Page 93 - Buried Pipe Design

Design of Gravity Flow Pipes 69

12 percent but more than 5 percent fines are neglected in ASTM D
2487 and the USCS and should be included. The gradation of class
II material influences its density and pipe-support strength when
loosely placed. The gradation of class II material may be critical to
the pipe support and stability of the foundation and embedment if
the material is imported and is not native to the trench excavation.
A gradation other than well-graded, such as uniformly graded or
gap-graded, may permit loss of support by migration into void
spaces of a finer-grained natural material from the trench wall and
bottom.
Class III comprises fine sand and clayey (clay-filled) gravels, includ-
ing fine sands, sand-clay mixtures, and gravel-clay mixtures. Soil
types GM, GC, SM, and SC are included in this class.
Class IV comprises silt, silty clays, and clays, including inorganic
clays and silts of low to high plasticity and liquid limits. Soil types
MH, ML, CH, and CL are included in this class. Note: Caution should
be used in the design and selection of the degree and method for com-
paction for class IV soils because of the difficulty in properly control-
ling the moisture content under field conditions. Some class IV soils
with medium to high plasticity and liquid limits greater than 50 per-
cent (CH, MH, CH-MH) exhibit reduced strength when wet and should
only be used for bedding, haunching, and initial backfill in arid loca-
tions where the pipe embedment will not be saturated by groundwa-
ter, rainfall, and/or exfiltration from the pipeline system. Class IV soils
with low to medium plasticity and with liquid limits lower than 50 per-
cent (CL, ML, CL-ML) also require careful consideration in design and
installation to control moisture content, but need not be restricted in
use to arid locations.
Class V includes the organic soils OL, OH, and PT as well as soils
1
containing frozen earth, debris, rocks larger than 1 in (40 mm) in
2
diameter, and other foreign materials. These materials are not recom-
mended for bedding, haunching, or initial backfill.
Soil-pipe interaction
Design of a buried conduit has a basic objective of adequate overall
performance at minimum cost. Overall performance includes not only
structural performance, but also service life. Minimum cost analysis
should consider all costs including lifetime maintenance.
Initial cost is often broken down into piping material cost and instal-
lation cost. Various pipe products have differing strengths and stiff-
ness characteristics and may require differing embedment materials
and placement techniques depending on the in situ soil and depth of
burial. Products which allow for minimum initial or installation costs