Preventing my garden draining into my foundations

[Andy H]

I am planning a single storey building which will sit on foundations around 1m below ground. I completed the excavation last summer and boarded up to start again this spring.

 

After a days rain the hole can contain 5-6 inches of water suggesting that alot of water is flowing in from my garden and neighbouring gardens.

 

Most of the research I have done on line suggests the use of a water permeable material for my back fill along with a french drain at the base of the foundation. Wouldnt this mean the garden water would then drain into the backfill around the building wall and foundations? That would be alot of water passing the wall and alot of pumping out.

 

It would seem more common sense to prevent the water getting around the below ground walls and foundations in the first place.

 

I have come across foam concrete which I thought may be able to serve the purpose of backfill and keeping the water away from the building.

 

Any clarification would be appreciated

 

Andy

Edited January 30, 2021 by Andy H
additional thoughts

[Russell griffiths]

I’m not understanding what you have done, you need to explain with a rough scale drawing showing a section and the heights you have. 

 

Not really sure why you are a metre into the ground. 

Is this just a simple garden room, office garage, workshop, a few more details needed including usage so we can gauge insulation levels and position and build method for the walls. 

[joe90]

Foundations sat in water is not a problem (I have a high water table), with our foundations I had to dig a small “sump” next to the foundations to enable a small pump to drain the water before the concrete was poured.

[Andy H]

56 minutes ago, joe90 said:

Foundations sat in water is not a problem (I have a high water table), with our foundations I had to dig a small “sump” next to the foundations to enable a small pump to drain the water before the concrete was poured.

I was intending having a sump and pumping out but think it makes more sense to prevent or reduce the water draining from the garden to the foundations, therby reducing the likelihood of water penetrating the building at some point - I am planning on appropriate waterproofing of course.

[Temp]

Our house is on three levels/steps. We tanked the uphill side of the steps and put in a perforated pipe into gravel. They lead out sideways to a surface water drain.

 

Will the walls also be retaining walls? If so they need careful design. 

 

 

 

Edited January 30, 2021 by Temp

[joe90]

9 minutes ago, Andy H said:

I was intending having a sump and pumping out but think it makes more sense to prevent or reduce the water draining from the garden to the foundations, therby reducing the likelihood of water penetrating the building at some point - I am planning on appropriate waterproofing of course.

It depends if the building itself, not just the foundations is below the water table. Our water table is high and when the trench was backfilled after the foundations were poured and brickwork started it was with perforated pipe in 50mm drainage stone and drained to a nearby ditch (French drain), if you’re  build is below the water table it’s a whole new ballgame, tanking etc.

Edited January 30, 2021 by joe90

[Andy H]

6 minutes ago, Temp said:

Our house is on three levels/steps. We tanked the uphill side of the steps and put in a perforated pipe into gravel. They lead out sideways to a surface water drain.

 

Will the walls also be retaining walls? If so they need careful design. 

 

 

 

The buildings walls will be retaining using steel reinforced concrete within ICF. I haven't decided whether to go for eps or woodcrete yet. I will be relying on a structural engineer to provide the exact specification of the wall.

[Andy H]

6 minutes ago, joe90 said:

It depends if the building itself, not just the foundations is below the water table. Our water table is high and when the trench was backfilled after the foundations were poured and brickwork started it was with perforated pipe in 50mm drainage stone and drained to a nearby ditch (French drain), if you’re  build is below the water table it’s a whole new ballgame, tanking etc.

The building and foundations are well above the water table. It just strikes me that my garden and surrrounding gardens are going to be dumping alot of water into the permeable back fill and put alot of demand on a sump pump. The foundation will be sat on clay so it wont drain away into the soil below the building.

 

Approx the first 1 foot of the soil is permeable top soil, below that is clay

[joe90]

2 minutes ago, Andy H said:

The building and foundations are well above the water table.

In that case it’s not a problem, as I said, foundations sat in water can be quite normal and any DPC will stop wicking up into the building. You could always do like I did and run the drainage pipe into you’re drains, that’s where the rain from the roof goes!!!

[Temp]

Is the FFL below ground? 

 

 

 

 

 

 

 

 

Edited January 30, 2021 by Temp

[Andy H]

1 hour ago, Russell griffiths said:

I’m not understanding what you have done, you need to explain with a rough scale drawing showing a section and the heights you have. 

 

Not really sure why you are a metre into the ground. 

Is this just a simple garden room, office garage, workshop, a few more details needed including usage so we can gauge insulation levels and position and build method for the walls. 

Russell I am building a 6m*12m building under permited development rights at the bottom of my garden, close to neighbouring boundaries so I am limited to a height above ground of 2.5m. I have dug down in order to get more height into the building whilst keeping within permitted development.

 

I am planning on an ICF build sat on a isoquick insulated slab.

 

I have attached a sketch fyi

 

Any comments would be welcome

[Andy H]

14 minutes ago, joe90 said:

In that case it’s not a problem, as I said, foundations sat in water can be quite normal and any DPC will stop wicking up into the building. You could always do like I did and run the drainage pipe into you’re drains, that’s where the rain from the roof goes!!!

My plan is to do the same as you - run the sump pump into the drains

[ProDave]

An explanation of what you are doing would help.  Digging a big 1M deep hole, shoring up the sides then leaving it over winter to fill with water is unusual.  Far better to pick a dryer season, dig and pour as quick as possible.

 

As it is filling with water and not draining away, I would not be sure you are not below the water table.  If so just where do you plan to drain it to?  Our ground is like this and at times.  Before we built I dug some 2M deep test holes for the SE and left one open over winter covered with a board and had a look down it from time to time.  I could find anything from a dry 2M deep hole to water almost at the surface depending on how much rain we had recently.

 

EDIT just seen more information in a post above.

 

Although this may be under permitted development, the size of it means you WILL need building regulations, so I suggest you stop, get a properly designed foundation plan and notify building control.

 

And digging 1M deep close to a boundary probably also invokes the party wall act.  How close are the nearest buildings?

[Andy H]

15 minutes ago, Temp said:

Is the FFL below ground? 

 

 

 

 

 

 

 

 

What is FFL??

[PeterW]

Finished Floor Level

[Andy H]

2 minutes ago, ProDave said:

An explanation of what you are doing would help.  Digging a big 1M deep hole, shoring up the sides then leaving it over winter to fill with water is unusual.  Far better to pick a dryer season, dig and pour as quick as possible.

 

As it is filling with water and not draining away, I would not be sure you are not below the water table.  If so just where do you plan to drain it to?  Our ground is like this and at times.  Before we built I dug some 2M deep test holes for the SE and left one open over winter covered with a board and had a look down it from time to time.  I could find anything from a dry 2M deep hole to water almost at the surface depending on how much rain we had recently.

Good evening Dave,

I started last summer and thought I would be able to get the foundations and ICF walls up but a combination of me being too slow, too ambitious and delayed by the structural engineer resulted in me making the decision to shut up for the winter.

 

I am now doing the prep work for a spring start and hopefully a water tight building before the end of summer.

 

I live in one of the highest locations in London and my understanding is that the water table in London is below the level of the London Underground. If the watertable was 1m below my garden I assume water would be seeping out through  the ground in the rest of London.

 

I presume the reason the water is not draining away is because it is sat on clay which is slow to drain?

[Andy H]

8 minutes ago, PeterW said:

Finished Floor Level

Yes the FFL will be around 0.75m below ground level in order to get more height within the building whilst fitting within permittesd development rights.

[Russell griffiths]

You are going to need a waterproofing system for those outer walls, this needs designing now with your icf supplier, and building regs designer. 

 

[Andy H]

Just now, Russell griffiths said:

You are going to need a waterproofing system for those outer walls, this needs designing now with your icf supplier, and building regs designer. 

 

Yes- I will be going overboard with the waterproofing plan - expect some future posts into the forum!

[Adsibob]

4 hours ago, Andy H said:

I live in one of the highest locations in London and my understanding is that the water table in London is below the level of the London Underground. If the watertable was 1m below my garden I assume water would be seeping out through  the ground in the rest of London.

 

I presume the reason the water is not draining away is because it is sat on clay which is slow to drain?

I also live in London and have this exact problem. Builder dug trenches in my garden for our extension’s foundations and also did some trial holes internally to check how low we could drop the floor of the existing house before we ran into the corbels, and both the trenches and the trial holes filled up with water. Structural engineer initially thought it was a high water table. I called a surveyor who inspected and assured me that was impossible, pointing out that I live halfway up a hill. It’s just London clay, lots of rainwater and not enough trees to drink it up.

 

Builder isn’t bothered about the water. He has a pump to suck it out so that he can get on with laying the foundations. But I’m still hoping February will be drier than January!

[Olf]

14 hours ago, Andy H said:

I am building a 6m*12m building under permited development rights at the bottom of my garden, close to neighbouring boundaries so I am limited to a height above ground of 2.5m

 

Did you try to get planning permission for taller structure first? It could be both cheaper and better than going with the underground struggles. I'm planning something smaller (~4.5x7.5) and as much as settled on wasting 1m space to stay away from the boundary (EWI, EPS not being 'significantly non-combustible'), I will start with attempting to get planning permission for a decent heght. At the end of the day, if I moved just extra 1 m away from the border, I could erect 4m high, 2.5m eaves without asking. I will however keep it under 30 m2 internal (as the size works for me anyway) to save on BC fees and hassle, even though it will be designed and built properly.

 

14 hours ago, ProDave said:

Although this may be under permitted development, the size of it means you WILL need building regulations

That's the main point. Also if you plan to add any services suggesting that it might be 'habitable', you're falling under the BC radar

 

[Hilldes]

We have a similar problem right now - based just outside the M25. The Soil Investigation reported no water in any of the boreholes (in May last year), London clay from 1.5m below ground level. For the piling earlier in Jan a few of the 32 piles had some water and only one pile had so much water it failed. Then onto trenhching for the ground beam and the trenches fill with water (from then ground not the sky). The groundworks contractor dug a well next to the trenches and put in a pump that runs 24x7, that keeps water level low in the trenches, but does not eliminate it. The plan next week is now to blind the trenches with a thin layer of concrete - not a cost we had budgeted for.

[Andy H]

6 hours ago, Olf said:

 

Did you try to get planning permission for taller structure first? It could be both cheaper and better than going with the underground struggles. I'm planning something smaller (~4.5x7.5) and as much as settled on wasting 1m space to stay away from the boundary (EWI, EPS not being 'significantly non-combustible'), I will start with attempting to get planning permission for a decent heght. At the end of the day, if I moved just extra 1 m away from the border, I could erect 4m high, 2.5m eaves without asking. I will however keep it under 30 m2 internal (as the size works for me anyway) to save on BC fees and hassle, even though it will be designed and built properly.

 

That's the main point. Also if you plan to add any services suggesting that it might be 'habitable', you're falling under the BC radar

 

I didnt go for planning for a couple of reasons - 1 By keeping at 2.5m above ground level the building will not be too visually imposing. 2. I was able to agree with a neighbour to aquire a very small section of their garden which meant I could build a larger building, but the condition was not to go above 2.5m.

 

I have appointed a building inspector based on recomendation and they have been helpful to date.

[Andy H]

17 hours ago, Adsibob said:

I also live in London and have this exact problem. Builder dug trenches in my garden for our extension’s foundations and also did some trial holes internally to check how low we could drop the floor of the existing house before we ran into the corbels, and both the trenches and the trial holes filled up with water. Structural engineer initially thought it was a high water table. I called a surveyor who inspected and assured me that was impossible, pointing out that I live halfway up a hill. It’s just London clay, lots of rainwater and not enough trees to drink it up.

 

Builder isn’t bothered about the water. He has a pump to suck it out so that he can get on with laying the foundations. But I’m still hoping February will be drier than January!

Its reassuring to hear your surveyor comments. Its quite surprising that a structural engineer would not know, given your location it couldnt be a water table problem!

 

Thanks   Andy

[Russell griffiths]

Have you considered digging a bigger hole and having a low retaining wall instead of having to waterproof your building walls

i did this on a garage that had a height restriction.