Separation and reinforcement layers

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Geotextiles

Geotextiles are rolled out over a prepared formation level in the formation tray, and then stone is tipped on top to build the path's base layer. The stone is spread out and compacted with the sheet stretching until full compaction is achieved. The softer the ground, the more stone is required and more strain is placed on the sheet. It is therefore important to use the right grade and strength of geotextile. Manufacturers will give information and recommendations on what grade to use. They are typically available in 100m long by 4.5 - 5m wide rolls. Some of the commonly used geotextiles for path construction are:

non woven felt terram

Non-woven felt...

woven polypropylene geotextile

Woven polypropylene...

Only use a geotextile if there is significant clay content in the soil (i.e. the soil is soft and ‘sticky’ when wet) or if the soil is very weak (i.e. it contains a lot of organic material such as peat). If the formation is hard or well drained and the sub-soil is granular with little or no clay content, a geotextile will not be required.

A geotextile can help to prevent vegetation growth through a path. ‘Terram’ and similar woven felt-type geotextiles are particularly resistant to vegetation. Some geotextiles are susceptible to damage by chemical weed killers so it is important to refer to the manufacturer’s information for details.

Geogrid

A geogrid is a tough moulded plastic mesh with large holes and generally greater strength than a woven geotextile. It can be installed on top of a geotextile in the formation tray, or, used on its own, directly on top of the prepared sub-soil. The mesh will hold the aggregate preventing it from moving downhill or sinking into the ground. This makes them of particular use for paths on sloping ground or crossing very soft ground.  Commonly used geogrid is made by 'Tensar'.

tensar triax geogrid

©Tensar TriAx TX geogrid...

Geocomposite

This is simply a combination of a geogrid laid over a geotextile to provide additional strength to a formation layer.

tensar geocomposite

©Tensar Geocomposite (TriAx geogrid on top of non-woven geotextile)...

Fascines

Fascines are tightly bound bundles of freshly cut brushwood which are laid across the path width and were the traditional predecessor to geotextiles. They have been used over many centuries and can still be used in areas where importing an artificial material may be inappropriate, being a constructive use of cleared vegetation. It is good practice to use fascines when crossing very wet ground as they can provide initial support to a geotextile sheet during base laying operations.

Geotextiles and geogrids can be used in the following situations:

Separation
To prevent base stone from disappearing into weak soil formation (geotextile)
To prevent soil pumping into base stone (geotextile)
To prevent the wash out of base stone from underground springs (geotextile)
To prevent vegetation growth through a formation (non-woven felt geotextile)
Reinforcement
To strengthen a base layer laid on a weak soil with high clay content (geotextile)
To float a base layer over a very weak soil (peat) (geocomposite)
To prevent migration of aggregate down the slope on gradients steeper than 1:12 (geogrid)
Filtration
To line filter drain or French drain trenches to prevent silting up of drain stone and perforated pipe with fine particles washed out of soil (geotextile)

 

Specification
Use a medium grade of geotextile sheet for most path applications (Autoway 120, Terram 1000, Terram 1500, Terram 2000, Lo Trak 16/15). Heavier grades are used for high loadings, such as paths with vehicle use or high volume of different user groups)
Overlap successive sheets of geotextile and geogrid by 1 metre along the formation tray length. Geotextile sheet edges should extend about 150mm beyond the formation tray edges and should be covered by verge soil or turfs to a minimum depth of 150mm. If a geocomposite is used, the geogrid should be facing upwards with the geotextile beneath, cut to the required path width within the formation tray. Geogrid edges should not curve up the tray sides
Cover the geotextile and geogrid with path base stone.  Minimum cover should be 150mm to prevent possible localised exposure on unbound path surfaces which are prone to frost-heave. Geotextiles and geogrids should not be visible after path verges have been landscaped.

 

Maintenance
A path should not require maintenance to the geotextile or geogrid once the path is constructed
Exposure of the geotextile or geogrid is an indication of other problems with the design or construction of the path

 

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