Floating concrete slab

[notrub]

Does anyone have comments about the use of these slabs as an alternative to pile driving please?  My single story house is built on 120x120x 40cm pads about every 4 meters with a 20x40 section continuous grade beam between, no piles.  The 10cm+ slab is tied into and  poured on top of this GB.  After 1 year no sign of any movement.  My friend is being told he must drive over 100 piles 5 meters deep at a cost of over 500,000 THB.  I think a floating slab should work OK Any comments please?

[bbradsby]

theres no one answer. every site is different. In the absence of a geotech engineer - and no typical owner-built Thai house project has one - you must trust the building designer and/or the local experienced contractor for what is done locally based on local conditions and the house geometry and weight. Chao Phraya bay mud, coastal alluvium, foothill alluvium mixed with rock at varying depths, solid rock hillside... all require difernt solutions. "Over 100 piles... over 500,000 THB" is useless info. How many piles.. and How many Baht, and exactly what location? Get real to get real answers.

 

[Crossy]

Is your friend in the same area with similar ground conditions?

 

 

[r136dg]

20 hours ago, bbradsby said:

Get real to get real answers.

 

100 piles 5 meters deep sounds commercial to me.

I wouldn't be afraid to build a modest home here on a floating monolithic slab (thick on the perimeter & wherever there's load bearing walls). But that would only be on stable virgin soil, not on a meter of fill in a rice field. 

Also only if using light weight roofing materials & building with 8" block that can be core filled.

But if subbing work out to locals you'd be best off sticking to what they know, no doubt! 

[cooked]

I built two houses in Switzerland on a concrete slab, on similar soil to what we have here PLUS having to consider the danger of frost going down to 80cm deep.

Not moved an inch.

Similarly here, I built an extension on top of a 20cm reinforced concrete slab above ground three years ago and it hasn't moved an inch in relation to the house itself, built on piles. At the edge I went down 40cm, 25 cm wide, before starting the slab.

Part of the reason for my doing this was to cut costs, meaning I did all the work. 

The neighbours spent ฿20 000 to get piles concreted in + all the steel structure for the roof erected, took three days. Dimensions about 9 metres by 10 metres. They still have to put in a concrete slab of course.....

[notrub]

All the soil around here is sand and I have never heard of a bedrock that exists at any depth at all.  Wells are drilled here down to 40 meters and there has never been a mention of a rocky band of any kind.

 

What I am getting at is the theory of a floating slab and how it must be made to 'float' on a sandy base?  If the slab is thick enough it should be able to settle evenly without breaking.   

 

Thanks rdg. for your comment.  I don't understand how a pile going straight down on to sand without a base footing of some kind can be an improvement on a thicker slab onto, say, 120x120 pads with a 40cm grade beam running between on the surface.

 

In other countries, where I have worked, core samples are taken to determine the depth of stable ground but nobody around here does that (that I have heard of). If there was a band of rock it would be common knowledge from all the well drilling locally here in Ban Mai Chaiyaphot, Isaan area.  I think the sand goes on to a great depth.  

 

I think the local contractors do what the other guy did without giving it much thought.  I see that these column piles are used a lot and they do drop a weight down to pack the base but still don't understand why that is better 3 or 4 meters down as opposed to a very thick monolithic slab on the surface.  I am going try to track down an engineer but don't know where I will find one yet.

 

There is a lot of wriggle room for other options when the quote for piles is 500,000

 

I found a supplier of 'super blocks' btw in Korat that are standard size but 14 cm deep and of very good quality too.  They are 10 THB each and delivered in quantities 1152 pieces.  

 

Thanks for your comment Cooked.  That is what I have in mind.  A 20 cm slab with 16mm rebar onto a grade beam on to pads is going to resist a lot of uneven settling.  I have built in Canada and France but there has always been some bedrock not too far from the surface.  Except in Calgary, Alta and the piles we used there were huge, maybe 1 or 2 meters in diameter.  I saw in Hong Kong a building site for a high rise and they were putting in piles dug by hand by a husband and wife team, 1 pile per team, but don't know what they were doing at the bottom or what soil conditions were down there.  They were deep, very deep.

[BEVUP]

In my Moo Baan (reclaimed land )

They drove piles only for the columns. This was for both single & double story houses

They just drive them until they stop

[Pogust]

This is how I am building my latest house. A floating "raft" foundation on filled land.

House built with lightweight load-bearing AAC blocks and no heavy roof material.

According to an engineer I asked this would work even in Bangkok area. House is 8 by 13 meters.

[bankruatsteve]

Put some pontoons under it and you're good to go anywhere.

[bbradsby]

On 4/3/2018 at 6:48 PM, r136dg said:

100 piles 5 meters deep sounds commercial to me.

I wouldn't be afraid to build a modest home here on a floating monolithic slab (thick on the perimeter & wherever there's load bearing walls). But that would only be on stable virgin soil, not on a meter of fill in a rice field. 

Also only if using light weight roofing materials & building with 8" block that can be core filled.

But if subbing work out to locals you'd be best off sticking to what they know, no doubt! 

For perspective, its not even a big [expat] house that requires around 100 driven piles. your 'commercial' is another man's 'modest' second home.

[bbradsby]

23 hours ago, notrub said:

All the soil around here is sand and I have never heard of a bedrock that exists at any depth at all.  Wells are drilled here down to 40 meters and there has never been a mention of a rocky band of any kind.

 

What I am getting at is the theory of a floating slab and how it must be made to 'float' on a sandy base?  If the slab is thick enough it should be able to settle evenly without breaking.   

 

Thanks rdg. for your comment.  I don't understand how a pile going straight down on to sand without a base footing of some kind can be an improvement on a thicker slab onto, say, 120x120 pads with a 40cm grade beam running between on the surface.

 

In other countries, where I have worked, core samples are taken to determine the depth of stable ground but nobody around here does that (that I have heard of). If there was a band of rock it would be common knowledge from all the well drilling locally here in Ban Mai Chaiyaphot, Isaan area.  I think the sand goes on to a great depth.  

 

I think the local contractors do what the other guy did without giving it much thought.  I see that these column piles are used a lot and they do drop a weight down to pack the base but still don't understand why that is better 3 or 4 meters down as opposed to a very thick monolithic slab on the surface.  I am going try to track down an engineer but don't know where I will find one yet.

 

There is a lot of wriggle room for other options when the quote for piles is 500,000

 

I found a supplier of 'super blocks' btw in Korat that are standard size but 14 cm deep and of very good quality too.  They are 10 THB each and delivered in quantities 1152 pieces.  

 

Thanks for your comment Cooked.  That is what I have in mind.  A 20 cm slab with 16mm rebar onto a grade beam on to pads is going to resist a lot of uneven settling.  I have built in Canada and France but there has always been some bedrock not too far from the surface.  Except in Calgary, Alta and the piles we used there were huge, maybe 1 or 2 meters in diameter.  I saw in Hong Kong a building site for a high rise and they were putting in piles dug by hand by a husband and wife team, 1 pile per team, but don't know what they were doing at the bottom or what soil conditions were down there.  They were deep, very deep.

ah ok, here's the useful info, if anecdotal. YOu're in sand.. not the typical condition for Thai houses - or in other countries. piles are useful if end-bearing on firm strata - or surface-bearing if driven into clay mud where firm stratum is too deep. Think "drinking straw in a thick chocolate shake." The straw stands by itself by surface friction. YO can build a house on that. This clay mud condition is the norm in LoS except for beach and hillside sites. Now, deep sand! Sand is great bearing for a raft slab. But its also about the reinforcement and grade beam design for the loads. not just concrete thickness. Concrete without proper reinforcement is very brittle. It has little flexural strength. Get it engineered, but as long as you're not in earthquake country, and youre not, deep sand is a great building platform. Just don't use it to mix your concrete! A small amount of sodium in the mix will destroy its longevity.

[notrub]

Thanks bbradsby,  I get it about a straw in a milkshake and that is great for shear but I still don't see how they want to go with piles without doing any investigation for what is down there first.  On a larger scale than we are talking about I have built an office block on piles in Canada.  The contractor just drove a 1+meter dia (?) pipe sleeve down 10 or 15 meters and then just pounded all the soil in the sleeve until they got to the bottom.  This formed a ball of compacted earth at thee bottom.and then we poured smaller dia concrete posts onto that.   But that equipment was big and the earth shook every time the weight was dropped.  

 

So your comment that a properly designed raft is good for construction on sand is what I am thinking too.  cooked had a good comment and I have seen some film clips on utube too that are encouraging.

 

But when I insisted on a monolithic slab onto a 20x40 grade beam for my house there was lots of 'stupid farang headshaking going on.  They were going to dig, by hand 24 (?) holes 120x120  6 meters deep and then pour a pad down there and then come off of that with 20x20 columns and then connect the result with a 20x40 grade beam at zero levation and then pour a slab, get this, inside the grade beam. So the exterior walls were to be built off the GB but the slab just sat inside without contributing anything to the structure.

 

We put in the 120x120 pads / footings at 70 cm depth and then poured the GB on top incorporating a monolithic slab which formed part of the whole structure to support the building. 

 

I like Thai people but the crowd around here are not able to depart from their standard methods of construction.  The contractor reluctantly did what I asked and after (only) 1 year there is no sign of movement at all.

 

Thanks for all your thoughts, very useful.