Water design often gets a bad rap for being more difficult than gravity pipe design because of the range of different fittings that are needed to complete a water system. Water fittings play a significant role in the overall constructability of your plans. We’re here to highlight a few constructability concepts that are important to keep in mind.
When water flow within a pipe changes direction such as around bends or in tees, the water creates a force called a water hammer against the pipe. Thrust blocking, made of poured-in-place concrete, counteracts this force and prevents the water main from shifting as a result of the water hammer. Thrust blocking must be placed on undisturbed earth and once placed, should not be disturbed by construction activities.
In some scenarios it is necessary to excavate behind existing blocking or place a tee in a location where blocking on disturbed earth is necessary. To secure the main from shifting in these situations we use a variety of techniques including temporary water mains, restrained joint pipe or connection sequencing.
2. Restrained Joint Pipe
Restraining is another method to secure the water main pipe from shifting. The most common way to restrain straight pipe runs is to switch out the gaskets that come from the pipe supplier in the bell end of the pipe with locking gaskets. These special gaskets have metal teeth embedded in them that grip on to the pipe like barbs, preventing the pipe from pulling apart.
To restrain pipe through fittings (tees, crosses, valves), restraining glands are used on a mechanical joint end type. Similarly to the locking gaskets, these glands have teeth that significantly increase the amount of force that can be applied without consequences, so much so that a bulldozer can be hung beneath a pipe that uses these restraining glands!
In order to secure water main using restraining so that thrust blocking is not required, we use a calculator provided by EBAA Iron, Inc. that shows how much restraining would be required on either side of your selected fitting to counteract the thrust force.
3. Temporary Water Mains
Temporary water mains are used during construction to aid in connection sequencing when service cannot be maintained because thrust blocking will be disturbed and therefore the water main cannot be kept at operating pressure. These temporary mains are constructed of PVC or HDPE pipe at smaller sizes than the permanent main. They are usually run along one side of the road in the gutter line but can also be shallow buried if they need to serve both side of the road. From the temporary main, service jumps are made, much like your typical water service in a permanent scenario. The existing water service is disconnected at the meter and the temporary service pipe is connected in its place.
4. Flanged Fittings
Most people are familiar with flange fittings, and they seem pretty simple on the surface. But if you are not keeping constructability in mind when using these fittings, they can create quite an issue in the field. A standard water main connection usually includes two 45-degree bends (or two bends of another angle), but if you flange these bends to a fitting that needs to be plumb, such as a valve, you are limited in the amount that you can shift the bends. Connections made with two bends are not always exactly aligned with the angle of the bends, so they are rolled in the field to have some upwards or downwards angle, reducing the horizontal angle of the bend. But if the flanged bend has a plumb fitting on it, you are limited to set rotation degrees of your bend based on how the bolts on the face of the flange are spaced.