Page 91 - Buried Pipe Design

Design of Gravity Flow Pipes 67

Plasticity Index

Liquid Limit
Figure 3.2 Plasticity chart. (Reprinted, by permission, from Asphalt
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Institute Soils Manual. )

A soil is given a descriptive name and letter symbol to indicate its prin-
cipal characteristics. (See ASTM D 2487 or any text on soil mechanics.)
Embedment materials listed here include the soil types defined
according to the USCS and a number of processed materials. ASTM D
2321, “Underground Installation of Flexible Thermoplastic Sewer
Pipe,” breaks down embedment materials into five classes. These
classes along with the USCS letter designation and description are
given in Table 3.1.
Class I comprise angular, to 1 in (6- to 40-mm) graded stone,
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1
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including a number of fill materials that have regional significance
such as coral, slag, cinders, crushed shells, and crushed stone. Note:
The size range and resulting high voids ratio of class I material make
it suitable for use to dewater trenches during pipe installation. This
permeable characteristic dictates that its use be limited to locations
where pipe support will not be lost by migration of fine-grained nat-
ural material from the trench walls and bottom, or migration of other
embedment materials into the class I material. When such migration
is possible, the material’s minimum size range should be reduced to
finer than inch (6 mm) and the gradation properly designed to limit
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4
the size of the voids.
Class II comprises coarse sands and gravels with maximum particle size
of 1 in (40 mm), including variously graded sands and gravels contain-
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2
ing small percentages of fines, generally granular and noncohesive, either
wet or dry. Soil types GW, GP, SW, and SP are included in this class.
Sands and gravels that are clean or borderline between clean and
with fines should be included. Coarse-grained soils with less than