Oop! That's 12 inches deep! Not 12 feet!!!

I wouldn't remove the top soil. If it's already pretty good, I'd just add mel's mix as needed to top off the beds.

If you want to kill off the weeds, you should try solarizing the plots. It works best during the height of summer. Wet down the weeds thoroughly, then cover the two beds with clear plastic, making sure the all edges are sealed against the ground. The sun should turn it into a sauna, cooking everything underneath, killing the weeds and any weed seeds, as well as some soil pathogens. It should be done in 6 weeks, depending on how hot and sunny it is.

It works great on tough weeds.

Here is a link that might be useful: How to solarize soil

If you need vermiculite Mc Connkey has the best prices.

Here is a link that might be useful: Mc Connkey

I would take out maybe the top 3 inches and fill it back up with pure compost, if the soil that's there looks good already, and skip the burlap sack step altogether.

Have you dug down in the beds and seen how deep the good soil goes? How is the soil under that, do you think the beds were double dug in the past? Pacific NorthWest tends to have glacial hard pan, my old gardening in Bellingham, WA took a lot of work for the 1st year.

Also, I think solarization will solve your problem and you won't have to waste any of the existing good soil.

How thick are the blackberries in the beds? They can be a real pain to reduce and get rid of.

CloneZero

Don't make this any more complicated than you have to. Simple options are:

1. Spray with Roundup, then turn the dead stuff under in a couple weeks.

or

2. Dig out all the weeds and grass. No sense discarding the top soil. No need to make that fancy "Mel's Mix" if the soil is decent.

If you have a good spade and garden fork digging out the weeds will probably take a few hours but will allow you to start using the beds this year. Once the weeds are out cover the soil with an inch or two of compost and add a quarter gallon of Steve Solomens's organic fertilizer per bed (4 parts cotton seed meal,1/2 part lime, 1/2 part bone meal, 1/2 part kelp meal). The lime helps combat the acidity of pacific NW soils and free up nutrients while the rest provides a great slow release fertilizer for your plants. Mix the compost and fertilizer into the top 3 inches of the soil and then water the beds well. Once that's done cover the beds with cheap clear plastic from home depot and secure the plastic around the edges with spare lumber, bricks or whatever you have to hand. Within a couple of weeks the soil will have warmed up and many of the weed seeds will have germinated. Remove the plastic and slice through the stems of the weed seedlings using a colinear or sickle hoe. This will kill them. Water the beds again and replace the plastic. After a couple more weeks you'll be able to kill off 90% of the remaining weed seedlings and plant your first plants into nice warm, rich soil. Good luck and let us know how it goes.

SH

Wow! What great advice. Thanks everyone.
So here's my plan:

1. I'll go get a bunch of clear plastic sheeting, and enough Mel Mix to top off 2-3 inches on each bed.

2. I'll dig out the current weeds, add the Mel Mix and some Steve Solomens's organic fertilizer (is this a brand, or what I need to mix myself?) to the top of the beds. My good soil goes down about 6 inches, but I suspect that the clay content is high. It's so wet that it's hard to tell at this point.

3. I'll wet the soil a little, and then cover with plastic, killing the germinating weeds as needed.

4. In about 6 weeks I should be able to start planting.

How does that sound?