Firming up a soft bottom

[NM_Shooter]

 

Knew that would get attention!  I have some questions about pier construction.

I'm curious... when digging the hole for a pier, what is the best way to make sure the bottom of the hole is tamped down and compressed?  Use something to pound it flat?  Put water in the hole and let it sit overnight? 

Also.. I've seen some interesting bell bottom attachments for sonotube.  Do they make such a thing to set concrete blocks on? 

In the past, when I've built a pier for sheds, or whatever, I dig my hole, then hammer 3 pcs of rebar into the ground so that about 8" are protruding up from the bottom of the hole.  I lay in two pcs of rebar in a "x" and tie them to the upright rebar about 6" from the bottom.  I then set my first cinder block perched on top of the ends of the three upright rebar, and tap it to make sure that it is level.  I do the same for the other holes, and make sure that the tops of the blocks are all on the same plane. 

Then I take two long sticks of rebar, and drive them into the ground through the center of the first pier, and leave them extended up as high as the total column height.  Once that is done, I fill the pier bottom with concrete and let that first course set before stacking up the pier height.

As a side note, I live in the desert, perched on top of sand.  Drainage is not an issue here.  Where i will be building a cabin is very wet.  I have avoided gravel in the past, as I was worried about additional compression.  Can I use the same technique in a wet area, or do I need to line the bottom of the hole with gravel for better drainage?

Thanks,

-f-

P.S.  full concrete piers is not really an option, not an easy way to get concrete or mix a bunch at the site.

[glenn kangiser]

You know I would recommend prospecting, NM.  Hiking up and down 40% grades will make things tighter than a bulls bum in fly time.

Crushed gravel will lock together at about a 45 degree angle out from the bottom edge of your support block or pier etc.

Lime is often used to treat clay soil on jobs I'm on  - firms it up so it no longer expands or contracts -- they work about 2" into the ground about a foot deep with a rotovator and pack it, but usually when you dig into undisturbed soil and get a firm bottom, you just want to clean it off and make it smooth -- you don't want to mess with it and make it loose.

[glenn kangiser]

Rather than the bell sonotube bottom for piers - you could use the gravel or sacks of cement and rebar poured in the bottom then set the blocks on it -- the effect of spreading the load is the same.  Pretty doesn't matter -- in this case.

[MountainDon]

I thought this was going to be a new years resolution type of thing. Hints tips etc.   

Pour one-half the concrete for the footer. Place rebar in an  X  or a  #  shape on the wet concrete. Pour the second half on top. Place the first concrete block on the wet concrete footer. Drive a couple lengths of rebar as you stated, or place the bent ends of them into the wet concrete. Carefully level that block and let it sit and cure. The next day you can dry stack the rest of the blocks and fill them with more concrete. Set your top anchor bolt.

A 16 x 16 footer about 8 - 9 inches thick should be good for about 4000 lbs. Tying the blocks to it with rebar and poured concrete should simulate the bell bottom effect without the expense. 

[NM_Shooter]

Pour one-half the concrete for the footer. Place rebar in an  X  or a  #  shape on the wet concrete. Pour the second half on top. Place the first concrete block on the wet concrete footer. Drive a couple lengths of rebar as you stated, or place the bent ends of them into the wet concrete. Carefully level that block and let it sit and cure. The next day you can dry stack the rest of the blocks and fill them with more concrete. Set your top anchor bolt.

Hi Don thanks.  What is the best way to ensure that the top of that block is on plane with the other first blocks in the other holes?  I have always tried to get within 1/4" (better when possible) and it always seems like a ton of work.  I guess this is where the sonotube folks have an advantage.  You can place the tubes and cut them to height!  But for blocks, it seems like you have to do the leveling at the first block in the hole or you are hosed.

I usually use the block-perched-on-rebar for tweaking the height of the first block, but there must be a better way. 

-f-

[Willy]

I have been known to use a post hole digger to dig angle holes at the bottom to get a firm ancor for posts I could not dig deep due to rocks. Not sure if it would help but diging a few holes on angles(like fingers) at the bottom would give you a large foot print, just be sure to put re/bar in the holes to tie them together.  Mark

[MountainDon]

What is the best way to ensure that the top of that block is on plane with the other first blocks in the other holes? 

I agree that trying to get those first blocks buried in the holes at the same level is a daunting task. In my case even worse because the terrain itself slopes and I want the bottom of the footers all 30 to 36 inches below grade.

My solution is this: Dig the footers to the depth needed. Don't worry about the elevation yet. Pour the footer. Place the first block. Build the column to where it's about one block above grade, give or take. Place an anchor bolt in the concrete.

Then using some of Mr. Simpson's post bases I'm placed PT 4x6 posts/piers. Those will all be different lengths ranging from something like a foot to three feet. Then using a reference stake more or less in the center of the layout I'll set the level lines at each post and trim them off. The taller piers will have plenty of through bolted bracing once the built up girders are in place.

I use a central reference point and set the level at each post from there as that results in less error. I rescued a friend who was going around the perimeter from one post to another. He must have read the water level a little high on each one becasue by the time he made it back to the start he was a full inch too high.

[glenn kangiser]

You could rough the blocks in then form enough around the top to laser them in - slide a tight plywood collar up or down a bit until level then pour the top inch or two to grade when you install your anchors.  That way block height could vary a bit but you could still fine tun it.

[NM_Shooter]

Then using some of Mr. Simpson's post bases I'm placed PT 4x6 posts/piers. Those will all be different lengths ranging from something like a foot to three feet. Then using a reference stake more or less in the center of the layout I'll set the level lines at each post and trim them off. The taller piers will have plenty of through bolted bracing once the built up girders are in place.

Thanks Don.. but no good deed goes unpunished.  What sort of beam do you use with this system?  I was leaning toward 3 pcs of 2X12 with ply sandwich as a beam.  Can you still stack that on top of the post?  do you use something like a "T" strap to hold it in place?  Let's see... the beam would be 4.5" plus 1", for a thickness of 5.5"..... does that mate up with a 4X6 post?  (seems close).

I am 6 months from digging a hole and can not seem to keep obsessively thinking about this cabin.  Thanks to all for patience with my incessant posting.

[NM_Shooter]

You could rough the blocks in then form enough around the top to laser them in - slide a tight plywood collar up or down a bit until level then pour the top inch or two to grade when you install your anchors.  That way block height could vary a bit but you could still fine tun it.

This is pretty darn clever!!!!

[glenn kangiser]

I have my moments.

You could just make the collars about a 1/16 to 1/8th smaller than the blocks - nail the top and spread /wedge the bottom down till you get proper grade.  If you are worried about it slipping, drive nails through the side to hit the top of the block.

[MountainDon]

I was leaning toward 3 pcs of 2X12 with ply sandwich as a beam.  Can you still stack that on top of the post?  do you use something like a "T" strap to hold it in place? 

Yes, build the beam using 3 layers of 2x material with two layers of 1/2" ply between.

Yes, such a beam/girder can be supported by a 4x6 (or 6x6) pier. Turn the 4x6 'sideways' so the widest dimension is under the beam. Mr. Simpson makes a couple of different plates than could be used. Any bldg dept would say okay to them. It would also be possible to cut T's out of plywood... I've already laid out a cut plan that has no waste. Don't know what way I'm going to go.

I think those sleeves that Glenn mentioned sound nifty. Would work nicely, but I want to mix the minimal amount of concrete.