After reading about Brunelleschi's Dome, my brother and I had an goal to build an arch out of snow. It doesn't follow his stone chain method, but it is a free standing arch.
Things you'll need, - Snow of the right consistency. Not too dry, not too wet. Packable. - Cool temperatures. This arch was built in warm temperature, which led to its short life of about an hour. - Support braces. We used a series of sawhorses, boards, and snow. - Ladder - Block mold. We used kitty litter buckets. It should have flat sides and have a slight taper. - Some sort of scrapping tool.
Find a flat surface and clear an area about 3'x3' down to the ground. For our arch we had a distance of about 14' from center of support to center of support. As you can see from the picture, I used an ice scrapper to measure an equal distance from a point we wanted as center. Also notice we used a larger form for the first two rows of the arch.
As you begin filling your forms and making your blocks, try and make them as compact as possible. Any weak areas of the block will eventually make your arch shift, trust me. Let the natural taper of your form begin to slowly form an arch. Do this by stacking every block with the small side of the taper towards the center of the arch. You will see it slowly bend towards the middle.
As the arch builds it's important to step back and adjust the pillars. It's snow, so it will shift and compact. Make sure you help the snow pillars get back to a true state, perfectly straight up and down from left to right. Obviously, keep the arch arching, but the front to back may need to be straightened.
Now the fun starts... Keep forming blocks and keep stacking. Keep adjusting front to back and keep stepping back to make sure both sides are arching in with the same degree and curve. As the pillars grow in, they will need support. You want to keep this as simple but as strong as possible. We used sawhorses with a simple board for the first layer. We supplemented smaller supports by using blocks of snow that didn't make the cut for the arch. Make sure you support the natural arch of the snow, do not over support it by pushing the curve up.
Keep stacking and adjusting from front to back. Make sure both pillars take the same shape and are working towards aligning at the top. As you keep staking, add another board for support. Then more snow supports then another board as you see if. This will vary depending on the size of your arch.
Also, you can see that we've added some front to back supports. Our blocks were very wet and heavy and compressed a great deal, which led to some serious front to back movement. We added some front and back buttresses to add support. We needed this to make this thing stand. You may need it too.
VERY IMPORTANT Be careful of hoop stress. Hoop stress is the outward force the arch sends due to the vertical weight on the curve. The goal is the properly direct all this stress into the ground. Do this by keep a smooth curve from the bottom of the curve to the top. If you have vertical sides on the lower portions of the pillars you will have increased hoop stress. If you have too much stress the arch will kick out the sides and collapse. Keep this in mind as you build.
Keep your arch growing until you have a space approximately the shape of a triangle on the top. This wedge piece is call the keystone. This locks in the arch and makes the entire arch lean against itself, thus eliminating the need for your entire support system. This piece needs to fit perfectly. After this piece is placed, it is your last chance to true up the arch. Make any serious adjustments now before the support system is removed.
Once the arch is completed and you've trued it up and you've filled any cracks with additional snow, it's time to remove your supports. Start at the top, and remove the snow from under the keystone. Then move to either side and take a little more snow out. As you remove snow, see how your arch reacts. Does it sink, settle, and compress more? This might be because you supported it too much. Basically, dismantle your support system very slowly. This is where most arches fail, even stone ones.
Watch how the arch changes shape over the one hour of it's life. We noticed where our weak spots were and how a curve that is not perfect is an arch in danger. Check out the final photos and video. The video is the last eight minutes of the arch's life. Each second is about one minute, until the crash. Be sure to watch the slow motion reverse crash. :)