Search the forums

Loading

Footers and bedrock

semipro's picture

First, sorry if this has been covered.  I searched the archives but did come up with a similar discussion.  However, if I've missed one, I'd appreciate a link to it.  


I'm building a T-type foundation for my 2 story garage.  The foundation will consist of a stepped, rebar reinforced, poured concrete footer with reinforced, filled block (CMUs) walls with a reinforced concrete slab above.  The site is sloping about 6 degrees and quite rocky which is one of the reasons the stepped footers makes sense.  


My question concerns how I build my footings around the large slabs of bedrock that I've encountered.  For the most part, is was able to excavate below my frost depth (24") in the trenches.  However, about 25% of the area of the trenches contain outcroppings that extend up into the trench by about 1.5' on average (to a depth shallower than the frost depth).  I need to build my footers around these outcroppings.   BTW; the soil and rock outcroppings on my site are structurally sound and suitable as foundation support.   I've consulted many references, including Taunton books, other books, the building codes, the local building inspector, engineering forums, and others.   I'm encountering a lot of conflicting information.  I want to appease the inspectors but I also want to build a bulletproof foundation.  The three options are basically these:


Option 1.  Per IBC, and my building inspector: isolate the rock from the footer using a thin sand layer (like 1/4")  or 6 mil. poly sheet and pour the footing over and around the rock.


Option 2.  Per some references, pin the rock to the footer using short sections of rebar, then pour around the rock.   This seems to conflict with Building Codes that preclude steel reinforcement within concrete lying within 2" of soil (where the term "soil" apparently includes rock).  Locally at least, engineers do not presribe pinning, maintaining that movement between foundation and rock should be allowed.   


Option 3.  A modified version of Option 1 but use a thicker layer of sand between rock and footer (more like 2").  The thought being that the sand will act as a "cushion" when differential settling occurs and more evenly distribute the force between ground and footer.    The concept of differential settling is that the added weight of the building sitting on the footers will compress the soil more than the bedrock causing a stress concentraion in the footer near the transition between rock and soil underneath.  


4. Others I haven't covered?


So, pin or don't pin?  If not, how much sand should be used to ioslate the rock and footer?  Or, do something else.


Please don't tell me to consult an engineer or geologist.    I value the knowledge that registered professionals have to offer but like to temper that with insights from experienced professionals (yeah, you guys) about real-world observations.   Unfortunately, the problems are usually buried and many foundations are over-built enough to compensate for design flaws. 


Thanks in advance for any constructive discussion and advice that this prompts.


Cheers, Semipro

(post #102008, reply #1 of 36)

Too bad you exclude an engineer 'cause that's who can give you the best answers.   (Yes, I am an engineer.................and a GC.)


Footings are sized based on the load carrying ability of the soil (e.g. you need a wider footing on sand than you do on rock).  Foundation depth is based on frost lines so you don't have heaving forces trying to break the footing and foundation walls.


Is there any chance that you're over thinking this problem?  It may be less expensive in the long run to just jackhammer (or even blast) thru the rock then form and pour one type of foundation rather than changing the design as you work your way around the perimeter.

(post #102008, reply #2 of 36)

just pin the rock into the foundation, thee term soil in relations to rock is wrong.

(post #102008, reply #3 of 36)

Option #2

 


 


Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming


WOW!!! What a Ride!


Forget the primal scream, just ROAR!!!

"Some days it's just not worth chewing through the restraints"

(post #102008, reply #4 of 36)

correct me if I missunderstand the situation.

It sounds like you have several knife edges or miinimountains of ledge upstrusting into and bey0ond your footing space.

If that is the case, I would use the jackhammer to remove those promontories so I could make the footer as consistant as possible'

We have a lot of ledge here and the worst condition is where a footer transitions from the ledge to soil. That soil must be compacted to nearly the same load bearing capacity as the ledge or risk trouble in the furture as settling induces stress across that spot.

So yours would have numberous changes - too much chance for trouble.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #102008, reply #5 of 36)

Well, seeing how your inspector and the building code both dictate Option #1, and you aren't willing to pay an engineer to sign and seal (and therefore take responsibility for) any other option, why is there even a question???  


Bob

"Brilliance!! That's all I can say- Sheer, unadulterated brilliance!!" Wile E. Coyote- Super Genius

(post #102008, reply #6 of 36)

I've never done anything other than pin the rock. which is noraml practice on the mountain sides of West Vancouver BC....not sure what they do where you live.


I've done a ton of work on rock, some work has taken an entire crew a week just to do the footings, the rocks we deal with aint gonna move, if they do the whole mountain and everything with it is coming down. 


if you have loose rocks it does make sense to remove them or deal with them in other ways.

                                                 

(post #102008, reply #7 of 36)

All: Thanks for the replies.  Here's some more info:


I've already consulted an engineer, actually more than one.  Their advice also conflicted which is why I wanted to get advice from those that come from a more practical, less theoretical background.  Just because an engineer is willing to stamp something doesn't means its the best way to do it.


The rock is very substantial and would be quite hard to remove.  It lines at least 25% of the trenches, in some places its quite flat.  It just extends above the frost line.  That is why I, and others, see no reason not to use it as foundation.  Why chisel or blast out what basically constitutes a natural footer? 


The question then becomes how to best connect the bedrock to the constructed footers.  As I expected, the answers so far are mixed.  Unfortunately, some of those that replied failed to give a substantiation for their advice.  Sorry, I guess I should have asked for that. 


One of the reasons I picked this forum is because I know I'll get more than just "do it that way because that's the way we've always done it" or 'do whatever the inspector will sign off on".   I want to do it right...and for the right reasons.

(post #102008, reply #8 of 36)

what better way is there to connect it to the bed rock than to pin it?


I think you believe this to be the case....without going back to your 1st post. teh other scenarios you mentioned treated the rock as if it wasn't a stable foundation platform...is that true?


if you have loose rock it makes sense to move it or create a big enough footing over top of it that your wall will not be impeded by any outcroppings.


IMO pin it....because it is done that way....it is done that way because if it is bedrock that aint moving, pinning to the bed rock is the sure way to make it so your walls won't move.


why anyone would want to put sand or plastic between the two is beyond me....


but that is just me and my experience.


PS...some of our walls where the rock has been flat enough, our kick plates have been directly to the rock. other areas we have made a flat (but not always level) footing. I have formed on alot of rock, there many different approaches to getting your walls onto the rock. But pinning it, has always been part of it.

                                                 

(post #102008, reply #9 of 36)

Keep in mind waht Piffin said about transitioning from soil to ledge. Remember you are only down 2' or less. If you have a clay that is expansable you will get lifting or settleling as the clay expands & contracts. At the same time the part on the ledge will be stationary.


I would be concerned about that transition to the point I would probably excavate to the stone all the way around even though it made the foundation more complicated. Then pin it all.

If you have a problem, don't just talk do something to set it right.

  Jim Andersen

(post #102008, reply #10 of 36)

"I would be concerned about that transition to the point I would probably excavate to the stone all the way around even though it made the foundation more complicated. Then pin it all."


I say yes to that....we always shoveled, blew ( with air blower) and swept away all the loose dirt.

                                                 

(post #102008, reply #15 of 36)

I have a foundation comming up with somewhat related conditions. At this time we know the rock is stable but haven't determined the pitch or regularity as it goes back into the bank. I plan to pin to the rock but have another question.


Why the concern over forming tightly to the rock? The similar ones we have done in the past we set the forms as close to the rock as was easy then poured a foot or so of concreate & let it oze under the form. Then when partially set finished pouring the wall. Yah' it was a cold joint but is that a problem where it isn't visable and is clean?


We essentially poured the footing so that at it's thinest point it was sufficently thick & leveled the whole footing to that. Then adjusted wall height/platform elevation from that point.

If you have a problem, don't just talk do something to set it right.

  Jim Andersen

(post #102008, reply #16 of 36)

Why the concern over forming tightly to the rock?


We don't consider gaps under 3/4" to exist.  Maybe if someone's pouring soupy it would be a problem, but that would be the least of their problems.  Structurally, cold joints are fine, long as there's rebar across it.


I've got a partial rock footer under a wall on my next house.  BI had no problem when I proposed it.  A little dirt outside the form and minimal "ooze".  3-4" slump works well.


PAHS Designer/Builder- Bury it!

PAHS works.  Bury it.

(post #102008, reply #11 of 36)

Why chisel or blast out what basically constitutes a natural footer? 


When it is cheaper and faster than forming steps in your footers.  At $150/day rental the hammer on the skidsteer make quick work of this type of rock & its a time saver & you won't need to keep thinking about it.


 


 


 


Treat every person you meet like you will know them the rest of your life - you just might!
Treat every person you meet like you will know them the rest of your life - you just might!

(post #102008, reply #12 of 36)

What kind of rock?

What kind of soil surrounds it?

Are these "slabs" of rock continuous or isolated from each other?

(post #102008, reply #13 of 36)

I was faced with the same problem in a house I was building in NH - I planned on pinning the footer per option #2.  After looking at the hassle of pinnning and trying to get forms to fit and an uneven basement, I determined that it would be best to blast - Bl;aster came in - overblasted about 1 " deep and I backfilled with 1" crushed stone.  Best decision - basement is dry as water has a good path to run off and while plasting had them put in trench to bring out the sewer pipe.  At a cost made things much simpler and better construction.

(post #102008, reply #14 of 36)

Bump.

Still thinking about this one?

(post #102008, reply #17 of 36)

1. You do not need to worry about the bedrock being above the frost line. It is not subject to frost heave, thus there is no problem with pouring your footings directly on top of bedrock. In fact, this can save you $$ in that a frost wall must be 12" thick and it doesn't take a lot of 12"-thick wall to cost you a lotta cubic in Ready-Mix.


2. Piffin's point about making a transition from bedrock to soil is the most critical element you need to deal with. We usually find that the bedrock tends to slope about 5-10% one way or the other, and just excavate down to it all around. If a part of that perimeter is above desired slab height, we step the forms down the slope as needed and do the best we can. Nothing's perfect, and we've learned that blasting or jackhammering out the rock to form the 'perfect' basement somebody drew in the plans is just an expensive way to create a subterranean 'swimming pool' under the house which will likely cause water rising up through the slab/F-wall joint during spring thaw or heavy rains.


If you're only a small amount above the desired slab elevation, just raise the whole house that much and pour the slab above that level. Then bring in a few truckloads of sand to build up the outside grade to the height-on-foundation that you want. Sand is cheap. And it drains very nicely, too.


3. Pin the footings and pour right onto the rock. While we sometimes don't bother with pins (if the rock is flat and the basement will be buried at least 2/3 of F-wall height) it's not hard to do and is the best way to tie the foundation to the planet. I have never heard of anyone putting a 'pad' of sand or plastic between the footing and the bedrock.



Dinosaur


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....


Dinosaur

How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #102008, reply #18 of 36)

You have gotten some good advice here but nobody has commented on something in your original post that caught my attention;

<<For the most part, is was able to excavate below my frost depth (24") in the trenches. However, about 25% of the area of the trenches contain outcroppings that extend up into the trench by about 1.5' on average (to a depth shallower than the frost depth). >>

Are you saying that your frost depth is 24" and you are hitting rock 6" down? If so, this is a problem. If not, please clarify.

<<Option 1. Per IBC, and my building inspector: isolate the rock from the footer using a thin sand layer (like 1/4") or 6 mil. poly sheet and pour the footing over and around the rock.>>

I'd like to hear the reasoning behind this. There are plenty of good reasons to isolate a slab, but isolating a footing is news to me.

<<Option 2. Per some references, pin the rock to the footer using short sections of rebar, then pour around the rock. This seems to conflict with Building Codes that preclude steel reinforcement within concrete lying within 2" of soil (where the term "soil" apparently includes rock). Locally at least, engineers do not presribe pinning, maintaining that movement between foundation and rock should be allowed.>>

'Bagg is right, soil and rock are not the same. Nonetheless, your footing does need to provide uniform bearing competence. If the rebar is not continuous, this will not happen. If you have ANY movement likely between the rock and the foundation you need to start this discussion over and reconsider your approach.

<<Option 3. A modified version of Option 1 but use a thicker layer of sand between rock and footer (more like 2"). The thought being that the sand will act as a "cushion" when differential settling occurs and more evenly distribute the force between ground and footer. The concept of differential settling is that the added weight of the building sitting on the footers will compress the soil more than the bedrock causing a stress concentraion in the footer near the transition between rock and soil underneath. >>

First off, the sand will not accomplish that for you. If you need to reduce the bearing pressure, let's devise another strategy. Secondly, as I think Piffin pointed out, if there is ANY chance that the soil will move and the rock won't, then again, we need to cut your bearing pressures.

OK, so what is the soil composition, and what kind of rock?

To add to the mix, why would you use CMU on a questionable site? A poured wall (ICF or standard) gives you far better opportunity for bridging.

[edit for spelling]


Edited 8/30/2006 12:48 am by Catskinner

(post #102008, reply #20 of 36)

"... nobody has commented on something in your original post that caught my attention;

<<For the most part, is was able to excavate below my frost depth (24") in the trenches. However, about 25% of the area of the trenches contain outcroppings that extend up into the trench by about 1.5' on average (to a depth shallower than the frost depth). >>

Are you saying that your frost depth is 24" and you are hitting rock 6" down? If so, this is a problem. If not, please clarify."

I did mention that, asking him if what he was describing was a series of knife edge projections up into the footer, and recommending that he use a jackhammer to get rid of them

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

PreviewAttachmentSize
AARX_4.gif
AARX_4.gif187.24 KB
Rock_excavation.jpg
Rock_excavation.jpg88.26 KB

(post #102008, reply #21 of 36)

I did see your reply, what I am trying to figure out is how far up into the footing this rock is. I can't tell from his description if the rock is 6 inches up from the bottom or 6 inches down from the top.

In any case, he hasn't answered yet, so I don't know if he even wants any more opinions so I'm just gonna wait until he posts again.

So what is that you are digging in? Doesn't look like it's coming out in very big pieces.

We just hammered out somewhere between 1,500 and 2,000 tons of limestone. That was slow.

(post #102008, reply #22 of 36)

It is what we call ledge here. much like shale or slate, but hard and doesen't flake as easy, but it does have a grain. Not granite.

The top 18" is always fairly easy to take because water has gotten in minor cracks thru history and frozen to have it fractured. But as soon as you get below the 18" mark, it gets a lot harder.

On that job, we had to get the big hammer in for the footer on one side because the ledge dropped off at a tall angle right thru the wall location, but the rest of the rock was fracturing pretty well so we kept going and got a good crawl space for the mechanicals.

There was one place where we did not pour footer all the way down to ledge, about 5' wide. We ran a second set of rebar and poured the footer qa bit deeper, and placed the access door right therre, so the load earing ended to either side of that gap in the structure, and we ran the drain line out under it through that gap in the ledge.

but you'll have to imagine it. Prospero won't accept an attachemnt this morning.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #102008, reply #23 of 36)

with only 25% of  your footing comming in contact with bedrock ( or boulders large enough to never move )  then 75% of your footing will not be in contact , but rather will be soil bearing


so it has to be one or the other


in this case i would not pin the footings to the rock


i would over excavate the entire area and put in  8" of crushed stone ( 3/4  - 1 1/2 " )


this also means i would hammer or blast the  25%


the sand / plastic  isolation technique is not going to change the bearing capacity of the rock areas.. they will still be 100% bearing .... and the soil areas will be less that 100%


you will get uneven settling & cracks


i'd also pour my walls instead of using CMU.. but either way i'd get even bearing under the footing


Mike Smith Rhode Island : Design / Build / Repair / Restore


Edited 8/31/2006 7:43 am ET by MikeSmith

Mike Smith Rhode Island : Design / Build / Repair / Restore

              www.mfsmithbuilder.com

(post #102008, reply #24 of 36)

what is it I'm missing...


we drill and pin the bed rock to the footers or they become part of the foundation...


providing it's bed rock and not just big rock or boulder..


 


 


Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming


WOW!!! What a Ride!


Forget the primal scream, just ROAR!!!

"Some days it's just not worth chewing through the restraints"

(post #102008, reply #25 of 36)

Here's another kind of bed rock. This is Sandstone in Okla. The shelf is at least 80' thick & is 200 million years old. This is absolutly stable.Because it was laid down under water It is nearly flat (about 3% slope). Ledge stone like Piffin's pix is severly eroded limestone. The softer layers erode out over a few million years or so and leave behind the harder layers (sometimes very hard) which by themself are not going to breakdown but are is small enough (500-1000lbs) that they are not locationally stable. That might also be the case with th big rocks seen in my pix. If I encountered them I would move them; but would pin to the shelf.

If you have a problem, don't just talk do something to set it right.

  Jim Andersen

PreviewAttachmentSize
Sandstone_Small.JPG
Sandstone_Small.JPG138.45 KB

(post #102008, reply #26 of 36)

I don't think you're missing a thing.

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #102008, reply #27 of 36)

correct me if I'm wrong...


the main concern is this rock is preventing the OP from putting the foundation work below frost level...


if the CC is pinned and bonded to the rock which the OP says is substantial and solid....  wouldnt that be the same as having the foundation below FG...


also..... wouldn't the 2" layer of buffer sand allow for water intrusion... same for the poly which both would allow for shift to happen


so I say..


dig to rock allowed limits..


shave off any rock intrusions into the verticale work...  


add pins as required..


form for CC instead of block...


clean off all loose material..


paint on bonding agent...


pour...


water proof outter foundation AND rock paying close attention to tne CC/rock seam to prevent water intrusion...



 


Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming


WOW!!! What a Ride!


Forget the primal scream, just ROAR!!!


Edited 8/31/2006 12:31 pm by IMERC

"Some days it's just not worth chewing through the restraints"

(post #102008, reply #28 of 36)

yes to all - good summmation. Maybe I said something that sounded like I was saying something different.
Even if the ledge is 6" down, when th ecrete is pinned and bonded, no water can get in to freeze and heave it away

 

 


Welcome to the
Taunton University of
Knowledge FHB Campus at Breaktime.
 where ...
Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #102008, reply #29 of 36)

ok... you're wrong.....


he said that 25% of the bearing area for the footings are solid rock


which means that 75% of the bearing area are soil bearing areas


 he will not have uniform bearing  and he cannot bridge the 75%.


 


to give an extreme example :


this is what created the leaning tower of Pisa.


if the ratio were reversed ( 75 % - 25 % ) then i'd think about pinning and bridging


but not given the OP's description


Mike Smith Rhode Island : Design / Build / Repair / Restore

Mike Smith Rhode Island : Design / Build / Repair / Restore

              www.mfsmithbuilder.com

(post #102008, reply #30 of 36)

And you, Sir, would be correct. <G>

We're really gettin' after this one considering the OP hasn't chimed in for a while.

I'm still waiting to hear from him on what exactly he is faced with, but in the meanwhile here are som more thoughts for debate.

The differnce in bearing competence between the soil and the rock is of course what will be his real problem. One way that he can successfully address this is to cut the bearing pressure and increase the foundation's ability to bridge.

I think he said it was a 2-story wall and a slab floor, I don't remember, but I will go back and look.

I think we're all agreed that if the rock really is stable, there is no reason why he can't use it. I also think we're pretty much in agreement that the bond breaker is a bad idea here.

So for the rest of the foundation -- assuming the soil really is reasonably good --if he over-excavates to a depth equal to the width of the footing (e.g. 2' deeper and 2' wider for a 2' wide footing) he will have cut his bearing pressure in half. If he was to place some CLSM (lean fill) in that hole and form the footing on top of it I think he will have largely addressed that concern.

Then if he throws out the idea of CMU and goes with a cast wall and gives himself #4 verts 4' o.c. and at least two #5 horiz. cont., he's probably going to be in pretty good shape, no?

(post #102008, reply #31 of 36)

All:


Sorry I haven't replied in a while after my first reply.  Things have been real busy.To answer a few questions.  


1. The rock is a hard limestone, the soil is a compact clay and sand mixture deemed locally to be suitable for standard footer loadings.


2.  The rock protrudes up into the trenches different amounts depending upon where  you are laterally.  In some places it does rise to a point where it will lie approx. 6" below the bottom of the proposed slab.  Most of the rock protrudes above the frostline by about a foot. The rock is rounded, somewhat smooth.  I'll post a picture when I can get one.  Its raining like crazy here right now.


Thanks to all.  I'm really impressed with the amount of consideration going into posts after what seemed to be a slow start, perhaps because my first post occured on weekend.