THE ANYWHERE COB WALL

What was the original material of Bangalore fort? Not all can respond to this quizzical question, but the answer is mud.

Cob Walls by Thannal, ThiruvannamalaiThe local chieftain Kempegowda moved his capital from Yelahanka to Bangalore in 1537, with a fort surrounding the market area called Pete. As was normal in those days, it was built with mud, possibly a thick solid wall. Increased threats by enemies made Chikkadeva Raya Wodeyar enlarge and strength it around 1700, still power moved to the hands of Maratha ruler Shahaji. When Hyder Ali claimed the territories around Bangalore, he got the mud fort replaced by stones in 1761, considering the changing technology of warfare.

This fascinating piece of history is also a part of history of mud. Mud walls have much to tell about how we lived in the past, for they sheltered the history of human civilizations. Incidentally, mud walls have a future too, in these days of climate change and ecological challenges.

Most of traditional mud walls were not built with sized bricks, but lumps of clay mixed with straw and lime, what we have today termed as cob construction – simply piling up lumps of local clay. Thick walls were the first human impulses towards shelter making to gain strength by thickness, stability by wider base, durability by multiple surface layers and passive cooling by thermal mass.

Though the walls are thick, they breathe being porous, hence can control indoor environment. The thickness also helps as moisture barrier, hence average indoor humidity can be maintained. Of course, we need to protect the base of the wall where rain water can splash back and protect the top of the wall with good overhangs, otherwise they tend to erode as can be seen in many village homes built carelessly. Lack of proper cleaning of clay may also lead to plant growth on wall from the roots left in the mud mix or if raw fertile soil is used, it may result sprouting of seeds settled on the wall during rains. Either of them is not a technology problem, but an operational one.

The wider and more compact the wall is, the greater load it can take. This system is called load bearing by compression, a cheaper and long lasting method.  Contrastingly, the RCC framed construction transfers load by tension members, which is costlier and has lesser life span.

The roof weight normally go vertically down, called axial loads, which can be well handled by cob. In case of sloping roofs and other lateral loads, proper wall plate beams are required. Many rural structures were not aware of this, where we can notice cracking of wall tops due to lateral loads. Of course, many of them were made thicker at the lower levels, to partially counter the lateral forces and also be able to transfer larger loads. People have also tried stone wall at the base, which actually works very well, often finished with mud plastering to blend with the wall.

In many ways, a cob house is like an anthill. Termites dig out small balls of earth and build atop, with porosity. It may appear primitive, but the way it is being done now scientifically with studied inputs makes it the most green.

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Posted on April 28, 2018, in designs, fundamentals and tagged , , . Bookmark the permalink. Leave a comment.

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