For many years, thrust blocks have been successfully used in distribution systems all over the world. Thrust blocks, however, are not without limitations. There is an argument that thrust blocks are the cheapest form of pipeline restraint. When all of the costs such as pouring, labor, and forming time are added to the price of thrust blocks, joint restraint systems can be as effective and less costly.
EBAA Iron will be exhibiting its joint restraint systems at ACE12, including the new Series 1900 PVC Restraint Systems engineered to accommodate the increased pressure ratings for C900 and C909 PVC pipe.
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 to resist the resultant thrust force at a change in direction or change of diameter.
Thrust block design is commonly determined by the equation Ab=Sf*T/Sb. Where Ab is the required bearing area of the thrust block, Sf is the safety factor, T is the resultant thrust force, and Sb is the bearing strength of the soil. As an example, for a 90 degree bend in a 12-inch ductile iron pipeline at 150 psi in a soil with a bearing strength of 2000 psf and a safety factor of 1.5:1 the required thrust block bearing area is almost 22 square feet roughly 6.6 feet by 3.3 feet.
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. When complicated bend combinations, vertical down-bends, parallel lines, dead ends, and future excavation possibilities become involved, the use of thrust blocks becomes very problematic.
Joint Restraints
