The Importance of Thrust Restraint

In an underground or aboveground Ductile iron pipeline, there are locations that experience what are known as “thrust forces.” These thrust forces occur when the direction or cross-sectional area of the waterway changes at any point in the piping system. These forces need to be restrained in order to avoid joint separation in the pipe.

Thrust Blocks

Thrust blocks provide resistance to thrust force by transferring the force to the soil through the large bearing area of the block. Thrust block design takes into consideration the appropriate bearing area of the block for particular conditions.

One of the most common methods of providing resistance to thrust forces is the use of thrust blocks. Resistance is provided by transferring the thrust force to the soil through the larger bearing area of the block such that the resultant pressure against the soil does not exceed the horizontal bearing strength of the soil. The design of thrust blocks consists of determining the appropriate bearing area of the block for a particular set of conditions. The parameters involved in the design include pipe size, design pressure, angle of the bend (or configuration of the fitting involved), and the horizontal bearing strength of the soil.

Restrained Joints

Balancing thrust forces in underground pipelines can also be accomplished with restrained joint systems when there is not enough space because of other utilities or structures, and the possibility of the soil behind a fitting will be disturbed.

A restrained joint is a special type of push-on or mechanical joint that is designed to provide longitudinal restraint. Restrained joint systems function in a manner similar to thrust blocks, insofar as the reaction of the entire restrained unit of piping with the soil balances the thrust forces.