Question
Hello Mark,  My house is on a small city lot (37.5x100).  The house probably sits about 8 feet above the street.  Accessed by two sets of steps, at the sidewalk and then at the porch. There is a generous slope to the lawn that is very difficult to mow.  I would like to install a low retaining wall at the sidewalk. I'm interested in using a "Trex" decking type of wood to weave in and out of posts set in the ground.  Is that a good application for that material? Expensive?  Do you have a better idea?  I am concerned with durability and costs.  Must be a wood type product, no stone or brick.  Thank-you for your time and patience.  SLK

Answer
Hello,

Use heavy railroad tie type timbers,at least 8" X 8".
Do not use "trex" and do not use any wood in the way you described (weaved between posts). This design will not work for a retaining wall.
Also,do not use landscape timbers.  They are too light and often not suitably treated for retaining wall use. Trex decking is not suitably treated for retaining wall use.  Creosote treated or railroad ties treated with ACQ (Alkaline Copper Quaternary) are suitable for the ground contact of retaining walls.  ACQ treated wood requires galvanized or stainless steel fasteners and should not come in contact with aluminum.  It will eat ordinary steel fasteners quickly and will corrode aluminum.

You can lay the ties horizontally or vertically. Support the bottom on a concrete footing if possible. Tie the pieces together with steel rods.  Drill a hole slightly smaller than the rod through the ties and insert the rod through the entire course of ties.  Install gravel behind the wall of a minimum 2 feet width.  You could extend the gravel to the top of the wall or cap the top of the wall with concrete or stone.  Tilt the cap back away from the wall or toward and over the wall or make it concave for the purpose of draining water away from the inside face of the wall.  If you do not want to use metal rods to connect the ties,connect the ties with 12 inch long spikes and then use treated planks across the length or height of the ties (depending on how they are arranged). The planking can be placed on the inside of the wall if you do not want them visible or you may use planking on both sides.  Or,you may use planks alone to connect the ties.

Do not let the angle of the soil behind the retaining wall exceed 30 degrees.

Tilt the ties back at an angle of about 10 to 20 degrees. This,along with tie backs,gives the wall greater holding power. The tie back can be individual ties layed perpendicular to the wall ties and extending back several feet.  The ties must go behind the gravel to solid,undisturbed earth.  Or,the tie backs can be steel cable run through the face of the wall ties and attached to a timber or concrete anchor beyond the gravel fill area. Use one tie back for every 16 square foot of wall.  The weight of the soil will bear on the timber tie backs and so give the wall resistance to overturning. This works on the same principle as a cantilevered retaining wall,where the weight of the soil behind the wall rests on a shelf attached to the wall.

Bury the bottom of the wall ties into the earth in front of the slope as much as possible,as this will add to the holding power.



You also might want to consider plastic ties and plastic coated ties.

A 6 inch diameter drain at the bottom of the wall would be desirable.  You must keep as much water as possible away from a wooden retaining wall because even treated wood will rot prematurely if kept constantly wet.

Drill weep holes in the face of the wall to allow water to escape from behind the wall.  

Good luck. Write back if you have further questions.

Mark Harshman
http://www.mahdrafting.com
Email: [email protected]

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