7. Road Design

Road design involves translating field location survey and other data into specific plans to guide construction. Design criteria must be flexible to allow for modifications to minimize erosion hazards under varying site conditions. This is the stage of development where various measures to control erosion and reduce off-site erosional impacts are incorporated into the road design.

Re-vegetation and associated practices are important considerations during the design process. In addition, future maintenance needs are an important consideration to assure stability and economical use of the completed road. If regular maintenance cannot be assured, this must be accounted for in the design so that undue erosion will not occur.

A number of possible erosion control practices can be included in the road design process:

1. Use as narrow a road as possible commensurate with traffic speed and safety requirements and erosion hazards. In certain situations it may be necessary to reduce speeds and provide for alternative safety measures (such as, restricted road use) to assure a narrow road in high erosion hazard areas.
2. Attempt to balance the volume of cut and fill material to minimize excavation. Use proper layer placement and compaction techniques wherever possible on fills to assure stability against mass failure.
3. Use full bench construction (no fill slope) where stable fill construction is impossible. Haul excavated material to safe disposal areas. Include waste areas in soil stabilization planning for the road.
4. Where full bench construction is impractical, properly designed retaining walls provide an effective but costly alternative to hold fill material.
5. Use the steepest slopes possible on cut and fill slopes commensurate with the strength of the soil and bedrock material as established by an engineering geologist or other specialist in soil mechanics. Benching cut slopes in areas of weak or erodible bedrock (such as, weathered granites) into a series of properly drained terraces provides opportunity for vegetation establishment and may even require less excavation.
   
   Properly designed road surfacing is often required to prevent excessive roadway erosion and maintain a usable road. The surface required depends on many factors such as the type and volume of traffic, strength of subgrade, service life, and materials available. Often, locally available gravels or crushed rock will serve the purpose. It may be desirable to surface both the road tread and the ditch in one operation

7.1 Road Drainage

7.1.1 Crossing Natural Drainageways

There are three methods for crossing natural drainageways: fords, culverts, and bridges. Factors influencing the appropriate crossing include construction and maintenance cost, equipment and supplies available, debris potential, stream size, contemplated road use and life, foundation conditions, and vertical position of the road relative to the stream.

1. Fords are attractive alternatives for crossing small streams, particularly in areas where large amounts of rock, sediment, and organic debris tend to plug bridges or culverts. Fords cause minimal disturbance to the stream channel, are inexpensive, and avoid many of the problems associated with bridge and culvert installation. Fords require stable channel bottoms able to support vehicles or channels that can be protected by gabions or paving (Figure 2).
    
2. Culverts (metal or wood) or bridges are required for channels where fords are impractical. Availability of construction equipment and materials, size of stream, potential for debris, terrain steepness, and reliability of the calculation for determining culvert capacity are some of the points to consider when deciding whether to use a culvert or a bridge at a given location. Other factors being equal, bridges are preferable, particularly in areas with debris or excessive sediment problems because the chances of failure are less.
   
   Structures should be large enough to carry the flows to which they are subjected within acceptable limits of risk. Costs increase rapidly with size so adequate local hydrological studies are needed. It is important to base the size requirement on the anticipated risk of failure rather than on the return interval of the flow alone (Table 3). The percent chance of failure established for a given structure will depend upon the anticipated economic and environmental hazards.
    
3. Roads should climb away from channel crossings in both directions wherever practical so high water will not flow along the road surface. Surface sloped sections of the road if necessary to reduce sediment movement directly into the stream.
    
4. Where adequate maintenance can be assured, install open top culverts or dips in the road surface to direct road runoff onto filter strips rather than directly into the stream.
    
5. Use rip-rap (placed rock), masonry headwalls or otherwise protect embankment and channel sides at drainage structures (Figure 3).
    
6. Increase the capacity of bridges or culverts in areas where debris, sediment, or both types of problem exist. In extreme situations, this may mean doubling the capacity of the structure.
    
7. Frequently maintained trash racks (grates) over the inlet and may be useful where floating debris tends to plug culverts.
    
8. If at all possible, use bridges in area where debris problems are severe and fords are impractical. Otherwise it may be necessary to construct rock- or gabion-protected fills with a dip to allow overflow in the event that culvert capacity is lost.

7.1.2 Drainage Along the Roadway

Drainage is needed along the roadway to remove water before it has a chance to concentrate and cause erosion. To help accomplish this, slope road surfaces laterally either outward or inward, depending on traffic needs and erosion hazards. Unfortunately, traffic can cause some rutting in the road surface that concentrates flow along the road in spite of the outsloping or insloping. Thus in many situations, additional cross-drainage measures are needed to interrupt this flow and divert it laterally before it has a chance to cause erosional problems.

1. Outsloping (sloping toward the downhill side of the road) of from 3-5% is preferable to insloping because it eliminates the need to develop facilities to dispose of the water draining down the inside of the road. Outsloping can be unsafe in some situations because of particular traffic requirements or unusual site conditions such as clayey road surfaces that are very slippery when wet. In addition, outsloping should only be used where runoff will flow off the road onto stable surfaces. Normally, this precludes the use of outsloping on fill portions of the road unless fill slopes are small and low in erodibility or, are well protected by mulches, vegetation, or both.
    
2. Insloping (sloping toward the uphill side) of the road surface is preferred to outsloping in areas of unstable fills, except in the case of a contour road where there is no chance for lateral flow along the road. Water draining from the road is carried along the inside of the road either on the road surface itself or more commonly in a ditch. Culverts are installed periodically to carry the water under the road. Some points to consider when designing an insloped road are:
   
   a) Avoid using ditches or keep ditches to a minimum width and increase the number of cross drains to reduce the total area disturbed by construction.
   
   b) Plan ditch gradients steep enough (generally greater than 2%) to prevent sediment deposition.
   
   c) Install culverts frequently enough to avoid accumulations of water that will cause excessive erosion of the road ditch and the area below the culvert outlet. Surface the ditch in areas of erodible material (such as, weathered granitics).
   
   d) Use a culvert size of at least 40 to 50 centimeters, depending on expected debris problems.
   
   e) Install culverts at the gradient of the original fill slope if possible: otherwise provide anchored downspouts to carry the water safely across the fill slope. Skew culverts 20° to 30° toward the inflow to provide better inlet efficiency and flow characteristics. Provide rock or other splash basins at the downstream end of culverts to reduce the erosion energy of the emerging water (Figure 4).
   
   f) Protect the upstream end of culverts from plugging with sediment by using sediment catch basins, drop inlets, changes in road grades, headwalls, and recessed cut slopes.
   
   g) Install the culvert deep enough to assure that it will not be crushed by traffic loads. This requires a depth of about 1.2 meters for metal culverts subjected to loads from large, loaded logging trucks.
    
3. In some areas, alternating inslope and outslope sections can be built into the road, especially if road grades are “rolled” (provide alternating adverse and favorable grades). In such instances, install dips or cross drains on the surface of the road to control erosion of the roadway.
    
4. It is usually necessary to construct cross drains in the road surface on either insloped or outsloped roads to help prevent erosion caused by water concentrations in ruts. Various types of cross drains are used, including open-top culverts and intercepting dips. Some points to consider when installing cross drains are:
   
   a) Spacing requirements - spacing depends on a number of factors such as road grade, and type of material. Guides for spacing are presented in Table 4.
   
   b) Open-top culverts are usually constructed of wood (Figure 5). They should be installed at a 30° angle downslope to promote self cleaning and make crossing easier (Figure 6). Culverts of this type must be properly maintained to prevent plugging and damage by traffic.
   
   c) Intercepting dips, when properly constructed, are cheaper to maintain and more permanent than wood, open-top culverts. Dip design depends on the kind and speed of the traffic using the road. The dip designs shown in Figures 7 and 8 allow road use by passenger autos traveling at speeds of approximately 30 kilometers per hour. On steeper roads it may be necessary to install open-top culverts (using the same design) in addition to dips to meet the cross-drainage spacing criteria shown in Table 4. A good discussion of dip design is given in reference 8.
   
   d) In addition to cross drain spacing, location of cross drains is an important factor to consider in minimizing sediment delivery to stream channels from either insloped or outsloped roads. Some location guides are presented in Figure 9.
    
5. Berms are required on the outside edge of the road at specific locations where alignment and grade characteristics cause excessive runoff from the road tread over the fill slope. Use compacted soil, soil cement, or asphalt mixtures to construct stable berms. Where necessary, use downspouts in the berm to safely carry water to the bottom of the fill slope. Locate downspouts at safe water discharge points and provide energy dissipators (rock basins, and so forth.) to further reduce erosion hazards (Figure 10).

< Back to Reducing Erosional Impacts of Roads

Last Modified: 07/05/2007