How to set drain for basement bathtub

I'm installing a bathtub in my basement. I have the drains laid out but have a few questions:

1) I have an 'access pit' setup right now to connect the tub drain to the house drain. Do I need to leave that accessible or can I fill in the whole pit (i.e. covering the j-trap and all) w/concrete? If I don't fill it, how do I make sure no ground water seeps up?
2) I need to use a studor vent for the bathroom since I can't tie into the roof vent. How high do I need to place that above the vanity drain. Can I simply put it under the vanity cabinent?

Thanks in advance for any assistance!

Code may require that you fill in the 'access pit' because of radon gas.
As far as ground water seepage, the concrete won't prevent a rising water table. The plastic sheet under the foundation (assuming you have one) is to inhibit water vapor. But rising water will come around and through the barrier and up thru the floor, regardless of what's there. If you ever have a water problem, you have to fix it with a sump or improving your land's drainage.

I know some places admit the use of a studor or other type of air admittance valve for bar sinks or islands - but I am not sure they're permissible for tubs and toilets. Check w/bldg offc, since this'll be specific to yr municipality.

How to set drain for basement

Thanks for your reply!