This section is based on BS EN 1295-1: 1998 Structural Design of Buried Pipelines Under Various Conditions of Loading. This standard gives guidance on the nationally established methods of design. For the UK this is detailed in ‘A Guide to Design Loadings for Buried Rigid Pipes’ published by HMSO 1987 and ‘Simplified Tables of External Loads on Buried Pipelines’ published by HMSO 1992.

The charts give the maximum depth of cover for single pipelines laid in a narrow trench where there will be relief of load due to friction at the fill/trench wall interface and where the pipe is laid in wide trench conditions.

Where special conditions apply, e.g. multiple pipelines in a single trench, pipelines supported on piles, trench backfilled with lean concrete and delayed removal of trench sheeting, the charts may not be applicable and advice should be sought from our Technical Department or reference made to the full publication.

Whilst the normal recommendation in the UK is to have a minimum of 1.2m cover (measured from the top of the pipe barrel to the finished road surface) under roads and 0.9m elsewhere, it is sometimes necessary to lay pipes at less cover and the charts have been prepared with a minimum cover of 0.6m in Roads and  Fields, etc. However, special precautions should be taken to minimise the risk of mechanical damage during future excavations and reinstatement of other services, agricultural machinery, etc.

The charts are based on the loadings likely to be encountered when installation, including reinstatement, is complete and care must be taken to ensure that the concentrated load due to construction traffic does not exceed the design surcharge loads given.

For a full structural analysis contact  our Technical Department who can provide  detailed calculations and recommendations on bedding design.

The load-bearing capacity of a concrete pipeline is dependent both on the strength of the manufactured pipe and on the support provided by the bedding. The bedding factor (Fm) is the ratio of the strength of the laid pipe to its laboratory crushing test strength.  The higher the bedding factor the greater is the load carrying capacity of a given pipeline.

The methods of bedding normally used with concrete pipes are: 

 Note:
1. Concrete cradles may extend to sides of trench.
2. Transverse steel to be 0.4% min of section at x-x (Fm 3.4).
3. Minimum strength concrete of 20N/mm².
4. Allow concrete to gain sufficient strength prior to trafficking.
5. Maintain pipeline flexibility by leaving gaps at pipe joints filled with compressible material.
6. Bedding beneath and at sides of pipe to be well compacted.
7. Bedding/backfill directly above pipe to be lightly compacted.
8. Dimension Y: minimum of 1/6Bc or 100mm under barrels, and 50mm minimum under sockets whichever is greater (400mm maximum). In rock etc. use 1/4Bc and 150mm minimum under sockets.

Key

Granular bedding material

Selected backfill material

In-situ concrete

* Denotes classic size.

Notes:

For a safe design the supporting strength of an installed pipeline must exceed the total external design load where the supporting strength is the product of the pipe crushing strength and the bedding factor and the total external load is the sum of the trench load, surcharge load and for pipes greater than DN600 the equivalent water load.
A minimum bedding factor required can be calculated from:

·               Unreinforced pipes (DN225-600 inclusive) = 1.25 Fse
·               Reinforced pipes manufactured to BS EN 1916, the Fse increases to 1.5, However in                  order to maintain design continuity and to cater for schemes  designed prior to the                    implementation of BS EN1916, CPM will continue to test reinforced pipes to BS5911 – 100          proof loads permitting continued use of the current Fse = 1.25

DN = Nominal diameter  Bc = External diameter   Bd = Trench width