Knowing what a thrust block is and what it does is critical to anyone in the water industry. A thrust block prevents separation of joints and pipe movement by transferring the resultant thrust force at a bend to the undisturbed soil behind the thrust block. The bearing strength of the soil is expressed in pounds per square foot. Therefore, the area behind the thrust block must engage enough soil area to resist the resultant thrust force at a change in direction.
A properly designed thrust block involves much more than dumping a load of concrete behind a bend. The design involves consideration of undisturbed soil, soil-bearing strength, test pressure, pipe size, fitting configuration and trench depth to determine the bearing area of the thrust block. Then, an installer must form up and pour the proper block. Care must be taken to prevent the concrete from covering the joints at fittings, the weep holes in hydrants and operating mechanisms of valves.
Thrust blocking should be used on all gasket joint pipe and fittings since there are great thrusts developed whenever there are changes in direction in the pipeline, dead ends, pipe sizes and valves. These thrusts can blow the joints apart, creating major problems.
The thrust block should be placed against undisturbed, fully compacted earth. It should have sufficient area on the soil side to restrain the thrust. The fitting should be placed over a large enough area to keep stresses on the fitting at a minimum. It should be designed knowing the intensity of the thrusts developed in the pipe and the load-bearing strength of the soil. Using the following tables, you can calculate the size of the thrust block for various pipe sizes.
Example: 4” pipe with a maximum pressure of 100psi at a 90 degree elbow in clay soil. Thrust developed = 1,800 lbs and load bearing strength of clay = 500 lbs.
1,800lbs
500 lbs. =3.6 square feet thrust block
Thrust blocking is important to any drinking water or wastewater piping system under pressure. By understanding reasons for thrust blocking and the math behind the concrete and helps you as collection system and distribution system operators to understand the importance of these necessary additions to the system.
Operation Matters 
