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how to calculate soil bearing capacity if softrock encountered in bore hole.

 
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nishant_bajpai1510
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PostPosted: Sun Jul 24, 2016 11:11 am    Post subject: how to calculate soil bearing capacity if softrock encountered in bore hole. Reply with quote

please consider the following strata:
GL to 4.5 m - sandy silty with clay & gravels(clay %-4,gravel %-15 to 16 of size approximately 10mm to 20mm )(non plastic)

4.5 m to further : manual bore not possible due to presence of softrock .
spt values : at 1.5m depth -15
                  at 3.0m depth -29
                  at 4.5m depth - >50( hence refusal )
ground water table was encountered at 3.0m depth .

My problem is how should I calculate soil bearing capacity as ?
what am doing is am calculating sbc according to shear and settlement considering spt values below 3.0m depth as 50 and applying correction of spt values and after that from avg spt applying all settlement correction and then calculating sbc according IS 8009.

is it correct method or spt correction and shear calculation is not valid in case of soft rock.if not valid then how to report sbc at 3.0m depth of foundation.

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bijoyav
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PostPosted: Mon Jul 25, 2016 2:28 pm    Post subject: Reply with quote

Is there a possibility that the WT increase further? In other seasons?


On rocks, theories of soil mechanics are not valid. Go for rock mechanics.
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es_jayakumar
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PostPosted: Tue Jul 26, 2016 12:45 am    Post subject: Reply with quote

At 3m depth, the N value is 29, as reported by you, which indicates medium to dense soil (cohesion-less) . I hope you need not connect it to the characteristics of rock, for calculating SBC at this depth.

E S Jayakumar
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bijoyav
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PostPosted: Tue Jul 26, 2016 6:55 am    Post subject: Reply with quote

For low rise building, one or two stories, an RCC framed structure with a column spacing 4mx4m, maximum load may be in the range of 400kN to 500kN.....
This can be placed at a 1.5m depth, even though the SPT is 15.... For sandy silt the value is not too low, if the topographical layout or underground water flow does not prevent from placing the foundation there. Also, do check for any possibility of differential settlement...i.e., variation in soil properties.

Look at the possibility of improving possibility of improving bearing capacity of soil like providing dense sand bed, or equivalent.

A low rise building with load bearing masonry with confining ductile elements placed on RCC strip can also be considered here, if it is a low rise one.

As you are aware, SBC depends on the type of foundation.

If the foundation is heavily loaded, do not look back. Extent the foundation to the rock suitably...provide deep foundation.

The above is my opinion.

BijoyAV
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nishant_bajpai1510
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PostPosted: Tue Jul 26, 2016 2:56 pm    Post subject: how to calculate soil bearing capacity if softrock encountered in bore hole. Reply with quote

Dear all

thank u for considering my problem .
I would like clear one thing as I have to report SBC at 3.0m depth for that what type of calculation I can show.

considering 3.0m depth and (3*3)square metre footing area influence zone would go up to 7.5m depth. considering presence of softrock after 4.5m depth calculation & correction of SPT not possible.

I have to calculate sbc under 25mm permissible settlement.

What I have been doing is that I would use all the SPT from 1.5 m to 3.0m depth and below that I assume them as 50 up to 10.0m depth. Calculate Avg of all spt after applying overburden and dilatency correction(even on spt after 4.5m depth), then with avg spt calculate settlement from IS 8009 ,then applying water correction (0.5) and fox & rigidity correction.
then I would get sbc for 25mm settlement which comes out to be approx. 20t/m2 to 21 t/m2.


But as spt & other corrections not valid on softrock  hence above calculation not correct...

so How to show calculation.?
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bijoyav
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PostPosted: Wed Jul 27, 2016 12:44 am    Post subject: Reply with quote

suppose concentrate on fig 9 of the IS. (there are many factors to be considered as mentioned above)

Assumptions made here: (1) Soil is isotropic homogeneous semi infinite . Actually it is not. After 4.5m, there is rock. Since rock does not settle you are in positive side.
for 25mm settlement, width required for N-29, is between 2.5m to 3m. You got the area of footing.
Now correlate shear parameters and SPT value and SBC can be arrived at. Check soil pressure is within SBC, considering axial load and moment due to gravity as well as EQ or wind as the case may be.

still I am wondering, why are you considering at 3m, where water is present?

bijoyav
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nishant_bajpai1510
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PostPosted: Wed Jul 27, 2016 8:17 am    Post subject: how to calculate SBC Reply with quote

Considering 3.0m depth because client requires Sbc only at 3.0m depth for (3*3)sqr mts & (7*7) sqr mts.

there is not much load , founadtion is for 3 legged ground based telecommunication tower.
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acsservicesllc
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PostPosted: Fri May 24, 2019 11:29 am    Post subject: Re: how to calculate soil bearing capacity if softrock encountered in bore hole. Reply with quote

nishant_bajpai1510 wrote:
please consider the following strata:
GL to 4.5 m - sandy silty with clay & gravels(clay %-4,gravel %-15 to 16 of size approximately 10mm to 20mm )(non plastic)

4.5 m to further : manual bore not possible due to presence of softrock .
spt values : at 1.5m depth -15
                  at 3.0m depth -29
                  at 4.5m depth - >50( hence refusal )
ground water table was encountered at 3.0m depth .

My problem is how should I calculate soil testing phoenix bearing capacity as ?
what am doing is am calculating sbc according to shear and settlement considering spt values below 3.0m depth as 50 and applying correction of spt values and after that from avg spt applying all settlement correction and then calculating sbc according IS 8009.

is it correct method or spt correction and shear calculation is not valid in case of soft rock.if not valid then how to report sbc at 3.0m depth of foundation.

Thanks for sharing this information!
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