Help! big hole found under the conservatory foundation!

[LLL]

I found a big hole under the conservatory foundation at the corner. It seems the sand has been washed away quite a lot and it is deep (seems about a metre, though it gets narrower at the bottom!). I can also see the brick pavement around is lower in that area than in other areas. Seems there is a thin layer of concrete under the bricks but this part is already suspended since the sand is washed away (still quite shocked it is only sand used for supporting the foundation!)

 

There is a downpipe but I can see a PCV pipe under and the rainwater may go somewhere else, so not sure that is the reason. I doubt it since the hole is very deep and sand should not be washed away anywhere deeper, then where is the sand and foundation? There are some flowers/bushes planted at the corner (facing another side), is it the reason? but the plants are not big. 

There is a crack on the same face of the wall but it is about 1.5 metres from the hole(can see from the pic), there is no sign inside the room. the conservatory was at least 20 years ago but I have no idea when exactly it was built. 

 

Can anyone advise if there are any safety issues? and can I put the concrete inside the hole myself to support the foundation? 

 

thanks a lot! 

 

[Russell griffiths]

Broken gulley below the rainwater pipe, or broken clay connection. 
 

fairly normal to see this. 
 

I’ve seen a kitchen extension drop 25mm due to a dripping pipe under a corner. 
 

you will need to take up a lot more paving and fix the broken pipe first. 
then back fill the hole paying great attention to compacting the fill material, or mass fill with concrete. 
 

fix the cause of the problem first. 

[ProDave]

I would want to know where that downpipe is going and where it expects the rainwater is going.

 

It is either a crude soakaway built way too close to the conservatory, or there is a pipe to take it away that has failed.

 

Either way to me it looks like a lot of soil has washed away and I would want to be lifting more blocks and digging a proper hole to investigate exactly what is happening before deciding how to fix it.

[LLL]

22 hours ago, Russell griffiths said:

雨水管下方的沟渠破裂，或粘土连接处破裂。 
 

出现这种情况很正常。 
 

我曾见过厨房扩建部分因角落下的滴水管而下降了 25 毫米。 
 

您将需要先进行大量的铺路并修复破裂的管道。 
然后填回洞，特别注意压实填充材料，或用混凝土填充。 
 

首先解决问题的根源。 

 

Translation is: 

The trench under the rainwater pipe has broken, or the clay joint has broken.

It is normal for this to happen.

I have seen kitchen extensions that have dropped 25mm due to a drip pipe under the corner.

You will need to do a lot of paving first and repair the broken pipe.
Then fill the hole back, paying special attention to compacting the fill material, or fill it with concrete.

Fix the source of the problem first.

 Thank you, Russell! your answer relaxed a bit my concern but still would like to ask, it is something I can DIY? I am a DIYer but never dealt with this kind of work/foundations. I think the pipe is fine but you are right, I will take up more bricks to see where the rainwater is going, what is the clay connection? Do you think there are any safety issues in short time or do you think it is better to call a structure engineer? 

[Russell griffiths]

Get more blocks up and do some digging, come back with what you find. 
It’s all a guess at the moment. 

[LLL]

53 minutes ago, ProDave said:

I would want to know where that downpipe is going and where it expects the rainwater is going.

 

It is either a crude soakaway built way too close to the conservatory, or there is a pipe to take it away that has failed.

 

Either way to me it looks like a lot of soil has washed away and I would want to be lifting more blocks and digging a proper hole to investigate exactly what is happening before deciding how to fix it.

Thank you ProDave. I appreciate your quick response. As I replied above, I plan to take up more bricks and see where the rainwater goes. I know I have asked above but also would like to seek your opinion is it something I can do myself or better to find a builder/structure engineer? I like DIY and if is it not too complex I want to fix it myself. 

I have seen the cricks have cut the bricks, see in the picture, which means the cross-bricks may be not functioning well, do you see any safety concerns?  

[saveasteading]

1 minute ago, LLL said:

something I can do myself or better to find a builder/structure engineer

At least carry on investigating yourself. Decision later.

[LLL]

Thank you all for your quick replies, all very helpful.

 

I did a bit more work and did a test by pouring water into the pipe, the pipe works fine and the water goes to another side of the garden far away from the foundation. So not the issue of the pipe I can see from the pic only the corner is not supported but the other places on the left are well-supported by stones and sand. Still can't understand what is the reason for such a big hole and such a deep hole. Does anyone have an idea?

 

I plan to use concrete to fill the hole, hope this is fine. 

 

 

Edited Sunday at 15:15 by LLL

[Alan Ambrose]

Can you see - is there any foundation under the brick 'plinth' wall anywhere? I'm thinking something that looks like a rough concrete wall maybe 3/4m deep with crushed stone underneath it. That is, is that hole an isolated problem, or is it the same along the entire plinth and there's no foundation at all?

[saveasteading]

23 minutes ago, LLL said:

hope this is fine. 

Good work. It would appear that there are no foundations, only that slab that we see.

I would want to dig the hole a it wider and deeper to establish this, or otherwise.

That will also allow you to pack concrete in better, as that is probably a good plan.

 

If it is just a slab with no thickening, , then it is a bodge job that didn't trouble the building inspector or a decent builder. So  you won't be doing any harm.

I don't see any sign of a damp proof membrane. Is it damp inside?

We can discuss the concreting later.

 ps 

@Alan Ambrose was writing concurrently.

Edited Sunday at 15:31 by saveasteading

[LLL]

13 minutes ago, Alan Ambrose said:

Can you see - is there any foundation under the brick 'plinth' wall anywhere? I'm thinking something that looks like a rough concrete wall maybe 3/4m deep with crushed stone underneath it. That is, is that hole an isolated problem, or is it the same along the entire plinth and there's no foundation at all?

Thank you. I forgot to say the floor inside the room start from the roughcasting part so the bricks and the slab below, in total like 0.5 metre, I thought it is the foundation. 
 

In addition, only the hole part there is nothing below (well, the slab below the bricks is suspending as you can see), the left side I can see sand and stone below the brick and slab, quite solid, but not concrete, do we count it as a foundation? 

[LLL]

14 minutes ago, saveasteading said:

Good work. It would appear that there are no foundations, only that slab that we see.

I would want to dig the hole a it wider and deeper to establish this, or otherwise.

That will also allow you to pack concrete in better, as that is probably a good plan.

 

If it is just a slab with no thickening, , then it is a bodge job that didn't trouble the building inspector or a decent builder. So  you won't be doing any harm.

I don't see any sign of a damp proof membrane. Is it damp inside?

We can discuss the concreting later.

 ps 

@Alan Ambrose was writing concurrently.

Thanks a lot, there are bricks above the slab I thought it is the foundation - the floor start from the roughcasting. The floor should be suspended to 0.4-0.5 metres from the ground since there are two doorsteps in front and I can see the floor is higher from ground. I believe though it is quite wet and cold nowadays but I thought that is the normal in winter time. 

[saveasteading]

36 minutes ago, LLL said:

bricks above the slab I thought it is the foundation -

No, there should be that distance , or close to, underground to where the the ground is stronger and away from the weather.

Yours seems to be 100mm or so of concrete at ground level. So it is built more like a garden shed.

There isn't much you can do. So dig that hole bigger so that you can prove it is very local, and get your arm into it to see how far it extends.

[ETC]

[ProDave]

That looks to me like there was a raft foundation but that rough edge looks like it may have been hacked back perhaps at the time the block paving was laid?

 

That lone brick under the concrete slab is a curiosity, just where the hole is?

 

I would carefully enlarge the opening to truly reveal the extend then try and get some pictures sown in the hole.

 

I bet that has never been seen by a building inspector, but that is not unusual for conservatories.

 

 

[LLL]

thank you all for all valid suggestions. I made a pic using flash, which can see a bit more inside, the hole seems big and narrows down deeper. Very interesting you can see that there are some bricks above the bottom of the hole to support the wall but suspended now. It seems the hole is only in this area, fortunately.

 

Still confused about the reasons why it is caused, maybe there was a tree stump that rotten?

 

Anyway, anyone can suggest how to make concrete? While waiting for your guys suggestions, I went to B&Q and bought some sharp sand, aggregator, and cement. Do I need to make the concrete very fluid so it can go to every corner even I can't reach? 

or I can use some stones I have in my garden (pebble) to put at the bottom and then use concrete on the top to support the wall

 

any suggestions are well come, thanks 

[ProDave]

I would first pour a few cans of water down or even run a hosepipe.  Where does that void under the bricks go?  If water runs forever it could be a bigger void?  Try probing into that space with drain rods?

 

Pure speculation, that conservatory has been built over the foundations of some previous structure.  Can you see any similar houses in the area that look to have some form of original looking single story building on the back or an outside privy or coal house etc?  That might give a clue what was there?  Old OS maps from just after the house was built might also be useful.

[saveasteading]

13 minutes ago, LLL said:

how to make concrete?

First do as @prodave suggests.

That is probably only for your comfort, and won't change much.

 

I often lecture people about not adding too much water to the mix. Very wet concrete shrinks quite a lot and is weaker.

In your case this doesn't really matter, but its still for the best.

And we do want the mix to permeate in to the depths of that hole.

 

Being pedantic I might suggest adding a plasticiser to the mix. This is  a liquid that bricklayers use. it produces bubbles which make the mix more fluid, and also have some detergent for the same purpose.

How much to add I don't know though...  your circumstances wont be on the bottle but will still be  a guide.

 

btw sharp sand is excellent stuff and stronger than building sand, but does not flow well. Building sand is better for that.

 

So you would first use a mix without stones I think, so that it flows into corners and gravity will do most of the work. you should agitate it with a stick though as this will encourage it to flow and it removes any air voids..

once that has stopped disappearing and is visible, then make a stronger mix with sand, cement and aggregate.  and fill the rest.

 

I can never remember mortar or concrete mix amounts but it will be on the cement bag.

 

Never pour concrete or use mortar unless the temperature is at least 3 degrees and rising, otherwise it fails dramatically.

 

 

 

 

 

[LLL]

15 minutes ago, saveasteading said:

First do as @prodave suggests.

That is probably only for your comfort, and won't change much.

 

I often lecture people about not adding too much water to the mix. Very wet concrete shrinks quite a lot and is weaker.

In your case this doesn't really matter, but its still for the best.

And we do want the mix to permeate in to the depths of that hole.

 

Being pedantic I might suggest adding a plasticiser to the mix. This is  a liquid that bricklayers use. it produces bubbles which make the mix more fluid, and also have some detergent for the same purpose.

How much to add I don't know though...  your circumstances wont be on the bottle but will still be  a guide.

 

btw sharp sand is excellent stuff and stronger than building sand, but does not flow well. Building sand is better for that.

 

So you would first use a mix without stones I think, so that it flows into corners and gravity will do most of the work. you should agitate it with a stick though as this will encourage it to flow and it removes any air voids..

once that has stopped disappearing and is visible, then make a stronger mix with sand, cement and aggregate.  and fill the rest.

 

I can never remember mortar or concrete mix amounts but it will be on the cement bag.

 

Never pour concrete or use mortar unless the temperature is at least 3 degrees and rising, otherwise it fails dramatically.

 

 

 

 

 

thank you very much. it is good to know from you do not use stone at the base but only a mix of cement and sand. i will put the mix of cement/sand/aggregator on top of that. The temperature is 5 degree at the moment but will go up to 10 degrees in the morning, I believe it is the same everywhere in the country. The issue is tonight might rain, but I can cover that area properly to stop rainwater flow in. 

[LLL]

42 minutes ago, ProDave said:

I would first pour a few cans of water down or even run a hosepipe.  Where does that void under the bricks go?  If water runs forever it could be a bigger void?  Try probing into that space with drain rods?

 

Pure speculation, that conservatory has been built over the foundations of some previous structure.  Can you see any similar houses in the area that look to have some form of original looking single story building on the back or an outside privy or coal house etc?  That might give a clue what was there?  Old OS maps from just after the house was built might also be useful.

thank you, that seems like a clue but I did not find any similar to this. I might ask my neighbour who lived here for more than 30 years and definitely knew something before this building. 

[Russell griffiths]

It’s good to see the drain isn’t broken, I think you need to do some more digging untill you find solid ground under there. 
can you tape your phone on a stick and put it right in there. 
 

how long have you lived there, that plastic pipe looks in very good condition, is it newish. 

Edited Sunday at 18:06 by Russell griffiths

[LLL]

On 12/01/2025 at 18:04, Russell griffiths said:

It’s good to see the drain isn’t broken, I think you need to do some more digging untill you find solid ground under there. 
can you tape your phone on a stick and put it right in there. 
 

how long have you lived there, that plastic pipe looks in very good condition, is it newish. 

thank you again Russell. I moved here not very long time ago and have no idea when the pipe was placed. When you say it is newish, how many years could it be? 

[LLL]

Hello, just an update, I have now filled the hole with concrete and hope that works.

Thank you all for your great suggestions, that helped a lot!