Hi
If you look at this thread, I put a diagram on it that will explain what you want to know.
http://www.cleanitup.co.uk/smf/index.php?topic=113480.0From what I understand, driveway paving companies build and price a driveway to the owners specifications. This is especially so when it comes to concrete driveways.
Eg. if they are concreting a driveway for you for cars only, they lay a 4" thick concrete slab. If you park a transit van on it, then they lay a 6" slab of concrete. Anything bigger is thicker still and usually reinforced with wire mesh. So your customer is probably right to show concern, especially if they chose the cheaper option.
Your van will weigh 3 tonnes fully laden, so I would stay away from parking on customer's drives if at all possible IMO. My van is slightly lighter than yours, and the only time I would put it on a driveway, especially concrete is if it was almost empty.
If you keep an eye open, you sometimes come across a drive that has two sunken tracks at the enterance- either due to being poorly layed or had a too heavy vehicle driven on and off it, especially when the underground is wet after heavy rainfall.
Spruce