Retaining walls required before garage foundation

[wertert]

Hi All

 

I'm continuing the thread I started in the 'introduce yourself' section.

I may have got carried away on the intro 😉

 

We plan to build a new garage but before we can really start we need to take care of a few retaining structures around the edges of the site.  One of them is to address this lovely retainer/planter which essentially retains the higher ground level on the other side of the stone wall.

 

it's about 1.2m out from the stone boundary wall and full of soil.  Take it all down in one go and I think we would see the wall collapse and landslip from next door.  As I mentioned in the linked post I am considering some kind of retaining structure built in sections or sheet piling.

 

1) Retaining structure.  We would take down the planter in ~650mm sections and build a foundation and retaining wall in sections, back fill before moving on to the next section.  Each section would be mechanically linked to the next.  The new sections could be made of standard concrete blocks and/or hollow blocks with reinforcement+concrete filled.  Something like..  

 

I'm favoring 4th design which would contain 2 or 3 rebar H12 starter rods. Buttresses adds further support and makes it easier to build/link together.  Madness / Overkill ?

 

2) Sheet Piling

2 options I as can think of

 

• Drive piles into the ground just in front of the retainer.  Dig out in front of the piles to install the foundation knowing the planter will not collapse. Once the foundation is in remove piles when safe to do so.
• Drive piles into the ground INSIDE the planter, so approx 1.1m above floor level and remove the planter stonework completely.  Leave the piles in place permanently to provide support.

 

I'd love to head about any other ideas from the forum.  Anything would be gratefully received. 

 

Edited April 19, 2023 by wertert
added pic of sheet piling

[saveasteading]

Nicd drawings.

I'd use number 2 with rods, and space the piers  at 1.5m or so.

You can't tell how much load might come from next door so better safe.

 

The easy way to do the rods is to do a plain footing, then drill and epoxy into it to suit block positions.  One in each pier and another half way.

Concrete fill will be a  chore but your phasing will reduce this.

 

Lapped coursings would be better. Otherwise use ties.

 

Put drain holes, preferably pipes into the face, at the first horizontal joint.

 

Hollow blocks are really rough, so either drape plants over or render, of both.

 

 

 

[wertert]

Hi Saveasteading

Thanks for getting back to me - drawings are done in sketchup which is great for this type of thing.  Web based and free version.

Did you mean piers on the outside ? or inside ?

How about outside ? ie acting as buttresses

 

So I could phase in 655mm sections. ie

Remove 655mm of retainer

Build foundation block approx 385mm depth ( 0.14 m3 )

Install bars ( epoxy )

lay blocks for section adding wall ties for next section in joints.  First joint add drainhole

Fill with concrete

Repeat.

 

I've got more piers than you recommended just to make each section identical.  Assuming this would make the wall stronger ? Bit more work.

 

We need 600mm below ground to line up with the raft foundation. I don't want to undermine this wall when we digout later for that.

 

[wertert]

I think you meant piers on the inside 😉 my bad.

[saveasteading]

It will work either way round, but inside is tidier and may need less digging. 

You could leave the top block of the buttresses off, to allow a full bed of soil and plants. Not much load is happening there.

 

655 is awfully close. Doubling that isn't going to be a risk.

 

When backfilling put gravel against the wall to aid drainage. A bit of plywood held 50mm off the wall, pour 300 deep gravel, backfill with soil to the same depth. repeat. Pull out the ply for reuse.

 

[wertert]

Hi Saveasteading.

I take your point on the 655mm.  Will be tight.  Tell you what, i'll add another block to meet you in the middle so 1095mm sections. I will have to build approx 5 sections to complete the wall

 

 

I flipped the piers to the to the other side.  I wanted to ask you about the rebar placement.  I think you mentioned one bar in the pier and one halfway ? Does the above look about right ? Is adding another one better or overkill ?

 

The design for the other retaining wall i'm building uses stepoc blocks and a much wider foundation ( unsuitable for this wall ).  Should I consider using mesh and starter bars as below instead of epoxy fixing in the rebar for the wall above.  Again, am I going into overkill mode ?

 

 

 

 

[saveasteading]

The red bar is structurally elegant but a pain to work with. They have to be exactly placed.

I would stick with epoxy and inserted bars. The pullout strength is huge. I can tell from the drawings that uou will be precise.

Mesh in the base is appropriate as the vertical bars will be trying to bend the base. One layer middle if being mean.

 

Still add 25mm drain pipes out of the wall above ground level in case your other drain fails. NB your land drain is going through the piers. Make it smaller, or settle for the front pipes.

[wertert]

Hi saveasteading & All

 

I started digging out for the first section over the weekend.  Good to get back on the tools. 

Still a lot to come out.  Skip arriving tomorrow.

 

Thinking slightly ahead, I'm aware that builders merchants stock 2 variations of the hollow block.  Standard and splitters.

 

Splitter on the far left. The design of the wall will mean I need approx 20% splitters as I don't fancy trying to cut a standard block in half.

Each section will need 15 standard blocks and 6 half blocks ( 3 splitters )

The merchants I've spoken to don't really differentiate between the 2 variations of blocks.  You basically get what's in the yard which isn't ideal. Jewson had no splitters.

Any ideas on a merchant who will supply a guaranteed ratio of splitters ? ie 2 in 8.

I'm assuming it's ok to use a splitter, unsplit in place of a standard block ? or could this create a weak spot ?

 

I aim to get enough supplies on site for the upcoming double bank holiday ( thanks Kings Charles ).  Last question would be on the concrete for the foundation and block fill.  I would normally use 10mm to dust ballast.  4 parts ballast to 1 cement.  I have a mixer here.  Any suggests on something better/stronger ?

 

So other than blocks, ballast, cement....

Rebar

Epoxy

A393 Mesh

Block/Brick ties - https://www.screwfix.com/p/sabrefix-wall-ties-50-pack/62409 ( look ok ? ) - or could use mesh as below.. or even make use of those edge slots in the hollow block walls. 10mm rebar would fit nicely

60mm land drain pipe ( covered with Land Drainage Geotextile Filter Sock )

10mm pea gravel.

Tonne bag of clean granular free draining material - any suggestions ? You mentioned gravel above. Would something angular be better ? MOT2 ?

25 mm coredrill and suitable pipe for front weep holes.

anything else ??

 

 

 

 

[saveasteading]

Ballast yes. 

Ties: either but these mesh ones look useful, esp at any day joints.

Gravel rather than type 1....we want free draining and self compacting, which gravel is.

Behind it backfill with earth or rubble but compacted. A manual tamper from wickes.

Ballast for concrete yes. Check the mix on the cement bag.  1 : 5 probably.

Compact it into the voids with a 2 x 2 or similar.

Splitters is fine for strength.

 

Get a nice new branded masonry bit. Just bigger than the bar, or you waste lots of epoxy.

Have spare nozzles for the epoxy.

What bar size? 12mm seems right to me. Rebar or threaded...either is fine.  3/4 into the concrete base.

 

[wertert]

Morning saveasteading & all.

Thanks again for the detail.  I'm planning on a concrete base which is 385mm thick so drilling in about 280mm for the rebar

The idea is the bottom of the wall base is level with the bottom of the proposed raft foundation.  ie we don't undermine the new wall when we dig down for the raft.

Possibly the base is too thick ?

I could add another row of hollows and make the base 170mm min thick instead ? Thicker base feels like the best option to me but would appreciate your advice.

 

On the subject of concrete i've always used ballast as we've discussed above.  I notice on my SE drawings ( for the other stepoc wall ) they are specifying RC35 concrete.  This has got me thinking that maybe I should investigate a C25 mix for the base and in-fill instead of ballast for this wall ? So cement + sharp sand + aggregate + water mixed in a magic ratio.  Worth considering ?

 

I've always wondered what aggregate you would actually use if making concrete without ballast ? 14mm clear/washed ? or smaller like a 6mm ? ( then add sharp sand etc ).  Most merchants want to sell you ballast as soon as you mention concrete.  BTW - my local pre-mix supplier will only do 0.5 m3 delivery as a minimum

 

[saveasteading]

1 hour ago, wertert said:

170mm min thick

Plenty, but do what suits you.

Laying blovks is less work than mixing concrete.

 

150mm or 125mm into the concrete is enough for the rods.

 

Ballast is also fine. It is a designed mix so that the small stones fit between the big ones, then the sand between them. Then the cement is simply an adhesive rather than a filler.

If you bought concrete it wouldn't be much different, except the price. 0.5m3 at £120 but then add 5m3 empty at about £40.  £260 ish.

Ballast. 

[wertert]

Hi All

 

few update pics for the first section.  Concrete tomorrow.  

 

 

Hollows should just about fit.

 

Ground was really rocky.  I'm knackered 😉 

[saveasteading]

12 hours ago, wertert said:

Ground was really rocky

That looks really dry and hard.

Perhaps try a breaker with a spade bit?

[wertert]

Hi All

Hope your bank holidays were productive.  Didn't get as far as I'd hoped due to constant rain.  I'm concerned about the soil/clay bank getting too wet and collapsing.

 

Few pics of the 2 courses we have so far.  Had to change the design slightly as there was no way to fit the '5 bay' design in.  Rebar is all epoxy'd in.  32mm drain/weep pipe installed.

 

Rookie mistake not digging enough out.  @saveasteading - you were 100% correct.  Very cramped in there.  The ground was incredibly hard and made up of clay and large stones.  Really tough going.  I may well invest in a breaker for the next section.

 

 

Next steps are to install the land drain ( 80mm ) around the back piers and try and try and curve it into the middle void.  This will be covered in 20mm shingle 1 block deep ( 215mm ).  I may also chuck in some geotextile to keep the shingle/drain as clean as possible.  Once that's in add 3 more courses.  I'm considering making the 6th course a straight run linking all the 5 sections ie no piers.  I'm also going to use some horizontal 10mm rebar ( courses 2 and 4 ) as the hollow blocks are notched.  I will leave ~20cm of rebar sticking out of the left hand side to act as a tie into the next section when it's built.

 

One question I have - When filling in the bays with concrete, once the wall is built, do I actually get more strength filling ALL the bays (6) or simply those bays with rebar (3) ? I only ask because I found a video on youtube where they did the latter. 

 

Would appreciate any feedback on the story so far.....

 

 

 

 

[Mr Punter]

With your drainage, the pipes need to go right through the blocks.

[wertert]

Hi Mr Punter

 

The 32 mm pipe does run through the block.   I didn't show this clearly above. Sorry about that.

 

 

The 80mm land drain is running behind the wall. Sound ok ?

[saveasteading]

1 hour ago, wertert said:

making the 6th course a straight run linking all the 5 sections ie no piers

Much tidier at the top and won't affect the strength significantly. 

 

Fill all the voids with concrete. Any advice to the contrary is from somebody tired of mixing concrete.

[wertert]

Hi All and Happy Saturday.

 

Couple of latest pics of section 1 which is basically finished pending more concrete fill and back fill with shingle.

 

 

Not a great time to run out of ballast

 

Couple of points/questions to the forum.

 

1) The top course is a straight run without piers. as you can see i've disrupted the laying pattern with a 3/4 block thinking this would help bind this section to the next section when it's built.  I hadn't really appreciated that any blocks laid from now on would NOT align fully with the block below. None of my front to back joints will be mortar'd as everything is now offset by 12cm.  ie only the front and back edges will be mortar'd - Bad idea ? weaker ?

 

2) same topic really.  Binding this section to the next one once it's built.  You may notice I have 2 sections of 10mm rebar sticking out the left hand side. I chucked in a couple of 90cm pieces as the blocks are notched and rebar is cheap.   I went for this over a wall starter ie https://www.screwfix.com/p/sabrefix-wall-starter-kit-stainless-steel/56037.  What do we think would be better ? The rebar strikes me as more robust.

 

Thanks as always for any advice / comments.  I will crack on with the next section tomorrow.  Bit of digging out and no Bank Holiday !

 

 

[wertert]

Hi All and Happy Friday.

 

Through i'd drop a picture of the wall progress.  Working on the 3rd section. 1 more planned after this so should start that this weekend.

Pleased with the overall progress but my cement mixer just died.  Drum bearing.

 

 

I think phasing the build into 4 sections has added alot of extra time but I still think it was the right thing to do given the potential downside if the ground moved at all.

 

Thinking slightly ahead to the next wall i don't need to worry about sections etc.  i'm trying to think of a block laying pattern to give me piers every 1.5/2 meters with minimal cuts etc.

Best I can come up with so far is below.  Half blocks marked with red.  View from the front and 2 views from the rear showing a partial pier and full.  Trying to minimise any longer vertical joins, best I can do is 3 courses.

 

 

How do the pros do this type of thing ? Alternative is simply build a flat wall with piers added separately with no cross/bonding blocks ie

 

 

 

 

 

 

Edited June 9, 2023 by wertert

[wertert]

Hi All

 

It's been a while!

Got the retaining walls all built but had some issues with building control which are now sorted.  The final concrete pour for the floor slab is happening next week then I can start on the walls.

 

We have decided to go with a cavity wall design for the garage with the retaining wall above making up the outer skin.  We will have a 100mm cavity and 4 inch blocks on the inside with ties every 450 centers ( screw in )

 

I have a question about DPC for the inner skin.   I was thinking to just put some standard 4" DPC on top of the first course but wondering if there something better I should consider ? Can't see a caviity tray working here as any liquid has nowhere to go.

 

 

This shows the inner block wall, cavity and retaining wall, left to right.  It's all built off approx the same level slab.

 

The building inspector has said he's not expecting a DPC in the retaining wall element.

 

Thanks all - any ideas ?