Materials & Soils

  1. Definition & Purpose 

Knowing the properties of the material you use for trail building is key for a quality trail experience and a sustainable facility. You can use local material or import specific material to build the trail or tread surface. This decision must be made during the planning process and in accordance with applicable laws and regulations, especially when developing trails in protected areas. 

1.1 Native vs. Imported Materials
Native
Imported
Native Sometimes referred to as “on-site materials”
Imported Materials that are imported to the worksite and not gathered or produced locally
Native Occures naturally within the environment or landform through which the trail passes
Imported Is appropriate when native materials are unavailable
Native Enhance the appearance of a trail structure and reduce the visual impact of the trail by blending it into the surroundings.
Imported Harvesting of native materials is detrimental to the resources
Native Should be of a sustainable quality
Imported To be considered if non-native materials are more cost effective or provide a safer, more durable product
Native _
Imported Must be durable and of an appropriate colour that blends with the area the trail traverses
Native _
Imported Can be contaminated with invasive and exotic weeds, pathogens, and organisms
1.1.1 Cost Benefit Analysis

To help determine which materials to use, perform a cost benefit analysis between the use of native and non-native materials. 

  • Utilizing native resources may reduce material and labor costs, particularly in remote locations where materials have to be transported over a long distance. 
  • Consider costs related to the purchase, harvest, and transportation of materials and restoration of the harvest site. In addition, evaluate the potential resource impacts of using native materials. 
  • Finally evaluate the durability, longevity, aesthetic, and architectural characteristics of the material.
  • Finally consider which materials may reduce future maintenance costs
1.1.2 Environmental Analysis and Permits
  • The use of any on-site materials must be disclosed in the project’s environmental documents
  • Must be reviewed by natural and cultural resource specialists and approved during the environmental review process
  • The use of native materials may require mitigation measures and control agency permits
  • All approvals and permits must be secured before native materials are harvested
  • Prior to the start of the trail project mitigation measures and permit conditions must be reviewed with the trail crew, including the location of all approved and permitted native materials and the marking or flagging of those sites
1.2 Specification of trail materials

When planning, the trail designer must decide on what materials to use during construction. When specifying materials, the designer needs to consider:

  • Local soil types
  • Local stone types
  • Climate, weather, and environmental conditions throughout the year
  • Trail difficulty 
  • Proposed amount of trail users
  • Trail user types – only bikes or equestrian or foot traffic
  • Will there be a trail maintenance crew?
  • Is there a maintenance budget?
  • How will the trail be accessed for maintenance?
  • What is the access for materials during the construction phase?

2. Material types

2.1 Rocks and Soil

Knowing about the local primal rock and soil types will help you understand the available materials and its impact on trail construction, such as the usage of tools. These basics should be considered in both planning and construction. Existing dirt or soil found on site can be a useful and free building material. Rocks and soils on site are always an ensemble of the formation of the local area, weather, and local plants. Basically, there is always a three-layered soil composition:

  • The mineral topsoil (A-horizon). 
  • The mineral transformation layer (B-horizon) 
  • The primal rock at the bottom (so-called C-horizon), 

These three layers consist of not more than 30% of organic substance and can be useful in trail construction. Soil is made from organic and mineral materials. Their relations define whether it’s an organic or mineral soil.

On top of these three layers, you find an organic layer. This is basically fallen leaves etc. Which are converted into mineral substances. That layer should be removed for the trail construction process because it is not to use it sustainably. Often, you will put it back on the trail edges after the construction work.

2.1.1 Rock

Rock can be used to construct retaining walls, bridge abutments, steps, causeways, drain lenses, armored drainage crossings, water bars, culverts, and energy dissipaters. 

Native 

It should be harvested from sources in the following order:

  1. Clearing and excavating the trail
  2. Trail reconstruction and trio maintenance
  3. Talus slope rock (fist size or larger), including beaches and ocean shores
  4. Forest floor
  5. Streams or rivers
  6. Quarry sites

Imported

  • Must have the appropriate color, texture, shape, size, structural strength, and durability to construct the intended trail structure
  • Must blend within the environment
  • Rock fill that is encapsulated within the structure and not visible from the trail can be of a type and colour that does not complement the native environment
2.1.1.1 Crushed Stone

Crushed stone or angular rock is a form of construction aggregate, typically produced by mining or quarrying a suitable rock deposit and breaking the removed rock down to the desired size using crushers. It’s often used in trail construction.  Different grades of processed rock may be used for trail substrate and the trail surface. 

Example:

Import build, blue grade, dynamic flow trail trail in Hemsedal Norway:

  • Base layer substrate:  Granite 30mm-200mm (clean)
  • Mid layer substrate:  Granite 30mm type 4 (clean)
  • Surface material:  Granite 0mm-4mm
2.1.1.2 Identifying rock types

For trail building it’s important to use the appropriate type of rock. The different characteristics of different types of rock can have an important impact on the trail quality. The best way to know what rock you’re facing is either a geomorphic map provided by local authorities or just visual recognition. Follow those links for an overview of some of the most important rock types:

2.2 Soil 

Soil can be used as fill material for a variety of trail structures including retaining walls, approach ramps, turnpikes, causeways, and step treads. It can also be used to cover exposed roots and rocks or help bind and soften the appearance of aggregate surfacing. 

Native

It should be noted that native soil types can vary widely depending on location. When using native soil, care should be taken to match the appropriate soil type with the purpose for which it is being used. If necessary, it may be better to use non- native materials for longevity and sustainability.

Soil should be harvested from sources in the following order:

  • From performing maintenance and construction on trail facilities
  • Excavated soil from trail or trail structure construction
  • Slough and berm material produced from restoring back slopes and/ or out sloping the trail bed
  • Cleaning of silt deposits from drainage structures.
  • Silt from point bars in streams or rivers where vegetation is not disturbed.
  • Soil borrow pits

Imported

  • Select a soil that has the appropriate color and texture
  • Review and evaluation must be performed to determine the appropriate soil type
  • The soil should be sterile and free from pathogens, as well as invasive and exotic plant seeds
  • If the soil is being used to augment aggregate surfacing, it should have sufficient quantities of clay to bond the aggregate together
2.2.1 Soil characteristics 

Physically, a soil can be defined by its grain size distribution and humidity, chemically by its composition (Figure 1).  Eventually the grain size defines the construction quality, water run-off and many others. The figure shows an overview on soils based on their share of clay (grain size <2µm), silt (2-63µm) and sand (63-2000µm). The smaller the grain, the higher the adhesive forces. This means a clay thread will have less erosion, can be compacted harder but will also have water run-off and less infiltration.

Figure 1: Soil texture triangle determination. Loamy soils possess a mixture of clay, sand, silt and humus (decomposing organic material). It is a very balanced soil resulting high quality material for trail building. Https://www.trugreen.com/lawn-care-101/learning-center/grass-basics/dig-deeper/soil-texture

2.2.2 Soil types (basic overview)

Clay:

  • Smaller than silt
  • The term “Clay” can refer to both a grain size and a mineral type
  • Clay minerals are sheet silicates that have a surface charge
  • Can incorporate soil water into the mineral structure
  • Easily forms into a ball
  • Stains your fingers
  • Dry clods are very hard
  • Good Ribbon test
  • Clay rich is essential to create the shapes required in modern flow and jump trails. Pump tracks, dirt jumps and bike parks rely heavily on clay based soils for their construction. 
  • Clay soil is sticky and will hold its shape. It retains moisture and nutrients, but has poor drainage and can impede root penetration when too compact or dry.

Sand:

  • You can see the grains
  • Gritty feel
  • Loose when dry
  • Forms a ball that crumbles when moist
  • (Fails a ball test)
  • Geotechnical gauge for approximate grain size.
  • High permeability, means good drainage
  • Good load bearing capacity
  • Increases shear strength of a soil mixture
  • No cohesion, meaning it does not hold shape
  • Erodes readily by fast flowing water
  • Narrow range of moisture contents where it is fun to ride 
  • Loose in dry climates

Silt:

  • Silt is basically rock flour or smaller sand
  • You can’t see the particles readily with the naked eye
  • Not sticky, and crumbles
  • Feels smooth or powdery
  • Dilatancy Test
  • Soil moisture retention is higher than sand (smaller pore size increases capillary suction).
  • This can help in soil mixes
  • Old school BMX track surfacing (fills cracks in clay base)
  • Dust when dry! (low dry strength)
  • Low/no cohesion so does not readily hold shape (similar to dry sand)
  • Erodes readily

Consider:

  • Texture refers to the size of individual soil particles
  • Clay and silt are the soil components with the smallest particles
  • Small particles tend to be muddy when wet and dusty when dry
  • Clay and silt don’t provide good drainage
  • Sand is made of large particles that don’t bind together at all and are very unstable
  • The best soil type is a mixture of clay, silt, and sand
  • If your soil is lacking any one of these, you can attempt to add what’s missing
2.2.3 Determinate (local) soil

Often you can find soil maps on your local authorities’ web-GIS platforms, or you talk to local farmers who know their lands. Maps and GIS are a valuable resource for experienced trail builders. Soil can be roughly tested in the field or more specifically via laboratory testing. 

(Some) Laboratory tests:

  • Moisture content test
  • Atterberg limits tests
  • Specific gravity of soil
  • Dry density of soil
  • Compaction test (Proctor’s test)

Field tests

  • Ribbon test
  • Moisture test
  • Mason jar method
  • Colour test
  • Dry crushing strength
  • Dilatancy Reaction
  • Toughness 
  • Stickiness 

Further reading: 

2.3 Soil Stabilisers

Liquid or powdered soil stabilisers bind the material particles together like a glue, they can be used to reduce erosion across a variety of soils and crushed stone aggregate trail surfaces. 

  • Different products available
  • Utilise the product that best meets their needs within the constraints of the project
  • Some soil stabilisers are organic and some soil stabilisers are chemical based
  • Some soil stabilisers may influence PH levels in areas surrounding the trail, trail corridor and water run off areas
  • Be aware of the impacts and only use soil stabilisers in suitable environments
  • Salt – Can be used to stabilise some soils and aggregate materials such as crushed granite.
  • Lime – Limestone or line dust can be used to stabilise clay soils.
  • Duste– organic, USDA bio certified
  • Soiltac – Soiltac is an engineered eco-safe, biodegradable, liquid copolymer used to stabilize and solidify any soil or aggregate as well as erosion control and dust suppression
2.4 Aggregate

Native

Similar to soil, aggregate can be used as fill material for a variety of trail structures, including retaining walls, approach ramps, turnpikes, causeways, and step treads as well as hardening a trail’s surface. It also can be used to repair holes or depressions in the trail tread or elevate the bottom tier of a rock retaining wall. Aggregate or gravel in any amount should be harvested from sources in the following order:

  • Trail or trail structure construction
  • Trail reconstruction and trio maintenance activities 
  • Scree slopes 
  • Beach area of lakes and oceans
  • Point bars on streams or rivers
  • Excavation borrow sites

Imported

  • Imported aggregate should have a texture and colour like the soil in the area through which the trail traverses
  • It should not produce a sharp contrast to the surrounding environment
  • Aggregate imported into an otherwise natural setting cannot be assumed to be free of contaminants
  • Must be evaluated for invasive, noxious seeds and pathogens. 
  • A monitoring element should be built into an aggregate surfacing project budget to ensure that any seeds transported into a site will be chemically or mechanically removed.
  • Imported rock should also be checked for its serpentine content, which could expose trail workers and users to asbestos dust.
2.5 Further readings: