Formation layer

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The formation layer is the prepared ground surface on which the path's base layer is laid and compacted. The hardness of the ground, depth of top soil and free draining properties of the sub-soil will determine which of the following approaches is adopted.

Full tray excavation

Ground vegetation and topsoil is removed to expose firm sub-soil and to form a formation tray to the specified width and depth. The tray should be rectangular in section with vertical sides. The depth of formation tray will depend on the strength of the sub-soil and the likely users of the path, but the final surface level is similar to the original ground level. Carefully store turfs and soil for use elsewhere.

Formation - full tray excavation

 

Semi-tray excavation

The path surface level is raised above the surrounding ground to aid drainage. Remove vegetation and excavate topsoil to form a tray for part of the total construction thickness. Prior to infilling, use excavated topsoil and stripped turfs to create a formation tray for the remaining thickness required. The tray should be rectangular in section with vertical sides and outer edges of the formation sloping down to the original ground levels.

Formation - semi-tray excavation

Specification
Excavate ground vegetation and topsoil to form X metre wide formation tray to maximum depth of X mm below ground levels. The tray should be rectangular in section with vertical sides and level base
Stripped vegetation and excavated topsoil to be cast and spread locally on site, either side of formation tray and landscaped into existing ground levels. If space is limited cart excess materials to suitable location on site for spreading and landscaping
If soft spots are present, excavate the area below formation level until the sub grade is stable. Back fill with lower quality granular sub base material to formation level and compact to refusal

 

Floated construction

On weak soil (high clay content) the path is ‘floated’ on geotextile (Geotextiles and geogrids). The degree of formation works will depend on soil composition. However, as a minimum, the vegetation and topsoil should be removed to a depth of 50mm to form a shallow tray. This approach is called ‘site strip’. If the soil is very wet and soft (e.g. peat) then the path can be built on a geocomposite laid in the shallow tray. This will provide the extra strength to support the weight of heavy machines, the built path and users. Avoid building the path straight onto vegetated ground as vegetation beneath will rot down causing the path to settle below ground levels – creating an uneven surface and potentially leading to drainage problems.

Formation - floated construction

 

Specification
Strip off ground vegetation and excavate topsoil to form X m wide formation tray to maximum depth of 50mm below ground levels
Use stripped vegetation and excavated topsoil to form raised path verges on either side of formation tray. Source extra topsoil on site. The tray should be rectangular in section with vertical sides and level base
If the formation level contains high clay content, lay and secure geotextile sheet in the formation tray. Geotextile sheet should line the base and both sides. Overlap joining sheets by 1.0m
If the formation level is very wet and soft, lay and secure geogrid on top of geotextile sheet. Geogrid should not protrude up the sides of the formation tray. Overlap joining sheets by 1.0m. Use a combined geocomposite instead of separate geotextile and geogrid.

 

Tips
Don’t excavate more than 150- 200 mm looking for a firm sub-soil. If the topsoil is so deep that more excavation is required then use the floated construction technique. This will reduce the amount of material produced during formation works and reduce the amount of imported stone fill required. Small localised soft spots may be excavated and back filled with stone to the formation level, or supported with geotextile and geogrid, if required
Soft spots can be difficult to identify before the formation layer has been excavated. As a contingency, it is useful to provide a provisional sum in the bill of quantities for excavating and back filling soft spots. Where a formation layer needs to be increased in depth because of soft spots, it is more cost effective to deal with the problem at this stage, rather than having to re-construct a section of path that has failed, because a soft spot was not dealt with when the path was first built

 

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© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh