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Fellow engineers,

I have encountered a rather peculiar problem. I will be happy to have your response. I will try to be as clear as possible.

1) A circular steel tank , weighing about 120 tonnes( inclusive of contents) , is supported on 4 'I' sections which rest on concrete pedestals of size 500 mm x 800 mm. The diameter of
        the tank is about 3.0 m.

2) The 4 concrete pedestals rest on annular mat foundation ( ID 1.8m, OD 4.2 m, 0.6 m thick )
      at about a depth of 2.5 m.

3) At the time of construction ( 1973), the top soil consisted of Black Cotton soil ( Expansive Clay & also impermeable ) up to a depth of 1.5 to 1.8 m, followed by a layer of weathered Basaltic rock of 0.3 to 0.5 m thickness. A good rocky strata was met with at a depth of 2.5 m & hence founded there. It was back filled with Black Cotton ( B.C.) soil due to its low permeability.

4) This tank is a part Aluminum production plant which heavily uses caustic soda. Despite several measures, due to leakages at various stages, the soda always finds its way to the ground. ( though it is let out through a steel lined gutter ). The caustic soda enters the pores/fissures of rock, crystallizes, expands & cracks the rock.It weakens & softens the rock.

5) Over years, caustic soda has leaked & reached the foundation level. It must have 'digested' the rock on which the footings rest, probably converting it in to 'soil'. This 'soil' is highly expansive in nature in the presence of caustic soda solution.And so is B.C. soil which was used for black filling. THUS BY 1996, THE WHOLE STRUCTURE GOT LIFTED BY GOOD SEVERAL INCHES!
     Surrounding flooring was also damaged.

6) It was decided to redo the flooring. By then, a research project was also undertaken by a renowned research institute in India to study the effect of caustic soda on BC soil ( still going on). The proposal to replace the back fill of BC soil was shot down on the premise that it is already saturated with caustic soda & would also act as impermeable layer. A reinforced concrete
    slab flooring was laid on top of this BC soil.

7) Now again, the floor is undulated. They want to use geomembranes to create an impermeable layer coupled with BC (!) soil. Hence the back fill has been taken out including that around the pedestal. NOW HORIZONTAL CRACKS IN RCC PEDESTALS ( ATLEAST IN 2, OTHER 2 NOT YET EXCAVATED OUT OF FEAR ) , 1 TO 2 MM WIDE, ARE NOTICED. ONE PEDESTAL HAS TWO HORIZONTAL CRACKS, FIRST ONE AT ABOUT 1.2 M BELOW GL & SECOND AT 1.6 M BELOW GL. THESE CRACKS GO ALL AROUND THE PEDESTAL. They don't look like fresh cracks & must
     have occurred during heaving.

    I have two questions:

1) What could be the mechanism by which the heavily loaded pedestals have developed horizontal cracks?

      2)  What may be the remedial measures.


a) I think there must be cyclic heaving & settlement ( heaving more than settling to produce net uplift ) due to variation in saturation levels of soda or moisture levels in surrounding BC soil.The downward movement of the pedestals may also be due to digested/weak soil underneath the

b) The BC soil has a high cohesion value & a significant normal force may also be getting developed on the pedestals due to swelling of the BC soil between RC concrete floor & the mat. During settlement of the pedestal, the reinforcement being rigid, may be moving down. And since the concrete near the soil is not free to move ( due to high cohesion & normal force ), it must be developing tension cracks to accommodate net movement of the member.

c) Also, these cracks are probably not because of differential settlement of mat as the bending moments generated should have produced cracks only on one side ( tension face ) of neutral
    axis. Besides, these are heavily reinforced pedestals.


     Dileep Kulkarni
     Structural Engineer, India.

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