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Measuring vehicle impacts on snow roads
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Journal of Terramechanics Journal of Terramechanics 50 (2013) 63–71 www.elsevier.com/locate/jterra
Measuring vehicle impacts on snow roads S. Shoop a,⇑, M. Knuth a, W. Wieder b a
Cold Regions Research and Engineering Laboratory, US Army, Engineer Research and Development Center, 72 Lyme Rd., Hanover, NH 03755-1290, United States b Science and Technology Corporation, 21 Enterprise Parkway, Suite 150, Hampton, VA 23666-6413, United States Available online 16 March 2013
Abstract The snow roads at McMurdo Station are the primary transport corridors to move personnel and material from the airfields servicing intra- and inter-continental air traffic for resupply. Thus, they are a critical transportation component and are particularly susceptible to deterioration during the warmest parts of the austral summer when above-freezing temperatures can occur for several days at a time. This study served to explore methodology that could quantify the impact of various vehicles, tires, driving speeds and maneuvers on the snow road conditions. Basic maneuvers were used to isolate the impact of turning, acceleration, braking and speed using spirals, circles, and straight-line testing on a flat, smooth snow pavement. In addition, a road course was set up to include corners and roughness using portions of the active snow roads for more realistic conditions. Measurements included snow surface strength both in and between tire tracks, tire track rut depth and width, and the height and width of the resulting snow piles adjacent to the tire tracks. Results indicate the impacts of driving speed and vehicle type including the importance of the tire and suspension components for preserving the road surfaces through the melt season. Since this type of testing had not been done on snow before, or using these vehicle types, the experiments yielded valuable guidance regarding what types of maneuvers, test surfaces, and measurements could most easily differentiate performance. Published by Elsevier Ltd. on behalf of ISTVS. Keywords: Strength; Disturbance; Rut; Rammsonde; Clegg; Maneuver; Turning; Corner; Speed
1. Background Transportation to and from McMurdo Station, Antarctica, is serviced by approximately 32 km (20 mi) of snow roads connecting the station and its airfields. In addition, the 1000-foot- (ft-) (305-m-) diameter Long Duration Balloon (LDB) Pad, serves as a large “paved” snow area for the launching of large, instrumented monitoring balloons. The construction and maintenance of these snow roads and the LDB Pad require approximately 4000 operator and equipment hours from September to February annually. These efforts are performed in a manner that relies heavily on the expertise of the operators on site with no specific prescription for snow road construction and maintenance currently in place. ⇑ Corresponding author. Tel.: +1 603 646 4321; fax: +1 603 646 4280.
E-mail address: [email protected] (S. Shoop). 0022-4898/$36.00 Published by Elsevier Ltd. on behalf of ISTVS. http://dx.doi.org/10.1016/j.jterra.2013.01.004
McMurdo snow roads are also subject to warm summer temperatures, which can severely reduce the road strength compounding any snow road construction or maintenance challenges, and making the snow surfaces more susceptible to failure. Consequently, in some years the snow roads fully support wheeled traffic for the entire summer season and in other years they cannot. In the worst case, nearly all transport of personnel and supplies to and from aircraft servicing McMurdo must be via a few specialized oversnow vehicles traveling at slow speeds. The Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH previously studied the processes used to prepare and maintain the McMurdo Station snow roads. Researchers witnessed activities in the December 2002–January 2003 time frame, and monitored existing snow road strength, maintenance, and vehicle fleet operations. The report documenting this work, ERDC/CRREL TR 10–5 Snow Roads at McMurdo Station, Antarctica [1],
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also contains a literature review of snow road construction methods, background on snow compaction and age-hardening, and a summary of McMurdo’s historic snow road construction and maintenance guidelines developed by the US Navy. That report concluded with recommendations to develop a modern snow road construction and maintenance program (also further developed in Shoop et al. [2]). Based on these studies, a series of tests were proposed to develop methodology to evaluate the impact of different types of vehicles on the snow roads under various weather and road conditions. Since this type of testing had not been done on snow or using these types of vehicles, similar studies of military vehicle impacts on Army training lands [3–8] and evaluating low impact military tires [12] provided guidance for test procedures. This report documents these vehicle-snow impact experiments. 2. Testing program Three basic types of tests were performed with four vehicles: spiral or circle test patterns to investigate the effect of turning; straight line constant speed, acceleration, and deceleration; and road course tests, which allowed both turns and speed variation. The spiral, circle and straight-line tests were performed on a flat, smooth area of prepared snow previously used as the LDB launch pad. Conversely, the road course was located on sections of existing snow roads and comprised a variety of curves and surface roughness, and was therefore more characteristic of the actual road conditions. Initial strength measurements were taken to characterize the test sites. After the vehicle trafficking tests, measurements included snow surface strength both in and between tire tracks, tire track rut depth and width, and the height and width of the resulting snow piles adjacent to the tire tracks. 3. Snow strength and impact measurements Two instruments were used to characterize the test sites and to monitor the snow strength changes during the vehicle impact testing: the Rammsonde Snow Penetrometer was used to measure a strength profile, and the Clegg Impact Hammer (Clegg) was used to measure the surface strength. The geometric impact on the surface was quantified in terms of rut depth and width and pile height and width. 3.1. Rammsonde snow penetrometer The Rammsonde was adapted by the US Army and others from an instrument originally used in the Swiss Alps. It has found extensive application for estimating avalanche danger and for determining allowable wheel loads on artificially compacted snow pavements. The device is a cone penetrometer consisting of a shaft with a 60° conical tip, a guide rod, and a drop hammer. For snow roads, the penetration cone has a diameter of 2.4 cm (0.94 in.) and height of 3.9 cm (1.54 in.); the total length of the penetrometer cone (to the beginning of the shaft) is 5 cm (1.97 in.). This
cone is smaller than what is used on natural, unprocessed snow and is more suitable for snow strength in the range seen on snow roads. It is also easier to use (both insert and remove) on compacted snow surfaces. The penetration force is obtained using a slide hammer of specific weight dropped from a measured height. The Rammsonde hardness number R is an index which indicates the snow resistance (in kilograms force, kgf) to the vertical penetration. The hardness reading at any depth represents the mean hardness through that depth. To characterize a given area a minimum of three Rammsonde profiles are collected and the results are averaged. 3.2. Clegg impact hammer Snow surface strength was measured using a Clegg Impact Hammer (Clegg). The Clegg consists of a cylindrical mass hammer that is dropped within a guide tube from a set height. The standard Clegg uses a 4.5 kg (9.9 lb) hammer mass. Two other hammer weights, the medium Clegg at 2.25 kg (5.0 lb) and the small Clegg at 0.5 kg (1.1 lb), were used in this testing program as it was determined that in many cases the 4.5-kg (9.9-lb) hammer was too heavy and the medium (2.5 kg) Clegg was determined to be the most suitable for characterizing the snow road strength. All of the Clegg hammers have the same diameter, 4.76 cm (1–7/8 in.). The Clegg is equipped with an accelerometer that measures the peak deceleration on impact. For the snow roads, the hammer is dropped five times at each location and the readings for each drop are recorded as Clegg Impact Value (CIV). Although the fourth drop CIV reading is usually used for soil strength calculations, an average of the third, fourth and fifth drop values were used in this case for the snow strength. In addition, the first drop value was investigated as an indicator of untouched or untamped snow surface strength. For each location, three separate tests are repeated within a 2-m (6.6-ft) circle and the average of these is reported. The Rammsonde and the Clegg are shown in Fig. 1. For additional details regarding the use of the strength instruments see Shoop et al. [1]. A more detailed analysis of the use of the Clegg on snow is given in Shoop et al. [10]. 3.3. Tire track rut and pile measurements Using the undisturbed snow as the datum, the width and depth of the ruts from each tire track, left and right, were measured with a straight edge cross piece and a meter stick or steel tape. The adjacent pile heights and widths were measured with a similar procedure (Fig. 2). These types of measurements duplicate those used for soil surface such as described in Haugen [8]. 4. Vehicles The four vehicles used during the vehicle impact testing are listed in Table 1 and shown in Fig. 3. Two of the
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warm weather when their high flotation tires allow greater over-snow mobility. 5. Test sites
Fig. 1. Rammsonde hardness test (top) and Clegg impact hammer test (bottom).
The LDB Pad is a 1000-ft- (305-m-) diameter pad used to launch high-atmosphere, long-duration balloons with various atmosphere and space payloads. The LDB Pad is maintained continuously throughout the season in order to provide a very smooth and stable platform for the heavy equipment used during balloon launches. Once the balloon experiments were completed for the season, the LDB Pad provided a smooth, uniform surface on which to perform comparative vehicle impact tests. Tests performed on the LDB Pad include the spiral and circle tests with all four vehicles, and the acceleration, deceleration, constant speed tests with the Fleet Operations Truck and the vans. Both of the large vehicles, the Terra Bus and the Delta, had difficulties performing the spiral maneuver with tight turning radii, so the spiral test was replaced by a breakout circle test for these two vehicles. Fig. 4 shows a plan map of the snow roads on the McMurdo Ice Shelf. In addition to the scripted tests, and to obtain information during a more realistic operation scenario, a small road course was developed at the junction of Williams and Pegasus cut-off Roads (the circled area in Fig. 4 map). The Terra Bus and Delta were used on the road course. The bottom portion of the figure is a high-resolution satellite photo showing the LDB Pad (the smooth snow circle on the right), and the triangular road course (shown to the left). All of the test courses had a smooth and level surface, although one side of the road course triangle was cut through sastrugi (snow ridges formed by wind) as can be seen on the imagery in Fig. 4 (and later generated “Bumps” as noted on Fig. 9). 6. Vehicle impact test maneuvers
Fig. 2. Measuring rut depth.
vehicles were tested with two different tires. When the temperatures are cooler and the roads are hard, the vans are the fastest and most comfortable ride to and from the airfields. The Terra Bus and Delta can handle many more people and cargo and are the primary people movers for the snow road transportation system, especially during
Spiral testing involved driving either the Fleet Operations Truck or one of the two vans in an ever-decreasing spiral starting at a given outside radius of 30.5 m (100 ft). The outside of the spiral was first measured and marked on the LDB Pad surface to serve as a guide for the driver. Every attempt was made to maintain a constant speed and produce a regular spiral. Measurements of the tire track rut depth and width, snow pile height and width, and the surface strength in and between the ruts were taken at points where the spiral crossed the radius to the spiral [9]. The distance to the left and right tire track, from the center point of the spiral, was measured at these points. For the Terra Bus and the Delta it was impossible to drive in a spiral of such a small radius and therefore larger circles on the outside of the spiral were used. These circles had diameters of 152 m (500 ft) for the Terra Bus, and 91 m (300 ft) for the Delta. The circles were marked to provide guidance to the driver. Testing proceeded until sliding was initiated.
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Table 1 Test vehicles. Vehicle
Tires
Weight kN (lbs)
Fleet Operations (Ops) Truck, Ford F350 (Old Tires) Fleet Operations Truck, Ford F350 (New Tires) Ford E350 Van 206 (old tires) Ford E350 Van 213 (new tires) Foremost Terra Bus “Ivan” Foremost Delta “Gale”
Denman Ground Hwy II steel belted radial, 36 14.5 R16.5LT Interco TRXUS STS 36 14.50 R16.5LT Cepek tires 17/40–16.5 LT Interco TRXUS M/T 38.5 14.5 R17LT Terra-Tire: Tubeless Nylon 66 44.00–25NHS Terra-Tire: Tubeless Nylon 66 44.00–25 NHS
27.9 (6262) 27.9 (6262) 41.4 (9300) 41.4 (9300) 298.0 (67,000) 186.8 (42,000)
Ford F350 Fleet Operations Truck
Ford E350 Van (typical)
Foremost Terra Bus "Ivan"
Foremost Delta
Fig. 3. Vehicles used in test program.
Using the light vehicles, acceleration/constant speed/ deceleration tests took place across a diameter bisecting the spiral. The vehicles accelerated to a specified speed, held a constant speed for approximately 61 m (200 ft), and then decelerated to a stop. Tire tracks were measured for rut depth and width, and the width and height of the snow piles accumulated next to the tire track. For a more operationally relevant test, the Terra Bus and Delta were also driven in a road course test. The road course tests utilized a short section of the snow road to the LDB Pad along with an unused section of snow road that resulted from re-routing the roads earlier in the season. Although the scope of the test program was limited by the availability of vehicles, personnel and uniformly prepared surfaces to use for comparison, due to daily station operations which have priority, a substantial number of tests and measurements were accomplished. A summary of all of the test types and combinations evaluated is given in Table 2. Keeping in mind that our goal was to determine
which type of testing and measurements would yield discriminating data, the results from each type of test maneuver are described below. Complete results are given in Shoop et al. [9]. 7. Impact analysis and results The data was analyzed to determine if we could quantify changes in snow strength or snow surface rutting based on the different vehicles, tires and maneuver conditions. The snow strength was compared in and out of the ruts at different locations along the maneuver. The impact was quantified in terms of rut depth (RD) and width (RW), and for the piles alongside the rut, the pile height (PH) and width (PW). From these measurements, a total impact can be calculated that represents the cross sectional area of all of the snow displaced by the vehicle (rut area plus pile area). The Total Vehicle Impact (TVI) value, for both the left (L) and right (R) vehicle tracks, is given by following equation:
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Fig. 4. Map of the snow roads and airfields on the Ice Shelf near McMurdo Station with test area noted in the circle (top); and a satellite image of the LDB Pad (circle) and road test course (triangular) (bottom). (29 October 2009 WorldView Satellite at 0.5 m resolution).
Total Vehicle Impact ðTVIÞ ¼ RDLþR RWLþR þ 1=2ðPHR PWR Þ þ 1=2ðPHL PWL Þ ð1Þ For soils, a predictive mathematical relationship between soil strength (using cone index values as a strength indicator) and rutting during a spiral maneuver with a varying turning radius is presented by Liu et al. [11]. No equation has yet been developed for vehicle rutting on snow, although perhaps our recent work with the Clegg as a method to assess the strength measurement could provide a suitable basis for future analytical development.
at speeds that were above 19 kph until the inner spiral, which was driven at 16 kph. Fig. 5 (top) shows the tire track rut depth decreasing with distance from the center, which could also be stated as rut depth increases as turning radius decreases. Pile heights and widths show similar trends (piles are higher and wider at smaller turning radius). The calculated TVI, a combined measure of rut and pile dimensions, is shown in the bottom of Fig. 5. This exponential decline in impact with distance is similar to findings from tests with military vehicles on training lands (Ayers et al. [7], and Affleck et al. [3]). 7.2. Circle tests C1, C2, and C5
7.1. Spiral tests Spiral test A1 completed four crossings across the radius using the Fleet Operations Truck with older Denman tires. The furthest set of tracks measured were 25.9 and 26.8 m (82 and 88 ft) from the spiral center point, left and right tires, respectively. The Fleet Operations Truck was driven
Circle test C1 was performed with the Terra Bus on the LDB Pad. The driver attempted to hold the vehicle at a constant radius on the marked circle, driving clockwise in fifth gear. Six circles were completed; however it was very hard to hold a true circle with the Terra Bus. Medium Clegg and rut and pile measurements were taken.
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Fig. 6. Clegg snow surface strength measurements for clockwise circle tests C1 and C5 with the Terra Bus and Delta, respectively.
45 40
Acceleration to 32 kph
35
Acceleration to 40 kph Constant speed 32 kph
cm
30
Constant speed 40 kph
25 20 15
Fig. 5. Rut and vehicle impact measurements from spiral maneuver A1 using the Fleet Operations Truck (older Denman tires).
Circle test C5 used the Delta personnel carrier driving in clockwise circles with a 91 m (300 ft) radius. The ruts left in this test were observed to be, subjectively, “very soft” (lower surface strength). Fig. 6 presents the medium Clegg (2.25 kg [5.0 lb]) data from tests C1 and C5 but there is no consistent trend in the ruts being softer (from more shearing) or harder (from compaction) than the surrounding snow, or from one side to the other of the vehicle. 7.3. Straight line tests (acceleration/constant speed/ deceleration) The acceleration/constant speed/deceleration tests took place on the LDB Pad. The vehicles were driven in a straight line bisecting the spirals and circles. The Fleet Operations Truck (old tires), Van 213 (new tires), Van 206 (old tires), and Fleet Operations Truck (new tires) were used for the straight line testing. For the tests done at two different speeds using the Fleet Operations Truck (Fig. 7), the imprint from the 32-kph imprint test was clearer with tread patterns discernible for most of the tests. At 40 kph, the tire track was deeper and the print was obscured by the tire shearing the surface of the snow. The 40-kph test consistently shows higher impact (higher rut depth, rut width, pile width) than the
10 5 0 Rut Depth
Rut Width
Pile Height
Pile Width
Measurement
Fig. 7. Rut and pile measurements for test B5, Fleet Operations Truck (new tires) straight line acceleration/constant speed/deceleration test.
32-kph test for acceleration, but this trend is not consistent for the constant speed portion of the test. A comparison between the two different tires used on the vans, and the new tires on the Fleet Operations Truck can be seen in Fig. 8. The van with the old tires had the highest or tied for highest impact in all categories, confirming the decision to upgrade the tires on the van fleet to lower impact tires. The difference between the Fleet Operations Truck and the van with the older tires is not as clear. Observationally, we noted that Van 213 with new tires bounced more, which could be a problem with creating impact on the roads during a more operationally relevant test; and the Fleet Operations Truck created some small washboarding. By visual inspection, Van 213 with new tires did slightly less damage to the snow surface during its acceleration test and also made clear track prints, while Van 206 (old tires) threw more snow and the tires sheared the surface, obliterating the tread print. The Clegg data shows lower snow strength in the ruts in most cases, but is not consistent.
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Table 2 Snow road vehicle impact testing program, December 2009. Date
Test
Vehicle
Field test #
Location
Data
16 December
Spiral counterclockwise, 24 kph
A1
LDB Pad
16 December
Circle counterclockwise, 37 kph, radius = 30.5 m Straight Line Acceleration/Constant Speed/Deceleration 40–43 kph Spiral Counterclockwise, 24 kph
A2
LDB Pad
A3
LDB Pad
Small Clegg, rut width and depth, pile width and height Small Clegg, rut width and depth, pile width and height Small Clegg
A4
LDB Pad
No measurable ruts
A5
LDB Pad
No measurable ruts
A6
LDB Pad
Small Clegg
17 December
Straight line acceleration/constant speed/deceleration 40 kph Straight line acceleration/constant speed/deceleration 32 kph Spiral Counterclockwise, 24 kph
Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Van 206 (Cepek tires)
B1
LDB Pad
17 December
Spiral counterclockwise, 24 kph
Van 213 (TRXUS tires)
B2
LDB Pad
17 December
Van 213 (TRXUS tires)
B3
LDB Pad
Van 206 (Cepek tires)
B4
LDB Pad
Fleet Operations Truck (TRXUS tires)
B5
LDB Pad
20 December 20 December 20 December
Straight line constant speed 40 kph, 6 passes Straight line constant speed 40 kph, 6 passes Straight line acceleration/constant speed: 32 kph acceleration, 40 kph acceleration, 32 kph constant, 40 kph constant Circle clockwise, radius = 152.4 m Circle clockwise, radius 6 152.4 m Road course 17 laps
Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height
Foremost Terra Bus Foremost Terra Bus Foremost Terra Bus
C1 C2 C3
20 December
Road course 21 laps
Foremost Delta
C4
20 December
Circle clockwise, radius = 91.4 m
Foremost Delta
C5
LDB Pad LDB Pad Road Course Road Course LDB Pad
16 December 16 December 16 December 16 December
17 December 17 December
Fig. 8. Rut and pile measurements for straight line constant speed (tests B3, B4, and B5, 6 passes at 25 mph).
7.4. Road course tests The Terra Bus accomplished 17 laps around the road course before the course became too rough to continue
Medium Clegg, rut depth and width No measurable ruts Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut depth and width
the test. The driver indicated that he achieved the maximum speed possible on the straight stretches, in sixth gear, but had to slow and take the turns in fifth gear. Fig. 9 shows the field notes from this testing. Rutting that occurred in the straight stretches seemed to be more obvious in the left tire tracks, perhaps due to the added weight of the fuel tank on that side. After the third lap, the course surface was noticeably changed; the bumpy area was soft, and slowed the vehicle down. After lap 10, the straightaway still looked good, and the corners were not too deeply rutted, but were slick. Final ruts were up to 55 cm (21.7 in.) deep and 150 cm (59.0 in.) wide. Fig. 10 presents the rut depth measurements at the final pass of the Terra Bus. For the Delta road course test, 21 laps were accomplished. The vehicle was driven in fourth gear, with the speed “maxed.” After 10 laps the only rutting observed was on corners. The straight stretches, however, became slightly bumpier. After lap 14, drivers noted that the corners began to feel like washboard from inside the vehicle. After lap 15, the vehicle began sliding laterally at the corner near the starting point. Fig. 9 shows additional field notes from this testing. The Delta left no measurable ruts or piles, only tire imprints.
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Fig. 9. Road course test notes and rut measurement locations (letters A–H).
are given in Fig. 10 showing rut depths exceeding 50 cm (19.7 in.). 8. Summary and conclusions
Fig. 10. Rut depth measurement for the final pass of the Terra Bus road course test.
For both the Terra Bus and the Delta the areas where the most damage occurred during the road test were the corners. The corners also required down shifting for the vehicles to negotiate them safely. The Terra Bus generally caused significantly more impact than the Delta, and in fact could not negotiate the course beyond 17 laps due to excessive rutting and roughness caused by the vehicle. The depths of the final rutting on the Terra Bus road course
During the austral summer, temperatures at McMurdo Station can be above freezing for several days at a time resulting in melting of infrastructure constructed of snow and ice. To make the best decisions regarding the construction, maintenance and use of the snow roads, a more formalized procedure and guidance, along with monitoring road maintenance, use and conditions, are needed. As part of this, understanding the impact of different vehicle types on the conditions of the roads is useful. Therefore, during the austral summer of 2009–2010 an exploration of how best to assess and quantify the impact of different vehicles on the road condition was conducted. Four different vehicles, some with various tires, were used in turning and straight-line maneuvers on a smooth prepared snow LDB launch pad; and the two of the larger transport vehicles were used on multiple passes on a test course made of existing snow roads. The project served to determine which maneuvers and measures can be helpful in distinguishing between vehicles. Measurements of the disturbance in terms of rutting and piles formed were helpful to determine speed effects, and
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turning effects, and which tires caused less damage to the surface. The road course provided a more operationally relevant test with compounded factors (turns, bumps, strength variations, etc.), and resulted in very clearly distinguishing which vehicle caused less impact and the types of maneuvers that cause the greatest impacts.
[4]
[5]
Acknowledgements The authors thanks the following personnel for their extra efforts and assistance with the funding, preparations, testing and measurements: Mr. George Blaisdell of the National Science Foundation Office of Polar Programs; Ms. Rosa Affleck of CRREL; Mr. Christopher Tomac, Mr. Alan Shaw, Ms. Julia Uberuaga, and Mr. William Sundee, from the Fleet Operations Ice Shelf Crew and the Shuttle Transportation Crew of Raytheon Polar Service Corp. References [1] Shoop S, Phetteplace G, Wieder W. Snow Roads at McMurdo Station, Antarctica. ERDC-CRREL Rpt 10-5; 2010. [2] Shoop S, Phetteplace G, Wieder W, Blaisdell G, Weale J, Knuth M. 2009. Snow Roads at McMurdo Station Antarctica. In: 11th European regional conference of the international society for terrainvehicle systems, Bremen, Germany; October 5–8 2009 [Paper 59]. [3] Affleck RT, Shoop S, Simmons K, Ayers P. Disturbance from offroad vehicle during spring thaw. In: Smith DW, Sego DC, Lendzion
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CA, editors. Proceedings of international symposium on cold regions engineering, vol. 12, unpaginated. Reston, Va.: ASCE; 2004 [unpaginated]. Althoff P, Thien S. Impact of M1A1 main battle tank disturbance on soil quality, invertebrates, and vegetation characteristics. Assessing the impact of military vehicular traffic on natural areas. J Terramech 2005;42(3–4):159–76. Anderson A, Shoop S, editors. 2005. Assessing the Impact of Military Vehicular Traffic on Natural Areas. J Terramech 42(3-4). Ayers P. Environmental damage from tracked vehicle operation. J Terramech 1994;31(3):173–83. Ayers P, Butler C, Fiscor A, Wu C, Qinghe Li, Anderson A. Vehicle impact study at Fort Riley, Kansas, October 2004. Unpublished report to Fort Riley. Haugen LB. Design and testing of a vehicle tracking system for monitoring environmental impacts at US Army training installations. MS thesis. Fort Collins, Colo.: Colorado State University; 2002. Shoop S, Knuth M, Wieder W, Preston M. Vehicle impact testing of snow roads at McMurdo Station, Antarctica. ERDC-CRREL Rpt, 2013. Shoop SA, Crandell J, Knuth M. Using a Clegg impact hammer to measure snow strength. In: Proceeding of the 15th international conference on cold regions engineering, Quebec, Canada; August 19– 22, 2012 p. 811–822. Liu K, Ayers P, Howard H, Jones R, Anderson A. Prediction of rut depth during military vehicle turning maneuvers using a modified sinkage numeric. Trans Am Soc Agric Biol Eng 2010;53(4):1019–24 [ISSN 2151-0032]. Ayers PD, Howard H, Anderson A, Li Q. Evaluation of low impact military tires. Tire Sci Technol 2006;34(4):256–74.
Journal of Terramechanics Journal of Terramechanics 50 (2013) 63–71 www.elsevier.com/locate/jterra
Measuring vehicle impacts on snow roads S. Shoop a,⇑, M. Knuth a, W. Wieder b a
Cold Regions Research and Engineering Laboratory, US Army, Engineer Research and Development Center, 72 Lyme Rd., Hanover, NH 03755-1290, United States b Science and Technology Corporation, 21 Enterprise Parkway, Suite 150, Hampton, VA 23666-6413, United States Available online 16 March 2013
Abstract The snow roads at McMurdo Station are the primary transport corridors to move personnel and material from the airfields servicing intra- and inter-continental air traffic for resupply. Thus, they are a critical transportation component and are particularly susceptible to deterioration during the warmest parts of the austral summer when above-freezing temperatures can occur for several days at a time. This study served to explore methodology that could quantify the impact of various vehicles, tires, driving speeds and maneuvers on the snow road conditions. Basic maneuvers were used to isolate the impact of turning, acceleration, braking and speed using spirals, circles, and straight-line testing on a flat, smooth snow pavement. In addition, a road course was set up to include corners and roughness using portions of the active snow roads for more realistic conditions. Measurements included snow surface strength both in and between tire tracks, tire track rut depth and width, and the height and width of the resulting snow piles adjacent to the tire tracks. Results indicate the impacts of driving speed and vehicle type including the importance of the tire and suspension components for preserving the road surfaces through the melt season. Since this type of testing had not been done on snow before, or using these vehicle types, the experiments yielded valuable guidance regarding what types of maneuvers, test surfaces, and measurements could most easily differentiate performance. Published by Elsevier Ltd. on behalf of ISTVS. Keywords: Strength; Disturbance; Rut; Rammsonde; Clegg; Maneuver; Turning; Corner; Speed
1. Background Transportation to and from McMurdo Station, Antarctica, is serviced by approximately 32 km (20 mi) of snow roads connecting the station and its airfields. In addition, the 1000-foot- (ft-) (305-m-) diameter Long Duration Balloon (LDB) Pad, serves as a large “paved” snow area for the launching of large, instrumented monitoring balloons. The construction and maintenance of these snow roads and the LDB Pad require approximately 4000 operator and equipment hours from September to February annually. These efforts are performed in a manner that relies heavily on the expertise of the operators on site with no specific prescription for snow road construction and maintenance currently in place. ⇑ Corresponding author. Tel.: +1 603 646 4321; fax: +1 603 646 4280.
E-mail address: [email protected] (S. Shoop). 0022-4898/$36.00 Published by Elsevier Ltd. on behalf of ISTVS. http://dx.doi.org/10.1016/j.jterra.2013.01.004
McMurdo snow roads are also subject to warm summer temperatures, which can severely reduce the road strength compounding any snow road construction or maintenance challenges, and making the snow surfaces more susceptible to failure. Consequently, in some years the snow roads fully support wheeled traffic for the entire summer season and in other years they cannot. In the worst case, nearly all transport of personnel and supplies to and from aircraft servicing McMurdo must be via a few specialized oversnow vehicles traveling at slow speeds. The Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH previously studied the processes used to prepare and maintain the McMurdo Station snow roads. Researchers witnessed activities in the December 2002–January 2003 time frame, and monitored existing snow road strength, maintenance, and vehicle fleet operations. The report documenting this work, ERDC/CRREL TR 10–5 Snow Roads at McMurdo Station, Antarctica [1],
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also contains a literature review of snow road construction methods, background on snow compaction and age-hardening, and a summary of McMurdo’s historic snow road construction and maintenance guidelines developed by the US Navy. That report concluded with recommendations to develop a modern snow road construction and maintenance program (also further developed in Shoop et al. [2]). Based on these studies, a series of tests were proposed to develop methodology to evaluate the impact of different types of vehicles on the snow roads under various weather and road conditions. Since this type of testing had not been done on snow or using these types of vehicles, similar studies of military vehicle impacts on Army training lands [3–8] and evaluating low impact military tires [12] provided guidance for test procedures. This report documents these vehicle-snow impact experiments. 2. Testing program Three basic types of tests were performed with four vehicles: spiral or circle test patterns to investigate the effect of turning; straight line constant speed, acceleration, and deceleration; and road course tests, which allowed both turns and speed variation. The spiral, circle and straight-line tests were performed on a flat, smooth area of prepared snow previously used as the LDB launch pad. Conversely, the road course was located on sections of existing snow roads and comprised a variety of curves and surface roughness, and was therefore more characteristic of the actual road conditions. Initial strength measurements were taken to characterize the test sites. After the vehicle trafficking tests, measurements included snow surface strength both in and between tire tracks, tire track rut depth and width, and the height and width of the resulting snow piles adjacent to the tire tracks. 3. Snow strength and impact measurements Two instruments were used to characterize the test sites and to monitor the snow strength changes during the vehicle impact testing: the Rammsonde Snow Penetrometer was used to measure a strength profile, and the Clegg Impact Hammer (Clegg) was used to measure the surface strength. The geometric impact on the surface was quantified in terms of rut depth and width and pile height and width. 3.1. Rammsonde snow penetrometer The Rammsonde was adapted by the US Army and others from an instrument originally used in the Swiss Alps. It has found extensive application for estimating avalanche danger and for determining allowable wheel loads on artificially compacted snow pavements. The device is a cone penetrometer consisting of a shaft with a 60° conical tip, a guide rod, and a drop hammer. For snow roads, the penetration cone has a diameter of 2.4 cm (0.94 in.) and height of 3.9 cm (1.54 in.); the total length of the penetrometer cone (to the beginning of the shaft) is 5 cm (1.97 in.). This
cone is smaller than what is used on natural, unprocessed snow and is more suitable for snow strength in the range seen on snow roads. It is also easier to use (both insert and remove) on compacted snow surfaces. The penetration force is obtained using a slide hammer of specific weight dropped from a measured height. The Rammsonde hardness number R is an index which indicates the snow resistance (in kilograms force, kgf) to the vertical penetration. The hardness reading at any depth represents the mean hardness through that depth. To characterize a given area a minimum of three Rammsonde profiles are collected and the results are averaged. 3.2. Clegg impact hammer Snow surface strength was measured using a Clegg Impact Hammer (Clegg). The Clegg consists of a cylindrical mass hammer that is dropped within a guide tube from a set height. The standard Clegg uses a 4.5 kg (9.9 lb) hammer mass. Two other hammer weights, the medium Clegg at 2.25 kg (5.0 lb) and the small Clegg at 0.5 kg (1.1 lb), were used in this testing program as it was determined that in many cases the 4.5-kg (9.9-lb) hammer was too heavy and the medium (2.5 kg) Clegg was determined to be the most suitable for characterizing the snow road strength. All of the Clegg hammers have the same diameter, 4.76 cm (1–7/8 in.). The Clegg is equipped with an accelerometer that measures the peak deceleration on impact. For the snow roads, the hammer is dropped five times at each location and the readings for each drop are recorded as Clegg Impact Value (CIV). Although the fourth drop CIV reading is usually used for soil strength calculations, an average of the third, fourth and fifth drop values were used in this case for the snow strength. In addition, the first drop value was investigated as an indicator of untouched or untamped snow surface strength. For each location, three separate tests are repeated within a 2-m (6.6-ft) circle and the average of these is reported. The Rammsonde and the Clegg are shown in Fig. 1. For additional details regarding the use of the strength instruments see Shoop et al. [1]. A more detailed analysis of the use of the Clegg on snow is given in Shoop et al. [10]. 3.3. Tire track rut and pile measurements Using the undisturbed snow as the datum, the width and depth of the ruts from each tire track, left and right, were measured with a straight edge cross piece and a meter stick or steel tape. The adjacent pile heights and widths were measured with a similar procedure (Fig. 2). These types of measurements duplicate those used for soil surface such as described in Haugen [8]. 4. Vehicles The four vehicles used during the vehicle impact testing are listed in Table 1 and shown in Fig. 3. Two of the
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warm weather when their high flotation tires allow greater over-snow mobility. 5. Test sites
Fig. 1. Rammsonde hardness test (top) and Clegg impact hammer test (bottom).
The LDB Pad is a 1000-ft- (305-m-) diameter pad used to launch high-atmosphere, long-duration balloons with various atmosphere and space payloads. The LDB Pad is maintained continuously throughout the season in order to provide a very smooth and stable platform for the heavy equipment used during balloon launches. Once the balloon experiments were completed for the season, the LDB Pad provided a smooth, uniform surface on which to perform comparative vehicle impact tests. Tests performed on the LDB Pad include the spiral and circle tests with all four vehicles, and the acceleration, deceleration, constant speed tests with the Fleet Operations Truck and the vans. Both of the large vehicles, the Terra Bus and the Delta, had difficulties performing the spiral maneuver with tight turning radii, so the spiral test was replaced by a breakout circle test for these two vehicles. Fig. 4 shows a plan map of the snow roads on the McMurdo Ice Shelf. In addition to the scripted tests, and to obtain information during a more realistic operation scenario, a small road course was developed at the junction of Williams and Pegasus cut-off Roads (the circled area in Fig. 4 map). The Terra Bus and Delta were used on the road course. The bottom portion of the figure is a high-resolution satellite photo showing the LDB Pad (the smooth snow circle on the right), and the triangular road course (shown to the left). All of the test courses had a smooth and level surface, although one side of the road course triangle was cut through sastrugi (snow ridges formed by wind) as can be seen on the imagery in Fig. 4 (and later generated “Bumps” as noted on Fig. 9). 6. Vehicle impact test maneuvers
Fig. 2. Measuring rut depth.
vehicles were tested with two different tires. When the temperatures are cooler and the roads are hard, the vans are the fastest and most comfortable ride to and from the airfields. The Terra Bus and Delta can handle many more people and cargo and are the primary people movers for the snow road transportation system, especially during
Spiral testing involved driving either the Fleet Operations Truck or one of the two vans in an ever-decreasing spiral starting at a given outside radius of 30.5 m (100 ft). The outside of the spiral was first measured and marked on the LDB Pad surface to serve as a guide for the driver. Every attempt was made to maintain a constant speed and produce a regular spiral. Measurements of the tire track rut depth and width, snow pile height and width, and the surface strength in and between the ruts were taken at points where the spiral crossed the radius to the spiral [9]. The distance to the left and right tire track, from the center point of the spiral, was measured at these points. For the Terra Bus and the Delta it was impossible to drive in a spiral of such a small radius and therefore larger circles on the outside of the spiral were used. These circles had diameters of 152 m (500 ft) for the Terra Bus, and 91 m (300 ft) for the Delta. The circles were marked to provide guidance to the driver. Testing proceeded until sliding was initiated.
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Table 1 Test vehicles. Vehicle
Tires
Weight kN (lbs)
Fleet Operations (Ops) Truck, Ford F350 (Old Tires) Fleet Operations Truck, Ford F350 (New Tires) Ford E350 Van 206 (old tires) Ford E350 Van 213 (new tires) Foremost Terra Bus “Ivan” Foremost Delta “Gale”
Denman Ground Hwy II steel belted radial, 36 14.5 R16.5LT Interco TRXUS STS 36 14.50 R16.5LT Cepek tires 17/40–16.5 LT Interco TRXUS M/T 38.5 14.5 R17LT Terra-Tire: Tubeless Nylon 66 44.00–25NHS Terra-Tire: Tubeless Nylon 66 44.00–25 NHS
27.9 (6262) 27.9 (6262) 41.4 (9300) 41.4 (9300) 298.0 (67,000) 186.8 (42,000)
Ford F350 Fleet Operations Truck
Ford E350 Van (typical)
Foremost Terra Bus "Ivan"
Foremost Delta
Fig. 3. Vehicles used in test program.
Using the light vehicles, acceleration/constant speed/ deceleration tests took place across a diameter bisecting the spiral. The vehicles accelerated to a specified speed, held a constant speed for approximately 61 m (200 ft), and then decelerated to a stop. Tire tracks were measured for rut depth and width, and the width and height of the snow piles accumulated next to the tire track. For a more operationally relevant test, the Terra Bus and Delta were also driven in a road course test. The road course tests utilized a short section of the snow road to the LDB Pad along with an unused section of snow road that resulted from re-routing the roads earlier in the season. Although the scope of the test program was limited by the availability of vehicles, personnel and uniformly prepared surfaces to use for comparison, due to daily station operations which have priority, a substantial number of tests and measurements were accomplished. A summary of all of the test types and combinations evaluated is given in Table 2. Keeping in mind that our goal was to determine
which type of testing and measurements would yield discriminating data, the results from each type of test maneuver are described below. Complete results are given in Shoop et al. [9]. 7. Impact analysis and results The data was analyzed to determine if we could quantify changes in snow strength or snow surface rutting based on the different vehicles, tires and maneuver conditions. The snow strength was compared in and out of the ruts at different locations along the maneuver. The impact was quantified in terms of rut depth (RD) and width (RW), and for the piles alongside the rut, the pile height (PH) and width (PW). From these measurements, a total impact can be calculated that represents the cross sectional area of all of the snow displaced by the vehicle (rut area plus pile area). The Total Vehicle Impact (TVI) value, for both the left (L) and right (R) vehicle tracks, is given by following equation:
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Fig. 4. Map of the snow roads and airfields on the Ice Shelf near McMurdo Station with test area noted in the circle (top); and a satellite image of the LDB Pad (circle) and road test course (triangular) (bottom). (29 October 2009 WorldView Satellite at 0.5 m resolution).
Total Vehicle Impact ðTVIÞ ¼ RDLþR RWLþR þ 1=2ðPHR PWR Þ þ 1=2ðPHL PWL Þ ð1Þ For soils, a predictive mathematical relationship between soil strength (using cone index values as a strength indicator) and rutting during a spiral maneuver with a varying turning radius is presented by Liu et al. [11]. No equation has yet been developed for vehicle rutting on snow, although perhaps our recent work with the Clegg as a method to assess the strength measurement could provide a suitable basis for future analytical development.
at speeds that were above 19 kph until the inner spiral, which was driven at 16 kph. Fig. 5 (top) shows the tire track rut depth decreasing with distance from the center, which could also be stated as rut depth increases as turning radius decreases. Pile heights and widths show similar trends (piles are higher and wider at smaller turning radius). The calculated TVI, a combined measure of rut and pile dimensions, is shown in the bottom of Fig. 5. This exponential decline in impact with distance is similar to findings from tests with military vehicles on training lands (Ayers et al. [7], and Affleck et al. [3]). 7.2. Circle tests C1, C2, and C5
7.1. Spiral tests Spiral test A1 completed four crossings across the radius using the Fleet Operations Truck with older Denman tires. The furthest set of tracks measured were 25.9 and 26.8 m (82 and 88 ft) from the spiral center point, left and right tires, respectively. The Fleet Operations Truck was driven
Circle test C1 was performed with the Terra Bus on the LDB Pad. The driver attempted to hold the vehicle at a constant radius on the marked circle, driving clockwise in fifth gear. Six circles were completed; however it was very hard to hold a true circle with the Terra Bus. Medium Clegg and rut and pile measurements were taken.
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Fig. 6. Clegg snow surface strength measurements for clockwise circle tests C1 and C5 with the Terra Bus and Delta, respectively.
45 40
Acceleration to 32 kph
35
Acceleration to 40 kph Constant speed 32 kph
cm
30
Constant speed 40 kph
25 20 15
Fig. 5. Rut and vehicle impact measurements from spiral maneuver A1 using the Fleet Operations Truck (older Denman tires).
Circle test C5 used the Delta personnel carrier driving in clockwise circles with a 91 m (300 ft) radius. The ruts left in this test were observed to be, subjectively, “very soft” (lower surface strength). Fig. 6 presents the medium Clegg (2.25 kg [5.0 lb]) data from tests C1 and C5 but there is no consistent trend in the ruts being softer (from more shearing) or harder (from compaction) than the surrounding snow, or from one side to the other of the vehicle. 7.3. Straight line tests (acceleration/constant speed/ deceleration) The acceleration/constant speed/deceleration tests took place on the LDB Pad. The vehicles were driven in a straight line bisecting the spirals and circles. The Fleet Operations Truck (old tires), Van 213 (new tires), Van 206 (old tires), and Fleet Operations Truck (new tires) were used for the straight line testing. For the tests done at two different speeds using the Fleet Operations Truck (Fig. 7), the imprint from the 32-kph imprint test was clearer with tread patterns discernible for most of the tests. At 40 kph, the tire track was deeper and the print was obscured by the tire shearing the surface of the snow. The 40-kph test consistently shows higher impact (higher rut depth, rut width, pile width) than the
10 5 0 Rut Depth
Rut Width
Pile Height
Pile Width
Measurement
Fig. 7. Rut and pile measurements for test B5, Fleet Operations Truck (new tires) straight line acceleration/constant speed/deceleration test.
32-kph test for acceleration, but this trend is not consistent for the constant speed portion of the test. A comparison between the two different tires used on the vans, and the new tires on the Fleet Operations Truck can be seen in Fig. 8. The van with the old tires had the highest or tied for highest impact in all categories, confirming the decision to upgrade the tires on the van fleet to lower impact tires. The difference between the Fleet Operations Truck and the van with the older tires is not as clear. Observationally, we noted that Van 213 with new tires bounced more, which could be a problem with creating impact on the roads during a more operationally relevant test; and the Fleet Operations Truck created some small washboarding. By visual inspection, Van 213 with new tires did slightly less damage to the snow surface during its acceleration test and also made clear track prints, while Van 206 (old tires) threw more snow and the tires sheared the surface, obliterating the tread print. The Clegg data shows lower snow strength in the ruts in most cases, but is not consistent.
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Table 2 Snow road vehicle impact testing program, December 2009. Date
Test
Vehicle
Field test #
Location
Data
16 December
Spiral counterclockwise, 24 kph
A1
LDB Pad
16 December
Circle counterclockwise, 37 kph, radius = 30.5 m Straight Line Acceleration/Constant Speed/Deceleration 40–43 kph Spiral Counterclockwise, 24 kph
A2
LDB Pad
A3
LDB Pad
Small Clegg, rut width and depth, pile width and height Small Clegg, rut width and depth, pile width and height Small Clegg
A4
LDB Pad
No measurable ruts
A5
LDB Pad
No measurable ruts
A6
LDB Pad
Small Clegg
17 December
Straight line acceleration/constant speed/deceleration 40 kph Straight line acceleration/constant speed/deceleration 32 kph Spiral Counterclockwise, 24 kph
Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Fleet Operations Truck (Denman tires) Van 206 (Cepek tires)
B1
LDB Pad
17 December
Spiral counterclockwise, 24 kph
Van 213 (TRXUS tires)
B2
LDB Pad
17 December
Van 213 (TRXUS tires)
B3
LDB Pad
Van 206 (Cepek tires)
B4
LDB Pad
Fleet Operations Truck (TRXUS tires)
B5
LDB Pad
20 December 20 December 20 December
Straight line constant speed 40 kph, 6 passes Straight line constant speed 40 kph, 6 passes Straight line acceleration/constant speed: 32 kph acceleration, 40 kph acceleration, 32 kph constant, 40 kph constant Circle clockwise, radius = 152.4 m Circle clockwise, radius 6 152.4 m Road course 17 laps
Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height
Foremost Terra Bus Foremost Terra Bus Foremost Terra Bus
C1 C2 C3
20 December
Road course 21 laps
Foremost Delta
C4
20 December
Circle clockwise, radius = 91.4 m
Foremost Delta
C5
LDB Pad LDB Pad Road Course Road Course LDB Pad
16 December 16 December 16 December 16 December
17 December 17 December
Fig. 8. Rut and pile measurements for straight line constant speed (tests B3, B4, and B5, 6 passes at 25 mph).
7.4. Road course tests The Terra Bus accomplished 17 laps around the road course before the course became too rough to continue
Medium Clegg, rut depth and width No measurable ruts Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut width and depth, pile width and height Medium Clegg, rut depth and width
the test. The driver indicated that he achieved the maximum speed possible on the straight stretches, in sixth gear, but had to slow and take the turns in fifth gear. Fig. 9 shows the field notes from this testing. Rutting that occurred in the straight stretches seemed to be more obvious in the left tire tracks, perhaps due to the added weight of the fuel tank on that side. After the third lap, the course surface was noticeably changed; the bumpy area was soft, and slowed the vehicle down. After lap 10, the straightaway still looked good, and the corners were not too deeply rutted, but were slick. Final ruts were up to 55 cm (21.7 in.) deep and 150 cm (59.0 in.) wide. Fig. 10 presents the rut depth measurements at the final pass of the Terra Bus. For the Delta road course test, 21 laps were accomplished. The vehicle was driven in fourth gear, with the speed “maxed.” After 10 laps the only rutting observed was on corners. The straight stretches, however, became slightly bumpier. After lap 14, drivers noted that the corners began to feel like washboard from inside the vehicle. After lap 15, the vehicle began sliding laterally at the corner near the starting point. Fig. 9 shows additional field notes from this testing. The Delta left no measurable ruts or piles, only tire imprints.
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Fig. 9. Road course test notes and rut measurement locations (letters A–H).
are given in Fig. 10 showing rut depths exceeding 50 cm (19.7 in.). 8. Summary and conclusions
Fig. 10. Rut depth measurement for the final pass of the Terra Bus road course test.
For both the Terra Bus and the Delta the areas where the most damage occurred during the road test were the corners. The corners also required down shifting for the vehicles to negotiate them safely. The Terra Bus generally caused significantly more impact than the Delta, and in fact could not negotiate the course beyond 17 laps due to excessive rutting and roughness caused by the vehicle. The depths of the final rutting on the Terra Bus road course
During the austral summer, temperatures at McMurdo Station can be above freezing for several days at a time resulting in melting of infrastructure constructed of snow and ice. To make the best decisions regarding the construction, maintenance and use of the snow roads, a more formalized procedure and guidance, along with monitoring road maintenance, use and conditions, are needed. As part of this, understanding the impact of different vehicle types on the conditions of the roads is useful. Therefore, during the austral summer of 2009–2010 an exploration of how best to assess and quantify the impact of different vehicles on the road condition was conducted. Four different vehicles, some with various tires, were used in turning and straight-line maneuvers on a smooth prepared snow LDB launch pad; and the two of the larger transport vehicles were used on multiple passes on a test course made of existing snow roads. The project served to determine which maneuvers and measures can be helpful in distinguishing between vehicles. Measurements of the disturbance in terms of rutting and piles formed were helpful to determine speed effects, and
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turning effects, and which tires caused less damage to the surface. The road course provided a more operationally relevant test with compounded factors (turns, bumps, strength variations, etc.), and resulted in very clearly distinguishing which vehicle caused less impact and the types of maneuvers that cause the greatest impacts.
[4]
[5]
Acknowledgements The authors thanks the following personnel for their extra efforts and assistance with the funding, preparations, testing and measurements: Mr. George Blaisdell of the National Science Foundation Office of Polar Programs; Ms. Rosa Affleck of CRREL; Mr. Christopher Tomac, Mr. Alan Shaw, Ms. Julia Uberuaga, and Mr. William Sundee, from the Fleet Operations Ice Shelf Crew and the Shuttle Transportation Crew of Raytheon Polar Service Corp. References [1] Shoop S, Phetteplace G, Wieder W. Snow Roads at McMurdo Station, Antarctica. ERDC-CRREL Rpt 10-5; 2010. [2] Shoop S, Phetteplace G, Wieder W, Blaisdell G, Weale J, Knuth M. 2009. Snow Roads at McMurdo Station Antarctica. In: 11th European regional conference of the international society for terrainvehicle systems, Bremen, Germany; October 5–8 2009 [Paper 59]. [3] Affleck RT, Shoop S, Simmons K, Ayers P. Disturbance from offroad vehicle during spring thaw. In: Smith DW, Sego DC, Lendzion
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