The B Street Permaculture Project
Natural Building Design
The B Street Farm had Lydia Dolman, an expert in earth building practices, here to instruct us on how to build a cob structure. We had several classes out to help build. Prof. Gundersen’s ENV 200 class and Prof. Van Buskirk’s Restoration Ecology class represented environmental studies who learned about using locally available, low-impact renewable materials. Art students came from Prof. O'Day's Sculpture and Ceramics classes to learn about harvesting clay from the wild and building large-scale ceramic structures. We also had some community members and their children come out to play and work. There is still lots to do so if you or anyone you know would like to try this fun activity, come on down!
“In the Mud”
Pacific University students took advantage of the opportunity to build an authentic cob (mud and straw) structure Last Friday at the B-street permaculture farm. The building will house a composting toilet that will, over time, turn waste into fertile soil for the farm. With cob expert, Lydia Doleman giving them guidance, the students and faculty began putting up walls for the composting outhouse to be. Cob building consists of combining earthly elements with water so that, after forming, they dry into a hard, durable material. Cob building is a very involved process that requires many hands, feet, and a willingness to learn and be creative.
Prior to the construction, Lydia took time to explain how the cob building process is different and similar to the construction of any other modern building. She discussed ow the cob has benefits for both the habitant and the environment. Students also took samples of the clay and silt on site and let them dry for a week’s time to see what ratio of sand to clay produced the strongest block. A cob mixture is made up of varying amounts of sand and dirt (clay) depending on the consistency of both products. The sand must be more jagged than smooth in order to grip itself together and form a tight bond to the clay. The dirt must have a good amount of clay in order to dry hard and bond to the sand. Too much sand will make the cob block too grainy and easily breakable. This also applies to the dirt in a mixture, if excessive; it will not have enough sand to bond with and will crumble. Therefore, the ratio of dirt to clay is determined by the composition of both and changes according to local conditions. After students found the best mixture for the cob (one and a half parts sand to one part dirt), and added straw to hold it together more tightly, they had the beginnings of their first cob blocks.
The labor began when about half a dozen tarps were laid around the building site and students took off their shoes, ready to sink their feet into the goop of the earth. A bucket of dirt form the site was poured onto each tarp, accompanied by one and a half of purchased sand. Half a bucket of water was then added in a manner reminiscent of pouring gravy over mashed potato “volcano”. The materials were then all mixed together by the feet of the laborers until they reached a consistency that was both moldable and sturdy. Goats or livestock in a pasture sometimes take the place of human feet, over time mixing the ingredients into cob by simply walking through them.
Once the dirt, sand, and water have been mixed thoroughly, straw is then folded and intermingled for extra strength. The mixture requires straw instead of hay because hay can harbor insects and is not as strong as straw. The straw also prevents too much heat flow through the walls once the structure is built. This step completes the cob mixing process.
As the concoction is completed, brick-sized spheres are then molded by hand. These balls (originally called cob) are where the cob building finds its name. Students and faculty formed tossing lines from the mixing tarps to the building area and started blocking the cob balls together to form walls. As cob flung through the air sporadically, often resulting in mud-splattered faces, the building began to take shape. The cob was not haphazardly dropped into place to accumulate into a wall, but intricately fashioned together. The cob was placed in a “stair-stepping” method, meaning that the wall went up in an orderly fashion, cob by cob, and took the shape of a small staircase. Where the cob joins the posts of the frame, the cob must be locked into place to prevent the cob from separating from the wood as it dries. Thus, nails were half pounded onto the frame to create an edge that the muddy blocks could grab onto. After being placed together, the wall was then kneaded together, fusing the straw of one cob ball to the mud of another. This action weaves the straw together throughout the entire wall, making for more support.
As the walls materialized, mixtures were consistently being produced. After about four hours of building, Students and faculty left to complete the construction at a later time. The cob walls must be built in sections, as stacking a wet wall too high can result in a collapse.
Article written by Robin Johnson