20x34 Universal Cottage near Briggs, TX (North of Austin)

[Txvineman]

Time to get to work!  We've learned a ton lurking on this forum, picked up the 20x34 universal two story plans, and have begun the process of removing prickly pear cactus from poorly maintained ranch land.  My wife and I both really like a house on piers for reasons aesthetic and practical, but we're currently debating how much of a PITA it could be to do it right vs. having someone else pour us a crawlspace or slab in a weekend.  We're on 30 acres, home site 1:20 slope, no serious caliche layers down at least 6 ft when we did the septic perc. test. (a rare thing here!) and none hit while using a tractor auger.   A highly unscientific, but representative soil sample from the test trench sidewall (jar and water) looks like sandy clay loam, and drains very well.  Of course there is limestone rubble and rock everywhere to turn an auger, but that's what rock bars and wives are for right??  

We have a 50hp tractor, box blade, Loader, and post hole auger at our disposal.  The more work we can do with that (as opposed to renting equipment or hiring it out) the better!  We'll post pictures as we go, and hopefully entertain all you who have done this already as we butt our heads against walls and try to do things 3 times the wrong way before getting it right in spite of all the good advice we'll probably get!

[Don_P]

I imagine footing depth there is 12", a couple of rips with 2 teeth in the box blade, then a tiller and shovel could dig that. At that point about 4 yards of concrete and a bit of rebar and you'd have a good continuous perimeter footing to uniformly bear on. With rebar turned up out of the footing at pier and corner locations block could be laid up as corners and as piers along each side, the block cells with rebar in them would be poured solid. The corners act as bracing and depending on the length of the pier sections they could also help brace the building. The goal would be to have the pier and corner sections wider than tall and well tied to the footing and to the girders that rest on them while still allowing good airflow. You would also be set up to easily make it a pier and curtain wall foundation, at which point the girders could go away.

[Txvineman]

Great plan!  for some reason i had sonotubes stuck in the brain.  Blocks on a footer sounds like far less labor than digging pier holes several feet deep, and getting a precise hole 20 or so times in a row with limestone chunks everywhere is bound to be largely a hand assisted tractor operation.  Shoveling shallow tilled earth sounds almost fun compared to that...

Yes footing depth is 12", and very unlikely to ever hit frost beyond probably 6" (it's been hitting the 60's and 70's in the daytime this week and people here are breaking out the puffy ski jackets).  I dont have a tiller at the moment, but I can cant the box blade sharply and scrape the top layer of soil out with the corner, then come back with a shovel. 

Can the footer be poured directly in the ground without using forms?  Seems like putting it tight against undisturbed earth would be a good thing, but would having irregular shaped sides be a structural issue later?  Clearly by this question you can tell i have minimal experience working with concrete...  Not trying to save a penny by not building forms, just time if its not needed for something poured at grade. 

My construction experience is in cathodic protection field installation and repair, so lots of steel and digging...water towers, pipelines, gas stations, stuff like that.  Residential construction is new to me (i've slapped together pole buildings and things like that no problem), but I do have knowledge of most tools, equipment, etc and can follow directions and blueprints.  trying to genuinely resist the urge to just wing it on the building my kids will sleep in...

[Txvineman]

Well we've dug holes in several places on the property and wondered where the limestone shelves are.  Answer: under the house pad...

http://flic.kr/p/pC6xy4

Six inches down and hard to tell because i didnt sweep it off, but solid limestone.  The tractor can rip out good chunks when it can grab a corner, but i drilled into it 6-8 inches with a hammer drill and did not punch through.  Is it safe to pour a continuous footer on limestone?  Ive read mixed opinions on doing this all over the internets.  It seems my options are move the house, break through the limestone and build, or scrape the shallow surface soil and pour a footer only 6 inches below grade? 

[Don_P]

If it is chunks I'd try to get down to 12", if it is "competent bedrock" then it is extending below frost depth, you can float over it on a skiff of sand or pin to it. Generally if it doesn't extend under everything I'd isolate from it with a bit of sand. If it is one rock under everything I'd bond to it. Limestone can be everything from punky nothing to marble, you'll have to judge for yourself what you're into.

You don't have to use forms if you can cut a nice square trench the size you need in the soil. Steel needs 3" of cover in concrete poured in direct soil contact.

Canting the blade might get some, I was thinking of removing all but a couple of teeth and dropping those down as far as you can, use it as a subsoiler to rip up the ground in the trenches, then shovel it out. If you have an old single bottom plow you can replace the moldboard with one of the box blade rippers and really get down deep enough. Subsoilers are usually cheap too if you can find one.

[Txvineman]

Thanks Don_P,

I ran the box blade with a single shank down as low as I could get it, and it will definitely rip earth, but the box is scraping the sides down as I go.  I tried a hard cant with the tine on an end but that puts the trench in line with a tire, giving me a single pass before I start compacting what i've just ripped.  Just a case of wrong tool for the job I think!  But you're right, subsoilers are cheap and I'll pick one up tomorrow.  That should get me right over the trench at proper depth. We'll see if its a continuous vein of limestone under the entire pad or not this week and go from there.  Thanks for the reply!

[Txvineman]

Well had some time to work.  We've got 6 inches of clay mixed with limestone rubble.  Under that I've been able to drill down ten inches with tools on hand and it's solid limestone. We're talking like marble.  Hammer drill working hard to penetrate.  It appears to be a solid vein of bedrock under the homesite.  If we clear off the clay for a continuous footer, build forms and pour it on the rock and anchor it to the rock, can we then backfill to proper depth to ensure frost protection around the footer?  We'd come up about 6 inches short of proper frost depth for our location

[Don_P]

When you pin to solid bedrock the rock is the footing and it extends below frost depth, no need to pile up dirt around the foundation.

[Txvineman]

Thank you, that makes sense.  I've cleared a rough footer trench and next weekend will set batter boards using steel pins drilled into the rock, and excavate by hand to get a nice clean surface for the concrete forms.  I'll have to back fill a little bit to make sure the drainage is right for the slope, but wont worry about frost heave.  I'm considering now clearing the entire interior of the foundation wall down to the limestone.  Is there a good reason to NOT do this?  Seems like it would be beneficial for a totally enclosed and conditioned crawlspace to have a solid rock floor (no problems with residual soil moisture wanting out, clean surface for working on, easy inspection for critters, extra couple inches of height)

[Txvineman]

Heres the roughed in trench, now considering scraping the entire area down to limestone.

Roughed out footer trench

and here's a chunk hacked out of the limestone with a rock bar

[Txvineman]

looking forward to this big sky view every night!

by Nigel Dunn, on Flickr

[Don_P]

Sorry I missed the jan 31 post, yes, if you can drain from the proposed crawlspace grade out to daylight downhill a little bit it would make a nicer space down there to work in.

[Txvineman]

Your time is much appreciated Don_P

discovered this document today, downloaded it but haven't yet read it.  Looks like a fairly comprehensive encapsulated crawlspace 101, funded by DOE

http://www.advancedenergy.org/portal/crawl_spaces/pdfs/Closed%20Crawl%20Spaces_An%20Introduction%20for%20the%20Southeast.pdf

hopefully it can be of some use to other DIY'ers.  As I look at the numbers for a good solid foundation wall, I'm considering just pouring the entire foundation and skipping the time-eating block wall aspect.  If I build the forms and set the anchoring pins, its a days work for an experienced crew to prep and pour the wall, then curing time and done.  block wall could eat some serious labor hours that I would rather spend on the rest of the house.  Fast, Cheap, or Good...pick two I guess! 

[UK4X4]

Clear to limestone for whole base, drill and secure rebar or all thread, make forms and pour in one hit
safe secure and dry in one operation would be my thoughts, 

Probably an ideal foundation site.......but think about levels for sewage  outlets and drainage of the crawl space, we put in sleeves prior to the pour for the utilities and drainage

[Txvineman]

Well the light colored area in the hole is continuous limestone for sure.  Now to clean up the rest of the loose rock, sweep out the dirt, and drill pin holes where the forms will be.  I've pinned the corners already so I don't lose them while in and out of the work area with the tractor.  This was slow going with a tractor, but honestly I couldnt see a light back hoe doing it any faster because of the nature of the rock we're pulling out.  many passes with the box blade and now we've got a definite 1 foot slope from the NE corner down to the SW corner, with undulating limestone in between.  My next question is this: Given our current plan of an enclosed and conditioned crawlspace, what are my best options?  I was thinking clean up the rock, toss some gravel in to level the low points, and covering this with vapor barrier that runs up the walls.  The nagging side of my brain, however, is saying "what if water gets under the barrier and settles in the gravel?"  Is this a problem?  as long as there is a hole on the downhill side for emergency flood drainage, and the hole is below the vapor barrier, shouldnt we be good to go?  I should also mention we are looking at a 3ft high wall on the downhill corner (so about 2ft on the uphill side above grade).