Retaining Wall advice

v.walia9

Hello, I am fairly new to this forum and received great help & suggestions to my first post. So, I'll summarise my plan here:

I've got a lovely blank canvas to start with, so I'm planning to tackle our sloping back garden which is 10m long and 6m wide. I want to build a 2ft high sleeper retaining wall in the middle of the garden behind the manhole as shown in the picture. The idea is to create a flat upper level, it'll be turfed & planning to keep the shed in the top right corner. Below are the steps I think I'll follow, please share your thoughts, opinions, issues or anything I'm missing:

1) Dig a trench for 200mm x 100m sleepers. The bottom sleeper will be halfway underneath the ground.

2) Lay the bottom sleeper course on compacted gravel base. Should I put a rebar through or lay them on concrete mix for fixing them in?

3) The rest of the courses to be laid with staggered joints (brickwork style) and screwed with stainless steel screws.

4) Line the inside of the retaining wall with membrane and backfill with gravel to avoid soil contact and help drainage. Would that be enough or do I need to install french drain pipe as well?

5) To prevent the brick wall on the left and fence on the right from damp/rot, I'm thinking of building a u-shaped retaining wall keeping it flush with the gabion at the back. I plan to leave a gap of about 100mm between the brick wall/fence and retaining wall and put some gravel in that gap to make it look tidy. Is it a good of doing it? Open to suggestions on this.

I've attached a couple of pictures - 1st of the land as it is now and 2nd a computerised image (created kindly by someone in this forum) of how I visualise it to be. Please note computerised image doesn't show the u-shaped retaining wall yet as it's something I only thought of last night.

Please share your views on this and any issues you think I need to be aware of. Thank you for your time and invaluable advice & apologies for the long post.

nick615

1 -  As this layout is presumably intended to last 'for ever', I'd instinctively avoid timber retention for the upper lawn as it's bound to decay at some point.  Stone effect Bradstone or similar would be my choice as a 'once and for all'.

2 -  I'm not familiar with the concept of a 4 inch 'U' shaped barrier between shed and wooden fence but, in the course of a year or so, or less, dust, leaves etc. will fly around in the wind.  Leaves will break up.  Dust will accumulate so, if there's anywhere where minute pieces can find a home where they're in contact with the fence and difficult to clear out, even if only washing down into the gravel you mention with rain, you face a risk.  Sorry to be a Job's comforter but better forewarned?

Nanny Beach

We used geniune railway sleepers in our last house, helishly expensive and heavy, but they wont degrade.

owd potter

Hiya.
We've discussed this project before.
OK, so you are going to build a 6m x 0.60m high sleeper wall half way up the slope, presumably with timbers laid on flat to match the wall at the front?
Couple of things to check before you finalise your plans.
Lift the manhole cover and see which way the drains run, you may need to bridge them with a lintol?
That is a lovely looking wall, is it yours and what is behind it? The soil at lower level is filled up against the brickwork, is there any reason you cannot do the same at higher level?
The fence is going to be an issue, always is when trying to build up to a wooden fence. Again, who's is it? 
Possibly you could site steps on the right side adjacent the fence or make a turf ramp so that you avoid soil or detritus collecting against the fence ?
As for the sleeper wall, I would construct a shallow concrete strip foundation beneath the wall, gravel will not provide sufficient stability to build upon. Depending on where you want to site steps, you may need to also construct a pier at the mid point for strength. Putting the steps in the centre will serve this purpose but then there is the fence to consider. You will also need to consider ways to fix and stabilise the sleeper wall at it's ends. This is also a function of where you site steps and whether you can utilise the brick wall.
Line the inside of the sleeper wall and backfill with granular material. Depends on volume of water anticipated but for what it's worth personally I would also install a drain routed to a soakaway as previously said.
I'm not really following your intention with a 'U shaped retaining wall'? Can you draw a quick sketch of your idea and I'll add any comments accordingly. 

v.walia9

Nanny Beach said:

We used geniune railway sleepers in our last house, helishly expensive and heavy, but they wont degrade.

Weren't they creosote treated? I've read that those treated with creosote are not good for plants etc.

v.walia9

owd potter said:

Hiya.
We've discussed this project before.
OK, so you are going to build a 6m x 0.60m high sleeper wall half way up the slope, presumably with timbers laid on flat to match the wall at the front?
Couple of things to check before you finalise your plans.
Lift the manhole cover and see which way the drains run, you may need to bridge them with a lintol?
That is a lovely looking wall, is it yours and what is behind it? The soil at lower level is filled up against the brickwork, is there any reason you cannot do the same at higher level?
The fence is going to be an issue, always is when trying to build up to a wooden fence. Again, who's is it? 
Possibly you could site steps on the right side adjacent the fence or make a turf ramp so that you avoid soil or detritus collecting against the fence ?
As for the sleeper wall, I would construct a shallow concrete strip foundation beneath the wall, gravel will not provide sufficient stability to build upon. Depending on where you want to site steps, you may need to also construct a pier at the mid point for strength. Putting the steps in the centre will serve this purpose but then there is the fence to consider. You will also need to consider ways to fix and stabilise the sleeper wall at it's ends. This is also a function of where you site steps and whether you can utilise the brick wall.
Line the inside of the sleeper wall and backfill with granular material. Depends on volume of water anticipated but for what it's worth personally I would also install a drain routed to a soakaway as previously said.
I'm not really following your intention with a 'U shaped retaining wall'? Can you draw a quick sketch of your idea and I'll add any comments accordingly. 

Hello, yes we did and I've now cemented the plan to an extent and just want to make sure there are no major issues with the plan. Mainly want to make sure that the walls on either side don't suffer because of the raised soil level on the 2nd level.

Yes I'm looking to use 200x100mm sleepers on their flat side. The brick wall is mine I guess as it's semi-detached house? The other side have some grass immediately and then car park for other flats. I have thought about putting the soil next to brickwork on the upper level as well but concerned that it may result in damp overtime?
I'm not sure about the fence ownership, would have to read the paperwork. I've been thinking different ways of saving the fence including building steps on the right or sloping raised beds on the right. Just want to keep steps in the middle for consistency with the wall at the front.
I've attached an image of what I mean by the retaining wall in U-shape. So, it'll be from the brick wall corner to fence corner leaving a 100mm gap to avoid raised soil contact with wall or fence. The u-shaped wall will create the 90 degree bend as well for strength. What do you think?

Blue Onion

A crescent shape would look nice, maybe half an ellipse?  More of an ovular crescent, to give a reasonable space for level seating area.  

owd potter

Right, I see what you are saying with your U shaped retaining wall. It would be fine if you want to do that but seems a bit of unnecessary work and cost all round. How are you planning to build this?
I would utilise the wall to build to and anchor the end of the sleeper wall to it with wall straps. These are galv steel L shaped straps about 600mm long used to mechanically fix wall plates to wall tops to tie house roofs down, get them at a builders merchant for a couple of quid each.
Prevention of damp penetration is a valid point, I would paint the wall below ground level with a bitumastic damp proofing paint and then fill up to the wall with granular fill as behind the sleeper wall.
The fence is another story that I don't have an easy fix for. The best most reliable fix is to install gravel boards and fill to those, but doesn't look an option here unless you want to replace half the fence so maybe go with your retainer at that side, but preventing all manner of crap from building up in the void and rotting the fence is a headache. Maybe consider building right upto the fence line and not leaving your 100mm void which may reduce the chance of build up and would give you a tidier line to work to when the inevitable time comes to replace the fence.

v.walia9

Blue Onion said:

A crescent shape would look nice, maybe half an ellipse?  More of an ovular crescent, to give a reasonable space for level seating area.  

I like that idea, but to achieve that the sleepers would need to be put in vertically to make the curve, which can be a bit tricky on the sloping ground..

v.walia9

owd potter said:

Right, I see what you are saying with your U shaped retaining wall. It would be fine if you want to do that but seems a bit of unnecessary work and cost all round. How are you planning to build this?
I would utilise the wall to build to and anchor the end of the sleeper wall to it with wall straps. These are galv steel L shaped straps about 600mm long used to mechanically fix wall plates to wall tops to tie house roofs down, get them at a builders merchant for a couple of quid each.
Prevention of damp penetration is a valid point, I would paint the wall below ground level with a bitumastic damp proofing paint and then fill up to the wall with granular fill as behind the sleeper wall.
The fence is another story that I don't have an easy fix for. The best most reliable fix is to install gravel boards and fill to those, but doesn't look an option here unless you want to replace half the fence so maybe go with your retainer at that side, but preventing all manner of crap from building up in the void and rotting the fence is a headache. Maybe consider building right upto the fence line and not leaving your 100mm void which may reduce the chance of build up and would give you a tidier line to work to when the inevitable time comes to replace the fence.

I'm quite hesitant with the u-shaped wall as well for it's extra work & money. But just thinking of ways to save the fence. I like the idea of painting the brick wall with damp proof paint. When you say fill with granular fill as retaining wall, do you mean fill the left side along the brick wall as well? Would I then need to secure a membrane to hold the fill in place? How much thickness would you recommend for the fill?
I got a couple of quotes to build the retaining wall (just the horizontal one not u-shaped), steps and turfing the upper section around the £3k mark including labour & material. It's a lot of money, so I want to do it myself, hence carefully planning things in advance.

Would these straps be good for anchoring to brick wall?
https://www.wickes.co.uk/Wickes-Galvanised-Restraint-Strap---600mm/p/246012
Ya fence is tricky, just concerned if your proposed L-shaped retaining wall (sleepers in the middle and along the fence on the right) would look aesthetically pleasing!! If I go with steps on the right, would they be enough to cover the raised soil along the fence? The current steps go 1200mm deep for 600mm high wall. 

Apologies for throwing so many questions at you. Just want to make sure I cover my base before putting the shovel in the ground.. Thanks

Nollie

Hi, I have noted some thoughts on your plans below, hope this helps...

1) Dig a trench for 200mm x 100m sleepers. The bottom sleeper will be halfway underneath the ground.

2) Lay the bottom sleeper course on compacted gravel base. Should I put a rebar through or lay them on concrete mix for fixing them in?

- personally, I would raise the compacted gravel bed up to ground height rather than half sinking the bottom sleeper, to prevent it rotting from below. It doesn’t need a concrete base, in fact that would move and crack as wood will expand when wet but concrete doesn’t.

3) The rest of the courses to be laid with staggered joints (brickwork style) and screwed with stainless steel screws.

- fine

4) Line the inside of the retaining wall with membrane and backfill with gravel to avoid soil contact and help drainage. Would that be enough or do I need to install french drain pipe as well?

- fine, I don’t think a french drain is necessary but if you are worried a line of seep holes drilled through the bottom sleeper, would be sufficient. 

5) To prevent the brick wall on the left and fence on the right from damp/rot, I'm thinking of building a u-shaped retaining wall keeping it flush with the gabion at the back. I plan to leave a gap of about 100mm between the brick wall/fence and retaining wall and put some gravel in that gap to make it look tidy. Is it a good of doing it? Open to suggestions on this.

- it is usual to leave a 100mm channel between a raised soil level and a building to avoid breaching the dpc of the building, so your instincts are right. But, if the wall is just a garden wall, with no structure attached the other side, I think Owd Potter’s solution of bitumen paint or such is sufficient. If it’s not your wall, but your neighbours, you may want to explain what you are doing to protect their wall.

The fence is more vulnerable to rot and also pressure from the weight of raised soil against it, so yes needs protecting. Again, gravel boards cemented in, in front of the fence should be sufficient, if you can get them high enough. If not, you could cement in a line of paving slabs to achieve the separation, or just build a short wall in front to contain the soil of the new raised level.