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Boulder interfering with isolated footing
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Manmohit Singh
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PostPosted: Tue Sep 14, 2021 8:09 pm    Post subject: Boulder interfering with isolated footing Reply with quote

Hi fellow Sefians,

We have encountered a huge boulder during excavations which is dividing the base of footing into 2 parts. We have tried removing the boulder but it huge and due to blasting limits it is no longer an option.

So, we are thinking about  anchoring the footing with the boulder by continuing the horizontal bars of footing into the rock (upto Ld) and vertical bars of column into the rock.

I have attached an image for representation purpose.

I want to know views of experts about this.

regards



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Plan of boulder interfering with footing
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vikram.jeet
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PostPosted: Wed Sep 15, 2021 3:27 am    Post subject: Reply with quote

A generic view :

The footing shall either rest completely on Boulder OR but preferably , it needs to be abutting Boulder but not resting on it , so that whole footing area is  on Soil. All other footings are on soil. It is not desirable to rest  foundation s of a building on two types of strata as unequal settlements dominate . Here the case is more typical with one footing, part on soil and part on Boulder .

Gravity loading - It is a favourable situation with eccentric footing fully resting on soil only, abutting a big Boulder , and passive resistance to counter eccentric moment from Boulder face is helpful.

Seismic Condition:  If higher seismic Zone , it is to be ascertained that the Boulder is part of any underneath  mountain range , or it is just some big independent Boulder . Advice of Geologist can be sought to know the presence of Boulder in the underneath .
If the Boulder is independent one in area , the EQ vibrations can generate heavy force in it , especially in horizontal direction and , footing structure , being  stitched to Boulder .
The higher EQ forces needs to be considered .
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vikram.jeet
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PostPosted: Wed Sep 15, 2021 8:28 am    Post subject: Reply with quote

It would be better to bypass the Boulder and the rcc column  coming on boulder be shifted elsewhere . If need arises column be removed from that location and instead two nos columns away from Boulder zone be planned.
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Manmohit Singh
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PostPosted: Wed Sep 15, 2021 1:10 pm    Post subject: Reply with quote

Thanks for reply.

Its the tower foundation and shifting boulder or footing is not possible unfortunately.

Our geologist has also certified it to be a huge boulder and recommended not to disturb it.
Further there is going to be around 3-4 m of backfill above this footing and boulder, which i think will further help our case.

So anchoring the half footing with the boulder looks like the most feasible solution at this point. I am still exploring more options.
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vikram.jeet
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PostPosted: Wed Sep 15, 2021 3:18 pm    Post subject: Reply with quote

If it is a tower foundation , all such apprehensions discussed in preceding posts ,  pertaining building foundation , can be ignored.

Mostly wind loading is critical in such structures . It would be better that alongwith horizontal anchoring , vertical anchors to be provided in portion of footing on boulder..

If deflection of tower is critical factor , footing needs to be checked as cantilevered from Boulder face and vertical anchors facilitating the cantilever action be provided accordingly. This may ensure soil any soil settlement beneath footing portion on soil.
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Manmohit Singh
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PostPosted: Wed Sep 15, 2021 5:16 pm    Post subject: Reply with quote

Thanks for reply.
Sounds reasonable enough.
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pvgraju
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PostPosted: Thu Sep 16, 2021 11:38 am    Post subject: Boulder interfering with isolated footing Reply with quote

Dear Sir,

Along with all the above this is suggested

1. Test boulder core and find out UCS, shear strength of boulder.
2. Then fill with rock of same material as boulder with same UCS, shear strength under the footing up to full depth of boulder. Put some fine rock pieces to fill gap between big boulder pieces.Else concrete of same strength as boulder can be used for filling.

By this uniform pressure distribution can be acheived.

Regards,

P.V.Gavarraju.
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satishkemkar
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PostPosted: Fri Sep 17, 2021 4:29 am    Post subject: Reply with quote

As per the sketch, the Footing resting on boulder is eccentric to the CG of boulder. If the load is excessive then Boulder may become unstable.

It is advisable to relocate footing so that Footing will be free from Boulder and uniform strata for all footings will be achieved. Hence avoiding differential settlement.
Provide Beams at ground level and raise Column / Pedestal as floating column from the Beam at the Current location of center line of column.

Sketch is attached for understanding.

Thanks

Satish



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pvgraju
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PostPosted: Fri Sep 17, 2021 7:08 am    Post subject: Boulder interfering with isolated footing Reply with quote

Dear Sir,

1. The rock material poured below foundation shall be binded with binding material. To achieve better interlocking original boulder surface shall be roughened. Then newly placed boulder will act in unison with original boulder.
2. Then an RCC footing can be cast on that which will act as foundation for tower.
3. All these loads will be transferred to boulder.
4. Then this original bolder with new boulder will act like masonry foundation.
5. Allowable stresses are to be checked in this by considering all load combinations.
6. In the same way overturning Factor of safety to be checked.
7. If any overturning found possibility of anchoring boulder into the soil to be explored.

Regards,

P.V.Gavarraju.
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suraj
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PostPosted: Sat Sep 18, 2021 9:17 am    Post subject: Rock Hammer stone portion encroaching foundation Reply with quote

Better use Rock Hammer to remove boulder encroaching foundation space, if it is feasible.
I do not think that it shall be a big issue to hammer it, if rock excavator is accessible to location
Is it igneous rock or metamorphic one?
Or, can foundation be raised to a level, at which rock does not clash?
If foundation can be raised, granular matrix soil can be designed & soil filled for improvement, so that soil fill bearing capacity is increased to required capacity
If soil fill is also, not possible, use concrete fill to that level in layers & avoid building loads to rock. No local rock should be used for transfer & spread of loads, according to elementary conception
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