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Building a cob house, need help with drain trench  RSS feed

 
                                        
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So, I am building a cob house and the first step is to dig the drainage trench around the house. We have a bit of a problem. There is some mountain run off during the spring (now)  that is pretty consistent. It hasn't stopped flowing.

My question is if we do the french drainage trench normally with a french drainage curtain trench as well, is it bad for your trench to act as a diverter trench? Meaning if there is a constant small flow through the trench in the spring time will that degrade my trench and allow my foundation to sink, crack, shift, ect...

It isn't very fast but it is consistent but of course will run faster when it rains. Also our water levels are higher this year than they have been in a while. A small creek is a big creek right now due to how much rain we have gotten.

Primarily there is some top soil for the first 3 inches and then clay as far down as we can dig. My frost line here is 3ft. so the minimum depth of the trench at any point will be 3 ft. Hope this information helps you to help me

Any and all ideas will be welcomed and appreciated.
Thanks.
 
gardener
Posts: 1244
Location: Okanogan Highlands, Washington
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books cat dog food preservation hugelkultur
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Ernie thinks that your trench should be fine as long as you put a sufficiently large pipe in the bottom of it, and of course a lot of good large gravel. You will also want a nice big curtain drain uphill of your house, to divert as much of the runoff as possible.
You want the house itself sitting on stable, dry earth as much as possible, with the drains and eaves protecting that dry pad. 

If that just isn't reality where you are, then the gravel trench footing allows the whole house to respond as a unit to any swell or shift in the ground.  Small, round houses do this better than large cornery ones.  And it might pay to make the dry-stacked stone courses of the wall a little extra-tall, just in case some settling does occur.  With cob you can patch almost any crack once it's stabilized, but of course it's nicer not to need to.

Before proceeding:
If you have the option of building on a saddle (small rise where the water is naturally diverted to both sides) you will have a much better chance of a stable foundation and a dry house.

We've seen cob sites where people really wanted to put their house in a hillside like a hobbit.  That's going to be a damp root cellar, not a cosy cottage.  First be sure that you are siting the house on the best-drained part of your available land, and then do what's necessary to protect it from any unavoidable runoff. 

If you want a more detailed consultation, feel free to email us, or give Cob Cottage Co a call.

Yours
-Erica
www.ErnieAndErica.info
 
                                        
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Erica and Ernie,

I can not thank you enough for responding!

To give you a better idea of the property the land is multilevel. Up above us we have a steep mountain then a smaller 2000 sqft area half circle coming out completely level (this is where we plan to build). From there it drops about 10-15 feet to a lower level, then drops 6 feet to a lower level and then to an even lower level where the creek runs.

I was thinking last night about running a separate trench from the very bottom of the steep mountain (the part that meets the land where we want to build) and having it wrap around the steeper mountain and diverting it to my medicinal herb garden to help water ect. Then of course having a separate trench for the house (curtain trench too). Are there any flaws in that thinking? Would it eat away the trench since it would be a drastic change? The runoff isn't fast, but I do understand the strength of water. What do you think?

We have natural sand, small rocks to large rocks. Do you think it would be okay to use large piping and fill in with medium sized rocks instead of gravel?

Also are there any other tips you can think of?

We helped build a cob house this past summer and periodically have gone back to help plaster ect, but in my short life I have realized experience is priceless, however I am very young and wisdom is something I have yet to acquire.

Yes as beautiful as a hobbit house may be, we opted out of doing that with cob We were thinking it would be fun to reuse an old shipping container though!

Thank you so much,

Joelle


 
pollinator
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my FIL built his house over a small spring and drained it with pvc to a pond near the house..the water ran all the time under his house but never near the foundation except where it went through the pvc under the foundation.
 
Erica Wisner
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And come to think of it, my grandma's homestead had a spring in their basement, which she loved because it kept things cool. Her folks were less than thrilled, but it's definitely a situation you can deal with as the house is still standing.

As long as you are catching as much water as you can, and diverting it around the house, you have the right idea.  Plan for bigger rain events than you would normally think of - it's not the day-to-day weather that will give you trouble, but those heavy flood or flash-flood events that happen less than once a decade.

If you are filling the trench as a curtain drain right away, with rock and pipe, you are building a channel that's as erosion-proof as we know how to make them.  You will definitely get some muddy runoff and erosion if you are digging trenches in the rain, but if you are immediately laying pipe and rock, the damage should be minimal. 

Where it comes out to daylight near your herb garden, consider doing a little cobblestone 'spring' as a feature to both contain erosion and give you access to a small pool of water before it drains off through the soil.  This will also let you monitor what's happening in your drain trench.  I don't know how steep things are, but the fact that you are planning to build on the level part and thinking about multiple drains to divert the water to more useful places, shows that you are on the right track.

Any rock bigger than about 2" should be fine.
Only problems I can think of with rocks instead of gravel is:
1) Making sure they are compactly settled before you start to build, yet allow plenty of 'air' space for water to move through, (round rock can be harder to pack than gravel) and
2) Making sure that no big rock corners are directly pressing on your pipe that might cause it to break or move out of alignment.  You can put some smaller rock around the pipe, or carefully line up the big rock to avoid the point-pressure situation.

If you have pointy rock, you are essentially doing a rough-and-ready fieldstone foundation in your trench.  Nothing wrong with that (some would consider it an upgrade even).  Just may take a little more fiddling.

-Erica and Ernie
 
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