In a saturated soil, why is the pore water pressure equal to hydrostatic pressure?
Terzaghi's principle states that the effective stress between solid particles is equal to the total stress $t$ minus the pore pressure $u$. $u$ is equal to the specific gravity of water $r$ times the depth of saturation $d$: $u = rd$. This is the exact same pressure that depth of water would have given no surrounding soil (i.e. it is equal to hydrostatic pressure).
So in total, the weight carried by the solid particles is whatever the water doesn't carry, and the water carries it's "natural" weight.
Why is this? What about water makes it so that it can only carry its natural weight. Or framed another way why can't the solid particles carry some of the water weight.