Page 185 - Trenchless Technology Piping Installation and Inspection

150 Cha pte r F o u r

of sanitary sewers. The earliest recorded use of clay pipe in the
United States was in Washington, D.C., in 1815. Clay pipe is not
used in pressure applications because of its inherently low tensile
strength.
Only 50 years ago, the sanitary sewer engineering community
followed the philosophy that leakage of wastewater through pipe
joints was an acceptable methodology for effectively transporting
suspended solids and reducing excessive flows within a sewer sys-
tem. Clay pipes were therefore designed with a low emphasis on the
effectiveness of their joints. This philosophy soon changed as engineers
realized the hazards posed by wastewater leakage to soils and
groundwater sources. The EPA’s role in reducing infiltration or inflow
(I/I) with the passing of several congressional legislations such as the
Water Pollution Control Act of 1972 and the Clean Water Act of 1977
were major factors in emphasis shifting to the requirement of water-
tight joints in sewer pipes. All factory-applied clay pipe jointing sys-
tems, whether on bell-and-spigot pipe or plain-end pipe, are designed
to provide resilience and flexibility to accommodate minor pipe
movement. All compression type jointing methods meet the require-
ments of ASTM C425, which requires that the joints should not leak.
With the proper installation, a clay pipe sewer system can meet stan-
dard infiltration or exfiltration requirements. Furthermore, an inde-
pendent study done by the University of Houston demonstrated that
the joints of vitrified clay pipe exceeded the industry standards. In
the arena of trenchless construction, clay pipe’s ability to withstand
high compressive loads and external abrasion has resulted in a sig-
nificant rise in its acceptance and use in pipe-jacking and microtun-
neling applications.

Manufacturing
Vitrified clay pipe is made of selected clay and shale that are aged to
various degrees, and blended in specified combinations. Large crush-
ing wheels grind the clay in a heavy perforated metal pan until the
finely ground clay passes through the perforations. The ground raw
materials are mixed with water in a pug mill. The mixture is then
forced through a vacuum, deairing chamber until a smooth, dense
mixture forms. The mixture is extruded under extremely high pres-
sures to form the pipe. After drying, the newly formed pipe is placed
in kilns and heated to temperatures of approximately 2000°F. The fin-
ished pipe then undergoes a QA/QC testing.

Applicable Standards
A relatively high minimum compressive strength of 7000 psi makes
clay pipe a good contender for jacking and microtunneling installa-
tion. In 1994, ASTM C1208 opened new doors for clay pipe in the
trenchless construction arena (see Table 4.8).