Bedding layer is composed of granular or concrete materials acting as a support under a pipe, and determined
by calculating external loads in conjunction with engineering specifications and requirements. In general, bedding has the following functions:
a.To provide uniform support under the pipe to ensure uniform pressure distribution in the bedding zone.
b.To maintain a correct gradient and level of the laid pipeline during and after installation.
c.To provide additional resistance force to the strength of the pipe against the external loads i.e. live + dead load) so that :
The protection class, depends on the following factors:
b.Type of Rubble
c.Type of external loads on the surface of the trench and its location.
d.Depth of trench.
e.Width of Trench
Pipe Strength x Bf > External Load x Fs
Fs = Factor of Safety
Bf = Bedding Factor
Standard types of Trench Bedding
a.Trimmed natural trench bottom (Class D):
When the trench bottom in appropriate soil ground and free from large stones, organic material or lumps of frozen soil and can be well compacted. It is enough to protect the pipes from external loads, and the protection of Bedding Factor is < 1 .1.
b.Flat Granular Layer (Class N):
When the trench bottom in appropriate soil ground and free from large stones organic material or lumps of frozen soil and can be well compacted. It is recommended option of pipes up to 300 and the nominal size of graded aggregate to be used is 14mm. It is enough to protect the pipes from external loads, and the protection of Bedding Factor is < 1 .1.
c.Flat Granular Layer (Class F):
This type of bedding protection is used when the layer of granular material at least 100 mm deep and single size gravel is used, containing graded gravel sizes from 5-14 mm for the nominal single size to be used for 100mm diameter pipes is 10mm, for 150mm diameter 10mm or 14mm, for 200 and 300mm diameter 10, 14mm or 20mm, for 350mm to 500mm diameter 14mm or 20mm and for 600mm diameter and larger, 14mm, 20mm or 40mm.It is enough to protect the pipes from external loads, and the protection of Bedding Factor will be < 1 .9.
d.180˚ Granular Bedding (Class B):
It is the best established and most widely used bedding for rigid pipes. Its main disadvantage is the need for import of selected granular material. Special care
should be taken for construction below the water table as excessive water flow through the bedding can lead to the washing out of fines from the trench walls leading to a reduction of side support for the bedding. In these cases, it may be necessary to construct clay or lean-mix water stops at manhole. The sizes to be used for 150mm diameter pipes are 5 to 14mm graded, for 200 to 500mm diameter 5 to 14mm graded and for 600mm diameter and larger 5 to 14mm graded, 5 to 20mm graded or 5 to 40mm graded. Bedding Factor 2.5
e.360˚ Granular Bedding (Class S):
This type of bedding is similar to Class B bedding but uses only single size granular material up to 0.3m above the pipe crown. Used when as-dug material contains large stones. The sizes to used are as 180° Class B. Bedding Factor 2.5
f. Concrete Cradle/Surround Bedding (Class A):
This type of bedding is used under certain conditions: When the highest possible supporting strength is required (e.g. pipes installed under a heavily trafficked road way. When the highest possible accuracy of level and gradient is required.- When it is impractical to remove the trench sheeting until after completion of the bedding.-To minimize any expected risk (near foundation, or any subsequent excavation). Sometimes the full concrete surround bedding is required (with or without RFMl). Bedding Factor from 4.5 – 2.6