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Is Soil Mechanics a science or art?

 
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htulpule
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Joined: 08 May 2009
Posts: 69

PostPosted: Thu Sep 18, 2014 4:43 pm    Post subject: Is Soil Mechanics a science or art? Reply with quote

Dear Sefians,

I have been designing foundations for structures taking for granted the SBC recommended by the soil reports prepared by soil investigating agencies.

However I ran in to some shocking situations recently

1)      At a site near the west coast of India, Pile foundations were recommended by the soil report. Accordingly we designed a large Steam Turbo Generator building requiring more than 300 piles of 750 dia and 20 m deep.  These piles cost nearly 1 lakh each ….. that is quite a cost apart from the pile caps. Almost immediately after the completion of this building, a separate contract for building a pipe rack less than 15 mtrs from this building came up. Another soil investigation agency was appointed and they recommended an SBC of 30 T/m2 at 3 m depth for 3m x 3m footing.  This would have been enough for the STG building to stand on isolated footings with much less cost

2)      Soil report for another site recommended 4.0 T of lateral load capacity for 800 dia piles owing to weak upper sol layers. Lateral load governed and number of piles required were so high that we ended up providing piled raft having more than 150 piles just to resist earthquake shears. Scared by the huge cost, the owner carried out another geotechnical investigation by another agency which recommended 20t of lateral capacity at the same site. the number of iples now are less than half and all isolated pile caps

Now this literally shakes the ”foundation” of all the design calculations we carryout for the structures.


Question then comes to mind whether the soil mechanics is really a science (pardon me Terzagi Sir) or a product of some personal judgment which has been overrated?


Or has it existed only because we all need to divide the responsibility as much as possible?

Request somebody with more insight in soil mechanics to illuminate, please

Thanks,

Himanshu
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es_jayakumar
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Joined: 24 Nov 2011
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PostPosted: Fri Sep 19, 2014 1:36 am    Post subject: Reply with quote

Dear Er. Himanshu,

I quote the words of Carl Terzaghi : " Soil mechanics is not an inexact science, but it is a science of inexacts". You can see there are many empirical formulae to calculate geotechnical aspects. The results may show considerable variation. Judgement by an experienced engineer plays a vital role in ascertaining the load carrying capacity of foundations. Geotechnical experts in the forum may please throw light on the specific query raised by Er. Himanshu.

E S Jayakumar
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veeran08
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Joined: 02 May 2012
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PostPosted: Fri Sep 19, 2014 10:12 am    Post subject: Soil Mechanics Reply with quote

Dear Engineers

It is sad to note such variations in soil report recommendations by soil testing agencies in India. The reasons are (my observations and opinions)

Poor drafting of tender for soil investigations to appoint an agency

Many owners /Client does not give importance to appoint good agency and expects very minimum no of boreholes due to cost

No in-house geotechnical Engineer in many engineering companies / consultants to prepare tendering, supervision and checking of soil report. (they are left to structural /civil engineers)

In some of International projects, soil testing agency only provides borelog data and lab tests results, as per the scope of tender, they will not recommend on SBC etc. and it is the Engineer of Consultant who has to interpret the results and arrive SBC, settlement calculations etc from their in-house geotechnical engineers.

Hence responsibility for taking BH and testing only lies with geotechnical contractor and final interpretation and judgment based on experience lies with Engineering Consultant since he has overall responsibility in these international contracts. Such system should be adopted in India too
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jshukla
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Joined: 16 Jun 2008
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PostPosted: Fri Sep 19, 2014 2:25 pm    Post subject: Reply with quote

Dear Colleague,

I dont know how to respond to you query since our soil investigation are not so mature even today. I also came across the cases where reports were modified to suite the intentions. Unlike mature practice, we dont have practice to write interpretative reports through experienced geotechincal engineers based on the factual reports.  What best i can say is that you should consult a fair geotechnical engineer who can advise you based on the reports you have. I am sure that looking to the field test data and laboratory realization he would be able to tell you which one to trust...

Best wishes,

Jay
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spsvasan
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Joined: 18 Dec 2008
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PostPosted: Sun Sep 21, 2014 3:10 am    Post subject: Re: Is Soil Mechanics a science or art? Reply with quote

Dear Er.Himanshu


I have come across some extremely good geotechnical consultants and also some very poor geotechnical agencies. The geotechnical head is usually well informed. But if the field personnel do not carryout tests and collect samples diligently, all the brilliance and experience of the geotechnical expert goes to waste. Hence, the work culture of the geotechnical agency becomes very important.

I have come across soil reports which are at complete variance with the site conditions, even at the location of the bore hole.  

I have also come across cases where structural engineers have gone against the soil report and the structure had developed distress on this account.

In my humble opinion, structural engineers should brush up their knowledge of soil mechanics and study soil reports seriously before adopting the recommendations. When we have doubts regarding the recommendations, we should definitely talk to the geotechnical consultant. We should not design a foundation when there is a nagging doubt about the soil report.

Regards
S.P.Srinivasan




htulpule wrote:
Dear Sefians,

I have been designing foundations for structures taking for granted the SBC recommended by the soil reports prepared by soil investigating agencies.

However I ran in to some shocking situations recently

Now this literally shakes the ”foundation” of all the design calculations we carryout for the structures.


Question then comes to mind whether the soil mechanics is really a science (pardon me Terzagi Sir) or a product of some personal judgment which has been overrated?


Or has it existed only because we all need to divide the responsibility as much as possible?

Request somebody with more insight in soil mechanics to illuminate, please

Thanks,

Himanshu
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View user's profile Send private message
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