Soil investigation for basement - cable percussion AND percussive window sampling rig?

[saveasteading]

1 hour ago, Alan Ambrose said:

just a matter of convincing a BCO that we've tried hard to do SUDS I think

In my view ( if I was the bco and interested in the subject), you should  show how flooding downstream  will be no worse than before, perhaps less bad. That is basically what SUDS is.

But chucking rainwater into a ditch will likely speed the water towards its destination, and so would be 'a bad thing'. Can you slow its journey? Barrels, swale, french drains, lagoon, limited flow outlet to ditch?

Cleaned sewage goes on a different loop and perhaps they join at the end.

[George]

Depends on how on the ball the Lead Local Flood Authority is but you might need to attenuate the outflow with a orifice chamber or hydro brake. 

 

Although usually just calling the ditch a 'swale' and the drain a 'overflow measure' will do the trick.

[Gus Potter]

On 21/03/2024 at 11:31, Alan Ambrose said:

Just to report back: the ground turned out to be quite hard boulder clay and the window sampling rig, which turned up first, only got down to 2.5m. So, the cable percussion rig was a good call and we moved 2x GW monitoring to those holes instead.

 

We also dug a percolation test pit by hand and the 300x300 hole in the bottom started slowing filling with water - and then kept on going. So, a sort of reverse perc test. A week later it was at 67cm in the main 1m deep hole! (It is protected from direct rainwater entry.) Our theory is it's using the slight sandy deposit to convey surface water. So, we'll need v good surface drainage for the winter and we're fairly close to proving that soakaways are out of the question.  

That's a good description.. reverse percolation test!

 

Clay soils can be quite variable. In a site investigation report you'll often see that they captilise the dominant component.. like.. silty sandy CLAY. The clay in capitals means the dominant component is clay. Also when you are sampling and doing boreholes you sometimes see a blank bit in the site bore / sampling record.. often described as "no sample recovered".. this is a big red flag in some respects, mainly as it relates to the design say of a basement and how you stop it from floating like a boat.. what is it (the no sample recovered part of the log) and how much water can it convey. On the other hand it could be a gift than can be taken advantage of in a drainage design if it is indeed a permeable layer of soil.

 

On 21/03/2024 at 12:23, saveasteading said:

Don't assume that yet. What level did the groundwater rise to?

Now there are soakaways that we a famiiar with. You dig a trench, fill it with gravel and a perforated pipe and the water goes downwards never to be seen again.. problem solved. But sometimes you can design them so that the water goes sideways and mixes with the natural ground water. This can work well where the additional flows are low. Often less so for surface water unless you can attenuate.

 

Funnily I did this on a self build on clay many years ago where I just added the water from the septic tank to the natural horizontal ground water flow. However you need to install it really carefully and it may some maintenance later.. it's often not a "do and forget solution". At the back of my mind is that the climate is changing.. more rain coupled with long dry spells so anything you do needs a bit of future proofing rather than just doing something that will pull the wool over the eyes of BC and the Environment agency say.. that a lot of folk are inclined to do.

 

To be able to design a "sideways" soakaway you need to establish the hyraulic gradient over the site. Simply if you dug 4 holes and took levels of the ground surface and the static water line in the holes after say a week you can plot that on a drawing to give you a rough 3D idea of what is going on under the ground... the hydraulic gradient. Yes you may have say ditches nearby but they won't give you all the info you need.

 

Alan.. To test this principle before embarking on detailed investigation.. what if you fill the hole you have dug from a bowser to bring the standing water up by say 200mm and then see how long it takes to revert back to the standing water level you see at the moment? It's not something that we could rely upon but it would let you see how something could work?

 

Now if you have that "no sampled recovered" bit in the site investigation log .. it could be a great permable layer at a decent depth that you can tap into to get rid of your water.

 

For all.. on a technical note.. I use the word permeable. This describes the overall behaviour of say a layer of soil. Say you have a layer that is made up of quartz gravel and sand / silt. The particles themselves are not porous.. like granite stones in a wall ("rising damp").. it is the gaps between the particles and density of compaction that is important and when we look at the layer as whole we call this permeability. Now the same principles apply when we are designing concrete for a basement so it is watertight. We want to reduce the cavities and the crack widths in the concrete. We reduce the cavities by selecting the right mix of concrete and reduce the cracking by using rebar and that makes the concrete more impermeable.

 

Often if you have a house that has no basement then the problem can be decoupled. But if you have a basement then you need to make sure that you are not going to channel water back towards the basement which is in effect a large sump.. that you will have to pump out or drain in some way. Also..if your water from the septic / digester tank / car parking area ends up (although diluted) next to your basement wall then you could alter the soil PH and sulphate content and that could cause a problem as when we design concrete under the ground we consider sulphate and PH value. Never mind that.. you don't want stinky toilet water against your house .. often happens if your visitors / relatives behave badly in the bathroom.

 

In the round if you look back over Alan's thread he is taking time to investigate and plan each step. This is good design.. you learn a bit.. review what you know, plan and execute the next step. It takes time but it's the best way of spending you money wisely.

 

In the round if you have a flat site say Norfolk / Linconshire or up in Scotland round about Stirling for example where you have these types of non consolidated clay soils (the list is quite long) then while you can get the structure to work you have to get the design to perform holistically. No point in say an SE doing a basement or a heavily loaded found just for it to get messed up as you can't get the drains and services to work.

 

For all. Alan's job I think is a good example of how if you work through the issues step by step in a holistic way then you don't waste your money.

 

A duck pond! I built a few for my self build and kept ducks.. etc. Some of the ducks were negligant mothers at times at times when broody so we would take a few eggs and stick them in the incubator. Once hatched we stuck them in the bath to help oil their feathers up, then later introduced them back into the adult population. It was great fun. The point here is that there are often solutions to rain and foul water disposal that can be fun. The SUDS guidance can be flexible once you get your head round the seven or so hundred pages!  The pond provides attenuation, dilution, habitat, a potential hydraulic head to drive water downwards and you can have a flow / oriface plate to regulate say discharge to a drainage ditch.

 

For me I'm an optomist, on a self build see what the options are that compliment say the type of garden you want, planting etc and how you want to live.. sometimes you have to live in a different way, learn new stuff and that can be fun too.

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited March 23 by Gus Potter
Typo.. I think?

[Alan Ambrose]

>>> To test this principle before embarking on detailed investigation.. what if you fill the hole you have dug from a bowser to bring the standing water up by say 200mm and then see how long it takes to revert back to the standing water level you see at the moment?

 

Very clever, I'll do this next time I'm at the plot.

 

Will run the duck pond idea past the wife