lumberpressure-treatedretaining-wall
I'm going to be building a retaining box for a gravel pad using 6×6's – 0.16 lb./cu. ft. MCA pressure treated lumber. The dimensions of the box matched up with available lengths at the lumber yard, so I'm not going to have to cut any of the pieces that I've ordered. Any ends that will be below grade are going to be up against clay soil.
Do I still need to dip/paint the ends with wood preservative, or will the PT chemicals alone be sufficient to prevent rot?
Unless you have a kiln to dry wood in, drying wood in a standard environment takes a really long time (if you buy firewood, usually you want ~2yr old wood!). I don't know that leaving it out for a couple weeks would dry it if it were so saturated that there is visible water coming out of it. It is recommended to let wood used for hardwoods sit a couple weeks to get accustomed to the environment as far as humidity goes, but it is not being dried any further.
I would recommend that you find a better source of wood that is dried properly. You also want to carefully select your wood, it might be that you just got a bad piece some how.
For outdoor projects, instead of PT lumber you might opt to try a wood like cedar which naturally resists the elements pretty well.
I would cut off the timber in the ground using a Sawzall type tool (reciprocating saw) that has a long rough cut blade installed. You can get blades as long as 12 inches that can easily reach into tight spots and cut the wood. Soil will wear the teeth away fairly quickly and so be prepared to replace the blade.
I've used this technique to cut roots free from stumps when removing a tree. Works very well.
Best Answer
The pressure treating commonly used today does not penetrate the lumber 100%. I'm assuming mostly financial reasons at play here (time, cost of materials, etc). I highly suggest any cut ends that you see untreated wood at the center, be treated with off the shelf, brush on preservative. You will be well served here. Why take that chance over a few extra minutes, and a relatively small amount of dollars on a presumably several hundred, to several thousand dollar investment?