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RLC32681 (Structural) (OP)
21 Oct 09 15:04
I already finished designing a pile foundation in which the flooring is considered as slab on grade. after driving all the piles in the ground, there was a change in the structural system, instead of using slab on grade we dicided to design it as structural slab because of settlement problem.

After checking the pile capacity that is considering the additional weight coming from the structural slab,grade beam and soil, i notice that the pile reaction exceeds the pile capacity. i am thinking to to add additional piles besides the critical pile making it 3 piles to distribute the pile reactions.

I want to hear your opinions about this. I appreciate if you can suggest other solutions.  
jdonville (Geotechnical)
21 Oct 09 15:42
Suggest you work with the geotech of record - they will help ensure that your are placing your supplemental piles effectively to get safe bearing out of the soils.

Jeff
 
hokie66 (Structural)
21 Oct 09 16:24
You might want to have the geotech provide the actual capacity of the piles based on the driving records and analysis of testing.  Often, piles as constructed have more capacity than specified.  If they are not stronger than specified, then additional piles would be necessary as you have assumed.  Whether you want the additional piles close to the existing or spaced between the existing will depend on the design of the floor.  
InDepth (Structural)
21 Oct 09 17:55
Sounds like a job for Mr. Micropile!
spats (Structural)
22 Oct 09 16:07
Geotechnical reccommendations are ALWAYS conservative. Work with your geotech to basically talk him into a higher safe pile capacity, as long as you're not too far off. I've done this on more than one occasion, successfully, at least related to safe bearing pressures. All you have to do allow a little more potential settlement, and slightly reduce the ridiculous safety factors.
BAretired (Structural)
23 Oct 09 23:04
I do not agree that the geotechnical safety factors are "ridiculous".  Keep in mind, soil is not a perfectly uniform, isotropic, elastic material.  

It should be no surprise that your piles are overloaded by adding one entire concrete floor, complete with live load.  The best advice I can offer is to follow the recommendations of your geotechnical engineer.  Add piles as required and you won't have to deal with settlement problems a year or two from now.

BA

ishvaaag (Structural)
24 Oct 09 8:46
To be more precise in my view on the goetehnical reports I don't think (those that I have met) are as bad on being overconservative on strength as in being overconservative in predicting settlement. They can be predicting 10 and 20 times what seen on buildings standing 50 or 100 years in the area (sometimes, of course, not always). So I think the great debate about this aspect of the geotechnical reports is that I feel that we must ask and receive the settlements perceived to be expected, as an average, if you want with 95% of reliability if you want, on the time of interest, and of course, this MUST show correlation with everything seen in the area (assuming remaining in the same range of nonuformity). The contrary is aberrant respect all practice known, for there's no reasonably economical way in common buildings to deal with, say, 15 cm differential settlements on 7 m bays, that, as well, are really not to be expected. This unwarranted checkmate by geotechnical reports to structural design must entirely be forfeited.

In short, we are not demanding ordinarily settlements anywhere that won't never be surpassed, but those that experience and reasonable computation show it is reasonable to expect.
RLC32681 (Structural) (OP)
24 Oct 09 12:45
i have some querry regarding the support between pile  and cap. I ask this question because it might solve the problem. for me, the connection between the pile and pile  cap can be consider as rigid connection thats why i assumed it as fixed but i heard some opinion that it can be supported as spring allowing the pile to move vertically.

any opinion i this issue would help me a lot. thanks in advance.
msquared48 (Structural)
24 Oct 09 13:02
Could you add more concrete grade beams to cut down the thickness of the slab, and the weight of the system the piles see?

Mike McCann
MMC Engineering

BAretired (Structural)
24 Oct 09 14:45
RLC32681,

If there was a soil report prior to design, why was the settlement problem not recognized prior to driving all of the piles?  What recommendations did the geotech make originally respecting the slab on grade?  What does he have to say about the decision to use a structural slab on grade now?  Is there another alternative such as removing questionable soil and placing engineered fill between piles?

You stated:

Quote:

for me the connection between the pile and pile  cap can be consider as rigid connection thats why i assumed it as fixed but i heard some opinion that it can be supported as spring allowing the pile to move vertically.

Are the driven piles timber, steel, precast concrete or other?  How are they attached to the pile caps?  What is the soil type at the site?

How can you expect a reasonable answer to your question based on the scant information you have provided?

BA

RLC32681 (Structural) (OP)
24 Oct 09 15:15
BA,

There is a soil report prior to driving of piles. It was only recommended there to use concrete piles. we just found out lately that there is an issue of settlement of slab in the neighboring structure. I try to re anlyze the structure using fixed support and spring for pile and i observe that the recation decreases thats why i rised a question regarding pile support. The soil bearing capacity is 60KPa.  
BAretired (Structural)
24 Oct 09 15:53
The fact that there is an issue of slab settlement in the  neighboring structure is a warning signal, but does not necessarily mean you have the same problem.

Request the geotech firm to do additional testing on your site and make recommendations as to the most appropriate measures to be taken to support the grade slab.  A structural slab is one possibility but there are others.

As to your question about boundary conditions at the support, a pile is not a fixed support.  It is a beam on an elastic foundation, the soil providing elastic reactions throughout the length of pile.  How close the support is to full fixity depends of the properties of the pile and the soil.

BA

hokie66 (Structural)
24 Oct 09 17:00
A geotechnical report showing a bearing capacity of only 60kPa should ring the alarm bells.  I would be surprised if the report did not include recommendations for support of the ground level slab.
RLC32681 (Structural) (OP)
25 Oct 09 11:58
BA

the piles are 6m apart, 16 m in length and 600mm diameter. The option that i did is to put a spring support (location of piles) along vertical axis with a spring onstant of k=AE/L and provide fixity on the horizontal axis and restained moment. I just want to clarify that the pile cap is not included in the model.

Just to ask some clarification on what you said " It is a beam on an elastic foundation, the soil providing elastic reactions throughout the length of pile". Does it mean that the pile is release in the horizontal axis?

Thanks.

RLC
BAretired (Structural)
25 Oct 09 14:31
RLC,

If it is a friction pile, the resistance to the vertical reaction is presumed to occur uniformly from top to bottom.  That may not be true if the soil characteristics are variable within the length of pile, but it is probably about as close as you can guess.  In that case, the vertical deflection at top of pile would be approximately FL/2AE, i.e. using the half length instead of the full length.

If the pile is an end bearing pile, the resistance to vertical load occurs at the bottom, so the vertical deflection at top of pile will be FL/AE + F*Ks where Ks represents a spring constant for a load on the soil.

When I said the pile is a beam on an elastic foundation, I was thinking about a moment applied at the top.  If the pile is pinned top and bottom and there is no soil around the shaft, the pile bends as a simple beam and its horizontal deflection can be calculated at any height by elementary theory.

Because the pile is surrounded by soil, it is not free to deflect sideways.  The soil will resist that movement.  If the soil is considered to be an elastic material, the pile may be considered as a beam on an elastic foundation.  The support could then be represented as a rotational spring plus a vertical spring.

It is not clear to me how this approach is going to significantly change the load on each pile.  If the additional reaction of the structural slab adds too much load to the piles, you must either add piles or find another way to support the slab on grade.

BA

hokie66 (Structural)
25 Oct 09 16:33
It might help us if we knew something about the geotechnical profile.  Also the type of pile.  It would be a rare situation for 600 mm bored piles, 16 m long, not to be able to support a single floor.  Is this a multistorey building?  I wouldn't worry about whether the pile to slab connection is rigid.  You don't want to assume the pile takes the slab bending.
BAretired (Structural)
25 Oct 09 18:30
hokie,

In RLC's first post he said the piles were driven in the ground.  If they are concrete piles, they must be precast.

I agree that the soil profile and a description of the building would be of interest.

 

BA

hokie66 (Structural)
25 Oct 09 21:46
BA,

You are right, he did say they are driven piles.  I would just ask him to confirm that, as 600 mm driven piles are not common.  Uncommon enough that I have never seen or heard of precast concrete piles of that size.  Could they be driven cast in place piles (Franki or similar)?  

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