Page 51 - Failure Analysis Case Studies II

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stiffer material. In homogeneous soils low DCP numbers infer dense materials. Figure 5 shows the
distribution with depth of the DCP numbers as inferred from the results shown in Fig. 4.
At three locations along the pipeline segment considered, test excavations were opened to depths
of 0.5-0.6 m below the pipe invert. The excavations were made at locations where DCP soundings
had been performed. Groundwater was encountered in each of the excavations. In order to enable
visual examination, water in the excavations was pumped out. The examination revealed the
following qualitative features in each of the test pits (see Fig. 6).
(1) Sand backfill of thickness between 10-35 cm was found below the pipe invert. It is noted that
the design called for the pipe to be placed directly on the gravel layer. The best available
information indicates that the pipe was laid out according to the design specifications.
(2) Below the sand backfill a layer of natural clay subgrade approximately 5-25 cm in thickness
was found. The thickness of this intermediate layer increases from the invert of the pipe towards
the trench wall (see Fig. 6(a)).
(3) Below the intermediate clay layer the gravel base was found, and below it, the natural clay
subgrade.

The sand backfill in the zone of the pipe haunches was found to be very loose, significantly less
dense than the sand fill in the upper part of the trench. The gravel layer was seen to be completely
impregnated by a mixture of the natural clay subgrade and the sand backfill.
Figure 7 shows very good correlation between the actual soil profile revealed by the visual
examination (Fig. 6) and the results of the corresponding DCP sounding shown in Fig 4. The
location of the discontinuities in the distribution of DCP numbers shown in Fig. 5 are generally
consistent with the layer boundaries in the lower portion of the trench profile. Breakpoint A shown
in Fig. 7 implies that the sand below mid pipe elevation (haunch zone) is considerably looser than
the sand above this level. Breakpoint A is a common feature of a11 the plots shown in Fig. 4 and
Fig. 5.
Despite variations in the absolute value of the DCP numbers, each of the sounding profiles shown
in Fig. 5 have the following common features:

(1) There is a marked increase in DCP number at depths between 75-145 cm below the pipe crown
which corresponds to the bottom part (haunches) of the pipe section.
(2) There is a marked decrease in DCP number at elevations corresponding to the visually observed
gravel layer below the pipe invert, followed by an increase in DCP numbers as the sounding
entered the natural clay subgrade.

6. INTERPRETATION AND ANALYSIS OF FAILURE

The vast majority of field measured pipe deflections (as shown for example in Fig. 2) exceed the
1.2% limit found to induce severe liner cracking of pipe sections in the laboratory. As a result the
extensive damage observed in the internal pipe liner in the field this is not surprising.
Steel pipes are usually considered to be flexible and they are designed in accordance with “flexible
design methodologies”. However, in the present case the deformations associated with such a design
far exceed the limiting capability of the inner pipe liner to withstand cracking. As a result, although
the pipe section may remain structurally sound, it loses its functionality due to cracking of the liner.
Although it is impossible to specify a sharp criterion defining a flexible pipe, the value of 2%
vertical deflection is often noted in the literature as the boundary between flexible and rigid pipes
14, 51; i.e. a flexible pipe should be capable to withstand 2% deflection without damage. According
to this criterion the present pipe does not belong to the flexible pipe category and should not have
been design based on this methodology.
It is worthwhile to note that design standards of flexible pipes allow vertical pipe deflections to
be as high as 5.0-7.5% [6,7].
The large vertical deformation of the pipe and cracking of the pipe liner appear to be related to
insufficient backfill stiffness as observed in the field investigation. The existing stiffness of the sand
backfill may be inferred on the basis of the DCP tests performed alongside the pipe. Using empirical