Yes, the geotechnical engineer should have a say in the program by which he will be making professional recommendations. If a client won't pay for a proper investigation, he won't get reasonable recommendations. However, the structural or civil engineer should be capable of giving initial direction to the geotech in many situations.
I've been out of geotech for a while working in civil. In every case where I have subbed out a geotech, I have given the geotech the depth and number of soil borings. If there are unforeseen soil conditions, I would expect a call during the exploration explaining the condition and providing recommendations for further exploration. If my program is way off, I would expect the geotech to laugh at me and tell me what we need to do. The geotech as a professional has the obligation to inform me if my program is inadequate, just as I have the obligation to inform a client if his design requirements are inadequate.
If the project is a dam or a high rise building, then yes, the geotech should be involved in the project from the beginning. If you have a low rise building and a parking lot, there is no reason to involve a geotech in the conceptual phase of the project. I would expect any structural or civil engineer to have a basic understanding of a typical project and to have knowledge of a reasonable exploration program.
You have a good point with the liability issue, fattdad.
Without other information, I would also use 25 feet deep. The borings should generally be taken to a depth of 5 to 10 feet below the depth of interest. For one boring, the cost difference is minimal between 20 and 25 feet. For a hundred borings, I would do most of them at 20 feet and just a couple at 25 or 30 feet if I expected the strata to be similar.
