Septic tanks in 18" ground water and running sand

I have 2, 2000 gallon holding tanks to install in 18" ground water and running sand. To top off house is existing along with trees and garage so not lots of room. Was thinking trench box 20x20 with steel plates on each end. Sounds good but rental guy says no. 20' spreader not available so I priced new pipe 3k he still saying not good idea I will get the box stuck. I have a cat 320 & 315 to work with. I'm thinking of buying 4 used steel plates and bolt the corners together should weigh about...

Septic tanks in 18" ground water and running sand

Septic tanks in 18" ground water and running sand