Sono Tubes in clay with high water table

[tmcmurran]

A few weeks back we were about to put down our piers, only to discover water at the 4 foot level.  We need to go down 4 to 5 feet due to the frost level in our area.

So far we have trenched one side of the building lot and an end to allow drainage and plan on installing a french drain type system on a crushed stone bed with weeping tile to be sure.

Question is, with it starting to get late in the season and wanting to get these piers into the ground would we be looking for trouble if we were to pour before we had the area dried out enough to prevent any additional movement?  Would adding a 6 inch crushed stone pad under the bases help?

I have a few images of the current trench, and a decent image of the soil type but do not see an option to attach files here

[MountainDon]

HOW TO DISPLAY IMAGES IN A TOPIC

[tmcmurran]

Ok, lets try the old Photobucket route

Current trench which has been draining for close to two weeks.



General slope of lot



Close up of what could be called soil

[MountainDon]

Q:
Is it known for sure that french drains will dry out the soil, or is this simply a hoped for result?
Is the soil moisture expansive?

I'm not a soil expert but I know enough to know that high water tables can be a problem with foundations. It may be that a pier type of foundation is a very poor choice as wet soil will generally have very low resistance to lateral movement of the piers as well as a low ability to support a vertical load.

The correct answers may require the enlistment of a geo-tech or soils engineer. One with local knowledge would be desirable in my opinion.

[tmcmurran]

Q:
Is it known for sure that french drains will dry out the soil, or is this simply a hoped for result?
Is the soil moisture expansive?

Others in the area have used both with satisfactory results while some have just back filled and laid a bed of crushed stone and used "elephant" feet for footings.  Not something I want to do for a 24x40 with loft.  The water issue came as a surprise to everyone in the area since we are higher than a few of the others on either side.

I'm not a soil expert but I know enough to know that high water tables can be a problem with foundations. It may be that a pier type of foundation is a very poor choice as wet soil will generally have very low resistance to lateral movement of the piers as well as a low ability to support a vertical load.

This is our concern as well due to the weather conditions here.  Subarctic where temperatures can sit well below -45c -50c  for weeks on end.

The correct answers may require the enlistment of a geo-tech or soils engineer. One with local knowledge would be desirable in my opinion.

The area "seems" to be slowly drying out to the left of the first trench now.  Others at the lake level have installed french drains with good results over 18 years with no movement on piers.  Now that is not to say we will not have issues though.  Seems like every lot is a little different.  Glacial till and clay.  We just won the lottery with clay

[MountainDon]

Clay sucks.    Lots of expansion and shrinkage with moisture changes. Little support when saturated. Checking with a soil engineer might be a wise first step. 24x40 is a house not a cabin    and IMO, deserves a careful study before forging ahead with Sonotubes.

[Huge29]

I think Don may be on to something here.  I would engage a soil engineer company.  Certainly not a desired expense, but when you consider that your project is an investment where memories will be created for multiple generations you won't ever regret spending a small percentage extra to do it correctly.  This is not a weekend project, but somewhere where you will go for decades and enjoy the pride of a job well done. 

[tmcmurran]

Well once we started to see what we were getting into we pretty much put a halt to everything.  I work at a mine and have access to the right people.  I guess I need to bring a sample in and see what they say.  What really gets me is the approved septic plan test holes "showed" layers of both sand and small areas of clay with no water to depths of six feet.  Makes you wonder if they even drilled the test holes in the first place or just sent out the invoice....

[MountainDon]

Does make you wonder.  """  On the other hand I have seen soil vary considerably within a distance of 500 feet. Our property has a huge deposit of pumice under the top layer. Our neighbor to the west ran into a large amount of hard dry clay when he went to dig a pit. I've never seen any on our property. 

Sounds like there should be someone at the mine who could give a good opinion on your local dirt.

[tmcmurran]

Does make you wonder.  """  On the other hand I have seen soil vary considerably within a distance of 500 feet. Our property has a huge deposit of pumice under the top layer. Our neighbor to the west ran into a large amount of hard dry clay when he went to dig a pit. I've never seen any on our property. 

Sounds like there should be someone at the mine who could give a good opinion on your local dirt.

I have also sent an email off to the survey company which drilled the test holes for the septic system to a depth of six feet to find out why the conditions vary so much from the excavations and the drill locations.  After all we dug up on two of the three test sites which reported no ground water, glacial till and sand with a 2 inch layer of clay at the five foot level.  Not all the way down through the entire area.  Waiting to hear the story they have for that.  Maybe they were lucky and hit the "good" spots :-)

[UK4X4]

wow- that looks way worse than mine !

Mines similar to that in spring I have the same grey mountain topsoil , same type glacial bolders- with a mix of sand silt and clay with water at 5-6ft, but right now its dry and toasty after the summer, ie with me its spring run off from the mountain above

when the soil is that wet - it has no sideways support and things will move

My soil report and test holes were money well spent- and outlined what would or would not work

In my case I had to over dig the whole foundation area 4ft deep backfill with crushed stone and multiple drains,

I presume from the pics behind the plot is a hill ? or road ?

To be that wet you must have water comming from above

A deep trench- wall higher up to divert the flow maybe required- if your fixed on posts....I can send you my soil report as an idea of what they recomended for me

[tmcmurran]

Well after getting a little deeper into things it appears that what will be required to ensure a sustainable structure is to remove most of the suspect material.  Install weeping tile under the proposed building area as well as the perimeter to keep any intrusive water from undermining the area.

Not what we wanted to face right off the start, but  estimates came back pretty decent.  Roughly $3,000 to $3,500 CAD to dig down and remove 3 to 4 feet of the saturated clay over an area roughly 30x50.  Install the weeping tile, lay crushed stone and back-fill with class B and sand.

So much for the tube idea now.  Looks like proper footing with knee wall.

The joys of building your future

[Ndrmyr]

Nothing, but nothing is more important to the longevity of our projects than our foundations.  In building my lake-side cabin, I poured a canti-levered reverse wing seawall, backfilled with crushed stone and compacted prior to pouring my slab.  It has withstood a 100 year rain (18" in about 30 hours) without a whimper, and not a single crack.  Originally I too was going to use sono-tubes.  ( I still have them and the bigfeet for the bottoms)  I believe that I made the right long-term decision.  It wasn't easy to invest into 47 Yds of concrete, but, it and the cabin on it is built to last and last.  You are making the right decision.  Far better than having your structure twist and slide.