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3.1 General
3.2 Determining ice thickness
Caution
3.3 Parked and stationary loads
3.4 Effects of speed
3.5 Cracks
3.6 Spring thaw
5. The use of snowmobiles on ice covers
Appendix A - Thickness of Good Quality Fresh Water Ice
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.
1.2.1 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.
2.1.1 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 ice.
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.
2.3.1 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 winter.
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.
3.2.1 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.
3.3.1 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 occur);
(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 conditions:
(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.
3.4.1 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).
3.5.1 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.
3.6.1 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 immediately.
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.
4.1.1 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.
4.2.1 The flooding operation is normally carried out with small lightweight pumps, rather than larger pumps which are less portable.
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 flooding.
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.
4.4.1 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 cold.
(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).
4.5.1 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 breakthrough.
(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 observed.
(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.
5.1.1 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.
5.2.1 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 conditions.
(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 slush.
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
