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joint restraint & thrust blocking

Water moving through a pipeline has significant force that tends to separate the joints at changes in flow direction (bends and tees), stops (plugs, caps, or closed valves), and changes in size (reducers). Something must be done to keep the joints from separating and to maintain the equilibrium of the pipeline. Three of the primary ways to do so are:

Mechanically  – megalug – used on both plastic and ductile iron pipe – done at all ductile iron fitting locations.

Bell Restraints – A tie-rod restraint has to be somehow protected against weakening by corrosion, so the equilibrium of the pipe is not jeopardized. This may be done with specially coated bolts, stainless steel bolts, or coating the tie-rod system with a tar or epoxy product.

Restraint is often required at the nearby spigot/bell joint within a certain distance of the 'thrust vulnerable' location. This thrust calculation/joint requirement is calculated by the engineer using factors such as pipe size, water mains intended working pressure, and type of thrust vulnerability (i.e. bend degree, vertical bends vs. horizontal bends, tees, end-of-line caps, etc) 

Restraint directly at the mechanical fitting joint

Also additional joint restraint is often required by engineers within a certain calculated distance of changes in direction (fittings) which with ductile iron pipe is often accomplished with a gripping/locking gasket mechanism.  This will resist joint pullout at the change in flow direction (tees, bends, etc), especially during pressure surges.  Gripping/locking gaskets, used in lieu of the standard gasket, grip the 'homed' pipe with metal teeth which dig into the iron pipe.  They are also commonly used in directional drilling and auger/casing bores as well. Gripping/locking gaskets are typically available in sizes from 4-30” diameter.

Intermittent hydrostatic pressure thrusts can occur with water mains. Thrust blocks are used in vulnerable locations (tees, bends, caps, etc) to resist joints pulling out. They are often used in addition to other means of joint restraint and are commonly poured concrete (formed or loose) but they may be precast blocks as well.  They must be sufficient in resisting the hydrostatic force based on the pipeline characteristics, type of soil, and bearing strength. Thrust blocks should be placed on undisturbed soil as forces in the pipeline are passed through the thrust block to the soil. Thrust blocks may be hand formed or earth formed.

Depending on pipe diameter, designed internal pressure, depth of cover, type of soil, trench construction type, fitting type, an engineer will determine how many feet a joint must be restrained in either direction to appropriately resist forces over a set period of time.

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