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Calculating SBC from N-value

 
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Sudhakaran
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PostPosted: Fri May 13, 2016 11:04 am    Post subject: Calculating SBC from N-value Reply with quote

Dear sirs,

In an investigation reports it only says the N-Values at different depth.   I have calculated SBC As per IS-6403. Is this calculation is correct?

I have attached the excel calculation for reference.

Thanks,

Sudhakaran.



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Zia
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PostPosted: Fri May 13, 2016 12:05 pm    Post subject: Safe Bearing Capacity Reply with quote

Hi,

The calculation may be correct.  But the question arises how you are going to construct footings at 6m depth?  Unless your structure includes two basement levels it would be very expensive to construct footings at 6m depth as retention of footing excavation faces and dewatering would be required (borehole log shows groundwater at ground level).

This calculation satisfies only shear failure criteria.  You will have to check settlement of footings, which could be very high under such high contact pressure.

Also it is very important to identify what type of soils are encountered. The value of angle of internal friction you have used in the calculation indicates clean sandy soils are present at the site.  This value could be different for clayey soils.

If there are no basements you may consider using bored piers (casings or drilling fluid might be required to support soft soils up to 6m) or continuous flight auger piles or screw piles or driven piles founded in dense soils (N => 50 blow/300mm) to support your structure.

Regards,
Ziauddin Mohammed
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Sudhakaran
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PostPosted: Fri May 13, 2016 2:13 pm    Post subject: Re: Safe Bearing Capacity Reply with quote

Thank you Zia,

This investigation is done for a foot bridge, bore hole location is at abutment ie, river side with a bank height about 4m. Depth of foundation will only 2 to 3m.

Sudhakaran.


Zia wrote:
Hi,

The calculation may be correct.  But the question arises how you are going to construct footings at 6m depth?  Unless your structure includes two basement levels it would be very expensive to construct footings at 6m depth as retention of footing excavation faces and dewatering would be required (borehole log shows groundwater at ground level).

This calculation satisfies only shear failure criteria.  You will have to check settlement of footings, which could be very high under such high contact pressure.

Also it is very important to identify what type of soils are encountered. The value of angle of internal friction you have used in the calculation indicates clean sandy soils are present at the site.  This value could be different for clayey soils.

If there are no basements you may consider using bored piers (casings or drilling fluid might be required to support soft soils up to 6m) or continuous flight auger piles or screw piles or driven piles founded in dense soils (N => 50 blow/300mm) to support your structure.

Regards,
Ziauddin Mohammed
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es_jayakumar
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PostPosted: Fri May 13, 2016 3:05 pm    Post subject: Reply with quote

Dear Shri. Sudhakaran,
It is understood that the footing is to be placed at a depth of 6m where, the N value is only 9. You get N>50 at 7m. What about the influence of the 1m thick layer below 6m ? Even at refusal, I think, you cannot adopt such an enormous value of SBC, unless it is a granitic rock. Even in hard laterite (N>50), the SBC reported by consultants seldom exceeds 50 T/Sqm. I hope, in geotechnical engineering, we have to exercise a fair degree of judgement over theory. Settlement criteria should also be taken care of while working out the SBC. Pure theory often leads to unrealistic values in such cases. Here it is worthwhile to quote Terzaghi : "Soil Mechanics is not an inexact science. But it is a science of inexacts"


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E S Jayakumar
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Sudhakaran
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PostPosted: Fri May 13, 2016 3:29 pm    Post subject: Reply with quote

Dear ES sir,

Thanks,
I am trying for a raft foundation abutment to abutment. In this case the pressure at base will only be about 40kN/m2

Also proposing soil stabilization using coconut piles and rubble.

will it be safe to place the foundation at 6m level?


Sudhakaran.


es_jayakumar wrote:
Dear Shri. Sudhakaran,
It is understood that the footing is to be placed at a depth of 6m where, the N value is only 9. You get N>50 at 7m. What about the influence of the 1m thick layer below 6m ? Even at refusal, I think, you cannot adopt such an enormous value of SBC, unless it is a granitic rock. Even in hard laterite (N>50), the SBC reported by consultants seldom exceeds 50 T/Sqm. I hope, in geotechnical engineering, we have to exercise a fair degree of judgement over theory. Settlement criteria should also be taken care of while working out the SBC. Pure theory often leads to unrealistic values in such cases. Here it is worthwhile to quote Terzaghi : "Soil Mechanics is not an inexact science. But it is a science of inexacts"


Regards,
E S Jayakumar
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es_jayakumar
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PostPosted: Sat May 14, 2016 4:49 pm    Post subject: Reply with quote

If the pressure on soil is restricted <40 T/ Sqm and you do some soil stabilisation, it is fine.


E S Jayakumar
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spsvasan
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PostPosted: Sun May 29, 2016 3:02 am    Post subject: Re: Safe Bearing Capacity Reply with quote

Sudhakaran wrote:
Thank you Zia,

This investigation is done for a foot bridge, bore hole location is at abutment ie, river side with a bank height about 4m. Depth of foundation will only 2 to 3m.

Sudhakaran.



Dear Er.Sudhakaran

Since the proposed foundation is along the river bed, please ensure that the soil below the foundation will not be scoured away during severe floods. Bearing capacity loses its meaning above scour level.

Regards
S.P.Srinivasan
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