Influence of age-hardening and strain-rate on confined compression and shear behaviour of snow

Journal of Terramechanics, Vol. 22, No. 1, pp. 37--49, 1985. Printed in Great Britain.

0022-4898/83 $3.00+0.00 Pergamon Press Ltd. © 1985 ISTVS

INFLUENCE OF AGE-HARDENING AND STRAIN-RATE ON CONFINED COMPRESSION AND SHEAR BEHAVIOUR OF SNOW RAYMOND N. YONG* and IOANNIS METAXAS t

Summary~The problem of assessing mobility performance of tracked or wheeled vehicles over snow is addressed in terms of the capability .of the snow to provide flotation and traction capabilities. To obtain input into analyses for energy transfer from the traction element to the supporting snow cover, it is necessary to describe confined compression and shear performances of the snowmrecognizing that a large density increase occurs under initial attack of the vehicle traction element. This study provides experimental input and diagrams of energy surfaces describing confined compressive and shear effects related to snow age and initial density.

INTRODUCTION

FoR development of the capability of tracked or wheeled vehicles to traverse snow-covered terrain, one of the necessary requirements is that track or wheel sinkage into the snow cover (i.e. snow pack), due to vehicular load, will not degrade traction to the point where immobility results. By and large, excessive track or wheel sinkage results in the development of (a) bellying effects where the belly of the vehicle interacts directly with the snow, and (b) high motion resistance due to frontal ploughing and belly interaction with the snow. In the extreme, excessive sinkage into the snow could produce a 'hang-up' condition where the vehicle belly rests entirely on the snow and total vehicle load is carried through load transfer in bearing via the vehicle belly. Another important requirement for development of vehicle mobility over snow is the capability of the snow pack to provide sufficient shear strength to enable production of tractive effort. It is clear that a very necessary ingredient in the evaluation of the capability of vehicles to perform over-snow and through-snow mobility, is a better understanding of (a) the response compression performance (of snow), and (b) the shear strength of the snow under confined compression. In the final analysis, the capability of snow as a material and also as a terrain 'cover', to provide flotation for effective transfer of traction forces is mandatory if vehicle mobility over snow-covered terrain is to be ensured. In the normal compression testing of materials for evaluation of stress-strain relationships, the general procedures used include unconfined compression or triaxial testing of representative test samples. Because loose snow cannot be readily handled without significant disturbance and local shear or collapse, it is generally difficult to assess shear strength of snow in conventional laboratory testing systems. To minimize or avoid the problems of local collapse, field testing has generally been performed. However, control of

*William Scott Professor of Civil Engineering and Applied Mechanics, and Director, Geotechnical Research Centre, McGill University, Montreal, Quebec, Canada. tGraduate Research Assistant, Department of Civil Engineering and Applied Mechanics (Geotechnical Research Centre), McGill University, Montreal, Quebec, Canada. 37

38

R . N . Y O N G a n d I. M E T A X A S

pertinent boundary and initial conditions in addition to problems of application of load, and proper determination of snow properties in the field have all contributed to hampering the progress of knowledge of snow performance under load. Observations in the field combined with plate load tests in the laboratory have recently confirmed that except in the case of densely" packed snow, plate loading of snow causes a vertical shear zone development--as has been shown by Yong and Fukue [1] (see Fig. l). Following on the work reported previously [1], this study (a) continues the investigation of the mechanical response behaviour of snow under confined compression as shown by element B in Fig. 1, and (b) the shear performance of the material, as represented by element A in the same figure. Particular attention is paid to the effects of (a) ageing of the snow, and (b) rate of load application. The motivation for the study recognizes the fact that the characteristics and properties of snow change as time progresses after initial snow deposition, which in turn will affect (vehicle) flotation and traction capabilities of the snow. A better knowledge and understanding of the response behaviour of the snow in confined compression and in direct shear, as represented by the response performance of Elements B and A respectively in Fig. 1, would contribute further to the analysis of snow traction capability of tracked and wheeled vehicles. Lood Snow s

Rigid plate

I I ELement A FIG. I.

ELement B

Failure modes in snow during the penetration of rigid plate.

EXPERIMENTATION

Snow material

Because of the critical need to test replicate samples of snow, the basic-,2tnowmaterial used in the study was artificially made in the laboratory by crushing 3-day-old ice with a pulverizer (Fig. 2). This is the same technique used by Yong and Fukue [1]. The grain site distribution of

INFLUENCE OF AGE=HARDENING

39

3 5

- A r t i f i c i a l . snow machine. I. Hopper 2. Crushed ice

4. Drum 5. Driving motor

3. Steal. vain

6. Artificial. snow

FIG. 2.

Schematic diagram of snow pulverizer.

the snow, labelled asfresh artificialsnow (Fig. 3) compares favourably with natural and Peter snow. With this procedure for production of snow, control on the production of initial densities of the test snow samples was achieved at a level of _-)-0.003 g/era 3. The freshly produced artificial snow served as basic snow material for all tests. In all, the seven snow types used in the experimental program were distinguished by the number of days of ageing after production of the basic snow. The seven types were identified as: (a) fresh snow----series 0, (b) 2-day aged snow--series 2, (c) 4-day aged snow--series 4, (d) 8-day aged snow--series 8, (e) 22-day aged snow--series 22, and (f) 30-day aged snow--series 30. To obtain the aged snow samples, freshly produced snow samples were kept in isolation boxes in the cold room which was maintained at a temperature o f - 13°C. After the prescribed number of days of ageing, the samples were removed for testing. The grain size distribution curves for the freshly produced and aged snows are shown in Fig. 4. From studies of the densities of fresh natural snow obtained from the field, it was observed that ageing of the natural snow for 1 month at a constant temperature of-13°C. +I°C., produced a density of about 0.30 g/era 3. Further ageing to 50 days after snow deposition produced snow densities of about 0.40 g/cm 3. Based on this information, it was decided that an initial density of about 0.35 g/cm 3for the test snow material could be used as a reasonable initial snow density test condition.

Test system The experimental test system used for confined compression studies is shown in Fig. 5. The confined compression apparatus is connected to a switch box which controls the loading

40

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Y O N G a n d I. M E T A X A S

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rate. Load is sensed by a load transducer and displacements resulting from load application are measured via a D C D T displacement transducer. Density samples were obtained from the test material using thin-walled brass tube samplers.

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INFLUENCE O F A G E - H A R D E N I N G

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Confined compression apparatus.

For confined compression tests, the samples were contained in lucite cylinders with common heights and inside diameters of 17.8 and 3.8 cm respectively. The wall thickness of the cylinders was 1.3 cm.

Compression testing All the snow samples (fresh and aged) were tested under three different rates of load application---denoted in terms of the loading velocities applied to the samples. The lowest loading velocity v] was 0.058 cm/s whilst the intermediate and highest velocities v2 and v3 were 0.158 cm/s and 0.216 cm/s, respectively. Three tests were performed at each loading velocity for each test sample.

Direct shear tests The samples used for direct shear testing were similar to those used for the confined compression tests. The test samples were contained in lucite cylinders with dimensions and shear collar system as shown in Fig. 6. Prior to shear testing, the lucite shear cylinder was placed in the shear casing shown in the figure. Note that the actual test shear casing was rigidly supported laterally by design clamps fixed to the left system rod, and that the right shear sleeve casing was also rigidly fixed to the piston. For direct shear testing, the lucite cyclinder containing the snow to be tested was positioned into the shear casing so that the shear plane in the lucite cyclinder was coincident with the shear plane of the fixed shear casing. As in the case of the confined compression tests, the same three velocities were used in application of the direct shear loads. The samples tested in the shear cylinders were prepared at densities of 0.35 g/cm 3 0.60 g/cm 3 in 0.05 g/cm 3 increments. These densities were obtained after ageing of the samples and with the aid of the confined compression device. From predetermined total height requirements, control on final density of the test samples was obtained by

42

R . N . Y O N G and I. M E T A X A S

I 2 3

6

[] Shear test set.-up [] I. Snow sample 2. Shear casing device 3. Load ceLL

FIG. 6.

4. Driving piston 5. Driving motor 6. Recording system

Direct shear apparatus

compressing the test sample with known initial density to a specified height which would yield the desired test density. RESULTS

AND

DISCUSSION

Confined compression behaviour A typical test result for the confined compression test isshown in Fig. 7. Load application to the test sample is represented on the ordinate. This is achieved by applying a constantvelocity downward motion of the loading head. As the sample compresses (in the confined

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Density T y p i c a l confined compression curve iLLustrating m i c r o f r o c t u r e s end b r i d l e p e r f o r m a n c e

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FIG. 7.

Typical confined compression curve illustrating microfractures and brittle performance.

INFLUENCE OF AGE-HARDENING

43

condition exercised by the sample casing), the density of the test sample obviously increases. Since the cross-section area of the sample is constant, the decreasing height of the sample is a direct registration of the increasing density of the sample. Figure 7 shows the typical density increase (arising from sample compression) in relation to load application--for one loading velocity. The particular point of interest is the 'chattering' load characteristic developed at a specific density range of the test sample. This type of performance is reflective of the continuous local load transfer and fracture of bonds occurring throughout the sample. As one local bond fractures, load transfer to other bonds occurs. Subsequent fracture of other bonds will continue the process of load transfer. The continuous process serves to change and increase the packing of the snow grains, to the threshold density (point A shown in Fig. 7) identified previously by Yong and Fukue [1]. By and large, this point is at the end of the highly visible 'chattering' zone. Minor 'chattering' continues beyond the threshold density, but with lesser magnitudes of load fluctuation. Figures 8-10 show the compression stress-strain results for the confined compression tests at the three loading velocities, in terms of days of ageing, plotted as a 'state surface'. The third axis identifying the age of the snow samples, 0-30 days, provides the 'state parameter'. As might be expected, the effect of ageing can be visibly demonstrated through increases in compression strength (resistance) of the lest samples. The differences in strength (resistance) for the same strain developed are significantly larger when the loading velocity is lowest, as seen by comparing the shapes of the surfaces shown in Figs. 8-10. It is useful to note that as the velocity of load application increases, the confined strength of the snow decreases for brittle performance. This can be attributed to the fact that as loading velocity increases, the ability for the sample to resist shear distortion via bonding between snow grains becomes considerably diminished. In slow distortion, load transfer to unbroken bonds coupled with a type of 'healing' process permits the snow to carry apparently higher loads, in contrast to higher rates of loading which obviously cause

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Age effect VI : 0.58 mm/s FIG. 8. Stress vs strain relationships for different ages of snow in confined compression.

44

R, N. YONG and I. METAXAS

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FIG. 9.

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Stress vs strain relationships for different ages of snow in confined compression.

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Stress vs strain relationships for different ages of snow in confined compression.

correspondingly higher rates of shear distortion. Figure 11 shows this effect well. It is observed that as the snow ages, the influence of rate o f load application is much greater, in contrast to the zero-day aged snow which shows very little strength degradation due to increased loading rates. Bond strength contribution to shear resistance is also a vital issue in the development of

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INFLUENCE O F A G E - H A R D E N I N G

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Stress vs loading velocity relationships in confined compression tests.

higher strengths in aged samples, as seen iri Fig. 11. The increase in strength at the lowest rate of loading is significantly large as the period of ageing is increased. As has been noted in many previous studies by various researchers, (e.g. Wakahama [2], Keeler [3], Hobbs and Mason [4], Colbeck [5,6], Yong and Fukue [ I], and Perla [7]) the transformation occurring in the ageing process which lead to the creation and enhancement of bonds between snow grains, would serve to provide a higher resistance to shear application. This has been confirmed in Figs. 8-11. Under ordinary field conditions, increases in the density of natural in situ snow will occur with increased ageing, thus providing another reason for higher strengths with aged samples. However, in the laboratory ageing process, because the samples were aged in the isolation box, the density increases with ageing were relatively small, less than 2% increase in density. Fukue [8] notes that below a density of 0.4 g/era athe strength of snow is quite small and that in the low density range, the strength of the snow depends more on grain structure than on density.

Direct shear test performance The test results obtained in direct shear testing, using the system described previously are shown in Figs. 12-14. These highlight the effects of ageing and density on the developed shear strength of the snow samples. These same figures can be viewed in terms of strain-rate effects. As was the case for the confined compression tests, the higher strain rates caused some degradation in strength of the samples, for the same physical reasons. This strength degradation was common for the various aged snows, as seen in Fig. 15. As in the confined comlSression study, the zero-day snow is relatively insensitive to the effects of loading rate. At longer periods of snow ageing, the effects of loading velocity become more pronounced. The trends are similar to those shown for confined compression tests.

46

R.N. YONG and I. METAXAS Sheor ~ e s ~ s

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FIG. 12.

S h e a r stress vs d e n s i t y r e l a t i o n s h i p s for d i f f e r e n t a g e s o f s n o w .

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S h e a r stress vs d e n s i t y r e l a t i o n s h i p s for different ages of s n o w .

Energy expenditure The energy expended in compression and shear constitutes a useful method in examining the response compression/shear behaviour of the snow material. Since the applied load is known, and since the distance of travel of the applied load is measured, the total work

47

INFLUENCE OF AGE-HARDENING

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Shear stress vs density relationships for different ages o f snow.

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expenditure involved in compressing the initial snow in the confining container to the final density under the load can be easily calculated. The energy state surface shown in Fig. 16 depicts the energy expenditure for confined compression for the lowest rate of loading. Similar surfaces can be generated for the other rates of loading. As might be expected, the energy surface shown in Fig. 16 matches closely the conf'med stress-strain surface shown

48

R.N. YONG and I. METAXAS

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Energy per unit volume required to c r e a t e compaction in confined compression . U1 :O,58mm/sec

FIG. 16. Energy vs density relationships for different ages of snow in confined compression. previously in Figs• 8-10, due obviously to the direct relationship between (a) strain and density, a n d (b) stress and energy• With Fig. 16, and o t h e r similar energy surfaces, we can n o w establish the a m o u n t of energy required to p r o d u c e c o m p a c t i o n at a particular initial snow density, and for a specific period o f snow m a t u r a t i o n (age). This is useful information for estimation o f the flotation capability o f the snow. In the case o f the direct shear tests, the m a x i m u m shear stress generated and the displacement o f the shear sleeve are k n o w n values• The shear energy surface s h o w n in Fig. 17

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Shear energy vs density relationships for different ages of snow.

INFLUENCE OF AGE-HARDENING

• 49

for the same lowest rate o f shear application bears some similarity to the compression surface s h o w n in Fig. 16. The preceding not withstanding, these are two distinctly different types o f surfaces. In Fig. 17, the surface shows the energy required to shear a snow sample o f a particular age and density. O n the o t h e r hand, the surface s h o w n in Fig. 16 indicates the a m o u n t o f energy required to compress a snow sample o f a particular age, f r o m an initial density o f 0.35 g / c m 3. CONCLUDING REMARKS The two elements shown in Fig. 1 which d e m o n s t r a t e the actual mechanisms o f s n o w p e r f o r m a n c e u n d e r load are indicative o f the flotation requirements for snow in vehicle mobility analyses. The tests p e r f o r m e d in this study have attempted to provide preliminary i n f o r m a t i o n leading to the application o f energy transfer mechanics for analysis o f vehicle mobility over snow covered terrain. The generation o f state energy surfaces for compression and shear are indeed preliminary and necessary for the further continuing studies which analyze the p r o b l e m o f energy transfer f r o m the traction elements o f the vehicles u n d e r study. Acknowledgements--The work performed in this study was in partial fulfilment of the study requirements for Defence Research Establishment Suffield (DRES) under contract arrangement with Department of Supply and Services (DSS), Canada. REFERENCES [ I] R.N. YONO,and M. FUKUE,Performance of snow in confined compression, J. Terramechanics 14 (2), 59-82 (1977). [2] G. WAKAH^MA,Metamorphisms of wet snow, Low Temp. Sci., Ser. A 23, 51-66 (1965). [3] C.M. KEELER,The growth of bonds and the increase of mechanical strength in a dry seasonal snowpack, J. Glaciology 8 (54) (1969). [4] P. V. H••Bs and B. •. M•s•N• Thesinteringand adhesi•n •fice•Phi•. Mag.. Ei•ht Ser. 9(98)• •8 •-•97. (•964). [5] S.C. COLBECK,Thermodynamics of snow metamorphism due to variations in curvature, J. Glaciology29 (94), 291-301 (1980). [6] S.C. COLBECK,An overview of seasonal snow metamorphism, Rev. Geophys. Space Phys. 20 (I), 45-61 (1982). [7] R. 1. PEAL^,Strength tests on newly fallen snow, J. Glaciology 8 (1969). [8] P.V. HOBBS,The effect of time on the physical properties of deposited snow, J. geophys. Res. 70 (16), 3903-07 (1965).