|An arch directs the weight of a structure out and around an opening which prevents the opening from collapsing under massive downward forces|
In that light I feel both great shame and great pride in my first attempts at grasping the technique of forming an arch.
To make the openings for the windows I am using salvaged tires as braces which keep the shape of the window opening and gives the bags a form to rest on as they are put in place. As the bags cure and solidify the forms can be removed. To get the tire out I just need to let the air out!
Since the tire is narrower than the width of the bags, my idea was to bend a piece of plywood over the tread so the base of support for the bags gets wider and flat. To my chagrin I learned the hard way that at that short a length plywood does not like to bend much at all.
To rescue the plan from total failure I cut the playwood into seperate sections, planning on having each section correspond to a layer of bags as they took shape up and over the tire.
1st attempt: Mixed results/how NOT to do it
As I started laying bags I soon found that the sides facing in toward the tire were not lining up with the plywood sections as well as I would like and I had to use a few extra slices of wood. The inner surfaces of the bags did not come out as flush as I would like but that isnt the real issue. The most important part of an arch is the placement of the keystone. The keystone locks all the other sections into place and gives the arch its load bearing abilities. It must be placed relative to the other "stones" so that it drives the force away and to the sides. If it is too low the compressive forces will drive the keystone below the support of the stones which brace against it and the arch may collapse. You can see in the picture above my placement of the keystone is exactly what you dont want. I plan to remove the center bag and replace it with three narrower and taller bags so that the force is distributed more evenly and so that the bags sit higher than those to the sides.
2nd attempt: Jackpot!
With the hard lessons of the first attempt under my belt I took the task of the second window with a bit more confidence. The results were far more satisfying both in a structural sense as well as psychologically. My only regret here is that I did not tuck in the open ends of the bags more neatly. Its not important for structural integrity but it drives me crazy to look at. Fortunately for you there are many examples of what earthbag arches look like when the builder takes the time to keep them pretty!
The bags which go over the shape of an arch are often called "fan bags" for their shape. The best option is to tamp the bags into their fan-like shape before lifting them into place. This gives them the form they need as well as makes it easier to tamp them into position. I did this in a sort of freestyle on the ground BUT if I were a smarter man I would have built a bag form which molds the bags into the perfect fan shape for making arches.
The bag form for making fan bags has sides that slant inward toward the bottom. this allows you to pack the bag filling into shape and creates a bag in the proper fan shape. If you plan on making arches over windows or doors I can not stress enough how nice it will be to have a form for molding the bags into just the right shape. Work smarter not harder!!
Information on making these types of box forms as well as many other tips tricks and techniques can be found all in one place in a book called Earthbag Building written by Donald Kiffmeyer and Kaki Hunter. This is the bible of earthbag building and no one should be without it. You can also read the whole thing as a series of slides at THIS LINK. Instructions for cunstructing box forms can be found in the appendices.
Lessons for the future
1. It was recommended to me later that soaking the plywood for 48 hours would have made it soft enough to form over the tires.
2. I could have use 2 tires or wider tires to make the solid base for the bags.
3. If I ever build another arch with earthbags Im definitely going to make a fanbag form! It can be done without but Im convinced now that the extra effort in building a box form is well worth it.
Some of the most important advice I received during this experience as well as throughout this process has come from the members of a facebook group dedicated to earthbag building. If you are just coming to earthbags as a construction option I strongly recommend joining the group and asking as many questions as you can. All the members are very friendly and eager to help, they have been as much a part of the group effort as anyone helping me on the ground and any information I share in this blog is greatly supplemented by the wisdom there. I added a link to the FB page in the menu bar section called "what is earthbag?" right below this blogs header. You can find the link toward the bottom in the section called "more information".