Construction Concerns: Water Mains

Gregory Havel discusses water utility construction using plastic pipe, the most common material used today for new municipal and private water systems that supply both hydrants and automatic fire sprinkler systems.

Article and photos by Gregory Havel

Plastic piping for water and chemicals became available during the 1960s. Polyvinyl chloride (PVC) pipe in larger sizes for water mains became available in the 1970s and in the last 10 years has replaced other materials as the most common in construction of new water mains and systems. PVC pipe does not rust or corrode, and is expected to have a longer service life than metal or concrete pipes.

PVC pipe for water mains usually is supplied in 20-foot lengths, meets the requirements of the American Water Works Association (www.awwa.org) and Underwriters Laboratories, and is usually blue in color. (PVC pipe for sanitary sewer is usually green.) Long runs of PVC water pipe are usually connected by “push-on” joints with rubber gaskets, similar to those used with ductile iron pipe. Since these connections are not mechanically fastened, thrust blocks or fittings must be installed at bends and at the ends of the runs, so that the water pressure does not push the lengths of pipe apart. This method is similar to that used for ductile iron and pipe of other materials. Visit www.awwa.org/awwa/sourcebook for product, installation, and engineering data.

Valves, flanges, and fittings for PVC pipe are usually made of ductile iron. To simplify the connections and to provide for mechanical strength, rubber-gasketed flanges with mechanical joint restraints have been developed. These use flanges, bolts, and gripping wedges tightened by set screws whose heads break off when the proper torque has been applied.

Photo 1 shows the end of a water main under construction. From left to right are shown eight-inch PVC water pipe; an 8 x 6 x 8-inch tee with a six-inch ductile iron valve and six-inch PVC piping that will be connected to a five-inch dry-barrel hydrant; a short piece of eight-inch PVC; an eight-inch ductile iron gate valve; a short piece of PVC; and a cap. All of the valves and fittings are connected to the PVC pipe with flanges that attach with gripping wedges, set screws, and bolts. Visit www.awwa.org/awwa/sourcebook for product, installation, and engineering data. When the water main is to be extended, the short piece of pipe with the cap will be removed from the eight-inch valve, and new pipe connected, without having to shut down the water main that is already in use. Some of the snapped-off blue set-screw heads are visible in the soil by the valve.

Photo 1. Click to enlarge

Since water mains are usually buried at least five feet deep in Wisconsin for protection from frost, valve boxes must be installed on top of the valves so that they can be operated from the surface of the street. Photo 2 shows these two valves in the background with the valve boxes in place. At the bottom of each valve box is the top of the bell-shaped fitting that covers the top of the valve, shown mostly covered with crushed bedding stone. The upper section of each valve box is threaded, and can be adjusted up or down to match the finished surface of the street. In the foreground is the new hydrant that is connected to the six-inch gate valve. The band of duct tape below the hydrant’s break-away flange holds the water main’s tracer wire (for locating the plastic pipe after it is buried) in place.

Photo 2. Click to enlarge