Do I need dead men in a 4ft lumber retaining wall that has 6x6x8 vertical beams holding the wall back?

Question

The wall will be 4 feet tall and consist of horizontal treated 2x8s that are held back with vertical 6x6s that will be burried 4 feet and have concrete holding them in. Some call the 6x6s 'vertical dead men' but will that be sufficient? The lenght of the wall will be about 80 feet.

Answer 1

I would

80' of wall is pretty long and any movement will stick out like a sore thumb.Along with dead-men I would also use geogrid to control movement.

Good Luck

Answer 2

What you are suggesting for a retaining wall is adequate.  Rather than dead men, which are difficult to tie into this type of retaining wall, I would add blind men instead.  

Blind men are simply 2 x 4's spaced about 3 - 4 feet on center in a vertical direction on the back side of the wall.  Your exposed 2 x 8's are screwed to the blind men to help prevent the 2 x 8's from moving as freely and creating unsightly gaps on the face of the wall.  To get the maximum strength from the blind men, be sure to fasten them on the 1 1/2" face rather than the 3 1/2" face.

Two other  things I would do to ensure the effects of hydraulic pressure do not over whelm your wall are;

1) install a series of weep holes about 2 inches in diameter and bout 3 - 4 inches above the lower side finish grade.  I would place two holes spaced equally between each 6x6 post

2) in order to direct any water runoff I would have a crushed stone siphon about 8 inches thick behind the retaining wall with a layer of geo-tech material against the retaining wall to prevent silt stains on the face.

Water, regardless of the source, rain fall run off, irrigation, or children playing with water is going to seek the path of least resistance.  That path exists as a super highway directly behind your retaining wall where the soils meet the wood.  

Most folks think it is all about the weight of the fill behind the retaining walls, when hydraulic pressure is actually the bigger force to be reckoned with as it exists in a horizontal plane where soil weight is mainly in a vertical direction.

EDIT for your additional info: screws are far superior, if you can stand the expense, purchase stainless steel or epoxy coated ( not powder coated ),,, for 2x materials, 3 inches is the min. length, 4 inches would be better but also harder to find.  Predrilling the 2x8's only takes a few seconds and will prevent the snapping off or torquing off the heads

On a 2x8 I would use 3 fastners per connection point ( at 6x6's or blind men )

Answer 3

The 6x6's should be more than sufficient, depending on the spacing and the fasteners.  I'd use 16' 2x8's, stagger the joints, and space the posts every 8'.  Attach them with 16d galvanized ring-shank nails.  Hope this helps.