That depends on quite a few things:
These are a few elements involved in creating an earthen form:
Site selection and preparation
At the very least you want level solid, compacted ground, that is not lower then the surrounding area. Your most likely going to want to dig at least a three inch trench the shape of the foot print of the structure, in which to form your footer. Choose an area that makes sense with the appropriate orientation relative to the sun, wind, traffic flow, etc. Move the straw bales around and sit on them visualize the end shape and how it will fit in to your space. What features can you build in to it? Can it be more then one thing, serve more then one purpose?
Can a wall be a bench, or a bench a wall for a garden, Can a wall be a bench and a water tank?
Who will use the space? how? how many?
Also consider access to materials , water, work area etc. Where will you mix? pile materials?.How will you get the materials to the site?
Footers can very from a rubble filled trench and pavers, to an urbanite (chunks of busted up concrete) filled trench, formed up with flashing, and infilled with quickcrete, or even a poured footer, or mortared stone, brick, or block. What ever footer material you choose, it has to be thick enough to support the weight of the structure, and dug in well enough to prevent under cutting, as well as high enough above grade to keep moisture from wicking up from the ground. The bottom of the cob, bales and plaster should be a minimum of 3″ from grade and as much as a few feet in snow country.
Straw Bales or Cob Pile
The method you choose, either straw bale with earthen plasters, or a cob pile, will effect both the materials cost, and the labor requirements. A straw bale core is quicker in the beginning stages, requires more straw bales, less adobe mix, and a thinner footer. Cob pile requires more adobe mix, less straw, and a more substantial footer, but it is more flexible as far as shapes.
Either method requires a plaster coat and some kind of weather resistant coating, even in dry areas, to slow deterioration. This can be done with linseed oil, lime plasters, and a number of other earth based coatings.
Depending on your climate, you may need to protect your bench with a roof. Horizontal surfaces are vulnerable to heavy rains and snow. Most of the work I am currently doing is in the Tucson Arizona area, also known as the upper Sonoran Desert where we get an average of 12″ of rain a year, and seldom more than an inch in a day. I do tough outer coats, topped off with a linseed oil earthen plaster that will be maintained on a yearly basis. I feel this will suffice here, and offers a good balance between upfront costs and ongoing maintenance.
Walls can be capped with stone, bricks, concrete, tiles, old pieces of roofing or planks of wood. You name it as long as there is adequate overhang to protect the wall. Benches need more cover in some areas with a ramada or gazebo roof covering the whole bench with a few inches of over hang at least.
So say you want a curved bench with a back about 4 feet tall and a full bales width to sit on. You want it to be able to sit 8-10 people comfortably facing a center piece such as a podium court yard or fire pit.
This bench is straw bales covered with cob done mostly by hand, no mixer, with only one day of workshop, to put on the rough cob coat. I put nearly 200 hours in to this project and bought a mixer near the end to take some of the work out of mixing alone.
This bench has a 6″ footer made of on site gravel and hardware cloth staked in with rebar. All the adobe mix was harvested on site The finish coat has linseed oil in it and the cap bricks are mortared in to place.
The materials were about $300.00 or so, the labor would have cost around $2000-$3000 depending. It could have been completed in 3 weekend work shops with set up and finish work done in between. The cost of the project if done workshop style would be around $1000.00-$1500.00, depending on the cost of the facilitator and how much paid work was needed. Most of the work can be done during the workshops, which should minimize labor costs.
This wall was built by Rhiwena and myself out of cob pile using a mixer and some on site materials harvested from basins created to direct the water away from the wall, and some adobe mix brought in.
The footer was urbanite mortared and filled with quickcrete. 8″-10″ thick and set in a trench. The finish coat was linseed oil and earthen plaster.
The materials were about $500.00 and we put about 300 man hours in to it. Labor would have been $3000 – $4500.00 depending. If this project were done workshop style over 3 or 4 weekends it could be done for about $1500 – $2000.00
These benches are straw bales covered in cob and earthen plaster. mostly done by hand work shop style with about a week of paid work for the footers and set up and 2 weekends of work shops. The cost of materials was about $400.00 The workshops and paid labor cost about $800.00 – $1000.00
The footer is urbanite infilled with quickcrete and formed up with 6″ flashing set in a 3 inch trench. The final coat will be linseed oil and earthen plaster.
These are just three examples of projects I have helped with. You don’t have to start this big, simple one or 2 bale benches can be made in a day for $50.00 or less. Your figures will depend on local costs of materials and labor source.
For me the least expensive and generally most fun way to do these types of projects is workshop style. Find a local earth builder or experienced work shop facilitator and gather your friends and neighbors. Make it a community building, and learning experience, have fun with it.
I’ll be expanding on these design examples in future posts with drawings, materials lists, and detailed instructions.
Start to finish in less than 60 seconds
360 cubic feet over 12 tons of adobe mix type soil some from on site
6 bales of straw
a couple hundred pounds of sand
and a gallon of linseed oil
About 2 months at 3 days a week for 2 people
The next steps will be prepping for the cap getting the final shape, and level, to the top of the core and smoothing out irregularities on the sides. The closer to finish the shape of the core the better. The scratch and finish coats require sifted materials, we want to use as little of these as possible.
Joe came up and helped a day, and learned how to do the mixes, and build up the core.
Having a third person to help with mixes made all the difference. The pace is much smoother and we got to catch breathers. This is the first day we got to the one foot of lift per day pace I wanted to be at from the beginning.
straw bale project at Joes slide show on Flickr
The yard is full of gravel so we are using that for footers and the soil has a lot of clay so we are using that for cob.
The Straw bales are spiked with rebar down in to the gravel footer, then sewn together with bailing wire to bond the whole structure together. All the gaps and seams are packed with chips of straw or loose straw.
Core form of straw bales is anchored and sewn together ready for cob.
First Layer of filler cob applied where the bench meets the wall