I just visited a project of mine today and as you can see we encountered a large piece of concrete and rebar debris a few feet down from the surface that prevented the soundwall post from reaching its proper depth. As a result we had to overexcavate the hole with an excavator and backfill the area before resuming the excavation of soundwall post. Encountering utilities or buried debris is not uncommon. Had this been rock or debris much deeper it would have resulted in a change order and quickly increase construction costs. The unknown below the surface is also one reason good footing designs are overdesigned to ensure minimal settlement.
Be sure to review your projects soil borings or hire a geotechnical engineering firm to perfom this analysis. Or you can contact me to find you one.