Soakaway location on a slope

[Adam2]

Am building (demo + rebuild) and planning site for soakaway crates. Garden is on a fair old slope and I was wondering if possibility to use the soakaways to help level up the bottom part of the garden - wanted to test my idea here before going to BC (independent). If relevant, f is 0.000016 and run-off will be ~440m² 

 

The existing garden is tiered and heavily overgrown aside from the lower area where there are 2 flatish areas of lawn (well pine needle covered moss) where the lower level is 1m down with an existing retaiing wall. I'm wondering if I could do the following:

 

1. On lower garden level, install a retaining wall - using gabions about 1m high

2. Remove the top ~800mm of soil for the area required from lower level

3. Put down base (if required) for crates

4. Install the drainage connection  into the crates for later connection into the drainage coming down from house

5. Install membrane around creates 

6. backfill with gravel etc around crates to fill in gap down to the new retaining wall (probably including some rubble from demolition)

7. replace the soil from (2) over/around the crates/rubble etc raising the finished level to match upper level - probably looking at 300mm of topsoil

 

Crates would be surrounded by between 2 & 3m of soil/rubble

Maybe would need a waterproof membrane inside the gabion retaining wall to help ensure water does not through them 

 

Couple of images one looking up from lower level and w looking down onto lower level.

 

Suggestions on improving etc appreciated

[Jeremy Harris]

Probably OK.  Wrap the crates in terram before covering, so they don't end up filled with backwashed sediment.

 

What's supporting that lower area, in terms of a retaining structure?  Might be an issue with the increased surcharge from the increased height of the lower area imposing an unacceptably high load on whatever is retaining it.  Not hard to do the calculations to see if it's OK, though.

[Adam2]

13 minutes ago, JSHarris said:

What's supporting that lower area, in terms of a retaining structure?

Same style of retaining wall as per the upper one in the first image. The furthest part of the lower area retaining wall (so the bit above the "W" in pic 2) is ~3m from the end of our garden so we could build up the gabions from the other side of the lower area and then not reply on the existing retaining wall supporting the lower area. Will have to see if we can get our demolition cost reduced if we use this much rubble!

 

This will all be about 30m from the house and involves a drop of about 8m - would we need some form of intermediate attenuation to avoid the high speed water delivery into the crates?

[Jeremy Harris]

The crates should work fine to attenuate the flow.

 

If the surcharge load from the crates and backfilled area is kept a distance back from the edge of whatever is supporting that lower area then there probably won't be a problem.  There's a critical angle back from the base  that determines this, and IIRC it's buried in the depths of Eurocode 7 somewhere (I have a copy of that on another PC).

[epsilonGreedy]

2 hours ago, JSHarris said:

Probably OK.  Wrap the crates in terram before covering, so they don't end up filled with backwashed sediment.

 

 

When I looked into this I was told terram is the wrong material and some more specialized uni directional permeable membrane was the correct stuff. Have you read anything to this effect?

[Construction Channel]

6 minutes ago, epsilonGreedy said:

 

When I looked into this I was told terram is the wrong material and some more specialized uni directional permeable membrane was the correct stuff. Have you read anything to this effect?

 

Technically you're supposed to use the correct white fabric stuff now a days. But in fairness it's only there to try and keep the silt out and it's all down to the local bco. 

 

We have made quite a few soak aways with just large hardcore in them recently and had them passed by bc. It all depends on the type of ground around it 

[Jeremy Harris]

45 minutes ago, epsilonGreedy said:

 

When I looked into this I was told terram is the wrong material and some more specialized uni directional permeable membrane was the correct stuff. Have you read anything to this effect?

 

 

Worth going back to basics and asking the question "What is the purpose of the membrane?"

 

In both the case of terram and any other membrane used to wrap around crates it's to reduce the migration of sediment.  Terram has been used for doing this for decades, albeit in reducing the migration of fines into a sub-base to stabilise this layer.

 

The only stipulation seems to be that any geotextile used to wrap crates should be non-woven and free draining (and terram is both non-woven and free draining).  Looking at the spec for something like Draintex (one product sold for wrapping crates), it seems near-identical to terram to me, and the spec even says it can also be used for any job where terram can be used.

[epsilonGreedy]

10 minutes ago, JSHarris said:

Worth going back to basics and asking the question "What is the purpose of the membrane?"

 

 

I had previously associated terram with a ground stabilization function.

 

Reading up further in the last 30 minutes the Terram product web site has sections for Ground Stabilization and Filtration though I could not work out which of their geo textiles were designed for those functions or both.

[Jeremy Harris]

The bottom line is that any free-draining, non-woven, geotextile will work for both wrapping crates and acting as an anti-fines migration barrier under a granular sub-base.  Until the advent of geotextiles, most soakaways were just holes in the ground filled with rocks.  Some of the houses we've lived in have had soakaways like this that have been working for a long time.  In the case of the second house we bought, the soakaways were probably as old as the house, which was late 17th c.

[PeterW]

Round here it’s not unusual to see the bulk bag of gravel lifted into the whole, bag and all ..! Terram over the top and the BCO is none the wiser ...!

 

Also had the opposite though when I “found” a soakaway that was basically 150mm of 20mm gravel over a metre square with a pipe seeming to disappear into it.... shame that was it and the soakaway was basically the sand under it ..!