Concrete slab moving - is it normal

[TheFew]

Hi,

I had a retaining wall built out of concrete boards set in H beams. The H beams were sunk and concreted in place and the textured concrete slabs were set between the beams to be a face for the wall. The whole area behind the wall was back filled with concrete. At the face of the wall the concrete was probably 2 feet deep and the rest of it was probably an 8 inch slab. The slab is probably 4m x 10m in size.

Before the concrete was poured concrete fence posts were set in place as were wooden posts of a gate. These were concreted in place before the slab was poured.

It seems that over the last year the slab has moved by about 8mm. This is visible from where it has moved from some of the wooden posts. It is also visible at the top of the gate, were there is now a good few cm gap (due to fulcrum action). Also the concrete fence posts are off vertical as are the H-beams. Not by much. By still!

So the question is, is it normal for a concrete slab to move as it presumably dries? Or should I be concerned?

Thanks.

 

[NoobCannon]

Pictures?

 

[TheFew]

Pictures?

Struggling to work out how to add some. Used to use photo bucket, but that no longer really works.

 

[Mcnumpty2323]

Struggling to work out how to add some. Used to use photo bucket, but that no longer really works.

use imgur and select BB code link

 

[jsmoke]

https://imgbb.com/

 

[TheFew]

Ok..

First three photos show the general arrangement. The third photo shows how the slab has receded from a post set against the house wall, moving towards the fence line. The last two show how the gaps in the gate have opened. The top of the gate has opened by 4mm in 2 weeks.

http://imgur.com/gallery/3TIdaj1

 

[NoobCannon]

Looks like shrinkage to me. The wood will move about now its hotter, may not be the slab moving at all?

 

[TheFew]

Looks like shrinkage to me. The wood will move about now its hotter, may not be the slab moving at all?

I really hope you are correct. However as mentioned some of the H beams that hold the wall in place lean forward by a few degrees as do some of the concrete fence posts. This could have happened as it was installed and drying and could always have been like that. I never checked.

It was only when I noticed the gap at the top of the gate starting that I took an interest and it has been getting slowly worse (the volt used to lock, now only the tip of the bolt touches the "hasp"). The fence section where the gate attaches has a number of gaps opening up where it seems little bits of the support structures are twisting and thus no longer square.

Hopefully it is just the wood.

Going to see what the builder who installed it thinks. Bit as always, it is good to get a spread of opinions.

 

[fish]

What was there before? If trees you would expect some movement as the ground sorts itself. It’s surprising the amount of movement due to heave and settling

 

[PlacidCasual]

How long has the concrete been there? What sort of soil do you have, is it heavy clay soils?

I would guess, but can’t know, that there is insufficient foundation for the weight and that the extended dry spell has caused soil shrinkage which with the weight has caused everything to move.

But without seeing the site it’s difficult to know for sure.

 

[TheFew]

Previously there was a block work retaining wall and a concrete slab! But it had been destroyed by a laurel which had broken he wall and caused the slab to drop. So the laurel was removed and the slab relaid. The majority of the area under the original slab was really sturdy as someone have gone to the trouble of sinking lots of 2m long scaffold poles vertically and concreting them in. So a new wall was erected and the area behind it compacted with scalping etc before the new slab was poured.

So yes. A tree did come out. It was projecting forward out of the wall, not an upright tree.

 

[TheFew]

How long has the concrete been there? What sort of soil do you have, is it heavy clay soils?

I would guess, but can’t know, that there is insufficient foundation for the weight and that the extended dry spell has caused soil shrinkage which with the weight has caused everything to move.

But without seeing the site it’s difficult to know for sure.

Concrete was laid a year ago next week!

Soil is kinda normal soil bit does become green/clay soil a meter of so down. The area was already fairly stable however as per my earlier post.

 

[Blackjack Davy]

Yeah shrinkage. Unless the post is obviously wobbly I wouldn't worry about it.

 

[j.col]

Those are concrete fence posts with rock face concrete gravel boards. these are not really designed for this type of job tbh but should be fine if the builder went very deep digging the holes for the posts, there is a lot of lateral pressure against that wall.

Also have a close look at the gravel boards for any cracks
A good way it to wait for a sunny day, hose down face of the wall and watch it dry, cracks will always stand out

I guess that behind the top posts there is higher land than your side?

If the ground is level with yours ignore below

If so it could be that the posts were fitted too near your concrete wall, under high wind, with so little gap between the posts and your wall, the concrete holding the top posts cant cope with the load on the top of the fence.
There is literally 100mm of concrete between the top posts and your wall, imo thats not enough to hold a fence that high

 

[TheFew]

Those are concrete fence posts with rock face concrete gravel boards. these are not really designed for this type of job tbh but should be fine if the builder went very deep digging the holes for the posts, there is a lot of lateral pressure against that wall.

Also have a close look at the gravel boards for any cracks
A good way it to wait for a sunny day, hose down face of the wall and watch it dry, cracks will always stand out

I guess that behind the top posts there is higher land than your side?

If the ground is level with yours ignore below

If so it could be that the posts were fitted too near your concrete wall, under high wind, with so little gap between the posts and your wall, the concrete holding the top posts cant cope with the load on the top of the fence.
There is literally 100mm of concrete between the top posts and your wall, imo thats not enough to hold a fence that high

Hi,

The rock faced boards are not held in BT concrete posts but by steel H beams. The beams have just been painted grey.

Some of the boards do have the off hairline crack. And have done so from day 1. My understanding was that these boards had internal metal reinforcement, bug I am probably wrong.

Both sides are mine. The boards are holding back a concrete slab that is about 3m to 4,m wide. Beyond that is the house!

The posts were installed when the wall was built and were concreted in, before the slab was laid on top. So they should be firm, but I guess the lever action caused by wind could be an issue!

 

[j.col]

Hi,

The rock faced boards are not held in BT concrete posts but by steel H beams. The beams have just been painted grey.

Some of the boards do have the off hairline crack. And have done so from day 1. My understanding was that these boards had internal metal reinforcement, bug I am probably wrong. they do have reinforcing inside

Both sides are mine. The boards are holding back a concrete slab that is about 3m to 4,m wide. Beyond that is the house!
Do you mean that that wall is holding back a 3m x 4m concrete slab?
and the top of this slab is the same height as the bottom of the fence? so basically that is a retaining wall?
if so a 9 inch wall should have been built, we would never use gravel boards to retain so much weight imo
but this isnt the problem


The posts were installed when the wall was built and were concreted in, before the slab was laid on top. So they should be firm, but I guess the lever action caused by wind could be an issue!
exactly, my guess is thery are either 4 or 5ft panels with 12" trellis on top?
if its 6 foot total that means there is 2 feet of the post below the top of the wall and your wall is 4 feet deep
so the bottom of the post is half way down your wall.
with it being so near the wall, you are correct about the lever action. if the posts were set further back from the wall, they would have been much stronger and you wouldn't have this problem
imo as the years go by this will get worse

 

[TheFew]

Here are some in construction pictures which help show the situation.

http://imgur.com/gallery/aZuhvGZ

http://imgur.com/gallery/sS9Pl63

 

[TheFew]

Yep. It is a retaining wall for the slab. And yes the fence is 6 foot. 5 foot plus trellis. Hopefully the pictures in the above post give a better understanding of how it was built.

 

[TheFew]

So it seems that the wooden post was twisting. So this has been replaced and it is much better.

I am still left however with a circa 2cm gap between the concrete slab and the wall of the house. So now I need to decide whether to leave it as a gap or to fill it with cement and sand (e.g. poscrete).

Just not sure if it is better to leave a gap or if water getting in at winter and freezing will cause any issues.

Thanks