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.
