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Why Does a concrete Slab Not get crushed under water

 
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someshwar ganti
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PostPosted: Mon Feb 11, 2019 11:42 am    Post subject: Why Does a concrete Slab Not get crushed under water Reply with quote

Hi all
  Let us consider a Hypothetical situation where a slab (1m x 1m ) of some thk is thrown into water in sea, I am sure by the time it reaches the bottom it does not get crushed (may decay in a million years)

Similarly, if the same were to be considered for a UG water tank with a  high water above the base slab, even though we do not design the base slab for the crushing force from the weight of water above it, it stays safe.

1. One may say that the ground is giving an equivalent upward thrust to that of the water column height,


   Then my question is why does a Concrete cube gets crushed in a UTM, does the same principal of upward and downward force neutralization not apply here ?
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abhio
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Joined: 08 Mar 2010
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PostPosted: Mon Feb 11, 2019 12:37 pm    Post subject: Reply with quote

Dear Er Someshwariji,

That's an interesting thought experiment. Let's try to put some numbers to it. The density of sea water is about 1027 kg/cu.m, and the depth of the Mariana trench is about 10994m. For simplicity, let's call them 1000 kg/cu.m and 11 km respectively. The hydrostatic pressure at this level is then 11000 t/sq.m, or 108 MPa, much greater than the compressive strength of most concrete. Will the concrete get crushed then? Most likely not, since the concrete is in a triaxial state of stress, being compressed uniformly from the sides as well as the top and bottom faces. We would have to consider the failure criterion as per Tresca/ Von Mises to figure out whether the concrete is crushed or not. Also, we would have to take into account the fact that internal pore water pressure equal to the external pressure will partially offset the external pressure.

These conditions don't apply in a UTM, where the concrete isn't laterally restrained by hydrostatic pressure, and is free to bulge out and crack.

Regards,

A S Oundhakar
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someshwar ganti
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PostPosted: Mon Feb 11, 2019 2:03 pm    Post subject: Reply with quote

abhio wrote:
Dear Er Someshwariji,
That's an interesting thought experiment. Let's try to put some numbers to it. The density of sea water is about 1027 kg/cu.m, and the depth of the Mariana trench is about 10994m. For simplicity, let's call them 1000 kg/cu.m and 11 km respectively. The hydrostatic pressure at this level is then 11000 t/sq.m, or 108 MPa, much greater than the compressive strength of most concrete. Will the concrete get crushed then? Most likely not, since the concrete is in a triaxial state of stress, being compressed uniformly from the sides as well as the top and bottom faces. We would have to consider the failure criterion as per Tresca/ Von Mises to figure out whether the concrete is crushed or not. Also, we would have to take into account the fact that internal pore water pressure equal to the external pressure will partially offset the external pressure.

These conditions don't apply in a UTM, where the concrete isn't laterally restrained by hydrostatic pressure, and is free to bulge out and crack.

Regards,

A S Oundhakar


In Continuation of the your explanation  say if a Concrete  PIPE is filled with some material closed  on both ends  and thrown into sea, will it get crushed ? as it will not be not be in the tri-axial state of stress as the internal filling material will be in a different stress compared to external pressure, if is crushed than perhaps we would not have been able to read these massages as many of our internet transmission is done through underwater cables (enclosed in pipes) across the world (chennai to singapore, mumbai to dubai etc) not to mention tonnes of different cables sea bed is having,
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