Many tank foundation repairs are needed due to shell edge settlement. The area directly under the shell takes much more load per square foot than the tank floor area, even when the tank is full. This problem is best solved by a concrete ringwall footing. A concrete ringwall spreads the shell load and decreases the psf on the grade below. A concrete footing does not lose strength like gravel does when subjected to the concentrated water exposure from rainfall collected on the tank roof, pouring down on the footing.
This repair can be done by leveling the shell to API specs, blocking it, excavating, forming and pouring the ringwall. The main floor area under the tank is usually ok and left as is. Where the whole tank and floor needs to be lifted, we show two examples.
A 250 ft. Diameter tank floor rusted out because the tank was always sitting in a pool of water. The rusted floor was cut out by others. We raised the tank 2 ft. in a unified vertical lift, using 40 jacks around the shell and 122 jacks on the 61 inner roof columns. We placed concrete pillars and rebar, then formed and poured the concrete ringwall with one pour. The inner columns were supported at 2 ft. and the jacks removed. Then a new 2 ft granular pad and new floor were placed.
A 45 ft. Dia. tank needed to be raised 2ft. and a monolithic pour was also specified. In this case, the jack bases were elevated above the ringwall, on 'bridge' cribbing, so they would be out of the way of the pour.
Signs your tank is settling:
Tank floor at shell is sunk into the ground, tilted, or out of plane
Tank floor at shell is not contacting the ringwall in places
Tank floor at shell lifts off the ground slightly when the tank is empty
Piping to tank is misaligned and under stress
Ovality in the shell indicated by diametrical measurement or binding of a floating roof
Visible flat spots at the top of the shell where the roof rafters contact the shell
Kinks or creases at the shell to roof joint
Tank roof plates are too tight, too loose, or buckled into waves