In the early 1900s the soil removed from the trench was placed back around the pipe by hand and carefully compacted. A “lamp test” verified alignment, but not pipe integrity and joint tightness after backfill. The dynamics of load development weren't as thoroughly understood as they are today. As a result, trench width and depth weren't designed properly, placing excessive pressure on the pipe. The four bedding classes — influenced by pipe strength, the unit weight and type of backfill, and the width of the trench at the top of the pipe — weren't developed until the late 1960s.
Today's consistent firing temperature of 2,000° F turns clay particles into ceramic, producing a new “vitrified” material with twice the three edge bearing strength of pipe manufactured a century or so ago.
Today, pipe designed for open-trench construction utilizes either a bell-and-spigot joint design with a factory-applied elastometic material or straight pipe barrels joined with rubber shear couplings and tightening bands. Similar to jacking pipe or bell-and-spigot pipe, these joints form a “leak-free” flexible compression fit when assembled. The advantages of utilizing a flexible joint on a rigid conduit have been widely demonstrated since these joints were first introduced more than 50 years ago. ASTM International C-425 states “joints shall not leak,” and according to Boschert, factory-applied joints manufactured in the last 50 to 60 years don't.
Impervious to virtually anything except hydrofluoric acid, clay requires almost no maintenance. As a result, until 1975 it was the dominant material specified for gravity sanitary sewer construction.
The 1980s ushered in the era of scheduled cleaning and maintenance. While many clay sewer systems are still in service today, others showed signs of leakage. Installed when inflow was intentional, they suffered from what's now considered poor installation and poor jointing but was standard practice when they were installed. As new materials, like PVC and high-density polyethylene (HDPE), were introduced and became accepted, clay fell out of favor.Midwest and East Coast installations
Though it's Portland's first modern installation, it's not the first in the Northwest. Seattle has been specifying vitrified clay exclusively for all new projects, largely because of its durability and performance in the region's dense glacial till.
On the other side of the country, Delaware's New Castle County initiated a program in 2004 to eliminate all combined sewer overflows from Brandywine Hundred, a cluster of villages and towns just south of the state's border with Pennsylvania, by December 2029. Four hundred miles of 8- to 54-inch clay and concrete interceptors are being upgraded with vitrified clay pipe up to 42 inches in diameter and HDPE and fiberglass-reinforced pipe for larger mains.
Though the traditional 3-foot length is still readily available and may be used in urban applications, bell-and-spigot pipe is available in 10-foot sections in diameters 21 inches and larger. “Many U.S. cities specify bell-and-spigot for their open-trench sanitary installations because they know how well it holds up,” Boschert says. “They want to put material in the ground with physical properties that do not change with age.”
Jacking pipe for trenchless installations can range in length from 2 to 10 feet depending on the jacking frame and size of the working shaft. The pipe is made with recessed ends coupled by stainless-steel collars at each joint. “It's the 21st century version of clay,” says Boschert. “Trenchless is a growing market for the clay pipe industry because the jacking pipe is cost-competitive in the short joint lengths typically associated with installation methods like slurry microtunneling and pilot tube microtunneling.”
Although both are guided trenchless methods used to install systems accurately on line and grade, there are significant differences.
Slurry microtunelling is a one-pass method that uses a tunneling head and water to grind and transport the spoil out of the bore in liquid form; the new pipe advances the cutting head, which is steered by a laser guidance system from a control station above.
Pilot tube microtunneling is a multiple-pass method that first installs a 4-inch tube that's steered from within the drive shaft by the operator; once the pilot tube is placed, a reaming head is attached followed by auger casings that transport the spoil out of the tunnel, creating the bore. Once the tunnel is cut, the new pipe is jacked into place advancing the auger casings into the reception pit.
Pilot tube microtunneling is becoming increasingly popular since its introduction in 1995 because of low equipment costs, a relatively small jobsite footprint, and the only shoring equipment required for an access pit is usually just a trench box. Vitrified clay jacking pipe has been the main material used in this method because of its high compressive strength (18,000 psi average), availability in the typical 1- or 2-meter (roughly 3- to 7-foot) lengths, and low-profile joints.
Omaha, Neb., recently finished installing 2,980 feet of 8- to 15-inch clay pipe via pilot tube microtunneling. A consent decree requires the city to reduce the annual number of combined sewer overflows from 58 to four by 2024, and the project was one of several pipeline separations where clay was specified. Other projects include 100 feet of 15-inch vitrified clay jacking pipe that was tunneled under a high-traffic road to tie into a manhole; 7,087 feet of 8- to 15-inch vitrified clay, and 18-inch vitrified clay jacking pipe in an industrial park.