Page 47 - Failure Analysis Case Studies II

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0 24 48 72 % 120
Distance along pipeline segment (m)

* examples of damage description from along pipe segment AA11-AA12
Fig. 2. Typical data obtained from damage survey.

4. GENERAL DESIGN PERSPECTIVE AND PURPOSE OF INVESTIGATION
It is common to define two major categories of soil-pipe systems:

Flexiblepipes. In this case the pipe is prevented from collapsing through the mobilization of soil
reaction. In order to mobilize the soil reaction the pipe must deform. A successful design in this
case depends on the ability of the pipe to retain its functional and structural integrity under the
deformation required to mobilize soil resistance. This case represents a typical soil structure inter-
action problem.

Rigid pipes. The common design assumption for this category of pipe is that their load carrying
capacity is independent of the reaction of the surrounding soil, and pipe deformation is neglected.

It is not obvious to which of the above categories the present pipe belongs. On one hand, being
basically a thin-walled steel pipe its unrestrained load carrying capacity is rather low, making it a
natural member of the flexible pipe category. On the other hand, the brittle inner concrete liner may
be damaged (cracked) at deformations below those required to mobilize sufficient soil reaction.
It appears, therefore, that the pipe under consideration represents a borderline case which does
not obviously belong to either one of the common design categories. Proper pipe design requires
analysis of the soil pipe system, rather than use of standard design methodologies.
The objective of the present investigation was to determine the cause(s) of damage and the areas
responsible. For this purpose it was necessary to determine mechanical properties of the pipe section,
and soil conditions in the field. A secondary objective of the investigation was to study the suitability
of the pipe as a structural shell for a more flexible insert which would act as a barrier between the
flowing corrosive sewage and the steel pipe. For this purpose it was necessary to evaluate the
structural integrity of the pipe in its present, damaged, condition.