I moved into a house in June and since that time have notice a water
puddle that forms in the middle of my basement on occasion. I have
tried to find a trend with when the water forms and cannot find any.
The water has formed in the summer, the fall, and now the winter
(despite the frozen ground in Minnesota).

My next step was to look for internal sources creating this water.
After trying every appliance, drain, facet, and bathroom, I cannot
find the source of the leak. I even placed a pan on the puddle and
found that the water is forming from below the concrete (the pan was
dry but under the pan was not).

After dealing with this for several months I finally decided to take
action. Having some knowledge of the workings of the house my
assumption was that the soil pipe from the kitchen and washer/dryer
which cuts beneth the floor to the main sewer return on the other side
of the house must be leaking. I scored the concrete floor and created
a 3 by 4 foot rectangular hole in the concrete floor. I expected to
see moisture in the soil at the very least and was quite shocked when
this was not the case. I tried digging down about 8 inches to find
the pipe and was unsuccessful. I hit clay and figured that I must be
deep enough.

So my questions are as follows:
1) How deep do they burying the soil pipes beneath the concrete of a
house? Minnesota house built in 1949? (More detail: I am about 6 or
7 feet from the main return line and about 15 feet from the kitchen
return line and washer/dryer line)

2) I am thinking that my best course of action is to find that pipe
and determine if this is the source of the water, should I be trying
or thinking of something else?

3) Is there anything else that I am missing?