[Ferro List] Catenary Dome

[Keith B]

Hi Peter,

For deep mine tunnels, lateral pressure commonly exceeds vertical, so 
rock bursts from the invert (floor and bottom hinge area) become major 
concerns.   For the shallow burials of earth sheltered structures, the 
biggest hazard is the condition of the material alongside, so again 
lateral pressure the principal concern.  By far the greatest danger is 
hydraulic pressure from saturated material, though the largest potential 
forces are from clay shrink/swell with moisture change.  Clay against 
the structure risks that, and damp clay does not compact protectively, 
instead it tends to ooze and move to where it's not wanted.  Damp but 
not wet sand bridges fairly well, but the key is drainage to prevent 
saturation and, ideally, measures to keep material dry to greatly 
increase internal friction. 

Minimum lateral pressure results from particulate matter consisting of 
particles which do not rotate easily.  Coarse, angular stones are best, 
of fairly even size rather than a graded consist.  Better air and water 
flow helps more than tighter packing.  Think sub foundation material and 
why gravel is used under water tanks, then add bridging action and 
effective friction from interlocking limiting lateral movement while 
transmitting force vertically..

kb


Peter Payne wrote:
> Best shape for underground: ...   ...A very sandy wet earth 
> might act similarly to water, whereas a clayey soil could compact around 
> the shape so as to put very little pressure or weight on it. The most 
> common situation would be: substantial vertical load, and also fairly 
> substantial lateral pressure. So, a shape that would be a cross between 
> a pointy arch and a sphere. ...
>
>