Structures Requiring Foundations Continued
Special Site Considerations
A trail in a spruce bog requires adapting wetland construction techniques to the site. The most applicable technique is the bog bridge on bents. However, unlike construction in most wetlands, the location of each pile will have to be adjusted in the field to avoid roots.
Although the upper layer of soil is organic, the underlying soil may not be. Dig test holes along the proposed route to determine whether end-bearing piles, friction piles, or a combination of both is the best technique.
The spacing between piers will vary, as will the angle of the bents to the trail centerline. Some of the tread planks will be shorter than normal, and others will be longer. Starting with tread planks that are twice the normal length will permit cutting short pieces to fit one location, leaving longer pieces for use elsewhere (figure 42).
Figure 42—A bog bridge on a pile foundation. Protecting tree roots
in a spruce bog requires careful placement of sleepers and piles.
Although this technique is described for northern spruce bogs, it may also have application in cypress swamps in the Southeastern United States and elsewhere.
If beaver are a problem, wrap piles with hardware cloth and staple it into place. The hardware cloth discourages beavers from chewing through the piles, timbers, or logs used in construction (figure 43).
Figure 43—Hardware cloth stapled around
piles helps discourage
beavers.
Old beaver ponds present something of a problem in bog bridge construction, especially in mountainous areas. The original soil may have been of glacial origin and capable of supporting end-bearing piles. However, beaver dams trap silt, which drops to the bottom of their ponds. While end-bearing piles may work well in some locations in such ponds, friction piles are needed elsewhere. When concrete end-bearing piles were used at one pond, some settled 1 to 2 feet in 5 years. After 10 years, all concrete end-bearing piles had to be replaced with log friction piles (figure 44).
Puncheon
Puncheon are essentially short-span footbridges or a series of connected short-span footbridges. The term puncheon means different things to different people. Puncheon on the Appalachian Trail is not the same as puncheon built in the Cascades, Rocky Mountains, or Sierras. Puncheon built in easily accessible areas may not be the same as that built in the backcountry. Puncheon can be used where the soil is wet but does not contain enough water to seriously hamper trail work. The one thing common to all puncheon construction is the use of sleepers.
Figure 44—Know your soil conditions. Concrete end-bearing piles settled
1 to 2 feet here and had to be replaced with these log friction piles.
Type 1 Puncheon
On the Appalachian Trail, 3- to 6-foot-long logs are commonly used for the sleepers. The sleepers are notched to receive one or two tread logs and then placed in a shallow trench. The tread logs are hewn, split, or sawn, roughly in half, to provide a level plane for the walking surface or tread. The tread logs are spiked or pinned in the notch of the sleepers (figure 45).
Figure 45—The rustic type 1 puncheon has sleepers and tread logs.
If the area to be crossed is longer than the logs available for the tread, the puncheon can be built as a series of connecting sections. Hiking any distance on single-tread-log puncheon can be unnerving because the hiker is looking down to avoid stepping off the tread. This is especially true if there is quite a drop from the tread to the ground or water below. Two tread logs placed side by side on longer sleepers will help. For two-tread-log construction, the inside face of each log should be hewn or sawn to butt closely to the adjacent log. A narrow gap between the two logs will help drain water, snow, and ice from the tread. This will reduce the chances of a slippery tread and retard decay.
Two or three small spacers can be nailed to the inside face of one of the logs to control the width of this gap. The spacers can be short, straight lengths of 2- to 4-inch-diameter branches or wood scraps, hewn flat on opposite sides to provide a piece of wood about 1 inch thick.
Type 2 Puncheon
In the Western States, puncheon uses log sleepers placed in a manner similar to that used on the Appalachian Trail. The sleepers are a few feet longer, however, and the space between them is spanned by two or three log stringers, or beams, spaced 1 to 3 feet apart (figure 46). The tread is made from 6- to 12-inch-diameter split logs, 4 to 6 feet long, or split planks. The split face becomes the tread. The bottom of the tread half-log is notched to rest on the stringer log, and the tread is spiked in place. If three stringers are used, do not spike the tread logs to the center stringer. The top of the three stringers will probably not be at the same height. Use a long carpenter's or mason's level to quickly determine the height of each stringer in relation to the others. Ideally the tread should be level from one side to the other. Handtools normally used in the field for construction make it difficult to get the tread perfectly level. Adjusting the depth of each notch, as needed, will allow for variations in stringer height. Shims under the decking also help to level the structure from side to side.
Half logs can be placed with their split sides facing up as a tread. Smaller half logs are placed split side facing down resting on the stringer and butted tightly against the tread log. These logs serve as brace logs, preventing the tread logs from wobbling. Succeeding tread log are butted snugly against the brace logs (figure 47).
If large logs are available, tread plank can be sawn from the logs, producing a number of pieces of plank of varying widths from one log. An Alaskan sawmill can be used at the site to produce planks with a uniform thickness. With this plank, there should be little—if any—need to notch or shim the stringers.
Excessive cross slope will make the surface very slippery. The meaning of excessive will vary, depending on the climate expected when the trail is being used. In a dry climate, the cross slope should not exceed ½ inch per foot of tread width; in a wet climate, or where snow, ice, or frost can be expected, the cross slope should be no more than 1/8 to ¼ inch per foot of tread width. If the trail leading to the puncheon is wet, no matter what the season, hikers will track mud onto the tread, making it slippery throughout the year.
Figure 46—Type 2 puncheon has
sleepers, stringers, and decking.
Figure 47—Rustic tread or decking made from
half logs (logs cut in
half lengthwise).
Type 3 Puncheon
Type 3 puncheon also uses sleepers to support the structure, but the material is sawn timber or lumber, which should be treated with wood preservative (figure 48). This construction is popular at more accessible sites where materials are easier to transport. The longevity of treated wood and the environmental consequences and labor of cutting trees onsite make the use of sawn, treated timbers increasingly popular at remote sites as well. Helicopters, packstock, all-terrain vehicles, and workers carry in the materials.
The sleepers can be either 6- by 6- or 8- by 8-inch-square timbers placed as previously described. Two or three stringers rest on the sleepers and may be toenailed to the sleepers and bolted or nailed to the stringer in the next span. The stringers may also be attached to the sleepers with steel angles and extended (cantilevered) a short distance beyond the sleepers.
Figure 48—Type 3 puncheon is constructed from preservative treated
timbers. The nailer bolted to the inside of each stringer
helps
prevent decay by concentrating screw holes and
associated decay in
the easily replaced nailers
instead of the stringers.
The size of the stringers is determined by the maximum weight they can be expected to support, which may be the snow load in snow country. For foot trails, usually the size of the stringers is calculated to support a live load of 85 pounds per square foot, the maximum weight expected for trail users standing on one section of trail. Heavier, wider puncheon is needed for horse and mule traffic.
On foot trails, the tread is often 2 by 6, 2 by 8, or 2 by 10 lumber nailed to the stringers. When three stringers are used, do not nail to the center stringer. The nails work their way out and pose a tripping hazard. The stringers are the most expensive and most difficult items to bring to the site. Do everything you can to extend their useful life; usually this means keeping them dry.
The tread will need replacement more frequently than any other portion of this type of puncheon. In some areas the wood tread will require replacement every 7 to 10 years. After three or four replacements of the tread, the top of the stringers will show signs of decay and wear. Water from runoff and condensation will follow the nails down into the wood, and repeated nailing in the same vicinity will soften the wood. To avoid this, a nailing board (nailer) of 2 by 4s or 2 by 6s can be nailed to the top or side of the stringer. A better solution is to bolt rough-sawn 2 by 4s or 3 by 4s to the side of the stringer with carriage or machine bolts. The bolts can be 2½ to 4 feet apart. The tread is nailed to the nailer instead of the stringer. Eventually, the nailer will require replacement, but the nailer is much easier to replace than the stringers. Esthetically, it is better to attach the nailers to the inside face of the stringers.
Puncheon Summary
The type 1 and 2 puncheon do not represent sustainable design. They damage the resource if onsite trees are cut to provide construction materials. Offsite timber materials may be from more sustainable commercial sources. The type 3 puncheon meets the criteria for sustainable design because the material used is more easily renewed. Although the tread may require replacement in 7 to 10 years, the heavy stringers have a much longer life expectancy.
All three types of puncheon are raised high enough above the ground to provide little interference with the movement of floodwater. The tread width of types 2 and 3 puncheon may affect the growth of plants under the tread.
Type 3 puncheon is the most likely of the three to meet accessibility guidelines.
Gadbury
Gadbury (figure 49), a structure similar to puncheon, was developed in the Pacific Northwest. Gadbury uses two half logs, as described for puncheon, and longer notched sleepers. The notch cut for gadbury must be about twice as wide as the notch cut for puncheon. The two half logs are placed on each side of the center of the notch with the flat surface up. Two full logs are placed in the notch on the outside of each of the half logs.
An experienced crew can construct gadbury without using spikes or steel driftpins. Such construction requires considerable skill and experience with woodworking tools. Lacking this experience, the pieces can be spiked or pinned together. Earth may be placed on the half logs and held in place by the full, outside logs.
Gadbury uses more wood than puncheon. From a standpoint of sustainable design, gadbury is less suitable than other techniques.
Figure 49—Gadbury is another rustic structure similar to puncheon.
Use peeled logs for gadbury.
Bog Bridge
A bog bridge is a form of puncheon. Normally, bog bridges have a single- or double-plank tread surface resting directly on mud sleepers (figure 50), cribbing, or piles. A puncheon, by contrast, will usually have stringers resting on the mud sleepers, with tread decking nailed perpendicular to the stringers.
Figure 50—A simple bog bridge with sleepers. This common
structure
is also called a single-plank boardwalk in coastal Alaska.
To add to the confusion over terminology, in coastal Alaska, bog bridges are called boardwalks, or step-and-run boardwalks if spacers are used to create steps (figure 51). In other places, the term bog bridge is synonymous with puncheon. In parts of the Rocky Mountains and Sierras, bog bridge equates to turnpike, a structure we described as a raised walkway of stone and fill material. We define bog bridges as a series of connected, short-span bridges close to the ground.
The tread of a bog bridge is usually treated, rough-sawn 3- by 12-inch plank that is 6 to 9 feet long. The plank parallels the centerline of the trail and rests on closely spaced, lightweight foundations. This means that the tread of the bog bridge can be closer to the ground, perhaps only 6 to 12 inches above it, providing 3 to 9 inches of clear space below the tread. There is little to block the flow of water (in either direction) below the plank, and little to resist the force of floodwater going over it. In the backcountry, bog bridges are normally one 12-inch plank wide. A plank this narrow does little to interfere with plant growth underneath. The span of each of these small bridges will vary with the type of wood used for the plank, the thickness of the plank, and the anticipated weight on the plank. In areas of heavy, wet snow, the snow may be the heaviest weight on the bridge. Snow load may be as much as 300 pounds per square foot in such areas.
Figure 51—The step-and-run technique is a way of keeping planks level
as
elevation changes. Level planks help reduce slipping in wet climates.
Bog Bridge on Sleepers
In its simplest form, the plank of the bog bridge rests on sleepers. A sleeper is placed in a shallow trench at right angles to the trail centerline. A second sleeper is prepared and placed in another trench 6 to 9 feet away. This distance is the span, which is determined from older installations or with the help of someone with carpentry or structural engineering experience. Place the plank flat in the notches of the sleepers, with one cut end centered in line with the centerline of the log. Mark the plank where it meets the centerline of the next sleeper and saw it to the proper length. The plank is nailed to the sleepers at each end with two 50- or 60-penny (appendix D), ring-shank nails driven through previously drilled pilot holes. This process continues across the wetland.
Bog Bridge on Cribbing
Occasionally, log or timber cribbing can be used to support the plank of a bog bridge. Plank can either be nailed to each of the top logs or timbers, or one large-diameter log can be notched and pinned to the top logs (similar to the sleepers described earlier). If the bog bridge is more than 2 feet high, the plank should be two planks wide for safety.
Bog Bridge on Piles
Another technique for building bog bridges is to rest the plank on pile foundations. The three types of suitable piles are endbearing piles, friction piles, and helical piles.
After installing a pair of bents or piers, pressure-treated 3- by 12-inch planks are nailed to the ledger or ledgers as described for the bog bridge on sleepers. The ledgers do not have to be notched. When piles are used, the plank may be more than 2 feet above the ground or water. In such cases, the tread should be two planks wide.
Bog Bridge Summary
Whether a bog bridge is built on sleepers, cribbing, or wood piles, it lends itself to backcountry construction. The materials are wood, steel washers, bolts, nuts, and nails. The pieces of wood are relatively small and can be carried by hand. Bog bridges as described here do not meet Forest Service accessibility guidelines, but are suitable where departures from these guidelines are allowed.
Boardwalk
For the purpose of this book, a boardwalk is a structure that uses widely spaced bents or piers as a foundation. Stringers, parallel with the centerline of the boardwalk, rest on the ledgers of the bents or piers. The stringers support the deck, which is usually 2 by 6 or 2 by 8 lumber laid perpendicular to the centerline and nailed or screwed to the stringers, or to nailers bolted to the stringers. Boardwalks usually have a curb or handrail along their edges (figure 52).
Basically, a boardwalk is a series of connected bridges, each with a span as long as is practical, perhaps 8 to 40 feet. At most wetland sites, longer stringers are not practical because they are difficult to transport. Also, building adequate foundations for the long spans often requires large pieces of specialized equipment that cannot negotiate unstable soil.
Figure 52—A typical boardwalk. Boardwalks are expensive
and
somewhat complicated, so seek the help of engineers and
landscape designers during planning.
Stringers
At least two stringers or beams rest on the ledgers and span the space between consecutive bents or piers. As the space between bents or piers increases, a third stringer, or heftier stringers, must be used. Three stringers are always better than two. There's safety in redundancy.
Long, thick stringers are more expensive than smaller ones. However, they permit the bents or piers to be farther apart. Studies of soil conditions and problems of construction access to the site will indicate the costs for stringers compared to bents or piers. Bring in some engineering help to figure out the most economical spacing of bents or piers. Large stringers should be bolted to steel angles that have been bolted to the ledgers. Nailers should be used to attach the deck, as described for type 3 puncheon (figure 53).
Figure 53—Details of boardwalk construction. Large stringers and
ledgers
connected with steel angles and nailers help increase the
life of the stringers.
Ideally, the bottom of the stringers of a boardwalk should be above high-water levels, but this is often impractical. To reduce maintenance, the design of the boardwalk should avoid interference with the flow of floodwater and floating debris. To check for evidence of flooding, look for clusters of dead, broken branches stuck in shrubbery or the crowns of trees. Bark on the upstream side of trees may be scraped or stripped off. The height of anticipated floodwater may seriously affect the design of a proposed handrail. Joists can be toenailed to the ledgers, or steel top flange hangers may be nailed to the ledgers to support the joists (figure 54). Top flange hangers reduce the distance between the deck and the ground below, perhaps eliminating the need for a pedestrian railing.
Pressure-treated wood that is now available is highly corrosive to untreated metal hardware. Hot-dipped galvanized treatment is recommended for all fasteners and hardware.
Figure 54—Supporting joists with top flange hangers helps keep a
boardwalk closer to the ground.
Boardwalk Summary
Often boardwalks, as described here, are found around visitor centers, heavily used interpretive trails, or at other high-use sites. The sophisticated construction and materials needed for a boardwalk are less appropriate in the backcountry where the trail user expects simpler, more rustic construction and more challenging facilities.
During floods, the posts and rails can catch debris and form a dam. In most situations it is better to build as little as possible that will have to resist the force of high-velocity floodwaters. A decision on how much or how little to build should be based on the type and age of the visitors who will use the finished facility—schoolchildren, senior citizens, day hikers, or backpackers. Professional geotechnical and structural engineers and landscape architects are needed for effective design of these big-budget structures.
