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Evaluate my Ansys model for Pressurized condition.

 
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Shivani
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PostPosted: Thu Sep 15, 2022 11:41 am    Post subject: Evaluate my Ansys model for Pressurized condition. Reply with quote

Dear All,

I am designing a pressurized tunnel for 35 m  of water head. The inverted D shape tunnel is about 6.1 m. The tunnel should hold 35 m head equivalent pressure.
I have modeled in ansys with thickess of  650 mm. I have designed in accordance to IS 3370_02_2021.
In updated IS 3370 its mentioned that working stress method has been removed and limit state method has been introduced because of which i have used the principal stress to calculate moment and crack width. if the stress is under the crack limit of 0.1 mm i have considered the section as pass. similarly, the area of steel has been provided to the section using the stress developed in the model.
Other than this please suggest me if other checks and design that needs to be done to consider the section as pass.
I would be highly grateful if i can get any guidance to this as i need to finalize this in very short time. Thank you.
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vikram.jeet
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PostPosted: Thu Sep 15, 2022 3:32 pm    Post subject: Reply with quote

Hydro tunnels carrying water at pressure are provided with concrete / rcc lining , but rock participation depending on type of rock mass is also considered  as per prevailing practice , for which BIS Code on Tunnel  lining IS 4880 (relevant part ) needs to be referred.

Generally Tunnels are excavated in Horse shoe shape / D shape , but lining is always  circular at inner face .

In case Tunnel is passing through soft strata ( soil)  / very poor / extremely poor strata , then rock / strata participation is neglected.
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Shivani
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PostPosted: Thu Sep 15, 2022 5:06 pm    Post subject: Reply with quote

vikram.jeet wrote:
Hydro tunnels carrying water at pressure are provided with concrete / rcc lining , but rock participation depending on type of rock mass is also considered  as per prevailing practice , for which BIS Code on Tunnel  lining IS 4880 (relevant part ) needs to be referred.

Generally Tunnels are excavated in Horse shoe shape / D shape , but lining is always  circular at inner face .

In case Tunnel is passing through soft strata ( soil)  / very poor / extremely poor strata , then rock / strata participation is neglected.




Thank you sir for your reply. As you said in hydro tunnels rock mass is considered however, there have been overbreak in and around the crown of the tunnel because of which the the rock strata participation has been neglected in this case. Therefore all the pressure needs to be contained by the concrete lining for which the model has been analyzed. I am concerned about the failure pattern of concrete and rebar. Am i correct to design a section to resist the moment and shear and check the crack width to be under permissible limit be sufficient to final the design ?
Thank you sir.
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vikram.jeet
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PostPosted: Fri Sep 16, 2022 3:20 am    Post subject: Reply with quote

For Tunnels carrying water like Head Race Tunnel / or permanent adits  to Power House and other underground structures   , the rock mass is strengthened  through Consolidation  Grouting after rcc lining is cast and achieved conc strength.  Also contact between lining and rock is sealed through Contact Grouting.

Contact Grouting  is done first , then consolidation Grouting to strengthen rock mass by prelocated holes in lining  . Holes upto 2D , D being Tunnel inner dia. are drlled in rock mass . Please refer relevant BIS code on Tunnel Grouting.

Grouting operations , though improve rock mass , but for design purpose rock participation  is considered based on Rock being VGood , Good,  Poor,  V poor and extremely poor. Design Engineer has to provide lining details for all these strata in drg . Field geologists identify the rock mass during Tunnel excavation and relevant detail is followed.

Please refer some drgs of Tunnel lining at internet.

( to be continued in next post )
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vikram.jeet
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PostPosted: Fri Sep 16, 2022 3:45 am    Post subject: Reply with quote

Tunnel Lining

In case you do not want to take rock participation  , the design will be conservative  unless rock mass is of very poor , extremely  poor , soil soft strata.

The prevailing practice in Hydropower  tunnels is to provide PCC lining for VGood , Good and even Poor rock masses ( taking rock participation  ) , keeping the Tensile strength of lining within permissible limit. For Lower Quality rock masses ( without taking  rock participation being very small & neglected)  , RCC lining is provided and crack control is to be ensured in line with IS 3370.

RCC lining is also provided at entry and exit reaches of tunnels conveying waters.

However u can do design as per instructions from your Superiors considering the project requirements . The above discussions is to be treated as Academic .
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vikram.jeet
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PostPosted: Fri Sep 16, 2022 3:56 am    Post subject: Reply with quote

Forgot to mention in previous post , Drgs for Tunnel Grouting  for both Contact as well as consolidation Grouting are also part of Tunnel Lining drawings and needs to be given alongwith necessary notes and instructions  regarding Grouting pressures ( as per BIS codes)  .
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Shivani
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PostPosted: Fri Sep 16, 2022 10:41 am    Post subject: Reply with quote

vikram.jeet wrote:
Tunnel Lining

In case you do not want to take rock participation  , the design will be conservative  unless rock mass is of very poor , extremely  poor , soil soft strata.

The prevailing practice in Hydropower  tunnels is to provide PCC lining for VGood , Good and even Poor rock masses ( taking rock participation  ) , keeping the Tensile strength of lining within permissible limit. For Lower Quality rock masses ( without taking  rock participation being very small & neglected)  , RCC lining is provided and crack control is to be ensured in line with IS 3370.

RCC lining is also provided at entry and exit reaches of tunnels conveying waters.

However u can do design as per instructions from your Superiors considering the project requirements . The above discussions is to be treated as Academic .




Thank you sir,
It is because the over break is more than 3 m along the crown, there is no possibilities of any kind of grouting and only way is to design is without taking rock participation. And yes as you have rightly said, the design will be conservative. Therefore i am trying to design such that the concrete lining will take all the pressure. With the introduction of updated IS 3370  where limit state is permitted i have derived the crack width from the developed principal stress and limited it to less than 0.1 mm. Other than that i have design area of steel as per the principal stress converted to the bending moment using the bending stress formula.
Thank you.
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vikram.jeet
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PostPosted: Fri Sep 16, 2022 2:57 pm    Post subject: Reply with quote

Tunnel design:
Further to add one thing , hope as a designer it has been duly taken care , regarding Tunnel empty situation during construction or during maintenancein future. It need to be designed for external pressure due to charging of area as result of Hydraulic gradient from upstream to downstream . OR Maximum Grouting pressure whichever is higher . Stresses can be enhanced by 33% as the condition is treated as temporary phase.

Old practice  of checking Tensile stress  in Concrete lining shall not be ignored and even if such calcs are not required to be submitted , a check need to be done.
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