Landfill Vs Stilts?

[Godders]

To avoid innundation problems during the rainy seasons, most housebuilders in Thailand raise the level of the site by importing umpteen lorryloads of infill. This takes a lot f time, money and means you have to wait a year or more for the soil to compact.

Recently, however, I have noticed a number of new houses springing up in my area built on the modern equivalent of the wooden stilts one sees supporting old traiditional wood Thai houses in flat, flood-prone areas. With these modern houses, the "stilts" are much shorter and made of reinforced concrete presumably supported bysolid concrete piers deep in the ground. The house floor is suspen

ded, with an airspace underneath which I image helps keep the house cool.

As I am planning to build, I would be interested in any input on these two very different solutions to

an age old problem. At first site the "stilts" solution looks the more inviting - no waiting for the land to settle and presumably cheaper than costly infill. But does it do the job!

[BKK90210]

For excessive rainfall area – elevated the house using filled dirt should do o.k.

But….

For torrential rain area or flood plain area – stilt house is a practical form of architecture. It’s sure to beat floods, prevents undercutting the foundation of the house by fast moving water, and minimizes major damage from mud and debris flows/avalances which start suddenly and move quickly.

Steel or concrete columns are used and should be embedded deep enough. The first floor is propped up generally 5 ft above the previous heavy flood level to prevent any risk of flooding or more accurately, debris avalanches & flows. This is a very practical form of architecture for the area that proned to mudslides and heavy floodings.

[Gary A]

If I were to build a house I'd do both. Building a house on pilings is very practical and to protect anything under the house it is a great idea to have the land built up. My wife already had a house so I had no choice. Her house is built on a slab on fill and is more than a meter above the road. I have never seen the road flood so it's not likely that we will ever have a water problem. Parking my car under a portion of the house sounds like a great idea.

Only her house was on high ground. The surrounding yard was still only a little above road level. I had trucks of fill hauled in and with the truck came a tractor with a blade to spread the dirt. It was ridiculously cheap.

[udon]

Hi Godders,

they are called pole houses or polehouses.

Concrete base and timber, concrete or steel poles.

This thread might help.

Have a read of the FAQs at polehouses.com

[Swelters]

Fill of course offers a dry yard area during rain and floods. As a house foundation it offers as well the advantage of using a simple concrete slab on grade floor. This is the usual US practice for inexpensive housing. But simply dumping fill and then waiting a year hoping it will become stable is not a great idea in my view even though it is standard practice here. Better to spread the fill in 6 inch layers and then roll it with heavy equipment. Then you can use it right away. Gravelly or sandy soils make better fill than sticky clay soils.

Reinforced concrete columns typically 20 cm by 20 cm are the heirs of the traditional Thai hardwood posts which were sunk about a meter into the ground and were durable enough to last in many cases 50 years or more. The concrete columns can be supported on piers typically 2 meters deep for most houses. The pier is basically just an underground extension of the column. A lot of Thai contractors use footings perhaps one meter square, one meter deep and these are fine too. The columns support reinforced concrete beams which make up a framework. The suspended floors are nowadays often made of handy lightweight precast reinforced concrete planks which can be purchased at supply stores rather than the formerly used poured concrete slabs.

Bangkok sites present very tricky problems due to deep deposits of soft clay below. Fill will usually settle as much as 50% of its thickness over several years after its placement regardless of how well placed and compacted. Areas of fast moving water or hillside areas subject to erosion or debris flows also are special cases.

Thai contractors ar pretty good at foundation work and it often is best to follow local successful practices.

Edited February 10, 2006 by Swelters

[chownah]

Godders,

From your post I gather that the problem you anticipate is water level rising from rainfall and not rapidly moving water like a river flowing which could be carrying debris which could damage your structure. If this is the case then either building an embankement or using stilts will work. Which is more effective depends on how much hassle you are willing to go through. If you build an embankement around your house and out to the road then in many floods you will be able to drive to and from your house and never have to wade through the water. If you build on stilts then you will have to wade some distance to get from the road to your house. If you want to build your house on an embankement then you do not have to wait for a year if you 1. have a tractor compact the fill somewhat when doing the fill work and 2. be sure that you dig down to the original undisturbed ground for placing the footings for the foundation. Typical construction in Thailand will have the footings for the foundation at about one or one and a half meters below the final ground level. Also, standard (good) building practice is to build the beams supporting the floor at ground level strong enough to support their loads even if the dirt is not supporting them at all and they are only supported by the foundation columns. Around where I live the typical footing is 50 cm square and about 80 cm below the final ground level....this is because the dirt here is strong enough for this and the houses are wood on the second floor so the houses are not so heavy...also the distance between adjacent columns is usually 4 metres or less. If you build two stories concrete and/or have greater distances between column and/or have weaker soil then the footings will need to be bigger and deeper.....check what your neighbors have and ask the contractors about this......if the soil is really bad you will have to go with driven piles.

Chownah

[Swelters]

OK, here is the essential distinction that is not being made here: Some places such as ALL of the Bangkok area and MAYBE some other coastal areas you must have deep foundations (eg 15 meter piles) to support almost all structures, because the soft clay below is going to settle irregularly both from the weight of the fill and the structure. You need either soil borings or good local experience and engineering to deal with these problems.

Other places, probably most up country dryland places, are free of this soft clay problem, and shallow foundations, either short piers or meter deep footings, are fine, including on/in fill IF it is compacted as suggested above.

Edited February 14, 2006 by Swelters

[chownah]

OK, here is the essential distinction that is not being made here: Some places such as ALL of the Bangkok area and MAYBE some other coastal areas you must have deep foundations (eg 15 meter piles) to support almost all structures, because the soft clay below is going to settle irregularly both from the weight of the fill and the structure. You need either soil borings or good local experience and engineering to deal with these problems.

Other places, probably most up country dryland places, are free of this soft clay problem, and shallow foundations, either short piers or meter deep footings, are fine, including on/in fill IF it is compacted as suggested above.

The last line of the post just above yours says, "if the soil is really bad you will have to go with driven piles." You are right, this is an essential distinction.

[Swelters]

OK, here is the essential distinction that is not being made here: Some places such as ALL of the Bangkok area and MAYBE some other coastal areas you must have deep foundations (eg 15 meter piles) to support almost all structures, because the soft clay below is going to settle irregularly both from the weight of the fill and the structure. You need either soil borings or good local experience and engineering to deal with these problems.

Other places, probably most up country dryland places, are free of this soft clay problem, and shallow foundations, either short piers or meter deep footings, are fine, including on/in fill IF it is compacted as suggested above.

The last line of the post just above yours says, "if the soil is really bad you will have to go with driven piles." You are right, this is an essential distinction.

Right. The reason I added the geographical imperative about Bangkok and environs is that typically the upper two or three meters of ground don't necessarily look like bad soil, you might have a meter of two of decent fill followed by another couple of meters of clay that has been dried out at some time or another and looks pretty firm. The root of the problem begins at about four or five meters down, where the clay is the consistency of gray toothpaste. This could come as an unpleasant surprise.

You can see the resulting problems in the photo below where the townhouse building on left is pile supported and the garage wall in front (with associated added fill ) is not.

Edited February 15, 2006 by Swelters