Geosynthetics for Trails in Wet Areas: 2008 Edition

Introduction

Trails in soft, saturated soils present special challenges for trail managers. Muddy trails cause problems for livestock and hikers, both of whom tend to skirt the edges of mud holes. The use along the edge of the trail increases the area being damaged. Improperly constructed trails in wet areas lead to erosion, soil compaction, sedimentation, multiple trails where only one is needed, and unhappy trail users. Traditional trail construction methods for wet areas include turnpike or puncheon. These methods have worked well where rock or wood materials are readily available. However, geosynthetics can increase the effectiveness of construction methods and offer additional alternatives.

Geosynthetics are synthetic materials (usually made from synthetic polymers) used with soil or rock in many types of construction. Their use has grown significantly in road construction for the past 40 years, and in trail construction for the past 15 years.

Guidelines on the use of geosynthetics in trail construction have not been readily available to trail managers. The information presented here applies some roads technology to trail design and construction in six categories:

General information on geosynthetic products
Basic geosynthetic design concepts
Specific design diagrams for trail construction over wet, saturated soils
A list of product manufacturers and recommended physical properties
Identification of unsuitable tread fill materials
Case studies

Photo of a path leading up to a bridge.

Geosynthetics--General Information

Geosynthetics have numerous uses in civil engineering. The basic functions of geosynthetics include:

Reinforcement--The geosynthetic acts as a reinforcing element in a soil mass or in combination with the soil to produce a composite that has improved strength and deformation properties. For example, geotextiles and geogrids are used to add tensile strength to a soil mass when these are vertical or near-vertical changes in grade (reinforced soil walls).
Separation--The geosynthetic acts to separate two layers of soil that have different particle size distributions. For example, geotextiles are used to prevent road base materials from penetrating into soft underlying subgrade soils, maintaining design thickness and roadway integrity. Separators also help to prevent finegrained subgrade soils from being pumped into permeable granular road bases.
Drainage--The geosynthetic acts as a drain to carry fluid flows through less permeable soils. For example, geotextiles are used to dissipate pore water pressure at the base of roadway embankments.
Filtration--The geosynthetic acts like a sand filter by allowing water to move through the soil while retaining the soil particles. For example, geotextiles are used to prevent soils from migrating into drainage aggregate or pipes while maintaining flow through the system. Geotextiles are also used below riprap and other armor materials in coastal and riverbank protection systems to prevent soil erosion.
Containment--The geosynthetic acts as a relatively impermeable barrier to fluids or gases. For example, geomembranes, thin film geotextile composites, geosynthetic clay liners (GCLs) and field-coated geotextiles are used as fluid barriers to impede the flow of liquids or gases.
Erosion Control--The geosynthetic acts to reduce soil erosion caused by rainfall impact and surface water runoff. For example, temporary geosynthetic blankets and permanent lightweight geosynthetic mats are placed over the otherwise exposed soil surface on slopes. Geotextile silt fences are used to remove suspended particles from sediment-laden runoff. Some erosion control mats are manufactured using biodegradable wood fibers.

Geosynthetic materials (figures 1 and 2) include geotextiles (construction fabrics), geonets, geogrids, and geocomposites, such as sheet drains and geocells. All these materials become a permanent part of the trail, but must be covered with soil or rock to prevent damage by ultraviolet light. Geosynthetic erosion control material also has important uses for slope and bank protection, but this report does not discuss those uses.

Manufacturers of erosion control geosynthetics are listed in the "Geosynthetic Product Information" section. Please contact the manufacturers for additional information. Geoblock, Lockgrid, EcoGrid and Grasspave2 are used for turf reinforcement and will be discussed. Because all these products are synthetic, their use in wilderness should be reviewed and approved before they are used.

Drawing of a trail fill diagram without geotextile. In the drawing the text reads, Aggregate cap, Shear force, Cross contamination leads to impacts from shear stress., Ground surface, Aggregate migration, Upward movement of soil, and Substandard soil base.
Figure 1--Trail fill material without geotextile. The
aggregate will lose strength as the fill material mixes
with the subbase.

Drawing of a trail fill with geotextile. In the drawing the text reads, Aggregate cap, Ground surface, Drainage, Seperation, Substandard soil base, and Geotextile layer.
Figure 2--Trail fill material with geotextile. The geotextile
layer enhances the trail performance by providing separation,
reinforcement, and drainage.

Geotextiles

Geotextiles (figure 3) are the most widely used geosynthetic. Geotextiles are often called construction fabrics. They are constructed from long-lasting synthetic fibers that form a fabric held together by weaving, heat bonding, or other means. Geotextiles are primarily used for separation and reinforcement over wet, unstable soils. They have the ability to support loads through tensile strength and can allow water, but not soil, to seep through. They can also be used in drainage applications where water flow is much greater than normal for wet areas. The physical requirements listed for all geotextiles in the "Geosynthetic Product Information" section are stringent enough that the products will work for properly designed high-flow drainage applications.

Photo of three different types of geotextile fabrics.
Figure 3--Geotextiles are made from woven and
nonwoven fabrics. Felt-like products are easier to
work with than slick products that are heat bonded,
woven, or made from slit film. Felt-like products are
easier to cut and their flexibility makes them easier
to place on curved trail sections.

Geonets

Geonets or geonet composites (figure 4) have a thin polyethylene drainage core that is covered on both sides by geotextile. Geonets are primarily used for drainage, but also may function as separation and reinforcement. Because geonets have a core plus two layers of geotextile, they provide more reinforcement than a single layer of geotextile.

Photo of a geonet that is between two layers of geotextile.
Figure 4--Geonets with the two layers of geotextile
shown are considered a geocomposite--the core
of geonet allows drainage to the sides that is normally
adequate for the seepage found under trails
in wet areas. The geotextile provides reinforcement
and separation.

Geogrids

Geogrids (figure 5) are made from polyethylene sheeting that is formed into very open gridlike configurations. Geogrids are good for reinforcement because they have high tensile strengths and because coarse aggregate can interlock into the grid structure.

Photo of a ruler next to a geogrid.
Figure 5--Geogrids are normally placed on top
of a layer of geotextile for separation from
saturated soils in wet areas.

Geocells

Geocells (figure 6) are usually made from polyethylene strips 50 to 200 millimeters (2 to 8 inches) high that have been bonded to form a honeycomb. The product is shipped collapsed so it is more compact. During installation, the material is pulled open and the honeycomb structure is staked to the ground surface. Each of the cells is filled and compacted. Compacting trail tread material within the cell increases the strength of the layer and reduces settlement into soft, saturated soils. Geocells are good for reinforcement and reduce the amount of fill material required.

Photo of a geocell.
Figure 6--Geocell usually has geotextile under it
for separation from wet, saturated soil. Normally,
the cells are filled with a soil that drains well.

Geocomposites--Sheet Drains

Sheet drains (figure 7) are a form of geocomposite material made with a drainage core and one or two layers of geotextile. The core of a sheet drain usually is made of a polyethylene sheet formed into the shape of an egg crate. The core provides an impermeable barrier unless it has been perforated by the manufacturer. Perforated cores are always covered with geotextile on both sides to prevent soil from clogging the drainage passages. Geotextile is bonded to one or both sides of the core to provide filtration and separation. When sheet drains are used under trail tread material, they provide separation, reinforcement, and drainage. Because sheet drains have greater bending strength than geotextiles or geonets, less tread fill may be needed above them. Sheet drains also can be installed vertically in covered trenches beside the trail to drain off subsurface water.

Photo of a ruler next to a geotextile on top of a cross section.
Figure 7--Geocomposites such as sheet drains have
a large cross section that allows drainage. If
geotextiles are placed under the trail tread, the
sheet drain should be oriented with the geotextile
on the bottom and the plastic core on top. This
orientation reduces the amount of fill needed.

Geo-Others--Turf Reinforcement

Other proprietary products used for reinforcement are considered geo-others. Typically, they are manufactured from recycled plastics to protect turf from rutting, erosion, and soil compaction. Geo-other products include Geoblock (figure 8), Lockgrid, EcoGrid, and Grasspave2 (figure 9). The MTDC report "Managing Degraded Off-Highway Vehicle Trails in Wet, Unstable, and Sensitive Areas" (Meyer 2002) has information on turf reinforcement materials and their installation.

Photo of a ruler next to a geoblock.
Figure 8--Geoblock, a very stiff material, is one of the many
products for turf reinforcement.

Photo of a ruler next to a Grasspave2.
Figure 9--Grasspave2 is another product for
turf reinforcement.

Basic Geosynthetic Design Concepts for Trail Construction in Wet Areas

Trails in wet areas often are unstable because they are saturated by subsurface moisture and precipitation. Geosynthetics help create stable trail surfaces by providing:

Separation--Geotextiles, geonets, and geocomposites (sheet drains) keep saturated, weak native soils from contaminating stronger, load-bearing trail surface materials. These materials allow water, but not soil, to pass through them.
Drainage--Geotextiles, geonets, and geocomposites (sheet drains) improve subsurface drainage to avoid saturation and weakening of the trail tread.
Reinforcement and Load Distribution--All geosynthetics provide some degree of tread reinforcement and load distribution. This may decrease the amount of imported fill material needed for trail surfacing.

Geosynthetics are relatively simple to use. Products that meet the physical requirements discussed in the "Geosynthetic Product Information" section are tough enough to be placed over small stumps that stick up from the ground surface after brush has been cleared for trail construction. Cutting stumps and brush to within a few inches of the ground usually is all that is necessary. Normally, joints in geotextiles, geonets, or geogrids should overlap at least 300 millimeters (12 inches). Sometimes sections of material are joined with pins or clips rather than being overlapped. All geosynthetics must be stored in their shipping wrappers until installation because they will deteriorate gradually when exposed to ultraviolet light.

Selecting good material for tread fill is very important. Organic, silt, or clay soils should not be used as tread fill because they become muddy when wet. Use firm mineral soil, coarse-grained soils, granular material, or small wellgraded angular rock instead. Soil from wet areas is normally not suitable for use as tread fill. Unsuitable organic soils are easily identified by a dark color and musty odor when damp. Many soils containing clays and silts are just as unstable, but such soils are more difficult to identify. The "Identification of Unsuitable Tread Fill Material" section discusses several methods for identifying unsuitable soils.

The amount of acceptable tread fill material you need over the geosynthetic depends on several site-specific factors (table 1).

In addition to the applications illustrated in the "Specific Design Applications" section, other combinations of geosynthetic materials are possible and perhaps preferable, depending on conditions at the site and the native building materials available there. Once you understand the function of the different types of geosynthetics and product capabilities, you may be able to identify many other applications.

Table 1-Factors affecting the recommended thickness of tread fill material over the geosynthetic material.
Factors Affecting Recommended Tread Thickness	Maximum Thickness Needed	Minimum Thickness Needed
Trail fill quality	Mineral soil with little rock, less than 20% silt or clay	Granular, free-draining materials
Trail tread surface	Horse or motorcycle	Foot traffic
Tread surface moisture content during traffic	Moisture content predominantly high	Moisture content predominantly low
Amount of foundation settlement	Continuously wet areas more than 2 feet deep	Intermittent soft, wet areas less than 2 feet deep
Geosynthetic alternative selected	Single layer of geotextile	Geotextile with other geosynthetics such as geocells
Trail surface crown maintenance	Less than annual	Annual

Specific Design Applications

Most of the applications 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. Curb logs or rocks may be needed to confine tread fill unless the 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 usually are placed directly on the ground without 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. Settlement decreases 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 10), than for geocell with geotextile (figure 11). 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, improve stability, and control erosion and sediment production. Additional tread fill may be needed to rebuild the crown after the trail settles initially. More imported fill will be needed to maintain the crown if tread wear is high. Alternatives are compared in table 2.

Table 2--Comparisons of alternative geosynthetic applications.
Construction Objectives	Geosynthetic Applications
	Geotextile only¹	Geonet only²	Geotextile sausage³	Geogrid with geotextile	Geogrid with geonet	Sheet drain with geotextile⁴	Sheet drain with geotextile⁵	Geocell with geotextile⁶
Separation (keep tread fill separate from poor soils)	B	B	A	B	B	B	NA.	A
Reinforcement (turnpike over deep layer of very weak soil)	D	D	A	B	A	B	NA.	A
Reduce quantity of imported fill material	D	D	B	B	A	B	C	B
Eliminate trail side ditching	D	C	A	C	C	B	D	B
Ease of product placement	A	B	C	C	C	B	D	C
Cost for geosynthetics	$	$$	$	$$	$$$	$$$	$$$	$$$$
Weight of geosynthetics:kilograms per square meter pounds per square yard	0.14 0.25	0.89 1.64	0.28 0.50	0.32 0.60	1.07 1.98	2.3 4.25	2.3 4.25	1.9 3.45
Alternative Rating Code: A=Best alternative; B=Better than most; C=Not as good as most; D=Least effective; NA=Not applicable; $ Least expensive => $$$$ Most expensive ¹Single layer of geotextile. ²Single layer of geonet. ³Geotextile with encapsulated free-draining rock. Rock can be large, single-size cobbles, down to relatively clean sands. ⁴Sheet drains under tread fill. ⁵Sheet drains or geonets for drainage cutoff wall. Extensive ditching required. ⁶Geocell with geotextile and permeable tread. Granular fill material required; weights are based on 100-mm-deep cells.

Drawing of where a geotextile or geonet would be placed in a boggy area. In the drawing the text reads, Geotextile or geonet, Direction of travel, and Tread fill material.
Figure 10--Typical placement of geotextile or geonet through flat,
boggy areas.

Drawing of a geocell with a geotextile. In the drawing the text reads, Optional geotextile layer under geocell, Permeable tread material, Direction of travel, and Geocell.
Figure 11--Geocell with geotextile and permeable tread material.

Geotextile or Geonet

Single-layer geotextile or geonet (see figure 10) separates fill material from saturated soils and distributes fill weight so less settling takes place. Because 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.

Geotextile With Encapsulated Free–Draining Rock

In the sausage technique (figure 12), 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 rock can be just one layer thick if drainage is all that is needed. For reinforcement, at least 75 millimeters (3 inches) of rock would be recommended. The geotextile is wrapped over the rock layer with a 300-millimeter (12-inch) overlap to ensure encapsulation, because settlement of saturated soil can pull the overlap apart.

Drawing of the encapsulation technique. In the drawing the text reads, Free-draining rock, Direction of travel, Tread fill material, and Geotextile (300 millimeters [12 inches] overlap at center).
Figure 12--The encapsulation or "sausage" technique, with
native rock used for drainage.

Geogrid With Geotextile or Geonet

Figure 13 shows geogrid placed on top of the geotextile or geonet to add bending strength to the system, decrease settling, and reduce 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 geogrid, better using 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 964 of the "Standard Specifications for Construction and Maintenance of Trails" (1996) for additional information.

Drawing of a geogrid with a geotextile or geonet. In the drawing the text reads, Geotextile or geonet, Fill material, permeable soil, or aggregate preferred, Direction of travel, Geogrid, and Pole rests against stakes.
Figure 13--Geogrid with geotextile or geonet.

Sheet Drains Under Tread Fill

Sheet drains under tread fill (figure 14) provide separation from saturated soils and distribute the weight of the trail tread 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's compressive strength and the fabric's tensile strength, reducing the amount of settling and 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 sheet drain’s capabilities. If the trail is on a grade or side slope, an outlet pipe or daylight section could provide drainage.

Drawing of a sheet drain under fill material on a trail. In the drawing the text reads, Geotextile, Any type of fill material (100 millimeters or 4 inches minimum thickness), Direction of travel and Sheet drain.
Figure 14--A sheet drain under fill material.

Sheet Drains or Geonets Used as Drainage Cutoff Walls

If a section of trail is on a side slope where subsurface water saturates the uphill side, a cutoff wall can be constructed to intercept surface and subsurface moisture (figure 15), helping drain and stabilize the trail section. This application is especially beneficial where the cut slope sloughs continually, filling 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 (rebar). Collector and outlet pipes can be made from flexible plastic pipe. Keep the top edge of the drain above the ground to capture surface runoff moving down the slope. Cover the exposed material with large rocks to protect the material from ultraviolet light. The collector pipe can be drained into an outlet pipe or with a sheet drain or geonet panel installed under the trail. This application requires ditching to intercept and drain water. Ditching is normally more extensive on flatter terrain.

Drawing of a sheet drain on a trail. In the drawing the text reads, Outlet pipe or sheet drain, Fill material, Direction of travel, Large rocks, Collector pipe, Timber, Seepage, and Sheet drain or geonet. Geotextile side faces uphill to intercept seepage.
Figure 15--A sheet drain or geonet used to intercept seepage.

Geocell With Geotextile and Permeable Tread Material

Geocell provides confinement chambers that distribute the trail tread loads over a wider area and reduce settling (see figure 11). Geocell works best in sandy soils, rocky soils, crushed aggregate, or free-draining rock, where it increases the tread's load-bearing capacity and prevents feet and hooves from punching holes into the trail. The geotextile provides separation between saturated soil and the tread fill material. Less tread fill will be needed with geotextile 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 side slope, moisture will drain through the permeable tread fill. Organic, silt, and clay soils are not desirable as fill for geocells because these soils will probably remain saturated and unstable, meaning they will not be strong enough to carry the loads on the trail. Geocell does not increase the load-bearing strength of clay or silt.