How to Add Drainage Behind Existing Concrete Retaining Wall

A common question I see: I have an existing concrete retaining wall that was installed without drainage. How do I add drainage to this wall to keep it from failing?

Before we proceed, note that this advice applies solely to solid concrete and cinder block walls. Segmental block walls and wood retaining walls require drainage to be built concurrently with the wall.

Evaluate Wall Stability

Before spending the time and energy adding drainage to an existing retaining wall, make sure the wall is worth saving. If the wall is leaning out or there are cracks in the concrete, these could be signs of irreversible damage. You do not want to go through all the effort of adding drainage to a wall that will just fail a short time from now.

Though there are methods to shore up a failing retaining wall, it is best to contact an engineer to evaluate these for your specific situation. If the retaining wall is structurally stable, then follow these steps to add drainage to the wall.

Coring Weep Holes

Mark locations for weep holes at a maximum of 6-ft spacing and 1″ to 3″ above the ground line. Try to avoid rebar (vertical steel reinforcement) when possible. For walls built out of cinder blocks, line the weep holes up with the grout lines to ensure rebar is avoided.

Use a core drill* to core 2″ diameter holes in the locations that you have marked. To penetrate the full thickness of the retaining wall, use a wet concrete diamond drill bit*. A core drill uses water to cool the drill bit and reduce concrete dust. Though water reduces dust, use a KN95 mask and eye protection during drilling.

Dig Drain Wells

At the top of the wall, use post hole diggers* to dig 12″ diameter holes centered at each weep hole. The holes need to extend below the level of the weep holes. Then, brush off the back of the wall and clean out the weep holes.

Cover the back of the weep holes (soil side down in the hole) with stainless steel wire mesh*. This will prevent material from spilling out of the weep holes.

Install Sand for Drainage

After the wire mesh is secured, pour concrete sand* into each hole. Concrete sand is a course sand that has great drainage properties. It also acts as a filter, meaning you can use it without needing to use filter fabric to separate it from the retained soil. Completely fill the hole with the sand and lightly compact it. Place filter fabric on top of sand to prevent organic material from seeping in.

Address Surface Drainage

Now that the groundwater can exit the wall through weep holes, it is time to address surface drainage. Reroute water away from the wall by using swales or ditches. Pipe gutters past the front of the retaining wall.

If the ground slopes toward the top of the wall, install a ditch directly behind the wall and route surface water off the wall in the quickest way possible. A spill way can be used if necessary to allow water flow over top of the wall at low points. Do not allow the ditch to drain into your drain wells as the drain wells are intended for groundwater only.

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