Drainage problems in elevated section of garden

[Jonathon]

Hi all

My backyard is about 15m by 15m, but the last 5 metres, along the back fence, is elevated about 2m higher than the rest of the property, with a retaining wall. This top section is grass and is about the same level as the land behind my property. However it's also the highest section of my land, with the overall block slopiong from back to front by about 4 metres.

This top section drains very badly, so whenever we get very heavy rain it pools on the surface and runs off into my neighbour’s property, causing problems.

As there are no pipes under the ground, I was thinking maybe I could drive holes into the ground to help the drainage – using a large drill bit to open a hole (like they do in the snow) then hammering a long metal stake into the ground to make a hole about 1.5-2m deep, in about 30-40 spots?

Are there commercial tools to do this? Should I be speaking with a plumber or a gardener?

Thanks
Jonathon

 

[AlmostBob]

a one inch drill, lengths of one inch conduit, and flyscreen
drilled a row of holes along the base of the retaining wall, capped the inward end of the conduit with screen to keep the dirt in, and hammered the pipe into the holes, there was a lot(really really) of water behind mine, even when it didnt rain, there had been no drainage built in when the DIYer DIhimself the retaining wall
merely opening the top wont provide a place for the water to go, I think, but would be great to aim the water at lower drains once there are any
5m*15m=75sqm == 75L/mm of rainfall, = a lot of water to drain

Mike Holmes( on homes) would dig out behind the retaining wall drop in screened ag-pipe and backfill, I didnt go that far,

 

[vaughan]

Try the cheap options first: add some soil to the top to give it a slight slope so the water will drain itself off.

 

[Jonathon]

Thanks - I think drilling holes in the retaining wall is cheaper than buying soil, but thanks for the suggestion.

 

[knightm]

Depends what the wall is made of too.

Drilling may work depending on what kind of soil is behind it. If it was solid clay for examply then the water will still pool and it wont drain well, which is why people put blue metal behind their walls when building from scratch. Its sandy you should be right.

 

[Jonathon]

The soil is clay-based. What's the blue metal and what does it do?

 

[knightm]

Blue metal = gravel. Generally dark blue/grey in colour, used base under railway track, in roads, and drainage. Anything from 5-20 mm is generally acceptable for residential drainage. I just built a 1 m high 26 metre long wall and put 12 tonne behind it with ag line at the bottom draining to stormwater drain. Several days after rain I can still go down to the street and see water being taken out of the hill above my house and deposited into the gutter, thus avoiding flooding of my house, which is on a slab cut into that hill. Very satisfying.

 

[Zeddy]

Can you describe what kind of retaining wall it is? Does it have sleepers or is it solid? Is it cantilevering with a toe at the front, or does it wrap around underneath to use the soil weight? There's a lot of unknowns here.

As it's 2 m high, I am surmising that it was engineered. It should already have subsoil drainage. Even if it didn't, I wouldn't start drilling it. You don't know where the reinforcement is. Last thing you want is to expose the reo to water, or break one.

My first thought would be to regrade the soil to slope away from the boundary and install an informal spoon or v-drain at the top of the retaining wall. Channel it away somewhere from there. This has the side benefit of making your slope more stable, but that might be just me being geeky as the chance of slip failure is probably low anyway provided it was engineered correctly.

This also means that if something goes wrong, you have done nothing to damage the wall and can still ask questions of the designer why the wall isn't performing. Drilling holes into the wall can be said to have possible effects on the strength of the structure.

I would not grade the soil to flow directly over the top, as you are likely to get mud and crap flowing over the face of your retaining wall.

 

[Zeddy]

Blue metal = gravel. Generally dark blue/grey in colour, used base under railway track, in roads, and drainage. Anything from 5-20 mm is generally acceptable for residential drainage. I just built a 1 m high 26 metre long wall and put 12 tonne behind it with ag line at the bottom draining to stormwater drain. Several days after rain I can still go down to the street and see water being taken out of the hill above my house and deposited into the gutter, thus avoiding flooding of my house, which is on a slab cut into that hill. Very satisfying.

This is good. Also worth mentioning is wrapping the ag drain with non woven geotextile fabric to prevent clogging of the drain in the long term.

However given the wall is already built, and it's two metres high x 15 m long in clay, I don't think it's very viable for him to start installing an ag drain by himself.

I should also point out to anyone doing this kind of stuff as a DYI, getting in a 2 m deep hole is very very dangerous.

 

[knightm]

There's a lot of unknowns here.

true

Last thing you want is to expose the reo to water, or break one.

true

My first thought would be to regrade the soil to slope away from the boundary and install an informal spoon or v-drain at the top of the retaining wall. Channel it away somewhere from there. This has the side benefit of making your slope more stable, but that might be just me being geeky as the chance of slip failure is probably low anyway provided it was engineered correctly.

that could be a good and easy option, a shallow trench across the surfact to the lower side of the block with a spoon drain or ag line in it that can bring water down at 1 point rather than many points in the wall. Less risk of compromising the wall and probably better visually.

 

[knightm]

This is good. Also worth mentioning is wrapping the ag drain with non woven geotextile fabric to prevent clogging of the drain in the long term.

Funny thing is my dad (retired professor of hydrogeology) shuns the cloth as a potential clogger after a few yrs when small bacteria and algae etc grow in it. I didn't use any in my wall, most people do.

However given the wall is already built, and it's two metres high x 15 m long in clay, I don't think it's very viable for him to start installing an ag drain by himself.

I should also point out to anyone doing this kind of stuff as a DYI, getting in a 2 m deep hole is very very dangerous.

Yep 2 metres is a lot of digging. Particularly in small space with walls seeping water I wouldn't want to be in there without a spotter with a shovel!!!

 

[knightm]

...might be just me being geeky...

I just reviewed my posts and I don't think its just you Zeddy! Love getting my drainage geek on

 

[Zeddy]

Funny thing is my dad (retired professor of hydrogeology) shuns the cloth as a potential clogger after a few yrs when small bacteria and algae etc grow in it. I didn't use any in my wall, most people do.

I can't really comment on that. All I know is that the geofabric is meant to be designed for it. I understand that bidim has a good reputation in this regard:

http://www.geofabrics.com.au/detailproduct.asp?id=76&pid=35

Yep 2 metres is a lot of digging. Particularly in small space with walls seeping water I wouldn't want to be in there without a spotter with a shovel!!!

I wouldn't get in there at all! One plumber I spoke to on site used to work with his son. His son was bent over putting some lagging on a pipe at the base of a chest high trench when it collapsed.

By the time they dug him out, he was dead.

 

[knightm]

I wouldn't get in there at all! One plumber I spoke to on site used to work with his son. His son was bent over putting some lagging on a pipe at the base of a chest high trench when it collapsed.

By the time they dug him out, he was dead.

Thats terrible!

 

[Zeddy]

Yeah, imagine having to explain that one to the missus...

 

[Jonathon]

Can you describe what kind of retaining wall it is?

I'm afraid I don't know the engineering detail, it was there when I bought the place. It's got a bluestone exterior and lt looks like it's got a lot of concrete behind it, but without digging up the soil I can't tell for sure. While it's 15m from side to side, that's made up of 2 retaining walls of about 5m each and steps in the midddle. I've attached a pic. The water is only pooling on the right-hand half, which is very slightly lower than the left side.

My first thought would be to regrade the soil to slope away from the boundary and install an informal spoon or v-drain at the top of the retaining wall. Channel it away somewhere from there.

Sounds like an ag-line might be the way to go. I reckon I could do it myself, but if I was to get in somebody to do it properly should it be a plumber or gardener/landscaper?

 

[AlmostBob]

Geez no, dont drill that, mine was sleepers leaning back about 75degrees into the slope

 

[vaughan]

That looks very new, and expensive: might be covered by insurance.

 

[Jonathon]

Interesting thought about insurance. It's a rental, I'll check my landlord insurance policy.

 

[vbplease]

I can tell you now no property insurance covers faults from faulty building practice. Recourse should be through the builder who should cover his work through his own ins. for 6yrs or so. My guess is that it's a proprietary design by the block company, they specify the drainage details - the builder has had his own idea or ignored the specs altogether
Nice backyard by the way