Mikepier wrote in
:

.

The dug hole can be 4" larger than the sump. Install non-woven
geotextile around the walls and bottom of the hole as well. Make it
tight and smooth, use large nails to hold the material in place while
you install the sump. Center sump and backfill space between the sump
and hole walls with coarse sand. Install the sand in layers of 4" to
6" and stop to pack it fairly tightly - don't pound hard, just enough
to make sure all lumps are broken up and the sand moves into all
gaps. So your layers will be; dirt wall of hole, geotextile, 2" of
sand, geotextile, wall of sump. You can see that if hole is only 4"
larger diameter than the sump there is only 2" to work sand into. You
have to work very accurately which is fine if your soil is sticky
enough to stand up by itself from when you first dig the hole untill
you get the sand backfill done. A larger hole makes it easier to work
but is not needed for the hydraulics.

If the sump does not have holes all the way to the bottom then drill
more holes lower down - a lot of 1/2" holes in two rings around the
bottom will give enough capacity. The rings should be 2 or 3 inches
above each other so the sump wall is not weakened

I've always heard to put 3/4 stone around the sump. What is the
advantage with sand?


The objective is too prevent fine soil particles from moving at all, not
into the sand/gravel layer and not into the sump itself. Gravel has
larger pore spaces and thus the geotextile could get pushed into the
voids - the cloth may tear in the future. Sand is easier to pack
tightly.

Gravel will interlock and create large pore spaces. You never put gravel
into tight spaces, it is for laying on surface layers. In addition;
clean gravel will interlock worse and what is called well-graded gravel
will
have a lot of small particles which fill the void spaces and greatly
reduce hydraulic conductivity.

If you are retricted for space it may be better to use an 18" diameter
sump than to place washer/dryer on top of the sump. You need to inspect
the
sump often to ensure that the float is not obstructed from moving up and
down. The appliances are heavy and have water or vent connections that
must be removed and replaced every time the appliance is moved. In my
experience, maintanance that is difficult is not going to happen.