Agi pipe drain - advice needed

Hi everyone,

I need to add some drainage in the subfloor area of my house as once the front lawn becomes saturated during periods of very heavy or sustained rainfall the water then moves downhill (it is on a significant sloping block) and unfortunately exits the soil just opposite the subfloor access door which leads to the downstairs rumpus room. There is a concrete path inside the subfloor area running down the length of the rumpus room wall. This path luckily slopes down towards the ideal exit point where I can connect to stormwater drainage outside.

The plan is to dig a 300mm deep trench around the edge of the concrete path and fill it with aggregate and slotted agi pipe.

The soil under the subfloor if not quite sand is very close to it, so my question is what sort of aggregate and agi pipe should I use? I am worried about the fine soil clogging the pipe. Should I use geofabric - thin layer of aggregate - agi-pipe - aggregate? What size agi-pipe and what type, flexible or hard plastic? I need to go around 2 90 degree bends so the flexible pipe would help here. Should I sock the agi-pipe as well? What size aggregate?

Any help would be appreciated. If you need a sketch or photos to help understand this a bit more let me know and I will try to post them.

I would put a layer of agg in the trench 1st, then Geofabric in a U shape. Then 100mm socked agi then cover with agg and fold the fabric over. Then cover the lot with more agg.

Thanks. What size aggregate would be best do you think?

I don't think it really matters. I've used both 10mm and 20mm agg.

10mm or smaller.

Hi Bashworth,

Thanks for the reply. I note that on your website I have read that you don't recommend geofabric for drainage of retaining walls etc, would you still recommend this course of action for this application, where the water movement, once it gets going after a lot of rain, is a sustained flow rather than just seeping down behind a retaining wall?

Also, a general question to everyone - how do you determine what size of agi-pipe to use? Is there any information around that mentions how much water the different sized pipes can absorb through the slots? Due to ease of installation I would prefer to stay at 65mm or below but obviously don't want any overflow from a backed-up pipe.

Have you checked for silt pit requirements?

Over the years I have seen many aggi drains stop working because the geofabric has become clogged by silt, especially with high groundwater flows. If the aggi pipe gets blocked (and this will take a long time if you use a small aggregate pipe surround) the pipe can be cleaned out with a high water pressure drain clearing device. If a geotextile layer gets clogged the only cure is to dig the whole thing up.

Unless you have to drain a very large area 65mm dia will be big enough although a better job will usually be 100mm as larger dia pipes can be laid to flatter gradients.

Thanks for the reply. Just wondering at what point you think a gradient becomes "flat" enough to warrant going with 100mm? The water flows out of the mini trench I have dug out fairly easily at the moment so there is a bit of a fall present.

I will be connecting this drain up to the stormwater system which will be improved with my backyard renovations so silt pits would be part of this. At the moment I just want to make sure the water gets out of the house!