Page 154 - Trenchless Technology Piping Installation and Inspection

120 Cha pte r F o u r

properties of pipe and soil to form a composite structure. Accordingly,
structural designs of underground pipelines include specifications for
the surrounding soil. The idea is for the system to be able to withstand
the imposed demands through a composite action of the pipe-soil
structure. If properly designed and constructed, this composite action
is often advantageous and can be used to enhance the load-carrying
capacity of pipelines. It follows that proper installation is extremely
important to the performance of underground pipeline structures. Of
particular importance are considerations for (1) proper selection and
compaction of soils for foundation, bedding, embedment, and back-
filling; (2) proper control over geometrical configurations of trench
excavations (width, height, and others); (3) control over appropriate
soil cover; (4) control of line and grade of the pipe; and (5) controlling
groundwater in the trench. In addition, appropriate care must be
taken during transportation, handling, and installation to avoid
cracks, gouges, buckling, and other forms of structural damage.
Soils, on the other hand, require different properties for different
subcomponents of an underground installation (e.g., foundation,
bedding, trench walls, and so on). Some of the important properties
considered in design include soil stiffness (modulus), density, type of
soil, and moisture content. Achieving the design properties in the
field requires careful monitoring of construction activities along with
testing and evaluation. The complexity of soil behavior introduces
considerable uncertainty in defining and measuring of soil proper-
ties. The problem is augmented by spatial variability of natural soils
combined with practical necessity to estimate the properties from
very limited amount of sampling and testing. While designs use lim-
ited information measured at few strategic points, construction must
be carried out over the entire spectrum of variability of soils. Coeffi-
cient of soil reaction (E’) represents soil variability and is not directly
measurable, but must be back-calculated using observed pipe deflec-
tions which might be dependent on the pipe depth also. ∗ For this
reason, E’ is not a property of soil alone (like Young’s modulus), but
is a function of the pipe-soil system. This integral treatment of pipe
and soil is common in current design standards. Therefore, that the
outcome of design and construction of pipelines is not the pipe alone,
but the entire pipe-soil structure.

4.3 The Pipe-Soil System
As said previously, pipelines, specifically those using flexible pipe,
are designed as composite structures with pipe and soil forming an
integral system to resist the applied loads. The pipe-soil structure
generally comprises the pipe, the in situ soil in the foundation and

∗ AWWA M45 “Fiberglass Pipe Design” Manual provides one of the most
commonly used procedure for determining the E’ values.