Page 39 - Reliability and Maintainability of In service Pipelines

28 Reliability and Maintainability of In-Service Pipelines

flowability as well as to ensure the fill is not too liquidated. For a flowable fill,
Portland cement or fly ash is typically used and formed into a slurry which flows
under the pipe, allowing a more uniform bedding and embedment.
It is essential that fill material is compacted with specific moisture content
based on the depth of the pipe burial. Efficient compaction ensures the usage of
well graded soils and can prevent piping, which is defined as the washing out of
soil by groundwater flow channels from underneath pipes. Well graded soils also
serve as a filter, preventing fine particles in the embedment from leaking out into
the trench walls and vice versa which can lead to instability of soils.
It is expected that the embedment should be able to support live loads above
pipes as well as the load of the backfill. To enable this, it is crucial that the pipe
is protected by the embedment, by forming an arch over the pipe. To maintain
this protective arch, the vertical compression of the soil needs to be controlled by
keeping the compressive strength to a low minimum of 40 psi and ensuring it
does not exceed the internal pressure of the pipe (Whidden, 2009).
As mentioned earlier, the deflection of the pipe ring is almost equal to the ver-
tical compression of the side soil. Therefore, it is important that the vertical com-
pression and shrinkage of the flowable fill which is placed as a side fill, does not
exceed the allowable limits of pipe deflection. Excessive deflection can occur if
the flowable fill fails to maintain the shape of the pipe and fails to support back-
fill. Therefore flowable fill is also required to hold an adequate bearing capacity
to ensure loads are supported and the risk for stress strain induced pipe deforma-
tion is significantly reduced.
To summarize in the context of the pipe soil interaction mechanism, the per-
formance limits are excessive compression and slippage of soils caused by the liq-
uefaction of soils, mixing of soil particles, caused by groundwater seepage which
contributes to soil instability, and the permeability of soils which affect water
seepage by further accelerating the problem.
Soils that are liquefied and are subject to movement, can cause an increased
susceptibility to larger strains that can exceed performance limits and cause addi-
tional stress which feeds into the failure mechanism of pipelines.
However, the liquefaction of soils caused by water seepage is not only limited
to causing pipe failure directly via misalignment. The exposure of flexible metal
pipes to water seepage can also contribute to corrosion and cause corrosion-
induced failure in pipes.

1.6 Deterioration of Pipes

It is understood that different types of pipes undergo different types of deteriora-
tion, due to their varying structural and chemical composition as well as various