Trail compaction

1. Definition & Purpose 

  • Compaction is “the exertion of force on something so that it becomes denser”
  • Creates a stable non eroding trail tread 
  • Compacts both the surface and subsurface material
  • Different techniques apply to compact different materials/ soil types 
  • Technics that can apply significant amounts of shear as well as compressive stress, are most effective

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2. Compaction techniques 

Method
Description
Method Static
Description A large stress is slowly applied to the soil and then released
Method Impact
Description The stress is applied by dropping a large mass onto the surface of the soil
Method Vibrating
Description A stress is applied repeatedly and rapidly via a mechanically driven plate or hammer. Often combined with rolling compaction (see below)
Method Gyrating
Description A static stress is applied and maintained in one direction while the soil is subjected to a gyratory motion about the axis of static loading. Limited to laboratory applications.
Method Rolling
Description A heavy cylinder is rolled over the surface of the soil. Commonly used on sports pitches. Roller-compactors are often fitted with vibratory devices to enhance their effectiveness
Method Kneading
Description Shear is applied by alternating movement in adjacent positions. An example, combined with rolling compaction, is the 'sheepsfoot' roller used in waste compaction at landfills.

3. Common methods for compaction used in Trail Building

Method
Technique
Description
Method Tamping
Technique Static
Description Mcleod or similar
Method Slapping
Technique Impact
Description Dirt jump style shovel slapping
Method Vibrating plate
Technique Vibrating
Description There are various sizes and models. Small plates weigh between 50 to 70kg, medium plates up to 90kg and heavy plates even more.
Method Machine attachment
Technique _
Description It is possible to get compactor attachments that can be fitted to construction equipment such as excavators which allow for machine compaction of berms and ttfs.
Method Roller
Technique Rolling
Description Rollers are used more commonly in import build urban bike park constructions where there are less obstructions to access and egress for the machine and generally flatter terrain and safer conditions.
Method Back blading with skid steer
Technique Static
Description Reverse the machine, dragging loose materials with you behind the blade. Monitor the amount of material being pulled behind the blade. As you reverse alter the angle of attack of the machine adding more weight to the back side of the blade, this will push the loose material into the ground and compact it.
Method Bucket smashing
Technique Impact
Description Using the back of the excavator bucket to push or punch into the ground to compact soils. Used in hard to reach areas, backslope, backs of elevated berms. The technique is normally followed by a hand crew who will add additional compaction manually.
Method Layering compaction
Technique _
Description Build up layers and compact these to create import build trails and features. 200mm layers. ‘Layer cake’
Method Direction of compaction
Technique _
Description In general it is recommended to criss-cross the direction of compaction. The last compaction has to be done in the riding direction to have a smooth feeling (same for narrow trails). Only berms are compacted vertically (going up and down from inside to outside edge) with a last shovel slapping in travelling direction to smooth the inter-lines.
Method Soil stabilisers
Technique _
Description Soil stabilisers are chemical, lime or clay-based additives to add into the soil.

4. Optimal soil texture for compaction:

  • 2 factors are essentials to create a hard and smooth trail surface
    • The compacting (take the air out of the material)
    • The cohesiveness (what links the particles together)
  • Compacting can be done with a rather wide scale of soil humidity from dry to wet but it will be hard to find that hard and smooth ground cohesion that is desired if the amount of water in the soil is not optimal. 
  • The best is to play with the natural elements and compact the trail a few hours after a rainfall, when the ground is still deeply moist but not wet. 
  • As this perfect situation is rarely happening when it is time to compact the trail, here are few tips to adapt:
  • Soil is too dry and sandy: 
    • If accessible with vehicles, bring water tanks and pipes. 
    • If not accessible, carry in water with camelbacks or portative containers
    • Water gently, making sure the water goes through the layers and does not wash over the top layer
    • Wait as long as needed for the water to drain through the soil, wait for a moist but not sticky texture. (can take up to 1 hour,  plan some other work in the meantime)
    • Compact in layers of maximum 20cm, criss cross the direction of compaction (if with a plate compactor), always do the last go in the rider direction. (Only berms are compacted vertically, up and down from bottom to top).
  • Soil is too wet and sticky:
    • Use a plastic sheet or carpet in between the ground and the compacting tool
    • Get rid of the water coming from the ground with a hard broom

See materials and soils for more understanding of the soil type properties

See compacting tools list

Soil stabilisers:

Description:
Some additives such as Soil-tac and Dustex have been developed to efficiently bind soil particles together and prevent tread to break down into loose particles and dust. 

Application:

  • These products have to be diluted in water and sprayed on an un-compacted layer of tread. 
  • Once the layer is evenly moist it can be compacted. 
  • The product needs to penetrate the surface material in order for it to soak deeper into the trail tread to be effective. 
  • Work with thin layers and repeat the operation as many times as necessary.

5. References 

Further reading: