- Email: [email protected]
Preconditioning of deep snowpack for off-road vehicle mobility
Journal of Terramechanics, Vol. 26, No. 1, pp. 83-11XI, 1989. Printed in Great Britain.
PRECONDITIONING
OF DEEP SNOWPACK VEHICLE MOBILITY
01122-4898/8953,(I(l+0.00 Maxwell Pergamon Macmillan pie. (~) 1989 ISTVS
FOR
OFF-ROAD
P. BOONSINSUK,* G . J. IRWIN,t R. N. YONG* a n d F. CAPORUSCIO*
Summary--This paper addresses several preconditioning techniques for strengthening a deep snowpack in order to support vehicular loadings. The criteria imposed on preconditioning a deep snowpack were: (1) only light commercially-available vehicles or equipment could bc used, (2) preconditioning would only be applied to the snow surface and (3) any additives to be used should be easily acquired in remote areas. Viable preconditioning techniques werc first evaluated in the laboratory using artificial snow. The techniques explored were surface loading (surcharging), heating and mixing with additives (sand and straw) followed by surcharging. The properties of the laboratory-preconditioned snow were evaluated primarily in terms of footing penetration resistance and Rammsonde hardness. Based on the laboratory results, preconditioning of a deep natural snowpack was carried out. Surcharging the snowpack was achieved by a BV206 Carrier. The preconditioning techniques studied were surcharging, heating and mixing with additives (salt and straw). Various ageing periods were imposed. The load-carrying capacity of the preconditioned snowpack was evaluated by multipasses of two wheeled vehicles (lltis and 5/4 ton truck). The results indicated that a soft deep snowpack can be preconditioned to the extent that it can support multipasses of wheeled vehicles.
INTRODUCTION THE NEED tO t r a v e l o v e r s n o w - c o v e r e d r e m o t e r e g i o n s of the n o r t h c o n t i n u e s u n a b a t e d with i n c r e a s i n g d e m a n d to d e v e l o p , for e x a m p l e , n a t u r a l r e s o u r c e s such as forests, p e t r o l e u m a n d m i n e r a l s . D u r i n g w i n t e r m o n t h s , t h e lack o f a w e l l - m a i n t a i n e d n e t w o r k o f r o a d s / h i g h w a y s m a k e s it n e c e s s a r y to use vehicles t h a t a r e specifically d e s i g n e d to travel o v e r o r t h r o u g h snow. Such vehicles a r e n o r m a l l y e q u i p p e d with t r a c k s to m i n i m i z e g r o u n d c o n t a c t p r e s s u r e s . In a d e e p s n o w p a c k , h e a v y t r a c k e d vehicles can b e c o m e i m m o b i l i z e d b e c a u s e of h a n g - u p failure which o c c u r s w h e n t h e s i n k a g e e x c e e d s the g r o u n d c l e a r a n c e o f the vehicle. T o p r o v i d e m o b i l i t y o f w h e e l e d vehicles, a r o a d must be c o n s t r u c t e d to s u p p o r t the high g r o u n d c o n t a c t p r e s s u r e s n o r m a l l y e x e r t e d by the wheels. W i t h the arrival of snow a n d c o l d w e a t h e r , the c o n s t r u c t i o n o f s n o w a n d ice r o a d s [1] p r o v i d e s the n e c e s s a r y trafficability for o n - r o a d vehicles. C o n s t r u c t i o n o f r o a d s r e q u i r e s special h e a v y m a c h i n e r y which first m u s t be a c q u i r e d a n d t r a n s p o r t e d to a r e m o t e site. T h e p r e c o n d i t i o n i n g o f n a t u r a l s n o w p a c k , on the o t h e r h a n d , p a r t i c u l a r l y d e e p snow, using c o m m o n l y a v a i l a b l e e q u i p m e n t m a y i m p r o v e t h e m o b i l i t y of o f f - r o a d t r a c k e d a n d w h e e l e d vehicles. In the p r e s e n t c o n t e x t , p r e c o n d i t i o n i n g t h e surface of d e e p snow entails the p r o d u c t i o n of a snow p a v e m e n t c a p a b l e o f s u p p o r t i n g v e h i c u l a r traffic.
*Geotechnical Research Centre, McGill University, Montreal, Canada H3A 2K6. tDefence Research Establishment Suffield, Ralston, Alberta, Canada TOJ 2NO. 83
84
P. B O O N S I N S U K et al.
On the premise that it is possible to improve deep snow trafficability for commercially available off-road vehicles through preconditioning, this study was aimed at developing techniques appropriate to field application. The objectives of the study were: (1) to investigate the effects of various snow preconditioning methods including the mixing of additive with the upper portion of the snowpack; (2) to determine the load carrying capacity of a preconditioned deep snowpack; and (3) to evaluate the trafficability of preconditioned deep snowpack. Both laboratory-scale and full-scale experiments were carried out to study various preconditioning techniques, the findings of which are addressed in this paper.
EXPERIMENTATION
The program of experimentation consisted of two parts, as follows:
Laboratory investigation This was established to explore alternative preconditioning techniques on a laboratory scale, to determine practical test procedures and to provide better control of test conditions. The parameters studied included: (a) (b) (c) (d)
surcharge pressures for compaction, 17 and 34 kPa; ageing after compaction, up to 8 days; application of heat; addition of sand or straw.
All laboratory tests were conducted using artificial snow which was prepared by pulverizing ice cubes into fine grains, similar to the technique previously used by Yong and Fukue [2]. Artificial snow was adopted because of the need of producing replicate snow samples for control testing..The grain size distribution of the artificial snow was not significantly different from certain types of natural snow [2, 3]. Similarly, the load-resisting behaviour of the artificial snow was not much different from that of natural snow [2]. Thus, the use of the artificial snow in the laboratory equipment provided the fundamental basis for planning field study. The artificial snow, which produced a typical distribution of particle size as shown in Fig. 1, was deposited loosely into a bin (1219 mm long by 102 mm wide by 1006 mm deep) having one transparent wall made of plexiglass, as illustrated in Fig. 2. The long length of the snow bin permitted to conduct several tests on precisely the same snow condition, thereby minimizing the effect of variation in snow properties. Tests performed included measurements of temperature and density, Rammsonde hardness, footing (plate) pressure and resistance to cone penetration. The load-carrying capacity of the artificial snow was measured by footing penetration which was carried out using a 74 mm by 99 mm rigid plate. The width of the footing (74 ram) was much smaller than the depth of the artificial snow (1006 mm) so as to satisfy the 'deep snow condition'. which required that pressure bulb development in the snow below a loading platform should not intersect the bottom rigid boundary. The development of a pressure bulb during footing penetration was visually observed by monitoring the deformation of a network of gridlines inscribed onto a vertical face of the artificial snow sample prior to testing.
PRECONDITIONING OF DEEP SNOWPACK
o,,
I00
--
5 0 mm from s u r f a c e zx160 mm from s u r f a c e o 360 mm from surface • 660 mm from surface
85
~
•
90-80--
~: 70-60-
4o~, n
30 20IC,0
02
I
03
o4o~
,
2
3
~
5
~ ~
Particle diameter (mm)
FIG. l.
Particle size distribution of artificial and natural snow.
~
1 1006 mm
~
~
G
r
~
d
Artificial
2 mm
s
snow
"~ PlexLgkass wall
j 1 2 1 9 J mm
FIG. 2. Test conducted in artificial snow container.
T h e test p r o g r a m and the test c o n d i t i o n s c o m p i l e d in Table with the f o l l o w i n g objectives:
1 w e r e drawn up
Test Nos. S1 to $3. T o evaluate the effects of ageing (10 min, 2 and 8 days, respectively) on 'undisturbed' artificial s n o w after its d e p o s i t i o n in the s n o w bin (Fig. 2).
P. B O O N S I N S U K et al.
86
TABLE 1. SUMMARYOF LABORATORYTEST CONDITIONS Test No.
S1 $2 $3 $4 $5 $6 $7 $8 $9 ttl H2 H3 H4 SD1 SD2 ST1 ST2
Preconditioning technique
undisturbed, no treatment
17 kPa surcharge
34 kPa surcharge
heating at surface, no compaction
mixed with sand and 17 kPa surcharge for compaction mixed with sand and 34 kPa surcharge for compaction mixed with straw and 17 kPa surcharge for compaction mixed with straw and 34 kPa surcharge for compaction
Air temperature* (°C)
Ageing
- 13 -13 -14 - 5
10 rain 2 days 8 days 10 min
-13 -13 -13 - 5 -13
2 days 8 days 10 rain 2 days 8 days
- 6 - 7.5 - 2 -13
10 rain 1 day 2 days 8 days
- 1
10 rain
- 1
10 rain
- 5.5
10 rain
- 5.5
10 rain
Remarks
reference
compaction by surcharging
* Snow temperature profiles are given in figures.
Test Nos. $4 and $9. To determine the effects of densifying the artificial snow by surcharge loadings and subsequent ageing. Test Nos. H1 to H4. subsequent ageing.
To investigate the influences of heat
1-m deep
application
and
Test Nos. SD1 and SD2. To evaluate the effects of adding sand to the top of the artificial snow, followed by compacting the snow with surcharge loadings. Test Nos. STI andST2. Similar to Test Nos. SD1 and SD2 but with straw in place of sand. Full-scale field in vestigation In order to implement the findings from laboratory experiments, a field trial with full size vehicles was conducted in a large area covered with a 700 to 800 m m depth of snow. For trials, a total of 21 road courses, each being about 200 m long, was prepared with the aid of a BV206 All Terrain Carrier. A designated n u m b e r of passes of the Carrier was followed by ageing the compacted snow, combined with additives (salt and straw) on certain courses or the application of heat, as listed in Table 2. The BV206 All Terrain Carrier is an articulated two-unit vehicle mounted with rubber tracks. The rear unit used in this study was loaded with sand bags to 8.9 kN which brought its ground contact pressure close to that of the front u n i t - - a p p r o x i m a t e l y 14 kPa. A low ground pressure vehicle of moderate weight and high mobility over deep snow, such as the BV206 Carrier, serves as a convenient device for preconditioning the
P R E C O N D I T I O N I N G OF DEEP SNOWPACK
87
e-
~
~
~
I ~
~
~"
I
e-,
"Z o 5 o
6
~
g 6
~
E
.. ~ .~
&o ~
," ~
~
,,¢
z
e~
I
,-¢
4-
+
'~"
~
ri
I~-
~-
I~
~
~
~
~
& ,,q.
,<
u~
,A~; > t~
. S o~
88
P. BOONS1NSUK et al.
snow for the benefit of less mobile (on-road) vehicles. U p o n completion of the test course construction, a four-passenger, four wheel drive vehicle (Iltis) was used as the test vehicle to evaluate the load-carrying capacity of the preconditioned snow in each course. The m a x i m u m ground contact pressure exerted by the Iltis was approximately 220 kPa using 6.50 R-16 tires. The n u m b e r of passes made by the Iltis on each course was monitored for strength and durability of a course, whenever more than 1 pass was possible. In addition, two other vehicles were used for testing, i.e. a 5/4 ton wheeled truck with a m a x i m u m ground contact pressure of 455 kPa and a tracked M 113 A r m o u r e d Personnel Carrier with ground contact pressure of 55 kPa.
L A B O R A T O R Y R ES U LTS
The ability of a deep snowpack to support vehicle loadings can be meaningfully measured by the footing penetration test (Fig. 2), while snow strength variation with depth can be effectively evaluated by the R a m m s o n d e hardness test [4]. The R a m m s o n d e device is a dynamic p e n e t r o m e t e r that measures snow resistance to the advancc of a cone driven by a drop hammer. The ram resistance or ram hardness n u m b e r is calculated as:
R -
Whn
+ W + Q
(1)
X
where
R W h n x
(2
= = = = = =
ram resistance, kgf, weight of drop h a m m e r , kgf, height of drop, cm, n u m b e r of h a m m e r blows, penetration after n blows, cm, weight of penetrometer, kgf.
Although both vane and cone devices were used (Fig. 2), the R a m m s o n d e p e n e t r o m e t e r was easier to operate in both laboratory and field, and yielded results that were easier to interpret. In this paper, the load-carrying capacity of the deep artificial snow will be expressed in terms of footing penetration resistance and ram hardness number. To compute the ram hardness n u m b e r at the surface (0-10 cm deep), the correction factors (4.7 for 0-5 cm and 1.6 for 5-10 cm) were included as suggested by Ueda et al. [4]. Artificial s n o w without preconditioning As a reference for comparison with preconditioned snow, the load-bearing capacity of the unpreconditioned artificial snow was evaluated by the footing penetration test at a rate of 15 mm/min (Fig. 2). The test was conducted after the artificial snow was deposited into a bin and allowed to age for 10 rain, 2 and 8 days, without further treatment. During ageing, the humidity of the walk-in cold room was kept continuously high by a humidifier. The density of the compacted snow was found to be quite uniform with depth, beginning at 0.40 Mg/m 3, when first deposited, to 0.50 Mg/m 3 at 2 and 8 days ageing. The density increase may be attributed to compaction under its own weight. Resistance to footing load on the unpreconditioned snow increased gradually as the footing penetrated into the snow (Fig. 3). The effect of ageing on thermally uniform snow
PRECONDITIONING OF DEEP SNOWPACK
Symbol Test No
24-0
89
Ageing
0
Sl
o
$2
0 rain 2 days
z~
S3
8 days
J
200 0_ v
160 2 o_
*g u2
120
80
40
i0
20
30
40
50
I
60
I
70
D,spLacement (ram)
FIG. 3. Results of footing penetration test in undisturbed unpreconditionedsnow. as tested was significant only beyond ageing periods of more than 2 days. In addition to intergranular bonding, resistance to footing penetration developed through densification of the snow underneath the footing. The affected snow mass or the pressure bulb thus formed extended vertically to about 4-5 times the footing width, i.e. about 300-350 mm below the 74 mm wide footing, as observed by the deformation pattern of the inscribed network of gridlines (Fig. 2). The lateral development of the pressure bulb extended slightly beyond the edge of the footing. Intergranular bonding was sufficient to prevent the snow from collapsing around the footing as it advanced into the snow, thus forming vertical walls at the edges of the footing. The rise in footing pressure (Fig. 3) with the advance of the footing plate into the snow may be explained by the snow densification and further development of the pressure bulb. The load-bearing capacity of the snow increased about 2-3 times when the ageing period was increased by 6-8 days. At the time of the footing penetration test, temperature within the snow varied slightly with depth. Among the three tests, the lowest temperature of-17°C was recorded in the 8-day aged snow, while that for the other two age conditions was -13°C. Precise control of snow temperature was limited by fluctuations in the ambient temperature of the walk-in cold room, particularly during periods of long ageing time.
Snow preconditioned by surcharge loading Natural deep snowpack can support only light vehicles which compact the snow as they progress; hence the idea of densifying deep snowpack by surface loadings from a grooming vehicle prior to passage by a test vehicle. A set of experiments consequently entailed the loading of the artificial laboratory snow (Fig. 2) uniformly over its entire surface (i.e. surcharging). Two surcharge pressures, 17 and 34 kPa, were used since they represented the low ground contact pressures of some off-road
90
P. BOONSINSUK et al.
vehicles. Surcharge loading was applied by gradually increasing the load acting on a rigid plate at the surface of the snow. When the applied loading reached the selected value, it was released, and the densified snow was left to age for 10 rain, 2 and 8 days prior to testing. The results of the footing penetration tests performed on the artificial snow preconditioned by surcharging and ageing (i.e. Test Nos. $4 to $9 in Table 1) are compared in Fig. 4(a) and (b). It is evident that the higher surcharge of 34 kPa led to higher load-bearing capacity (Fig. 4a). Ageing significantly increased the strength of the artificial snow densified by the lower surcharge of 17 kPa but only slightly improved that of the snow preconditioned at the higher surcharge of 34 kPa when the ageing period was greater than 2 days (Fig. 4b). For similar artificial snow, the increase in shear strength of the snow due to ageing was found to be more pronounced when the snow density was in the range of (t.40 and 0.55 Mg/m 3 [3]. This range is in agreement with the resulting densities produced by the lower surcharge of 17 kPa (Fig. 5). At a common footing displacement of I0 ram, surcharging strengthened the laboratory snow to the extent that it could resist a footing pressure of more than 500 kPa, as opposed to the footing pressure of 100 kPa in unpreconditioned snow (Fig. 3). Surcharging or surface compaction produces the densification necessary to form a road pavement within the snowpack. The density and temperature profiles of the artificial snow preconditioned by surcharging and ageing, presented in Fig. 5, indicate a relatively uniform distribution in both properties. In general, the density of the preconditioned snow ranged from 0.50 to 0.58 Mg/m ~, compared to the initial density of 0.40 Mg/m 3. The temperature profiles which were measured at the time of testing snow strength illustrate the equi-temperature nature of the snow used in laboratory experiments. Any effects of snow temperature on load-bearing capacity in these tests are, it is believed, overshadowed by the effects of surcharge level and ageing. For example, the snow preconditioned by 17 kPa surcharge without ageing (Test No. $4) offered the lowest resistance to footing penetration (Fig. 4a) despite having the lowest snow temperature (--14°C). However, it should be noted that fluctuations in snow temperature occurred during the ageing period due to interruptions in the walk-in cold room. Variations were as high as +5°C during the longest ageing period of 8 days. Possible effects of thermal history on footing resistance were not investigated. The ram hardness profiles depicted in Fig. 5 reveal the strength increase due to preconditioning by surcharging and ageing. All preconditioned snow samples possessed higher ram hardness than the unpreconditioned snow without ageing (R = 30 in Test No. SI). The ram hardness numbers were, in general, uniform with depth after passing through some variations in the top 300 mm. The highest ram hardness of more than 400 was attained in the snow subjected to the highest degree of preconditioning, i.e. using 34 kPa surcharge and 8-day ageing (Test No. $9). High surcharge loading and long ageing periods coincided evidently with high values of ram hardness. Variations in ram hardness number in the upper 31)0 mm of the snow were possibly a consequence of (1) non-uniformity of densification particularly in close proximity to the surcharge loading plate and (2) fluctuations of temperature and humidity in the cold room. Snow preconditioned by heat application The benefits of promoting stronger bonds between snow grains through heat application and refreezing followed by ageing were investigated. The surface of the
PRECONDITIONING OF DEEP SNOWPACK
,°-I!
91
90o i
/
800
700 o_
Symbol_ Test No [] o
600
500
i
S4 $5 S6 S7 S8 $9
• • •
o
SurchargekPa Ageing I0 min I 2 days 8 days I 0 min I 2 days I 8 da~/s I
17 17 ]7
34 34
34
aO0
300
200
tOO
I
i
20
I
30
I
40
I
50
]
60
70
DispLacement (ram)
Symbol I000
Surcharge Footing kPa displacement (mm)
o
17
5
• o
34-
5 tO I0
17 34-
•
v
800 O ~
&
600
~ ~
~°°// ~
400
ol
UI------
//f/
/
J
2
~
. . . . . . . . .
1
4
i
6
-o--
I
Ageing time (days)
FIG. 4(a).
Results of footing penetration test in snow preconditioned by surcharging. (b) Variation of footing pressure with ageing.
92
P. BOONSINSUK Density [ M g / m 3 } 04
08/-15
06
Temperature
(°C)
-I0
-5
et al.
Ram 0
IO0
hardness 200
number
300
400
'.tzj.
I00
200 300 E
--
4-00
d]
500
(
600 700
~] 0 o
SI $4 $5 $6 $7 $8 S9
* • •
800
0 17 17 17 34 34 34
I0 min IO min 2 days 8 days I0 min 2 days 8 days
Fl(;. 5. Profiles of density, temperature and ram hardness of snow preconditioned by surcharging.
artificial snow (Fig. 2) was heated by two 1200-watt hot-air blowers for 20 min. Prior to heating, dye had been sprayed on top of the snow surface so as to observe the extent of percolation by melting snow. Under the imposed conditions of heating, the melting snow as observed to percolate down to a depth of about 400 mm. Subsequent to heat application, the snow was aged for periods of 10 min, 1, 2 and 8 days before testing. The load-bearing capacity of the heated artificial snow (Fig. 6) was significantly increased when compared with the unpreconditioned snow (Fig. 3), even without appreciable ageing. Densification and stronger intergranular bonding were undoubtedly achieved by heating and refreezing of the melted snow. The minimum ageing period of
800J
Test
Ageing
,¢oo~ I
600I
&
Sym bat
/J
Ill
•
~I
G
, day
H3
2
days
ks_ 200
0
•
[
I0
1
20
", '
40
I
50
~L)
?C
D''Sptacement ( m m )
FJ~;. 6. Results of footing test in snow preconditioned by heating and ageing.
PRECONDITIONING OF DEEP SNOWPACK
93
one day considerably strengthened the snow, while longer ageing periods of 2 and 8 days produced only slight improvements in load-bearing capacity. The density profile shown in Fig. 7 indicated some increase in density caused by ageing. The ram hardness number of the top 300 mm, which was within the zone of percolation in melting snow, was much higher than those at depths below that zone. Not surprisingly, the higher ram number corresponded to the artificial snow sample aged one day at the lowest temperature. Resistance to footing penetration (Fig. 6) was, however, not affected by the variations in temperature and ram hardness. This can be explained by the fact that a substantially greater snowmass within the pressure bulb contributed to resisting the footing penetration in comparison to that resisting Rammsonde penetration which affected only a relatively small mass within close proximity to the 40 mm diameter cone of the Rammsonde penetrometer. The pressure bulb underneath the 74 mm wide footing, in general, extended to a depth of about 300 mm below the footing and coincided with the zone affected by heating.
Snow preconditioned by mbcing additives The anticipated benefits of mixing the surface layer of a snowpack with additives are densification, reinforcement of the snow structure, strengthening of the bonds of grains, or some combination of these. In practice, the additives chosen for mixing should be locally available and economical to acquire so as to minimize construction cost. Furthermore, the additives should not be difficult to handle in the field and should not require special heavy machinery for spraying and mixing with snow in an attempt to satisfy the requirements of preconditioning a deep snowpack in some remote area. Wuori [5] reported that additives such as direct heat, water and sawdust may be effective when dry processing and other compaction methods are inadequate. Two types of additive were used in the present laboratory experiments, silica sand (passing sieve no. 10) and straw. Sand would increase the density of a snowpack because of its higher specific gravity, while straw may be expected to improve the distribution of stresses within the snow mass through its tensile strength and interconnection between
Density 0
04
I00
(Mg/m
06
I
3)
08/
Temperature
15
-I0 [
/
(°C)
Ram 5O
-5 !
'1~
I
hardness I00
number 150
20O
I ~ i'l
I
2OO
3OO
E 4OO C:] 5OO
600 -
700 -
FKI. 7.
/
°\ Symbot Tes"c Ageing No
-
• a •
H I H2 H3 H4
I0 I 2 8
min day days days
Profiles of density, temperature and ram hardness of snow preconditioned by heating.
94
P. B O O N S I N S U K et al.
fibres. Each additive was sprayed manually on top of the artificial snow (Fig. 2) such that snow grains were visible among sand particles or straw fibres. A thin layer of fresh artificial snow was then sprinkled to partially cover the snow surface already mixed with additives, followed by spraying sand or straw. The process described was repeated until the top layer of snow mixed with sand or straw was about 250 mm thick. Surcharge loading (17 or 34 kPa) was then applied to compact the snow, followed by strength testing without appreciable ageing. The load-bearing capacity of the snow preconditioned by adding sand was improved when the surcharge loading was increased from 17 kPa to 34 kPa as depicted in Fig. 8. The increased resistance in the snow/sand mixture is believed to be due principally to an increase in density of the snow with little contribution, if any, from intergranular bonding. Sand acts primarily as a filler of voids and would tend to inhibit, not enhance, bonding. By contrast, the snow mixed with straw exhibited higher load-bearing capacity for the lower surcharge loading (17 kPa) preconditioning when the footing penetration was more than 22 mm. Such behaviour probably reflected the non-uniformity in straw fibre strength, straw distribution in snow mass and nonhomogeneity of the snow/straw mixture. Unlike sand, which is cohesionless, straw was expected to enhance the tensile strength of the snow and to distribute the footing load to the snow adjacent to the footing. However, the advantage offered by additives (i.e. sand and straw) was not clear when compared with the snow preconditioned by surcharging without additives (Fig. 4). At the lower surcharge loading of 17 kPa, the inclusion of straw led to the highest load-bearing capacity of the preconditioned snow (Figs 4 and 8). At the higher surcharge loading of 34 kPa, the highest load-bearing capacity was found in the snow preconditioned only by surcharging without additives. It is apparent that sand is not an efficient additive in snow because of its cohesionless nature. When straw is uniformly and homogeneously distributed in the snow mass, its reinforcing effects with surcharging may be expected. The density profiles presented in Fig. 9 indicate clearly the effects of surface surcharging since the density of the snow mixed with sand decreased from the surface
700
SymbotAddltweSurcharge(kPa) _
Goo"~ 5O0 I:1_ a,:
="
sand
,7
z~
Sand
34
•
Straw S
t
17
I
J ~
.
'
~
40o 590 45
8
u_
-"
2©0
-J,--
~00
, 0
L 20
3'0
i 40
DispLacement
FIG. 8.
i 50
I GO
i 70
80
(mm)
Results of footing test in snow preconditioned with additives.
P R E C O N D I T I O N I N G OF D E E P S N O W P A C K
Density(Mg/m3] 00~4 015 016 0[7 0 8 I00
Temperature(°C) Ii ~'/l!
95
Ram hardnessnumber ~*Ix1 5 I00 ~~"~ 0 150 ~
200
•
300~-.~ 400
/il.l [t /
600
700~
FK;. 9.
/\ i.I-l - - = ,
sD~O A~i~_veSurcharge(kPa,
[] a •
SD2
,7
Sand
STI Straw ST2 Straw
34
17 34
Profiles of density, temperature and ram hardness number in snow mixed with sand or straw.
downwards until it reached a relatively constant value beyond about 250 mm depth. The highest density of the snow mixed with sand was close to 0.80 Mg/m3 which was much higher than the 0.58 Mg/m 3 found in snow without additive (Fig. 5), due obviously to the higher specific gravity of sand grains. The density of the snow mixed with straw was not totally dependent on the surcharge loading as the density under the lower surcharge loading of 17 kPa could be higher than that produced by the higher surcharge loading of 34 kPa. As a result, the ram hardness of the former was higher than that of the latter, similar to the load-bearing behaviour (Fig. 8). The fact that temperature was lowest for the snow preconditioned with straw and 17 kPa surcharge (Fig. 9) may have contributed to its superior load-bearing capacity and ram hardness. It is evident that using the proper additive can indeed strengthen a deep snowpack. Straw has shown its promising potential although its fibre strength is highly variable and uniformity of distribution in snow cannot easily be achieved.
Summary of laboratory snow preconditioning results Based on the preceding results of deep snow preconditioning under laboratory conditions, the following observations were made: (a) It is possible to precondition a deep, soft snowpack such that it can support a contact pressure of about 150 kPa. Viable preconditioning techniques can be one of the following: compaction by light surface loadings (less than 35 kPa), heating and mixing snow with appropriate additives followed by compaction. Ageing will further enhance strengthening of the preconditioned snowpack. (b) The highest load-bearing capacity was achieved by either compaction with surcharge loading (34 kPa was used in this study) or heat application followed by at least 1 day of ageing. (c) Due to limitation of sample size and equipment in the walk-in cold room, the advantages of additives could not be established conclusively. Nevertheless, straw
96
P. B O O N S I N S U K et al.
appeared to be a promising reinforcing additive capable of increasing the tensile and shear strength of snow. (d) In these tests, the load-bearing capacity of the preconditioned snow was not significantly affected by temperature as long as it remained well below zero. (e) The ram hardness number was a good indicator for identifying the variation of snow strength with depth. When correlated with resistance to footing penetration, the ram hardness number indicates load-bearing capacity of the deep snow as illustrated in Fig. 10, which combines all information available from the tests listed in Table 1. The ram hardness number of each test was averaged from the data within the top 250 mm of the 1-m deep snow, while the resistance to footing penetration was taken at 5 mm footing displacement. Although the data shown in Fig. 10 are scattered, a higher ram hardness number normally indicates a higher load-bearing capacity. It is noticeable that a footing pressure of 500 kPa or above was achieved by the heated snow (Test Nos. H2, H3 and H4) and the 34 kPa surcharged snow (Test Nos. $8 and $9). Ice might develop during refreezing of the heated snow and stronger bonds might be generated because of the high humidity of the walk-in cold room during ageing. Such conditions might not be representative of actual field situations. The 'artificial snow' line is therefore considered to be a better indicator of the load-carrying behaviour of the artificial snow tested under laboratory conditions. 8 0 0 --
700
0 H4
o $9
600
c
OH2
H3
500
Z
0
$8
0
E E Lo
400
• MIJttl - " " [:)asses / "ot wheeled" / vehicles / /possibLe /
o $6
0
Laboratory result (with Test No.)
g 3oo / OS5 200
/ ~/57
/
//
B FieLd
/ / /
result
OST2
/ J/
Artificial
snow
/ HI I00 --
SD2 //o o o
/
/ /
~/
°ST I o SDI
1 0
$4
/
I
l
I
J
t
I
50
iO 0
150
200
250
500
Ram
FIG. lO.
hardness
number
Relationship between resistance to footing penetration and ram hardness number.
PRECONDITIONING OF DEEP SNOWPACK
97
Using the two-dimensional test arrangement as displayed in Fig. 2 and a footing size of 74 mm by 99 mm, the development of a pressure bulb visually observed from the deformation pattern of the grid lines inscribed onto the snow surface was, in general, limited to within 4-5 times the footing width (74 mm) below the footing. The lateral extent of the pressure bulb was slightly beyond the edge of the footing. Such an observation was made at the end of the test, thus the footing pressure was relatively high. The 'deep' snow conditions imposed in this study were satisfied since pressure bulbs did not touch the bottom rigid boundary. The pressure bulb itself may be considered as an important element in preconditioning a snow pavement on which vehicles would, in a field situation, be operated.
FIELD TEST RESULTS To apply the knowledge gained from the laboratory results, a field trial for preconditioning deep snow was carried out in February 1987 a few kilometres north of Quebec City. The snowpack, on a level 1 km 2 area of test range, was 660-800 mm deep with a highly stratified structure. Ice crusts were present, varying in thickness from 10 mm near the surface to 70 mm at the base. Snow had accumulated over the previous 3 months during which changes in air temperatures and precipitation produced alternate conditions of isothermal and temperature gradient metamorphism. In the 3 week period of field testing, the air temperature fluctuated from 0.6°C to -27.3°C as listed in Table 2, while no new snow precipitation was recorded. Wind was occasionally strong resulting in moguls and wind slabs. It was in general observed that the nonhomogeneous natural snow differed from the uniformity of laboratory (artificial) snow in such physical properties as structure, density, temperature and grain size profiles with depth. The density of natural snow ranged between 0.2 and 0.4 Mg/m 3, i.e. lighter than laboratory snow. Thermal conditions progressed from temperature gradient to isothermal conditions during trials. The grain size of natural snow ranged predominantly between 1 and 2 mm, i.e. coarser than the artificial snow whose grains were largely between 0.2 and 1 mm. Taking into account such differences, projections or predictions about the behaviour of natural preconditioned snow based on results of laboratory snow were intended to serve only as a guide to field experiments. Based on the laboratory findings that deep snowpack can be effectively preconditioned by low surcharge loadings, the BV206 Carrier with 14 kPa ground contact pressure was used as the principal preconditioning equipment in the field. On its first pass over hitherto undisturbed snowpack, the sinkage of the BV206 Carrier was about 150-200 mm, leaving in its wake a track footprint. At least 3 passes along the same course were necessary to produce a uniform swath of compacted snow. The BV206 Carrier could easily travel over the undisturbed snowpack, while wheeled vehicles (i.e. Iltis and 5/4 ton truck) could not float, despite the presence of underlying ice lenses. All test courses listed in Table 2 were essentially prepared by the BV206 Carrier under a variety of artificially imposed conditions. The conditions of test course preparation were determined by the use of mixing additives such as salt and straw, as well as heat application. Sand as an additive was eliminated from further consideration based on laboratory results. Salt, on the other hand, was thought to be a potential substitute for direct heat application. Road salt and straw were manually deposited on the snowpack from the BV206 Carrier after which at least one further pass was made for mixing the salt and/or straw with the
98
P. BOONS1NSUK et
al.
snow matrix. A H e r m a n - N e l s o n (BT-400 Series Type H I ) heater was mounted in the rear unit of the BV206 Carrier, and two hoses were used to apply hot air from the heater to the snow with or without mixing additives. The heater was set at a heat output of about 2 x 105J at an air delivery rate of 0.23 to 0.46 m-Vs and a discharge air t e m p e r a t u r e varying from 65°C to 138°C. For the 200 m long course, the period of heat application was almost one hour. The course was then allowed to age before vehicular testing. Using the snow preconditioning techniques described, the resulting snow road surface was not smooth but conformed to the track footprint of the BV206 Carrier. In addition, the road surface was not level on account of numerous soft spots. Although snow from an adjacent area could be deposited on the road to make the surface level, it would require the use of additional heavy equipment which would deny the purpose of snow preconditioning in remote regions with minimum equipment. Thus test courses were treated as simulating real, uneven terrain. After preconditioning the natural snowpack, the 200 m test courses were travelled by one of the two wheeled vehicles, i.e. the Iltis with 220 kPa ground contact pressure and the 5/4 ton truck with 455 kPa ground contact pressure. The n u m b e r of passes made by the two wheeled vehicles are listed in Table 2, together with the ram hardness numbers averaged from the values measured within the top 300 m m of the preconditioned snowpack. The value of 300 m m was chosen to represent the extent of pressure bulb development since ram hardness numbers were normally higher in the top 300 mm of the snowpack than below this level. From the field trial tests, the following observations were made: (1) The natural undisturbed snowpack was unable to support any wheeled vehicles having a ground contact pressure of 220 kPa (Iltis) or more. The density of the 600-800 mm thick natural snowpack varied from 0.21 Mg/m 3 near the surface to 0.45 Mg/m s near the base, while the density of the artificial snow used in the laboratory study ranged from 0.40 Mg/m 3 to 0.50 Mg/m s. The ram hardness n u m b e r of the natural snow measured about 20 below a depth of 200 m m and was undetectable above the depth. For the artificial snow, the ram hardness number was approximately 30, capable of supporting a footing pressure up to 50 kPa at about 40 m m sinkage. In comparison, the natural snowpack reinforced by several underlying ice crusts was capable of supporting the BV206 Carrier with 14 kPa ground contact pressure at about 150-200 m m sinkage and the M l 1 3 Carrier with 55 kPa ground contact pressure at about 300 m m sinkage. (2) Preconditioning the natural snowpack by a few passes of the BV206 Carrier and ageing for 3 to 4 days could stiffen the snowpack such that it was able to support up to 8 passes of the wheeled vehicle Iltis having a ground contact pressure of 220 kPa and 3 passes of the 5/4 ton truck with 455 kPa ground contact pressure. The density of the snowpack was increased from about 0.25 Mg/m s in the top 300 mm of the natural snowpack to about 0.44 Mg/m 3 after multiple passes of the BV206 Carrier and ageing. The ram hardness n u m b e r of the preconditioned snowpack capable of supporting these vehicular loadings varied between 150 and 200. For comparison, a ram hardness number of 150 would support 1 pass of a tire inflated to 690 kPa (100 psi) carrying 98 kN (10,000 lb) wheel load, and a ram hardness n u m b e r of 200 would support 2 passes of the same tire (Wuori, 1963). The wheel load of the Iltis was 10.8 kN (1100 lb), using a tire inflation pressure of 269 kPa (39 psi), while the wheel load of the 5/4 ton truck was 15.2 kN (1550 lb) with a tire inflation pressure of 414 kPa (60 psi). Thus the results obtained in this study are comparable with those given by Wuori [5].
PRECONDITIONING OF DEEP SNOWPACK
99
(3) Using salt and straw as additives to the natural snow, the test courses preconditioned with these additives significantly outperformed those preconditioned without any additives. The Iltis succeeded in negotiating a 6 ° slope in Course No. 11 (Table 2) mixed with straw. The combining of straw and salt, followed by mixing and compacting by the BV206 Carrier, produced a test course which was strong enough to support 10 passes of the Iltis with 3 passengers (Course No. 12) even at melting ambient temperatures. Such performance contrasted vividly with that on snow preconditioned without straw wherein excavation-sinkage quickly immobilized the vehicle. Longer ageing periods should be at sufficiently cold temperatures. Road salt in particular required several days to extensively melt snow and permit re-freezing. Uniform distribution of additives could be achieved by using appropriate light equipment in lieu of manual operation. (4) Heat application to the natural snowpack has also proved to be a viable preconditioning technique since one of the heated courses (No. 9) could support up to 10 passes of the Iltis. The corresponding ram hardness number was in the 120-170 range. When compared with the other snowpacks preconditioned only by the BV206 Carrier passage followed by ageing (Course Nos. 2-4), the benefit of heating was, however, not significant. (5) It is characteristic of a wheeled vehicle to travel within its own rut after the first pass. Thus in multipass testing, wheel sinkages increased due to excavation slip until soft underlying snow was reached, consequently immobilizing the vehicle. The BV206 Carrier may be used to resurface the damaged snow surface so as to restore the course for further passes subsequent to age hardening. (6) The ram hardness numbers of the preconditioned snowpacks in the field were generally higher than those obtained in the laboratory, possibly due to the difference in snow condition, presence of ice crusts, thermal profile and preconditioning technique. Nevertheless, the relationship between the load-bearing capacity and ram hardness number of the natural snowpack is comparable to that observed in the laboratory as exhibited in Fig. 10. In the field a preconditioned snowpack was capable of supporting a few passes of a wheeled vehicle having a ground contact pressure of up to 455 kPa when its ram hardness number was between 150 and 200. In the laboratory where the artificial snow was tested in two-dimensional boundary conditions, the preconditioned snow was able to support a footing pressure of more than 250 kPa when its ram hardness number was 100 or more. The ram hardness number of the field snowpack lay close to the range observed in the laboratory snow (Fig. 10). As a comparison it may be noted that Adam [6] reports a ram hardness peak near 250 when ageing a compacted snow road for about 42 hours at air temperatures above -20°C. (7) The preconditioned snowpack in the field provided improved flotation for the tracked Ml13 Carrier with a ground contact pressure of 55 kPa. In the natural snowpack without any preconditioning, the sinkage made by the first pass of the Ml13 Carrier was approximately 250-300 mm. The sinkage was reduced to about 100-150 mm in the Ml13 Carrier's first pass over a preconditioned snowpack. Due to its aggressive track configuration, each pass of the Ml13 Carrier easily damaged the preconditioned snowpack, however hard the crust. Nevertheless, preconditioning a snowpack can improve mobility of the tracked Ml13 Carrier by reducing sinkage and increasing speed. (8) By visual inspection of a snow pit cross section of a preconditioned snowpack, the BV206 Carrier loadings could not break the ice lenses located in close proximity to the ground. The 'deep' snow condition which required that the extent of the pressure
I00
P. BOONSINSUK et al.
bulb should not reach the bottom rigid boundary (i.e. the frozen ground) was therefore satisfied. Note that in practice, the 'deep' snow condition refers specifically to a snowpack of sufficient depth that pressure induced densification at its surface is entirely independent of the underlying bottom rigid boundary. Such preconditioning treatment contrasts with conventional methods of road construction where a strong substrate may be built up from the base in dense layers.
CONCLUSIONS
From the results obtained in both laboratory and field investigation on snow preconditioning, the following conclusions can be drawn: (1) A soft deep snowback in a remote area can be preconditioned by using commercially available equipment such that it can support light wheeled vehicles. (2) The simplest preconditioning technique is to disaggregate and densify a natural snowpack by a light tracked vehicle (about 14 kPa ground contact pressure) making a few passes over the snowpack, followed by at least 1 day ageing. Additives such as road salt and straw can be mixed with the snow by the tracked vehicle in order to enhance strengthening of the snowpack. At ambient temperatures between -20°C and 0°C, salt would produce free water which would form a hard surface crust when re-frozen at very low ambient temperatures, while straw would reinforce the snow surface against tensile and shear failure. Straw also delays the effects of excavation sinkage. (3) Although the surface of the snow road built by the tracked vehicle is not smooth, it can provide sufficient bearing strength for a wheeled vehicle and permit mobility over deep snow. Ride comfort and speed may have to be sacrificed as would be expected in off-road travelling. (4) Despite differences between the laboratory artificial snow and the natural snow studied, both laboratory and field trial results exhibit similar load-carrying behaviour. The laboratory tests can be reproduced in the field with comparable results. Laboratory simulation of snow preconditioning may conveniently be used as a first step prior to full scale field implementation in order to develop control techniques at minimum cost. Acknowledgement--The authors wish to express their appreciation to the Reviewers for their input. The field portion of this study was performed at CFB Valcartier and Defence Research Establishment Valcartier, Quebec. The cooperation of all participating personnel from both establishments is gratefully acknowledged. The services of Mr J. J. Fitzgerald of DRES for snow classification studies are also acknowledged.
REFERENCES [1] [2] [3] [4] [5] [6]
K. M. ADAM, R. F. PIOTROWSKI, J. M. COLLINS and B. T. SILVER, Snow roads for pipeline installation on the Arctic pilot project, Proc. Third Int. Speciality Conf., Cold Regions Engineering Northern Resources Development, Edmonton, Alberta, Vol. II, pp. 603-618, 1984. R. N. YONG and M. FUKUE, Performance of snow in confined compression, J. Terramechanics 14, (2), 59-82. R . N . YONG and I. METAXAS, Influence of age-hardening and strain-rate on confined compression and shear behaviour of snow, J. Terramechanics 22 (1), 37-49. J. UEDA, P. SELLMANN and G. ABELE, USA CRREL snow and ice testing equipment, Special Report 146, Cold Regions Research and Engineering Laboratory, New Hampshire (1975). A. F. WUOR1, Snow stabilization studies, in Proc. Conf. Ice and Snow." Properties, Processes and Application, (Edited by W. D. Kingery), pp. 438-458, M.I.T. Press, Cambridge, Massachusetts (1963). K. M. ADAM, Report on Norman Wells winter road research study, Winnipeg Interdisciplinary Systems Ltd., Winnipeg, Manitoba (1978).
PRECONDITIONING
OF DEEP SNOWPACK VEHICLE MOBILITY
01122-4898/8953,(I(l+0.00 Maxwell Pergamon Macmillan pie. (~) 1989 ISTVS
FOR
OFF-ROAD
P. BOONSINSUK,* G . J. IRWIN,t R. N. YONG* a n d F. CAPORUSCIO*
Summary--This paper addresses several preconditioning techniques for strengthening a deep snowpack in order to support vehicular loadings. The criteria imposed on preconditioning a deep snowpack were: (1) only light commercially-available vehicles or equipment could bc used, (2) preconditioning would only be applied to the snow surface and (3) any additives to be used should be easily acquired in remote areas. Viable preconditioning techniques werc first evaluated in the laboratory using artificial snow. The techniques explored were surface loading (surcharging), heating and mixing with additives (sand and straw) followed by surcharging. The properties of the laboratory-preconditioned snow were evaluated primarily in terms of footing penetration resistance and Rammsonde hardness. Based on the laboratory results, preconditioning of a deep natural snowpack was carried out. Surcharging the snowpack was achieved by a BV206 Carrier. The preconditioning techniques studied were surcharging, heating and mixing with additives (salt and straw). Various ageing periods were imposed. The load-carrying capacity of the preconditioned snowpack was evaluated by multipasses of two wheeled vehicles (lltis and 5/4 ton truck). The results indicated that a soft deep snowpack can be preconditioned to the extent that it can support multipasses of wheeled vehicles.
INTRODUCTION THE NEED tO t r a v e l o v e r s n o w - c o v e r e d r e m o t e r e g i o n s of the n o r t h c o n t i n u e s u n a b a t e d with i n c r e a s i n g d e m a n d to d e v e l o p , for e x a m p l e , n a t u r a l r e s o u r c e s such as forests, p e t r o l e u m a n d m i n e r a l s . D u r i n g w i n t e r m o n t h s , t h e lack o f a w e l l - m a i n t a i n e d n e t w o r k o f r o a d s / h i g h w a y s m a k e s it n e c e s s a r y to use vehicles t h a t a r e specifically d e s i g n e d to travel o v e r o r t h r o u g h snow. Such vehicles a r e n o r m a l l y e q u i p p e d with t r a c k s to m i n i m i z e g r o u n d c o n t a c t p r e s s u r e s . In a d e e p s n o w p a c k , h e a v y t r a c k e d vehicles can b e c o m e i m m o b i l i z e d b e c a u s e of h a n g - u p failure which o c c u r s w h e n t h e s i n k a g e e x c e e d s the g r o u n d c l e a r a n c e o f the vehicle. T o p r o v i d e m o b i l i t y o f w h e e l e d vehicles, a r o a d must be c o n s t r u c t e d to s u p p o r t the high g r o u n d c o n t a c t p r e s s u r e s n o r m a l l y e x e r t e d by the wheels. W i t h the arrival of snow a n d c o l d w e a t h e r , the c o n s t r u c t i o n o f s n o w a n d ice r o a d s [1] p r o v i d e s the n e c e s s a r y trafficability for o n - r o a d vehicles. C o n s t r u c t i o n o f r o a d s r e q u i r e s special h e a v y m a c h i n e r y which first m u s t be a c q u i r e d a n d t r a n s p o r t e d to a r e m o t e site. T h e p r e c o n d i t i o n i n g o f n a t u r a l s n o w p a c k , on the o t h e r h a n d , p a r t i c u l a r l y d e e p snow, using c o m m o n l y a v a i l a b l e e q u i p m e n t m a y i m p r o v e t h e m o b i l i t y of o f f - r o a d t r a c k e d a n d w h e e l e d vehicles. In the p r e s e n t c o n t e x t , p r e c o n d i t i o n i n g t h e surface of d e e p snow entails the p r o d u c t i o n of a snow p a v e m e n t c a p a b l e o f s u p p o r t i n g v e h i c u l a r traffic.
*Geotechnical Research Centre, McGill University, Montreal, Canada H3A 2K6. tDefence Research Establishment Suffield, Ralston, Alberta, Canada TOJ 2NO. 83
84
P. B O O N S I N S U K et al.
On the premise that it is possible to improve deep snow trafficability for commercially available off-road vehicles through preconditioning, this study was aimed at developing techniques appropriate to field application. The objectives of the study were: (1) to investigate the effects of various snow preconditioning methods including the mixing of additive with the upper portion of the snowpack; (2) to determine the load carrying capacity of a preconditioned deep snowpack; and (3) to evaluate the trafficability of preconditioned deep snowpack. Both laboratory-scale and full-scale experiments were carried out to study various preconditioning techniques, the findings of which are addressed in this paper.
EXPERIMENTATION
The program of experimentation consisted of two parts, as follows:
Laboratory investigation This was established to explore alternative preconditioning techniques on a laboratory scale, to determine practical test procedures and to provide better control of test conditions. The parameters studied included: (a) (b) (c) (d)
surcharge pressures for compaction, 17 and 34 kPa; ageing after compaction, up to 8 days; application of heat; addition of sand or straw.
All laboratory tests were conducted using artificial snow which was prepared by pulverizing ice cubes into fine grains, similar to the technique previously used by Yong and Fukue [2]. Artificial snow was adopted because of the need of producing replicate snow samples for control testing..The grain size distribution of the artificial snow was not significantly different from certain types of natural snow [2, 3]. Similarly, the load-resisting behaviour of the artificial snow was not much different from that of natural snow [2]. Thus, the use of the artificial snow in the laboratory equipment provided the fundamental basis for planning field study. The artificial snow, which produced a typical distribution of particle size as shown in Fig. 1, was deposited loosely into a bin (1219 mm long by 102 mm wide by 1006 mm deep) having one transparent wall made of plexiglass, as illustrated in Fig. 2. The long length of the snow bin permitted to conduct several tests on precisely the same snow condition, thereby minimizing the effect of variation in snow properties. Tests performed included measurements of temperature and density, Rammsonde hardness, footing (plate) pressure and resistance to cone penetration. The load-carrying capacity of the artificial snow was measured by footing penetration which was carried out using a 74 mm by 99 mm rigid plate. The width of the footing (74 ram) was much smaller than the depth of the artificial snow (1006 mm) so as to satisfy the 'deep snow condition'. which required that pressure bulb development in the snow below a loading platform should not intersect the bottom rigid boundary. The development of a pressure bulb during footing penetration was visually observed by monitoring the deformation of a network of gridlines inscribed onto a vertical face of the artificial snow sample prior to testing.
PRECONDITIONING OF DEEP SNOWPACK
o,,
I00
--
5 0 mm from s u r f a c e zx160 mm from s u r f a c e o 360 mm from surface • 660 mm from surface
85
~
•
90-80--
~: 70-60-
4o~, n
30 20IC,0
02
I
03
o4o~
,
2
3
~
5
~ ~
Particle diameter (mm)
FIG. l.
Particle size distribution of artificial and natural snow.
~
1 1006 mm
~
~
G
r
~
d
Artificial
2 mm
s
snow
"~ PlexLgkass wall
j 1 2 1 9 J mm
FIG. 2. Test conducted in artificial snow container.
T h e test p r o g r a m and the test c o n d i t i o n s c o m p i l e d in Table with the f o l l o w i n g objectives:
1 w e r e drawn up
Test Nos. S1 to $3. T o evaluate the effects of ageing (10 min, 2 and 8 days, respectively) on 'undisturbed' artificial s n o w after its d e p o s i t i o n in the s n o w bin (Fig. 2).
P. B O O N S I N S U K et al.
86
TABLE 1. SUMMARYOF LABORATORYTEST CONDITIONS Test No.
S1 $2 $3 $4 $5 $6 $7 $8 $9 ttl H2 H3 H4 SD1 SD2 ST1 ST2
Preconditioning technique
undisturbed, no treatment
17 kPa surcharge
34 kPa surcharge
heating at surface, no compaction
mixed with sand and 17 kPa surcharge for compaction mixed with sand and 34 kPa surcharge for compaction mixed with straw and 17 kPa surcharge for compaction mixed with straw and 34 kPa surcharge for compaction
Air temperature* (°C)
Ageing
- 13 -13 -14 - 5
10 rain 2 days 8 days 10 min
-13 -13 -13 - 5 -13
2 days 8 days 10 rain 2 days 8 days
- 6 - 7.5 - 2 -13
10 rain 1 day 2 days 8 days
- 1
10 rain
- 1
10 rain
- 5.5
10 rain
- 5.5
10 rain
Remarks
reference
compaction by surcharging
* Snow temperature profiles are given in figures.
Test Nos. $4 and $9. To determine the effects of densifying the artificial snow by surcharge loadings and subsequent ageing. Test Nos. H1 to H4. subsequent ageing.
To investigate the influences of heat
1-m deep
application
and
Test Nos. SD1 and SD2. To evaluate the effects of adding sand to the top of the artificial snow, followed by compacting the snow with surcharge loadings. Test Nos. STI andST2. Similar to Test Nos. SD1 and SD2 but with straw in place of sand. Full-scale field in vestigation In order to implement the findings from laboratory experiments, a field trial with full size vehicles was conducted in a large area covered with a 700 to 800 m m depth of snow. For trials, a total of 21 road courses, each being about 200 m long, was prepared with the aid of a BV206 All Terrain Carrier. A designated n u m b e r of passes of the Carrier was followed by ageing the compacted snow, combined with additives (salt and straw) on certain courses or the application of heat, as listed in Table 2. The BV206 All Terrain Carrier is an articulated two-unit vehicle mounted with rubber tracks. The rear unit used in this study was loaded with sand bags to 8.9 kN which brought its ground contact pressure close to that of the front u n i t - - a p p r o x i m a t e l y 14 kPa. A low ground pressure vehicle of moderate weight and high mobility over deep snow, such as the BV206 Carrier, serves as a convenient device for preconditioning the
P R E C O N D I T I O N I N G OF DEEP SNOWPACK
87
e-
~
~
~
I ~
~
~"
I
e-,
"Z o 5 o
6
~
g 6
~
E
.. ~ .~
&o ~
," ~
~
,,¢
z
e~
I
,-¢
4-
+
'~"
~
ri
I~-
~-
I~
~
~
~
~
& ,,q.
,<
u~
,A~; > t~
. S o~
88
P. BOONS1NSUK et al.
snow for the benefit of less mobile (on-road) vehicles. U p o n completion of the test course construction, a four-passenger, four wheel drive vehicle (Iltis) was used as the test vehicle to evaluate the load-carrying capacity of the preconditioned snow in each course. The m a x i m u m ground contact pressure exerted by the Iltis was approximately 220 kPa using 6.50 R-16 tires. The n u m b e r of passes made by the Iltis on each course was monitored for strength and durability of a course, whenever more than 1 pass was possible. In addition, two other vehicles were used for testing, i.e. a 5/4 ton wheeled truck with a m a x i m u m ground contact pressure of 455 kPa and a tracked M 113 A r m o u r e d Personnel Carrier with ground contact pressure of 55 kPa.
L A B O R A T O R Y R ES U LTS
The ability of a deep snowpack to support vehicle loadings can be meaningfully measured by the footing penetration test (Fig. 2), while snow strength variation with depth can be effectively evaluated by the R a m m s o n d e hardness test [4]. The R a m m s o n d e device is a dynamic p e n e t r o m e t e r that measures snow resistance to the advancc of a cone driven by a drop hammer. The ram resistance or ram hardness n u m b e r is calculated as:
R -
Whn
+ W + Q
(1)
X
where
R W h n x
(2
= = = = = =
ram resistance, kgf, weight of drop h a m m e r , kgf, height of drop, cm, n u m b e r of h a m m e r blows, penetration after n blows, cm, weight of penetrometer, kgf.
Although both vane and cone devices were used (Fig. 2), the R a m m s o n d e p e n e t r o m e t e r was easier to operate in both laboratory and field, and yielded results that were easier to interpret. In this paper, the load-carrying capacity of the deep artificial snow will be expressed in terms of footing penetration resistance and ram hardness number. To compute the ram hardness n u m b e r at the surface (0-10 cm deep), the correction factors (4.7 for 0-5 cm and 1.6 for 5-10 cm) were included as suggested by Ueda et al. [4]. Artificial s n o w without preconditioning As a reference for comparison with preconditioned snow, the load-bearing capacity of the unpreconditioned artificial snow was evaluated by the footing penetration test at a rate of 15 mm/min (Fig. 2). The test was conducted after the artificial snow was deposited into a bin and allowed to age for 10 rain, 2 and 8 days, without further treatment. During ageing, the humidity of the walk-in cold room was kept continuously high by a humidifier. The density of the compacted snow was found to be quite uniform with depth, beginning at 0.40 Mg/m 3, when first deposited, to 0.50 Mg/m 3 at 2 and 8 days ageing. The density increase may be attributed to compaction under its own weight. Resistance to footing load on the unpreconditioned snow increased gradually as the footing penetrated into the snow (Fig. 3). The effect of ageing on thermally uniform snow
PRECONDITIONING OF DEEP SNOWPACK
Symbol Test No
24-0
89
Ageing
0
Sl
o
$2
0 rain 2 days
z~
S3
8 days
J
200 0_ v
160 2 o_
*g u2
120
80
40
i0
20
30
40
50
I
60
I
70
D,spLacement (ram)
FIG. 3. Results of footing penetration test in undisturbed unpreconditionedsnow. as tested was significant only beyond ageing periods of more than 2 days. In addition to intergranular bonding, resistance to footing penetration developed through densification of the snow underneath the footing. The affected snow mass or the pressure bulb thus formed extended vertically to about 4-5 times the footing width, i.e. about 300-350 mm below the 74 mm wide footing, as observed by the deformation pattern of the inscribed network of gridlines (Fig. 2). The lateral development of the pressure bulb extended slightly beyond the edge of the footing. Intergranular bonding was sufficient to prevent the snow from collapsing around the footing as it advanced into the snow, thus forming vertical walls at the edges of the footing. The rise in footing pressure (Fig. 3) with the advance of the footing plate into the snow may be explained by the snow densification and further development of the pressure bulb. The load-bearing capacity of the snow increased about 2-3 times when the ageing period was increased by 6-8 days. At the time of the footing penetration test, temperature within the snow varied slightly with depth. Among the three tests, the lowest temperature of-17°C was recorded in the 8-day aged snow, while that for the other two age conditions was -13°C. Precise control of snow temperature was limited by fluctuations in the ambient temperature of the walk-in cold room, particularly during periods of long ageing time.
Snow preconditioned by surcharge loading Natural deep snowpack can support only light vehicles which compact the snow as they progress; hence the idea of densifying deep snowpack by surface loadings from a grooming vehicle prior to passage by a test vehicle. A set of experiments consequently entailed the loading of the artificial laboratory snow (Fig. 2) uniformly over its entire surface (i.e. surcharging). Two surcharge pressures, 17 and 34 kPa, were used since they represented the low ground contact pressures of some off-road
90
P. BOONSINSUK et al.
vehicles. Surcharge loading was applied by gradually increasing the load acting on a rigid plate at the surface of the snow. When the applied loading reached the selected value, it was released, and the densified snow was left to age for 10 rain, 2 and 8 days prior to testing. The results of the footing penetration tests performed on the artificial snow preconditioned by surcharging and ageing (i.e. Test Nos. $4 to $9 in Table 1) are compared in Fig. 4(a) and (b). It is evident that the higher surcharge of 34 kPa led to higher load-bearing capacity (Fig. 4a). Ageing significantly increased the strength of the artificial snow densified by the lower surcharge of 17 kPa but only slightly improved that of the snow preconditioned at the higher surcharge of 34 kPa when the ageing period was greater than 2 days (Fig. 4b). For similar artificial snow, the increase in shear strength of the snow due to ageing was found to be more pronounced when the snow density was in the range of (t.40 and 0.55 Mg/m 3 [3]. This range is in agreement with the resulting densities produced by the lower surcharge of 17 kPa (Fig. 5). At a common footing displacement of I0 ram, surcharging strengthened the laboratory snow to the extent that it could resist a footing pressure of more than 500 kPa, as opposed to the footing pressure of 100 kPa in unpreconditioned snow (Fig. 3). Surcharging or surface compaction produces the densification necessary to form a road pavement within the snowpack. The density and temperature profiles of the artificial snow preconditioned by surcharging and ageing, presented in Fig. 5, indicate a relatively uniform distribution in both properties. In general, the density of the preconditioned snow ranged from 0.50 to 0.58 Mg/m ~, compared to the initial density of 0.40 Mg/m 3. The temperature profiles which were measured at the time of testing snow strength illustrate the equi-temperature nature of the snow used in laboratory experiments. Any effects of snow temperature on load-bearing capacity in these tests are, it is believed, overshadowed by the effects of surcharge level and ageing. For example, the snow preconditioned by 17 kPa surcharge without ageing (Test No. $4) offered the lowest resistance to footing penetration (Fig. 4a) despite having the lowest snow temperature (--14°C). However, it should be noted that fluctuations in snow temperature occurred during the ageing period due to interruptions in the walk-in cold room. Variations were as high as +5°C during the longest ageing period of 8 days. Possible effects of thermal history on footing resistance were not investigated. The ram hardness profiles depicted in Fig. 5 reveal the strength increase due to preconditioning by surcharging and ageing. All preconditioned snow samples possessed higher ram hardness than the unpreconditioned snow without ageing (R = 30 in Test No. SI). The ram hardness numbers were, in general, uniform with depth after passing through some variations in the top 300 mm. The highest ram hardness of more than 400 was attained in the snow subjected to the highest degree of preconditioning, i.e. using 34 kPa surcharge and 8-day ageing (Test No. $9). High surcharge loading and long ageing periods coincided evidently with high values of ram hardness. Variations in ram hardness number in the upper 31)0 mm of the snow were possibly a consequence of (1) non-uniformity of densification particularly in close proximity to the surcharge loading plate and (2) fluctuations of temperature and humidity in the cold room. Snow preconditioned by heat application The benefits of promoting stronger bonds between snow grains through heat application and refreezing followed by ageing were investigated. The surface of the
PRECONDITIONING OF DEEP SNOWPACK
,°-I!
91
90o i
/
800
700 o_
Symbol_ Test No [] o
600
500
i
S4 $5 S6 S7 S8 $9
• • •
o
SurchargekPa Ageing I0 min I 2 days 8 days I 0 min I 2 days I 8 da~/s I
17 17 ]7
34 34
34
aO0
300
200
tOO
I
i
20
I
30
I
40
I
50
]
60
70
DispLacement (ram)
Symbol I000
Surcharge Footing kPa displacement (mm)
o
17
5
• o
34-
5 tO I0
17 34-
•
v
800 O ~
&
600
~ ~
~°°// ~
400
ol
UI------
//f/
/
J
2
~
. . . . . . . . .
1
4
i
6
-o--
I
Ageing time (days)
FIG. 4(a).
Results of footing penetration test in snow preconditioned by surcharging. (b) Variation of footing pressure with ageing.
92
P. BOONSINSUK Density [ M g / m 3 } 04
08/-15
06
Temperature
(°C)
-I0
-5
et al.
Ram 0
IO0
hardness 200
number
300
400
'.tzj.
I00
200 300 E
--
4-00
d]
500
(
600 700
~] 0 o
SI $4 $5 $6 $7 $8 S9
* • •
800
0 17 17 17 34 34 34
I0 min IO min 2 days 8 days I0 min 2 days 8 days
Fl(;. 5. Profiles of density, temperature and ram hardness of snow preconditioned by surcharging.
artificial snow (Fig. 2) was heated by two 1200-watt hot-air blowers for 20 min. Prior to heating, dye had been sprayed on top of the snow surface so as to observe the extent of percolation by melting snow. Under the imposed conditions of heating, the melting snow as observed to percolate down to a depth of about 400 mm. Subsequent to heat application, the snow was aged for periods of 10 min, 1, 2 and 8 days before testing. The load-bearing capacity of the heated artificial snow (Fig. 6) was significantly increased when compared with the unpreconditioned snow (Fig. 3), even without appreciable ageing. Densification and stronger intergranular bonding were undoubtedly achieved by heating and refreezing of the melted snow. The minimum ageing period of
800J
Test
Ageing
,¢oo~ I
600I
&
Sym bat
/J
Ill
•
~I
G
, day
H3
2
days
ks_ 200
0
•
[
I0
1
20
", '
40
I
50
~L)
?C
D''Sptacement ( m m )
FJ~;. 6. Results of footing test in snow preconditioned by heating and ageing.
PRECONDITIONING OF DEEP SNOWPACK
93
one day considerably strengthened the snow, while longer ageing periods of 2 and 8 days produced only slight improvements in load-bearing capacity. The density profile shown in Fig. 7 indicated some increase in density caused by ageing. The ram hardness number of the top 300 mm, which was within the zone of percolation in melting snow, was much higher than those at depths below that zone. Not surprisingly, the higher ram number corresponded to the artificial snow sample aged one day at the lowest temperature. Resistance to footing penetration (Fig. 6) was, however, not affected by the variations in temperature and ram hardness. This can be explained by the fact that a substantially greater snowmass within the pressure bulb contributed to resisting the footing penetration in comparison to that resisting Rammsonde penetration which affected only a relatively small mass within close proximity to the 40 mm diameter cone of the Rammsonde penetrometer. The pressure bulb underneath the 74 mm wide footing, in general, extended to a depth of about 300 mm below the footing and coincided with the zone affected by heating.
Snow preconditioned by mbcing additives The anticipated benefits of mixing the surface layer of a snowpack with additives are densification, reinforcement of the snow structure, strengthening of the bonds of grains, or some combination of these. In practice, the additives chosen for mixing should be locally available and economical to acquire so as to minimize construction cost. Furthermore, the additives should not be difficult to handle in the field and should not require special heavy machinery for spraying and mixing with snow in an attempt to satisfy the requirements of preconditioning a deep snowpack in some remote area. Wuori [5] reported that additives such as direct heat, water and sawdust may be effective when dry processing and other compaction methods are inadequate. Two types of additive were used in the present laboratory experiments, silica sand (passing sieve no. 10) and straw. Sand would increase the density of a snowpack because of its higher specific gravity, while straw may be expected to improve the distribution of stresses within the snow mass through its tensile strength and interconnection between
Density 0
04
I00
(Mg/m
06
I
3)
08/
Temperature
15
-I0 [
/
(°C)
Ram 5O
-5 !
'1~
I
hardness I00
number 150
20O
I ~ i'l
I
2OO
3OO
E 4OO C:] 5OO
600 -
700 -
FKI. 7.
/
°\ Symbot Tes"c Ageing No
-
• a •
H I H2 H3 H4
I0 I 2 8
min day days days
Profiles of density, temperature and ram hardness of snow preconditioned by heating.
94
P. B O O N S I N S U K et al.
fibres. Each additive was sprayed manually on top of the artificial snow (Fig. 2) such that snow grains were visible among sand particles or straw fibres. A thin layer of fresh artificial snow was then sprinkled to partially cover the snow surface already mixed with additives, followed by spraying sand or straw. The process described was repeated until the top layer of snow mixed with sand or straw was about 250 mm thick. Surcharge loading (17 or 34 kPa) was then applied to compact the snow, followed by strength testing without appreciable ageing. The load-bearing capacity of the snow preconditioned by adding sand was improved when the surcharge loading was increased from 17 kPa to 34 kPa as depicted in Fig. 8. The increased resistance in the snow/sand mixture is believed to be due principally to an increase in density of the snow with little contribution, if any, from intergranular bonding. Sand acts primarily as a filler of voids and would tend to inhibit, not enhance, bonding. By contrast, the snow mixed with straw exhibited higher load-bearing capacity for the lower surcharge loading (17 kPa) preconditioning when the footing penetration was more than 22 mm. Such behaviour probably reflected the non-uniformity in straw fibre strength, straw distribution in snow mass and nonhomogeneity of the snow/straw mixture. Unlike sand, which is cohesionless, straw was expected to enhance the tensile strength of the snow and to distribute the footing load to the snow adjacent to the footing. However, the advantage offered by additives (i.e. sand and straw) was not clear when compared with the snow preconditioned by surcharging without additives (Fig. 4). At the lower surcharge loading of 17 kPa, the inclusion of straw led to the highest load-bearing capacity of the preconditioned snow (Figs 4 and 8). At the higher surcharge loading of 34 kPa, the highest load-bearing capacity was found in the snow preconditioned only by surcharging without additives. It is apparent that sand is not an efficient additive in snow because of its cohesionless nature. When straw is uniformly and homogeneously distributed in the snow mass, its reinforcing effects with surcharging may be expected. The density profiles presented in Fig. 9 indicate clearly the effects of surface surcharging since the density of the snow mixed with sand decreased from the surface
700
SymbotAddltweSurcharge(kPa) _
Goo"~ 5O0 I:1_ a,:
="
sand
,7
z~
Sand
34
•
Straw S
t
17
I
J ~
.
'
~
40o 590 45
8
u_
-"
2©0
-J,--
~00
, 0
L 20
3'0
i 40
DispLacement
FIG. 8.
i 50
I GO
i 70
80
(mm)
Results of footing test in snow preconditioned with additives.
P R E C O N D I T I O N I N G OF D E E P S N O W P A C K
Density(Mg/m3] 00~4 015 016 0[7 0 8 I00
Temperature(°C) Ii ~'/l!
95
Ram hardnessnumber ~*Ix1 5 I00 ~~"~ 0 150 ~
200
•
300~-.~ 400
/il.l [t /
600
700~
FK;. 9.
/\ i.I-l - - = ,
sD~O A~i~_veSurcharge(kPa,
[] a •
SD2
,7
Sand
STI Straw ST2 Straw
34
17 34
Profiles of density, temperature and ram hardness number in snow mixed with sand or straw.
downwards until it reached a relatively constant value beyond about 250 mm depth. The highest density of the snow mixed with sand was close to 0.80 Mg/m3 which was much higher than the 0.58 Mg/m 3 found in snow without additive (Fig. 5), due obviously to the higher specific gravity of sand grains. The density of the snow mixed with straw was not totally dependent on the surcharge loading as the density under the lower surcharge loading of 17 kPa could be higher than that produced by the higher surcharge loading of 34 kPa. As a result, the ram hardness of the former was higher than that of the latter, similar to the load-bearing behaviour (Fig. 8). The fact that temperature was lowest for the snow preconditioned with straw and 17 kPa surcharge (Fig. 9) may have contributed to its superior load-bearing capacity and ram hardness. It is evident that using the proper additive can indeed strengthen a deep snowpack. Straw has shown its promising potential although its fibre strength is highly variable and uniformity of distribution in snow cannot easily be achieved.
Summary of laboratory snow preconditioning results Based on the preceding results of deep snow preconditioning under laboratory conditions, the following observations were made: (a) It is possible to precondition a deep, soft snowpack such that it can support a contact pressure of about 150 kPa. Viable preconditioning techniques can be one of the following: compaction by light surface loadings (less than 35 kPa), heating and mixing snow with appropriate additives followed by compaction. Ageing will further enhance strengthening of the preconditioned snowpack. (b) The highest load-bearing capacity was achieved by either compaction with surcharge loading (34 kPa was used in this study) or heat application followed by at least 1 day of ageing. (c) Due to limitation of sample size and equipment in the walk-in cold room, the advantages of additives could not be established conclusively. Nevertheless, straw
96
P. B O O N S I N S U K et al.
appeared to be a promising reinforcing additive capable of increasing the tensile and shear strength of snow. (d) In these tests, the load-bearing capacity of the preconditioned snow was not significantly affected by temperature as long as it remained well below zero. (e) The ram hardness number was a good indicator for identifying the variation of snow strength with depth. When correlated with resistance to footing penetration, the ram hardness number indicates load-bearing capacity of the deep snow as illustrated in Fig. 10, which combines all information available from the tests listed in Table 1. The ram hardness number of each test was averaged from the data within the top 250 mm of the 1-m deep snow, while the resistance to footing penetration was taken at 5 mm footing displacement. Although the data shown in Fig. 10 are scattered, a higher ram hardness number normally indicates a higher load-bearing capacity. It is noticeable that a footing pressure of 500 kPa or above was achieved by the heated snow (Test Nos. H2, H3 and H4) and the 34 kPa surcharged snow (Test Nos. $8 and $9). Ice might develop during refreezing of the heated snow and stronger bonds might be generated because of the high humidity of the walk-in cold room during ageing. Such conditions might not be representative of actual field situations. The 'artificial snow' line is therefore considered to be a better indicator of the load-carrying behaviour of the artificial snow tested under laboratory conditions. 8 0 0 --
700
0 H4
o $9
600
c
OH2
H3
500
Z
0
$8
0
E E Lo
400
• MIJttl - " " [:)asses / "ot wheeled" / vehicles / /possibLe /
o $6
0
Laboratory result (with Test No.)
g 3oo / OS5 200
/ ~/57
/
//
B FieLd
/ / /
result
OST2
/ J/
Artificial
snow
/ HI I00 --
SD2 //o o o
/
/ /
~/
°ST I o SDI
1 0
$4
/
I
l
I
J
t
I
50
iO 0
150
200
250
500
Ram
FIG. lO.
hardness
number
Relationship between resistance to footing penetration and ram hardness number.
PRECONDITIONING OF DEEP SNOWPACK
97
Using the two-dimensional test arrangement as displayed in Fig. 2 and a footing size of 74 mm by 99 mm, the development of a pressure bulb visually observed from the deformation pattern of the grid lines inscribed onto the snow surface was, in general, limited to within 4-5 times the footing width (74 mm) below the footing. The lateral extent of the pressure bulb was slightly beyond the edge of the footing. Such an observation was made at the end of the test, thus the footing pressure was relatively high. The 'deep' snow conditions imposed in this study were satisfied since pressure bulbs did not touch the bottom rigid boundary. The pressure bulb itself may be considered as an important element in preconditioning a snow pavement on which vehicles would, in a field situation, be operated.
FIELD TEST RESULTS To apply the knowledge gained from the laboratory results, a field trial for preconditioning deep snow was carried out in February 1987 a few kilometres north of Quebec City. The snowpack, on a level 1 km 2 area of test range, was 660-800 mm deep with a highly stratified structure. Ice crusts were present, varying in thickness from 10 mm near the surface to 70 mm at the base. Snow had accumulated over the previous 3 months during which changes in air temperatures and precipitation produced alternate conditions of isothermal and temperature gradient metamorphism. In the 3 week period of field testing, the air temperature fluctuated from 0.6°C to -27.3°C as listed in Table 2, while no new snow precipitation was recorded. Wind was occasionally strong resulting in moguls and wind slabs. It was in general observed that the nonhomogeneous natural snow differed from the uniformity of laboratory (artificial) snow in such physical properties as structure, density, temperature and grain size profiles with depth. The density of natural snow ranged between 0.2 and 0.4 Mg/m 3, i.e. lighter than laboratory snow. Thermal conditions progressed from temperature gradient to isothermal conditions during trials. The grain size of natural snow ranged predominantly between 1 and 2 mm, i.e. coarser than the artificial snow whose grains were largely between 0.2 and 1 mm. Taking into account such differences, projections or predictions about the behaviour of natural preconditioned snow based on results of laboratory snow were intended to serve only as a guide to field experiments. Based on the laboratory findings that deep snowpack can be effectively preconditioned by low surcharge loadings, the BV206 Carrier with 14 kPa ground contact pressure was used as the principal preconditioning equipment in the field. On its first pass over hitherto undisturbed snowpack, the sinkage of the BV206 Carrier was about 150-200 mm, leaving in its wake a track footprint. At least 3 passes along the same course were necessary to produce a uniform swath of compacted snow. The BV206 Carrier could easily travel over the undisturbed snowpack, while wheeled vehicles (i.e. Iltis and 5/4 ton truck) could not float, despite the presence of underlying ice lenses. All test courses listed in Table 2 were essentially prepared by the BV206 Carrier under a variety of artificially imposed conditions. The conditions of test course preparation were determined by the use of mixing additives such as salt and straw, as well as heat application. Sand as an additive was eliminated from further consideration based on laboratory results. Salt, on the other hand, was thought to be a potential substitute for direct heat application. Road salt and straw were manually deposited on the snowpack from the BV206 Carrier after which at least one further pass was made for mixing the salt and/or straw with the
98
P. BOONS1NSUK et
al.
snow matrix. A H e r m a n - N e l s o n (BT-400 Series Type H I ) heater was mounted in the rear unit of the BV206 Carrier, and two hoses were used to apply hot air from the heater to the snow with or without mixing additives. The heater was set at a heat output of about 2 x 105J at an air delivery rate of 0.23 to 0.46 m-Vs and a discharge air t e m p e r a t u r e varying from 65°C to 138°C. For the 200 m long course, the period of heat application was almost one hour. The course was then allowed to age before vehicular testing. Using the snow preconditioning techniques described, the resulting snow road surface was not smooth but conformed to the track footprint of the BV206 Carrier. In addition, the road surface was not level on account of numerous soft spots. Although snow from an adjacent area could be deposited on the road to make the surface level, it would require the use of additional heavy equipment which would deny the purpose of snow preconditioning in remote regions with minimum equipment. Thus test courses were treated as simulating real, uneven terrain. After preconditioning the natural snowpack, the 200 m test courses were travelled by one of the two wheeled vehicles, i.e. the Iltis with 220 kPa ground contact pressure and the 5/4 ton truck with 455 kPa ground contact pressure. The n u m b e r of passes made by the two wheeled vehicles are listed in Table 2, together with the ram hardness numbers averaged from the values measured within the top 300 m m of the preconditioned snowpack. The value of 300 m m was chosen to represent the extent of pressure bulb development since ram hardness numbers were normally higher in the top 300 mm of the snowpack than below this level. From the field trial tests, the following observations were made: (1) The natural undisturbed snowpack was unable to support any wheeled vehicles having a ground contact pressure of 220 kPa (Iltis) or more. The density of the 600-800 mm thick natural snowpack varied from 0.21 Mg/m 3 near the surface to 0.45 Mg/m s near the base, while the density of the artificial snow used in the laboratory study ranged from 0.40 Mg/m 3 to 0.50 Mg/m s. The ram hardness n u m b e r of the natural snow measured about 20 below a depth of 200 m m and was undetectable above the depth. For the artificial snow, the ram hardness number was approximately 30, capable of supporting a footing pressure up to 50 kPa at about 40 m m sinkage. In comparison, the natural snowpack reinforced by several underlying ice crusts was capable of supporting the BV206 Carrier with 14 kPa ground contact pressure at about 150-200 m m sinkage and the M l 1 3 Carrier with 55 kPa ground contact pressure at about 300 m m sinkage. (2) Preconditioning the natural snowpack by a few passes of the BV206 Carrier and ageing for 3 to 4 days could stiffen the snowpack such that it was able to support up to 8 passes of the wheeled vehicle Iltis having a ground contact pressure of 220 kPa and 3 passes of the 5/4 ton truck with 455 kPa ground contact pressure. The density of the snowpack was increased from about 0.25 Mg/m s in the top 300 mm of the natural snowpack to about 0.44 Mg/m 3 after multiple passes of the BV206 Carrier and ageing. The ram hardness n u m b e r of the preconditioned snowpack capable of supporting these vehicular loadings varied between 150 and 200. For comparison, a ram hardness number of 150 would support 1 pass of a tire inflated to 690 kPa (100 psi) carrying 98 kN (10,000 lb) wheel load, and a ram hardness n u m b e r of 200 would support 2 passes of the same tire (Wuori, 1963). The wheel load of the Iltis was 10.8 kN (1100 lb), using a tire inflation pressure of 269 kPa (39 psi), while the wheel load of the 5/4 ton truck was 15.2 kN (1550 lb) with a tire inflation pressure of 414 kPa (60 psi). Thus the results obtained in this study are comparable with those given by Wuori [5].
PRECONDITIONING OF DEEP SNOWPACK
99
(3) Using salt and straw as additives to the natural snow, the test courses preconditioned with these additives significantly outperformed those preconditioned without any additives. The Iltis succeeded in negotiating a 6 ° slope in Course No. 11 (Table 2) mixed with straw. The combining of straw and salt, followed by mixing and compacting by the BV206 Carrier, produced a test course which was strong enough to support 10 passes of the Iltis with 3 passengers (Course No. 12) even at melting ambient temperatures. Such performance contrasted vividly with that on snow preconditioned without straw wherein excavation-sinkage quickly immobilized the vehicle. Longer ageing periods should be at sufficiently cold temperatures. Road salt in particular required several days to extensively melt snow and permit re-freezing. Uniform distribution of additives could be achieved by using appropriate light equipment in lieu of manual operation. (4) Heat application to the natural snowpack has also proved to be a viable preconditioning technique since one of the heated courses (No. 9) could support up to 10 passes of the Iltis. The corresponding ram hardness number was in the 120-170 range. When compared with the other snowpacks preconditioned only by the BV206 Carrier passage followed by ageing (Course Nos. 2-4), the benefit of heating was, however, not significant. (5) It is characteristic of a wheeled vehicle to travel within its own rut after the first pass. Thus in multipass testing, wheel sinkages increased due to excavation slip until soft underlying snow was reached, consequently immobilizing the vehicle. The BV206 Carrier may be used to resurface the damaged snow surface so as to restore the course for further passes subsequent to age hardening. (6) The ram hardness numbers of the preconditioned snowpacks in the field were generally higher than those obtained in the laboratory, possibly due to the difference in snow condition, presence of ice crusts, thermal profile and preconditioning technique. Nevertheless, the relationship between the load-bearing capacity and ram hardness number of the natural snowpack is comparable to that observed in the laboratory as exhibited in Fig. 10. In the field a preconditioned snowpack was capable of supporting a few passes of a wheeled vehicle having a ground contact pressure of up to 455 kPa when its ram hardness number was between 150 and 200. In the laboratory where the artificial snow was tested in two-dimensional boundary conditions, the preconditioned snow was able to support a footing pressure of more than 250 kPa when its ram hardness number was 100 or more. The ram hardness number of the field snowpack lay close to the range observed in the laboratory snow (Fig. 10). As a comparison it may be noted that Adam [6] reports a ram hardness peak near 250 when ageing a compacted snow road for about 42 hours at air temperatures above -20°C. (7) The preconditioned snowpack in the field provided improved flotation for the tracked Ml13 Carrier with a ground contact pressure of 55 kPa. In the natural snowpack without any preconditioning, the sinkage made by the first pass of the Ml13 Carrier was approximately 250-300 mm. The sinkage was reduced to about 100-150 mm in the Ml13 Carrier's first pass over a preconditioned snowpack. Due to its aggressive track configuration, each pass of the Ml13 Carrier easily damaged the preconditioned snowpack, however hard the crust. Nevertheless, preconditioning a snowpack can improve mobility of the tracked Ml13 Carrier by reducing sinkage and increasing speed. (8) By visual inspection of a snow pit cross section of a preconditioned snowpack, the BV206 Carrier loadings could not break the ice lenses located in close proximity to the ground. The 'deep' snow condition which required that the extent of the pressure
I00
P. BOONSINSUK et al.
bulb should not reach the bottom rigid boundary (i.e. the frozen ground) was therefore satisfied. Note that in practice, the 'deep' snow condition refers specifically to a snowpack of sufficient depth that pressure induced densification at its surface is entirely independent of the underlying bottom rigid boundary. Such preconditioning treatment contrasts with conventional methods of road construction where a strong substrate may be built up from the base in dense layers.
CONCLUSIONS
From the results obtained in both laboratory and field investigation on snow preconditioning, the following conclusions can be drawn: (1) A soft deep snowback in a remote area can be preconditioned by using commercially available equipment such that it can support light wheeled vehicles. (2) The simplest preconditioning technique is to disaggregate and densify a natural snowpack by a light tracked vehicle (about 14 kPa ground contact pressure) making a few passes over the snowpack, followed by at least 1 day ageing. Additives such as road salt and straw can be mixed with the snow by the tracked vehicle in order to enhance strengthening of the snowpack. At ambient temperatures between -20°C and 0°C, salt would produce free water which would form a hard surface crust when re-frozen at very low ambient temperatures, while straw would reinforce the snow surface against tensile and shear failure. Straw also delays the effects of excavation sinkage. (3) Although the surface of the snow road built by the tracked vehicle is not smooth, it can provide sufficient bearing strength for a wheeled vehicle and permit mobility over deep snow. Ride comfort and speed may have to be sacrificed as would be expected in off-road travelling. (4) Despite differences between the laboratory artificial snow and the natural snow studied, both laboratory and field trial results exhibit similar load-carrying behaviour. The laboratory tests can be reproduced in the field with comparable results. Laboratory simulation of snow preconditioning may conveniently be used as a first step prior to full scale field implementation in order to develop control techniques at minimum cost. Acknowledgement--The authors wish to express their appreciation to the Reviewers for their input. The field portion of this study was performed at CFB Valcartier and Defence Research Establishment Valcartier, Quebec. The cooperation of all participating personnel from both establishments is gratefully acknowledged. The services of Mr J. J. Fitzgerald of DRES for snow classification studies are also acknowledged.
REFERENCES [1] [2] [3] [4] [5] [6]
K. M. ADAM, R. F. PIOTROWSKI, J. M. COLLINS and B. T. SILVER, Snow roads for pipeline installation on the Arctic pilot project, Proc. Third Int. Speciality Conf., Cold Regions Engineering Northern Resources Development, Edmonton, Alberta, Vol. II, pp. 603-618, 1984. R. N. YONG and M. FUKUE, Performance of snow in confined compression, J. Terramechanics 14, (2), 59-82. R . N . YONG and I. METAXAS, Influence of age-hardening and strain-rate on confined compression and shear behaviour of snow, J. Terramechanics 22 (1), 37-49. J. UEDA, P. SELLMANN and G. ABELE, USA CRREL snow and ice testing equipment, Special Report 146, Cold Regions Research and Engineering Laboratory, New Hampshire (1975). A. F. WUOR1, Snow stabilization studies, in Proc. Conf. Ice and Snow." Properties, Processes and Application, (Edited by W. D. Kingery), pp. 438-458, M.I.T. Press, Cambridge, Massachusetts (1963). K. M. ADAM, Report on Norman Wells winter road research study, Winnipeg Interdisciplinary Systems Ltd., Winnipeg, Manitoba (1978).