Building earthquake resistant clay houses
by Gernot Minke and Hans-Peter Schmidt
1. Building Material
HPS: Driving around Nepal after the earthquakes, one of the first things to notice is that most steel reinforced concrete buildings are still standing with only minor damages while many traditional clay and clay-brick buildings crumbled or were severely damaged. This raises the logical and compassionate question: Is it possible to build earthquake resistant houses with clay or must Nepal replace clay and clay-brick when rebuilding the country?
Minke: It is not the building material clay or clay-brick which causes the collapse in an earthquake but the structural system and the design of the houses. For instance in Mendoza, Argentina old houses with thick rammed earth walls withstood all earthquakes of the last centuries, whereas all modern buildings built of adobe (unburned clay bricks, mud bricks) or burnt bricks collapsed. I saw also in Bam, Iran after the heavy earthquake in 2003 that some of the adobe vaults resisted, whereas all brick and even steel-skeleton structures collapsed.
Do you have an easy test to check if the clay-soil found close to the building site of a house is suitable as building material either for bricks, rammed earth walls, or joints between bricks or plaster?
Minke: Clayey soils are always different and there are simple field tests to check their quality. They should have enough clay content but also enough coarse sand and fine gravel to provide stability. If the soil has too much clay and silt (very fine sand), the soil will get many cracks after drying. If it has enough coarse particles it will dry without getting cracks.
Is it better, from an earthquake resistance perspective, to use burnt clay bricks or build with massive rammed clay-soil walls?
Minke: There are generally two possibilities to build earthquake-resistant houses with earth. Either using thick massive walls from rammed earth or large adobes or using a flexible structure of timber or branches filled with earth, like the wattle-and-daub-system. Fig. 1 shows such a house which withstood a heavy earthquake in Guatemala. In this case the kinetic energy of the earthquake was absorbed by deflection. Either thick and massive walls or thin and flexible walls!
Fig. 1: Clay house build with a wattle and daub structure after a heavy earthquake in Guatemala with no vissible damages (Minke, 2000).
Does it help to add some cement to a local clay-soil mixture? And if yes, how much cement should be added?
Minke: No, it is not necessary to add cement, only if you use earth plaster and want to make it water-resistant. But it might be dangerous if you add only up to 5% of cement to the earth, than the mixture is less resistant against compressive forces than without cement!
Are there other locally available materials to add to the clay-soil mixture to improve it's strength like cow dung, straw, lime etc.?
Minke: If the soil has little clay-content it is good to add cow dung, also to make an earth-plaster more resistant against rain. Fine natural fibers help to reduce shrinkage during drying.
2. Placement of the building site
Most of the building sites in Nepal are steep or terraced on steep slopes. How should a house be placed in this difficult topography?
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