Compaction or removal of wet snow by traffic

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Publisher’s version / Version de l'éditeur:

Highway Research Board Special Report, 115, pp. 97-103, 1971-01-01

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Compaction or removal of wet snow by traffic

Schaerer, P. A.

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Compaction or Removal of Wet-Snow by

Traffic

P . A. Schaerer

Observations were made of the free-water content of snow on r o a d s and i t s behavior under traffic. A Tapley d e c e l e r o m e t e r was used to m e a s u r e the skid r e s i s t a n c e of the snow-covered r o a d s . It was found that snow having a free-water content of l e s s than 15 percent was compacted by traffic and formed a slippery s u r f a c e on which a deceleration value of 0.30 was mea- s u r e d . Snow with a free-water content between 15 and 30 percent was usually not compacted but remained on the road in a soft, loose s t a t e and gave deceleration values of 0.35 to 0.42. The exact behavior depended on other variables such as depth, shape, and s i z e of the g r a i n s . Snow with a f r e e - water content of 30 percent w a s removed by traffic. Deceleration values between 0.40 and 0.50 w e r e m e a s u r e d on this s u r f a c e , and a value of 0.60 was m e a s u r e d on the b a r e , wet pavement. The observations confirm that chemicals need to be applied only in an amount sufficient to produce 30 p e r - cent melting if a d e c r e a s e of the skid r e s i s t a n c e can be tolerated. A melting of 15 percent would prevent the snow f r o m being compacted into an i c e c r u s t but usually would not cause i t t o b e removed by traffic, and thus plowing would be required.

Wet snow deposited on a r o a d and exposed to traffic will e i t h e r be compacted into

a h a r d and slippery snow-ice l a y e r , r e m a i n on the pavement in a loose state, o r be removed t o the s i d e by the action of vehicles. The condition that develops depends on the p r o p e r t i e s of the snow and c h a r a c t e r i s t i c s of the traffic. In 1966, the Division of Building R e s e a r c h of the National R e s e a r c h Council of Canada began to obtain information on whether the snow is compacted, remains loose, o r is removed. Observations w e r e made on city s t r e e t s in Ottawa during one winter, and limited observations w e r e c a r r i e d out i n the following y e a r s a t Rogers P a s s , British Columbia. It was not possible, however, to continue the p r o g r a m in i t s full extent o r t o bring i t t o a stage where final conclusions could b e drawn. This paper contains p r e l i m i n a r y r e s u l t s only.

P u r e snow can contain water in liquid f o r m only if i t s t e m p e r a t u r e is 0 C. This water may be f o r m e d on the snow as i t f a l l s through the atmosphere o r i t may r e s u l t f r o m r a i n o r f r o m the p a r t i a l melting of snow on the road. Snow can a l s o be melted partially by the application of chemicals, and in this c a s e the liquid is a solution. Wet snow that contains chemicals in solution can have a t e m p e r a t u r e lower than 0 C a d is the type of wet snow most frequently observed on r o a d s .

OBSERVATIONS O F SNOW CONDITIONS

T h e r e a r e , on the average a t Ottawa, 60 days p e r winter with snowfalls m o r e than 0.25 c m , and on 5 days the amount of new snow exceeds 10 c m . Plowing during snow- f a l l s usually e n s u r e s that snow on roads does not accumulate deeper than 7 cm. The principal observation made during and immediately following snowfalls, either before o r after plowing, was to classify visually the s t a t e of the snow on the road s u r f a c e by using the following c r i t e r i a :

1. Compact snow-Pavement is covered with a sheet of dense snow that does not b r e a k o r move under the action of the traffic;

2 . Loose snow-There is no apparent bond between the snow and the pavement, and the snow is broken into chunks o r is a cohesionless m a s s that moves under the wheels of passing vehicles but is not thrown to the side of the road; and

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3. Loose snow removed-The wheels of moving vehicles throw the snow to the side, and loose snow f r o m the road accumulates on the shoulder.

The properties of snow that w e r e m e a s u r e d w e r e f r e e water content, depth, density, temperature, and type of c r y s t a l s .

Free-water content, F , was determined as the weight of the liquid divided by the total weight (liquid plus solid), e x p r e s s e d in percent.

where

W = weight of liquid (pure water o r b r i n e ) and I = weight of i c e .

The hot water calorimeter was found to be the most convenient instrument f o r measuring the free-water content. Because there w e r e significant variations i n the f r e e - water content over s m a l l distances, i t was s o m e t i m e s n e c e s s a r y to collect in a bucket about 5,000 g r a m s of snow f r o m different spots, mix i t , and draw f r o m i t 2 to 3 small samples of about 300 g r a m s and melt them in the c a l o r i m e t e r .

The free-water content is the variable that h a s the s t r o n g e s t influence on the condition of snow on the road. I t s effect is discussed in a s e p a r a t e section.

Snow depth on the road was m e a s u r e d by r u l e r a t 10 points selected a t random. All observations w e r e made with snow depths between 0.8 and 7 cm. Within this range, depth had no significant influence on whether snow was compacted, remained loose, o r was removed by traffic. T h e r e appeared to be a change in behavior, however, f o r very thin l a y e r s of snow, 0.2 to 0.5 cm deep.

L a y e r s of this thickness w e r e not removed as readily as snow m o r e than 0.5 cm deep because traffic packed i t into the voids of the pavement where i t f o r m e d a slippery surface. The number of observations on very thin l a y e r s , however, was insufficient to draw definite conclusions.

Measurements w e r e made of snow density, but, because density was strongly influenced by the free-water content and varied with location and time, this formation was found to be of little value.

Snow temperature was m e a s u r e d with a shielded g l a s s thermometer. I t s value was always consistent with the free-water content, but i t appeared to have no d i r e c t relation to the behavior of wet snow.

The density of traffic on roads where the observations w e r e made was between 10 and 140 vehicles p e r hour. Rather than density, the total number of vehicles passing over the snow determined the s t a t e of the snow. About one-third of the vehicles w e r e trucks; the reason f o r this high t r a c k r a t i o is that the observations w e r e made n e a r industrial s i t e s . The speed of traffic was between 30 and 45 km/hr (18 to 27 mph), a

normal speed on snow-covered r o a d s in cities.

INFLUENCE O F FREE-WATER CONTENT O F SNOW

Whether wet snow is compacted o r removed under the action of traffic depends on the cohesion between the snow g r a i n s and their adhesion to the pavement. The study h a s confirmed that both cohesion and adhesion a r e strongly influenced by free-water

content. Figure 1 shows a plot of observed free-water content against the amount of traffic and the condition of the snow. The values fall into the following 3 zones:

1. Zone A-Initial free-water content l e s s than 15 percent: snow usually compacted on the road;

2 . Zone B-Initial free-water content between 15 and 30 percent: snow r e m a i n s on the the road in a loose condition and is removed by traffic a t a slow r a t e ; and

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0

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 TOTAL NUMBER OF VEHICLES

OBSERVATIONS o COMPACT SNOW LOOSE SNOW

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a SNOW REMOVED ZONE C

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Figure 1. Effect of free-water content and traffic on the cornpactibility of snow on roads.

3. Zone C-Initial free-water content greater than 30 percent: traffic moves the loose snow to the side rapidly.

The shape of the snow crystals appears to influence the behavior of snow. Rounded grains were moved more readily than needle-shaped o r dendritic crystals. This influence is evident only when the snow is fresh. About 2 hours after snow was deposited all the grains had developed a round shape. Zone A in Figure 1 contains observations of snow that was loose and had a free-water content lower than 15 percent. It consisted of graupel and ice pellets.

A similar influence of crystal shape h a s been reported in measurements of the water- holding capacity of snow. The water-holding capacity is the maximum free-water content that is retained in the snow without producing runoff. de Quervain

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has observed for dense new snow a water-holding capacity of 20 to 30 percent; f o r fine-grained old snow, a capacity of 10 to 20 percent; and for coarse old snow, a capacity of 5 to 10 percent. Gerdel (2)

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r e p o r t s 13 to 19 percent f o r new snow and 0.4 to 6 percent f o r old snow.

The British Road Research Laboratory h a s recommended f o r snowfalls the application of 0.0125 l b sodium chloride per inch of new snow, per square yard, per deg F below freezing on roads with a traffic density of m o r e than 50 vehicles per hour, if the resulting wet snow is to be removed by traffic

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The recommended amount of chemical would produce between 30 and 40 percent melting, which would agree well with the present observations.

OBSERVATIONS OF FRICTION COEFFICIENT

In addition to evaluating the quality of the road surface by observation of the behavior of snow, we took measurements of the friction coefficient. This was determined with a Tapley decelerometer mounted in a light truck driven

at

a speedof 30mph

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(48 km/hr). Full brakes were applied TABLE 1

and the maximum deceleration and cor- SUMMARY OF OBSERVED FRICTION COEFFICIENTS

responding friction coefficient read from

Condition of Snow on the Road Friction Coefficient

the instrument. The results obtained (percent)

with the Tapley decelerometer a r e in-

Compacted wet snow 25-30

fluenced by the type of vehicle and the

Compacted dry snow 30-35

driver. In order to minimize equipment

and human error, the same vehicle and Loose. d w new snow Onbare pavement _{1 to 5 c m deep} 32-38

the same operator were used throughout 5 to 10 cm deep 38-43

the tests after preliminary experiments Loose, wet snow (free-water content

had shown that the chosen operator could l5 to 1 to 30 5 percent) cm deep

obtain reproducible results. The Tapley Pavement completely covered 33-38

decelerometer, however, was found un- Center bare 2 f t wide 38-46 Center bare 6 ft wide 44-56

satisfactory for making observations on

snow- and ice-covered roads. It was wet snow On bare pavement (free-water content greater than

considered that better results could be 30 percent) 42-49

obtained with a trailer-type, friction- Bare wet pavement 60-70

measuring apparatus.

There i s usually a great variation in the friction coefficient of roads having ap-

parently uniform snow conditions, depending on whether one o r several wheels of the test vehicle happen to be in direct contact with pavement o r ice at the instant the brakes a r e applied. It was found necessary in the present study to make 8 observations on the same road section in order to obtain a mean value that had a 95 percent confidence limit.

More variables influence the friction coefficient than the degree of compaction of snow on the road. Insufficient observations were made to delineate them and establish their relation to friction. Table 1 gives a general picture of the friction coefficients that were observed. All observations were made on roads with asphalt pavement.

CONCLUSION

Observations of winter road conditions typical of urban a r e a s in eastern Canada indicate that, during light snowfall o r after plowing deep snow, chemicals should be applied or pavements should be heated at such a rate that at least 30 percent of the snow is melted. Traffic will move the resulting wet snow to the side of the road. Chemicals should not be applied a t a rate that produces l e s s than 15 percent melting, because the resulting wet snow would be compacted into a slippery layer.

It would be useful to continue studies of the friction coefficient of snow- and ice- covered roads in order to determine the influence of different treatments on traffic safety.

REFERENCES

de Quervain, M. Ueber den Abbau der Alpinen Schneedecke. Internat. Assn. of Scientific Hydrology, Assemblde Ge'ne'rale d'Oslo, Vol. 2, 1948, pp. 55-68. Gerdel, R. W. Physical Changes in Snow-Cover Leading to Run-Off, Especially

to Floods. Internat. Assn. of Scientific Hydrology, Assemble'e GQne'rale d'Oslo, Vol. 2, 1948, pp. 42-54.

Nichols, R. J., and Price, W. I. J. Salt Treatment for Clearing Snow and Ice. The Surveyor, Vol. 115, No. 3368, 1956, pp. 886-888.

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Formal Discussion

S. F. Ackley

Several points a r e not c l e a r to m e in M r . Schaerer's paper, and elaboration on these points may aid in the evaluation of the data obtained from e i t h e r an operational o r a r e s e a r c h viewpoint. T h e r e a r e a number of difficulties in using hot water calo- r i m e t r y f o r the 2-component s y s t e m s encountered on salted roads, and discussion by M r . Schaerer on how these difficulties w e r e overcome would b e appreciated. Specif- ically, m y questions a r e as follows:

1. Was the specific heat of p u r e water (1 cal/gm C) o r the specific heat of a s a l t solution (0.88-0.96 cal/gm C, depending on concentration) used in the calculation?

2. Was the melting point of p u r e i c e o r a depressed melting point caused by the presence of s a l t used in the calculation?

3. Because these contributions a r e salt-concentration dependent, how was the

concentration determined to f i x the values of the specific heat and depressed melting points ?

4. What was the construction of the c a l o r i m e t e r used f o r these m e a s u r e m e n t s ? Specifically, what was the accuracy of the field temperature measurements, and was the m a s s of hot water used f a i r l y l a r g e compared to the s a m p l e s i z e ?

My purpose in asking these questions is to allow an a s s e s s m e n t of the advantages of this system over density m e a s u r e m e n t s f o r a s i m i l a r c a s e . M r . Schaerer h a s presented numbers f o r free-water content with an e r r o r of about 1 percent (Fig. 1) in h i s paper. I have done a "worst case" calculation (assuming a specific heat of 0.9 cal/gm C, a melting temperature of -5 C, 500 g r a m s of hot water in the calorimeter a t a temperature of 50 C, and a 300-gram s a m p l e of snow plus 30 percent f r e e water a t -6 C ) and have found that, if the f a c t o r s I have mentioned a r e completely ignored,

an

e r r o r of 10 percent i n the m e a s u r e d f r e e - w a t e r content would b e quite possible. Although I do not feel that the numbers given by M r . Schaerer have this l a r g e an e r r o r , the range possible f o r e r r o r indicated by the calculation is certainly g r e a t e r than the

1 percent shown, especially in a field situation. A m o r e complete description of the measurement procedure might give a b e t t e r i d e a of the accuracy of the numbers p r e - sented and allow a m o r e f a i r comparison with the density method to determine f r e e - water content.

Schaerer

The observations reported i n the paper w e r e made a t t e m p e r a t u r e s between -3 and 0 C, and tlle concentration of chemicals in the wet snow was usually l e s s than 1.5 percent. T e s t s w e r e c a r r i e d out i n the laboratory a t lower t e m p e r a t u r e s and with higher concentrations of chemicals. Because of the difficulties mentioned by M r . Ackley, i t was decided not to include in the study these e x t r e m e conditions.

F o r a l l observations made on salted roads, the amount of sodium chloride o r cal- cium chloride was determined in the laboratory with s a m p l e s of about 500 g r a m s of wet snow. The snow was melted, and the meltwater was filtered and then evaporated i n the drying oven. F o r s o m e s a m p l e s the concentration of s a l t w a s determined by measuring the e l e c t r i c a l conductivity of the solution.

The specific heat of the s a l t solution was used in the calculation of the free-water content. Because of the low concentration of s a l t , the specific h e a t was assumed to be 0.98 cal/gm C. The melting point depression due to the p r e s e n c e of chemical was a l s o considered. A correction was a l s o made f o r impurities of dust, sand, and r o c k chips usually contained in the snow on roads. These m i n e r a l s could amount to 3 percent of the snow; a specific heat of 0.25 cal/gm C was assumed f o r them.

The c a l o r i m e t e r that was found to b e b e s t suited was a wide-neck thermos flask, volume 2,000 cm3, with a cork stopper. It is most important to work always in the s a m e temperature range and with the s a m e amount of water, which m u s t a l s o b e used to determine the heat capacity of the calorimeter. The following a r e the c h a r a c t e r i s t i c s of the water and mixture used in our measurements:

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Temperature of, hot water, 35 C

Temperature of mixture after melting, 5 to 10 C

Amount of hot water, 1,000 grams Amount of snow, 300 to 400 grams

The temperature was measured with a mercury thermometer graduated in %O C.

The greatest heat loss and possibility of e r r o r occurs when the cork stopper is r e - moved and the snow added to the hot water. With some practice this can be done quickly and the e r r o r kept within reasonable limits.

The number of observations that could be carried out on the road was too limited to make an evaluation of the influence of all possible e r r o r s . It would appear that the variation in the free-water content of the snow on a road produces a greater e r r o r than inaccuracies of the calorimetric measurements. The information presented in the paper should only be taken a s an indication of the trend in the relationship between free-water content and the condition of the snow. Additional studies must be carried out before a detailed analysis of the effect of traffic on wet snow can be undertaken.

Informal D i s c ~ ~ s s i o n

Lorne W. Gold

What was the particle size of the salt, in other words, the screen size used? Schaerer

We have used sodium chloride that meets the specification of the Ontario Department of Highways. Can you tell us what this is Mr. Brohm?

David Brohm

The salt we normally use for highway work is coarse-crushed, maximum size somewhere around

'/4

in.

Gold

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Some European countries use very fine salt, s o tests run there would give results that a r e almost meaningless because there is not a wide range of particle sizes to get down to the interface.

Brohm

Our coarse-crushed salt specification, which is not much different from that com- monly used in the United States, does permit a fair amount of fines. It covers a fairly broad band. You cannot really pin i t down unless you have an analysis of the salt being used.

M. E. Volz

Apparently there is a great deal of discrimination among definitions of wet snow. Am I correct that your definition is snow with a water content in excess of 30 percent? Schaerer

Wet snow is any snow that has a certain amount of liquid. Volz

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Schaerer

Anything with 1 percent and more.

H , R. ~ i v i s i l d

I noticed from the graph that traffic intensity has hardly any effect. Schaerer

The tests were run up to a maximum of 4 hours immediately after snow deposition. Glenn Balmer

We have made tests with a skid trailer on snow and obtained results very comparable to yours.

Schaerer

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