Beware of clay soils in Philippine construction.

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Coming from a place where it gets very cold in the winter (Upstate New York), we realize that footers have to be deep enough, usually four or five feet, so that freezing soil does not heave and damage buildings.  When we built our house in the Philippines, we puzzled over our engineer’s plans which called for column footers to be three or four feet below the original level of the soil.  Why in a place where it never freezes do footers have to be so deep?  Likely such deep footers are not necessary in many parts of the Philippines, but our property is underlain by many feet of exceptionally dense clay.  This is typical of “rice land” in the Philippines.  If you have sandy or rocky soils just ignore what we say, but of course you may have different construction issues.

As noted, our area has heavy clay soils dozens of meters deep.  During the dry season these soils crack and shrink. This shrinking and cracking can exert tremendous force on structures not supported by deep foundations —  foundations well below the area affected by the seasonal shrinking and expanding of the clay.  Only structures with footers deep enough to be stable will escape these forces.

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Soil movement by shrinking clay can tear apart concrete construction. These cracks can extend a foot or two or more  below the surface.

We have had an exceptionally dry summer in 2016. We first learned of the dangers of shrinking clay when the lovely concrete platform our workers built for us around our well and water pump was broken apart by the heaving clay beneath it.  The foundation of this platform was one hollow block set on edge and a concrete footer The cavities in the block were filled with concrete.  Reinforcing bar tied the elements together.

Concrete platform around our well and water pump.

Concrete platform around our well and water pump.

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As you can see in these photos of the platform’s demolition, it was not lightly made. Nonetheless, it was destroyed over its five year lifespan by the seasonal expanding and shrinking of the soil on which it sat. Our worker is shown breaking up the platform. For the time being, the platform has been replaced by topsoil. The well itself was not affected as it extends some 25 feet into the ground and hence is well supported.

 

 

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Here you can see a lovely tiled concrete stoop being removed. It was very strongly made of 1-2-3 concrete with rebar. It was not damaged by the clay soil, but it had no below grade footers and moved with the shrinking and expanding clay soil which was underneath it. The movement f the slab  threatened the many water and drainage pipes running underneath it.  That’s why it had to be removed. Building on a slab may be fine on sandy soils (Florida?) but do not seem suitable for the shifting clay of Philippine ricefields.

Catch Basins.    Our house has an interconnected  system of underground concrete catch basins which collect water from the roof and gray water from the bathrooms and kitchen and direct it away from the house so the area around the house does not become wet, muddy or worse.  The catch basins themselves are very strong but as the clay soils dry out and shrink, the catch basins sink.  When the rains come and the clay soil expands, they move upwards again.  As you can imagine, with plastic pipe connecting the catch basins to the house; the house stable, the catch basins moving, pipes can do do break, but they are underground so you may not be aware of what is happening, but this photo shows the effect of sinking catch basins on gutter downspouts.

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But the main event is the destruction of our storage building by these forces.

After the house was completed we had some big roof trusses left over due to an engineering change. (A change we never understood.) We also had some 16mm rebar left over. So, we decided to build a shelter for our car.  We really did not have proper plans, but we went ahead and constructed four large columns which supported concrete beams reinforced with that leftover 16mm rebar.  Then we took the leftover trusses and used them to make a roof structure which we subsequently roofed with the same metal roofing we used on the house.

The garage columns had proper footers and have not moved.

The garage columns had proper footers and have not moved.  Disclosure – this is a footer from the house.  We did not have a photo of the garage footers.  They were the same.

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Garage frame up and ready for roof trusses.

The leftover roof trusses were sized for the house, not for the much smaller garage but I was loathe to cut off the ends of the trusses.  The oversized trusses allowed bigger area protected by a bigger roof — for “free”.  We did cut the ends of the trusses on the side of the garage shown here, but left the truss overhangs on the fence side of the garage  This gave us a garage which was much too large for our single car.  At that point the garage floor was just gravel.clay_soils-1-6

clay_soils-1-7Why not use the extra space for a much-needed storage room and a small laundry area?  Everything was impromptu at this point and that was a mistake. The garage building itself has deep footers and has been totally stable, even though some small earthquakes.

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Storage and laundry area being added into extra space under garage room.

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Completed storage room with iron door.

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and laundry area

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

However we did not put in proper footers and columns for the storage room.  As a result the rear wall of the storage room/laundry are has sunk several inches and is tearing apart the storage area.

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What you see here on the right is one of the garage columns which is resting on proper footers. On the left you can see that the rear of the storage room  is sinking, pulling the whole storage room structure apart. The entire building will have to be demolished and replaced.

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At the time we built this room, we did not see the need for the deep four foot footers we used elsewhere.   This “shortcut” will cost us money in demolishing the old structure and in building a new one.  Had we taken the time to put in even two columns on proper footers, the building would probably have served us for many years. Lesson:  hire an engineer to ensure whatever it is you are building has proper footers!

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Despite the problems, the garage is looking good.

Comments (3) Write a comment

  1. This is the reason why people in the old days dig up the soil and expose the rock underneath, then use the clay to make cob homes, until concrete becomes fashionable.

    Reply

    • In our area, the clay can be hundreds of feet deep. In essence there is no rock underneath. In other parts of the Philippines, bedrock is available.

      Reply

  2. That is terrible! Like you I guess, I had no idea… Just wondering, would keeping your yard watered be enough to stop that? I have seen a lot of newly built places with sidewalks that turn into a rocky path after a few years. I thought it was just cheap cement. The place I live now was built right at the time of Yolanda and I don’t think they put much of any footers in. Interesting story Bob. Thanks.

    Reply

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