Page 38 - Reliability and Maintainability of In service Pipelines

Introduction 27

compaction of the embedment is also a component that affects vertical strain, and
thereby plays a role in the ring deflection ratio. It is observed that vertical strain
decreases as compaction of the embedment increases. However, although it seems
logical to implement a highly compacted embedment to maintain a stable ratio, it
is also important to consider that improper embedments such as those with high
strengths can lead to damage and failure of pipes. In the case of soil movement,
high strength embedments are prone to cracks, leaving pipes to be the concen-
trated target of high stresses, and with higher stresses leading to a larger deflec-
tion, exceedance of allowable limits, and finally failure. Ultimately it is a cycle
that continues, with one factor affecting the other.
The usage of proper embedment is an important concept that affects the
pipe soil interaction by ensuring the surrounding environment of the pipe will
not cause any additional stress and strain that can impair the structure of the pipe.
One of the issues that causes pipes to buckle and collapse is the misalignment of
pipes in soils. The embedment ensures that the pipe is kept aligned in place on
the bedding and protected from external loads. However, issues arise with soils
subjected to improper fill materials in the embedment, high loads that exceed
allowable limits, compaction issues, soil permeability, high water tables, seepage,
creep, and temperature fluctuations.
Another major issue caused by loose soils, high water tables, saturation of
soils under the water table, and external loads, particularly vibrations from traffic,
is the liquefaction of soils. The “liquefied” state of soils can cause the pipe to
float out of alignment and cause the pipe to collapse. This becomes very easy as
liquefied soil unit weight is twice the unit weight of water.
Water seepage into soils can also cause soils to liquefy and resemble quicksand
which causes flotation of pipes and the liquefaction of the soil embedment.
Furthermore, the soil under the pipe can be washed out through groundwater flow
channels caused by the seepage, thereby resulting in a significant change within the
overall structure of the affected pipe. This is especially common in areas where tem-
perature fluctuations and water content constantly changes, which present a “free-
ze thaw” action, causing a “creep” in the surface layers of the soil. These condi-
tions allow ground to landslide and greatly affect the pipelines underneath.
Due to the inevitable factors such as those mentioned above, liquefaction of
soils may not be entirely preventable, although the risk of soil embedment becom-
ing liquefied can be minimized in order to protect pipelines.
Ensuring that the correct material is used in the embedment can minimize the
risk of factors that may cause the pipes to leak or collapse. It is important to
ensure that the fill in the embedment is “flowable” and that the granular material
which is used for fill purposes is well-graded. Large rock particles are generally
avoided to prevent slippage, and a nonuniform surface, while fine rock particles
are used, generally being less than 12% of total embedment material, to allow