The original mud wall
Re-discovering the virtues of cob construction, a mud wall so thick that it can take all load without getting eroded by rains for a long time.
All of us have heard of mud walls, possibly the wall material for the majority of standing structures in India today including all rural buildings. Of course, the increasing presence of burnt bricks and cement blocks are a threat to the popularity of mud, yet traditional mud walls are around us. But how many of us have heard of the term ‘cob walls’?
Dictionary definitions apart, cob walls are the original approach humans used to construct walls with, even before learning how to make mud bricks to specific sizes to sun dry or kiln-burn them. For many thousands of years, cob walls dominated the early human settlements, including those in Babylonian or Gangetic plains.
Have they lost their relevance today? Partly yes, but not entirely. In many rural areas, people continue to build with cob, but in cities, it has almost vanished. Curiously, now there are increasing attempts to revive this technology by groups such as Thannal and Marudam in Thiruvannamalai, Sacred Grove at Auroville, Made in Earth, Mudhands and Biome in Bengaluru and few others in different parts of India.
We are rediscovering the virtues of cob construction. Simply stated, it is a mud wall built so thick that it can take all load without getting eroded by rains for a long time. Some surface moisture, peeling and erosion may occur, which can be repaired periodically. The volume of the wall acts as thermal mass, keeping the building cool in summer and warm in winter. With high compressive strength due to the thickness, cob walls are also earthquake resistant.
Having said this, haven’t we heard, seen or been to such structures like shrines, choultries, monuments, heritage homes or even palaces? Of course, many of us have been there to wonder about the thick walls, not realising their possible potential today. Often these walls were thicker at base, tapering as they reach the top at least in one side, often on both sides.
Cob walls do not need much structural calculations, great construction skills or big budgets. A lump of clay is called cob, so building with them must have been an easy discovery during early civilizations. The fact that even after construction became regular and formalised, cob continued widely, seen even now from lower Himalayas to coastal south India, proves its time-tested qualities.
Cob walls make minimal demands of clean sub-soil with less clay and more silt, space to accommodate thick walls, some local additives to reduce cracking and material options for final finish. Typically, cob walls are plastered with mud mixed with straw, though in poorer homes they were left un-plastered, with mud slurry and lime wash as crack fillers. If we walk into an old house with plastering peeled off, we can see the original clay lumps revealing the origins of wall construction.
While cob uses local soil to build, it is not the same as the other mud architecture techniques. Building with double hand size mud balls dipped into additively mixed mud slurry as the joint binder is seen some rural areas, which is very close to cob practices. Rammed earth walls could be mistaken as a cob. But they are much thinner at 9 inches only, while cob could be 18 to 24 inches thick. The other differences being the smoother surface of rammed earth, very few surface cracks, no need for plastering and such others.
The Adobe system depends upon mud bricks made to size, sun-dried at the site itself and used to build with. Stabilised mud blocks are also sized blocks, with cement and quarry dust added to gain different properties like strength, thinner walls, durability and better surface density. Cob walls are much simpler than all these.
The re-discovered cob walls are not merely repeating the rural practices, but have attempted technical improvements. There also has been deeper studies about the causes behind surface cracking with solutions; possibility of avoiding thicker base where the whole wall width could be same; different options for additives for the mud; scientific modes of quantifying the components of mud into clay, silt, sand and such others to accordingly decide the right kind of proportion ideal for the cob wall.
These methods of improvisation re-validate the use of cob walls again to claim a position in sustainable architecture. It is time we look into it.