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Water Main Joint Restraints
By: First Supply
Water main joints are susceptible to movement, and ultimately failure, when impacted by a sudden thrust of water pressure. Problems start when the velocity and weight of water cause fittings to pull away from water main pipes during a fire flow or similar event. To prevent pipe joint separation, concrete restraints called thrust blocks can be placed up against the underground pipe fittings. A thrust block disperses the force that hits a joint and transfers it to the surrounding soil. When provided with enough space, a thrust block can effectively resist thrust forces at a bend or end of the main.
Mechanical joint restraints (also known as retainer glands) have become popular for water main installations. Mechanical restraints for ductile iron and cast iron pipe restraints are split for easy installation. These restraints were originally developed for use with cast iron and ductile iron water mains. Mechanical joint restraints for cast and ductile iron pipe where promoted as a cheaper, labor saving option than concrete thrust blocks.
When PVC was introduced for water main piping, manufacturers had to develop a compatible restraint system. Current recommendations for awwa c900 PVC pipe include using serrated restraint rings combined with thrust blocks. This two-prong approach of using serrated rings with thrust blocks avoids placing extra stress on the PVC where it contacts wedge restraints. If thrust forces are too high, contact points will develop cracks that will eventually cause piping failure. This method prevents cracks to maintain system integrity.
Adding Thrust Blocks to Mechanical Joint Restraints
When installing piping for a fully restrained installation, mechanical joint restraints should be combined with thrust blocks. A combined restraint system will provide more protection when underground pipes are impacted by water hammer or thrust forces. Mechanical restraints and thrust blocks still allow for slight movement before the teeth of the restraint engage for a mechanical joint restraint or the soil compresses with a concrete thrust block. For safety, their thrust restraint threshold must be calculated based on the potential thrust force for a given water main.
Mechanical Joint Restraints
Mechanical joint restraints use a flange (also known as a gland) and a gasket to make a watertight seal. The gland incorporates set screws and lug-style clamping devices, designed for restraining, which are serrated to grip the outside diameter (OD) of the pipe to prevent it from separating from a ductile iron mechanical joint bell restraint.
Mechanical Joint Restraint Configurations
Internal pressure exerted on pipes is known as thrust force. The joints alone my not provide enough resistance to the force created by water pressure on the fitting. To prevent failure, most joints need to be restrained in some manner. Mechanical joint restraints rely on the additional restraint and stability provided by the friction resistance and the density of the soil around the pipe. This makes mechanical joint restraints ineffective for aboveground applications. Additionally, if the surrounding ground is too wet or unstable, too much movement will occur.
Using mechanical joint restraints instead of concrete thrust blocks can save money by reducing the cost of labor and materials. Mechanical joint restraints are a good choice is situations where thrust forces will be greater than the holding strength of the set screws and lugs during a fire flow. To be safe, joints should always be specified for the worst-case conditions.
Mechanical joints with set screws and slip-on flanges are sealed with a rubber type gasket that’s compressed between the spigot end and the pipe and the adapter flange. This type of joint won’t resist the thrust force of water at a change in direction. Calculations are necessary to determine the full potential force based on the rating of the water line and the offset or the number of joints being restrained.
Thrust blocks, external friction clamps with tie-rods and mechanical joint restraining glands are the most common methods of water main restraints. Restraining glands shouldn’t be used unassisted unless an engineer has calculated the maximum forces and soil samples have been analyzed to determine stability.
Friction clamps can be bolted around the pipe on both sides of mechanical joints when thrust is evaluated and determined to be a concern. Friction clamps are utilized with steel tension rods (tie-rods) across the joint, resulting in clamped/rodded joint combination that prevents separation.
Thrust Block Design
When designing or installing a thrust block, care should be taken to prevent the concrete form covering the joints at the fittings. Factors such as soil bearing strength, anticipated thrust pressure, pipe size, fitting configuration, and trench depth to determine the bearing area of the thrust block should be considered. When designing or installing a thrust block, care should be taken to prevent the concrete from covering the joints at the fittings, the weep holes of the hydrants, and the operating mechanisms of valves. The concrete must dry before the pipe is charged with water and tested.
When water mains aren’t designed or installed properly, joint separations can occur. Some joint failures have been on water mains after they enter a building where a mechanical joint restraint is used inside a building. These types of joints are not recommended for unrestrained, aboveground applications where excessive water hammer forces can cause joint failures and extensive water damage.
On PVC pipe, mechanical joint restraints can be used as the pipe enters a building connecting to main building shut-off valve. Extreme thrust forces my cause failures in these cases. Using additional thrust blocking, anchors or restraints to resist the pull-out of the transition joint from PVC to ductile iron or other material within the building.
When a force beyond the capability of the mechanical joint restraint is exerted, it will disengage leaving stress cracks inside the pipe. Interior joints, without additional thrust restraints have been known to fail and when they do, it is usually on the utility side of the building isolation valve. This usually exposes the mechanical room or the entire basement to flooding.
In situations where concrete thrust blocks were not used, water main separation happens underground. In these cases, proper soil bedding backfill and bolt adjustment on the underside of the water mains are difficult to accomplish in poor weather conditions. Adding thrust blocks provides the extra security necessary to prevent joint failure.
When mechanical joint restraints are used without concrete thrust blocks, there is a potential for water main failure if the installation is not done to specifications or if the maximum thrust restraint pull-out forces exceed the design forces used for the specifications.
For best results engineers should be required to perform hydraulic thrust force calculations to determine what the potential thrust forces can be for the water mains with fire flows and ensure that the thrust forces are within the limits of the mechanical joint restraint system specified. The installation also requires torquing the set screws to the specified level. A field engineer or construction inspector should witness or perform torque tests as part of the installation.
Added Security for Water Main Installations
Costly failures can be avoided by using a concrete thrust block placed behind an elbow or tee in a piping system. This method is a reliable way to prevent thrust forces from moving a water main or causing pipe joints to separate.