How to fix holes in retaining wall?

[Zog]

Hi,

My retaining wall (built long before I bought the house) has lots of holes and the soil behind is falling through and making holes in the yard.

I expect I need to use concrete or mortar but how do I fill a vertical hole?

[JacobZ]

Hi @Zog,

At present, it sounds like the water is flowing through the retaining wall, so if you were to plug up those holes, that water would need to go somewhere, or it would eventually start to push the retaining wall over. You'd want to make sure that by plugging the gaps, you aren't adversely affecting the drainage from the retaining wall.

Do you know if there is any drainage, such as a trench of drainage gravel and an agi pipe, installed behind the retaining wall?

If not, then you would need to look at the drainage before doing anything to the wall. It is likely going to be more important to address the drainage behind the wall than to plug it up, as if the drainage is not addressed, that water would sit behind the wall applying pressure to the back of it and eventually pushing things out of place if it has nowhere to go.

To address the gaps in the retaining wall, typically, a mason would use mortar as opposed to concrete. To do this, you would need to create a stable base to apply the mortar to. This could be achieved by wedging medium-sized stones between the large stones, then plugging the smaller gaps with Drainage Gravel as much as you can. Once you have a more stable base, you could apply standard mortar over the gaps to fill things in and lock everything in place.

Allow me to tag our knowledgeable members @Dave-1, @Nailbag and @AlanM52 to see what they think.

Jacob

[Dave-1]

Good Afternoon @Zog 

So I have a question to start with, 'How much soil are you comfortable with potentially coming through your wall?"

If you want no soil to come through then its a remove and redo (not really high on the list) Highest effort for this one.

If you want the larger holes to be filled, then I would remve a shovels blade with from behind that section of the wall and dig down to a bit below the hole. Then would fill the holes with slightly larger stones so they act as wedges and then fill around the second later of plug stones with chunky gravel.

This will allow water to still travel through but stop most of the soil from passing through easily. Easiest effort for this one depending on how many holly sections you have)

If you want an in between fix, then you could use a 20mm square mesh and dig down behind the rocks (in say 1.5m lengths along the wall) and then lay the mesh against the rocks as a finish filter and then back fill with scouria that is 20mm in width at least. That will stop a large amonty of soil, plus allow water to pass through. You could even use some weed mat against the messh to help stop the fines from sifting through. Medium effort for this one.

Dave

Dave

[Nailbag]

Hi @Zog 

I would first suggest the same as @JacobZ and start by filling the gaps with mortar and see how successful that is. The wall is quite short, so if still requiring more leak protection, you could then consider digging a small trench on the inside of the wall and lining the inside of the rocks with builders plastic. This would stop 100% of any seepage.

Nailbag

[AlanM52]

Hi @Zog,

I have been there and done that and when needed... go back and do the same thing again, like for the last thirty years.

I am going to show you photos of our rock walls that surrounds 3/4 of our two story house on a slab situated on a sloping block on the low side of the street - phew. Built by a stonemason something like fifty years ago the top has concrete garden edging and rocks are mortar pointed.

Utility side of the house.

  

Movement over the years causes the mortar to become loose and fall out.

There are no issues behind the wall.

  

Along the front porch, fence paling edging I'll come back to that.

  

Up the drive and down the other side.

  

Around the corner of the carport and down the side of lounge and dining.

I did that paling edging in 2022.

To reinforce the metal edging that I installed in 2015.

Because the amount of earth coming in was pushing that over.

Interesting observations I have made over the years is that most of what comes in is not from the rock wall but from above off the front yard. It's a dry version of the muck that fills the gutters and the carport drain.

Sorry for the ranting I am known for that and by now most people have left the room.

Back to your job... follow the advice here and if you don't have any 'filling' buy some bags of mixed pebbles.

I use one of those to inject the mortar into the holes.

And I usually do the tinting thing so the mortar blends in with the rocks.

Cheers

[Zog]

Thanks for those suggestions.

@JacobZ I don't know if there was any drainage installed behind the wall. There's a drain pipe below the gravel in the foreground of the photo and there's drainage in other areas of the yard so I'm inclined to think there is some drainage.

There is only 3 metres of yard between the wall and the back fence and the fence is at the top of a slope - it slopes downwards from the fence on both sides so the water tends to run off quickly and not soak in if that makes a difference. Maybe I could put some ag pipe just below the surface behind the wall to catch the water running off ?

@Dave-1 Thanks, the medium effort solution sounds good.

@Nailbag Thanks, the wall is about 14 metres long and about 80cm high.

@AlanM52 Thanks, that's some good work in your yard. My house was built in the '90s and from the look of the retaining wall, it wasn't done by a mason as the rocks don't fit together and they didn't fill the gaps - they just used enough mortar to stick the rocks together. The caulking gun suggestion is brilliant.

[EricL]

Hello @Zog 

Installing the agi-pipe just below the surface will definitely help out a bit, but it is the flow of water that we can't see that's the worry. Typically, agi-pipe drains are placed at the back bottom of the retaining wall. This catches the water running inside the soil and redirects it away from the base of the retaining wall. 

Just to be sure that there is a drain pipe installed behind your retaining wall I suggest digging a small hole with a Cyclone Post Hole Pincer. This will allow you to dig an inspection hole behind your retaining wall and see if there is a drain pipe in place. Once you have confirmed that there is an agi-pipe in place you can then proceed with the repairs to your retaining wall. If you are not keen on digging, I propose trying to track the possible outlet of the agi-pipe behind your retaining wall.

If you need further assistance, please let us know.

Eric

[Zog]

Thanks @EricL 

I'm not keen on digging a hole as there's lots of rocks underneath the top soil. I'll try to find if there's ag pipe joining the pipe under the gravel as I think that's the most likely place it would lead to. The pipe under the gravel joins the stormwater and leads down to the street gutter.

[EricL]

Hi @Zog 

That just might be the join your looking for, but it would be best to double check that it is running at the back of your retaining wall.

Eric