Maximum spacing of Piles

[voi]

Hi
Can someone indicate to me the maximum spacing of piles. The piles we are dealing with are supported both by end bearing and friction. Most codes are on minimum spacing.
thanks

 

[Ron]

The spacing of piles is dictated by the load they will carry. Your pile capacity will be limited by the soil conditions. Once you figure out how much load each pile can take, then you start figuring how you apply the load to those piles....whether through pile caps, individual columns resting on a single pile, or pile supported mats, etc.

 

[massagung]

@voi : for practice purpose minimum spacing pile to pile at one pile cap the range about 2.5 to 3.5 of Pile Diameter, this condition to reduce heave effect on driven piles or to reduce sliding of soil on bored piles(geotechnical point of view), and depend on carry of load to calculate/consider excentricitate of moment on group pile that transfered by coloumn(structural view).


M.A.

 

[voi]

Thanks for the input. The project that I am involved is because of the heavy loading's ( 25 floors ) and the soil conditions ( firm rock is 60 ft below ground)we have to support the building on a pile cap supported on bored piles. The contractor has available 3 ft diameter casings. The minimum spacing that we can use is say 3 Pile diameter which works out to be 9 ft. My calculations show that the group effect of the piles is less than the allowable individual piles at about 6 pile diameters. So is this maximum spacing acceptable?

 

[VoyageofDiscovery]

Group effects are an obvious occurrence, so design the piles for that effect. If the pile size, soil and configuration cannot handle the loads, you need to consider another practical system (ie raft foundation). We typically do not design for a maximum spacing as this would inherently lead to an uneconomical design, however speak to the structural engineer with respect to this and communicate your problem to see what he/she suggests for alternative foundation systems for the structure.

HTH

VoD

 

[massagung]

i agreed with voyageofdiscovery said the maximum spacing is uneconomical design,

Voi :
1. what is the formula that you use to calculation group piles to get efficiency group factor (coverse labare, vesic, or..)
2. can you share the layout of group pile and working load on single pile and also working load will be supported by pile group?

generaly, if spacing pile to pile (centre-centre) more than 3 times of pile perimeter, group reduction factors need not be applied,
i.e : Pile Dia = 3ft, -> 3 x pi() x 3ft = 28 ft --> no need to applied reduction factor. (source: code of practice for foundation_2004_HK)

[CMMIW]
this indicate no maximum spacing pile in pile group, if the spacing more than 3 times pile perimeter the group pile axial capacity is sum of axial single pile.

M.A.

 

[voi]

massagung,
Does the code say 3 x pile diameter ( 9 ft) or 3 x pile perimeter ( 28 ft) in my case? . The difference is quite huge.

 

[massagung]

the code written if spacing pile to pile more than 3 x pile perimeter is no need to applied group reduction factor.

M.A.

 

[aeoliantexan]

Will your piles extend to the bedrock or be supported entirely in the soil? I don't usually consider group effects when piles bear on good rock.

You said they piles will depend on end bearing and friction. I don't know your load magnitudes, but two 18" piles will have the same perimeter as one 36" pile and need only half the concrete.

What is your individual pile design load? What are the group loads? What is the soil type?

 

[voi]

Hi all,
Thanks for all the contributions. I have managed to obtain the information I needed. Basically there is no limit on maximum spacing of piles but is governed by the pile cap design which becomes uneconomical if the spacing is too large. Tomlinson ( Pile Design and Construction Practice) covers it very well and in detail.He also reports on the Swedish code i.e for end bearing + friction piles the minimum spacing should be.
< than 10 m depth .... 3 x diameter
10 -25 m .... 4 x diameter
>25 m .... 5 x diameter

voi

 

[Kpaudel]

Very great information...According to AASTHO LRFD pile c/c spacing must be 4d or more...3d is not good practice.

 

[BigH]

You should also do a literature review. Polous has several good articles on pile supported raft foundations.

 

[fattdad]

I find it hard to believe that there will be any real contribution from skin friction when the end bearing is on rock. Too much strain would be required to moblize the full potential strength from shaft friction. There'd be some eccentricity concerns at the ground level if the spacing is too great, so I'd want to discuss the structural approach with the structural engineer from the onset. If you are moblizing only the end bearing, how does spacing factor into this problem. Ultimate bearing on the rock is clearly a concern, but if you look at the equivalent raft acting on the rock and you have capacity, who'd care whether the spacing is 2d or 10d? Maybe I'm missing something, but this is end bearing on rock, eh?

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Would surmise, if rock,a lot depends on the degree of weathering. You would develop both end bearing and side friction if stratum was cohesionless. Of course, most of the side friction would have to overcome before substantial end bearing could develop.

 

[dik]

Fatdad... the difference in stiffness is the reason that most people do not combine bearing with skin friction.

Dik