence or collapse, pro- sure and cast development during thaw; and (3) meas- ducing a “cone-in-cone” structure. In addition, a sedi- urement of the initial ice wedge and the final cast. ment lining (ϳ3 mm thick) formed around the lower The objectives of this paper are to (i) describe the sides of one cast, the sediment deriving from within the new experimental methodology using scaled centrifuge ice wedge. The cast was reported to be “larger and modelling, and (ii) discuss the results from one test in deeper” than the former ice wedge, and Rudzinska con- order to demonstrate the potential of this new approach cluded that the degree of sediment subsidence depends to studies of ice-wedge casting. A companion paper on texture and the number of freeze-thaw cycles. will describe the results from all six tests and evaluate The approach taken in Rudzinska’s experiment their significance to the cryostratigraphic record applies only at the small scale at which it was con- (Harris and Murton in prep.). First we describe a previ- ducted. To apply the results to the field scale it is ous experiment to simulate ice-wedge casting in order essential to reproduce the correct self-weight stresses to highlight the necessity of correct scaling of stresses associated with thaw of the upper few metres of soil. in thawing soil. This can be achieved either by carrying out full-scale laboratory experiments or, more simply, by centrifuge 2 PREVIOUS EXPERIMENT modeling in which 1/N scale models are thawed at N times gravity. The centrifuge scaling laws applicable To our knowledge, only one previous laboratory experi- to simulating the degradation of the upper metres of ment has attempted to simulate ice-wedge casting, as permafrost are described by Harris et al. (2001a, b).

807 3 EXPERIMENTAL METHODOLOGY Model ice wedges were 150 mm high and tapered uniformly downward from a maximum true width of The present experiments were carried out in the 50 mm. During thaw in the centrifuge at 30 gravities, Cardiff Geotechnical Centrifuge Facility. This com- these were equivalent to prototype-scale wedges 4.5 m prises a beam centrifuge of radius 2.8 m capable of high and 1.5 m wide. Six models were constructed to accelerating a model package with a mass of up to 1/30 scale (Table 1). Host sediments comprised medium 1000 kg at an acceleration of 100 gravities. The test sand (models 1 and 2), natural loessic silt (models 3 package is contained within a strongbox of external and 4) and silt-clay mixtures (models 5 and 6). dimensions ϳ750 mm 550 mm 500 mm built of Gravimetric moisture contents ranged from ϳ15% steel with Perspex sides. (model 1) to 60% (model 6). Models 3 and 5 had mois- Six ice-wedge models were constructed in a ture contents between the Plastic Limit and Liquid polypropylene test box of internal dimensions 750 mm Limit, whereas in models 4 and 6, moisture contents long by 450 mm wide by 500 mm deep that fitted exceeded the Liquid Limit. closely inside the centrifuge strongbox. Freezing of the Centrifuge tests lasted between approximately model soil took place within a chest freezer, and the 8 hours (model 1) and 22.5 hours (model 6). Soil models were thawed from the surface downwards in drainage was allowed during thaw, except in model 2. the geotechnical centrifuge under an acceleration of Temperature and pore-water pressure were recorded 30 gravities (g). Thus, dimensional scaling during at 10-second intervals on a data logger. Time-lapse thaw between model and prototype was 30. photographs were taken through the clear Perspex A layer of sand ϳ20 mm thick formed the base of front of the strongbox, and videos through the front each model. Above it were laid successive layers of and top of the model in order to document the forma- soil ϳ20–35 mm thick. The top of each layer was tion of ice-wedge casts. After complete thawing of the carefully flattened and the layer was frozen before the wedge and host soil, the models were allowed to drain next was added. Marker layers 2–3 mm thick and of for 2–4 weeks before they were vertically sectioned different colour and/or texture to the soil layers were and structures within them were measured, sketched placed between them in order to highlight any defor- and photographed. mation structures formed during thaw. The soil layers and markers were laid on both sides of an aluminium mould with external dimensions identical to the internal dimensions of a similar mould used to form the model ice wedges. After the uppermost soil layer had been laid and frozen, the mould was removed from the soil, leaving a wedge-shaped trough into which a previously-frozen model ice wedge was inserted. Any gaps between the sides of the wedge and trough, usually no more than 1 to 2 mm wide, were filled with chilled water and frozen prior to commencing thaw of the ice wedge. Finally, a model active layer 25 mm thick was placed above the top of the wedge and moistened with warm Figure 1. Vertical section through ice-wedge model 6, water in order to expedite thaw of the model. showing location of pore-water pressure transducers and Instrumentation buried in the soil during model thermocouples adjacent to the ice wedge, and of marker horizons in the host silt-clay soil. construction comprised six miniature pore pressure transducers (Druck PDCR-81, 350 mbar), and up to 14 Type K thermocouples. The transducers were placed at 50 mm and 100 mm distances from the edge Table 1. Ice-wedge model parameters. of the ice wedge in order to determine if (1) excess Moisture pore pressures resulted during thaw of the ice wedge Model Grain size content* (%) Cover soil and host soil (2) whether these were associated with lateral or vertical hydraulic gradients. Monitoring of 1 medium sand 15 sand pore pressure also facilitates interpretation of soil 2 medium sand 20 sand rheology and hence casting mechanisms during thaw. 3 Pegwell silt 40 Prawle silt 4 Pegwell silt 20 Prawle silt The thermocouples permitted monitoring of the rate † † of thaw and the pattern of isotherms adjacent to the 5 2/3 silt, 1/3 clay 30 2/3 silt, 1/3 clay 6 2/3 silt, 1/3 clay† 60 2/3 silt, 1/3 clay† thawing ice wedge. The positioning of transducers and thermocouples in model 6 is shown in Figure 1. *Approximate gravimetric moisture content. †Kaolinite.

808 4 RESULTS 4.2 Pore pressures

To illustrate the results of these casting experiments, we Pore pressures generated during thaw consolidation present some data on temperature, pore pressures and are illustrated in Figure 3 for transducers located at cast geometry from model 6. This model simulated frozen depths of 62.5 mm, 100 mm and 137.5 mm thawing of ice wedges in ice-rich silt-clay host materials below the surface (Fig. 2), scaling at 30 g to 1.875 m, (Table 1) with a silt-clay active layer. The ice content 3.0 m and 4.125 m, respectively. During thaw, the in the host sediment was high, so the latent heat compo- model surface lowered by 87.5 mm, equivalent to nent of soil thawing was greater than in the other mod- some 41% of the model frozen thickness; there was no els, and the time for thaw was correspondingly long ice within the cover soil, and much less settlement in (22.5 hours). Since time for seepage force similarity this layer than in the frozen host sediments. Thus, and time for heat transfer both scale in the centrifuge as although pore pressures approximated hydrostatic rel- 1/N2, (where N is the number of gravities at which the ative to the frozen model depth, at the point of thaw test is run), no scaling conflicts arise in modelling the and thereafter, the pore pressures recorded were thaw-consolidation process, and model time is reduced nearer geostatic than hydrostatic as a result of (a) low by a factor of N2 compared with prototype time (Harris initial soil density and (b) the reduction in overburden et al. 2000, 2001a, b). Thus at 30 g, the thaw period thickness as consolidation progressed. Near-surface scales to 843.75 days, or 2.3 years for the prototype. pore pressures fell only slightly through the thaw phase, reflecting continued upward migration of water 4.1 Temperature from below. Pore pressures at greater depths remained high during the period when the adjacent ice wedge Since ice content of the host soil was 60% dry weight, thawed. Thus, it is likely that the frictional strength of but the wedge consisted only of ice, the latent heat the soil was low, facilitating deformation as the void required to thaw the wedge exceeded that of the soil, left by the thawing wedge filled with soil from above. slowing penetration of the thaw front above the ice wedge compared with the adjacent host soil (Fig. 2). 4.3 Geometry of ice-wedge cast Thaw of the model ice wedge was delayed by approximately 1.8 hours (scaling to approximately 67 days) The model ice-wedge cast from test 6 is shown in compared with soil 50 mm away from it (1.5 m at pro- Figure 4. The mean maximum true width was 42 mm totype scale). Thaw-consolidation processes would (std dev. 7mm; n 14) and the mean height was 84 mm therefore be expected to have at least partially dissi- (std dev. 4mm; n 14), equivalent to prototype-scale casts pated excess pore pressures in the thawed soil by the with a mean maximum true width of 1.26 m wide and a time the adjacent wedge had melted, stiffening the mean height of 2.32 m. The model values represent a soil, and reducing the potential for deformation around narrowing of 17% relative to the initial maximum true the wedge void. width of the ice wedge, and a shortening of 44% relative

60 ABC Active Layer 40 2 hours 20 A

0 B 14 hours -20

C Pore Pressure (kPa) -40

-60 18 hours

-80 Thaw front 0 5 10 15 20 25 Time in hours (model scale) TC Figure 3. Pore pressures recorded by three transducers TC and PWP during test 6. Arrows indicate time of thaw. Transducer Figure 2. Thaw-front geometry interpolated from ther- locations are shown in Figure 2. Transducers A and B were mocouple data, test 6. TC thermocouples, PWP pore- 50 mm from the sides of the ice wedge, transducer C pressure transducers. Ice wedge is shaded. 100 mm.

809 coefficient. The resulting cast geometry also differed markedly, reflecting contrasting degrees of host soil deformation. Clearly, we are unable to represent precisely the regime of permafrost degradation associated with ice- wedge casting, since this process may occur over many years, with seasonal surface freezing punctuating pro- gressive downward thaw or with complex histories involving, for example, partial ice-wedge thaw and renewed permafrost aggradation (Murton and Kolstrup 2003). However, the approach taken here does allow us to investigate the effects of host soil geotechnical prop- erties, and provides insights into the factors influencing the geometry of Pleistocene ice-wedge casts.

ACKNOWLEDGEMENT

The research was funded by a grant from the Royal Society.

REFERENCES

Black, R.F. 1976. Periglacial features indicative of permafrost: ice and soil wedges, Quaternary Research 6: 3–26. Dylik, J. 1966. Problems of ice-wedge structures and frost- fissure polygons, Biuletyn Peryglacjalny 15: 241–291. Harris, C. & Murton, J.B. in prep. Scaled centifuge model- Figure 4. Vertical sections at 20 mm (a) and 100 mm (b) ing of ice-wedge casting. through the ice-wedge cast in model 6. The central part of Harris, C., Murton, J.B. & Davies, M.C.R. 2000. Soft- the cast comprises grey cover soil that descended into the sediment deformation during thawing of ice-rich void left by the thawing ice wedge. Downturned marker frozen soils: results of scaled centrifuge modelling horizons extend into the cast. experiments. Sedimentology 47: 687–700. Harris, C., Rea, B. & Davies, M.C.R. 2001a. Geotechnical centrifuge modelling of gelifluction processes: valida- to its initial height. Above the cast was a trough whose tion of a new approach to periglacial slope studies. mean maximum width was 214 mm (std dev. 29; n 14) Annals of Glaciology 31: 263–269. and mean maximum depth was 16 mm (std dev. 1mm; n Harris, C., Rea, B. & Davies, M.C.R. 2001b. Scaled physi- 16). Adjacent to the cast was a crack whose mean width cal modelling of mass movement processes on thaw- was 18 mm (std dev. 4mm; n 12) and whose mean depth ing slopes. Permafrost and Periglacial Processes 12: 125–136. was 113 mm (std dev. 7mm; n 10). Harry, D.G. & Gozdzik, J.S. 1988. Ice wedges: growth, thaw transformation, and palaeoenvironmental significance, Journal of Quaternary Science 3: 39–55. 5 CONCLUSIONS Huijzer, B. & Vandenberghe, J. 1998. Climate reconstruc- tions of the Weichselian Pleniglacial in northwestern Scaled centrifuge modelling facilitates detailed inves- and central Europe. Journal of Quaternary Science tigation of the behaviour of thawing soils. In this 13: 391–418. example we have simulated the thawing of an ice Jahn, A. 1975. Problems of the Periglacial Zone (Zagad- wedge, and the morphology of the resulting ice-wedge nienia strefy peryglacjalnef). Warsaw: Panstwowe cast. In the case described here, pore pressures gener- wydawnictwo Naukowe. ated by thaw-consolidation processes were high, and Murton, J.B. & French, H.M. 1993. Thaw modification of reduced the strength of the thawed host soil suffi- frost-fissure wedges, Richards Island, Pleistocene Mackenzie Delta, western Canadian Arctic, Journal of ciently for significant deformation during casting. In Quaternary Science 8: 185–196. earlier tests with host soils of differing ice content and Murton, J.B. & Kolstrup, E. (2003, in press). Ice-wedge granulometry, thaw-consolidation ratios differed sig- casts as indicators of palaeotemperature: precise nificantly, reflecting different rates of thaw-front pen- proxy or wishful thinking? Progress in Physical etration and different values of the consolidation Geography 27.

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