family of aggregate piers (geopiers/vibropiers/impact piers/etc...) are tested by plate load tests, normally a 36" plate load test is used. Please note that a 10 ft long pier will show you the same result as 20 ft long pier because the size of plate is too small. Therefore, make sure that getechnical engineer is involved throughout the design and construction process and hire a contractor who won't cut corners to get the project.