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Stability of well foundation on heavily scoured bed.

 
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saroj
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Joined: 24 Jan 2014
Posts: 6

PostPosted: Tue Jan 28, 2014 12:33 pm    Post subject: Stability of well foundation on heavily scoured bed. Reply with quote

Dear Sefians

Due to negligence or error, a bridge at some hilly area was designed with well foundation of depth 20 m from the river bed, but during sinking, bed rock was found below 12 m from the present bed level. It is very difficult to continue sinking and probably that's worthless. The hydraulic analysis shows heavy future scour of about 8 to 9 m from the present bed level.

Now, can we analyze the stability of the well in such condition? What could be the appropriate solutions? I am thinking of anchoring the bottom plug with grouted steel bars but not sure how to evaluate the tension in the rods.

Would be grateful if someone help me!

Thanks

Saroj Bhattarai
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bkballa
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Joined: 29 Nov 2010
Posts: 11

PostPosted: Wed Dec 10, 2014 5:56 am    Post subject: Re: Stability of well foundation on heavily scoured bed. Reply with quote

saroj wrote:
Dear Sefians

Due to negligence or error, a bridge at some hilly area was designed with well foundation of depth 20 m from the river bed, but during sinking, bed rock was found below 12 m from the present bed level. It is very difficult to continue sinking and probably that's worthless. The hydraulic analysis shows heavy future scour of about 8 to 9 m from the present bed level.

Now, can we analyze the stability of the well in such condition? What could be the appropriate solutions? I am thinking of anchoring the bottom plug with grouted steel bars but not sure how to evaluate the tension in the rods.

Would be grateful if someone help me!

Thanks

Saroj Bhattarai


Saroj Sir,

One probably need more data than you provided to check the stability of well (superstructure details, bearing detail, substructure details, RL of various points, HFL/LWL level, etc). Soil and Hydraulic data may also be needed.
Having said that 4.0 m of embedded depth of well is probably not enough. IRC stipulates 1/3 (HFL-Max. Scour Level) as minimum embedded depth. This may not need to be adhered in case of hilly condition, even then 4.0 m may not be enough.
As you have rightly pointed "anchoring" is required. Is it possible to consider micro-pile from current bottom level of well foundation.

Sorry, I probably was not of much help. Just trying to keep the conversation going in the topic. I also like to know from other "sefians" if there are any "innovative" solutions to the problem mentioned by Saroj Sir.

Thanks.
Biswa Balla
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saroj
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Joined: 24 Jan 2014
Posts: 6

PostPosted: Wed Dec 10, 2014 8:08 am    Post subject: Reply with quote

Dear Balla jee


Thank you for your reply. In fact i posted this long ago while solving a problem for a Hydro Electric Project (access road bridge), to know if anyone has some particular solution. You are right about the IRC provision about the grip length, which is essential for the stability of the well by active and passive pressures. But in absence of soil around the well we have to replace the resisting pressure by some other mechanism, such as anchors.


Without any surrounding soil, the well stands as a large hollow column with eccentric loads and other horizontal forces. So I prepared a model of the well with shells in Sap2000. Applied the horizontal forces and moments in different combinations and checked the positive and negative reactions at the support restrains. After several trials I arrived at suitable spacing of the supports, which would be the anchor points. Then to resist the "pulling" force in each support, anchor rods were recommended with proper grouts. Another mesh of re-bars was also anchored inside the well steining and embedded in the bottom plug. The anchor rods were hooked into the re-bar mesh.

Additionally, scour and bed protection with cut off walls, check dam and bed matting was also recommended. The contractor has already finished the work and hopefully the bridge will stand well during its design life!

Thank you for your concern.
Saroj Bhattarai
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