Construction Materials
- Choosing Materials
- Logs
- Lumber and Timber
- Decay-Resistant Wood
- Recycled Plastic
- Hardware
- Geosynthetics
- Nonslip Gratings and Grit-Treated Mats
Choosing Materials
Materials used in trail design should be appropriate for the setting. Steel, plastic, concrete, and asphalt may be appropriate in an urban greenbelt, but out of place in the backcountry. Log construction, stone masonry, and dirt trails are appropriate in a primitive, backcountry setting, but out of place in a city.
The Forest Service recognized this problem in the late 1970s and developed a system called the Recreational Opportunity Spectrum (ROS).
The ROS system establishes seven types of recreational land uses and describes the level of development, management, and construction materials suitable for each of them. The ROS principles may appear overly structured, but their application should result in construction and management that is compatible with the environment surrounding a wetland trail whether that trail is in a remote area, an urban greenbelt, or another setting. The ROS concepts are too detailed to include here, but they should be understood by anyone planning to design and construct wetland trails.
Logs
Wood from logs cut onsite is commonly used in trail construction, but wood is susceptible to attack by insects and fungi. Bark separates from the wood. The gap collects water and provides shade and protection for insects and fungi. Peeling off the bark reduces the likelihood of these attacks. Depending on local conditions, removing the bark may double the life of a log.
The bark can be removed by hand or machine. Using a draw knife or bark spud is the traditional way of peeling logs. The random scrape marks left on the peeled logs gives them a rustic appearance. Machine peeling "chews" the bark and some of the wood in a spiral pattern. The finished pieces are almost uniform in size, with a machined appearance that lacks the rustic character of peeled logs.
Wood that is exposed to the weather or is in contact with the ground will eventually require replacement. In wetlands, a flood, a heavy snow, a buildup of ice, fallen trees, or animal damage may shorten the life of wooden materials. Trees growing near a wetland site are unlikely to provide a sustainable source of logs for replacement structures. Even in remote areas, logs cut from trees growing in the vicinity may not be the best choice of materials.
Using logs cut onsite for trail construction is an inefficient use of wood and does not represent sustainable design. Tearing up areas near a site and destroying the character of the wetland makes no sense. Today, responsible trail crews are taking commercially obtained logs and other wood materials to remote wetland sites by boat, horse, mule, off-highway vehicle, by hand, or by helicopter, even when adequate material is growing a few feet from where it could be used. Sometimes materials can be hauled in more easily over snow during the winter for use the following summer.
Lumber and Timber
For the purposes of this text, lumber is wood that has been sawed and planed into uniform pieces with a minimum dimension of 2 inches or less. For instance, a 2 by 6 is a piece of lumber. Timber is wood that has been sawed into more or less uniform pieces, with a minimum dimension of at least 3 inches. Usually, timbers have not been planed smooth.
It helps to understand how logs are processed into lumber and timbers. Logs run through a sawmill are typically sawed into standard-size pieces, usually 1-inch thick or in increments of 2 inches. Common sizes are: 1 by 4, 1 by 6, 2 by 4, 2 by 6, 2 by 8, and 4 by 4. The pieces can also be cut into 3-inch stock. However, such nonstandard timbers would not be readily available at the local lumberyard. Most 4-by-4, 6-by-6, and larger timbers are cut from the center of the log. Generally 1- and 2-inch materials are cut from the outside of the log.
After the pieces of wood are cut from the log, they are referred to as rough sawn. The first step produces a piece that is sawn on its two widest faces. The bark remains on the narrow edges. At this point the piece is described as rough sawn and waney edged. The edges are not parallel or square. Waney-edged wood is used for rustic siding. Waney-edged lumber can be special ordered (figure 68).
Next, the piece of wood is run through another saw, the edger, that trims the edges square and to a standard 2-inch dimension. The piece of wood is now rough sawn on all four sides and is full size—a 2 by 4 is 2 inches thick, 4 inches wide, and as long as the log.
The pieces are cut to standard lengths. Normally, the shortest pieces are 8 feet long. Longer pieces are cut in multiples of 2 feet, up to 16 feet. Rough-sawn lumber or timbers can be ordered. A piece of rough-sawn, 2-inch lumber is considerably heavier than the finished lumber normally carried at a lumberyard. Rough-sawn pieces are not completely uniform.
Figure 68—Lumber terminology.
Depending on the capability of the sawmill, similar pieces may vary 1/8 to 3/8 inch from each other. The pieces will not have a smooth surface, and the edges will be sharp and splintery.
Finally, the rough-sawn pieces are run through a planer. The planer removes enough wood to smooth the surface on all sides and to produce standard-size pieces. After planing, a 2 by 4 is 1½ inches by 3½ inches and is described as S4S (surfaced four sides). The size after the lumber has been surfaced on all four sides is referred to as nominal size.
Most 2-by-4 material is usually run through a special planer to round off the corners. This process is called edges eased and reduces the chances of splinters when handling the wood. Edges eased can also be specified for other dimensions of lumber and the smallest dimension timbers, but must be special ordered.
Waney-edged material should be less expensive than roughsawn because it requires less processing. Rough-sawn material should also be less expensive than nominal-size material because it has not been through a planer or had the edges eased. If the imperfections of waney-edged or rough-sawn material are acceptable, there is no point in specifying the nominal size material for a project. Why pay for someone to turn wood into sawdust and shavings that you can't use? Besides, the additional work results in a weaker piece of wood.
Wholesalers sell wood by the thousand board feet. A board foot is 12 inches by 12 inches and 1 inch thick, or 144 cubic inches. The board footage of lumber and timber is determined at the time the piece of wood is rough sawn. See appendix E for a table of board feet contained in most standard sizes of lumber and timber, and for various standard lengths.
Decay-Resistant Wood
Using decay-resistant wood will greatly increase the life of the material and reduce maintenance. Some species of trees are naturally decay resistant. Wood from other species can be treated with preservatives to extend its life. Depending on the climate and the location of the piece of wood in the finished work, construction without decay-resistant wood may last only 7 to 10 years, while installations of naturally decay-resistant woods may last 70 years or more.
Naturally Decay-Resistant Wood
The most common decay-resistant species include the various cedars, redwood, baldcypress, black locust, honeylocust, and some white oaks. A tannin found in the wood of these trees colors the heartwood and makes it decay and insect resistant. The sapwood of the same tree is almost white and is not resistant. The wood of Douglas-fir and the white oaks does not contain a toxin, but it is dense enough to repel some fungus and insect attacks.
Preservative-Treated Wood
Using chemically-treated wood in wet environments may mean the structure lasts 30 years instead of 7 to 10 years. It is important to know which chemical treatments are appropriate, and whether or not they cause adverse health or environmental effects.
The subject of chemically-treated wood is complex, and is an area of continuing research and product development. Follow the recommendations in the Best Management Practices for the Use of Treated Wood in Aquatic Environments (Western Wood Preservers Institute 2006). Another comprehensive source of information is Preservative-Treated Wood and Alternative Products in the Forest Service (Groenier and LeBow 2006).
In a nutshell, there are several good reasons to use preservative-treated wood in wet areas and few reasons not to use them. All of the treatments effective in wet areas must be applied under pressure in a factory to exacting standards. The exception is copper naphthenate, which can be applied carefully and sparingly with a brush and is good for spot treatment. Both oil-type and waterborne preservatives are suitable for wet environments from a standpoint of preserving wood, but the person specifying materials needs to know the characteristics and effects of each type of preservative before deciding which to use. Water-soluble preservatives, such as borates, are not suited for wet environments. The borates do not permanently "fix" to the wood.
Workers need to take safety precautions when handling or disposing of treated wood. Treated wood should not be burned. Some States and other jurisdictions may also impose disposal restrictions. Best management practices call for proper collection and disposal of treated wood debris and sawdust.
Each of the preservatives containing copper imparts a color that disappears in time. Normally, the color disappears within 2 years, but depending on site conditions and exposure, the process may take several months to 3 or 4 years. One of the most popular preservative treatments, chromated copper arsenate (CCA), is no longer used if anyone is likely to contact the preservative-treated wood. Replacement treatments are more corrosive than CCA, so hot-dipped galvanized hardware and fasteners are recommended to prevent corrosion.
Recycled Plastic
Many manufacturers of recycled plastic are producing this material in the shapes and dimensions of standard wood lumber and timber products. Some of these products are being marketed as premium deck coverings. Recycled plastic can be worked like wood. It can be sawed, drilled, nailed, screwed, bolted, and painted. Although the surface is smooth, it is not slippery.
The properties of some recycled plastic may present unexpected challenges and disappointments. The material can be up to three times heavier than wood. By itself, 100-percent recycled plastic has little strength. It must be reinforced with a steel backing or core to have any structural value, increasing its weight and introducing another material.
Plastic is decay resistant. The thermodynamic properties of plastic—how much it expands and contracts in the heat or cold—are quite different from those of concrete, steel, or wood, the materials that would normally be used with recycled plastic. The surfaces of some recycled plastic severely degrade in sunlight. The problems of strength, thermodynamics, and ultraviolet degradation are being studied. These problems have resulted in new, improved formulations of recycled plastic. These products have not yet withstood the test of time.
Some recycled plastics contain sawdust or another form of wood fiber or fiberglass. These composites are usually stronger and do not have the same thermodynamic problems as most 100-percent plastics. When sawed or drilled, the exposed sawdust and wood fiber may be just as subject to fungus and insect attack as untreated wood. However, wood fibers completely encased in plastic will be decay resistant. A problem is created when any of the recycled plastics are drilled or sawed in the field. Unlike wood, the shavings and sawdust will not decompose. This problem can be resolved by drilling and sawing over a large plastic sheet and carrying the shavings out, the same process that is recommended for disposing of treated wood residues.
Recycled plastic is not a traditional construction material. It may be inappropriate where a rustic appearance is important. Recycled plastic costs 50 to 300 percent more than treated wood. The increased weight of plastic will be reflected in higher shipping and onsite construction costs. One advantage of this plastic is that it does not support combustion.
Hardware
Connectors
The nails, bolts, washers, nuts, and other connectors used for outdoor construction should be made of corrosion-resistant steel. Hot-dipped galvanizing provides more durable protection than electroplating. Products commonly available at most building supply stores are electroplated. It is especially important to use galvanized or stainless steel connectors on wood that has been treated with waterborne preservatives containing copper.
Nails
Most nails used in trail construction are ringshank nails, barn spikes, or occasionally, roofing nails. Ringshank nails have closely spaced circular rings around the shank of the nail. These nails rarely work loose and are very difficult to remove if driven incorrectly. The steel is quite brittle. It will usually break off if it is bent or hit on the side. Nails are sized by the penny, an old form of measurement. See appendix D for gauge (thickness), lengths, and number of nails per pound for each size. Barn spikes are from 8 to 12 inches long, with a wide thread making a complete revolution around the shank every 4 to 6 inches.
Bolts
Bolts are used for constructing bents. Bolted connections are better than screwed connections because the bolt passes completely through at least two timbers or a timber and a steel plate or angle. Both ends of the bolt are visible and can be tightened if the wood shrinks. Three different types of bolts can be used: carriage bolts, machine bolts, and long bolts that are custom cut from threaded rod (called all thread).
Carriage bolts were used to construct wooden wagons and carriages. A square portion of the head of a carriage bolt penetrates into the wood, preventing the bolt from turning when it is tightened. Carriage bolts were originally used with oak, a hardwood that did not allow the bolt head to turn. Carriage bolts are effective with most woods, except for softwoods such as redwood and western redcedar. Carriage bolts do not require washers between the head of the bolt and the wood, but a washer is needed between the nut and the wood. Carriage bolts may be up to 12 inches long.
Machine bolts have a hexagonal head that is flat on the top and bottom. Machine bolts require steel between the head and the wood and between the nut and the wood. The steel can be either a washer or a steel angle or plate. Machine bolts may be up to 12 inches long.
All-thread rods are available in lengths of 2, 3, 6, and 12 feet and diameters of ¼ to 1 inch. The rod, threaded for its entire length, is useful where long bolts are needed. The appropriate length is cut from the long rod with a hacksaw, and a nut and washer are attached to each end. Bolt cutters should not be used to cut the rod. They will mash the threads, making it impossible to attach the nut (figure 69).
Figure 69—Fasteners (from left): carriage bolt,
machine bolt, all-thread
rod, and lag screw.
Lag Screws (Lag Bolts)
Most people working with these connectors refer to them as lag bolts. Manufacturers call them lag screws. Regardless of their name, they usually have a square or hexagonal head, a threaded tapered shank, and a sharp point. They must be tightened with a wrench. They are made in lengths from 1 to 8 inches and diameters from ¼ to 5/8 inch.
Washers
Four types of washers are suitable for working with wood in a wetland trail: flat washers, fender washers, lockwashers, and malleable iron washers. Flat washers are the most commonly used. They are placed between the wood and nuts and between the wood and the head of machine and lag bolts. The washer prevents the bolt head or nut from being drawn into the wood. Fender washers are wider than flat washers, but they have the same purpose. Fender washers are used if the wood or other material is soft. Lockwashers are not a closed circle; they are cut once and the ends are offset on one side or the other. They are used with the other washers and against the nut to prevent the nut from loosening.
Malleable iron washers are much larger and thicker than other washers. These washers were used when large-diameter bolts joined logs and heavy timbers in traditional rustic construction. Malleable iron washers can be used with 3/8- to 1-inch-diameter bolts.
Nuts
Nuts fit over the threaded ends of carriage and machine bolts and all-thread rods. They must be used against a washer or a piece of structural metal. Nuts are either square or hexagonal, with a round, threaded hole in the center to fit over the bolt or rod. Locknuts fit more snugly on the bolt than common nuts. They are used when vibration may loosen a common nut. Locknuts function better than lockwashers, but they are not as readily available.
Wood Screws (Deck Screws)
A screw is threaded and tapers to a point. The use of a screw determines the desired shape of the screw's head and point, and the material from which it is made. There are perhaps 100 kinds of screws, but wood screws are the ones most likely to be used in wetland trail construction. Wood screws are used to attach tread plank to a nailer, or an interpretive sign to a post. The head of a wood screw is wedge shaped to penetrate into the wood without protruding above the surface. Most screw heads will either have a recessed slot or cross to accommodate a standard screwdriver or a Phillips-head screwdriver. Hot-dipped galvanized steel, stainless steel, and brass screws should be used for trail work.
Most stainless steel deck screws are produced with a hexagonal recess in the head to accommodate an Allen wrench, which makes them somewhat vandal resistant. Other vandalresistant screws require special screwdrivers for removal. These screws are best for installing signs.
Steel Reinforcing Bars
Steel reinforcing bars used for driftpins must be protected from the weather and the copper in wood treated with preservatives. Epoxy-coated steel reinforcing bars are available from suppliers of heavy construction materials. Usually these suppliers sell only to contractors. Epoxy can be purchased from some mail-order companies. The crew building the trail can cut the uncoated bars to size and dip the ends and paint the bars with the epoxy compound. The epoxy coating will resist saltwater corrosion. Before epoxy compounds were available, steel driftpins were protected with a thin layer of heavy automobile grease. The grease also made driving the driftpins easier.
Staples
Heavy steel fence staples, ¼ to ½ inch in size, are useful for attaching hardware cloth to wooden piles used for bog bridge and boardwalk in areas frequented by beavers. Staples can also be used to attach geotextile fabric to wood.
Hardware Cloth
Figure 70—Two sizes of hardware cloth.
Geosynthetics
Geosynthetics are synthetic materials used with soil or rock in many types of construction. Geosynthetics can improve construction methods and offer some alternatives to traditional trail construction practices.
The Missoula Technology and Development Center produced a detailed report, Geosynthetics for Trails in Wet Areas: 2000 Edition (Monlux and Vachowski 2000), about these versatile products. The following information is summarized from that report. MTDC updated the plan in 2007.
Geosynthetics perform three major functions: separation, reinforcement, and drainage. Geosynthetic materials include geotextiles (construction fabrics), geonets, sheet drains, geogrids, and geocells. All these materials become a permanent part of the trail, but they must be covered with soil or rock to prevent ultraviolet light or trail users from damaging them.
Geotextiles, sometimes called construction fabrics, are the most widely used geosynthetic material. They are made from long-lasting synthetic fibers bonded to form a fabric. They are primarily used to separate trail construction materials from wet, mucky soil and to reinforce the trail. They have the tensile strength needed to support loads and can allow water, but not soil, to seep through. Nonporous geotextiles can be used in drainage applications to intercept and divert groundwater. Felt-like geotextiles are easier to work with than heatbonded, slit-film, or woven products that have a slick texture.
Geotextiles are often used in trail turnpike or causeway construction. They serve as a barrier between the silty, mucky soil beneath the fabric and the mineral, coarse-grained, or granular soil placed as tread material on top of the geotextile. The importance of separation cannot be overemphasized. Once mineral soil contains about 20 percent of silt or clay, it takes on the characteristics of mud—and mud is certainly not what you want for your tread surface. Most geotextiles commonly used in road construction work for trail turnpikes. The fabric should allow water to pass through it, but have openings of 0.3 millimeters or smaller to prevent silt from passing through.
Geotextile is sensitive to ultraviolet light. It readily decomposes when exposed to sunlight. When geotextile is not exposed to sunlight, it lasts indefinitely. Always store unused geotextile in its original wrapper.
Geonets or geonet composites (figure 71) have a thin polyethylene drainage core that is covered on both sides with geotextile. They are used for all three functions—separation, reinforcement, and drainage. Since geonets have a core plus two layers of geotextile, they provide more reinforcement for the trail than would a single layer of geotextile.
Figure 71—The net-like core of
geonet allows drainage.
Sheet drains are a form of composite made with a drainage core and one or two layers of geotextile. The core is usually made of a polyethylene sheet shaped like a thin egg crate. The core provides separation, reinforcement, and drainage. Since sheet drains have greater bending strength than geotextiles or geonets, less tread fill is often needed above them.
Geogrids are made from polyethylene sheeting that is formed into very open grid-like configurations. Geogrids are good for reinforcement because they have high tensile strengths, and because coarse aggregate can interlock in the grid structure. Geogrids are normally placed on top of a layer of geotextile for separation from saturated soil.
Geocells (figure 72) are usually made from polyethylene strips bonded to form a honeycomb structure. Each of the cells is filled with backfill and compacted. Geocells are good for reinforcement, reduce the amount of fill material required, and help hold the fill in place. Geocell usually has geotextile under it to provide separation from saturated soils. The grids need to be covered with soil so they will never be exposed. Exposed geocells present a substantial hazard to vehicles due to loss of traction, and can cause hikers or packstock to trip.
Figure 72—Geocell being laid in courses for a bridge approach.
When the approach is completed, the geocell will not be visible.
Nonslip Gratings and Grit-Treated Mats
Gratings are normally used for walking surfaces at industrial sites and boat docks. They may be useful where a slippery tread in a wetland trail has become a problem, or where this problem can be anticipated because of deep shade, heavy rainfall, or icy conditions.
Gratings are made in a variety of sizes from steel, stainless steel, aluminum, and fiberglass. Some manufacturers use fine serrated teeth on the surface of the grating to prevent users from slipping; others use small, round, raised knobs on the surface; still others embed silica grit. The gratings can be attached to an existing deck or used by themselves in the original construction.
Other options to reduce the likelihood of users slipping on the trail include the use of strips of rubber-like material with a non-skid surface. The strips adhere to clean decking. When wood is painted, stained, or sealed, a nonskid additive (sold at paint stores) can be mixed with the paint, stain, or clear sealer before they are applied.
Silica-treated fiberglass mats are available from some of the grating manufacturers. They come in thicknesses of 1/8 to ¾ inch and in panel sizes of 5 to 12 feet. Fiberglass can be sawed to size. Holes can be drilled for nailing or screwing fiberglass to wood planks.
Most gratings are extremely expensive, well beyond the budgets of most trail projects. The exceptions would be for wetland trails at very heavily used sites such as visitor centers or for short interpretive trails.
In Alaska, slippery surfaces are a reality on miles and miles of boardwalk. The Forest Service Alaska Region's Trails Construction and Maintenance Guide (1991) offers several ways of dealing with this problem. These methods are described next.
Roughened Wood Surface
Use a saw or adz to cut grooves perpendicular to the line of travel. Make the cuts deep enough to be effective, but not so deep that they hold enough water to cause decay.
Mineral Paper
Mineral paper is available in a 9-inch width in 50-foot rolls. This tar-fiberglass material is tacked down every 3 to 4 inches along each edge with galvanized roofing nails. Mineral paper should be used on pressure-treated wood because it will hasten the decay of untreated wood. If properly installed, it has given good service for up to 10 years. Mineral paper is inexpensive and easy to replace.
Fishing Net
Nylon fishing net (No. 96 Bunt Web) has been used successfully in the Alaska Region and has been found to be durable and effective. Make sure the net is properly stapled to each pressure-treated plank before delivering and installing the planks. Use an air-driven pneumatic stapler (that can be rented with an air compressor) to drive galvanized staples. Staple at 4-inch intervals to keep the net from bunching and creating a tripping hazard. The netting can be applied in the field using hand-driven galvanized fencing staples.
Neatly hide all edges underneath the walking surface of the plank or logs. Black 1- to 2-inch mesh netting has been used successfully on trails in Alaska. The color blends into the landscape. Used net material can usually be obtained free from net hangers in most Alaska fishing towns.
Cleats
Cleats, narrow boards screwed or bolted perpendicular to the tread at step-sized intervals, are an effective way to reduce slipping, especially on slopes. Metal cleats are common on steep gangways leading to docks subject to tidal fluctuations.
