Drainage - Relationship to Foundations

[berty3000]

Hello,

I've had this drainage pipe installed down the side of my house to remove excess surface water from front garden to rear garden area where it drains much better. The pipe has some perforations in the top half to also collect water from down the side which would get some pooling in rainy conditions. The bottom of the trench is probably 6 bricks below dpc at the point nearest on the photo, going up to about 4 bricks below further away. The trench was filled with pea gravel, membrane over the top, and finished in a few inches of gravel, the top of which finishes 2-3 bricks below dpc from one end to the other. The trench will get water in it when it rains although the pipe will carry some of that away if it fills up...

Is any of this likely to be a problem for the house, foundations etc, given position and depth? House is 1961 bungalow.

Thanks

 

[berty3000]

Any thoughts on this one?

Many thanks

 

[noseall]

Any thoughts on this one?

Many thanks

For that pipe to be a problem, your concrete would need to be sitting about six courses up from your glove!.

You are safe.

 

[berty3000]

Thanks Noseall! I've been fretting about it.

I gather the assumption is that the footings are likely to be another few courses further down below the bottom of the trench? I think at its lowest point the trench is about 6 bricks below dpc.

What about the trench having surface water sat in it from time to time as a result of rain, and being more concentrated than if it had just fallen across a flat soil surface? Would this make any odds? Also, I don't know if this technically makes the trench a soakaway in that case and those should be further from dwellings... but that is presumably for other reasons?

 

[noseall]

What about the trench having surface water sat in it from time to time as a result of rain, and being more concentrated than if it had just fallen across a flat soil surface? Would this make any odds? Also, I don't know if this technically makes the trench a soakaway in that case and those should be further from dwellings... but that is presumably for other reasons?

Make no difference what so ever. All the masonry below ground, including the foundation concrete is permanently wet. No dramas. The most vulnerable masonry is at (or just above) ground level, regards any structure, but for different reasons.

 

[berty3000]

How is it that material gets washed from beneath footings? Is that when the footings are exposed and then some kind of pressured flow, like from leaking pipe, attacks the soil underneath?

Also, just a question about the principles... when building regs require 1m or whatever depth for foundations, I assume they mean depth below surface? Is this because the material to the side of the foundations and upwards to the surface has a functional role in keeping the foundations stable? In theory, could you have entirely gravel at the side of a house say 7 courses below DPC but above footings, or does it have to be more consolidated material?

 

[noseall]

How is it that material gets washed from beneath footings? Is that when the footings are exposed and then some kind of pressured flow, like from leaking pipe, attacks the soil underneath?

Also, just a question about the principles... when building regs require 1m or whatever depth for foundations, I assume they mean depth below surface? Is this because the material to the side of the foundations and upwards to the surface has a functional role in keeping the foundations stable? In theory, could you have entirely gravel at the side of a house say 7 courses below DPC but above footings, or does it have to be more consolidated material?

It's more to do with guaranteeing decent bearing strata and avoiding 'vegetative matter' etc. Protection from frost has some bearing but other important factors supersede.

 

[noseall]

How is it that material gets washed from beneath footings?

Very rare. You would need a muti-fronted attack for that to occur - i.e. poor ground, leaking drain or water main, continual source of water etc.

 

[berty3000]

Thanks for the info, noseall. Filled some gaps in my understanding!

 

[berty3000]

Make no difference what so ever. All the masonry below ground, including the foundation concrete is permanently wet. No dramas. The most vulnerable masonry is at (or just above) ground level, regards any structure, but for different reasons.

Hey noseall,

Just one last question. When the building regs say a soakaway should be at least 5m from a building - is this because of the need for the regs to provide a catch-all safety net for a range of situations? Different sizes of soakaway and different types of property and footings, etc?

So in many cases, a 1m deep soakaway 2.5m from the house won't make any odds because it wouldn't be below the footings or in the angle of repose, and soil type wouldn't be an issue. But in other cases like shallow foundations, etc it could be a source of instability. Have I got that right?

 

[noseall]

Hey noseall,

Just one last question. When the building regs say a soakaway should be at least 5m from a building - is this because of the need for the regs to provide a catch-all safety net for a range of situations?

Basically yes.

I've lost count the amount of soakaways I've excavated ten foot away from buildings (or less).

 

[cwhaley]

When I bought my house, it transpired that rain water was being discharged down the drainpipe and underneath the drive. When I had the driveway re-lay, there was a large cavity underneath the drive and all of the foundation blocks in that corner were eroded with missing mortar. We had it put into a soakaway at the end of the driveway with a setup like yours. No issues years later.

 

[cdbe]

You sound like a worrier - just like me!

I live on a busy main road in a 1930's house, I'm replacing the roof at the moment and I was sat up there yesterday when a couple of particularly big lorries went past, I felt the roof under me physically shake, like a small earthquake - I'm sure the entire house is just going to fall down, I know it hasn't in the last 90 years but I'm convinced it's going to happen in the next few weeks!

 

[cwhaley]

You sound like a worrier - just like me!

I live on a busy main road in a 1930's house, I'm replacing the roof at the moment and I was sat up there yesterday when a couple of particularly big lorries went past, I felt the roof under me physically shake, like a small earthquake - I'm sure the entire house is just going to fall down, I know it hasn't in the last 90 years but I'm convinced it's going to happen in the next few weeks!

It was worrying to see the foundations being eroded away! But yes, I do have a tendency to overthink and worry too much. At the moment it's laying in bed at night overthinking jobs I've got to do to my car, next it'll be whatever job I've got on in the garden.

A little bit of movement is good!