Which of the following is NOT a method for jointing two lengths of drainage pipe?

Plastic welding is primarily used for joining thermoplastic materials, often in various manufacturing processes, but it is not a standard method specifically employed for jointing drainage pipes. In drainage systems, common jointing methods include rubber rings, which provide a flexible seal, and cement mortar, which is used in the joining of concrete pipes due to its strong bonding properties. Silver brazing, which involves joining metals by melting a filler metal at a temperature above 450°C, is also not typically employed for drainage pipes, as the materials used in drainage systems are generally designed for other jointing techniques that accommodate movement and potential ground shifts.

In the context of drainage, the primary methods reflect the need for both stability and flexibility, which is why rubber ring and cement mortar are preferred choices, while plastic welding does not align with the requirements or materials of traditional drainage pipe systems.