Replacing a 70-Foot Timber Retaining Wall with Allan Block in Delta
A leaning, insect-damaged timber wall on a slope in North Delta, rebuilt in Allan Block. What the job involved and why the homeowners moved away from wood.
This past summer we replaced a 70-foot pressure-treated timber retaining wall on a sloped lot in North Delta. The wall was about nine years old. It was still standing, but sections were leaning and the wood had insect damage. Here's what the job involved.
Timber or block?
We priced the job both ways. Timber costs less up front, and on some properties it's the right call. On a steep slope, with a wall that was already failing after nine years, the homeowners looked at what they'd taken out and decided against building with wood again. They went with Allan Block Classic for the full length.
Getting the old wall out
The old timbers and the soil behind them had to come out before anything new could go in, and all of it was hauled off site. We moved the new wall back toward the house by about six inches, and the extra soil went with it.
We ran a mini-excavator from the lawn side. Near a row of evergreens growing close to the wall we switched to hand digging, so we could work around the roots.
Base, drainage and reinforcement
Most of a retaining wall's strength is in the parts you can't see afterward.
A compacted crushed stone base under the first course.
Clear drain rock behind the wall for its full height.
Perforated drain pipe at the base. The old outlet pipe was broken, so the new one daylights down the slope.
Geogrid reinforcement, laid back into the soil between courses and tying the wall into the slope.
If water can't get out from behind a wall, it pushes on the wall. Most of the failed walls we see failed because of water, and block doesn't change that. The drainage still has to be done right.
Permits
The wall stayed under 4 feet of exposed height, and for this job no building permit was required in Delta. Rules vary by city and by site. Taller walls, and walls carrying a load such as a driveway or a building, can need an engineer and a permit, so check before you assume.
Finishing the job
An aluminum fence ran along the top of the wall. It came off before the work started and went back on afterward, with the posts core-grouted into the new block. The last step was topsoil and seed along the strip of lawn we'd disturbed, then cleanup.
Thinking about replacing a wall?
If your timber wall is leaning, soft or showing insect damage, it's worth having someone look at it before it gets worse. We'll walk it with you, tell you honestly whether it needs replacing, and give you a written, itemized quote for the options that make sense.
