foundation for clay soil?

[redsonya]

Hey Folks,

Im new here to the forum.

I bought a piece of land out by the Olympic Penninsula in WA
last year and Ive been doing as much research as I can.
I want to build a 200 sq ft cabin but the soil is mostly clay
and I had wanted to originally build on pier blocks however now
that I discovered clay soil Im wondering if I should go with deeper footings?
What do you guys think? Most of the information I have found has been based
on foundations for houses in areas that freeze but the cabin is a lot lighter then a house and my area
is not in a frist zone but it rains and rains and rains..

So any suggestions would be welcomed and appreciated 

[Squirl]

My view is if you want it to last, put the footings below the frost line no matter what the soil.  There are exceptions to this, but I don't think they would apply for your purposes.  It sounds like that wouldn't be to deep.  Going wider or with more footings is for difficult soils, not usually deeper, unless you are going to hit a different kind of soil deeper.  If there is no frost line where you are at, still go below the layer of topsoil.  This is filled with organic matter and not really suitable to build on.  With a 200 sq. foot cabin it doesn't sound like you would exceed the weight limit of just a few good piers.  I assume you meant the precast 8 " blocks.  You can build or pour a slightly larger footing for them to sit on top of if you have very difficult soils.  A 12-16 in. block under each would be more than enough.  16 in. is the minimum for the worst quality soils under the guidelines.  You could also use concrete pavers to spread the load across a greater area.  For an extra days work and $50-$100 you could probably alleviate your fears of shifting for the rest of the buildings life.  If the spot you are building in turns into a swamp when it rains, you might want to try a different spot or do some site work to grade the area to drain water better.

and 

[glenn kangiser]

Be sure that you keep in mind that in clay soil any hole becomes a tub of water. 

Gravel or sand is porous so any water that gets in will be staying there for quite a while if it is used as backfill depending on the actual properties of your particular clay.   Drainage around the cabin to prevent water from getting under it and gutters can all help.  Possibly concrete piers could be good or concrete around foundation grade treated wood though I would prefer solid concrete for dug footings in clay.

[MountainDon]

I don't know a lot about clay other than as Glenn pointed out water makes it unstable. I believe a full perimeter, steel reinforced with steel bent around the corners, poured concrete footing is the starting point. Wide enough and thick enough to be a monolithic ring. From that one could build a block wall, a poured concrete wall, build concrete block piers, pour sonotube poured concrete piers... with any of those tied into the footing with rebar, bent and embedded in the wet concrete footing with el's up into the wall/pier.  I believe those would give the best chance in clay.

[redsonya]

Thanks for the input, I like the idea of the concrete pavers making the load bearing surface wider and more stable we use this in scaffolding in the form of base pads for outdoor stages at work also too bad they dont have screw jacks
I can put through the pier blocks so I can adjust the foundation depending on the season the swelling and settling of the clay..

I have also been thinking about drainage originally I was thinking of a basic French drain but, as pointed out before clay soil just loves to hold on to water so Im wondering if I should try to change the soil under the foundation first and compact it and try to build it up a bit so the water would run down away from the foundation as it likes to gather at the surface.. I was thinking crushed gravel or larger crushed rock thats of a builders grade..

any ideas?

Thanks again.

[MountainDon]

Try and obtain advice from good local foundation contractor; ones who have been in business in that locale for some time. I've noticed that some parts of the country have lots of companies specializing in foundation repairs. Parts of TX, for example. Somebody like that would know a lot about what works and would have seen a lot of what has not been successful.

[glenn kangiser]

You cannot change the soil type under the foundation to make that clear.  

I assume you meant on top of the clay or using the clay and grading away from the building pad.  Any rock and sand in an excavation will become a bathrub in the clay full of water and sand or rock, causing the clay under it to swell and move even more.

A french drain around the site draining water via a drain pipe and sloped down hill draining to daylight can keep the building area more free of water.  French drains under it if used would also have to drain down via a sloped trench and drainage pipe.  

Good clay will nearly always hold water and not allow it to leave from an excavation unless a physical ditch sloping down hill with a french drain in it or not as in a perimeter drainage slope or ditch is provided.  If the site is graded properly away from the house then the clay will also help to prevent water penetration into the soil under it, provided it is not extremely expansive and deeply cracked.

Note that I paid an engineer thousands of dollars for this advice in a similar situation but on flat ground for a large building project.  Other experience is from my hard clay soil in the mountains.  As Don mentioned, a local engineer will know most about your clay or soil type.

[redsonya]

All good advice

Also there are others building cabins in my area luckily no weekend microsoft warriors but
mostly outdoorsmen so I will also go out there and ask them what their plans are and for the ones
that are already built I will ask how its been working for them.

The weather here is a constant not too hot not too cold so luckily the clay will not dry and crack
however it is earth quake territory so some reenforcing might be a good idea as I can see clay turning
to soup and me and the cabin headed down hill 

[bayview]

   You may need to stabilize the soil or build on pier and beam.   If building on expansive clay soil.   Piers must be to dug to bedrock.   

/.

[diyfrank]

I wouldn't be to worried just because its clay. The clay in Washington is a lot different then the clay in other places like Texas.

Is your soil the hard sandy cemented clay or the soft gooey stuff? Most of the houses built here are on clay and use a standard perimeter footing and 2" wall. Most of the good ground I've seen you only find in the foot hills (sand/gravel).  The Glacial till around here is mostly clay but very hard and is what almost everything gets built on. The clay in the valleys and farmlands get pretty gooey.  Are you saying you're on a steep hill side? They get some land slides from time to time, that would be my main concern.
A 200 sq ft building is fairly small.

[redsonya]

Thanks Frank

Nice to see someone from Mt.Vernon here, I grew up in Bellingham now in Seattle after living in Norway for years and I really cannot wait to get out of the city and build my little cabin. Seems like a few of the cabins near me have been built straight on the ground with pressure treated timber and another guy is overhauling a shipping container up the hill. Luckily it is the harder cement style clay you were talking about and the guys down the way from me say its totally ok to build on but I was still concerned because the land sits on an old logging grade between the ocean and the olmpic national forest it hasnt been logged in 70 years since the park became protected but I still wonder about what the logging has done to the soil in a way what I would like to do is plan the placement of my little cabin and out buildings wisely and then plant some cedars and hemlock (the local trees) around the perimiter to protect and help hold in the soil as well as to just balance my humanity by planting trees because they are more of a best friend then a dog could ever be. We have made shelters fire clothes food beer books and medicine from trees and it just seems the right thing to do for them and for us.

[Pine Cone]

 

Our project can't be that far away from yours.  Ours is south of Port Townsend.  We are on soils made from glacial til with a high clay content in places, including where we dug some of our piers.  So far no problems, but the cabin has only been up for a bit more than a year now, so even if there were problems we might not know it yet.

You can find out more about what we did here...
http://countryplans.com/smf/index.php?topic=8030.0

Things we did learn along the way...

Careful site evaluation pays huge dividends.  Our final cabin site is on a just ever-so-slightly higher place. 

We dug a french drain across most of the property above our cabin site and that dried up a wide variety of problem places downhill of the drain. 

We also dug a french drain around 3 sides of our cabin and deck foundation.  That french drain ends 2-feet beyond the edge of the deck on one side, and follows a natural drainage away from the foundation on the other side.

We have fairly large overhangs on all sides of the roof, at least 2 feet beyond the foundation holes, so little if any rain falls inside the area defined by the french drain.  Rain gutters take the water from the room and dump it below the cabin and deck. 

Most of our neighbors have built on skids or on foundation blocks just laid on the surface of the soil.  Most have been there for 30-40 years with no apparent problems.

Most importantly, we need some pictures of your property    Easier to answer questions with more knowledge about your site, including tree and vegetation cover and what sorts of topography and slopes you are dealing with.  Besides, we all like to look at the pictures....

... I was still concerned because the land sits on an old logging grade between the ocean and the Olympic national forest it hasn't been logged in 70 years since the park became protected but I still wonder about what the logging has done to the soil...

If it was really logged 70 years ago, that predates chainsaws and lots of the bigger logging equipment.  That also probably means it has only been logged once, and compaction and other negative impacts will be pretty minimal by now. 

If a steam donkey or rail system was used you can probably still see the skid trails created by cables pulling logs in a fan-like radial pattern.  The old skid trails will be about 6-10 feet wide and will be compacted.  Often the soil will be raised on either side of the skid trail, making the  trail a drainage ditch part of the year.  After 70 years, even the compacted areas shouldn't be too bad.  I once lived in an A-frame that was built next to a skid trail created somewhere between 1890 and 1930.  You could tell what it was, but it really wasn't a problem.

As a forester, I'm all for planting trees, but you should find out what soil type you are on before you worry too much about needing to "hold the soil" near your cabin.  If you have deep-seated soil stability problems trees alone won't help much.  I would be more worried about historic and recent road construction and whatever site grading and modifications you plan associated with building.