[Ferro List] Community size ferrocement underground water tanks forBolivia.

[Doug Lacy]

Ron -
 There seems to be a lack of a usable models for anything but that is getting better with ferrocement thanks to FEN and the publishing of free information about the technical details for ferrocement.
I have considered underground tank techs and have done some ferro ditches and have some ideas that may be helpfull.  

If the hole can be carfully shaped to act as a back form for the LFC technique, much like a swimming pool hole but tighter on the details, underground may have some advantages.  

If the edges and side of the hole form are not stable and tend to crumble away because of too much sand, an inexpensive water diluted liquid polymer binder called SoilSement can be sprayed on the edges, side and bottom to harden and stabilize the soil edges so it can be shaped with better precision.  A sacraficial layer of inexpensive plaster may be needed to create a smooth geometry for the lining of the hole so the more expensive water proof mix can be conserved.   The closer you can get with carving the hole the better and if the shaping is real tight, you can start your LFC tech with the rich mix right on the damp earth. 

A practical shape for a sub terrainian tank may be an elogated bread-loaf looking shape that is 8.5 feet deep at the center, 8.5 feet wide, 60 feet long with a shallow arch cap that is a 20 percent lateral slice of a circle.  This shape should be wide enough so that workers are not in too much danger of being trapped by a cave in yet narrow enough that you don't have to worry about driving heavy equipment over your subterainian tank. The shape is also an easy shape for two workers to plaster while working a short green seam and being able to execute a substantial section of a more complete geometry of the shape, in otherwords the cold joint will be easy to control because it is kept in proximity of the workers and is in a tight area that can be kept fresh and tarped as opposed to very spread out circular shape where one seam must be abondoned for a few days.
This tank should be about 34,000 gallons.
The shell area surface area will be about 1,500 square feet or about 40 Portland bag mixes if the shell thickness is kept under 3/4 of an inch and each bag of portland yeilds one 3 cubic foot wheel barrel. 
 If two and half layers of expanded metal lath is used, it will use 240 sheets of lath.  The arch cap will use a roll of remesh and possibly 6 sticks of 5/8 rebar that are 20 feet long, cut in half and flexed into the arch profile at five foot centers along the length.  6 inch Remesh blocks will work better for rigidifing the arch and you can get most of those out of the left over remesh steel.  This tank shape is easy to plaster and can be burried withoun any other internal supports.  I would put the man-hole lids at either end so you can create some cross-draft and seperate any intake and out take water ciculation lines that may be used to help condition the water if that is neccessary.
Doug 


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