Surcharge Due to Friction Piles

[InDepth]

OK Eng-Tips I need some serious help!

Question:
1) Is there a method to calculate surcharge pressures due to friction piles? (Not FEM or SSI, something like a Boussinesq but for piles)
2) What type of load would you expect? vertical shear stress? lateral/horizontal load? or both?
3) What type of temporary shoring system would you use?

Site Conditions:
Silty Sands (SM) and/or sand (SW or SP) with “very low” expansion potential. “Temporary excavations for construction of the proposed buildings may expose alluvium that is prone to localized raveling. Looking at the boring logs, within the depth of excavation varies between 4 and 18 blows/6in.

Existing Building:
As built drawings are not available. Existing contract document specify length of piles by "30 ft into natural grade." Thus uncertainty in length. These are 4 pile 24" diameter groups at 30'-0" OC. The structure is approximately 4 stories. Estimated loads on pile cap are between 400 and 600 kips unfactored.

Proposed New Building / Shoring:
Depth of new building is between 16'-0" and 24'-0" max below existing pile cap. Existing pile caps are within 5'-0" of new wall...i.e. really close. Proposing a Soldier Beam and Tieback System, but I think I may need Sheet piling (using Giken Silent Piler).

What the geotech recommended:
Vertical shear stress = 0.3 ksf @ Grid XXX & 0.7 ksf @ XXX. Only Shear Stress from perimeter area of pile group parallel to shoring. Soldier Beam Shaft Friction: 200 psf, for min 10 ft vertical embedment. Geotech indicate that values have an FS = 1.0....thus how do you develope 0.7 ksf in 0.2 ksf soil?

Any comments appreciated!

 

[escrowe]

Hmmmm...

1) If you mean stress change over depth, yes and no.
The answer depends in part on settlement that may have occurred following installation of the (drilled or driven?) piles. That would suggest the depth of the neutral plane, below which positive pile shear stress would develop. A lot of blanks to fill in. Ultimately you're just guessing.

2) What kind of structure(s) are these existing piles supporting? 4 x caps 30 ft on center? Exterior columns?

3) Do I read correctly that the project requires 24 feet of excavation in predominantly loose to dense sandy (non-cohesive?) materials within 5 feet of existing 30-ft piles supporting a 4-story building?

A shoring system designed for the active case could allow movement of the soils surrounding the piles, and trigger settlement. I think you need a very stiff retention system, designed for the at rest earth pressure case.
Not sure about the numbers in your last paragraph.

If the existing perimeter piling is not providing lateral/moment support, fine.
If the piles are resisting lateral thrust (wind) then that load may be presumed to transfer to the retaining structure.

You might consider permeation grouting or compaction grouting to stabilize the existing foundation soils prior to excavation.

Don't forget the existing floor slab. Is it structural or slab on grade?
Relatively small movement of your retaining system could allow a slab-on-grade system to settle.

 

[DRC1]

If the pile is truely suported by skin friction, then break the piles into segments and aclaculate the imposed lateral load on the wall due to the unit load, treating it as a point load using Bousenesq. You will have to make some assumptions about the pile lengths. The existing condition quote makes me wonder if you do not have fill underlying your site.

If you site is on piles, I would not be inclined to use the silent piler. First, they generally have a limited capacity of 20-40 tons. Once you reach that resistance, you can not drive further. I would consider driven piles as opposed to vibrated or soil nailing. If you want a nonvibrational method, that may be your best bet.

I too do not understand your last paragraph.

 

[msucog]

i'll barely skim this thread and simply note that it may be worth the effort to use ndt methods to get a handle on the approximate length and integrity of the installed/existing (and apparently undocumented) piles (or at length a couple of them). it will be a pain since you will likely need to excavate a couple feet out just below the caps so that you can have access to the pile itself. if it's cost prohibitive, you might first weigh your options to see what happens if you rely on that 30' natural ground number and it turns out that it's actually only 10' (or damaged piles). it's just a thought from a different perspective.

 

[InDepth]

Thanks for the feedback. Here are a few clarifications:

escrowe questions

1) Yes, pile cap supports exterior building columns. The pile cap has four 24" diameter drilled-CIP.

2) Yes, bulkhead will be within 5 foot of pile cap.

3) Yes, lateral force (wind and seismic) and vertical force should be considered. Since the shoring system is temporary, I was going to assume only vertical force would apply. The permanent structure, though, would need to take both vertical and lateral surcharge.

4) I considered jet and chemical grouting to increase the frictional resistance below the excavation line. Hayward Baker indicated that chemical was not feasible, but that jet grouting is possible.

5) SOG is actually a structural slab on grade.

6) Yes, I think a stiff system like a secant pile wall with tiebacks may be the best solution.

 

[InDepth]

DRC1 Questions/Clarifications

1) Great idea! I was thinking about using a segmented Bousinnesq. Mactec suggested this method to me, even though they aren't the geotech on the project. What is confusing me is that the above suggested method is a horizontal surcharge load. What the geotech provided was a vertical downdrag force. From an academic point of view what should the force be - horizontal or vertical?

2) Yes, from previous existing and demo drawings, there use to be an existing parking structure that was demoed. I suspect the material below the building is fill. I am also on the banks of an old river. All buildings adjacent are also on friction piles.

3) Yes, sheet piles does not sound apealling based on everyone's comments. Would you really soil nail beneath an existing building? Wouldn't you have movement because a soil nail system is passive? Or were you thinking I should use "tensioned" soil nails (Golder approach).

4) Here is the geotech's response to the last paragraph of my initial posting:

-> The FS for friction resistance of 200psf for minimum embedment depth of 10ft is 1.0. As you aware, the friction resistance is a function of overburden pressure which is a function of depth. As mentioned in our report the friction resistance of 200psf is estimated based on an embedment depth of 10ft. The 700 psf is based on the assumption that the total length of existing pile is 30 feet and that the entire working load of 150 kips is supported by skin friction. Since the average overburden depth for the existing piles is much greater than that used for the soldier beams, the resulting friction resistance is much higher

 

[InDepth]

msucog response:

Yes, looked into using a

1) Pulse Echo Method - ideally used when the pile top is accessible (usually for new pile). Testing manufacturers indicated that using the hammer from the side with a pile cap or grade beam attached gives mixed results.

2) Parallel Seismic Instrument - The operation is fairly straightforward the main drawback being the necessity of installing the parallel tube. For this situation I would have access to only 2 piles...but could make an assumptiuon on the 3rd and 4th. But at least I can assume +/- 2 feet

 

[escrowe]

Just occurred to me that 'temporary' is an oft' abused term.
Murphy says the wind will howl and the waters rise before your project is complete. :]

 

[InDepth]

Yes, absolutely right...I abused the term on this one. What we should actually be doing is a probabilistic analysis/force adjustment based on a return period appropriate for the expected life of the structure. (Wind and Seismic)

i.e. For 2 year shoring, using a 0.1%in50yr seismic load (~~~). For a 40 yr life permanent system, use a 10%in50yr, else if it serves as an essential structure, use a 2%in50yr seismic load. etc...etc...

 

[DRC1]

You would definately have a horizontal component to the load. I really had not thought about the vert. component. Generally this would only effect end bearing, which is typically not a problem. I have used the segmental loads and bousinesq to compute lateral loads. One can definately appreciate a spreadsheet.
If you have a building material fill penetrating obstruction may be a problem eepecailly if you are trying to limit vibrations.
I would use tension anchors for soil nailing. Talk to expert installers, such as Moretrench or Haywood Baker or Nicholson, to name a few.
After rereading your geotech's statement a few times I am concerned that he is only giving parameters to support the vertical load, which on soldier beams isnot the controlling factor, but rather horizontal load is. Generally for horizontal load, the required embedment must be solved for the desired case, usually FS = 1.5. I think your geotech may not be familar with shoring systems

 

[aeoliantexan]

Let me add a few thoughts to the other responders' contributions:
1. ACIP piles develop lateral stresses due to the fluid concrete pressures. Lateral stresses may then increase due to dilation as the vertical load develops skin friction. No one knows just what the present lateral stresses are, but you can be pretty sure that if they are relaxed, the piles will settle.
2. Friction piles in sand can settle due to vibration when piles are driven nearby. General the piles settle with each blow and stop when driving stops, so the settlement can be monitored and driving stopped if necessary. Of course, you may then need an alternate approach.
3. If the soil to be excavated is supporting vertical load, the shoring must take over that function as well as supporting the lateral pressures with very little deflection.
4. You may not want to stake your livelihood on complex analyses, especially when someone not under your control might do something unexpected during construction that really affects the behavior of the shoring system. I would seriously consider underpinning the existing building on piles that bear well below the new excavation. Some type of drilled, cased micropile may be best.Don't fight an enemy you can go around.

 

[InDepth]

As a reference, here are two papers I found that are related to the above discussion that may be of use to others:

1) DFI/ADSC - "Manual for Non Destructive Testing and Evaluation of Drilled Shafts"

2) BRIAUD, J.L., TUCKER, L.M., “Horizontally Loaded Piles Next to a Trench,”Proceedings of an ASCE Session on Foundations for Transmission Towers, Atlantic City, April, 1987.

 

[palmahouse]

I did not have time to read all the posts - hope this helps:

I have dealt with a similar problem before. I took the total axial load on the pile, divided the pile into segments (maybe 5-foot-long segments), distributed the load over the segments using some hand-waving - with higher loads at the top of the pile and decreasing with depth (ignoring soil-strucutre interaction and neutral plane concepts - because in the short term, the soil will feel the full load in this manner). For instance, if you have 5 segments, you could put 50 percent of the load on the top segment, 30 on the second, 10 on the third, 6 on the 4th, and 4 on the 5th. Then, you can do an analysis based on elastic theory to estimate both the horizontal, vertical, and shear stresses at all points in space relative to the pile. I think I used the computer program "STRESS" for this. Then, you need to think about whether the subsurface structure you will apply the surcharge loads to can deflect - and in that case, you may want to consider ruducing the stresses. In my case, it was a basement wall that was free to rotate, so I used 3/5 of the calculated value (the approximate ratio of active/at-rest earth coefficients).