pile group effects reduction factors in bedrock

[dan00]

Currently the CBC requires reduction factors to account for group effects
- on lateral behavior when pile spacing is less than 8D (D-pile diameter)
- on axial behavior when pile spacing is less than 3D (1810.2.5).
Local and federal agencies came up with different reduction factors. Have you seen any group effects reduction factors tailored for different foundation earth materials (i.e. alluvium, bedrock)? A colleague suggested using reduction factor of 1 for lateral behavior when pile spacing is more than 4D, but he couldn't provide any document to support it. Thanks.

 

[SlideRuleEra]

Go to this page of my website and download Bethlehem Steel H Piles.

For point bearing pile recommendations, see page 33,34 & 35.

For friction pile recommendations, see pages 35 & 36.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dan00]

Thank you SlideRuleEra for your response. The link you provided was useful for group effects on axial capacity.
How about the lateral behavior? CBC requires to consider group effects on lateral behavior when spacing is less than 8 pile diameters. Shall these reduction factors be any different for piles in bedrock? Or shall we use at all reduction factors for piles in bedrock?

 

[Okiryu]

dan00, even you have bedrock (which is the bearing layer of your piles), I expect that you have some soils above it which are not appropriate to carry your loads (that's why the need of piles). So, I think that you may have soils within the zone of influence for lateral loaded piles (about 8D to 9D). In this case the typical reduction factors for soils will apply. The Reese and Van Impe book on lateral loaded piles has good details about lateral capacity reduction for pile groups.

If this is a marine structure (for example a pile supported pier), where bedrock is very shallow, I would use some reductions for the lateral subgrade reaction of the rock. Here in Japan, we have reduction factors that account for this. The Japanese methodology is also explained in the Reese and Van Impe book.

 

[dan00]

Okiryu, you're right, there is some soil present, not just bedrock. Are you suggesting to use the reduction factors only for the portion of the pile embedded in soil and no reduction for the portion of pile embedded in bedrock? L-pile actually allows to change the p-modifier with depth. Thank you for your input.

 

[Okiryu]

Yes. I think that soils affected by lateral loading on piles are the soils that are within 8D or 9D (D, diameter of pile). See Duncan and Evans for reference. I do not remember the exact paper title, but I think it is something like this: Characteristic Method for analyzing lateral loading on piles.

I think that if your bearing layer (bedrock) is deep, it may not be affected by lateral loading and therefore reductions may not be required at this level.

I would also like to hear the opinion of other geotechs...

 

[dan00]

Okiryu, when you say the soils affected by lateral loading on piles are the soils that are within 8D or 9D, you refer to horizontal distance, right? How about this scenario: Pile foundation needed for foundation setback from slope face and because of the presence of 5'-10' of uncertified fill at the top of the slope. The bedrock is relatively shallow. Shall reduction factors be used for bedrock? If yes, are they different from the factors published by AASHTO, NAVFAC, etc. Thank you.

 

[Okiryu]

dan00, I was referring to the vertical distance. For example, you may need to reduce the lateral coefficient of soil reaction for the soils located within a depth of 8D or so. One question: if the bedrock is relatively shallow, are there any reasons why not go for a shallow foundation system? If the bedrock is relatively shallow but still deep for a shallow foundation system, perhaps you can also use soil replacement or soil improvements of the soils above the bedrock and then place your footings there.

 

[dan00]

Okiryu, upper soil improvement is an option, but there are situations where using deep foundation is the best option (e.g. uncertified fill/soft soil present in the upper 10 feet, new construction adjacent to existing structures which are on shallow foundations, underground utilities that cannot be surcharged by shallow foundation, etc). Regardless the scenario, if the bedrock is within upper 8D-9D, my question is: shall group effects reduction factors be used for bedrock and what are these factors? Thank you.

 

[Okiryu]

dan00, the Japan Road Association recommends a reduction facto "m" in the coefficient of horizontal subgrade reaction:

m = 1 -0.2 * (2.5 - s/b), s < 2.5 b

where s = center-to-center distance between piles and b=pile diameter.

Reese and Van Impe book indicates that the above equation is less conservative than other methods but I am located in Japan so I have used it for my projects.

If you find other reference, we will be more than happy if you can share them with us..