Displacement piles and effect on surrounding soil 1

[AussieGeo]

Hi all,

I am currently attempting to figure out the increase in strength one would see in soils adjacent to a helical pile after installation.

When screwing in the helical pile (e.g. 200mm shaft - closed tip), the shaft obviously displaces a certain amount of soil - let us assume for now we are talking about a stiff clay. As you can imagine, this is similar to the effects of driving a pile although the installation process would possible mean very different stresses in the surrounding soil.

Can anyone discuss or point out a paper which discusses the increase in radial stress/strain as a result of the soil displacement?

I am hoping i can use this increase in strength to argue that we will achieve higher shaft capacities, as the soil immeadiately surrounding the shaft will exert increased pressure upon the pile. I am also interested in how much extra torque would be required to screw in the pile depending on groud conditions (as it would obviously be easier to displace large amounts of soft clay compared to a stiff clay).

Thanks in advance

 

[dgillette]

Hi AussieGeo. I'd be hesitant to count on much from the shaft. Wouldn't the soil immediately adjacent to the shaft be completely remolded by the spinning, and the soil within the footprint area of the flights get disturbed a little by the auger passing through?

The torque calculation would probably be pretty easy - just the cylinder area times the radius times the remolded strength, or, maybe I don't understand the geometry.

Coincidentally, I'm having some small ones installed next month as part of the repairs and improvements on my house. They will go through slopewash and crappy fill into very soft claystone. Some will anchor counterfort walls to prevent further movement of the basement wall, and some will be foundation for a small part of the house that was built badly the first time and is being replaced.

DRG

 

[PSlem]

I discount any friction after first coupling as bolts and coupling wipe an annulus in our silts and discount the shaft for a distance equal to the helix diameter as downward movement can develop tensile cracks resulting in lateral stress reduction per Ellison (1977), so shaft friction is only a little over 10% of capacity. For a 2.875"(.12' radius)shaft with a 7' lead in a soil with c of 2625 PSF and ? = 0.3 x 2625 = 788 PSF Perimeter = 2.875/12 x ? = 0.75’
(0.75’ x (7’-1.17’)) SF x 788 PSF x 0.12' x 0.43(remolded) = 180#'. This pile was put in with 3000#' of torque.

One paper I can think of is

http://www.scribd.com/doc/5021455/C...E-DISSIPATION-AFTER-HELICAL-PILE-INSTALLATION

Another is

http://www.magnumpiering.com/shared/content/pdfs/Helix Manual Capacity web.pdf

 

[PSlem]

Also excellent paper

http://www.almita.com/_pdf/Load_Testing_in_Sensitive_Fine_grained_Soils_Christopher_Weech.pdf

 

[BigHarvey]

Displacement can lead to significative improvement of mechanical properties of granular material because the displacement goes together with a voume reduction or compaction but not for fine grained soil where displacement occurrs without volume change.
In this type of soil you might observe a small improvement but it is really nominal . For stone columns, for example, you do not consider any improvement of the soil betwen the columns( you just have an inclusion of better material and a different load transfer mechanism ).
I believe this kind of approach leads to the limits of the method and this is how you get involved in a failure.

 

[oldestguy]

Sounds like a "touchy" situation.

Do you have possibilities for increasing the pile numbers, deepening, etc.?

If so, how about some initial load tests to prove or disprove your predictions? That will then help all to settle on production piles required.

Early tests on piles with temporary soil weakening followed by some delayed tests will help to resolve the sensitivity situation.

 

[AussieGeo]

Thanks all for your input - very informative. Those papers are a fantastic place to continue my thinking! I would agree with BigHarvey and conclude that for granular, there may be something there (although any help may be wiped out by the bolts during installation) but there wont be much help for cohesive materials.

I guess all the answers lead me to focus my attention on a further question with regard to the installation torque (which is obviously related to the pile capacity).

How much torque would be generated by the shaft if installing in cohesive materials? Would it be significant? PSlem pointed out that he discounts any resistance after the first set of bolts but i am wondering if the compressive effect of the shaft means we should include all the shaft as acting. As installation torque has often been related to capacity, any increase in torque readings may give (somehwat) misleading capacity values?

Looking forward to some more very interestign responses.

 

[PSlem]

As to installation in sands, there is a recent helical pile FEA paper that plotted 12" shaft piles without and with a 36" helix. At a settlement of 50mm and a friction coefficient of 0.0 he got 42K and 280K. At 0.15 he got 52K and 290K. At 0.3 he got 60K and 375K. So 15-18% for shaft. He modeled them as soil replacement shafts rather than displacement as he said that is what they typically are, Most piles will have a smaller shaft/helix ratio than 12"/36" so shaft % of load will be less. I would not get excited about friction.

Here's how I'd break down a 14" helix on a 10' 2-7/8" OD shaft in silt and you can see adhesion was about 8% and numbers worked out.
A 14" helix in 21 blow silt with a c of 21 X 125 = 2625 PSF
Using a hex shaped helix so edges are 7.75” and area is 1.08 SF.
Bearing
3.5" Coupling (3.5^2 - 2.875^2)/144 x ?/4 = 0.02 SF
0.02 SF x 9c x 0.43 x (0.43 x 21 +6) / (21 + 6) / 8?:1 = 5 lbs-ft
Pile tip (2.875/12)^2 x ?/4 = 0.045?SF
0.045 SF x 9c / 8?:1 =42 lbs-ft
4.5” bolts stick out 1” and are assumed to wipe a 4-1/4” area per revolution as they will clear material between bolts also. 1”x 4.25” / 144 = 0.03 SF. They will react directly with torque at r = (1.75” + 0.5”) / 12 = 0.1875’
0.03 SF x 7.5c x 0.1875’ x 0.43 = 48 lbs-ft
Helix edge 5.875" x 3/8" = 0.0153SF
(1.4375 + 5.875/2)/12 = 0.365’
0.0153 x 7.5c x 0.365’ = 110 lbs-ft
Helix resultant is acting at an angle to the vertical of (tan-1(3/2.875?)
+ tan-1(3/14.09?)) / 2 = 11.1 degrees

The vertical force is (1.12 - 0.045)SF x 9c / cos(11.1 degrees) = 25,881#
Helix top friction of 0.20 acting at
midpoint of area, 1.4375 + (7.045 – 1.4375) x ?2/2 = 5.4” = 0.45’
25,881# x 0.2 x 0.45' = 2329 lbs-ft
Adhesion
? = 0.3 x 2625 = 788 PSF
No adhesion is counted within a distance equal to the helix diameter, due to shadowing per Zhang.
2.875/12 x ? = 0.75’
10' Lead (0.75’ x (10’-1.17’)) SF x 788 PSF x 0.12' x 0.43 = 269 lbs-ft
Helix bottom 1.08SF x 788 PSF x 5.4/12 = 383 lbs-ft for a total 3186 lbs-ft
The 10’ pile was installed to 3000 lbs-ft and tested to 25,000# at 10% of average helix diameter. 25,000# / 3000 lbs-ft = 8.3#/lbs-ft for a Kt.
Pile ultimate capacity is 1.08 x 9 x 2625 + 0.75 x (10’- 1.17’)) SF x 788 PSF x 0.43 = 27,760#. Pile capacity to calculated by Std. Brng. Eqn. was 90% Pile torque to calculated was 94% and pile deflection by FEA was 1.4”.

 

[PSlem]

Total torque was 3186 lbs-ft by calcs. It got accidentally cut out.