TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 45

WINTER AIR TEMPERATURES IN RELATION TO FROST

DAMAGE IN ROADS

by P T Sherwood and P G Roe

The views expressed in this Report are not necessarily those of the Department of Transport

Pavement Materials and Construction Division Highways and Structures Department Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1986

ISSN 0266-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on 1 st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Page

Abstract 1

1. Introduction 1

2. Types of frost damage 1

2.1 Frost-shattering 1

2.2 Frost-heave 1

3. Assessing frost penetration 2

3.1 From measurements in the road 2

3.2 From measurements of air temperature 2

3.2.1 Frost index 2

3.2.2 Calculation of frost penetration 2

4. Examination of the meteorological records 5

4.1 Selection of sites 5

4.2 Data extracted 5

4.2.1 Frost index 9

4.2.2 Duration of freezing period 9

5. Discussion of results 9

5.1 Winter severity 9

5.2 Risk of frost-heave 11

6. Further examination of meteorological records 11

6.1 Discussion of results 12

7. Conclusions 14

8. Acknowledgements 15

9. References 15

© CROWN COPYRIGHT 1986 Extracts from the text may be reproduced, except for

commercial purposes, provided the source is acknowledged

AN E X A M I N A T I O N OF WINTER T E M P E R A T U R E S IN RELATION TO FROST D A M A G E IN R O A D S

A B S T R A C T

Very cold winters sufficiently severe to cause widespread damage to road foundations occur infrequently in Great Britain. A design procedure for new roads has been adopted which minimises the danger of frost-damage to subgrades by ensuring that the materials used in the construction of the road are not susceptible to frost-heave. Consequently, frost- susceptible materials are banned from the top 450 mm of construction throughout Great Britain. This approach relies on the assumption that for design purposes road pavements may be regarded as being at equal risk to frost penetration.

This report describes a study of meteorological records for various sites (from Cornwall to the Scottish Highlands) during the period 1959-1981. The frequency and severity of cold spells were examined with regard to the risk of frost penetrating deeply into a road. The results show that the present assumption of equal frost-penetration risk is untenable. Consideration should be given to reducing the thickness of non- susceptible construction required at a given site (with consequent economic benefits) on the basis of an examination of the meteorological records for the locality. Some further investigation is needed, because, in addition to finding the frequency of cold spells, the way in which successive spells combine to influence frost penetration must be taken into account.

1 INTRODUCTION

Frost damage to the surfaces of roads can occur in almost any winter in Great Britain but very cold winters sufficiently severe to cause widespread damage to road foundations occur only infrequently. Nevertheless it is necessary to adopt a design procedure for new roads that minimises the danger of frost damage by ensuring that the materials used in the construction of the road are not susceptible to frost-heave.

In a report published in 1967 (Croney and Jacobs 1967) it was concluded that the depth of frost penetration was related to the near-ground night temperature and the maximum day temperature. This report claimed that meteorological records showed that during prolonged frosts these factors did not vary systematically or very significantly over the British Isles. It went on to conclude that although local conditions of exposure had some influence on frost penetration a figure of 500 mm was likely to be generally applicable.

As 500 mm was considered to be the maximum penetration ever likely to occur, a penetration of

450 mm was suggested as a realistic figure to use for design purposes. Since 1969 the basis of the design procedure (Department of Transport 1975) has therefore been to require that all materials used within 450 mm of the road surface must be non-frost- susceptible. The test that is used to establish whether or not a material is frost-susceptible is the TRRL frost- heave test, which was introduced by Croney and Jacobs in 1967, but has since been modified (Roe and Webster 1984).

The review by Croney and Jacobs was published only four years after the winter of 1962/63 which was the coldest winter recorded in Great Britain for more than 200 years. That winter undoubtedly influenced their conclusions and, as it was fol lowed by a 16-year period during which there were no severe winters, they have been criticised for adopting an over-cautious approach, It has been suggested that the 450 mm limit might be varied or not applied so generally. Because of this and because of the changes that have been made to the TRRL frost-heave test a more detailed examination of temperature conditions in Great Britain has been made to see if any changes are required to the existing specification for preventing damage to road foundations. This report describes the results of the investigation.

2 TYPES OF FROST D A M A G E

2.1 FROST-SHA'n'ERING The most common form of frost damage to roads is frost-shattering. This occurs when the water penetrates into the surface of a road and expands as it freezes in the voids of the road pavement layers. Because all public highways have a sealed surface there is no need for a freeze-thaw test to measure the susceptibil ity of materials to this kind of damage. Provided the road is kept sealed and well drained such damage should not occur, and if it does, it is a result of faulty construction or lack of maintenance. It is most common where a bituminous surfacing is nearing the end of its useful life and the remedy is, of course, to ensure that the surface is well sealed.

2.2 FROST-HEAVE More serious than frost-shattering is the problem of frost-heave. This can occur during prolonged periods of freezing when the temperature within the unbound layers of the road pavement falls below 0°C. When this occurs one of the conditions for frost-heave is met and, if other conditions allow, water wil l be drawn from the water-table into the zone which is in the

process of freezing where it will form ice lenses. As these form, further water is drawn into the freezing zone, more ice is formed causing considerable expansion and as a result, heaving of the road surface may occur. After thawing, the foundations of the road are weakened by the excess moisture and the whole pavement structure may fail.

For frost-heave to occur three conditions must be satisfied:

(a) the materials in the frozen zone must be susceptible to frost-heave;

(b) water must be able to move freely from the water table into the freezing zone;

(c) frost must penetrate into the unbound layers of the road pavement and/or subgrade.

If any one of these three is absent heave will not occur, and any procedure for avoiding heave damage should keep this in mind.

Condition (a) for frost susceptibility is tested by the TRRL frost-heave test (Roe and Webster 1984). Condition (b) is restricted by relying on good drainage. However, since this may not be possible throughout the life of the road the design procedure makes the pessimistic assumption that the water-table is always high. Whether or not this assumption is justified is open to question and laboratory investigations (Burns 1977) have shown that when the water-table is lowered no heave will occur even with frost-susceptible materials.

The extent to which frost will penetrate the road is the major unknown factor. The present procedure relies on the conclusion based on the work of Croney and Jacobs that penetration of 450 mm is likely to occur at least once during the 20 year design life of a road anywhere in Great Britain. The remainder of this report is concerned with a reappraisal of this conclusion on the basis of a study of temperature records made available by the Meteorological Office.

3 A S S E S S I N G FROST P E N E T R A T I O N

3.1 FROM MEASUREMENTS IN THE ROAD

Frost penetration into a road pavement depends on the temperature levels experienced, the duration of the cold spell and the thermal properties of the road pavement layers. To establish the geographical and yearly variations of frost penetration it would be necessary to monitor temperature conditions at a large number of road sites throughout the country. This would be a large undertaking but even if it were practicable the results would be of limited value because of the infrequent occurrence of severe winters.

Results are available for some road sites but these are too few to enable any general conclusions to be drawn.

3.2 FROM MEASUREMENTS OF AIR TEMPERATURE

In the absence of detailed records of temperature measurements in road pavements an alternative approach has to be adopted so that use can be made of the extensive records of air temperature measurements that are available. This problem is not confined to Great Britain and research has been carried out in many countries to find how air temperature measurements can be used to predict frost penetration; for example a review of the literature (Moulton 1969) listed more than 150 references relating to the prediction of frost penetration.

3.2.1 Frost Index: Nearly all the research carried out on this topic has been based on the calculation of a 'Frost Index' which is a measure of the severity and duration of a cold period. The frost index is measured in units of degree days usually calculated in terms of the time during which the temperature is below 0°C. Thus a day with an average temperature of - 4 ° C would count as 4 degree days provided that the temperature was continually below 0°C. The magnitude of 'coldness' for any selected period is obtained by adding all the freezing indices together for all the days in that period. For example, a frost index of 20 degree days could be made up from 20 days at - 1°C or 10 days at -2°C or 5 days at - 2 ° C followed by 10 days at - 1°C provided that during these periods the temperature stayed below 0°C. If the temperature rises above 0°C during a cold period the calculations become more complicated; this aspect is considered later in Sections 4.2.2 and 6.

In order to gain an impression of the frost index values likely under British conditions data were provided by the Meteorological Office for 16 sites in Scotland for the 20 year period 1959/60-1978/79. Table 1 gives some of these data; frost index values are tabulated for nine of the winters ranked in order of severity, ie the three most severe, three mildest and three average. The values range from 1 degree day on the coast during the mildest winter to 302 degree days in the Highlands in the most severe winter. Even during the very severe winter 1962/63 there is a range from 29 to 302 degree days.

3.2.2. Calculation of frost p e n e t r a t i o n :

Most calculations of frost penetration from air- temperature measurements are based on the theoretical linear relation that exists between frost penetration and the square root of the frost index. The most widely used formula for relating these two factors is the modified Berggren equation (Moulton 1969) (Kersten 1959):

==1

111 < I--

>..

~b

-0

o

>

0

c-

c -

p ~

t ~

d,

m

o

e-

t.-

r -

t-.

-1

c -

o o

¢.-

-.,1

m >

x

e,-

v o

0

¢-

>

O0

o9

~o

p~ co~ co~ (.o

0

O~ ~ t~

p~

r~O v P~

0 p~A

(D

0")

00 v P~

~v

t.tJ a~ D~

.J

LIJ

< Z

m o ~ o ~ ~ o ~ o ~

~ ~ 0 ~ ~ ~ 0 ~ ~

(D ¢=" > .~-

i i ~E£ 6 ~ ~ ' ~

kO

C.O

O0

I.D 0

O0 ILl

where h =depth of frost penetration into a uniform soil in feet

k =thermal conduct ivi ty of the soil in BTU/sq f t /deg F / f t /h r

F= frost index in degree Farenheit days L = volumetric latent heat of fusion of ice in

BTU/cu ft and k =a correction coefficient which depends on

the thermal properties of both the frozen and unfrozen soils.

[The units have been left in imperial form for ease of reference.]

When all the factors in the equation are known, good agreement between calculated and measured frost penetration can be obtained. Unfortunately, with the diversity of materials that may be found in the various layers making up the road structure, it is not possible to make more than an estimate of frost penetration from the frost index except in those few instances where the thermal properties of all the layers can be assessed.

Figure 1, (which is derived from Johnson, Berk et al 1974), gives an indication of how the thermal properties of a soil affect frost penetration. This shows, for example, that penetration can vary between 200 mm and 650 mm for a frost index of 100 (~/I = 10). The main reason for this variation is the manner in which thermal conductivity varies with density and moisture content illustrated in Figures 2 and 3 (from Aldrich 1956). Another important factor affecting penetration when soil conditions are constant is the residual heat from different summer and autumn conditions before winter begins.

The values of frost penetration in Figure 1 correspond well with French work (Rouques and Caniard 1975) which reports that the relation between frost penetration and frost index can be written in the form: --

H = Ax/I

where H = penetration, in centimetres I =frost index in degree C days A = is a factor which may vary between 3 and

10 with an average value of 5.

0

° E, 1 0 0

200 m

300

400 --

m

m

900 - -

1000

1100 B

1200

1300

500 E

g 600 ¢g

~ 7 0 0

o 8oo L I .

1 2 3

Fig.1

(Frost Index) y2 (Degree C days)

5 6 7 8 9 10 11 12 13 14 15 16

I I I I I I I I I I I

1. Clay and damp top soil

2. Si l t and Sandy Silt

3. Si l ty Sand

4. Sand and Si l ty Sandy Gravel

5. Gravel and Sand

6. Glacial Ti l l

7. Wel l -drained, Sandy Gravel

8. Port land Cement Concrete

'1

2

3 4

I I i I I I 25 50 75 100 150 200

Frost Index (Degree C days)

Relation between Frost Index and frost penetration into snow-free homogeneous materials (Derived from Johnson, Berk et al. 1974)

40

E

t~

E o

30

20

10

0 I.

0

Unfrozen soil

I /

/ / / ;

~ d

I Sandy soil 70 80 "Yd = 90 90 ~ ~ Si l t and clayey soil I /

/ 100 ")'d = Dry Density in Ib/ f t 3

/ 110 i / 110 100 Ib / f t 3 = 1.602 Mg/m 3

/ / i / / ' / / 140

0.5 1.0 1.5 2.0 2.5 k u

Fig.2 Thermal conductivity, ku, for unfrozen soil (Aldrich, 1956)

40

v

(J

30

20

10

Frozen soil

_ / / ,

/ / / /

0 0.5

Fig.3

70 I / 80

/ / 90 90 - - - -

/ i-lOO / / / / / . , / ~ / / / /

Sandy soil

Si l t and clay soil

~/'d = 100

110

120

-~ ' - - - - - '~ , - I I 1.0 1.5 2.0

kf

Thermal conductivity, kf, for frozen soil (Aldrich, 1956)

130

140

2.5

If the suggested value of 5 is taken for A this formula gives a calculated frost penetration of 500 mm for a frost index of 100 degree C days; this is the same value as that given in Figure 1 for a sandy gravel.

Although, in the absence of data on the thermal properties of the road pavement, it would be possible to make only a rough estimate of frost penetration from the frost index, such an estimate would still be of value in considering the design procedure. It would enable conclusions to be drawn about the relative extent of frost penetration in different parts of the country which, as the data in Table 1 would suggest, could vary widely. In particular the estimate of penetration would establish whether the present assumption of a maximum penetration of 450 mm in all parts of the country regardless of geographical location was well-founded. As a first stage a more detailed examination of the meteorological records was therefore carried out to find the extent to which winter temperatures varied with geographical location and from year to year.

4 EXAMINATION OF THE METEOROLOGICAL RECORDS

4.1 S E L E C T I O N O F S I T E S

Air temperatures are recorded throughout the year at many sites in Great Britain. The Meteorological Office was asked to select 50 sites for which computerised records were available which would provide a reasonable coverage of the country, broadly following the motorway/trunk road network. The weather stations have been given code numbers and their approximate locations are illustrated in Figure 4.

4.2 D A T A E X T R A C T E D For each of the sites the records were examined and the following data extracted for the period 1959-1979, where available.

!°

, d

® ®

O'

t S IL J s.,o

P

@

.

I

IT I

I

| |

@ @@.

) @

%® ®

®

% @ (9@

® .

Fig.4 Locations of meteorological stations whose records were examined

6

No. of winters when frost index

Weather Station 25.1-50.0

1. Rosewarne 2. Exeter 3. Wincanton 4. Boscombe Down 5. Easthampstead 6. Heathrow 7. Gillingham 8. East Mailing 9. Dover

10. Rhoose

11. Filton 12. Cheltenham 13. Letcombe Regis 14. Rothamstead 15. Wattisham 16. Woburn 17. Rugby 18. Newtown Linford 19. Nottingham 20. Penkridge

21. Aberystwyth

22. Ruthin 23. Keele 24. Ringway 25. Sheffield 26. Preston 27. Huddersfield 28. Pontefract 29. Hull 30. Harrogate

31. Durham 32. Cockle Park 33. Newton Rigg 34. Carlisle 35. Dumfries 36. Eskdalemuir 37. Prestwick 38a. Glasgow (Renfrew) 38b. (Abbotsinch) 39. Turnhouse 40. Stirling

41. Perth 42. Faskally 43. Dalwhinnie 44. Aberdeen 45. Tomatin 46. Inverness 47. Wick 48. Benbecula

50.1-75.0

1 11 6 9 3 8 5

12 0 0

11 10 7 7 9 8 4 8 5 6

(°C days) was in range given

0 0 3 4 2 1 1 1 1 1

0 2 5 5 4 5 2 6 1 4

8 10 8 6 9 8 7 5 8

7 9 7 7 9 1

10

4

4 3

4 3 0 6 0 3 7 4

2 4 2 0 1 0 2 0 6

5 3 3 4 6 4 5

6

9 4

5 3 0 3 0 7 6 0

TABLE 2

Analysis of winter frost index values for weather stations studied

75.1-100.0

0 0 1 1 2 0 0 1 0 0

1 1 1 1 1 2 1 0 0 3

0

2 1 0 0 2 1 0 1 1

2 0 5 4 1 4 1

5

2 4

5 7 1 3 0 2 0 0

100.1

0 1 0 1 0 1 1 1 0 0

1 1 1 2 1 2 1 2 1 1

0

1 1 1 1 1 1 1 0 2

1 1 3 1 1

10 1

1

2 2

2 5 2 0 2 0 0 0

+

Period covered

(between 1957 and 1979)

59-78 59-78 63-79 59-78 71-78 59-78 59-78 59-78 59-75 59-75

59-78 59-78 60-79 59-78 59-78 59-78 59-78 59-78 59-77 59-78

59-76

60-78 59-78 59-78 59-78 59-78 59-73 59-78 59-78 59-78

59-78 59-78 59-78 61-79 59-78 59-78 59-78 59-65 67-78 59-68 61-79

59-78 59-78 74-78 75-79 75-79 59-78 59-78 59-78

Missing date

74-76

67/8, 73/4

67-70 69-71, 73/4

73/4 68-70, 74/5

64-71

73-74 61/2, 65/6, 75/6

61-63, 65-67, 69-71 70/1, 73/4

7112 6011 6819, 71/2

66/7

66-70

75/6 70/1 75/6 76-78 76-78 74/5, 76/7

Weather Station

1. Rosewarne 2. Exeter 3. Wincanton 4. Boscombe Down 5. Easthampstead 6. Heathrow 7. Gillingham 8. East Mailing 9. Dover

10. Rhoose

11. Filton 12. Cheltenham 13. Letcombe Regis 14. Rothamstead 15. Watt isham 16. Woburn 17. Rugby

18. Newtown Linford 19. Nott ingham 20. Penkridge

21. Aberystwyth 22. Ruthin 23. Keele 24. Ringway 25. Sheffield 26. Preston 27. Huddersfield 28. Pontefract 29. Hull 30. Harrogate

31. Durham 32. Cockle Park 33. Newton Rigg 34. Carlisle 35. Dumfries 36. Eskdalemuir 37. Prestwick 38a. Glasgow (Renfrew) 38b. (Abbotsinch) 39. Turnhouse 40. Stirl ing

41. Perth 42. Faskally 43. Dalwhinnie 44. Aberdeen 45. Tomatin 46. Inverness 47. Wick 46. Benbecula

T A B L E 3

Analysis of continuous freezing spells at weather

Number of occasions when freezing spell lasted for number of days

in range given

1-9 10-14 15-19 20+ I I I

22 0 0 0 2.4 54 0 0 1 3.5 72 3 0 1 3.5 83 4 0 2 4.0 27 1 0 0 3.1 69 1 0 1 3.3 67 0 1 1 3.5 74 3 1 1 3.8

-24 3 0 0 4.1

18 1 0

83 2 0 89 1 0 95 5 1 96 5 1 82 5 0 98 3 1 55 3 0

0

1 1 1 1 2 1 2

1 1 1

0 1 2 0 1 0 0 1 0 0

0 0 1 0 0 2 0

0

0 1

95 4 1 67 2 0 97 4 1

27 2 0 82 3 1

100 3 0 79 3 2 77 3 0 84 1 1 72 2 2 81 5 0 65 4 0

108 4 2

108 3 2 74 5 3

110 5 2 86 4 1

106 3 2 174 5 4 96 3 0

127 2 0

106 6 1 98 1 1

Average length of freezing

spell (days)

3.7

3.3 3.4 3.5 4.0 4.3 3.9 4.4

4.2 3.8 4.2

3.8 4.2 3.0 3.6 4.2 3.7 3.7 3.2 4.0 3.8

3.3 3.4 3.5 4.1 3.9 3.9 3.3

3.1

3.5 3.8

122 4 1 0 3.6 150 4 2 1 3.6 66 3 3 2 4.5 93 5 0 0 3.6 61 5 1 1 4.2 85 3 0 0 3.5 95 5 0 0 3.2 49 0 0 0 2.6

;tations studied

Longest freezing

spell recorded

(days)

5 37 22 36 12 21 2O 21 11

10

37 36 35 36 35 35 36

36 36 38

13 38 36 19 4

18 19 34 14 19

17 19 20 18 17 35 13

10

15 2O

17 31 29 13 28 10 13 7

Period covered

59-78 57 -78 61-79 59-78 71-78 59 -78 59-78 59 -78 59-78

59-78

57-78 59 -78 60-79 59-78 59-78 59-78 59-78

59-78 59-78 59-78

59 -78 59-79 59 -78 59-78 59 -78 59-78 59-77 59-78 59-78 59-78

59 -78 59-78 59-78 61-79 57-78 57-77 57-78 57 -66 66-78 59 -78 60-79

59-78 59-78 73-79 59-78 73-79 59 -78 57-78 57-78

Missing data

Dec 70, Feb, Mar 72

Nov 64-Dec 70, Mar 72

Jan-Dec 71

Nov-Dec 61

21 Feb 72

Jan 67-Feb 70, 18 Feb 72

Mar 60

4.2.1 Frost index: The cumulative degree days below zero (ie frost index) in each winter was determined for each site and the data arranged as a frequency distribution in Table 2. (The average daily temperature was taken as being half the sum of the night minimum and the day maximum.)

4.2.2 Duration of freezing period: An important factor in the development of frost-heave is the duration of the freezing period. The records were also examined in this respect and the frequency distributions of the freezing period lengths were determined. A freezing period was defined as being from the time the temperature fell below 0°C until it rose above this value, except that if two successive cold spells were separated by not more than 3 days during which time the temperature did not exceed + 1°C, they were regarded as one. The distributions are recorded in Table 3.

5 DISCUSSION OF RESULTS

5.1 WINTER SEVERITY The results in Tables 2 and 3 shows how the severity of British winters varies from year to year and with geographical location; this may be illustrated (in terms

of frost index) by taking the mean frost index of all the weather stations studied for each winter, Figure 5. This clearly shows the variation; mean frost index ranged from 10 to 130 degree C days with 1973/4 particularly mild and 1962/3 exceptionally cold. The overall mean over the whole period is 50 degree C days, in good agreement with the data in Table 1.

The geographical variation of severity for the sites investigated is illustrated by Figure 6, which shows the mean frost index for the whole period at each station. As would be expected there is considerable variation with western coastal sites being warmer and inland upland sites colder than average. Every site had some frost index in every winter.

In terms of freezing period duration, spells of 1-9 days were common. The average freezing period length ranged from 2-5 days at the warmer sites to 4-5 days at the colder. The longest periods recorded all occurred during the exceptionally severe winter of 1962/63 and show considerable variation around the country the shortest being 5 days at Rosewarne in Cornwall and the longest being 38 days at Penkridge in Staffordshire and Ruthin in N. Wales. All sites except Rosewarne and Benbecula had experienced at least one freezing period of 10 days during the period examined.

"~ 100

~ 8o D

- - 6 0

u _

N 4 0

140

120

20

59/60

Overall mean = 50 degree days

61/2 63/4 65/6 67/8 69/70 71/2 73/4 75 /6

Winter

Note: Data for 78/9 (from Table 1 augmented wi th data for 3 other Scottish and English sites) included for comparison

Fig. 5 M e a n Frost Index over sites studied for each winter

77/8

.# eo

~.° -LL

Degree C days

~ 150

100

50

0

Unshaded bar indicates signif icant gap(s) in data fo r that site

B. ~.,.J

0

~ °

. 0

D~ me

°£]. ° ~

i S

" ~L. "==

I |

I "" , / |

r-l°

~ e ~°

.B

a . [L .~"" me

.B

Fig.6 Mean yearly Frost Index for the sites studied

10

Croney and Jacobs had concluded that at least 10 days of continuous freezing are required to bring about conditions that might give rise to frost-heave in the unbound layers of the road pavement. They estimated that winters that produce such conditions are likely to occur in most parts of Great Britain at least once every 11 years. The data, in Tables 2 and 3 do not invalidate the latter conclusion but they suggest that, in their assumption that temperature conditions are similar over the whole country during severe frosts, Croney and Jacobs were strongly influenced by the 1962/63 winter, which had happened only four years before they published their report.

The 1962/63 winter was the coldest recorded in most parts of Great Britain for more than 200 years. As roads are designed for a 20 year life, a once in 200 year event should not be allowed to have an undue significance. However, before a change can be made to the existing situation an assessment needs to be made of the probably risk to a road during its life- span.

5.2 RISK OF FROST-HEAVE In order to use frost index data to assess the risk of frost-heave occurring in roads some assumptions are necessary:

(i) the water table is high

(ii) there are at least 250 mm of bound construction (where there is less the risk of damage is greater).

A single cold spell having a frost-index of about 50 degree days would typically be needed to produce a penetration into the unbound layers of the road (x=40,JI mm ie 40,J50-~280 mm). The data in Table 2 cannot be used for the purpose of calculating frost penetrations because it is cumulative for the whole winter and does not distinguish between a long very cold spell and the sum of a number of much shorter cold spells. For example at Penkridge during the period 1959-78 there were four winters when a frost index between 50 and 75 degree days was recorded, three in which a frost index of 75-100 degree days was recorded and one where the frost index was in excess of 100 degree days.

The frost index in excess of 100 degree days occurred in 1963 when 38 days of continuous freezing was recorded. This would be enough to give a frost penetration in excess of 250 mm but for the other winters it is impossible to deduce from the data in Tables 2 and 3 the extent to which frost penetration in excess of 250 mm occurred during other winters at Penkridge. A similar situation exists for most of the other sites and all that can be deduced from Tables 2 and 3 is that in the period 1959-1978:--

(i) Rosewarne, Aberystwyth and Benbecula did not record a frost index of more than 50 degree days and hence frost penetrations in excess of 250 mm were unlikely to be experienced.

(ii) all other sites experienced at least one occasion when frost penetrations in excess of 250 mm probably occurred.

(iii) If the winter of 1962/63 is excluded Exeter, Dover, Rhoose, Sheffield and Hull did not experience winters likely to have caused frost penetrations in excess of 250 mm.

Even from these limited conclusions it is evident that the requirement that frost susceptible materials should be excluded from the top 450 mm of the road pavement is unduly conservative for much of the country. The problem lies in the identif ication of those areas where the requirement could be modified. This is discussed in the next section of the report.

6 FURTHER EXAMINATION OF METEOROLOGICAL RECORDS

As already mentioned the data in Tables 2 and 3 cannot be used to assess frost penetration because the frost indices are cumulative for whole winters. To gain a better impression it is necessary to know the number of separate occasions during a winter in which periods of freezing occur that could give rise to significant depths of frost penetration. As a first step towards this, data from seven sites were examined in some detail to find the individual cold spells that occurred at these sites during the period 1959-1981. Each cold spell was defined as the time from when the mean daily temperature fell below 0°C until it rose above 0°C. The results of this examination are given in Table 4.

A comparison of the results in Table 4 with those in Tables 2 and 3 shows how an assessment of the cumulative frost index for a whole winter gives a false impression of the likely degree of frost penetration. Taking the figures for Rothamstead as an example, Table 2 shows 15 occasions when the frost index was above 25 degree days compared with six occasions listed in Table 4. Table 4 also shows that frost indices in excess of 50 degree days are uncommon and only Braemar in the Highlands of Scotland recorded this value more than once.

Unfortunately whi lst the figures in Tables 2 and 3 give a pessimistic impression, those in Table 4 give an optimistic one because they completely ignore the cumulative effect of two cold spells being separated by a short milder period. For example Table 2 shows that Rothamstead experienced 36 days of continuous freezing from 23rd December 1962 until 27th January 1963; there was then a short break of three days but temperatures fell below freezing again from 30th January until 8th February. In this particular example there was another 18 days of continuous freezing from 14th February until 3rd March. Clearly in such instances the second and subsequent cold spells should be added to the first spell but the problem lies in deciding when

11

TABLE 4

Individual frost indices at 7 stations during the period 1959-1981

Cold spell* Total No. of Degree degree

Station from to days days days

Rosewarne -- -- 0 0 0

Aberporth 17. 1.63 26. 1.63 10 28 28

Ringway 23.12.61 4. 1.62 14 32 11. 1.63 27. 1.63 17 61 93

Wick 24.12.61 1. 1.62 8 25 6. 1.63 15. 1.63 10 26

15. 2.78 22. 2.78 8 26 77

Turnhouse

Rothamstead

5. 1.59 17. 1.59 25.12.61 5. 1.62 11. 1.63 19. 1.63 13. 2.69 20. 2.69 15. 2.78 22. 2.78 7.12.81 14.12.81

15.12.81 21.12.81

23.12.62 27. 1.63 30. 1.63 8. 2.63 14. 2.63 3. 3.63 30.12.78 8. 1.79 21. 1.79 1. 2.79

8.12.81 15.12.81

13 12 9 8 8 8 7

36 11 18 10 12 8

37 38 29 27 45 25 42

137 25 35 35 26 29

243

287

Braemar 29 cold spells recorded 7-29 27-165 1,650

*Cold spell defined as a period between the t ime when the mean daily temperature fell below 0°C until it rose above 0°C

cold spells should be added together and when they should be regarded as separate incidents.

It was therefore decided to make a more detailed examination of three of the sites to ascertain how the effect of 'bridging periods' when temperatures were marginally above 0°C affected the frost index calculations. The examples taken were for bridging periods of 1, 2 and 3 days and for each of these periods allowing the mean daily temperature to rise to 1°, 2 ° and 3°C. Degree day analyses which allowed for these nine types of bridging were carried out for Rothamstead, Aberporth and Braemar. The results for Rothamstead are given in Table 5 and summaries for all three stations are given in Table 6.

6.1 D I S C U S S I O N OF RESULTS It is of interest to note that the results in Tables 5 and 6 show that if bridging periods are not taken into account false conclusions can easily be drawn about the severity of a cold spell. However, once some bridging is allowed, the frost index generally is not

particularly sensitive to the bridging criterion adopted apart from evidence from the already very severe 1962-3 winter.

The results of the limited survey showed that the introduction of bridging periods in the calculation of frost indices had a significant effect on the apparent severity of the cold spells. Table 6 shows that at Rothamstead there was only one incident of a frost index in excess of 50 degree days but three incidents if a bridging period of 1 day in which the temperature rose to 3°C between cold spells was allowed. (The apparent decrease if up to three days were allowed arose because two cold spells with frost indices of 137 and 64 merged together to give a frost index of 201 degree days).

Two of the three sites chosen represented extreme conditions for Great Britain. Aberporth on the coast of Cardigan Bay did not experience any cold spells during the period 1959-1981 when the frost index exceeded 50 degree days; Braemar in the Scottish Highlands reported 13 such incidents even when no allowance was made for bridging periods. Rothamstead

12

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T A B L E 6

Frequency of cold spells at three meteorological stations 1959-1981

Station

Aberporth

Rothamstead

Braemar

Frost Index (degree

days)

50

50 100 200

50 100 150 200

No Bridging

0

1 1 0

13 5 1 0

No of occasions when given frost index was exceeded, using different bridging criteria

Mean daily temperatures rising to up t o : - -

1 ° C for : - - 2°C for : - - 3°C for : - -

1 day 2 days 3 days

0 0 0

2 2 2 1 1 1 0 0 0

12 12 12 5 5 5 2 2 2 0 0 0

1 day 2 days 3 days

0 0 0

3 3 2 1 1 1 0 0 1

14 16 15 5 5 4 2 2 1 0 0 1

1 day 2 days 3 days

0 0 0

3 3 2 1 1 1 0 0 1

14 16 17 4 5 4 3 3 2 0 0 1

which is more typical of inland, lowland Britain reported one cold spell of 137 degree days in the winter of 1962/63 and if allowance is made for bridging periods during the winter a frost index of 201 degree days was recorded. Apart from this winter the only other winter in which a frost index in excess of 50 was recorded was the winter of 1981/82 when, if allowance is made for a short bridging period of one day at 1°C, a frost index of 56 degree days occurred.

7 CONCLUSIONS The results of the more detailed survey do not alter the conclusions drawn in Section 5.2 and suggest that for most of Britain frost penetrations of 250-300 mm will occur several t imes during the design life of a road but penetrations in excess of this, although uncommon, need to be considered. However, even from the limited study of the temperature conditions that prevail in Great Britain it is clear that the present assumption made for design purposes, that road pavements should be treated as being at equal risk to frost penetration, is untenable.

There are certain areas, notably on the south-west and western sides of the country, where there is little risk of frost penetration into the unbound layers of the road. In these areas a requirement that materials used within 450 mm of the surface should not be frost susceptible as measured by the TRRL frost-heave test is not necessary. It would be appropriate in such localities to reduce the thickness of non-susceptible material that is required to about 200 ram. Conversely there are other areas such as the Scottish Highlands

where penetrations of about 450 mm are likely to be commonplace and no relaxation of the present requirements could be contemplated.

For most of inland, lowland Britain frost penetration into the unbound layers of the road is likely to occur at least once in the design life of the road. However whether or not frost will penetrate as far as 450 mm is open to question and it may well be that in many cases some lower figure for maximum penetration could be used. There would be considerable consequent economic benefits if the amount of non frost-susceptible construction could be tailored to suit the likely frost penetration.

Clearly there are places (Aberporth, for example) where an examination of the records could quickly show that the risk of a frost penetration in excess of 200 mm is negligible, in which case it would be acceptable to use frost-susceptible materials nearer the surface than 450 mm. For most cases, however, the situation would be less clear-cut. Further research is required to follow up the work described in this repoR, using the extensive computerised records of air temperatures t o : - -

(a) determine appropriate bridging criteria for the definition of continuous freezing spells;

(b) Using these criteria investigate the frost indices obtained from a wide sample of sites, and hence establish the risk of frost penetrating to given depths in different areas;

(c) identify how this information might be used to establish at the design stage what thickness of non frost-susceptible construction is needed.

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8 ACKNOWLEDGEMENTS The assistance of the Meteorological Office in making available the temperature records included in this report is gratefully acknowledged as also is the help of Mr J Burns of the Scottish Branch of the Laboratory. The work was carried out in the Pavement Materials and Construction Division (Division Head: Mr G F Salt) of the Highways and Structures Department of the Laboratory.

9 REFERENCES ALDRICH, H P (1956) Frost penetration below highway and airfield pavements. Highway Research Board Bulletin 135, Washington.

BURNS, J (1977). The effect of water table on the frost-susceptibility of a roadmaking material. Department of the Environment Department of Transport TRRL Report SR 305: Transport and Road Research Laboratory, Crowthorne.

CRONEY, D and J C JACOBS (1967). The frost susceptibility of soils and road materials. Ministry of Transport RRL Report LR 90: Road Research Laboratory, Crowthorne.

DEPARTMENT OF TRANSPORT (1975). Specification for roads and bridge works. Department of Transport, London, HMSO.

JOHNSON, T C, BERK, R L, CAREY, K W and C W KAPLAR (1974). Roadway design in seasonal frost areas. US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory Project 20-5. Hanover, New Hampshire.

KERSTEN, M S (1959). Frost penetration: Relationship to air temperatures and other factors. Highway Research Board Bulletin 225, Washington.

MOULTON, L K (1969). Prediction of the depth of frost penetration. A Review of the Literature. Engineering Experiment Station, West Virginia, Report 5.

ROUQUES, G and L CANIARD (1975). Gel et degel des chausses. Note d'lnformation Technique. LCPC Paris. Laboratorie Central des Ponts et Chausses.

ROE, P G and D C WEBSTER (1984). Specification for the TRRL Frost Heave Test. Department of the Environment Department of Transport TRRL Report SR 829: Transport and Road Research Laboratory, Crowthorne.

SHERWOOD, P T (1981). British experience with the frost-susceptibility of roadmaking materials. Frost I Jord No. 22, November 1981, OSLO. The Norwegian Committee on Permafrost.

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