What is the weight of usual dump truck?

You'd have to define "usual". Dump trucks can range from a one ton pickup chassis all the way up to a Caterpillar 797F off-road dump truck rated to haul 400 (short) tons.

Assuming you're referring to a tandem axle dump truck, of the sort used for on-road applications, 20,000 - 25,000 lbs., depending on a number of factors, such as dump body type, chassis, length, what material the dump body is made from, etc.