If crossing wetlands cannot be avoided, contact your State natural resources agency for rules and regulations, which may vary from State to State. The landowner is strongly advised to use the services of a forester and a professional engineer to develop complete design and construction specifications for roads through forested wetlands.
Forested wetlands can be divided into three types: mineral soil wetlands, shallow peat wetlands, and deep peat wetlands. Roads in both mineral soil and shallow peat wetlands may be constructed using conventional road construction techniques for cut and fill, and drainage structures. Special construction methods must be used for roads on deep peat wetlands.
This section gives general recommendations for planning, designing, and constructing roads in any of these wetland types. Geotextiles can be used to solve drainage problems in any wetland type. Recommendations are also given for road construction during winter, which can minimize adverse effects to the wetland and reduce costs.
|Minimize total wetland road mileage when wetlands must be crossed, while still meeting landowner objectives.|
|Determine the type and depth of wetland subsoils to ensure proper road design and construction.|
|Minimize width of roads consistent with maintaining safety and road design considerations. Provide turnouts, as appropriate, at intervals to accommodate two-way traffic. On deep peat wetlands, road fill slopes should be 3:1 or flatter to spread out road loading and minimize failure (Figure 9, Crowned Fill Section).|
|Construct all road embankment fills with material free of stumps, roots, and duff.|
|Design upland road approaches to wetlands so surface runoff is diverted before entering the wetland.|
|Anchor temporary structures at one end to allow them to move aside during high-water flows.|
|Remove temporary fills and structures to the extent practical when their use is complete.|
|Employ sediment control techniques (such as silt curtains) to prevent sediment movement to open water when placing fill during construction.|
|Provide adequate cross-drainage by employing one or both of the following techniques:|
|1.||Construction methods that allow free water flow throughout the entire roadbed (Figure 22).|
|2.||Culverts or other cross-drain structures at each end of each wetland crossing and at intermediate low points. Space culverts or other cross-drain structures at maximum 300-foot intervals to ensure adequate cross-drainage through the roadbed (Figure 23).|
Choosing the appropriate road construction technique depends on a knowledge of water table position, zone of water flow, and type and strength of wetland soils. With any road construction technique in wetlands, culverts, ditches, or both may be necessary.
|Follow these recommendations when constructing ditches on wetland roads:|
|Construct ditches, where necessary, to intercept and carry surface and subsurface water (the top 12 inches) to, through, and away from culverts. Unditched openings should be left midway between culverts (Figure 23).|
|Avoid having ditches create additional outlets that will result in drainage of the wetland.|
|Additional methods used for drainage ditches are listed under guidelines that follow for crossing specific wetland types.|
Wetlands with mineral soils include those wetlands having fine-textured (clay or silt), slowly permeable soils to sandy soils overlaying impervious subsoils or hardpans. Road building across these wetland types employs conventional road construction techniques for road fill and drainage structures.
These weak mineral soils can be excavated and backfilled with clean granular soils, or they can be covered with clean granular fill and allowed to compress and displace. Additional fill is added to keep the roadbed at the desired grade.
Culverts and ditches are installed to minimize disruption of normal water flow across the landscape and transport water through and away from the roadbed. Install culverts of sufficient size to handle hydrologic flows for the site and for long-term maintenance needs. If ditches are needed, construct them immediately adjacent to the toe of the fill slope. Filled areas in flow planes should be designed to allow high flows to pass unimpeded.
Figure 22. Proper roadbed construction in peat wetlands allows free water flow. (Redrawn and adapted from Minnesota Department of Natural Resources 1995, p. 50)
Roads in wetlands with peat soils less than 4 feet deep may be constructed using conventional construction methods. The conventional road construction method consists of excavating the shallow peat and then backfilling with clean granular material. The excavated peat can be used to flatten the roadbed fill slope. Excess peat should be hauled away and disposed of at an approved upland disposal site.
Figure 23. Adequate cross-drainage throughout the road length is essential in deep peat wetlands. (Redrawn from Minnesota Department of Natural Resources 1995, p. 51)
|Another accepted road construction method
involves placing granular fill material directly onto the peat surface. The
weight of the fill material displaces (or pushes aside) the weaker peat until
the strength of the subsoils is sufficient to bear the weight of the fill
material and vehicle loadings. As final settling occurs, additional fill may be
needed to maintain the desired road grade.
With both methods, culverts and ditches are installed to intercept surface and subsurface water flow, transporting it through and away from the roadbed. Most subsurface flow occurs in the top 12 inches of the peat.
|Follow these recommendations when placing culverts:|
|Install culverts that are a minimum of 24 inches in diameter buried halfway below the soil surface (Figure 24). The upper half will handle surface storm flows and the lower half will handle normal subsurface water flows. Failure to bury the lower half of the culvert will cause subsurface water to pond on the upstream side of the road and to kill trees.|
|Place culverts at the low points of the wetland to allow surface water flows to pass through the road embankments. If ditches are needed, construct them immediately adjacent to the toe of the fill slope.|
Roads in wetlands with peat soils greater than 4 feet deep can be constructed using special construction methods that do not require excavation and backfilling. These methods use geotextile fabrics, special embankment structures (such as lightweight road fills, extra-wide road bases, or log corduroy layers), and the inherent strength of the underlying peat layers to resist slip failure and resultant road failure (Figure 22).
Road failure in deep peat wetlands can range from the gradual sinking to the sudden loss of the road into the wetland. When such failures occur, water flow through the peat wetland is greatly disturbed, which can result in large areas of flooding.
Figure 24. Proper culvert height is lower in wetlands than in other forest conditions, to accommodate subsurface flow. (Redrawn and adapted from Minnesota Department of Natural Resources 1995, p. 55)
|Recommended construction methods
generally specify that a layer of geotextile fabric be placed on the peat
surface. Road fill is then placed over the geotextile. To provide additional
strength and adequate cross-drainage, special materials such as log corduroy,
chunkwood, or wood chips may be added in the lower portion of the fill (Figure
The specific road structure needed depends on the strength of the peat layers underneath the road. The determination of shear strength is critical in designing a sound, safe, and economical road. Consult the services of a registered civil engineer to accurately determine shear strengths, conduct field tests, and provide design specifications.
Some deep peat wetlands with peat layers too weak to support a roadbed will require traditional excavation and backfill methods. Because of the high cost of traditional construction methods, as well as adverse effects to the wetland, it is best to avoid building on these weak peat wetlands.
Cross-drainage through the roadbed in a deep peat wetland is normally slowed or halted as a result of the compression of the peat layers by the road embankment, rutting of the peat surface by construction equipment, or road failure. This slowing of cross-drainage can cause flooding on the upslope side of the wetland and drying on the downslope side.
|Cross-drainage can be maintained
by the proper installation of culverts and drainage layers. In all cases, the
construction objective is to provide a stable road surface while maintaining
the free flow of water through the roadbed.
The following construction techniques can prevent or minimize adverse impacts to deep peat wetlands:
|Construct road embankments when the peat is frozen. Construction on frozen peat avoids ruts and other damage to the topmost root mat layer which normally contains considerable shear strength. Damage to this root mat can greatly reduce the strength of the upper peat layers and reduce the ability of wetland subsoils to hold up the weight of the roadbed and vehicle loads.|
|Maintain a separation between the toe of the embankment fill slope and the ditch when constructing ditches parallel to the roadway. The separation distance should be at least three times the depth of the peat (Figure 23), to prevent or minimize disturbance of the inherent strength of the top layer of peat containing the root mat.|
|Provide ditches to facilitate water flow into and out of culverts (Figure 23). Construct ditches using flotation devices, such as timber mats, or schedule construction during frozen conditions, to prevent or minimize adverse impacts on wetlands and minimize damage to construction equipment.|
|Construct ditches using flotation devices, such as timber mats, or schedule construction during frozen conditions, to prevent or minimize adverse impacts on wetlands and minimize damage to construction equipment.|
|Obtain advice from professional engineers on designing cross-drainage ditches for permanent seasonal roads across deep peat wetlands.|
Geotextiles and their uses are described in a separate section later in this guide. Roadbeds that use geotextile fabrics should be prepared to protect the woody root mat by flush-cutting trees and brush and leaving nonmerchantable material in place. The first geotextile fabric should be laid loosely over the cut material. Then proceed with road construction using log corduroy, a rock drainage layer, or lightweight road fill. Construction techniques follow.
|Place trees parallel to each other, side by side, and perpendicular to the roadbed direction.|
|Cover trees with clean road fill or gravel.|
|If log corduroy is to
be used for cross-drainage, apply geotextile both above and below the corduroy.
If log corduroy is not to be used for cross-drainage, other cross-drainage
structures should be considered (Figure 22).
|Rock drainage layer|
|Place 12 inches of rock (4 inches or less in diameter) over the geotextile, followed by another layer of geotextile. The rock layer will settle into the top 12 inches of the wetland, providing the pore space for water passage through the roadbed.|
|Place clean road fill
or gravel (typically 18 inches deep) on top of the rock.
Lightweight materials may be incorporated into the core of the road embankment fill to lessen the total weight of the road embankment when constructing on weak peat wetlands. Lightweight materials include chunkwood, wood chips, and sawmill residues, among other materials. Materials with known potential to leach toxic substances, such as construction debris, treated wood, tires, asphalt, or other petroleum-laden materials are not suitable for use.
|Place the lightweight materials over geotextile fabric to form the core of the road embankment fill, followed by another layer of geotextile fabric over the lightweight materials.|
|Cover the core with at least 18 inches of granular sand or gravel.|
|Install culverts and
ditches, if necessary, to allow surface and subsurface waters to pass through
the road embankment (Figure 23).
Roads are often constructed across wetlands in winter to take advantage of frozen ground conditions. Follow these recommendations to cross all wetland types in winter:
|Plan the road layout to maximize operating efficiency and minimize site disturbance.|
|Select the shortest practical routes that minimize potential problems with drifting snow and the crossing of open water.|
|Tamp and pack the wetland area wider than needed for the driving and working area if sufficient frost is not present. This additional space will allow for turnouts, snow removal, and parking.|
|Avoid crossing open water or active springs. If these are unavoidable, temporary crossings are preferred. These can be ice bridges, temporarily installed bridges or culverts, or timber mats.|
|Avoid using soil fill.|
|Install any structures that will block water flow so they can be easily removed before ice breakup. If the streams are navigable or require a permit to cross, removal may be necessary at the end of each winter of operation, not just at the end of the timber contract.|
|Use planking, timber mats, or other support alternatives to improve the ability to support heavy traffic.|
|Anchor temporary structures at one end to allow the structure to move aside during high-water flows.|
|Remove temporary fills and structures to the extent practical when use is complete.|
|Avoid clearing practices that result in berms of soil or organic debris building up on either side of the road clearing. Such berms can disrupt normal water flow.|
|Provide buffer strips near open water.|
operations on any portion of the road where ruts exceed 6 inches below the
water surface for a continuous distance longer than 300 feet. Resume operations
only when conditions are adequate to support equipment. This practice will
minimize blockage of cross-drainage through porous peat and prevent or minimize
down-road channelization (Figure 25).
|Cease equipment operations on any portion of the road where ruts exceed 6 inches below the water surface for a continuous distance longer than 300 feet. Resume operations only when conditions are adequate to support equipment. This practice will minimize blockage of cross-drainage through porous peat and prevent or minimize down-road channelization (Figure 25).||
Figure 25. Ruts deeper than 6 inches below the water table in porous peat can take up to 20 years to heal. (Redrawn from Minnesota Department of Natural Resources 1995, p. 63)
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