That was weird!
We were just doing our job here in the building and it was like the scene from the first Jurassic Park when the T-Rex is walking up. The ground vibrated and there was a loud booming sound, and it was much like foot steps with a short interval in between booms. It got me curious enough to go look out the front door. There was a sim-trailer trying to back up into a business down the street. The rear bumper was dragging on the concrete driveway from the steep angle. The rear wheels of the trailer were too far forward. The driver got out, and released the rear wheels. He got back in the truck and pulled forward and the wheels stayed right where they were but the trailer moved ahead. When he got the rear wheels to the back of the trailer he got back out and flipped the lock and then backed up the rig and there was clearance since the trailer overhang was minimal.
The rear bumper must be STRONG and STOUT and well built to make that much racket scraping over concrete and not bend backwards. THe concrete driveway is well built as well or it is damaged now.
It is astonishing how loud and booming that sound was.