What is under a (sand) desert?

Question

I mean, a desert is basically just a huge amount of sand but where does it stop, what is beneath? I googled it and it says there’s groundwater but what does that mean, and (how) is that even possible?

Edit: So, actually the main question I wanted to know is if there’s a exact border between sand and rock, or if the sand just gets denser and denser until it’s rock (but I believe the weight must not be that big), like Saturn or something.

(Yes, I know what a desert is but I specifically need to know about this certain type of desert)

In: 1165

Answer 1

From geotechnical persepct: the soil or sand void ratio(voids for liquid to get into) to solid particulates in a volume), along with porosity, is directly related to ground water flow. Water pressure is proportional with height, more water column, more water pressure.

So all that boils down to, deoending on how the sand layers are, and other comments and a google search can tell you. The diff layers of sand, soil, dirt, rocks, clay, all can contain water. Ground usually sits on clay, and stones. All together, there can be hundredsnof meters of soils and rock layers that can allow for water ‘flow’. In sand its like wet sand at the beach, water can flow through due to porosity, and that is done through sand that evebtually can get compacted by the flow, and we get stuff lile ‘quick’ sand, or like low bearing pressure sand. There are tons of engineering courses on groundwater flow, geotechnics, etc. I did this as part of my civil enginerring, but i am a structurak engineer atm. So ground water flow, is for geotechnic guys. Thank god, as ‘getting’ geotechnical engineering isnt easy, often cited as harderst units for eng degrees are the geotechnics. I failed one out of three units, but i improved immensly the second time around which is great as i do retaining walls and sometimes dams too. But think of it like a system, not like individual layers, as ‘layers’ often intersect, there are disctinct lines and levels of soils and rocks. But water flows through them differently due to their physical composition and porosity.