Buba
A total creep
Typical septic tanks are not very sturdy. And at the depicted thickness - I'd guess that they are proof against pistols and intermediate rounds and smaller artillery fragments. I'd not expect them to withstand rifle bullets.
Rain culverts - as these often are burried at shallow depth under roads - actually may be more resistant than septic tanks which are not expected to "work" but to stay "static" (I lack engineering terminology here).
Rain culverts - as these often are burried at shallow depth under roads - actually may be more resistant than septic tanks which are not expected to "work" but to stay "static" (I lack engineering terminology here).