Bridging in Arctic and Subarctic Environments
Subsurface exploration and structural design are very important when building structures on permafrost. Successful military construction on permafrost depends on how the permafrost is used as part of the construction. Surface transportation is totally undependable during spring breakup and fall freeze. The change of seasons in arctic and subarctic environments creates problems that are not common in more temperate climates. Extreme cold primarily affects bridge foundations and substructures.
12-1. Permafrost describes permanently frozen ground at variable depths below the surface. It underlies about one-fifth of the land area of the world (Figure 12-1). Permafrost may exist as a continuous material or as an island or lens within unfrozen material. The deepest layers of permafrost are in the polar regions and may extend as deeply as 1,300 feet. In arctic and subarctic regions, it is the most troublesome condition encountered when constructing foundations.
Figure 12-1. Permafrost Regions (Northern Hemisphere)
12-2. Permafrost can be used effectively as a foundation for construction. However, improper construction methods may allow undue amounts of heat to transfer into the permafrost, thereby thawing and weakening its supporting properties. Nonfrost active gravel and sand are effective for controlling ground swell and ice formations in fills and bases. Use of these materials allows potentially dangerous groundwater to break through the surface and freeze, where its cyclical freezing and thawing will do no harm.
12-3. In areas of extreme cold, transportation is best completed during the extreme cold season. Some considerations are discussed below.
SUMMER CONDITIONS (EARLY JULY TO EARLY AUGUST)
12-4. Difficulties during the summer months are due to the lack of soil bearing capacity from inadequate underground drainage. Permafrost prevents the groundwater from percolating to levels that allow surface soil to dry readily. Transportation is limited to tracked vehicles operating on established roads and railroad beds. The surface soil does not dry until late summer, and even then, drying may be only superficial. Much of the drainage pattern of arctic terrain resembles swamps, shallow lakes, and slow-flowing watercourses.
FALL-FREEZE CONDITIONS (LATE AUGUST TO EARLY SEPTEMBER)
12-5. The ability of roads to carry traffic during a freeze-up depends on the condition of the roads' surfaces before they freeze. Hard-surface roads are generally only affected by icing on the surface. Where no hard surface exists (as is characteristic of roads in these regions), the most important factor is whether the road surface has been regularly maintained. Heavy snowfall immediately following a fall freeze may fill the ruts and level the roads' surfaces. If the early winter has little snowfall, deep, frozen ruts will be more prominent.
WINTER CONDITIONS (LATE SEPTEMBER TO EARLY APRIL)
12-6. Surface transportation conditions during the winter are better than at any other time of the year in arctic and subarctic regions. Continuous, extremely low temperatures freeze the surfaces of seas, lakes, rivers, streams, swamps, and tundra, which allows vehicles to breach these obstacles without bridging assets. However, the same low temperatures make equipment operation and maintenance difficult.
SPRING-BREAKUP CONDITIONS (LATE APRIL TO LATE JUNE)
12-7. As winter snows melt, the active soil layer becomes completely saturated. Temperatures are warm enough to begin thawing and breaking up the snow and ice on which skis or sleigh runners have been operating. Surface soil does not dry in the spring.
12-8. Warm winds, as well as heat from the sun, have a deteriorating effect on a snow mass. Within a few days, the warmed surface loses much of its bearing capacity. Tracks and sled runners cut deeply into the ground surface and make ruts in which water accumulates and further destroys the ground's bearing capacity.
12-9. The penetration of meltwater through the snowpack into the surface eventually carries heat into the subgrade. This deterioration may be further aggravated by rainfall. During the initial period, night frosts will temporarily improve the surface. To avert complete destruction of roads, traffic is usually restricted to night travel with light loads.
12-10. Groundwater in the surrounding areas cannot percolate below the permafrost. The groundwater either remains in place to form surface mud and water or runs off the slopes and across roadways, filling the ditches. This action greatly reduces the repose of the terrain and results in landslides. Melting snow often results in snowslides. The rapid runoff may result in devastating flash floods. These situations may delay or suspend bridging operations.
12-11. Engineers use a great quantity of explosives in areas of extreme cold, because many jobs that would normally be done by machinery in warmer climates are accomplished with explosives. The cold affects the operation of explosive components differently.
12-12. Military explosives are generally unaffected by intense cold. However, they become less sensitive to shock, are somewhat difficult to detonate, and are not as powerful when exposed to extremely cold environments.
12-13. Commercial explosives become less sensitive until they freeze, after which time they become extremely dangerous. Commercial dynamite is currently manufactured with freezing depressants to lower the temperature range at which its sensitivity and reliability are affected. Black powder has no moisture and, therefore, cannot freeze, but it is extremely dangerous to use. See FM 5-250 and TM 5-852-1 for additional information on explosives.
12-14. Explosive accessories (such as blasting caps and machines) are hardly affected by the cold except for being slightly sluggish. However, standard silver-chloride, dry-cell galvanometers are impractical. Tactical firing devices are adversely affected.
12-15. Extreme cold makes steel brittle. The most important consideration is impact loading. The properties of chromium-nickel types of stainless steel are actually improved by extreme cold.
12-16. The strength of wood is relatively unaffected by extreme cold. However, the impact strength of wood is reduced considerably because the wood's moisture content is reduced by the environment. Wood normally contains between 10 and 20 percent moisture. This moisture content may drop to nearly 1 percent in extreme cold because of the relationship between relative and absolute humidity. Reduced moisture content causes shrinkage, and the nail-holding property of wood is greatly reduced. Nails also tend to split the wood more easily.
12-17. Concrete is not adversely affected by extreme cold, provided it does not freeze before the curing process is complete. The problems of protecting fresh and uncured concrete from freezing are exactly the same as in cold portions of the US except for the amount of heat that must be provided. See TM 5-852-1 and FM 5-428 for information on cold-weather concrete placement. It is not practical to place concrete during weather colder than 0°F.
12-18. Most construction equipment can be adapted for use in arctic and subarctic conditions. The equipment will not be as effective, and the operator must be aware of the limitations of the equipment when used in extremely cold environments. The efficiency of the equipment depends on the knowledge and skills of the operator and the maintenance technician.
12-19. Schedules may be hampered by weather. Using proper precautions will allow a construction crew to still achieve a relatively high progress rate.
BRIDGE FOUNDATIONS AND SUBSTRUCTURES
12-20. The best locations for bridges in extremely cold environments are areas where the water is deep, the channel is narrow, and the banks are high. Some of the more pertinent effects and remedial measures for building foundations and substructures are discussed below.
12-21. Precipitation over most of the arctic is about 8 to 10 inches per year, but it varies widely depending on geographic location. Snow and ice fogs are the prevalent form of precipitation. Although usually very light, precipitation reduces lowland prairies to swamps except where the soil is rocky. Generally, rocky areas have a slow evaporation rate and little or no underground drainage.
12-22. Precipitation in the subarctic is much heavier than in the arctic. Some coastal areas receive as much as 150 inches of precipitation per year. Most of this moisture occurs as rain during the period from March through November. This rainfall supports a lush growth of trees and other plants. Interior regions receive an amount of precipitation comparable to that of the Midwestern US, with most of that falling in late summer.
STREAMS AND RIVERS
12-23. The flow rates of streams and rivers in extremely cold environments vary widely. The daily water crests are more pronounced in the warmer regions than those in the colder regions. The crests in the warmer regions may vary as much as 7 to 10 feet daily. Seasonal variations in arctic and subarctic regions are numerous. Flash floods are common during spring breakup, and the waterways carry great quantities of ice. The water level is generally low during the fall, and streams fed by snowmelt may cease to flow entirely (Figure 12-2). During the winter, some streams quit flowing and freeze to the bottom. Wide floodplains with indeterminate and meandering channels are common.
Figure 12-2. Dried Streambed
12-24. Ice and slush form during a fall freeze and act abrasively on bridge substructures. Ice also forms and clings to substructures, building sufficient weight to obstruct the water flow. When the water recedes, a bridge may collapse. Ice also adheres to other ice on the water surface. This condition (called valley icing) is most common in water with permafrost under their channels.
12-25. During the spring, large sheets and blocks of ice float downstream, exerting tremendous pressures against bridge substructures. Final bridge failures from ice accumulation usually occur during ice breakup.
12-26. Local bridge construction materials are abundant in some regions and nonexistent in others. The temperature, the precipitation, the amount of sunlight, and the geologic history all affect the availability of natural materials in a local area. Accumulated snowfall is the biggest deterrent to finding sufficient local resources.
12-27. In some subarctic regions, timber is so bountiful that it hampers military operations and construction. Timber is usually available within a range of 50 to 100 miles. In other areas (particularly the arctic), timber is nonexistent. Deciduous trees are present near the timber line and coniferous trees are common in northern parts of the subarctic region. Timber is a viable construction material for arctic construction if allowances are made in the structural design for seasonal changes.
12-28. A wide variety of rock outcrops exists in the arctic. Rock composition ranges from hard, Precambrian to Canadian shield to soft, sedimentary rock. Rock outcroppings in the arctic and subarctic regions represent all hardnesses and textures created by geological processes. In flat-frosted and tundra regions, rock outcrops are scarce. The frequency of outcrops increases in hilly and mountainous regions. Above the 75th parallel in the Canadian Archipelago, rock outcrops are common and are often badly fractured.
SAND AND GRAVEL
12-29. Subsurface deposits of sand and gravel are present in arctic and subarctic regions. Glacial action has produced many deposits of these materials, especially in riverbeds and deltas. Sand and gravel may also be obtained from the seacoasts and lake shores, backwaters, and meandering river channels. The quality of inland and shoreline deposits is not as high as those of glacial deposits, because glacial deposits are often a matrix of silt and vegetable matter, thus having better construction characteristics.
SNOW AND ICE
12-30. Snow and ice are useful construction materials. Additional information on using ice as a construction material is discussed later in this chapter.
12-31. The existence and arrangement of permafrost under river channels depend on many factors. Always conduct a permafrost survey at a bridge site before beginning construction. The extent of the survey depends on the size, importance, and expected durability of the proposed bridge. Base the design of the bridge on criteria applicable to the active layer (the area where cyclical thawing and freezing occur) in its thawed condition. Core sampling (boreholes) provides an engineer with a profile of the river and the bridge approaches. Extend a cross section, 15 to 20 feet below the proposed foundation base unless bedrock is encountered at a shallower depth. Determine the characteristics of any bedrock by using a test-hole penetration of at least 5 feet (Figure 12-3). All surveys should show—
- A profile of the riverbed.
- The permafrost table.
- The active-layer depth and its relationship to the permafrost table.
- The subchannel drainage.
- The geological structure of the site.
- The site's soil composition and texture.
Figure 12-3. Borehole Exploration of a Bridge Site
12-32. Trestle bents are unsatisfactory as intermediate supports for any but temporary bridges because their footings rest on the very unstable active layer. They are impractical because they have too many supports that are susceptible to attack by ice and too many obstructions that block water flow and ice passage. When placed across shallow, still water, trestles are susceptible to variable upward thrusts that may wreck a bridge.
12-33. Pile bents are the most suitable bridge foundations in extremely cold environments because they can be driven down to stable soil. Treated timber; precast, reinforced concrete; steel piling; and steel pipe are satisfactory pile materials. Precast, concrete piling is excellent because there is less ground thawing after placement than with other pile materials. Generally, reinforced concrete and steel piles should be uniform in cross section along their entire length. The heat conductivity of steel helps to thaw ice accumulation on the member. The considerations discussed below may affect the use of pile bents.
Bearing and Upthrust
12-34. Since the bearing capacity of frozen ground is high, piles placed in permafrost will support tremendous loads. Figure 12-4 shows the adverse action of settlement and upthrust on piles or other foundations based on permafrost. Figure 12-5 shows the action of a pile that is properly extended into permafrost. Generally, extend piles into permafrost to a depth at least twice the thickness of the active layer. To minimize upthrust, place timber piles butt down.
Figure 12-4. Effects of Settlement and Upthrust
Figure 12-5. Pile Installation in Permafrost
12-35. Permafrost prohibits the use of normal pile-driving techniques. The piles have to be placed in drilled or thawed holes. Drill the holes with directional explosive charges. Accomplish thawing with a water or steam pipe, advanced downward as the ground thaws. Drive the pile in the normal manner.
12-36. Because of severe structure wear on any component exposed to floating ice and the danger of collapse from ice accumulation, place as few piles in the actual waterway as possible. This will require longer spans and extensive protection for intermediate supports. Greater spacing requirements favor steel as a pile material.
12-37. Pile fenders, sheathing, and modified construction help to protect against ice damage. Drive fenders in a diamond pattern, upstream from the piles they protect. Sheathe the piles to provide a smooth surface and to prevent ice from snagging and damming up in front of the piles. Omitting the diagonal bracing below the waterline or ice line will allow ice to pass the piles more freely.
12-38. Construct piers on top of pile foundations. Cap the piles with concrete, timber, or steel superstructures. Bailey-type panels are also satisfactory for pier construction. Solid concrete piers usually require no protection.
12-39. Avoid using rock-filled cribs as piers for midstream supports unless they are supported by rock foundations near the water surface. Rock cribs are subject to heaving and induce icing. Use rock cribs as intermediate supports only for temporary bridges.
12-40. Foundation instability and normal wear and tear of intermediate supports in extremely cold environments may require planned replacement of piers after each spring breakup. Make every attempt to salvage superstructures before breakup occurs. Periodically replacing damaged piers and supports makes total bridge failure less likely.
12-41. The soil condition, the active-layer depth, the permafrost table, and the bridge type will determine the type of abutment required. If the soil's condition shows that heaving and settling are unlikely, follow normal abutment construction techniques. Special abutments are necessary when a bridge site has poor soil conditions, permafrost, or an active-layer depth of 6 feet or more.
12-42. Piles provide satisfactory abutment foundations because they can be rapidly placed and they eliminate the need for excavation. If the permafrost table is depressed sufficiently so that the piles will not reach the permafrost, use temperate-zone practices for abutment foundations.
12-43. The use of concrete abutments is limited by the construction season and their effect on the thermal regime. Figure 12-6 shows how threat engineers have modified abutments to compensate for arctic construction. If the permafrost is so deep that placing a foundation is not feasible or if the permafrost is expected to thaw, use construction practices applicable to the site in its unfrozen state.
Figure 12-6. Abutment Installation in Permafrost
12-44. Apply the insulation principle to foundations when the abutment is to be frozen into the permafrost. Use the following procedure:
Step 1. Excavate the area for the foundation to a depth of at least 3 feet below the permafrost table in coarse-grained sands and gravels and at least 5 feet below the permafrost table in fine-grained sand, silt, and clay soils.
Step 2. Level the bottom of the excavation with a 4- to 10-inch layer of moist sand. Place a wooden or precast-concrete slab over the sand. This platform reduces permafrost thawing due to the heat generated from concrete setting and prevents the concrete from mixing with ground materials.
Step 3. Place the abutment. If the abutment foundation is wider at the bottom than at the top, make the taper from bottom to top smooth with no steps in the active layer. After the concrete sets, fill the space between the abutment and the excavation sides to the top of the permafrost table with wet sand. This layer of wet sand will freeze and anchor the abutment in the permafrost layer.
Step 4. Make sure that the abutment has a smooth surface to prevent the backfill from freezing to the foundation. Backfill the abutment portion in the active layer with coarse gravel.
Step 5. Plant shrubs or other protective vegetation around the portion of the abutment exposed to the sunlight to reduce solar-heat absorption.
12-45. If it is impractical to excavate the permafrost, use the procedure shown in Figure 12-7. This method requires excavating to the permafrost table. Drive the piles into the permafrost as described earlier in this chapter, leaving at least 18 inches of each pile exposed above the permafrost table. Place a layer of wet sand around the piles to a depth equal to one-third of the exposed height of the exposed piles. Doing this prevents the heat that is generated by concrete setting from thawing the permafrost. Pour the abutment, covering the remaining portion of the piles with concrete.
Figure 12-7. Abutment Installation on Permafrost
12-46. When constructing retaining walls, backfill them with coarse material to ensure good drainage. This reduces the normal groundwater level and the possibility of icing around the wall. Finish all retaining walls as smoothly as possible to prevent ice accumulation.
12-47. Ice will accumulate against bridge supports and cause serious damage if left unchecked. It may be necessary to remove ice accumulation from bridges and drainage structures at regular intervals. See previous discussion in this chapter on using explosives.
12-48. Chopping is an expedient method of removing ice buildup from a bridge. Chopping is inconvenient as it needs to be periodically repeated. Do not depend on this method as a permanent solution to icing problems.
12-49. Spreading sodium chloride (common salt) or calcium chloride on the accumulated ice will break down the ice. The ice should then drop off the bridge and continue downstream.
THAWING (HEATING WATER)
12-50. Channeling and heating the water flowing under a bridge will thaw the ice that is threatening to block the water flow. Heat the water above the bridge opening with steam pipes laid above or through the bridge opening. An expedient method of heating water is to use steel drums filled with fire. Cut one end off the drums, weight them with rocks, and place them upstream from the bridge. Burn oil, wood, or another suitable fuel in the drums to provide the heat. Intermittent fires should provide enough heat to keep the bridge opening clear of excessive ice accumulation.
THAWING (MODIFYING A WATER CHANNEL)
12-51. Deepening and straightening a channel will allow free flow of deep water under a bridge. The deeper the water, the less chance there is for serious bridge icing to occur.
DRAINING EXCESS WATER
12-52. Use drainage ditches and low barriers to drain any excess surface water away from a bridge site. Drainage ditches require continual maintenance to ensure their effectiveness. Low barriers are effective as a temporary control and require repeated installation. Both methods are only effective for small volumes of water.
INSULATING BRIDGE COMPONENTS
12-53. Insulating potential icing areas to prevent ice accumulation is often effective at bridge sites. The insulation retards ice formation and allows the water to flow freely throughout the winter.
12-54. Light timber canopies covered with moss or boughs (built across the stream channel and against each side of the bridge) can retard ice formation on a bridge's substructure. Build these canopies before the first frost sets in. Ensure that they extend 20 to 100 yards upstream and 20 to 50 yards downstream. Winter snow cover on the canopies serves as an insulating layer, keeping the water flowing under the bridge and preventing ice from forming on the canopies. Remove canopies before the spring breakup to prevent damming and potential bridge damage.
12-55. Another method of insulating is to allow a canopy of ice to form above the normal water level. Dam the stream below the bridge site and allow the water to raise and form a 6-inch ice sheet. After the desired thickness of ice accumulates, remove the dam and allow the water to return to its normal level. There will now be an air gap between the underside of the ice and the water level, acting as an insulator to keep the water from freezing. Allow snow to accumulate on top of the ice canopy to enhance its insulating abilities. Snow fences may be needed so that the snow will drift onto the ice canopy or so that the snow will not blow away. Mark the ice canopies to prevent them from being damaged by attempted crossings.
12-56. Extremely cold conditions generally dictate long, clear spans. Locate superstructures high enough above the high-water mark to prevent them from being damaged or forming obstructions to high water or ice accumulations that may build above the normal high-water mark. Do not design bracing that extends below the waterline or is in the path of water or ice.
12-57. Panel-type truss bridges (such as the Bailey bridge) are the best bridges for arctic applications. The main advantage is the availability of standard military truss sections. Use steel trusses whenever possible.
12-58. Timber-truss bridges are the most satisfactory timber bridges for arctic use. Erecting them does not require much more time than that required for trestle- or pile-bent bridges. Use available timber if it is suitable for constructing trusses.
12-59. Simple steel trusses made of standard, rolled shapes are very practical if the steel is available. Rolled sections are especially valuable for short-span bridge construction. Use plate girders for longer spans. Girders require substantial piers and abutments. Arch, suspension, and cantilever bridges meet arctic bridge requirements (long spans and clear waterways). However, their use is generally limited to permanent, peacetime construction. Simple pony trusses or A-frame structures are adequate for small bridges.
12-60. Fording operations in arctic and subarctic environments are generally more difficult than in temperate areas. The techniques are similar, but there are seasonal considerations. Extremely low temperatures and ice obstacles, especially during spring breakup and fall freeze-up, limit fording operations. Stream velocity and depth change with the seasons and the time of day.
12-61. During summer, a stream's depth and velocity may rise due to glacier or ice-cap melt. Fording of streams and rivers fed by glaciers and ice caps is feasible only when the volume and velocity of water are at their minimum. Permafrost on stream banks and riverbanks may make approaches difficult to develop and use. For example, ramps may be necessary because approaches may not be able to be cut to the proper grade (Figure 12-8). Make provisions to limit the use of the ford during unfavorable times and to provide assistance (vehicle recovery, drying facilities, and so forth) when difficulties arise during actual crossings of equipment and personnel.
Figure 12-8. Expedient Ramps
12-62. Fording during the winter is impractical because ambient air temperatures are damaging to the equipment. Some watercourses (particularly those flowing in broad floodplains) have open channels that continually shift. As the stream direction shifts under the ice, many portions of the ice are left unsupported, creating valley ice. Valley ice makes fording very treacherous. Equipment that breaks through valley ice is difficult to recover. If fording operations are necessary during winter, mark the route, remove all valley ice, and ensure that the fording is through the active stream channels. Maintain continual reconnaissance upstream to determine probable shifts in the water channels. Channels may have to be dammed or diverted to complete a proper fording operation.
12-63. Keep the entire ford area clear of ice to a point below the actual ford area. Vehicles should be operated by experienced drivers only. Drivers should use the lowest gear necessary, maintain a constant speed, and avoid using the brakes (they will freeze closed). After each fording operation, warm and dry the wheels, rollers, engine, brakes, and clutch in a heated shelter, if possible. The alternative is to keep the vehicle moving quickly.
12-64. Rivers with low velocities, lakes, and deep swamps will freeze sufficiently to allow ice crossings in winter and well into spring. Develop approaches and reinforce and maintain crossings as discussed below.
12-65. Ice along the shoreline is usually thin and weak. Provide a smooth approach from the shoreline to the thicker ice that is capable of supporting traffic. A timber ramp, with the shore end resting in a prepared cut or on a fill and the ice end supported on a timber mat, makes a very satisfactory approach (Figure 12-9). When the banks are high and the obstacle is not excessively wide, a well-compacted and well-frozen fill of snow and brush will provide an effective approach.
Figure 12-9. Expedient Approach
12-66. During periods of low temperature, increase the thickness of ice by removing the snow cover and allowing the ice to thicken by natural methods. Clear a 150-foot strip along the entire path of the intended crossing. Allow sufficient time for the ice to thicken. A weak ice sheet will increase in thickness by inch during a 12-hour period at 5°F. In general, the temperature must be less than 15°F for this method to be effective.
12-67. This method involves building snow dikes along each side of the desired route about 2 times the desired road width. Flood the space between the dikes to a depth of about 1 inch and let it freeze. The freezing process takes about 2 hours for a 1-inch thick layer at 5°F. Build the ice in 1-inch increments to permit rapid freezing. The only disadvantage to this reinforcement method is that the capacity of the induced ice is only one-half that of ice resulting from the natural process.
12-68. The simplest method of reinforcing ice is to lay level layers of snow on top of the desired road and add water to freeze them in place. Allow each layer to freeze before adding subsequent layers. Further reinforcement is possible by adding materials such as brush, straw, or chicken wire between every 2 to 4 inches of frozen snow. Table 12-1 lists data for reinforcing ice.
12-69. Many materials are effective for reinforcing ice bridges (logs; wood-plank, corduroy mats; deck components of military floating and fixed bridges; or pierced, steel planks [PSPs]). The ice surface must be leveled before using these materials. Next, firmly anchor the reinforcing material to the ice and cover it with snow. Finally, flood or spray the snow cover with water and allow it to freeze to the original ice surface. Ensure that any metal is painted white and is fully covered to inhibit solar absorption. Also, cover the completed bridge with enough snow to provide a wearing surface. Planks or small logs can also be frozen into place to form runways or tracks for vehicles and sleds. Make each track about 3 feet wide and cover them the same as the other materials mentioned above.
12-70. Frequently check the ice thickness and the water level beneath the ice. Note any cracks, the need for further reinforcement, the snow clearance, the approach condition, and the adequacy of the wearing surface. Ice capacity is reduced by continual use. A heavily used crossing may have to be relocated.
EXPEDIENT ICE-BRIDGE CONSTRUCTION
12-71. During periods of continual low temperatures (5°F or colder), it is possible to use open areas or ice in standing or slowly flowing water to bridge gaps. Engineers create ice bridges by floating large sections of ice (cut from the rear of the ice pack) into a transverse position across the water gap. Ice bridges are only an expedient temporary measure. Water movement under a floating section of ice can erode the bottom surface and weaken the section. Engineers will need to monitor the ice section continually and determine its bearing capacity. The best location for obtaining a section of ice is where the ice extends for an appreciable distance from the bank and does not vary in thickness or quality. There should be no underwater obstacles near the site. Survey the depth and the bottom conditions to ensure that they are adequate.
12-72. Underwater topography is very important when constructing ice bridges, planning for repairs and rescue, and estimating bridge longevity. If there is insufficient ice along the bank and the temperature is below freezing, an alternate crossing of limited capacity can be built. Tie brush or small trees to a rope or cable, stretch the cable across the waterway, and allow the water to freeze onto the cable to form a crossing. Use the procedures described below for building an ice bridge.
12-73. Spend the time necessary to determine the required bridge dimensions properly and to complete the layout. Table 12-2 gives data on typical ice-bridge sizes and the resources necessary for construction. The float must fit the notches created in the layout to be effective. The float should be one-half to one-third as wide as it is long. The minimum width for a single column of soldiers is 15 feet. The notch in which the float fits on the far bank should be 20 to 26 feet deep. The depth of this notch depends on the thickness of the bank ice, the length of the bridge, and the water currents. The notch should be deep enough to prevent splintering and breaking as the float is guided into position.
12-74. After completing the layout, remove the excess ice (Figure 12-10A) by sawing it free and then floating it downstream. Make double cuts (6 to 8 inches apart) on the ends of these sections to facilitate their removal. After removing the excess ice, install mooring lines on the float and cut it free.
Figure 12-10. Ice-Bridge Layout
12-75. Using the mooring lines, carefully guide the float into its final position (Figure 12-10B and C). Bridge the banks and gaps at the end of the float with available timber or prefabricated balk and decking. Mark the traffic lanes and post the bridge capacity.
EXPEDIENT ICE-BRIDGE REINFORECEMENT AND REPAIR
12-76. Use the same methods of reinforcement for ice bridges as previously described for ice crossings. The float will eventually freeze into the notches on both sides and ends, and the float will gradually increase in size as new ice freezes onto its sides. If no bottom-surface erosion is present, the bridge's stability will increase through natural processes. An additional reinforcement method is to add ice blocks to increase the ice bridge's thickness. Cut these blocks from ice at least 200 yards downstream. Place the blocks on top of the existing ice bridge, pack snow around them, spray water on the entire reinforcement, and allow the blocks to freeze together. Reinforcement is also possible by adding M4 balk, Bailey panels, or other standard bridging resources. However, these components will be impossible to salvage.
12-77. The most probable condition requiring repair is broken floats. These breaks can be repaired by stitching or racking the broken pieces together (Figure 12-11). Follow these steps to repair a break:
Step 1. Bore holes completely through the ice along each side of the crack.
Step 2. Place a doubled-over, two- or four-strand rope through each pair of adjoining holes.
Step 3. Install small boards between the rope and the ice under each hole.
Step 4. Remove the slack from the rope and tie the rope ends.
Step 5. Place a rack stick between the strands of rope and tighten the stitch.
Step 6. Repeat the process for each pair of holes along the crack.
Figure 12-11. Ice Stitching
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