First step in a foundation job, and the foundation is a precast one that goes into the hole with a crane doing the heavy lifting.
But here is the task at hand for the moment.
The hole is all dug. Nice big thing, level bottom, about 75 feet long by 40 wide. A number of precast column and loadpath pads must be placed. Two sizes, 24x24x4.5 inches, and the larger ones are 36x36x6.
See the attached pic for the array of pads. The centerlines shown are work line references. I’m not showing the dimensions because they would gum up the picture.
The first thing you need to do is locate all these and spray-blaze the dirt for the excavator to do 6″ and 8″ deep scoopouts for the gravel fill that goes under the pads. After the gravel goes in, tamped, leveled, and shot to depth, you need to set the pads to within an inch or so, x and y, of their plan locations.
How would you proceed?
Replies
If the hole is big enough (enough overdig), then get some batter boards in there so you can set strings to locate the perimeter, the corners, and the x/y centers of each pier. The batter boards need to be nicely done and sturdy, so that the strings can be set, removed, and reset with a high degree of accuracy. Normally one would set the concrete forms for the footing and stemwall first, and then use those locations to measure out the piers. With the precast system it sounds like you have to do it in reverse, unless you want to set the walls first and then do the pier digging by hand.
Batterboards don't have to be down in the hole, either. If nothing else, you could just string centerlines on each axis and pull all you measurements off of those.
How accurately for grade and center do these pads have to be located?
Of the 20 or so pads you see, maybe half will actually sit under the Superior Walls precast panels. For grade, we'll want to shoot those at +/- 1/8"
All the others will be buried under the floor slab and can be plus zero, minus up to maybe 1/2".
For line (not grade), we'll take the centers at maybe +/- 2" of plan, and they can be cocked (rotated) up to as much as our artistic sensibilities can stand.
If I had my surveyor's total station unit, I would upload my CAD data into it, do one setup where I could see all the locations, give my helper the target device, and start shootin'. Ten minutes. Done.
Here's an image of the foundation plan, in which you can see the pads.
Sure are a lot of pads and in an odd array. Looks like a school play ground I did. A couple of things that seem a little odd. You are spray blazing to thaw the areas? Does that mean there is frost in the hole? Generally, for footings, we like to use undisturbed ground. The combination of frost and compacted gravel would worry me. Is there a reason they are not using a perimeter footing? Seems like it would take less time and provide a stronger base. What will happen when the slab is poured? Do you just back fill slightly around the foundation sections and let it flow under? Don't you have frost issues with the back fill? What will keep the pre-cast sections from being pushed in without a keyed footing? The pads are rather thin also. Maybe I am totally misunderstanding the situation.
If you are up to it, the whole job can be set up with a transit. It is basically just finding the hypotenuse from the instruments location. If you are set up on the intersection of the red lines, each pad center can be described as a corner of a rectangle in relation to the red layout lines. In other words, each pad is x number of feet (inches) from the horizontal line and y number of feet (inches) from the vertical line. you simply need to figure the hypotenuse to the center of the block, from the intersection of the two layout lines. You could take it one more step by setting the compass head at 0° - 180° and 90° - 270°. You would have to figure the angle of degree for each pad. On large jobs, the engineers do this for you. In your case it would be much easier just to run some lines on the red layout lines and measure, just like you would when squaring up. You run three tapes, one horizontally, one vertically and one for the hypotenuse. When all three measures come together, you have the center. You only need the transit to set the layout lines in a 90° grid this way. While it is there you can shoot the pads to grade or use your laser. Do you understand what I've written? A simple calculator has the square root function but I'm sure you know how to figure a hypotenuse.
Beat it to fit / Paint it to match
This is a digression from the topic, but if you are curious about precast foundations, and have a high speed connection, go here http://www.superiorwalls.com/
There is a lot there to see. The time lapse video of a job going in in a couple hours is worth watching.
Here in far north upstate New York, these foundations are used with modular doublewides, up to huge multimillion dollar estate homes with all the trimmings. In the small area I am working, half the starts in the last sixteen months had precast foundations, and sell prices of the homes were in the $1 million plus range.
And as for winter weather, check the weather channel each day this time of year. One town over, Saranac Lake, often has the country's lowest early morning temperature. We know how to protect our newly-excavated undisturbed soil from frost, and it takes big trailer-drawn machinery, and lots of fuel.
"Here in far north upstate New York..."
Strictly as an aside - I got a chuckle when I read that, Gene. I don't know where Saranac Lake is, but my family goes back several generations in Cape Vincent. That far enough upstate?
"precast concrete"? Sounds like a winner. You weld sections together, or what?
Bolt them together. As I said to the other poster, go to the Superior Walls site, and go to the interactive page. You'll see one going together. High speed connection required, though.
Interesting idea. I didn't notice that the wall panels were being placed on pads. Are they there and I'm not seeing them? Do you have to frame the deck and pour the slab before backfilling? The crane is at your expense? How are the panels priced, per square foot? I would guess they are available in various sizes for stepped walls. Do you have to be careful when backfilling not to fracture the exterior skin, can you use common barrow or will grapefruit size rocks in the mix be of concern? How many places have you used them on and for how long? If you are using propane blankets, no wonder the cost is high. I had to use them on a commercial job after equipment drove the frost down 3'. Propane is out of sight up here for cost. I tried some hi-hat heaters on a job, sucked down 1000# in a week. How much is ready mix per yard in your neighborhood? Thanks for the link.Beat it to fit / Paint it to match
Superior Walls says over 50,000 installations are in place since they started this about 20 years ago. It looks to me like it is only happening in the northeast and upper midwest, but creeping into the western parts of NC.
The work is always done as a total service package by a licenced franchisee through his dealer/installer. That means that the GC gets a hole ready, gets the gravel in, levels and tamps the gravel as best he can, then the Superior Walls sub comes in with the trucks and the crane and crew.
Most simple foundations need no pads under the walls. The walls bear on the bed of tamped gravel, 6" minimum thickness. Perimeter drains in filter fabric are run all around outside. Wall flanges at top and bottom are 10" width.
There is no such thing as pricing by the square foot, or by the lineal foot. You'll get a price for the job, and it will be higher than a formed and poured job.
Panel heights (from my franchisee) are 4/0, 4/8, 8/2, 9/0, and 10/0. The 4/8 has an 8" deep lip ledge on the inside and is typically used for a garage.
We'll do this job in warm weather, and when it's set, we'll do the basement slab right away. Rebar hooks are cast into the bottom lip of the panels to tie the walls to the 4" floor slab.
We'll build the deck atop the walls beginning the day after the floor pour. Then and only then can we backfill. And yes, we want to be careful with the big stones.
Three things we don't have to do when using this system: foundation waterproofing, studwalls inside for basement finishing, or basement wall insulation.
And one final benefit. These things are made straight, true, and with precision. In three days of erection time for my big complicated mess, we'll have a foundation we can build on, without error. Can your 'crete sub get this one right for you, in just three days?
It would take a crew of 5 or 6 guys a few weeks to form and pour that. The main advantage I see is that you get continuous concrete and rebar. I assume they could engineer a precast system for the west coast, but the inspectors here sure do like beef!
What I am starting to see here is modular buildings, i.e. all of the floor, wall, and roof panels are prefab in sections and brought in on trucks to be craned into place and assembled.
Gene,
I've breifly looked into this sytem in the past, and have seen one up close.
I was not thrilled with the joinery at the corners, but I am looking at it from a carpenters view point.
I think that with the deduction for waterproofing and the benefit of the interior ins. and studding that it is a cost effective system.
Are you 100% confident on the waterproof??
What will you be doing for footing and foundation drainage?
EricI Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
Full perimeter socked corrugated plastic "tile," 6" diameter, around outside, then piped to daylight.
Our overhangs are all 42 inches, and we'll put drainage under the driplines.
But best of all, we are building in glacial till, and everything that has been dug in the adjacent building lots showed only gravelly sand, an occasional boulder, no underground flows, and excellent drainage. I've built twice in this subdivision, and it is typical when excavation holes are open and we get a heavy rain, to see the puddles disappear 30 minutes after the rain stops.
And with 3000 psi bearing pressure for the soil type, the 6" compacted gravel base under the walls, the walls can handle 4000 plf loading, way under what we will be stacking on top.
I once dug a foundation and sold the bankrun as approved septic fill to the excavator. Actually, he sold it as such, but talk about easy digging.
No clay in that till? I know that it is not uncommon to find it in those types of conditions.
I'm still curious as to how you feel about how waterproof these foundations are.
EricI Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
Over 50,000 in place and still going strong? Sounds like pretty good technology to me.
Precast in the same factory conditions as for septic tanks, hollowcore planking, underground electric ducting, and water tower components, with walls made with 5000 psi 'crete, joints double sealed with the best sealants, I don't expect problems.
Their number one claim, right from the start, is "warm and dry."
And no, we won't hit clay. Ever been up to the high peaks region?
Ever been up to the high peaks region?
My wife and I took the ferry from Burlington to Port Kent, then rode (our bicycles) to 9n, then took 86 to lake Placid. That was the last leg of our honeymoon journey.
We did some riding while we were there, I'm not sure I can recollect where we rode though.
What is considered the High Peaks?
Did you figure out how you are going to set those pads? I'm not sure I 'get' them all, but not seeing the floor plan it's a little mysterious. Even then, I am often found scratching my head for several hours until I have an 'AHA!'.
I have on my drawing board a set of Susanka plans for the Maple Forest House.
A freind of mine bid on some cabinetry and millwork several years ago, and still has the plans. I am looking at them just as a matter of interest.
Didn't you post pics or links to the house you are to build?
EricI Love A Hand That Meets My Own,
With A Hold That Causes Some Sensation.
A surveyor instrument system called "Total Station" would be just the ticket for doing a tedious layout like that required for digging for, and setting, the precast pads.
A total station instrument gives the survey crew full x-y-z locating capability, and backed up by the PC-driven software back in the office, allows all site detail to be uploaded into the field hardware.
With the field "transit" set up on a tripod, the operator establishes his base position by sighting a receiver target held by his assistant on two visible benchmarks, and then a program is run to compute the instrument's position and zero bearing angle.
The unit reports with an audible tone received through an earpiece, when the operator has the receiver target at the precise x-y-z location sought.
Operator and assistant communicate with on-head mikesets to coordinate movement and location.
For each uploaded point to be located, the operator is given a bearing and azimuth, and dials the scope to the required settings. He then steers the assistant, who holds the receiver target, commanding things like, "toward me," or "back," or "up," and so forth, until the target is placed right. The unit will give the beep tone, and the assistant places a flag mark. On to the next.
But I don't have a Total Station. I might, however, get the Pacific Laser System PLS5x, and receiver, to do the layout.
With the PLS5x, I would establish my x and y direction work lines, and place flags at all the dimension locations along the axes where pad centerlines intersect them, then backshoot to origin with the PLS5x at each flag, then tape out along the laser's right angle line to the determined location, to flag each pad's center.
Here's an example. Let's say that pad AA is at x = 27'-2 1/2" and y = 7'-3". With the PLS5x set over the zero point (main axes intersect) and its main laser sighted down the x-axis, I tape out 27'-2 1/2" with the target in hand, and when it is hit by the laser, place a flag at the mark. Now I am at the y centerline of pad AA. By placing the PLS5x over the flag mark, aligning the main laser back to the zero point (I stuck the target back there), I now have a 90 degree laser pointing to pad AA's location. I take the target and tape out 7'-3", fish for the beep from the receiver, and there's my AA centerpoint location.
One down, 19 to go. But it will go faster than in my example, because I will locate all the points along both axes in two operations.
BTW, regarding the high peaks, they are commonly though of as the group of highest mountains, over in the large backcountry wilderness where Mt. Marcy is. As the crow flies from Keene Valley to Lake Placid, he is passing over the high peaks region.
As long as we're talking about precast concrete panels...
I understand how the system works (This Old House used it on their current project), but I have a question:
Does anyone know how it works if you want a walk-out basement? If you don't have a hole all the way around, can you still use the system?
Thanks
Burt
Take a look at the pic attached. It is a model of a precast foundation that will be delivered this spring.
It is configured under a house with an attached garage and a walkout basement. You can see the 4/0 frostwalls across the back where the walkout wood-framed walls will be built atop the slab. Note how the frostwalls extend back under the 8/2 walls a little ways.
The foundation could have had full height 8/2 panels clear across the back, providing the design would be to stone-veneer the rear walls. It is quite easy to have the precast factory place window and door openings in the walls, and to cast in brick ties.
In this case, since the design is to have wood siding at the lower rear walls, it is more cost-effective to do the foundation like shown.
Great, thanks.
Would the slab be poured at the top of the frostwalls?
Yep.
Thanks again...perfect answer to my question.
I once saw a rep for the Total Station layout a simple floor plan in a parking lot. It was pretty amazing; I can only imagine how much time you could save once you familiarized yourself with the thing. I was doing foundations at the time and we were a bit concerned with the accuracy of the Station for laying out wall corners on the footing. It would be perfect for more rough measurements, such as footings, pads, etc. I didn't look at the site for Superior, but I'm going to. How do they assure accuracy over that of poured walls?
How is a precast foundation as or more accurate, for line and grade, than one formed and poured?
Don't get me started on that one. My last experience with a poured job was awful.
But as regards the precast system, it's analogous to these: how is a truss plant able to make identical trusses with such accuracy? How can a panelizing plant make wall sections that are more square, straighter, and more true, than the average hand-framed job? Why do all the parts fit together so well in your Porsche or Lexus?
The answer is clear. All have factories driven by computer-aided engineering, with expensive and specialized machinery and equipment to manufacture their products, all under the guidance of quality control procedures.