Trial pits and water table

[Jomider]

"When is a soakaway not a soakaway... when it's a hole" I get that - thanks Tony for the post and for all the excellent material on the site.

I was all set up to build a nice land drain with a 0.6m3 aquacell soakaway. But then... my site investigation

My question is this - I dug a trial pit and left it overnight - I didn't go to the full 1.2m, just 0.5m and about 20cmX20cm

When I dug the pit the subsoil was really quite dry, so I was hoping the water table was deeper

Anyway this morning, there's still over 30cm of water in the pit - the level has only dropped by 18cm.

Does this definitely mean

1. I have hit the water table at 18cm below surface, or

2. The soil is just not very porous (I know...it's clay!!)

I'm thinking that if it's 2., a deeper hole could generate more hydrostatic pressure - 0.2-0.3 Bar and drain, or even better I may hit a more porous sublayer.

If it's 1., then the I assume the project is off.

Any thoughts please.

[Pablo]

It's not something I could give an answer to without seeing the site itself. When you dug the hole were you careful not to puddle the soil if you did mash the wet sub soil up this could be whats preventing the water getting away.

[Jomider]

Thanks - the sub soil was quite dry and I just dug it straight out pretty cleanly.

During the day the water level has dropped by just under 1cm per hour, so I think it might be just impermeable soil...

I have dug another pit to about 1m to leave dry overnight and see if it fills with water - so far still dry after 4 hrs. Not sure if it will fill up if it's below the water table as the soil is so impermeable, but it might get damp I suppose.


I'm hoping if the groundwater is way below, I can dig the 3 Aquacells in vertically to give me a narrow 3m deep soakaway with high pressure at the base if it does fill up.