Found this on the LASCO support site. It even has part of the ASTM specifications in it. ------------------------------------------------ Thrust Blocking The question occasionally rises, "Do solvent cemented systems need to be Thrust Blocked?" To answer this question we must first understand what thrust blocking is. Then we need to understand the differences in design between Solvent Joined, and Gasket (O-ring) systems. The force of water within piping can be great enough to cause separation of certain types of joints. Thrust blocking is the procedure of placing a solid backing (such as concrete) behind a joint in order to hold the joint together. The joint is held in place once the pipe, joint and concrete is buried in the ground. In a solvent cement system, the joined parts are cemented together. This means they are fused and/or bonded together into a self-restraining entity. The same happens when steel pipe and fittings are arc welded together. The American Society for Testing and Materials (ASTM) socket dimensions used in Schedule 40 and Schedule 80 fittings provide in excess of twice the socket depth necessary to constrain the thrust forces generated by the system pressures. Therefore, thrust blocking is not required on a solvent cement joint. In a Gasket or O-ring joint system the parts are not cemented or fused together. They remain separate entities. An O-ring is required to halt seepage of water from the joint. Because they remain separate entities the only restraining forces that are available are friction and burial. However, neither of these forces is capable of counter balancing the force created by the internal hydraulic pressure of the system. It is for this reason that thrust blocking is required on a Gasket or O-ring system joint. Some references to this subject are noted in the ASTM Standards* (D 2774 & F690). ASTM D 2774 - Standard Practice for Underground Installation of Thermoplastic Pressure Piping. 5.3.1 Thrust transmitting joints such as heat fused or solvent cemented, should be capable of restraining maximum anticipated pipe pull-out forces which may be generated by internal pressure or pipe expansion/contraction, or both. Gasket and other non-thrust transmitting joints should be restrained by means of a properly engineered external restraints or joint restraint devices (see 7.3) 7.3 Thrust Restraint-- When installation piping systems with joints which cannot transmit the anticipated maximum longitudinal thrust, thrust restraint may be necessary at certain points in the system, such as changes in direction, or terminal ends, to prevent joint disengagement. ASTM F 690 - Standard Practice for Underground Installation of Thermoplastic Pressure Piping Irrigation Systems 6.Thrust Blocking 6.1 When installing piping systems that include joints that are self restraining (for example, elastomeric seal type) thrust blocking may be necessary at certain points in the system, such as changes in direction, in order to prevent possible disengagement of the fitting from the pipe.