Just finished a retaining wall and will be putting about 12 - 14 " of
soil on top of an area 10' x 20'. currently filled with grass.

Question ??? Can I just put weed block down and dump 14" of
soil on top of the existing grass. Will this kill the grass
completely. Or should I use Round-Up or a herbicide first to kill
the grass.

I'd like to finish the job quickly, and hate to wait for the round-up
to work... if the grass dies under the load of soil that's good
enough for me.

Your advice???

Chuckle With 12"-14" of soil going over top of it, I sincerely doubt you
have much to worry about unless if you haven't cut the lawn yet this year.
On the off chance that there's something else a little more aggressive
growing in there, cut it short, cover the area with 5-10 pages of newspaper
(only the regular section, not the glossy) then fill it in -- this will add
to the soil as well.

FWIW, as you fill this area in you may want to add organic material (i.e.,
compost) in with the new topsoil. This will help build healthy soil for
planting purposes. What will you be planting over the area? If it's a
vegetable garden now would be an excellent time to prep the perfect planting
soil.

James