Installing Your Trampoline at Ground Level
Measure the size of your trampoline. Most rectangular trampolines run 10 feet to 14 feet and round usually start at 12 feet. The depth of the hole should be 3 to 4 feet deep, depending on the height of your trampoline. In most cases you want the trampoline to stand an inch or 2 above the ground, allowing for air flow, which will produce better bouncing. The width of the hole should match the size of your trampoline and the masonry blocks that will act as your side walls to prevent the dirt from caving in over time. Add an additional inch for mobility.
Dig your hole with a mini loader, backhoe or shovel.
Install a perforated drainage pipe (HDPE) that draws rainwater away from the hole and disperses it throughout the length of the pipe. Install the pipe in the center of the circular foundation, to minimize standing water and puddling. It is also best if the outlet of the pipe drains where there can be no chance of flooding in case of a large rainfall. A standard storm drain ditch would work well.
Build the masonry wall. Spread mortar in between the bricks to ensure that they stay in place. Let dry four to six hours.
Set up the trampoline frame inside the hole. Add the mat to the frame by attaching it with the springs. Place safety pad over springs and tightly secure it so that it does not move around a lot.