From HeatSpring.com PV Boot Camp
Using Ironridge's ground mount Design Assistant, an array tilt angle of 35° (close to local latitude of 38°) the minimum hole depth is 7.5' but with tilt angle of 15°, the hole depth is just 5'.
Is it cost effective to dig a deeper hole, using more concrete in able to use latitude as the tilt angle?
That is a question with many answers and some of them do not have to do with the cost of the concrete.
Many people tilt at an angle less than optimal, so that they have less inter-row shading and can fit more in a smaller space. Also lower tilt angles work better in the summer when electricity is often more valuable. Another thing is that some people like the aesthetics of a lower slope.
With a steeper tilt angle, there are more wind forces at work on the array and concrete is required. The question here is how much does the deeper hole filled with concrete cost vs. the extra kWh produced from the PV? How efficient is your PV? If the PV was expensive, it makes more sense throwing money into a hole (concrete) in order to squeeze all you could out of that PV. If the PV is less expansive, like it is now compared to 10 years ago, then the lower tilt angle starts to make sense.
Personally, I would probably look at how much space I had and wanted to use. If I want my array compact, I would look more into the lower tilt angle.
I am at latitude 38 myself right now in the San Francisco Bay Area. So is Athens Greece. We should all get together on the 38th day of the year.
Or perhaps you should use a human pile driver and forget the concrete, like this: