Standard types of Trench Beddings
1. Trimmed natural trench bottom (Class D)
Material selected from the as-dug soil from the trench free from large stones, organic materials or lumps of frozen soil and can be well compacted.
Bending Factor 1.1
2. Flat Granular Layer (class F and N)
Consist of a layer of granular material at least 100 mm deep. When single size gravel is used, the bedding is Class F (1.9) and when compacted graded granular material is used, the bedding is Class N (1.1).It is recommended option of pipes up to 300 DN. For Class N the nominal size of graded aggregate to be used is 14mm to 5mm. For Class F the nominal single size to be used for 100mm diameter pipes is 10mm, for 150mm diameter 10mm or 14mm, for 200mm and 300mm diameter 10mm ,14mm or 20mm, for 350mm to 500mm diameter 14mm or 20mm and for 600mm diameter and larger, 14mm, 20mm or 40mm.
3. 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 14 to 5mm graded, for 200 to 500mm diameter 14 to 5mm graded or 20 to 5mm graded, and for 600mm diameter and larger 14 to 5mm graded, 20 to 5mm graded or 40 to 5mm graded.
Bedding Factor 2.5
4. 360° Granular Bedding (Class S)
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 size to used are as 180° Class B.
Bedding Factor 2.5
5. Concrete Cradle / Surround Bedding (Class A)
This Type is used under certain conditions:
a) When the highest possible supporting strength is required (e.g. pipes installed under a heavily trafficked road way.)
b) When the highest possible accuracy of level and gradient is required.
c) When it is impractical to remove the trench sheeting until after completion of the bedding.
d) To minimise any expected risk (near foundation or any subsequent excavtion). Sometimes the full concrete surround bedding is required (with or without RFMT).
Bending Factor 4.5 - 2.6