Pile Driving 3

[VanNuysDave]

I am looking for good references related to preboring prior to installing and driving precast prestressed concrete piles. The question is 'how large should the diameter of the prebored hole be in relationship to the pile diameter (or cross sectional area)and what effect does the prebore have on the lateral capacity of the pile?

 

[PEinc]

FHWA HI 97-013, Design and Construction of Driven Pile Foundations, Workshop Manual - Volume 1, Section 9.10.2, page 9-157, states that "McClelland et at. (1969) reported that a decrease in shaft resistance over a predrilled depth can range from 50 to 85% of that calculated without predrilling, depending opon the size of the predrilled hole. Hence, the probable reduction in compression, uplift, and lateral capacity from jetting or predrilling should be evaluated whenever predrilling or jetting is being considered."

Could not find any recommendation for the pre-drill diameter.

 

[BigH]

Normally, in those cases of our company where this was done, the pre-bore is slightly smaller than the pile being driven. You are most likely putting in a displacement pile and the prebore will remove a lot of the volume that will need displaced - thereby minimizing the disturbed mass of the soils through which you are driving. I would suggest that the prebore be about 80% of the pile diameter. As for the lateral loading effects - it would depend on a number of items such as sensitivity of the clayey soil, the natural o/b Su, etc.

 

[Focht3]

Hmmm, the pre-bore diameter - and the consequences of its size - have a lot to do with the soil conditions, pile size, installation technique, etc. I'm uncomfortable with blanket prognostications...

The main idea is to use the smallest pre-drill size hole that is practical so that the lateral stresses remain as high as possible. Along the Texas Gulf coast, I limit the pre-drill to 75% of the shaft width/diameter, and it has to be at least 6 inches (150 mm) smaller than the pile shaft width/diameter.

Cheers!

 

[VAD]

Hello VanNuysDave:

PEinc, Big H and Focht3 have provided useful comments on the size of the prebore. The comments reflect that the soil and pile should be in intimate contact for the best desirable performance related to capacity etc. However, I am not sure that the lateral capacity will be adversely affected even if the bore allowed you to drop the pile in the hole as a snug fit. This comment assumes that driving would not improve the ground conditions and disturbed ground would in time return close to existing conditions before ple installation.

How does change in fabric of the soil influence shaft resistance is largely unknown or debatable. Effective stresses at pile shaft-soil interface and the zone of influence resulting from pile installation are worthwhile considerations that still pose difficulties in design and anticipated performance.

In relation to the McClelland finding on the shaft resistance,while this is plausible and good information,the effect will also depend on the characteristics of the soils and concept of design used as the numbers derived from calculations are based on what approach is used and as well the values that we conjure up for use, pile surface characteristics, and time.

We all have different ideas based on the soils we encounter and the how the foundation is expected to react under concieved loading conditions, hence the assessment of what is important to the intent of the foundation guides your design.

The findings of others along with your experience, judgement and/or gut feelings would in my opinion serve to guide your selection of values. Your problem seems to be related to driveability to achieve a desired embedment. Once you can get your pile to the desirable depth, the other concerns are, in my opinion, of less consequence and a numbers game.

 

[Focht3]

Hmmm, I can go along with a good bit of what VAD had to say, but the following statement was too much for me:

...Once you can get your pile to the desirable depth, the other concerns are, in my opinion, of less consequence and a numbers game.

...of less consequence and a numbers game? Property and lives rely on that "numbers game"!

Before we get too cavalier about McClelland's work, let us remember that their opinion (whose principal author I know quite well - and is a past Terzaghi lecturer [among many other awards]) was based on the results of hundreds of detailed geotechnical studies and full scale load tests. We're not dealing with speculation here. While you may not agree with all their analyses, the basis of their work is quite substantial. I've seen (many of) the files myself.

Okay, is the size of the predrilled hole important? You betcha! I'm still looking for the pile installer that can drill a perfectly plumb, uniform, straight hole and install a perfectly straight pile in a perfectly plumb orientation. That "animal" doesn't exist! If you care about lateral resistance, you need to have a "tight" hole all around the shaft - the soils around the pile add to resistance. Read everything that Lymon Reese, Hudson Matlock, and Mike O'Neill have written on their field observations of actual lateral load tests - the effect is quite clear. Better yet, those of you in the U.S. should attend GeoInstitute meetings and talk to them in person. You will get an appreciation for these men that no technical paper can convey. (And you may get a chance to meet Ralph Peck before he passes. He is a true gentleman and I will miss him when he's gone.)

The appropriate question, in my opinion, is not "Is a tight hole desirable?" The proper questions are, "How tight can I get the hole and still get the pile installed in a reasonable time and at a reasonable cost? Will this amount of "snugness" be sufficient for the proposed structure?" These are not easy questions to answer!

{Focht3 descends from his soap box}

 

[VAD]

Hello Focht3

Sorry to have raised your ire on my statement. Indeed, I recognize the contributions of the various eminent geo-professionals quoted and have met and had discussions with Ralph Peck etc.and do not attempt to discredit their findings and opinions. However, I also have my opinions as well based on my experience and have undertaken pile design etc on numerous sites and have been able to monitor the performance of many structures. I have also seen very different numbers coming out of designs deopending on which information one uses and the approach one takes to design.

These are of course hinged on what the risks are. Indeed, the higher the risk the more cautious one should be and research the valuable information that is in the literature. I am, however, convinced that we fail to recognize many times all the ingredients that are related to design, construction and performance of a foundation. Too often we operate in separete compartments one providing values,the other designing, and another constructing, with each party completely oblivious of what consideration the other has taken.

Of course, the more experienced one is the more one tries to link the processs before recommending values. Constructability more than often dictates what numbers or considerations we have to eventually invoke.

My objective is that while there is much information available one has as well to embrace the project or design that one is involved with from a load-soil structure interation view point and take into consideration all the factors that would come into play and concieve the consequences of such on performance.

One should not be afraid to push the leading edge inspite of what is already in the literature etc. We are all mortals and those before, ourselves and those to follow will not be infallible. I still believe that there is much more to learn in the piling design arena especially whenever so often we discover that some considerations that we have accepted as gospel truth are shown a different twist by someone.

Again, the contributions of the eminent geotechnical professionals are well recognized and they have contributed immenseley to our understanding of the subject. However, this should not deter us from having our opinions or questioning what they or others have done in relation to what we are involved with. It is the only way to learn. There is really no fun in this business if all we do is to use numbers established by someone else, the design becomes as I said a numbers game. Talk to many young graduates and they will say they can design a piled foundation etc. True enough, as the formulae are straight forward. Ask them to step back and determine what concepts have led to the numbers that they have chosen then most are lost.

I can go on and on but this is not the intent of the response to the question raised. I close in saying that the geotechnical subject is one of creativity that allows one to think of the unknowns and question even what has been found to be working satisfactory. Our quest for understanding and clarity should be the ultimate which can only come from freedom of thought while recognizing what others have found but not chained to those findings. I must also admit that we all have travelled the same path during our fledgling years.

Finally, I agree with you 100% that Dr. Peck is a true gentleman. Fortunately or unfortunately, many others who we have recognized as eminent in this field have their own axes to grind and depending on the soap box, push their theories etc and as such have contributed to our state of learning today. With the exception of the confusion that the various theories and opinions create there is nothing wrong with that, hence my own views on the subject.

Your response to my comments has been most interesting.

Cheers

 

[MRM]

Is Dr. Peck in having health problems? I haven't heard anything and that's why I ask.

 

[VanNuysDave]

Gentlemen:
I really appreciate your thoughts on my "prebore" question. May be a little more info on the specific conditions will allow you to clarify your comments or raise more ire!
The soil profile is 30 feet of seismically liquefiable hydraulic fill and native silty sands overlying dense interbedded sand and gravel (non liquefiable). The piles will be 45 feet long and derive no fricional resitance from the upper 30 feet - actually this upper layer creates significant downdrag and a very large moment due to about 15 feet of the hydraulic fill and surface crust being subject to lateral spreading. Blow counts on the 24 inch square indicator piles without preboring exceed 150 blows well short of the required emeddment depth. Preboring 75% of the pile diameter has only slight improvement and preboring with 26 inch diameter auger allows the 24 square pile to achieve the required embeddment and resistance. Maybe you now see the reason for my question - the 26 inch prebore seems a little high but works practically - thats why I'm looking for references related to this topic.
Again - many thanks for all your input.

 

[Focht3]

VAD:

We are on the same page after all. I appreciate your detailed response - particularly the portion about "pushing the envelope." I have never been accused of being a "yes" man! And while I deeply appreciate the many contributions made by the likes of Ralph Peck et al, we should challenge their work and conclusions for soundness and completeness.

MRM:

I don't know of any specific health problems troubling Dr. Peck, but he's 91 (I think.) At his age, things can happen. Quickly.

VanNuysDave:

Are you working near the Santa Ana River? Your problem sounds like one I encountered near SR 91 (Riverside Fwy.) and the OCTC Eastern Corridor intersection. Nasty site - got both HSA (CME 95) and wash rotary (Failing 2500, I think) stuck in the cobbles at depth (about 95 feet.)

How much lateral lateral capacity do you need? How have you modelled the soil? (the part that will provide lateral resistance, that is.) The answers to these questions will color our suggestions -

If your lateral load requirements are substantial, then you may be looking a predrilling a 38 inch hole, setting the pile, then grouting it in place. Expensive, but it can be done. And you will know that the pile is installed at a given depth and has not been damaged by the installation process. This has an advantage over simply casting a pier, because you can coat the upper 30 feet of the pile with bitumen or other material to reduce the downdrag force. (This won't help the lateral spreading problem.)

Is this a bridge? Is site improvement feasible? (I'm thinking specifically about deep dynamic compaction.)

Sorry if I stressed anyone with my previous posting; but a raw nerve was (accidentally) touched. Guess I've dealt with too many architects and structural engineers that have no respect or appreciation for geotechnical engineering.

Cheers!