Dear All, I need some advise regarding pile raft foundation on soft soil. In normal practice, do we design in such a way that the piles are evenly spaced? (If so then pile underneath s the column might be overstress) Back to top gpsarathyy ...
Posted: Fri Jan 06, 2012 1:14 pm
Post subject:
Dear Sefian, The pilecaps are designed assuming the pilecaps are behaving as a rigit element. Hence the forces from the column will be evenly distributed to the piles. If the pilecap is so big, then the stiffness of the pilecap to be considered for the analysis and design of the pilecap and checking the pile reactions. Joined: 28 Jun If the pilecap is big and the pilecap is also less thickness, sure there will be local 2010 Posts: 406 stress concentration in pilecaps and the local pile will be stressed more than the Location: other piles. chennai
For this you can go for FEM analysis using Plate elements and spring value for the pile support. The FEM approach can only give you the correct distribution of stresses based on their stiffness. _________________ G.PARTHASARATHY M.E. CEng(UK), MICE., MIE., CHENNAI
[email protected] Back to top sspawar ...
Posted: Fri Jan 06, 2012 3:06 pm
Post subject: Re: PILE RAFT
Generally Piles below Pilecaps and columns above pilecaps are geometrically so oriented that load coming by columns could be distributed evenly over pile cap and to the piles.
In a Pilecap if there is 4 columns/pedestals with 4 piles which are placed just below columns/pedestals, the case should not be imagined as piles are overstressed. In a pilecap if there is 3 piles with 2 columns then geometrically cg (centre of verical reactions/forces) of pilecap, piles and columns should be one. Joined: 05 Jun In case of raft, pile arrangment should be so that load can be transferred evenly 2009 rather than could create any moment action due to eccentricity. Though pilecaps are Posts: 1157
rigid element and its stiffness if designed so piles and columns or walls can be eccentric.
Like in case of Chimney foundation or cylinderical stem (shaft) of any structure annular beam rested over piles is more realstic, then a raft. Where in case of any frame structure building, flexibility in pattern of piles below raft can be allowed if stiffness is being attained. Few piles may concentric while few may be away from load lines. Where in case of water retaining structure over earth, raft could be piled with an even pattern. regards jameschan wrote: Back to top jc ...
Posted: Wed Jan 11, 2012 6:30 pm
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Dear sspawar, Joined: 07 May 2010 Posts: 178
So what you meant is for building frame system, it's better to put more piles underneath column position as the axial load is high?
Back to top sspawar ...
Posted: Thu Jan 12, 2012 5:26 am
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Raft fdns are designed to spread load over the larger area of low bearing earth and it requires piles evenly. If case is not this it can be individual footings. regards
Joined: 05 Jun 2009 Posts: 1157
Back to top jc ...
Joined: 07 May 2010 Posts: 178
Posted: Thu Jan 12, 2012 6:55 am
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I don't really agree with that. To spread the pile evenly in raft foundation is reasonable when it's a water retaining structure as the load is evenly distributed. For RC frame structure, the load is concentrated in column area and if we spread the pile evenly, some piles near the column area will be overstressed. I have tested this by using FEM modeling.
Back to top suresh_sha rma
Posted: Thu Jan 12, 2012 9:42 am
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In my view the raft in the case quoted by you, is a substitute of pile cap. It is first necessary to know as to why the necessity of raft over the piles arose in substitution of usual pile cap. Then only the solution can be found out. The problem can be Joined: 23 Mar tackled if the load CG, pile CG and column load CG coincide. The lay out of the raft and pile has to be decided accordingly. 2011 ...
Posts: 737
Back to top vikram.jeet General Sponsor
Posted: Thu Jan 12, 2012 10:29 am
Post subject: PILE RAFT
Pile cap cum raft In case the strata at which bottom of pile cap rest , is not susceptible to liquefaction and has countable sbc , the advantage is taken of this . Net load to be taken by piles is worked after deducting the load supported by this strata beneath pile cap and in such cases , the pile cap is pile raft.
Even distribution The rigidity of pile cap plays major role in even distribution.Pile cap must be thick enough for this. As per IRC 78 on bridge foundations, the thickness of Joined: 26 Jan pile cap must be minimum 0.60m or 1.5 times pile diameter whichever higher. 2003 Posts: 2212
best regds vikramjeet
Dear All, I need some advise regarding pile raft foundation on soft soil. In normal practice, do we design in such a way that the piles are evenly spaced? (If so then pile underneaths the column might be overstress) -Posted via Email Back to top jc ...
Posted: Thu Jan 12, 2012 11:28 am
Post subject:
Pile raft is used because the soil is medium stiff and the actual soil pressure is higher than the allowable if pure raft is used. Joined: 07 May 2010 Posts: 178
So, i need to provide friction piles in order to reduce the soil pressure and to control settlement. It's not feasible if normal pile cap is used. Or else, we just go for the simple way.
Back to top suresh_sha rma
Posted: Thu Jan 12, 2012 2:17 pm
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I fail to visualise the the function of raft in your case when it is not supporting the structure meaning thereby that the structure loads are not being tranferred into the raft. The load transfer to the soil is through pile foundation. If the raft is transferring Joined: 23 Mar the load to the pile then it is a large pile cap connecting all the piles together. If the scenario is as above the the piles should be closely spaced and the raft should be 2011 ...
Posts: 737
designed as all other rafts are done with in which columns can be positioned at any place. In actual design we make so many assumptions to simplify the design process. Even in isolated footings it is assumed that the loads are uniformly distributed to the founding strata. In reality it can not be so. If you apply the finite element method for analysis for isolated footing it can be seen.
No. The raft is not transferring all loads to piles. As i said, the soil is medium stiff, if i design a pure raft then the bearing capacity is exceeded. So, in this case, the raft is just like partially sitting on soil and partially sitting on pile. Back to top sspawar ...
Posted: Thu Jan 12, 2012 7:53 pm
Post subject:
As per the load /stresses pattern pile arrangement can be designed. Some times different dia of piles also used accordingly or by varying depth and steel %age, different capacity of piles also provided in a single raft. regards
Joined: 05 Jun 2009 Posts: 1157
Back to top suresh_sha rma ...
Joined: 23 Mar 2011 Posts: 737
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Posted: Fri Jan 13, 2012 8:56 am
Post subject:
I feel the question is imaginary. The foundation i.e. raft, pile, pile cap all together for the situation illustrated by Jameschan can not exist in reality.
jc ...
Joined: 07 May 2010 Posts: 178
Posted: Fri Jan 13, 2012 11:11 am
Post subject:
Please don't simply say i am bluffing. Please take note that it's realistic. Many massive structures in here are designed as pile raft in such a way that the raft is resting on soil and pile to control settlement. If you have no idea on this then please listen to other opinions.
Back to top suresh_sha rma
Posted: Fri Jan 13, 2012 12:38 pm
Post subject:
I provisionally agree that you have a real problem as illustrated by you. Please do not get hurt by the word "imaginary". Any thing and every thing is first imagined and then executed. Nothing can be executed without imagination. Many a times the Joined: 23 Mar imagination fails to take off. All scientific innovations that we enjoy today have gone through many failed imaginations. Will you please post the file showing the 2011 Posts: 737 arrangement of foundation to help me to actually understand the matter. That will be a help to me to remove my illusions. ...
Thanking you in advance Back to top gpsarathyy ...
Posted: Fri Jan 13, 2012 1:13 pm
Post subject:
Dear Jameschan, The pile raft approach is seems to give economical solution to the size and quantity of the foundation and pile numbers. May I know, for what kind of foundation you are going with this approach? Joined: 28 Jun 2010 Posts: 406 Location: chennai
In General, the Engineers will not mix the Pile and Raft for the bearing pressure check, since the piles are more rigid than the bearing foundation, the rigid element attracts more forces. Here the raft thickness also comes into picture for distrubuting the forces. We will check for bearing type foundation or pile foundation and not combining both as you said. I like to know more about this, in what are all cases we can go with this kind of approach for foundations? As said by Mr. Vikram Jeet, the soil shall not be liquifiable. In future you should not allow any trenches are excavation near these kind of foundations, which will will reduce the allowable bearing pressure of soil, the load transfer will go more to the piles than through raft. _________________ G.PARTHASARATHY M.E. CEng(UK), MICE., MIE., CHENNAI
[email protected]
Back to top sakumar79 ...
Posted: Fri Jan 13, 2012 1:38 pm
Post subject:
Dear Sirs, The Piled Raft foundation is a relatively new concept (at least in India, as far as I know) wherein the total load of the superstructure is partly carried out by piles Joined: 18 Apr through skin friction and the remaining load is taken by raft through contact with the 2008 Posts: 450 soil.
Regarding the original query, (whether piles are equally spaced or not), first of all, let me say that I am no way qualified to answer the question. I have not done any projects where it has been used. I believe that finite element modelling is used to model the raft, pile and soil and the design is attempted to ensure uniform settlement within limits and the load sharing between pile and raft is calculated. Since this is more of a geotechnical exercise than a structural design exercise once the loads are known, I would strongly recommend that you consult a geotechnical engineer (or geotechnical engineering firm) with prior experience in the design of piled raft. Alternatively, you may also consult geotechnical professors in the universities who have worked on this concept... Yours sincerely, S. Arunkumar Back to top Dr. N. Subramani an General Sponsor
Posted: Fri Jan 13, 2012 9:58 pm
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Dear All, I agree with the comments made by Er Arunkumar. I am also enclosing a report which may be useful. Best wishes Subramanian sakumar79 wrote:
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TC18_Report_Piled Raft-July_2001.pdf Joined: 21 Feb 2008 Posts: 4416 Location: Gaithersburg, MD, U.S.A.
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Back to top jc ...
Posted: Sat Jan 14, 2012 3:40 am
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Dear gpsarathyy, Joined: 07 May 2010 Posts: 178
Back to top
We use pile raft is because we want the raft and pile work together to carry the superstructure. This is the main intention but you said the other way round. Please read the article attached by Dr N. Subramanian then you will understand what i am talking about. Really thank you for your help Dr N. Subramanian & Er Arunkumar. It is glad that both of you understood what i am talking about.
suresh_sha rma ...
Posted: Sat Jan 14, 2012 8:13 am
Post subject:
Dear Dr. N S
you have posted an excellent article on pile-raft foundation. The article has been Joined: 23 Mar written in the year 2001. After going through the article I conclude that the matter was at that time still in evolution stage. By this time I suppose the design procdure 2011 Posts: 737 might have crystallised. I find the article in order and acceptable with some discretion to be applied by the designer. This would definitely be economical and suitable for weak soils. The article has a perfect answer for the query raised by Mr. Jameschan. I would like to know from all you that whether the pile raft design is being practised in India. Is there any code National(BIS) or Internationa on this topic?
Dear Er Sureah Sharma, Unfortunately my earlier post giving several references is not showing. I wish to say that there are a number of articles appearing in Foundation Engg. Journals- some of them published electronically- for example see Elec Journal of Geotechnical Engg. http://www.ejge.com/In dex_ejge.htm Also IITM Soil Mech div. is doing extensive research on this topicContact Prof. S R Gandhi or Prof. Dr. G. R. Dodagoudar Best wishes NS suresh_sharma wrote:
Back to top gpsarathyy ...
Posted: Mon Jan 16, 2012 11:54 am
Post subject:
Dear Jameschan, Thanks for highlighting the concept of the Pile raft system. After going through the material attached by Dr NS. it was clear about the Pile raft system. Joined: 28 Jun 2010 Posts: 406
1. Now the next question is how far it is used in India?
Location: chennai
2. For what kind of foundation we can go with this type ? (Power plant structures, Oil and gas structures etc., - need not give the site or project name) 3. Whether this approach is accepted by the Indian clients or approvers? (how frequently used in the design or very rarely used). 4. Any specific code is available in Indian Standards? As of now we are going for the raft foundation or pile foundation not by combing both which are conventional methos, this new approach will reduce the piles and give optimised design interms of reducing piles. On the other hand if you see from the raft foundation, adding piles will be increasing the cost of the foundation. Dr. NS sir, Thanks for uploading the Pile raft material which clear my doubt and also helps to improve the knowledge in the Pile raft foundation design analysis. _________________ G.PARTHASARATHY M.E. CEng(UK), MICE., MIE., CHENNAI
[email protected]
Back to top vikram.jeet
General Sponsor
Posted: Mon Jan 16, 2012 3:01 pm
Post subject: PILE RAFT
National building code of India clause 8.2.5.3 speak about the pile cap laid on firm strata and indicate about the functioning of cap as pile raft. best regds vikramjeet Joined: 26 Jan 2003 Posts: 2212
.I would like to know from all you that whether the pile raft design is being practised in India. Is there any code National(BIS) or Internationa on this topic? -Posted via Email Back to top ibarua
General Sponsor
Posted: Tue Jan 17, 2012 2:32 pm
Post subject: PILE RAFT
17th Jan 2012
Joined: 26 Jan 2003 Posts: 996
Here, the problem is the apportionment of loads between the piles and the raft. In case of an isolated group of pipes, it is generally assumed that all the load is sustained by the piles and nothing is transferred to the soil below the pile caps. However, in the case of a raft, the raft itself will transfer some load directly to the soil below. Can anyone cite a reference /references for computation of such apportionment of load between the piles and the raft?
Indrajit Barua. From: vikram.jeet Sent: Fri, 13 Jan 2012 09:02:08 To:
[email protected] Subject: [SEFI] Re: PILE RAFT Pile cap cum raft In case the strata at which bottom of pile cap rest , is not susceptible to liquefaction and has countable sbc , the advantage is taken of this . Net load to be taken by piles is worked after deducting the load supported by this strata beneath pile cap and in such cases , the pile cap is pile raft. Even distribution The rigidity of pile cap plays major role in even distribution.Pile cap must be thick enough for this. As per IRC 78 on bridge foundations, the thickness of pile cap must be minimum 0.60m or 1.5 times pile diameter whichever higher. best regds vikramjeet
Dear All, I need some advise regarding pile raft foundation on soft soil. In normal practice, do we design in such a way that the piles are evenly spaced? (If so then pile underneaths the column might be overstress) -- ÂÂ Posted via Email Back to top sspawar ...
Posted: Tue Jan 17, 2012 4:07 pm
Post subject:
Dear Sefians, Though Raft with Piles are very common in India and outside for heavy structures. Particularly for Heavy Industries and in coastal areas where SBC is very low. Joined: 05 Jun 2009 Posts: 1157
In stiff soil raft can be designed to bear and transfer the load directly but in case of heavy works, generally this part is neglected. Because stress differences are considered very high in between pile and soil below raft. In residential buildings grad slab and grad beams are designed on this concept, but when frame structure is built, fill below slab is ignored as part of receiving strata. I attached here an old drawing, which shows raft with piles of different capacity. Here designer is not considering the raft as to transfer the load direct to ground. During discussion one thing also I observed, that sometimes matter has been left on the geologist or it is said that it is not part of structural engineering.
In my view Geo-technical reports are basic parameters, where design of a structure could not be left over on that. It does not matter that it is case of piling or raft or column. If designer is finding that certain parameter or results are required from soil investigation, then it can be asked for, but complete exercise could not be ignored. One thing more I would like to say raft foundation and raft and raft with pile foundation all terms are being used frequently here, and some fellow are only considering raft as raft foundation, where it is a type of pile cap also. Regards Warning: Make sure you scan the downloaded attachment with updated antivirus tools before opening them. They may contain viruses. Use online scanners here and here to upload downloaded attachment to check for safety.
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Back to top pravin.pai
General Sponsor
Posted: Tue Jan 17, 2012 4:53 pm
Post subject: PILE RAFT
Structurally, after the settlement occurs for the pile cap, the load will come on the piles. So wahatever sbc we know at pile-cap bottom multiplied by the pile-cap area (minus area of piles) will be the load taken by pile cap and the rest will be by piles. regards pravin Joined: 25 Aug 2008 Posts: 64 Location: Mumbai
On Tue, Jan 17, 2012 at 4:20 PM, ibarua wrote:
Posted via Email Back to top
sarfaraj.husain ...
Joined: 26 Jan 2003 Posts: 90
Posted: Tue Jan 17, 2012 5:07 pm
Post subject: PILE RAFT
Mr. AA pile under / near to column will not be subjected to bending stress ......it will have only direct stress....in that case pile away from column may be overstressed due to combination of axial & bending stresses.... sarfraj
[SEFI] Re: PILE RAFT ibarua to: general 01/17/12 04:16 PM Please respond to general
17th Jan 2012 Here, the problem is the apportionment of loads between the piles and the raft. In case of an isolated group of pipes, it is generally assumed that all the load is sustained by the piles and nothing is transferred to the soil below the pile caps. However, in the case of a raft, the raft itself will transfer some load directly to the soil below. Can anyone cite a reference /references for computation of such apportionment of load between the piles and the raft? Indrajit Barua. From: vikram.jeet Sent: Fri, 13 Jan 2012 09:02:08 To:
[email protected] Subject: [SEFI] Re: PILE RAFT Pile cap cum raft In case the strata at which bottom of pile cap rest , is not susceptible to liquefaction and has countable sbc , the advantage is taken of this . Net load to be taken by piles is worked after deducting the load supported by this strata beneath pile cap and in such cases , the pile cap is pile raft. Even distribution The rigidity of pile cap plays major role in even distribution.Pile cap must be thick enough for this. As per IRC 78 on bridge foundations, the thickness of pile cap must be minimum 0.60m or 1.5 times pile diameter whichever higher. best regds vikramjeet Dear All, I need some advise regarding pile raft foundation on soft soil. In normal practice, do we design in such a way that the piles are evenly spaced? (If so then pile underneaths the column might be overstress) -- ÂÂ
-Posted via Email Back to top suresh_sharma ...
Joined: 23 Mar 2011 Posts: 737
Posted: Tue Jan 17, 2012 8:20 pm
Post subject:
Sri Barua, If a portion of the foundation load is to be transferred to the sub base through raft then connection between the pile and raft has to be made accordingly. The usual ebedment of Ldc of pile reinforcement in the pile cap has to be changed to Ldt if the raft also becomes an active member of load transfer. Many more things have to be accounted for as per paper posted by Dr. NS.
Back to top suresh_kumar ...
Posted: Tue Jan 17, 2012 10:04 pm
Post subject:
Respected Sefians Pile raft can be classified as 2 types Joined: 19 May 2009 Posts: 79
A. Piled raft for settlement reduction. B. Piled raft for load transfer. A chapter-15 in Foundation engineering by Mr.PC varghesse have details these kind of Piled raft. A best example for this kind is Turbogenerator bottom raft over weaker soil having SBC less than 100kn/sqm. Regards N. Suresh kumar
egarding approach for design of pile raft, - - - - -- as a personal view The Pile raft concept will definitely economise the design requirements of piles (their length/dia) , but as far as design of pile raft itself is concerned I think it has to go by the design as pile cap especially for rigid pile caps Load apportionment - Piles are deep foundations and sustain more loads with little settlement (in comparison to raft). Thus loads will be mostly shared by the piles even beyond their working capacity (but within ultimate reach) Still system of pile -raft has overall capacity to sustain loads since raft strata will mobilise capacity gradually. The capacity of raft strata will be fully mobilized only when piles reach their ultimate capacity and hence in working stage it will be a nearly passive assurance from raft strata.(since piles are active in taking almost entire working loads). Just considering a simplified Example Column load = 200 tonne Strata sbc= 10T/m2 No of piles =4 Nos Area of pile cap = 2.0 X 2.0 = 4 sqm
A ] In case pile raft concept not used working Capacity of pile required = 200/4 = 50 tonne ultimate capacity of pile required = 2.5*50=125 tonne Design of pile cap is based on piles reaction of 50 tonne B ] In case pile raft concept is used Capacity of strata= 10*4=40 tonne Balance capacity from piles= 200-40=160 tonne working Capacity of pile required = 160/4 = 40 tonne ultimate capacity of pile required = 2.5*40=100 tonne Saving in piling already evident. Only For purpose of Pile raft design - - Actual scenario;-Design of pile cap needs to be still based on piles reaction of 50 tonne(though working capacity is 40 tonne) This actual scenario is also critical in face of hypothetical scenario of design as pile raft considering 40 tonne pile reaction and 10 tone/m2 base pressure from pile raft. Case of very Flexible raft: But in case of very flexible pile raft (with piles spaced much apart) , the apportionment will be complex. Here again the design as pile cap considering full loads taken by piles may be conservative approach for purpose of pile raft design . (though in actuality loads will be shared very early between raft strata and piles in very flexible raft ) the viewpoint is personal and may be dis-regarded otherwise but discussion for better understanding of structural concepts must continue. best regds vikramjeet
-Posted via Email Back to top padamkr
SEFI Member
Posted: Wed Jan 18, 2012 8:38 pm PILE RAFT
Post subject:
Dear mr vikramjeet ,
Joined: 26 Jan 2003 Posts: 4
SBC of raft is based upon 75mm settlement where as pile is considered to be failed at 12 mm settlement . In the example below 10t/sqm shall be mobilized at 75 mm settlement . at this settlement, piles can not take load to their full capacity . your comments please ? regards
padam kumar S-205 CHETAK COMPLEX PKT-B&E MARKET ,DILSHAD GARDEN DELHI-95 01122132424,
On 18 January 2012 15:38, vikram.jeet wrote:
Posted via Email Back to top padamkr1
SEFI Member
Posted: Wed Jan 18, 2012 8:55 pm PILE RAFT
Post subject:
Dear mr vikram jeet,
Joined: 26 Jan 2003 Posts: 18
SBC of raft is based upon 75 mm settlement where as pile is considered to be failed at 12mm settlement . In the said example sbc 10t/sqm shall be mobilized at 75 mm setttlement . At this stage piles shall not be in a position
to transfer assumed capacity of pile to soil . Your comments please ? Padam Kumar S-205, Chetak Complex, Pocket B & E Market, Dilshad Garden, Delhi - 95 Ph - 22132424 From: vikram.jeet To:
[email protected] Sent: Wednesday, 18 January 2012 3:38 PM Subject: [SEFI] Re: PILE RAFT Regarding approach for design of pile raft, - - - - -- as a personal view The Pile raft concept will definitely economise the design requirements of piles (their length/dia) , but as far as design of pile raft itself is concerned I think it has to go by the design as pile cap especially for rigid pile caps Load apportionment - -
Piles are deep foundations and sustain more loads with little settlement (in comparison to raft). Thus loads will be mostly shared by the piles even beyond their working capacity (but within ultimate reach) Still system of pile -raft has overall capacity to sustain loads since raft strata will mobilise capacity gradually. The capacity of raft strata will be fully mobilized only when piles reach their ultimate capacity and hence in working stage it will be a nearly passive assurance from raft strata.(since piles are active in taking almost entire working loads). Just considering a simplified Example Column load = 200 tonne Strata sbc= 10T/m2 No of piles =4 Nos Area of pile cap = 2.0 X 2.0 = 4 sqm A ] In case pile raft concept not used working Capacity of pile required = 200/4 = 50 tonne ultimate capacity of pile required = 2.5*50=125 tonne Design of pile cap is based on piles reaction of 50 tonne B ] In case pile raft concept is used Capacity of strata= 10*4=40 tonne Balance capacity from piles= 200-40=160 tonne working Capacity of pile required = 160/4 = 40 tonne ultimate capacity of pile required = 2.5*40=100 tonne Saving in piling already evident. Only For purpose of Pile raft design - - Actual scenario;-Design of pile cap needs to be still based on piles reaction of 50 tonne(though working capacity is 40 tonne) This actual scenario is also critical in face of hypothetical scenario of design as pile raft considering 40 tonne pile reaction and 10 tone/m2 base pressure from pile raft. Case of very Flexible raft: But in case of very flexible pile raft (with piles spaced much apart) , the apportionment will be complex. Here again the design as pile cap considering full loads taken by piles may be conservative approach for purpose of pile raft design . (though in actuality loads will be shared very early between raft strata and piles in very flexible raft )
the viewpoint is personal and may be dis-regarded otherwise but discussion for better understanding of structural concepts must continue. best regds vikramjeet -Posted via Email Back to top ibarua
General Sponsor
Posted: Thu Jan 19, 2012 12:51 pm PILE RAFT
Post subject:
19th Jan 2012 Re.: RAFTS ON PILES The issue is not as simple and straightforward as it has made out to be. Joined: 26 Jan 2003 Posts: 996
Firstly, in the case of individual pile caps, the cap is cast on soil disturbed by pile boring /driving. The site is generally cleaned up, leveled, compacted to whatever degree possible, and then the pile cap is cast. In such cases, the soil directly below the cap may be assumed to have very little or no shear strength. Therefore, it would not be wise to apportion any load to the soil under the cap, and this is the practice usually followed. Secondly, please realise that a raft mat is not a rigid plate -- it's flexible. Consider the usual raft (without piles): the maximum intensity of soil reaction will be in the vicinity of the superstructure columns. The zone of influence of such column load depends on the thickness of the raft. As you move away from a column, the intensity of soil pressure decreases. This is to say that the distribution of pressure below the raft is not uniform. Coming to a raft with piles, it's evident that the bulk of the load will be transferred to the pile group due to the flexible nature of the raft, depending on the thickness of the raft. The question is: how much? That's the rub. Indrajit Barua. From: vikram.jeet Sent: Wed, 18 Jan 2012 15:38:14 To:
[email protected] Subject: [SEFI] Re: PILE RAFT Regarding approach for design of pile raft, - - - - -- as a personal view The Pile raft concept will definitely economise the design requirements of piles (their length/dia) , but as far as design of pile raft itself is concerned I think it has to go by the design as pile cap especially for
rigid pile caps Load apportionment - Piles are deep foundations and sustain more loads with little settlement (in comparison to raft). Thus loads will be mostly shared by the piles even beyond their working capacity (but within ultimate reach) Still system of pile -raft has overall capacity to sustain loads since raft strata will mobilise capacity gradually. The capacity of raft strata will be fully mobilized only when piles reach their ultimate capacity and hence in working stage it will be a nearly passive assurance from raft strata.(since piles are active in taking almost entire working loads). Just considering a simplified Example Column load = 200 tonne Strata sbc= 10T/m2 No of piles =4 Nos Area of pile cap = 2.0 X 2.0 = 4 sqm A ] In case pile raft concept not used working Capacity of pile required = 200/4 = 50 tonne ultimate capacity of pile required = 2.5*50=125 tonne Design of pile cap is based on piles reaction of 50 tonne B ] In case pile raft concept is used Capacity of strata= 10*4=40 tonne Balance capacity from piles= 200-40=160 tonne working Capacity of pile required = 160/4 = 40 tonne ultimate capacity of pile required = 2.5*40=100 tonne Saving in piling already evident. Only For purpose of Pile raft design - - Actual scenario;-Design of pile cap needs to be still based on piles reaction of 50 tonne(though working capacity is 40 tonne) This actual scenario is also critical in face of hypothetical scenario of design as pile raft considering 40 tonne pile reaction and 10 tone/m2 base pressure from pile raft. Case of very Flexible raft: But in case of very flexible pile raft (with piles spaced much apart) , the apportionment will be complex. Here again the design as pile cap considering full loads taken by piles may be conservative approach for purpose of pile raft design .
(though in actuality loads will be shared very early between raft strata and piles in very flexible raft ) the viewpoint is personal and may be dis-regarded otherwise but discussion for better understanding of structural concepts must continue. best regds vikramjeet
-- ÂÂ Posted via Email Back to top knsheth123
Bronze Sponsor
Posted: Thu Jan 19, 2012 5:53 pm PILE RAFT
Post subject:
Dear Sefian
Joined: 26 Jan 2003 Posts: 91
The design of Piled Raft is a fairly complex Soil Structure Interaction problem. The variables includes Raft Stiffness, Pile Stiffness, Soil Stiffness. Hence the results of Raft analysis and Pile Analysis which are based on different criteria can not be added using Principle of Superposition. The independent analysis of pile as well as raft to account for soil structure interaction is carried out using Soil spring model. Using Spring model it does not account for Raft Soil Pile Interaction and Pile Soil Pile interaction. Hence the problem demands a 3-D elasto-plastic non linear model for soil, pile and raft, including models for interface element between pile-soil & raft-soil. Extensive work is carried out by H. G. Poulos, M. F. Randoph, Reese L. C, Matlock, Coyle etc. A Report for this type of problem by Prof. H. G. Poulos is attached for reference. Prof. H. G. Poulos delivered an Expert Lecture at SGSITS-Indore on Design of Burj Tower Foundation system as Piled Raft (Indian Geotechnical Society). I would high light some important points as given below: With a Piled Raft on Sand. The capacity of the system is significantly increased and the final settlement will be lesser than that of a pile group only. The reason is with the provision of Raft, the stress level at the pile interface is increased. The resistance offered by sand is proportional to the confining pressure. For sand, Shear strength = confining pr. * tan (phi). so if superposition of Raft capacity for reduced allowable settlement is used will yield safe
results. Where as the Piled raft on clays should be treated as Pile Group only without taking any advantage of superposition. The reason is shear strength of interface is not improved for clay. For Clay Shear Strength = cohesion. More over with the provision of raft, stresses in upper strata is will increase, resulting in actual sssettlement on the higher side than for the pile group only K. N. Sheth
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Back to top Dr. N. Subramanian General Sponsor
Posted: Fri Jan 20, 2012 1:28 am Re: PILE RAFT
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Dear Prof. Sheth, Thank you for your comments. I uploaded the same report a few days ago! Regards NS Joined: 21 Feb 2008 Posts: 4416 Location: Gaithersburg, MD, U.S.A.
knsheth123 wrote: Posted via Email
Back to top vikram.jeet
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Joined: 26 Jan 2003 Posts: 2212
Posted: Sat Jan 21, 2012 10:24 am PILE RAFT
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Raft strata will also simultaneously mobilise the capacity right from outset ,but will be negligible in the initial stages and gradually increases with settlements of pile cap(raft) depending upon settlements of piles..At point when piles settles 12mm , the pile- raft strata capacity mobilised has to be based on corresponding 12mm settlement only and ,not 75mm usual adopted for raft foundation , I think.
best regds vikramjeet
Dear mr vikram jeet,SBC of raft is based upon 75 mm settlement where as pile is considered to be failed at 12mm settlement . In the said example sbc 10t/sqm shall be mobilized at 75 mm setttlement . At this stage piles shall not be in a position to transfer assumed capacity of pile to soil .Your comments please ?Padam Kumar -Posted via Email Back to top hemal ...
Posted: Sat Jan 21, 2012 7:32 pm PILE RAFT
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Joined: 01 Apr 2008 Posts: 108
Back to top knsheth123
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Posted: Sun Jan 22, 2012 10:24 pm PILE RAFT
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Dear Hemal
Joined: 26 Jan 2003 Posts: 91
For a raft allowable settlement is more. 75mm for sand and hard clay, 100mm for plastic clays (IS 1904) for RCC Structures. The primary reason is differential settlement is very much reduced with the use of raft. A raft on sand is a most advantageous solution. For a foundation having width 5 to 6 m and larger, the settlement does not increase significantly (Stiffness of sand increases with confinement). The allowable settlement is also more. The bearing capacity from shear criteria increases significantly with increased width ( "0.4 B gama Nv" part of SBC equation). So we have advantage from both the criteria. A clay is not generally preferred as settlement is based on pressure bulb size which increases with increasing width. So SBC from settlement criteria reduces with increased width. SBC from shear Criteria is not affected much by increased width as for phi = 0, BC Factor Nv = 1. K. N. Sheth Posted via Email
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hemal ...
Posted: Mon Jan 23, 2012 8:22 pm PILE RAFT
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Dear K.N. Sheth sir, Joined: 01 Apr 2008 Posts: 108
Regarding my previous post actually, i was trying to mention that actual settlement for raft will be less as compared to isolated footings, as also supported by you. As mentioned by you present approach to design piled raft foundation is to model whole building along with RAft and pile and applying soil spring (winkler's bed) below raft and lateral spring along some points on side and vertical spring at end for piles. Calculation of lateral spring for pile involves estimation of lateral subgrade modulus (ksh) for pile. There are many approach (D'Appolonia's approach, Bowl's approach etc.) to estimate ksh. One more approach is Ksh=As+Bs*Zn.Difference between value estimated by each approach is very large. So, lateral spring value calculated is also different. Many people model pile with single vertical spring (appx 20*D) and single horizontal spring 0.1*vertical spring. It is observed that effect of spring value (Kv or Kh) on design forces (B.M.,S.F etc) is less but on displacements is very large. Many geotech-software (OASIS GSA RAFT, ELPLA, ENSOFT PILE GROUP, FLAC3D etc.) have ability to model Pile and raft with soil but no facility to model superstructure. Superstructure loads from columns and shear walls are entered separately in such software, which are derived from structure analysis software with assumed support condition. So, it will be raft-pile-soil interaction but not the soil-pile-raft-structure interaction. So, my question is : which approach shall be used, so that we can have actual soil-pile-raft-structure interaction? Regards Hemal On Mon, 23/1/12, knsheth123 wrote: Quote:
