I’m planning to add 6 in. (152 mm) of shotcrete to an existing 12 in. (305 mm) wall of a below-surface concrete tank to accommodate the removal of an existing middle support slab. The soil grade is approximately near the top of the existing tank wall. I’ve been told that since the existing wall is preloaded with soil, adding shotcrete will not increase the strength of the thickened wall and that the only way the wall will act as a whole (based on 18 in. [457 mm] thickness) is if the retained soil load is removed, then the shotcrete is added, and then soil is put back in place. Is this assessment accurate? Is there a way make this wall work as 18 in. (457 mm) without removing the existing soil?
Stress distribution from external loads through the tank wall with the shotcrete lining will depend on the geometry of the tank and the structural function of sections to be removed. A professional engineer experienced in shotcrete and concrete tank design should be consulted to ascertain the structural capacity of the completed wall. It would certainly be important to create a good bond plane by roughening the surface and removing any loose or fractured materials and using sufficient drilled dowels to make the existing 12 in. (305 mm) wall and new 6 in. (152 mm) overlay work well together. Also, it might help to specify the use of a shrinkage reducing admixture.
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