Drainage under block paving

[Gary Prentice]

A quick question for the landscape fraternity.

 

Is it common practice to instal a perforated plastic land drain under block paving?

 

A client has a garge built into a hillside, the rear of which is backfilled with stone. At the base a land-drain was laid which runs around to the front corner and then under the block paving and towards the sewers.

 

After eight years the drain is blocked under the paving and water is now being forced upwards between the blocks. This water is mainly near a mature sycamore ( positioned within the paved area) but there is also a 10-15 yr old cypress hedge close by, another individual cypress and a climbing hydrangea growing up the front wall of the house.

 

I'm no expert, but I think this is poorly designed and the run under the paving shouldn't have been perforated. Could the contractor have thought that surface water would perculate through the paving into the land drain to be moved away? Would that be a normal practice or consideration? :confused1:

[Ian Leach]

I've never seen or used perforated drainage under block paving, you can use it say behind the garage then solid pipe to the sewer for example as all the block work should be permeable so anything should soak through.

 

Do you know the pipe is going to the sewer??, or is it just acting like a soak away and now it's blocked and coming through the perforated holes

[Peter]

Block paving is not water permeable. Surface water needs to be moved to a collection point and drained through a grid.

 

As Ian says, the pipework under the paving should be solid, it's possible that fine sand from the block paving sun base has got into the drain. More likely they've blocked from the discharge end imo. The solid pipe should also have rodding points at every change of direction.

[5 shires]

A quick question for the landscape fraternity.

 

Is it common practice to instal a perforated plastic land drain under block paving?

 

A client has a garge built into a hillside, the rear of which is backfilled with stone. At the base a land-drain was laid which runs around to the front corner and then under the block paving and towards the sewers.

 

After eight years the drain is blocked under the paving and water is now being forced upwards between the blocks. This water is mainly near a mature sycamore ( positioned within the paved area) but there is also a 10-15 yr old cypress hedge close by, another individual cypress and a climbing hydrangea growing up the front wall of the house.

 

I'm no expert, but I think this is poorly designed and the run under the paving shouldn't have been perforated. Could the contractor have thought that surface water would perculate through the paving into the land drain to be moved away? Would that be a normal practice or consideration? :confused1:

 

Unless the block paving is permeable block paving system there is no point in installing any kind of under ground drainage system and even if the paving is permeable it wouldn't work that well.obviously the guy has try'd to be clever and it has done a job for a while but now all the sharp sand has washed through and filled the perforated 4" ground drainage pipe. With out seeing the lay out it would be difficult to asses but eco drains are best used with standard block paving and run of into existing service drain if possible and if not a good constructed to allow run of to flow into.

[Ian Leach]

Block paving is not water permeable. Surface water needs to be moved to a collection point and drained through a grid.

 

As Ian says, the pipework under the paving should be solid, it's possible that fine sand from the block paving sun base has got into the drain. More likely they've blocked from the discharge end imo. The solid pipe should also have rodding points at every change of direction.

Oh yes it is well the newer stuff is, that's why the council don't like you concreting stuff. Everything has to be permeable due to flooding.

[Peter]

What do they put under "permeable" block paving?

 

Any compacted layer of sand or mot type limestone isn't going to let any water through.

 

Drives don't have to permeable, they have to have some way of dealing with surface run off without discharging onto the street or into the sewers.

[bankhouse builder]

As said before standard block paving isn't permeable, water coming up through the joints is usually due to ponding of water underneath the paving surface in the subbase and the sand bed the sand gets water logged and turns to something like semolina pudding and starts to come up through the bricks and the paving starts to sink.

The drain if perforated could have roots in in it and the perforated coils can easily become blocked if there is a lot of sediment going down it especially if there isn't a lot of fall.

IMO I would lift the block paving replace the perforated under the block paving with 100mm solid underground pipe and let the perforated pipe run into a roddable back inlet bottle gulley this would act as a silt trap and keep things clear plus there are less joints for any roots to get into on the solid plastic pipe

[Gary Prentice]

Can you compact sand?

 

Serious question, as my understanding of the partical size means that it has a lot of pore space. I also understood that block paving was acceptable under planning conditions/restraints as a permeable (semi-permeable?) surfacing. But thinking about it more, most hard surfacing goes over a compacted base or sub-base that rather reduces the permeability anyway.

 

The onsite builder, not the original contractor, suggested that as the land drain pipe comes in 50m lengths, it may have been convenient to use (inappropriately) with little thought of root intrusion in the future.

 

The next stage is some exploratory digging which I'm going to be involved with as the sycamore is TPO'ed. I suspect the whole lot will end up being dug out and re-installed properly.

 

Thanks for every- ones confirmation of my thoughts.

[Gary Prentice]

As said before standard block paving isn't permeable, water coming up through the joints is usually due to ponding of water underneath the paving surface in the subbase and the sand bed the sand gets water logged and turns to something like semolina pudding and starts to come up through the bricks and the paving starts to sink.

The drain if perforated could have roots in in it and the perforated coils can easily become blocked if there is a lot of sediment going down it especially if there isn't a lot of fall.

IMO I would lift the block paving replace the perforated under the block paving with 100mm solid underground pipe and let the perforated pipe run into a roddable back inlet bottle gulley this would act as a silt trap and keep things clear plus there are less joints for any roots to get into on the solid plastic pipe

 

We think there's a high probability that root intrusion has blocked the pipe. The area where water is coming up doesn't get vehicles over it because of the layout and there's no visible distortion of the blocks, there or elsewhere on the paving.

 

I was wondering if it would be possible to 'bore' the pipe to remove any roots and then use a smaller diameter solid pipe internally, to save lifting all the blocks, but suppose that wouldn't be a practical proposition?

[Ian Leach]

What do they put under "permeable" block paving?

 

Any compacted layer of sand or mot type limestone isn't going to let any water through.

 

Drives don't have to permeable, they have to have some way of dealing with surface run off without discharging onto the street or into the sewers.

Hope this helps https://www.google.co.uk/url?q=https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/7728/pavingfrontgardens.pdf&sa=U&ved=0ahUKEwiFnq7Ao4jQAhXIAsAKHQx2AegQFggPMAE&usg=AFQjCNHRDtwzdIIeE6IV20ppqt7p7ncdKw