Page 25 - Buried Pipe Design

Introduction and Overview 3

Soil has been used as a construction material throughout history. It is
used for roads, embankments, dams, and so forth. In the case of sewers,
culverts, tunnels, and other underground conduits, soil is important, not
only as a material upon which the structure rests, but also as a support
and load-transfer material. The enveloping soil transfers surface and
gravity loads to, from, and around the structure. Much has been written
about soil mechanics and soil structure interaction. Such variables as
soil type, soil density, moisture content, and depth of the installation are
commonly considered. If finite element analysis is used, many soil char-
acteristics are required as input to the mathematical soil model. These
soil properties are usually determined from triaxial shear tests.
Standards organizations such as the American Association of State
Highway and Transportation Officials (AASHTO) and the American
Society for Testing and Materials (ASTM) issue standard test methods
for classifying soil and for the determination of various soil properties.
Of the various methods of soil classification, the Unified Soil
Classification System (USCS) is most commonly used in the construc-
tion industry. Complete details on this system can be found in any
textbook or manual on soils engineering. 1
Soils vary in physical and chemical structure, but can be separated
into five broad groups:

Gravel. Individual grains vary from 0.08 to 3 in (2 to 75 mm) in
diameter and are generally rounded in appearance.
Sand. Small rock and mineral fragments are smaller than 0.08 in
(2 mm) in diameter.
Silt. Fine grains appear soft and floury.
Clay. This very fine-textured soil forms hard lumps when dry and is
sticky to slick when wet.
Organic. This is peat.

Soils are sometimes classified into categories according to the abil-
ity of the soil(s) to enhance the structural performance of pipe when
installed in the particular soil. One such classification is described in
ASTM D 2321. 2
The project engineer often requires a soil survey along the route of
a proposed pipeline. Information from the survey helps to determine
the necessary trench configuration and to decide whether an imported
soil will be required to be placed around the pipe. Soil parameters such
as soil type, soil density, and moisture content are usually considered
in a design. Soil stiffness (modulus) is an extremely important soil
property and is the main contributor to the pipe-soil system perfor-
mance. Experience has shown that a high soil density will ensure high