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A flexible pipe such as WEHOLITE is by definition a pipe which will deflect when subjected to external loads (traffic, ground water changes, soil settlement etc.) as opposed to a rigid pipe, which carries all external loads by itself. The degree of deflection of a flexible pipe will depend on external loads, support from the surrounding soil and pipe stiffness.

There are several methods of calculating the deflection in buried flexible pipelines. Most of them are based on the Spangler Formula.

Fig. 1

A flexible pipe absorbs external loads and deforms to a certain extent. A rigid pipe, on the other hand, by definition cannot deform. When external loads increase sufficiently, the rigid pipe will finally crack, after which it starts to behave like a flexible pipe.

Fig. 2

Pipes installed underground react to soil settlements along the length of the pipeline. Loads / deflections vary from place to place. A flexible pipe reacts to additional settlements / loads by bending, while rigid pipe reacts by angular deformation in the joints.

WEHOLITE is designed by limit state principles in most cases in accordance with BS EN 1295 - 1 : 1998 - structural design of buried pipelines under various conditions of loading, although other design methods including finite element analysis have been used for special applications.

The limit states used are:

• Serviceability limit states represented by a limiting deflection expressed as a maximum percentage change in the vertical dimension.

• Ultimate limit state represented by the strength of the pipe as determined by the buckling behaviour of the pipe.

This design method forms part of the WRc Approval for WEHOLITE pipe and ASSET International Ltd will provide detailed calculations conforming to this method for any proposed installation upon request.