Joint Restraints Offer Alternative to Thrust Blocks
A properly designed, restrained pipeline essentially turns the pipeline itself into a thrust block, using the bearing strength and frictional resistance of the soil to counter the thrust force. Parameters used to determine the amount of pipe that must be restrained to resist thrust forces underground are pipe size, pipe type, test pressure, fitting type, trench type, depth of bury, soil type, and safety factor.
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.
|PVC restraint systems are engineered to accommodate the increased pressure ratings for C900 and C909 PVC pipe.|
A properly designed, restrained pipeline essentially turns the pipeline itself, into a thrust block, using the bearing strength and frictional resistance of the soil to counter the thrust force. The same basic parameters required to determine the size of the thrust block are used to determine the amount of pipe that must be restrained to resist thrust forces underground. These parameters are pipe size, pipe type, test pressure, fitting type, trench type, depth of bury, soil type, and safety factor. With this information and the various design equations, it is possible to calculate the length of piping that must be restrained.
Continuing with the previous 90 degree bend example but using restrained length design instead of thrust blocks, a comparable 12-inch ductile iron pipeline with a 3 foot depth of bury in a silty-sand with a type 5 trench compaction the restrained length requirement is 28 feet. That is two restraints at the fitting and one restraint on the first line joint back on each leg of the fitting for a total of four restraints.
Thrust blocks are commonly used with push-on fittings. This type of fitting is difficult to install, and when pipe is cut in the field extra time and effort are required to bevel the end of the pipe to enable insertion of the spigot into the fitting bell. This requires powered equipment or special rigging tools.
|Mechanical joint assemblies are adaptable to changes in fabrication and can be diassembled to make adjustments|
On the other hand, the mechanical joint is very adaptable to changes in fabrication in the field. Pipe cut in the field is easily inserted into the MJ bell and the joint is made by tightening some bolts. Disassembling a mechanical joint to make changes or adjustments is simple and straight forward.
Very little extra effort is required to restrain a mechanical joint fitting after the t-bolts are tightened. EBAA mechanical joint restraint products will be on display at ACE12. They use accepted assembly procedures to seal the joint, then the simple tightening of wedges to restrain the joint. This requires no special tooling or rigging and is performed by one man with hand or power tools. Complex fitting configurations are fabricated outside of the trench and lowered into place for final connections.
Once the line is restrained and buried, the line is ready for testing. This hastens the construction process and prevents the need for trenches to remain open for long periods of time. This is not possible with unrestrained joints that require thrust blocks.
Can you really eliminate thrust blocks using joint restraint? Based on years of experience, the answer is "yes." The use of the EBAA Iron mechanical joint restraint products and proven design procedures allows for reliable installations that effectively eliminate the need for thrust blocks.
"EBAA® is an exhibitor at the ACE® '12 event and can be found at Booth No. 3402"