5-03 Safety Guide for Operations Over Ice

[Version franšaise]


Safety guide for operations over ice





Properties of ice covers

Ice formation

Ice colour

Ice thickness

Bearing capability of ice


Determining ice thickness


Parked and stationary loads

Effects of speed


Spring thaw

Preparation of ice bridges

Building techniques




Operating precautions

The use of snowmobiles on ice covers


Operation precautions




Appendix A - Thickness of Good Quality Fresh Water Ice

afety guide for operations over ice

1.    Introduction

1.1   General

1.1.1      Ice covers are used for transportation routes, as a surface on
           which structures can be erected, and for the temporary storage
           of materials.

1.1.2      This guide is concerned primarily with fresh water ice bridges,
           which are intended to support a gross vehicle weight of no more
           than 25 tons (22.5 tonnes).  An ice bridge can be a natural
           untouched ice cover, a built-up, or a combined reinforced and
           built-up crossing route.

1.1.3      When loads are expected to exceed 25 tons (22.5 tonnes) or when
           operations will be conducted over salt water ice covers, advice
           should be sought from the Geotechnical Section, Division of
           Building Research, National Research Council of Canada, Ottawa,
           Ontario, K1A 0R6.

1.1.4      Information on the safe use of ice covers for aircraft
           operations is available from Transport Canada.

1.2   Purpose

1.2.1      The purpose of this safety guide is to:

      (a)  specify rules of good safety practice for all Public Service
           employees engaged in operations on ice covers;

      (b)  provide information on the thickness of ice required to support
           moving and stationary loads;

      (c)  specify methods for determining ice thickness and quality; and

      (d)  outline approved methods for the preparation and maintenance of
           ice bridges.

2.    Properties of ice covers

2.1   Ice formation

2.1.1      Ice forms on fresh water when the surface temperature falls to
           zero degrees Celsius, or at lower temperatures if dissolved
           impurities are present.  While the underside of the ice cover
           in contact with the water will remain close to the melting
           temperature, the upper surface will be nearer to the
           surrounding air temperature.

2.1.2      The date of annual freeze-up, the rate of ice growth, and the
           quality of the ice cover depend on various factors such as air
           temperature, solar radiation, wind speed, snow cover, wave
           action, currents, and the size and depth of the water body. 
           Generally, small lakes and slow-moving streams freeze over
           earlier than larger lakes or fast moving streams.

2.1.3      While there are many different types of ice, the two types of
           major concern are:

      (a)  clear ice - formed by the freezing of water;

      (b)  snow ice - formed when water-saturated snow freezes on top of
           ice, making an opaque white ice which is not as strong as clear

2.2   Ice colour

2.2.1      The colour of ice, which may range from blue to white to grey,
           provides an indication of its quality and strength:

      (a)  clear blue ice is generally the strongest;

      (b)  white opaque ice (snow ice) has a relatively high air content,
           and its strength depends on the density:  the lower the density
           the weaker the ice; but high density white ice has a strength
           approaching that of blue ice;

      (c)  grey ice generally indicates the presence of water as a result
           of thawing, and must be considered highly suspect as a
           load-bearing surface.

2.3   Ice thickness

2.3.1      The other major factor determining the bearing capability of
           ice is its thickness.  Care must be taken when determining the
           thickness of ice covers to ensure that the readings are
           properly taken and are an accurate representation of the area
           under consideration.

2.3.2      Currents have a distinct bearing on the temperature required to
           form ice.  Rivers and channels with strong currents may remain
           open all winter despite low air temperatures.  Springs can
           cause currents, and also be the source of warmer water;
           currents can also cause variations in ice thickness without
           changing the uniform surface characteristics.

2.3.3      When selecting the site of an ice bridge, currents and springs
           should be located and avoided.  Frequent checks of the ice
           thickness should be made in areas suspected of being affected
           by currents.

2.3.4      Ice under an insulating snow blanket thickens very slowly even
           in low temperatures.  A heavy snow cover, before significant
           ice growth, may cause the ice to remain unsafe throughout the

3.    Bearing capability of ice

3.1   General

3.1.1      The load bearing capacity of ice covers depends on the quality
           of ice, its thickness, ice and air temperatures, temperature
           changes and solar radiation.

3.1.2      Clear blue ice is the standard of quality against which other
           types of ice are compared.  White opaque ice, or snow ice, is
           normally considered to be only half as strong.

3.1.3      Ice covers may consist of alternate layers of clear ice and
           snow ice, and each layer should be measured so that the
           effective thickness may be calculated.  For example, an ice
           cover with a total thickness of 8 inches (20 cm) consisting of
           a 4 inch (10 cm) layer of clear ice and a 4 inch (10 cm) layer
           of snow ice would have an effective thickness of 6 inches
           (15 cm).

3.1.4      The strength of ice is generally increased by low temperatures. 
           The increase is progressive from zero to minus eighteen degrees
           Celsius and remains fairly constant below this point.  However,
           a marked drop in temperature can temporarily cause internal
           stress in an ice cover and reduce its bearing capacity.  This
           can often occur during overnight periods when the temperature
           is much lower than the preceding average for the day.

3.1.5      The removal of snow from an ice cover during periods of low
           temperature has an effect similar to a marked temperature drop.
           The bearing capacity of ice should be considered to be reduced
           by 50 per cent for 24 hours after these conditions.

3.2   Determining ice thickness

3.2.1 Prior to use, the ice should be measured to determine whether its
      effective thickness is adequate to support the expected load.  The
      graph presented in Appendix A should be used as a guide to the
      required thickness for the loads involved.

3.2.2      To initially determine effective ice thickness, the rule of
           thumb "one inch (2.5 cm) of clear blue ice for every thousand
           pounds (450 kg)" may be used.


Ice that is less than six inches (15 cm) thick should not be used for any
crossing.  Because of natural variations, thickness may be less than
2 inches (5 cm) in some areas.

3.2.3      The effective thickness can vary considerably in an ice cover. 
           In particular, dangerously thin areas can occur due to currents
           in the covers of rivers and estuaries, and on lakes near the
           inlet or outlet of rivers and streams.  Careful attention
           should be given to reduced ice thickness close to shorelines
           and around ridges and leads.

3.2.4      The thickness can be determined by drilling test holes spaced
           at a maximum of 50 feet (15 m) apart in rivers, and 100 feet
           (30 m) apart on a lake.

3.2.5      Crossings should be checked for ice thickness once a week when
           average air temperatures vary between -15 and -5 degrees
           Celsius; and daily when the temperature is above -5 degrees
           Celsius.  Checks can be less frequent when ice thickness
           substantially exceeds requirements.  A new hole should be
           drilled for each ice measurement.

3.2.6      Ice that is no longer supported by water, due to lowering water
           levels, may be too weak to support the loads to be applied;
           conversely, a rising water level can result in the formation of
           two ice layers with an intervening water layer.  Ice thickness
           tests will reveal these conditions.

3.3   Parked and stationary loads

3.3.1      Ice behaves elastically under moving loads; that is, the ice is
           depressed while loaded but recovers its original position after
           the load has passed.

3.3.2      With a stationary load the ice surface will sag continuously
           and may fail, depending on the strength of the ice cover.  The
           safe bearing capability for stationary loads should be
           considered to be 50 per cent less than that for moving loads.

3.3.3      The sequence of failure for stationary loads is as follows:

      (a)  radiating cracks form at the bottom of the cover immediately
           beneath the load (and ultimately propagate through the cover);

      (b)  circular cracks form at the upper surface of the cover at some
           distance from the load (noticeable sagging of the ice may

      (c)  the ice shears in a circle immediately adjacent to the loaded
           surface (failure may be imminent).

3.3.4      The initial radial cracks may not be of immediate concern if
           the load bearing capacity of the ice is substantially higher
           than the load.  However, prolonged application of the load
           should cause concern about possible ice failure.

3.3.5      Stationary loads should be moved under any of the following

      (a)  when radial cracks develop;

      (b)  if noticeable sagging is observed;

      (c)  if the rate of sagging increases;

      (d)  if continuous cracking is heard or observed;

      (e)  if water appears on the surface of the cover.

3.3.6      The accumulation of drifted snow, often caused by stationary
           loads, may mask the indicators listed in paragraph 3.3.5 as
           well as increase the static load on the ice.  Vehicles should
           be parked at least 5 lengths apart and in such a way that snow
           drifts do not interfere with other vehicles.

3.4   Effects of speed

3.4.1      When a vehicle travels over an ice cover, a hydrodynamic or
           resonance wave is set up in the underlying water.  This wave
           travels at a speed that depends upon the depth of the water,
           the thickness of the cover and the degree of elasticity of the
           ice.  If the speed of the vehicle coincides with that of the
           hydrodynamic wave, the stress on the cover due to the wave
           reinforces that due to the vehicle, and can increase the
           maximum stress in the ice to the point of failure.  The wave
           action tends to crack the ice in a checkerboard pattern.

3.4.2      Particular care should be exercised when approaching or
           travelling close to shore, or over shallow water, because of
           more severe stressing of the cover due to reflection of the
           hydrodynamic wave.  Roads and vehicle approaches should meet
           the shoreline at an angle of not less than 45 degrees.

3.4.3      If the weight of a loaded vehicle is one-half or less than that
           determined from Figure 1 as safe for the thickness of the ice
           being used, speed is not critical.  When the weight is greater,
           and for ice thickness less than 30 inches (75 cm), speed should
           be carefully controlled and in general be kept below 10 m/h
           (15 km/h).

3.5   Cracks

3.5.1      The ice usually has many cracks made by thermal contraction or
           movements of the ice cover.  Except at the thaw period cracks
           do not necessarily indicate a reduction in the load-bearing
           capability of the cover.

3.5.2      A dry crack with an opening of less than 1/8 inch (0.32 cm),
           which does not penetrate very deeply into the ice cover, will
           not cause serious weakening.  Where a single dry crack in
           excess of one inch (2.5 cm) is noted, loads should be reduced
           by one third; for intersecting cracks of this size the loads
           should be reduced by two thirds.  Dry cracks should be repaired
           by filling with water or slush.

3.5.3      A wet crack indicates that the crack penetrates completely
           through the ice cover and therefore affects the load bearing
           capacity, which should be reduced by one-half in the case of a
           single wet crack.  If two wet cracks meet at right angles the
           reduction is to one-quarter of that for a good cover.  Most wet
           cracks refreeze as strong as the original ice cover; however a
           core sample should be taken to ascertain the depth of healing.

3.5.4      Due to normal thermal contraction, cracks sometimes form at the
           middle of a road in the direction of travel; but these do not
           seriously reduce the bearing capability if they remain dry.  If
           cracks form parallel to the road, at the sides, they do
           indicate over-stressing (perhaps by snow deposits from clearing
           operations) and possible fatigue due to excessive traffic.  If
           such cracks develop, particularly if they are wet, road use
           should cease at once, and not be recommenced until the cracks
           are healed.

3.5.5      Fluctuating water levels may produce cracks near and generally
           parallel to the shoreline.  These cracks are often accompanied
           by a difference in the levels of the floating and the grounded
           ice.  If these cracks are wet, loads should be reduced
           accordingly.  With extreme level differences, appropriate
           bridging repair (flooding, reinforcing) may be necessary.

3.6   Spring thaw

3.6.1      Ice covers will begin to decay in the spring as the ice warms
           and begins to melt.  The ice will thaw in the sunlight, but in
           the early spring may refreeze at night.  Intensive thawing
           begins only in atmospheric temperatures above freezing.

3.6.2      Snow is a poorer thermal conductor than ice.  A covering of 3
           to 4 inches (7.5 to 10 cm) of clean snow on an ice bridge will
           reduce significantly the solar radiation penetrating the cover,
           thus prolonging the period of use.

3.6.3      Travel over an ice bridge displaying water on the surface
           should be executed with great caution and only if absolutely
           necessary.  If mild weather continues and the water disappears,
           it may indicate that the ice is honey-combed, in which case the
           use of the area as an ice bridge should be discontinued

3.6.4      If the average air temperature has been above zero degrees
           Celsius for three days or more, then use of an ice-bridge
           should cease.

4.    Preparation of ice bridges

4.1   Building techniques

4.1.1      A marked route over a natural ice cover can be utilized as an
           ice bridge, but since this may not provide sufficient strength
           for repetitive use, various techniques may be used to increase
           the safe load-bearing capability.

4.1.2      When temperatures are low and early winter use is not required,
           ice thickness can be increased by keeping the intended crossing
           snow-free, or by compacting the snow so that its normal
           insulating qualities are diminished.  The natural rate of ice
           growth will thus be accelerated and the required thickness will
           eventually be reached.

4.1.3      If there is a need for a bridge when temperatures are not low
           enough to obtain the necessary natural thickness by the time of
           required use, the ice thickness can be increased by flooding: 
           adding water on top of the existing ice cover.

4.2   Flooding

4.2.1      The flooding operation is normally carried out with small
           lightweight pumps, rather than larger pumps which are less

4.2.2      Flooding may be started as soon as the natural ice is about
           3 inches (7.5 cm) thick and strong enough to bear the weight of
           persons and pumps.  The initial flooding should be limited to a
           depth of about one inch (2.5 cm).

4.2.3      Subsequent floodings for "lifts" should be limited to that
           depth of water that will freeze within 12 hours.  As a rule of
           thumb, an average air temperature of -18 degrees Celsius will
           freeze 2 inches (5 cm) of water overnight.  With average
           temperatures of -31 degrees Celsius or lower, lifts may be
           increased to 3 1/2 inches (9 cm).  Wind or snow on the surface
           will increase or decrease the freezing rate respectively.

4.2.4      Thicker lifts can lead to a layer of water between the old ice
           surface and the new layer of ice.  When covered by succeeding
           lifts of warm water, this layer may not freeze until well after
           the bridge has been completed.  Such lifts may also overload
           and crack the existing ice cover.

4.2.5      To achieve maximum strength in the bridge, any snow cover
           should, if possible, be removed before each flooding operation. 
           However, dragging or packing the snow to an even thickness and
           then flooding - "slushing" - provides a thicker sheet in less
           time but the resulting ice is not as strong.

4.2.6      If banks of snow are constructed on each side of the bridge to
           contain the flooding, they should be at least 150 feet
           (45 metres) apart; however, a 200 foot (60 metre) wide bridge
           is preferable.

4.2.7      Snow banks may leak after freezing has begun so that a crust of
           ice is formed with an air-filled void between it and the
           initial ice cover.

4.2.8      Flooding should take place from the bridge centre line, letting
           the water feather out to seek its own level.  This method also
           provides a wider bridge surface.

4.2.9      Ice formed by the flooding process will be stress-free if each
           lift is allowed to become completely frozen before the next

4.3   Reinforcement

4.3.1      An ice bridge built in more temperate climates or intended for
           repeated use may be reinforced with grasses, brush or logs. 
           Such a bridge can then take a greater load for the same
           thickness, being held together by the reinforcing inclusions. 
           It can heal itself more easily after cracking and is less
           likely to fail catastrophically.

4.3.2      One disadvantage to reinforcement is the added time and effort
           required for construction.  Another is the effect of local
           radiational heating of the reinforcing inclusions, particularly
           during the spring thaw, which will increase the rate of decay
           of the bridge.

4.3.3      It is preferable to locate the reinforcing items in the bottom
           portion of the final ice bridge; they should be placed and
           frozen in as early as possible.

4.3.4      Reinforcing logs, properly placed in an ice bridge, will make
           possible a reduction of ice thickness of up to 25 per cent.

4.4   Maintenance

4.4.1      On completion, the following rules should be observed in order
           to increase the safety and life of the ice bridge:

      (a)  The bridge must be kept clear of excessive snow, and the snow
           banks kept well back, with slopes of no more than a ratio of 1
           to 5.  The weight of snow banks can weaken the ice underneath
           and form relatively deep ditches by slow sagging, and therefore
           should be levelled out if higher than 3 feet (1 metre) or two
           thirds of the ice thickness, whichever is the larger.

      (b)  A covering of 3 to 4 inches (7.5 to 10 cm) of compacted snow
           will give good traction and will also provide a cushion.  Glare
           or snow-free ice breaks up rapidly under traffic in extreme

      (c)  The surface should be kept clear of dirt or other dark
           material, such as oil spots, which will absorb solar radiation
           and melt into the ice.  Puddles of water also absorb heat from
           the sun and should be "repaired" by filling with snow.

      (d)  The ice bridge should be checked for cracks daily and on foot,
           and its thickness measured as outlined in article 3.2.  A
           longitudinal crack more or less down the centre line may occur,
           particularly if the ice thickness has been increased by
           flooding.  If dry, this crack is not serious.  Wet cracks
           should be repaired immediately and loads reduced until the
           refreezing process is completed (see article 3.5).

4.5   Operating precautions

4.5.1      Following are a number of general precautions which should be
           taken when testing for ice thickness or crossing ice covers:

      (a)  All persons involved in operations over ice covers should be
           familiar with the hazards involved, the precautions to be taken
           and the basic rescue techniques required in case of a

      (b)  Single persons or single vehicles should not venture onto an
           ice cover when there is no help at hand.

      (c)  When testing, persons on foot should carry long poles, to be
           used as an aid to rescue in case of a breakthrough, or
           alternatively be securely roped together, with minimum spacing
           of 50 feet (15 m).

      (d)  Light vehicles used during test periods and initial build-up
           should be equipped with an extended frame of logs to provide
           support if the vehicles break through the ice cover.

      (e)  A rope at least 50 feet (15 m) long, or equivalent to water
           depth, with a float, may be attached to test vehicles as an aid
           to marking and recovery.

      (f)  Vehicle doors and cab hatches should be removed or lashed open;
           seat belts must NOT be worn.

      (g)  Adequate spacing must be maintained between vehicles; it is
           recommended that an interval of at least 100 feet (30 m) be

      (h)  Vehicle speed should not normally exceed 10 m/h (15 km/h) in
           order to avoid the effects of the hydrodynamic wave, nor should
           speed be less than 1 m/h (1.5 km/h) in order to avoid the
           effects of stationary load.

      (i)  Where practicable, precautionary and speed limit signs should
           be erected at each end of the ice bridge, and the route across
           the ice cover clearly marked.

      (j)  Where practicable, precautionary and speed limit signs should
           be erected at each end of the ice bridge, and the route across
           the ice cover clearly marked.

      (k)  Equipment required for rescue operations, such as "mats"
           (chained or wire-linked small logs or heavy planks as a
           platform for rescue vehicles) jacks, hoists, etc., should be
           available near by.

      (l)  Frequently it is the second vehicle in a convoy which
           encounters ice failure problems.  Before a second heavily
           loaded vehicle proceeds along the ice bridge, it is advisable
           to have it preceded by a more lightly loaded vehicle to check
           the route.

      (m)  For a period of 24 hours after a marked drop in temperature, or
           following the removal of snow from the ice cover during periods
           of low temperature, loads should be reduced by 50 per cent and
           night-time travel should be discouraged.

5.    The use of snowmobiles on ice covers

5.1   General

5.1.1      Drownings resulting from snowmobiles going through ice are the
           greatest single cause of fatalities arising out of the use of
           these machines.  However, snowmobile operations over ice covers
           can be conducted safely by using common sense and observing the
           basic precautions.

5.1.2      As the total load - machine, operator and ancillary gear - may
           weigh approximately 500 pounds (225 kg) or more, a substantial
           thickness of ice is required for support.

5.1.3      Difficulties in control, steering and stopping are increased on
           snow-free ice, particularly at higher speeds.

5.2   Operation precautions

5.2.1      The following is an outline of some of the basic precautions:

      (a)  Where there is an alternative, single machines should not be
           operated unaccompanied over ice covers.

      (b)  Should single machine operation be unavoidable, the shore base
           should be notified of the route to be taken, the destination
           and probable time of return.

      (c)  Operations should not be conducted over ice covers less than
           6 inches (15 cm) thick.

      (d)  Operators should know of and avoid locations where currents or
           springs may cause dangerous thinning of the ice cover.

      (e)  Fog may indicate the proximity of open water; speed should be
           reduced and great care taken.

      (f)  When unexpectedly encountering open water normal action is to
           slow down, brake gently and turn away; otherwise, turn as
           sharply as possible.  If a turn cannot be made in time or a
           skid results, the operator should roll off the machine.

      (g)  Glare from the sun and ice may obscure obstacles or dangerous
           areas; anti-glare sun glasses should be worn under these

      (h)  Operations at night or at high speeds should be restricted to
           well-marked and known safe trails or crossings.

      (i)  Unless essential, snowmobiles should not be operated on ice
           bridges or roads with other types of traffic.

      (j)  Avoid operating over slush or water-covered ice; but if
           unavoidable, ensure that the tracks are cleared of ice and


Additional technical information concerning ice formation and its use is
available in the following publications:

      Publication CL1-7-71
      Freeze-up and Break-up Dates of Water Bodies in Canada
      Information Section
      Central Service Directorate
      Atmospheric Environment Services
      Environment Canada

      Technical Memorandum No. 56
      The Bearing Strength of Ice
      National Research Council

      Research Paper No. 469, NRCC 11806
      Use of Ice Covers for Transportation
      National Research Council

Information and advice may be obtained also from the National Research
Council of Canada, Division of Building Research, Geotechnical Section,
Ottawa, Ontario K1A 0R6.

This chapter replaces chapter 5-3 of PMM volume 12.


Enquiries should be directed to the responsible officers in departments
headquarters, who in turn, may seek interpretation from the following:

      Safety, Health and Employee Services Group
      Staff Relations Division
      Human Resources Policy Branch
      Treasury Board Secretariat

Appendix A
Thickness of Good Quality Fresh Water Ice