The first question is do you intend to use the steel shell to carry any of the load. If you do, you should be concerned about corrosion. At .08mm/yr, the shell would be reduced to .25 inches of shell thickness in approx. 40 years, which may be the limit of the design life. However if you figure...
Wow! This is a very tricky problem. You probably need to sheet the excavation. The fine sand has little shear strength, and with a 25 foot head and satuated condtions, could easily blow. The sheeting would probably need to be tied back, and the depth of the toe of the sheets would be controlled...
A thickened slab is a portion of a slab that is deeper than the rest of the slab. This is usually done to increase capacity in a specfic area. In a warehouse for example, a slab is usually thickened at the truck bay door because this area sees more activity than the rest of the floor. If not...
