Geosynthetics for Trails in Wet Areas
Section 3: Specific Design Applications
- Geotextile or Geonet
- Geotextile With Encapsulated Free-Draining Rock
- Geotextile With Poles or Logs
- Geogrid With Geotextile or Geonet
- Sheet Drains Under Tread Fill
- Sheet Drains or Geonets Used as Drainage Cutoff Walls
- Geocell With Geotextile and Permeable Tread Material
- TrailMaster With Geotextile
Most all of the techniques shown can be integrated into standard trail turnpike construction specifications. To simplify the illustrations, not all the components of a complete turnpike (ditches, curb rocks, or logs, etc.) are shown. For many trail locations through flat muddy sites, the traditional trail side ditch and long outlet ditch will not be necessary. Curb logs or rocks are still needed to confine tread fill except where tread fill materials are quite granular. Shoulders must be maintained to keep geosynthetics covered to protect them from ultraviolet light and traffic abrasion. The figures are simplified cutaway cross-sectional views of the trail. They normally look much better on paper than they do during construction.
Geosynthetics are usually placed directly on the natural ground without prior excavation. Many of the illustrations show the various applications with a sag in the native soil surface along the center of the trail alignment. This sag is caused by adding the weight of the tread fill. The actual amount of settlement is very site specific and depends on soil type, level of saturation, and weight of tread fill used. Less tread fill can be used over geosynthetic products that are rigid or have high bending strengths because the weight of fill is distributed over a larger area. Settlements are decreased when less fill is needed to obtain a stable tread surface. For example, much more tread fill is required for a single layer of geotextile (Figure 7), than for geocell with geotextile (Figure 13). In this example, the cost of importing tread fill must be compared to the increased cost of the geocell.
All alternatives that use tread fill should have a crowned or outsloped surface to help shed water quickly and improve stability and control erosion and sediment. Additional tread fill may be necessary to rebuild the crown after initial settlement. More imported fill will be necessary to maintain the crown if tread wear is high. Alternatives are compared in Table 1.
| Evaluation Criteria of Construction Objectives | Geosynthetic Application | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Figure 7¹ Geotex | Figure 7² Geonet | Figure 8³ | Figure 94 | Figure 105 Geotex | Figure 106 Geonet | Figure 117 | Figure 128 | Figure 139 Geocell | Figure 1410 | |
| Separation (keep tread fill separate from poor soils) | B | B | A | A | B | B | B | NA | A | C |
| Reinforcement (turnpike over deep layer of very weak soil) | D | D | A | A | B | A | B | NA | A | B |
| Reduce quantity of imported fill material | D | D | B | A | B | A | B | C | B | A |
| Eliminate trail side ditching | D | C | A | B | C | C | B | D | B | A |
| Ease of product placement | A | B | C | C | C | C | B | D | C | A |
| Low-cost geosynthetic | A | C | A | B | C | C | C | C | D | D |
| Cost for geosynthetics per square meter... per square yard... |
0.68 0.57 |
4.05 3.42 |
1.36 1.14 |
2.04 2.71 |
4.13 3.47 |
7.50 6.32 |
7.50 6.26 |
7.50 6.26 |
10.08 8.45 |
16.68 14.07 |
| Weight of geosynthetics: kilogram per square meter... pound per square yard... |
0.14 0.25 |
0.89 1.64 |
0.28 0.50 |
0.42 0.75 |
0.32 0.60 |
1.07 1.98 |
2.3 4.25 |
2.3 4.25 |
1.9 3.45 |
3.0 5.40 |
| Alternative Rating Code: A=Best alternative; B=Better than most; C=Not as good as most; D=Least effective; NA=Not applicable | ||||||||||
| 1Single layer of geotextile. | 4Geotextile with poles, logs. Must have small trees onsite. | 8Sheet drains or geonets for drainage cutoff wall. Extensive ditching required. |
| 2Single layer of geonet. | 5Geogrid with geotextile. | 9Geocell with geotextile and permeable tread. Granular fill material required; costs and weights are based on 100-mm-deep cells. |
| 3Geotextile with encapsulated free-draining rock. Rock can be large, single-size cobbles, down to relatively clean sands. | 6Geogrid with geonet. | 10TrailMaster with geotextile. Curb logs are required. |
| 7Sheet drains under tread fill. |
Geotextile or Geonet
Single-layer geotextile or geonet (Figure 7) separates fill material from saturated soils and distributes fill weight so less settling takes place. Since geonets cost more, use them only where drainage and subsurface moisture conditions are worst. Avoid using organic, silt, or clay soils for trail tread material because little subsurface drainage will occur and the trail tread will become muddy in wet weather. Rocky soils or crushed aggregate should be used as a tread material if possible. These materials retain much of their strength when saturated. Excess surface moisture can drain off through these permeable materials if the trail is located on a grade or side slope.
Figure 7-Typical placement of geotextile or geonet through flat, boggy areas. Side or lead-off ditches may not be needed.
Geotextile With Encapsulated Free-Draining Rock
(Sausage Technique, Figure 8). The geotextile provides separation from the saturated soil, and the rock provides drainage for excess water. Twenty-five-millimeter (1-inch) flexible plastic pipe outlets for subsurface water may be desirable where trails are constructed on very flat terrain to avoid the "bath tub" effect. If the trail has grade or is built on a sideslope, other drainage options exist. The rock may be single-size material from pea gravel size to cobbles (75 to 300 millimeters or 3 to 12 inches), or it may be a mixture of rock materials that does not contain silt or clay. The free-draining rock can be placed to a thickness equal to the maximum size rock if only drainage is desired. If reinforcement is also needed, at least 75 millimeters (3 inches) of rock is recommended. The geotextile is wrapped over the rock layer with a 300-millimeter (12-inch) overlap to ensure encapsulation, since settlement of saturated soil can pull the overlap joint apart.
Figure 8-Encapsulation or "sausage" technique where native rock is used for drainage.
Geotextile With Poles or Logs
Figure 9 provides a system that requires less imported fill and resists being pushed down into soft saturated soils. No subsurface drainage is provided with this design, although longitudinal drainage may occur along the poles if the trail is on a grade. Another approach is to place interior logs perpendicular to the trail after cutting them to lengths equal to the trail width. This method does not utilize log bending strengths as effectively and is more labor intensive. An outlet pipe or daylight section would provide drainage where trails are on a grade or side slope.
Figure 9-Poles or logs wrapped with geotextile.
Settling of saturated soil is minimal; the turnpike structure is light weight since it is primarily wood; the bending strength of wood distributes dead load (tread fill) and live loads (traffic); and wrapping trees together with geotextile distributes concentrated live loads.
This alternative is attractive for areas that have wood and not much rock for obtaining drainage. If the trail alignment is very swampy, this alternative has strong advantages because the flotation and bending strength of wood is utilized. Keeping wood continually wet or dry is necessary to control rotting. Otherwise the life of the structure will be cut to less than half. A layer of geotextile placed down the centerline over the logs will help keep the wood saturated and also keep individual logs from coming up through the trail tread surface.
Geogrid With Geotextile or Geonet
Figure 10 shows geogrid placed on top of the geotextile or geonet adds bending strength to the system and decreases settling. This reduces the amount of fill material required. Very little drainage is required with this design, unless geonets are used or the tread material is permeable (rocky soils or crushed aggregate). The geogrid should be pulled taut to remove wrinkles before staking. The stakes and poles provide some pretension of the grid, to better utilize its strength. The geotextile or geonet provides separation from the saturated soil and keeps the drainage paths along the bottom of the fill material from clogging. See Section 913 of the Standard Specifications for Construction of Trails for additional information.
Figure 10-Geogrid with geotextile or geonet.
Sheet Drains Under Tread Fill
Sheet drains under tread fill (Figure 11) provide separation from saturated soils and distribute the trail tread weight to limit settling. Install the product with the plastic core side facing up and the fabric side facing down. This orientation takes advantage of the plastic core compressive strength and the fabric's tensile strength, which will reduce the amount of settling and also the amount of tread fill required. Twenty-five-millimeter (1-inch) diameter flexible plastic pipe can be used as a drainage outlet to take full advantage of the drainage capability of the sheet drain. If the trail is on a grade or side slope, an outlet pipe or daylight section could provide drainage.
Figure 11-Sheet drain under fill material.
Sheet Drains or Geonets Used as Drainage Cutoff Walls
If the trail section is on a sideslope where subsurface water saturates the uphill side of the trail, a cutoff wall can be constructed to intercept surface and subsurface moisture (Figure 12) and help drain and stabilize the trail section. This application is especially beneficial where cutslope sloughing continually closes ditches. The sheet drain or geonet should be installed within 1 meter (3 feet) of the trail's edge. The proper depth of the collection pipe and location of the sheet drain can be determined by probing the saturated soil with a short length of Number 4 reinforcing steel. Collector and outlet pipes can be made from flexible plastic pipe. Keeping the top edge of the drain above the ground will capture surface runoff moving downslope. Cover the exposed material with large rocks to protect it from deterioration from ultraviolet light. The collector pipe can be drained into an outlet pipe or with a sheet drain or geonet panel under the trail section. This application requires ditching for proper interception and drainage of water. Ditching is normally more extensive on flatter terrain.
Figure 12-Sheet drain or goenet used to intercept seepage.
Geocell With Geotextile and Permeable Tread Material
The geocell provides confinement chambers that distribute the trail tread loads over a wider area and reduce settling (Figure 13). This works best in sandy soils, rocky soils, crushed aggregate, or free-draining rock. The net effect is to increase the load-bearing capacity of the tread and prevent feet and hooves from punching down into the trail. The geotextile provides separation between saturated soil and the tread fill. Somewhat less tread fill is required because settling is reduced. There is no subsurface drainage if the trail is on flat ground. If the trail has a grade or is built on a sideslope, drainage will occur through the permeable tread fill. Organic, silt, and clay soils are not desirable fill for geocells because these soils will likely remain saturated and unstable, and thus not strong enough to carry the loads placed on the trail. Geocell itself does not increase the load-bearing strength of clay or silt.
Figure 13-Geocell with geotextile and permeable tread material.
TrailMaster With Geotextile
TrailMaster with geotextile (Figure 14) is made from 3-millimeter (one-eighth-inch) thick polypropylene with 16-millimeter (five-eighths-inch) diameter extruded holes about 32 millimeters (1¼-inches) on center. This material is quite rigid, although it can be rolled up for easier transport. It is widely used in horse stalls to keep hooves dry.
Figure 14- TrailMaster with geotextile.
It can be used without fill material since it has significant bending and tensile strength and is durable enough to resist abrasion from pack animals and even motorcycles. Since the material is used as the trail surface and grasses can grow through, it also resists erosion better than tread fill materials. The geotextile allows water, but not soil, to be pumped through the TrailMaster . The water collected on the surface can be removed by an outlet ditch at the lowest point on flat trail sections, or by using rolling dips or outsloping on other trails. Since the surfacing will deflect under the weight of stock, rebar with hooks on the upper ends must be used to anchor the geotextile and TrailMaster to the ground. Rebar staples are normally needed for transverse joints. The poles provide a curb to keep traffic on the surface and also help anchoring. Settling should be minimal since the weight of TrailMaster is only 2.8 kilograms per square meter (0.57 pounds per square foot). The high price of this product may be offset by not having to import fill for a turnpike section. It can be cut with a hand saw. Turns in the trail are normally cut on angles, which makes constructing winding trail alignments more difficult.
Before placing a large order, it's a good idea to obtain a sample for examination. This is a manufactured material and does not blend well with natural forest environments. Since installation is relatively quick, it may be especially useful as a temporary surface until a more aesthetically pleasing alternative is constructed.
