Help with drainage issue after digging and laying sub-base near house wall

[LLL]

Hi everyone,

I’m currently working on hardening a small area right next to my conservatory wall. It was covered by soil and plants. The total area is about 3–4 m². I dug down roughly 25 cm and found the bottom to be quite firm — it seems mostly compacted sand when it was built. 

To prepare the base, I first added a layer of larger pebbles (about 3-5 cm thick), then covered that with Type 1 MOT sub-base and compacted it. Now it is still 15cm lower than the pavement. However, after a day of heavy rain, I noticed standing water in several spots — still raining. 

It looks like the water isn’t draining down through the base, possibly because the ground underneath is already saturated or contains clay. Is this a problem? if I just add more sub-base and sharp sand and pave the bricks at the same level as the existing one, will it cause any problem to the room? GPT is suggesting me install a short French drain by using a perforated pipe and clean gravel) to direct the water away from the house wall. But how far and deep should this be? I don't want to make the project too big. 

 

thanks 

 

[Redbeard]

AFAIK Type 1 'MOT' has lots of fines in and can almost set like concrete. It is not what you want for a permeable base. My contractors used 300-500 40mm limestone with a permeable membrane over, then 'permeable block paviours' (which have an extra gap formed by nobs on the sides) with large (3mm-sh) grit in the resultant gaps. It performs excellently, although the natural ground below the limestone is clay and bedrock. If I sling a bucket of water onto the paving it just goes *slurp* and it has gone.

 

Try putting "can MOT type 1 form a permeable base?" into a search engine, though, and you will get the whole spectrum, from yes, very much so to 'no, it is impermeable'. My experience suggests that the latter is the safest one to believe.

 

Others' experience might be different...

 

And as for the advice re a French drain, 'to direct the water away from the house wall', it has to have a destination. What is it? Is it a soakaway? If so is it the requisite distance from buildings and boundaries?

Edited Sunday at 17:31 by Redbeard
added re French drain

[LLL]

4 hours ago, Redbeard said:

AFAIK Type 1 'MOT' has lots of fines in and can almost set like concrete. It is not what you want for a permeable base. My contractors used 300-500 40mm limestone with a permeable membrane over, then 'permeable block paviours' (which have an extra gap formed by nobs on the sides) with large (3mm-sh) grit in the resultant gaps. It performs excellently, although the natural ground below the limestone is clay and bedrock. If I sling a bucket of water onto the paving it just goes *slurp* and it has gone.

 

Try putting "can MOT type 1 form a permeable base?" into a search engine, though, and you will get the whole spectrum, from yes, very much so to 'no, it is impermeable'. My experience suggests that the latter is the safest one to believe.

 

Others' experience might be different...

 

And as for the advice re a French drain, 'to direct the water away from the house wall', it has to have a destination. What is it? Is it a soakaway? If so is it the requisite distance from buildings and boundaries?

Thanks a lot, I believe you are right. I thought it is big and it shows in B&Q website but it came with same very fine stuff. However the water is not caused by it since if I dig I can see water is just not drain away below the sub base. The below clay is probably the problem. I believe the water it above the bricks of the crawl space of my sun room. Just do know if I just keep it and put the bricks on, can it cause an issue to the room in long term. It will be no water on the top or the pavement since the water can go away somewhere lower from the pavement, but underneath, it is just blocked I believe. 

[LLL]

Can any help with this? Is it a problem if I just leave it and put the sand and bricks on? 

thanks 

[markc]

55 minutes ago, LLL said:

Can any help with this? Is it a problem if I just leave it and put the sand and bricks on? 

thanks 

Loads of places have high water tables and digging a hole results in a pond, but filling the hole prevents water from collecting so it’s not noticed.  You would only need a drain if you had water coming into a basement etc.

go ahead, fill the hole and lay the toppings.

[LLL]

1 hour ago, markc said:

Loads of places have high water tables and digging a hole results in a pond, but filling the hole prevents water from collecting so it’s not noticed.  You would only need a drain if you had water coming into a basement etc.

go ahead, fill the hole and lay the toppings.

thank you very much!