| Lessons learned in Snow Arch |
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| Written by Ryan Hunt | |||
| Wednesday, 16 February 2005 05:39 | |||
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Notes from arch building on 2-12-05 We learned: The snow did not “cure” at the 50 degree temperature to be able to hold the weight of the snow above it. It settled as the arch form was lowered and then slid down one side of the form. The snow was very wet and very heavy and would’ve frozen into a rather dense and hard material if we let it get cold. But as the snow thrower threw it, it wasn’t hard enough to bare the 3 PSI at the bottom. We need to do some measurements of the snow density and strength as it was blown and as it was packed, and how it would be when frozen. The hopper we put on the bobcat snow blower worked. When it got clogged up, jiggling the bobcat arm up and down did a great job of getting it going again. The snow blower was difficult to aim, especially since it was aiming into a 4 foot wide arch. Pete eventually got better at it with practice, though. Making sure it wasn’t shooting too high was also a challenge. We placed a piece of plywood at the top of the form to act as a back stop for a while. We were afraid that this was causing too much of a seam at the top, though. First, we built up both sides 16” thick with the bobcat just dumping snow into the form, and then working the chunks out and homogenizing it by jabbing the shovel into it repeatedly. When the outside piece was removed, there were still visible gaps at the surface where the snow had been lumpy. We built both sides up about 7 feet this way and then had to quit till the next day. On Saturday, Pete brought his snow blower. We used that to finish making the first arch. As the form was lowered, everything worked beautifully. Then we moved the form over and tried it again. That piece went up smoothly. The snow blower processed all the chunks out of the snow, making a spray of snow that half stuck where it landed and half flowed down hill up against the outside form. Where it would build up, some snow needed to be removed when it got too thick. And the snow needed to be helped down into the space between the outside form and the arch sometimes. When we lowered the form for the second arch length, we decided to try dropping one side at a time. The jacks in the front got lowered more because I let them down a bit faster than Pete let down the back ones. That made the form tip away from the first arch, and cause all sorts of nasty cracks in the snow of the first arch, too. As we let down the back jacks completely, the snow collapsed and slid down the north side of the form. After this, we rebuilt the north side, and let it set for an hour while we got lunch and diesel for the bobcat. When we came back, we let the form down by lowering the back just a little faster than the front and tipping the form so it pressed the new piece against the previous piece. The results were no better this time. The north slide avalanched. So we tried to clear it and move the form back under the original piece of arch. That arch had shifted/leaned a bit to the south and had cracks at the top. As we pressed on the arch to try and get it back into shape, it failed catastrophically. The fact that it did fail catastrophically was a bit unnerving and disheartening. After that, we decided to try making the arch the full 2 feet thick. Doug noted after we got snow up, that it took quite a bit more snow. I though it was easier to guide the outside thickness of the snow since we didn’t have to follow the line 8 inches inside the edge of the form. We built a new outer form piece to be this outer guide and it worked pretty well. After this was put together and the back end piece was removed, we examined the profile of the snow arch. We could already see some radial cracks near the top. As Doug and Pete lowered the jacks (very gently) I noticed that the snow arch was lowering right along with the form. Then the south side failed and slid off the form. The problem was that the snow was not carrying the estimated 3 PSI at the bottom. The cracks at the top before we moved the form were from the snow settling at the base under the pressure of the snow above it. The remaining snow on the North side of the form was very soft to the touch and could tolerate a bit of packing. While the snow blower did a great job of making a uniform material, and in the 50 degree warmth of the day, it was a nice, sticky, formable snow, it was also a snow that wouldn’t harden till it got cold again. Snow density and strength: According to a study done in Sweden of the ice hotel, the density and strength of the machine made snow was: Temp Density Kg/M3 (lb/ft3) Compression strength MPa (PSI) -5 C 532 (33.2) 0.598 (86.1) -10 C 558 (34.8) 0.681 (98.8) -11C 550 (34.3) 0.879 (127) Source: http://epubl.luth.se/1402-1528/2002/13/index-en.html Based on this, which was obviously frozen snow instead of the wet snow we were using, our snow was really dense, but really soft till it got colder. Improvements: 1. Electric Jacks to better lower all four corners smoothly 2. A funnel at the top of the form to catch the snow blower spray would be nice 3. Better side forms to make efficient use of the snow while making the thickness consistent 4. Better understanding of snow and its density and load carrying capacity. Do this with a short study 5. Some sort of packing mechanism to help pack the snow as it’s applied. This might be a person in a snow-suit and mask and big boots to ride up the arch and just stomp the snow. Or it might be a machine like a jumping tamper. 6. Longer form. Instead of making a 4 foot length at a time, I think 8 or 12 or 20 feet at a time might make more sense. It would take more jacks and capital in the form, but much less overall motion and hassle. The amount of “tipping” when the jacks are lowered unevenly would be less and it would happen less often. Plus, if the snow has to set over night to get hard enough, you would be making better progress than just 4 feet at a time. 7. Pete suggested maybe just piling the snow along side the form and snow blowing it like usual instead of having a hopper on the snow blower. That is another thing that would be easier with a bigger form. 8. If the form gets any taller or even deeper, it would be nice to include a set of steps or stairs right on the form to help the workers access the top and work the snow. Transferability to building with Cob: Lots to be determined there. Many lessons are applicable, but most of the information will probably not be used because of different techniques that we may choose for building from a mud like material.
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