Concreteing behind a retaining wall advice!

ian103

Id be tempted to mix it yourself, you are going to be paying a lot for a pump / uncarried concrete in the wagon plus a loss of 1/4m3 concrete or so in the pump that just geta washed out. 1 option would be a semi dry mix but in my wiew if access is a problem Id have a couple jumbo bags ballast, anf mix that up and then have another couple of bags. If you want the ease of the pump, I would prop the walls at 2m centres wimber and acrow props but Id check the various cost options first

phil24_7

Furts wrote: »

I would need to see a clear photo of the wall construction before hazarding any comment. Too many risks involved in your proposal!

phil24_7

ian103 wrote: »

Id be tempted to mix it yourself, you are going to be paying a lot for a pump / uncarried concrete in the wagon plus a loss of 1/4m3 concrete or so in the pump that just geta washed out. 1 option would be a semi dry mix but in my wiew if access is a problem Id have a couple jumbo bags ballast, anf mix that up and then have another couple of bags. If you want the ease of the pump, I would prop the walls at 2m centres wimber and acrow props but Id check the various cost options first

It'll be about £500 for pumping but this will be offset slightly as premixed (c40) concrete will be a bit cheaper than me buying everything in bags (site conditions mean that dumpy bags are not practical, it would take too much labour to get it where I need it) I would also need some king of scaffolding/tower for the mixing otherwise I'm gonna need to throw it in by the bucket!

There wouldn't be much, if any wastage as I will be needing some footings for 6 or 7 metres of wall so what I don't use in the wall will be used in the footings, very little is wasted in the pump too as they pump a ball through when they are done, forcing what is in the pipe out of the end.

phil24_7

Incidentally I had a structural engineer here for an unrelated project (a couple of wall removals) and I took the time to pick his brains on a few different ideas for various projects and when asked about this he said 2 courses at a time would be best. When pushed on whether I could get away with 4 courses if it was supported he said...possibly! lol

He said I could OSB over the wall, tie it together with a frame and the use diagonal posts from the top and horizontal post at the bottom to the course of blocks on the other side of the pond. I could also screw/bolt the frame to the base and/or steps as well as fixing the ends of the supports to the base (either directly or via a frame).

After he left I had a think and I could also hammer stakes into the ground just beyond the concrete base and fix diagonal supports to these as a belt and braces approach.

Furts

phil24_7 wrote: »

Incidentally I had a structural engineer here for an unrelated project (a couple of wall removals) and I took the time to pick his brains on a few different ideas for various projects and when asked about this he said 2 courses at a time would be best. When pushed on whether I could get away with 4 courses if it was supported he said...possibly! lol

He said I could OSB over the wall, tie it together with a frame and the use diagonal posts from the top and horizontal post at the bottom to the course of blocks on the other side of the pond. I could also screw/bolt the frame to the base and/or steps as well as fixing the ends of the supports to the base (either directly or via a frame).

After he left I had a think and I could also hammer stakes into the ground just beyond the concrete base and fix diagonal supports to these as a belt and braces approach.

Looking at the photos and interpreting these comments it sounds like you have a single skin of blockwork. It looks like a tanking membrane in the gap, so there will be no ties through it. Behind is a brick wall...all a bit vague so you would have to interject.

If I have judged this correctly you have a bad piece of design and workmanship. This is not the way to build retaining walls even before coming onto the topic of insulation, condensation and damp.

I would be site mixing a semi dry sand cement mix then gradually and evenly filling behind the wall.

phil24_7

Furts wrote: »

Looking at the photos and interpreting these comments it sounds like you have a single skin of blockwork. It looks like a tanking membrane in the gap, so there will be no ties through it. Behind is a brick wall...all a bit vague so you would have to interject.

If I have judged this correctly you have a bad piece of design and workmanship. This is not the way to build retaining walls even before coming onto the topic of insulation, condensation and damp.

I would be site mixing a semi dry sand cement mix then gradually and evenly filling behind the wall.

Yes...and no.

The wall behind finishes mid way up (maybe a bit higher) the new wall and is compacted soil/shillet (sp) below that. This new wall is just to aid the support of that wall and hold back the 4 meters of ground between this and the house. Their is a cellar on the other side that is less that a metre above the level of the pond base so its not like it is retaining much (10mx4mx2m of soil, with retaining walls on all sides) There is mesh rebar running up behind the wall that is tied into the base and chemifixed into the steps at either end, and wall ties from the outer skin of wall to the mesh. There is no inner skin and the large gap is going to be fully filled with concrete.

The DPM is to stop damp coming through the wall (though this will be negligible do to the ground being retained having a concrete cap on it as well as drainage that comes out above the wall that I am building.). This DPM goes under the concrete bases' DPM.

All concrete will be C40 so will be hard to get a uniform mix on site. Incidentally my structural engineer saw no real problems with what I have done or am doing. Granted, it would have been better to rip down the old wall and build a completely new wall all the way up but that would have been a s**t load of extra work for very little gain. Is a block wall and 285-450mm of C40 concrete going to be weaker than a cavity wall filled with 100mm of C40 concrete?

The wall I am retaining has been undermined for the best part of 2 years (maybe 3, I can't honestly remember) and hasn't gone anywhere as it is not holding back a great deal.

Insulation, condensation and even damp don't come into as it is an external wall in my garden and a pond will be built in it!

Regards
Phil

Furts

I am uncertain what you have, but am assuming there is mesh in the cavity. The bottom line here would be structural concrete requires compaction, and nobody builds a stretcher bond block wall expecting it to resist compaction, or even act as a shutter. That is why I said your design and workmanship was flawed.


Clearly you have a fixation with C40 concrete, though being pragmatic it escapes me why this should be so. I still stand by my previous observation - a sandy cement mix (because there appears to be tanking) carefully placed, which if too wet will push over your wall leaving a big mess! Mix this with a modicum of care and it should be adequate - you can achieve an equivalent of structural concrete by producing a 1:3 mix, or stronger if you wish.

ian103

Id agree the pump will be approx £500 plus vat, looking at the picture Id be wary about filling that in one go, unless it was propped correctly, acrow props fixed to the floor and timbers / boards supporting the walls, and possibly 1m ccs as its only 100mm blocks. Is it an option to concrete the other footings and partially fill the cavity? Youll still need a way of controlling the pump line and distributing the concrete in the cavity. Unfortunately there doesnt seem to be a simple solution other than site mixed concrete poured in with a bucket.

phil24_7

ian103 wrote: »

Id agree the pump will be approx £500 plus vat, looking at the picture Id be wary about filling that in one go, unless it was propped correctly, acrow props fixed to the floor and timbers / boards supporting the walls, and possibly 1m ccs as its only 100mm blocks. Is it an option to concrete the other footings and partially fill the cavity? Youll still need a way of controlling the pump line and distributing the concrete in the cavity. Unfortunately there doesnt seem to be a simple solution other than site mixed concrete poured in with a bucket.

Thanks Ian.

I wouldn't fill it in one go, I would be looking to do 2/3rds (4 courses, 2.25m3).

It would be difficult to do things the other way around as they would need to blow the pipe out, take a few bits off, then extend the pipe through a garage and over my decking (much better done before everything gets dirty!), though it wouldn't be impossible. I don't think I need 1.75m3 of concrete for my footings though, it would probably be closer to 1.5m3.

I could be a right pain and get accelerated drying stuff (with fibres if needed). This could be done 2 courses, then the footings, the the second 2 courses. Tea and biscuits may ease the pumpers pain!

Mixing on site and loading with buckets is not easy...I'm an out of shape 38! lol

How would be best fix acros to the floor?

What is 1m ccs?

The pump guys have a large length of flexible rubber pipe for the last piece so moving the pipe down the length of the cavity wont be a problem. They did the concrete base the new wall is built on.

Cheers.

ian103

1m centres between props, you could bolt a timber to the concrete slab and then naik the acrow base plate to the timber to prevent it slipping or use a longer prop and stake it into the soil beyond the building (the timber bolted down woukd be easiest) you need to create a triangulated shore, for rigigidity. So a vertical timber, a horizonal timber and a raking timber to prop, if you are part filling I would do less temporary propping.