There’s growing interest in lightweight bag fill materials as alternatives to more labor intensive soil-filled earthbags. While lightweight materials such as scoria, perlite, etc. are not as stable as tamped earth, their higher insulation value and labor saving advantages offer tantalizing possibilities for certain designs, locales and site conditions.
We’re currently investigating vetiver grass as another lightweight alternative fill material. Vetiver grows abundantly in tropical regions. It provides valuable erosion resistance in the rainy season, and can be cut in the dry season and used for thatch, or combined with clay or lime to make blocks or earthbags. Useful properties include natural resistance to termites and moisture. “The leaves of vetiver are coated with wax, and have a unique scent that repels insect and fungal attacks.”1
Vetiver is tough stuff and will grow over head high. This quote from Wikipedia explains why vetiver works so well to control erosion. “Unlike most grasses, vetiver does not form a horizontal mat of roots; rather, the roots grow almost exclusively downward, 2–4 meters. This makes vetiver an excellent stabilizing hedge for stream banks, terraces, and rice paddies. The close-growing culms also help to block the runoff of surface water.”2
You may be familiar with straw/clay – a lightweight mixture of straw drizzled with a slurry of clay. Straw/clay is typically used between post and beam or studs. It has just enough clay to hold it together so it provides maximum insulation. The mix we’re testing is quite a bit denser because we want increased bearing capacity. We want the blocks and bags of vetiver to be load-bearing: to support the load of the entire wall and a lightweight roof. This is achieved by adding additional clay and packing the mix tightly into the form or earthbag. In summary, our vetiver/clay mix should have greater termite and rot resistance and greater bearing capacity than straw/clay.
Useful tips: Mix large batches on tarps or in pits once you’ve determined the proper ratios. Use dry vetiver grass chopped into pieces approximately 8” (20cm) long. At first, add thin clay slurry with enough water to soften the grass. Then add a thick mix of wet clay. Soaking for a few hours or overnight will improve working properties, but is not required. Mix well and pack tightly into forms or earthbags. Forms can be most any size. Ours is 10”x14”x6” high. (24cmx35cmx15cm high)
We haven’t tested vetiver/lime yet because lime is not available in our region. We hope to buy some on a trip to a bigger city in the near future. And, we’ll be adding vetiver videos at YouTube very soon.