Sub-base and existing drain pipes - How to deal with existing buried pipes

[Alpaca]

Had a search on the forum but couldn't find the answer to this.

We're about to lay a gravelled area along the edge of the house, where I know there are quite a few buried PVC pipes (both for foul water and rainwater). I'm pretty sure the pipes are not buried hugely deeply and are not encased in concrete. Besides being a bit worried that they are not protected enough from heavy loads on the surface, I also don't want to squash the pipes when I compact the sub-base. Am I worrying too much, or should I be trying to put a concrete layer around them? If so, what's the best order to tackle things? Dig out the whole sub-base first, or try to find and just dig around all the pipes first so that I've got trenches to pour concrete into.

[jwill]

How deep are they below where top of sublease will finish?

[Tony McC]

What's the purpose of the planned sub-base? Are you eventually going to be running vehicles over the buried pipes?