Page 158 - Trenchless Technology Piping Installation and Inspection

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illustration of the load transfer mechanism for both types of pipes
due to the vertical soil pressures. As can be seen from Fig. 4.3, rigid
pipes sustain vertical loads by virtue of the material strength alone
and with very little deflection. On the other hand, flexible pipes
tend to deflect and use the horizontal passive resistance of the soil
on the sides. As described in the following sections, this difference
in behavior has important consequences in analysis, design, and
installation of pipelines.

4.3.2 Soils
Soils are formed over several geologic periods through physical,
chemical, and temperature effects (collectively known as weathering)
on rocks and minerals. For engineering purposes, soils are often
classified as boulders, cobbles, gravel, sand, silt, and clay depend-
ing upon the particle sizes and Atterberg limits. Particle sizes for
different soils can vary over a broad range with boulders having
sizes in excess of 300 mm and clay having sizes less than 75 μm
(Howard, 1996). This wide variation in particle sizes results in a
wide variation of mechanical properties of soils. Furthermore, natu-
ral deposition resulting from weathering often entails a highly vari-
able soil composition. Although, in localized areas such as along a
short pipe trench, the soil composition may appear more constant,
the contractor must always be alert to changes in the characteristics
of soils exposed during pipeline construction.
The first step in determining properties of soil is soil identifica-
tion. ASTM D2487-06 describes a system for classification of soils for
construction (see Table 4.3), which is a version of the Unified Soil
Classification System (USCS). According to this system, four basic
soil types have been identified, gravel, sand, silt, and clay. Boulders,
cobbles, and organic soils such as peat, and others, are ignored because
they are generally not used in pipeline construction (Howard, 1996).
The USCS classification groups soils according to particle sizes into
several categories such as silty gravel (GW), silty sand (SM), elastic
silt (MH), and so on (see Table 4.3). These categories range from
purely gravel, sand, silt, or clay to categories that are a combina-
tion of these basic types (e.g., silty sands, [SM]). In addition to pro-
viding an estimate of soil properties, such categorization gives the
experienced contractor tools to compare a site with other locations
and determine the required equipment and methods of construc-
tion (ASCE, 2009).
The desired properties of soil in pipeline construction are dif-
ferent from other types of construction owing to the integral nature
of the pipe-soil structure. In most other types of construction, soils
are expected to have enough bearing capacity to sustain the imposed
dead weight of the structure. In pipeline construction, however, the
weight of the pipeline is typically less than the weight of the earth