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.
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.
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.
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 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.
Why 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.
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.
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!