raft construction, a convert

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raft construction, a convert

Postby dozer » Thu Aug 18, 2005 10:19 am

Another thing is I've been over to look at the beginnings of Attila's house construction. Similar, foundation wise, to Cruzing's Spanish Style villa, it has in the end made a convert out of me. Let us call it 'raft foundation' as one of the posters termed it. Basically, in Attila's situation the ground is excavated to a depth of about a meter, a 60 centimeter pad is laid down, then one top of the pad a reinforced footing is poured inbetween a frame. On top of course will be load bearing walls, he happens to be using q-con blocks, but could easily be heavy cinderblock with reinforced concrete fill (but rule out regular cinderblock or small red brick). The advantage to this method depends on the soil conditions where your house is constructed. It reduces the problem of inadequate depth on the foundation posts (normal construction method) or bad soil conditions which cause settling cracks or worse. The downside is you eliminate a large percentage of contractors because in my view it is so different any contractor can't do it without previous experience doing it this way. (There are exceptions, Mr Cruzing was successful in getting his 'never done it this way before contractors' to do it successfully).

I'll be posting a few blog article on this soon, one in which Attila will supply some pics of the early foundation work.
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indoor swimming pool

Postby RobinT » Thu Aug 18, 2005 8:23 pm

:D

with alll that structural concrete, your basement could serve as an indoor swimming pool :D

I presume the 60cm concrete pad has reinforcement?

With good foundations you try to equalise the ground pressure as fas as practical to avoid differential settlement (or else you design for differential settlement as they did with lime mortar mix for brickwork in cheap victorian housing in London (a large basin of clay btw) :wink:

how much does this form of construction cost?
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raft construction

Postby dozer » Thu Aug 18, 2005 9:45 pm

One added expense would be the construction cost in that I believe you would be limited in the pool of contractors capable enough (or should I say experienced with this method) to take the project on. As far as materials, in Attila's case there is more rebar used than in pillar construction, according to the resident expert (mr. cruzing) probably overkill. Anyway, using this build as an example I would say about 30% more concrete and about double the rebar compared with pillars. Time wise about the same about of time needed.

It will become more clear once I have the pictures to post.
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raft construction pictures 1

Postby Attila » Sun Sep 04, 2005 4:17 pm

Here are some pictures to illustrate what dozer describes.
Attachments
carport2-bottom.jpg
the foundation (in the green oval)
carport2-bottom.jpg (32.41 KiB) Viewed 22662 times
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step 1 to 7

Postby Attila » Sun Sep 04, 2005 5:05 pm

After all the theory now the real story. Here is a foundation without columns, and without to much publicity there are many more like that. Some farang builders prefer it this way, and the ABDs do not complain or stop them, but smile the typical Thai smile when they see something they do not know :wink:

The ground here is a kind of sand, there was no need to fill in earth. There is a slight slope, so any rain water will flow away from the house anyway.

(the last pic is in the next post, there is a max of 7 attachements)

The worker there with the white paper in the hand, well, it is the foreman holding the plan developed in Archicad. He is actually using it to make sure the trenches and the plan are in sync.
Attachments
foundation-step1.700.jpg
Digging the holes, or better trenches, under every loadbearing wall. At the end the ground of these trenches is compacted with the little machine (see the picture of a later step)
foundation-step1.700.jpg (171.32 KiB) Viewed 22675 times
foundation-step2.700.jpg
The sides of the 60 cm wide foot are fixed with cinderblocks, and some thicker rebar is put in.
foundation-step2.700.jpg (181.02 KiB) Viewed 22657 times
foundation-step3.700.jpg
The 60 cm wide and 20 cm deep foot is filled with concrete.
foundation-step3.700.jpg (68.27 KiB) Viewed 22630 times
foundation-step4.700.jpg
On the 60 cm foot the sides of the foundation wall (20 cm) are build using regular cinderblocks. These walls are exactly under the loadbearing walls of the house.
foundation-step4.700.jpg (106.11 KiB) Viewed 22621 times
foundation-step5.700.jpg
After the sides are finished, the rebar is put in.
foundation-step5.700.jpg (173.75 KiB) Viewed 22598 times
foundation-step6.700.jpg
Filling in the concrete. It is pushed down and compacted a little to flow around and under the rebar.
foundation-step6.700.jpg (111.1 KiB) Viewed 22580 times
foundation-step7.700.jpg
The earth is filled in, and compacted with this machine. The detail shows a distance holder, to be put under the rebar, to help keeping it away from the ground when the concrete is filled in.
foundation-step7.700.jpg (178.5 KiB) Viewed 22558 times
Last edited by Attila on Mon Sep 05, 2005 1:09 am, edited 2 times in total.
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step 8

Postby Attila » Sun Sep 04, 2005 5:12 pm

The last step, soon the slab is ready to build the walls.
Attachments
foundation-step8.700.jpg
Concrete and concrete and even more concrete. Important is not to let the rebar sit on the ground, but to make sure it is lifted up a little. Then keep it wet while it dries, otherwise it might get cracks.
foundation-step8.700.jpg (93.02 KiB) Viewed 22549 times
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Postby runker » Sun Sep 04, 2005 11:41 pm

what about the plumbing?
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Postby Attila » Mon Sep 05, 2005 1:01 am

runker wrote:what about the plumbing?


That was quite easy, as all the bath rooms and kitchen are on the back side. On the picture of Step 7 you see the blue pipes in the upper left corner. They are going through the floor slab / will be in the walls.

I did not build with a crawl space, there are supposed to be many cobras in the area, the neighbors dog is catching one from time to time. Now I love snakes (the animal ones only :wink: ), but my wife not, so she decided "no crawl space under the floor!".
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Postby John » Fri Sep 23, 2005 7:23 pm

I have never seen anything so crazy. With so much concrete, block, rebar and labour time wasted you would have been better off sinking in one huge reinforced raft to internal floor level and just building right off it.
The guy who specified the rebar and foundation sizes forgot to switch out of column mode when designing that project.

I would love the see the calcs sheet, is there a roof top car park or garden ?
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Postby Attila » Sat Sep 24, 2005 1:11 am

John wrote:I have never seen anything so crazy. With so much concrete, block, rebar and labour time wasted you would have been better off sinking in one huge reinforced raft to internal floor level and just building right off it.
The guy who specified the rebar and foundation sizes forgot to switch out of column mode when designing that project.

I would love the see the calcs sheet, is there a roof top car park or garden ?


It is a 2 story house.

No problem with the time, it was not done by me, but by Thai workers, who had been happy to have a paid job :wink:

But actually, it was done quite fast. A neighbor, who is doing a column based foundation, is now working on it much longer than it took to get this foundation done.

And yes, they said they would put more rebar in than necessary. I'm glad to get this confirmed.

Your proposal of "one huge reinforced raft" is actually what I did discuss with them also. However with the slope of the land which we have here, I would have had to build some foundation walls anyway, to get it high enough. So this way seemed to be easier.
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Postby dozer » Sat Sep 24, 2005 8:46 am

With so much concrete, block, rebar
This is indeed the comment which seems to be made when people see a house being built with raft construction (Cruzings and this one), like -- what are you building, an airport or a high rise building? But actually even though there is a bit more material being used it isn't as much as you would think. A house of this size would need several reinforced foundation columns, since these are all eliminated the need for all that rebar goes away. All in all I would guess if you took all of the pluses and minuses the material budget increases not more than about 20,000 baht, probably less than this. If you look at the above pictures, remember step 7 and 8 show flooring being poured and would be the same in either method (raft or column).
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foundation

Postby cruzing » Sat Sep 24, 2005 10:05 am

The one thing we did differently than Attila, is to use the earth as our form for the 1 meter pads and footings.

Then concrete blocks were laid for a crawlspace, then the subfloor was installed, the floor poured and the walls built.

With his he just doesn't have the crawlspace.
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Postby Itchy » Wed Jan 11, 2006 2:25 pm

I shall be employing raft construction method when we start the build on our place.

I have a few questions for which I hope you (Attila) can help with answers.

What sizes/spacings of Rebar did you use in each of the following sections:

Sub footings?
Sub footing risers?
House base floor?

Having built on a raft did you then use standard load bearing wall construction above the raft or did you follow the Thai practice and put up a reinforced concrete frame followed by infill?

If you used load bearing walls did you have any trouble getting drawing aprovals?

As I say I plan to use a raft, I shall also be adding a cellar and I hope to be able to employ double skin red brick wall construction. My current idea is to build a two story Italian Villa style house - and I hate those concrete frames.


For those who think this raft construction is overdone, you need only look at the damage rain water and termites can do to the soil structure under standard Thai construction houses. My estimate is that most Thai houses are built to last around 30-40 years and are pretty much unstable after twenty years or so.
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Postby Attila » Thu Jan 12, 2006 2:26 am

Itchy wrote:I shall be employing raft construction method when we start the build on our place.

I have a few questions for which I hope you (Attila) can help with answers.

What sizes/spacings of Rebar did you use in each of the following sections:

Sub footings?
Sub footing risers?
House base floor?


I do not know, they had been chosen by the builder. Some experts might be able to say from the pictures.

There is a house build now here nearby, he is using much thicker rebar, but admitting that it is an overkill.

I will ask the builder of my foundation, and another one, who made my roof, but usually also makes this type of foundation.

Itchy wrote:Having built on a raft did you then use standard load bearing wall construction above the raft or did you follow the Thai practice and put up a reinforced concrete frame followed by infill?


No posts, no concrete frame, but loadbearing walls, done the Western way, as it is done in Europe and the U.S. / Canada. I'll try to organize my pictures of that process in a similar way as I have done here with the foundation ... soon

I used 20 cm Q-Con blocks as loadbearing walls. You can also use the thicker concrete blocks, they can be loadbearing too.

Itchy wrote:If you used load bearing walls did you have any trouble getting drawing aprovals?


Well, I did not start any discussion with the approval people. First of all that would have made it much more expensive. As soon as they would have seen my white farang face they would have reacted on the $ sign tatoo on my forehead, which only they can see. (see also dozer's story about the "planning czar" on this site) So my wife did handle that part all alone, getting the Thai price, saving a lot of baht :D

So they made the plans the Thai way. With the usual concrete posts.

And the builder did build it without ever looking at them (as he said he usually does), but using MY plans, which had the same dimensions as the official plans, but all the farang extras, additional bathrooms, etc. And no concrete posts (except two round posts in the living room area, supporting a beam and so allowing for more open space.

They came looking one day, and complained that there would be the wall weakened at a place where there was a hole in the wall to let the cable go through for a lamp outside, and that in the area under a beam. They made that comment and left. Another area, which was much more critical, they did not comment, but I did reinforce it anyway, that was where there was not much wall left in the living room wall due to too many windows and sliding doors.

When they came looking it seemed that they did try to sell the services of one of them to poison the foundation, to keep the insects out of the wood. Which I don't understand, because we did build with concrete and steel and tiles, and not wood.

Speaking with some builders, which use loadbearing walls, they usually say that when they see that there are no posts they usually show the typical shy Thai smile which they show when they do not understand something... but that's it.

Now you can also do it the way Cruzing did which was to make it clear to them that there are no posts needed, but then you might better speak some good Thai and have some architect knowledge...

Itchy wrote:As I say I plan to use a raft, I shall also be adding a cellar and I hope to be able to employ double skin red brick wall construction. My current idea is to build a two story Italian Villa style house - and I hate those concrete frames.


If you go down into the earth anyway to make your cellar then you do not need the stripe foundation under the slab. You only need to make your slab (floor of the cellar), and you can build on that your walls directly. You are deep enough anyway, probably much more than 1 meter. Just make the slab strong enough. That's the usual way I have seen it in Europe and Canada.

Make your deep hole. Fill a 20 cm concrete slab in it with a lot of rebar. And then you can build the walls directly on it, using either the thicker concrete blocks or a double wall of the thin breeze blocks which you then fill with conrete and some steel.

Itchy wrote:For those who think this raft construction is overdone, you need only look at the damage rain water and termites can do to the soil structure under standard Thai construction houses. My estimate is that most Thai houses are built to last around 30-40 years and are pretty much unstable after twenty years or so.


They depend on the posts / feet to not sink in, but they usually do. The problem is that some sink more than others, that gives a tension to the concrete frame, and cracks in the walls.

A strong slab foundation, with or without a stripe foundation under it, does not get this tension, but sinks in evenly, no tension. In colder climates they put the stripe foundation under the slab mainly to compensate for the bigger movements of the upper parts of the ground in the winter, when it gets frozen and de-frozen again in spring.

Itchy wrote:... My estimate is that most Thai houses are built to last around 30-40 years and are pretty much unstable after twenty years or so.


I asked a (very nice and good) Thai worker how long the conwood stuff would last which he was putting under the eve of the roof. The answer was that this would be good stuff, it would last 20 years :wink:
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Postby Itchy » Thu Jan 12, 2006 2:44 am

Thanks for taking the time to reply Attila, I'll look forward to seeing more photos when they are available.

I'm currently knocking out sketches ready for doing some material and stress calculations, not rocket science and more for my own benefit than for planning aproval.

My soil condition is alluvial silts that have been settled for at least a hundred and fifty years. I know this because I have an old well that was inspected by the local water authority and then by a guy from the local university (water authority had informed the university of the well's existance) The guy from the university spent hours in and out of the well and then proclaimed it as being at least 150 years old.

I've done some basic stress tests on the soil to figure out the weights and I have done some bore tests to confirm the water table is at 5.3 meters below grade - hence I feel confident that the cellar will remain dry.

I'll be posting some information once I start my build, meanwhile I'm working with paper, pencil and balsa wood to develop various models of options.


Again thanks for the feedback which I really do apreciate.


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