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Determining the depth of embedment of driven piles

VAD (Geotechnical) (OP)
10 Apr 03 18:06
Determining the depth of embedment of a pile (toe elevation)is one of the prerequisites for calculating its load capacity. How do you determine this depth for a driven steel pile?.

1. Do you undertake test pile driving at the site before design?.

2. Do you assess the depth from the subsurface characteristics and past experience? What field testing provides this confidence SPT, Cone Penetrometer etc? Does lab testing play a role. If so, what information is relied on.

3. Do you invoke the pile hammer characteristics in estimating this design depth?. How is this utilized in your design.

4. How do you concieve the type of pile to be recommended i.e H-section, open end pipe, closed end pipe etc?.

5. How far off have you been in your estimate of depth of embedment?.

6. How do you ensure that your design depth will be achieved in the field?.

7. If the design depth is not achieved what do you do?.

This question is general in nature in relation to design and constructability of driven piles.
Ron (Structural)
18 Apr 03 7:37
VAD...The selection of the tip elevation is done first through an analytical process based on the soil conditions, the needed capacity of the pile, the type of pile selected and the capability of the encountered soils to resist the load through skin friction, end bearing, or both.

The selection of the pile type is done through an iterative process of selecting the one that best fits the situation, considering total length of pile, splicing capability, compatibility with expected load resistance mode (friction or end bearing), then tempered with local availability and practice.  For instance, suppose you have a condition where most of the load will be resisted by skin friction and the pile length is anticipated to be 40 feet.  For this application, an augered concrete pile would likely be a better choice than a steel pipe pile.  You will have to resolve the selection by a prioritization of capacity, conditions, availability, cost, and practicality.

There are several ways to estimate pile capacity through charts, equations, or by computer solution.

In one respect the pile hammer is both a means to install the pile, but also a means to "test" the capacity of the pile.  You would expect to impart more energy into installing and seating the pile than the structure would ever impart to the pile.  Once a driving criterion has been established for your particular pile at your particular site, then you would expect to receive the desired capacity if the criteria are met.  This can be tested either during pile installation with dynamic testing (PDA) or through a static load test of selected piles after installation.

Can you be off in your estimate of pile lengths?  Of course.  That depends on the variability of the soils encountered and whether or not you reach a bearing layer with the piles.  Soil strata are not necessarily nice horizontal lines with uniform and distinct divisions.  The subsurface stratigraphy is at least as variable as the surface topography.

Unless you are driving to a specific depth criteria (for instance to reach a bearing layer or to accommodate scour), you are more concerned with achieving the desired capacity than the depth.  If for some reason the piles are not carried to sufficient depth and you do not have the desired capacity, you have to re-analyze the existing capacity to see if you can live with the result or if you will need to drive the same pile further after splicing or if you need to drive additional piles adjacent to those already driven.
Focht3 (Geotechnical)
18 Apr 03 11:49
I agree with Ron - and have a few other comments and suggestions.

Use a spreadsheet to do your analyses.  You can analyse a whole range of depths at a pass, then change the pile diameter in one cell - voila! - a new set of results.  [This was the first problem I solved with a spreadsheet - on VisiCalc, later on Lotus 1-2-3.]

Responding to your questions -

1. Do you undertake test pile driving at the site before design?.

Not usually.  Mobilization costs are too high to make this practical most of the time.

2. Do you assess the depth from the subsurface characteristics and past experience?

Yes.

What field testing provides this confidence SPT, Cone Penetrometer etc? Does lab testing play a role. If so, what information is relied on.

This depends a lot on the ground; CPT doesn't work too well in hard clays, cemented sands, or gravels.  SPT doesn't give the continuous profile available from a CPT rig with electronic logging.  Lab testing does have a role - especially in clays and rock.

3. Do you invoke the pile hammer characteristics in estimating this design depth?. How is this utilized in your design.

Sizing the pile hammer has more to do with construction than design.

4. How do you concieve the type of pile to be recommended i.e H-section, open end pipe, closed end pipe etc?.

Simple answer?  Experience.  

   H-piles: a hard layer is "near" the surface.
   open-end pipe pile: capacity is largely by friction, or penetration is important because the pile will be
      subjected to lateral loading.
   closed-end pipe pile: capacity is largely by friction, but you need bearing across the full pile end and
      can't count on a plug to form.

5. How far off have you been in your estimate of depth of embedment?.

Hard to tell - no failures to date.

6. How do you ensure that your design depth will be achieved in the field?.

That's the contractor's duty, although I must comment on his "means and methods."  And note when his "means and methods" will destroy the intent of the pile.  Example: jetting for a friction pile is a very bad idea.

7. If the design depth is not achieved what do you do?.

Work with the owner and contractor to figure out what's wrong.  This usually entails a PDA.
BigH (Geotechnical)
18 Apr 03 16:59
I'll pop in with your kind permission! - if I remember right, Franki (or the American Piling Institute) used to have a little booklet that they would publish a few times a year.  Back in the 70s I remember seeing an article on estimating pile penetration based on soil boring results (admitting that the article may have been from the late 60s).  Can't help you out with any more details as I have no access to my library.  Secondly, there may be an underlying, but unsaid, practical basis for some of the points in the orignal question - that is, there might be a situation where if the pile goes deeper than is estimated from the stratigraphy, there might might be a need to weld on an extension for very little additional penetration (an expense).  I saw this once the Vancouver area in that the soils to about 65 ft were "low end (N = 20)" compact and then became dense (N>45).  The standard lengths of the piles obtainable were 60 ft - so the designer "designed" for 60 ft rather than taking the pile down into the dense layer beyond 65 ft.  In the end, when the length of the standard pile was reached, it didn't have a low enough "set" - basis of acceptance to the (cough) pile driving formula.  It was necessary to weld on an extra pile length; the pile ended up stopping at about 70ft. (I had to set the criteria as per the pile driving formular or geotechniques methods whichever came first.) I won't go into how the designer used H-piles rather than closed-end steel tubes in the sands . . .  (see Chellis for a practical solution when H-piles don't "set up" in sands.)
VAD (Geotechnical) (OP)
18 Apr 03 19:22
Thank you for the responses provided so far. Just in passing although the PDA is touted as the preferred test for capacity determination, the results do depend on the energy imparted by the hammer. In some cases, if the hammer is not properly sized/rated the results fall short of what is determined from static analysis. I have found also that the not so preferred driving formulae do as well provide reasonable results. However, experiences of others may be different.

I rely heavily on a good understanding of the ground conditions, experience and imagination. The latter comes first before calculations, which very often can be done once or twice.

The comments provided have been interesting. This type of information would be pertinent as well at the academic level to jump start new graduates until they acquire their own experiences or develop their yardsticks for developing good pile designs.A piling contractor once mentioned to me that an awful lot of pig iron is thrown into the ground at bridge sites as he has observed over the years. I guess we can counter argue this statement but at times I have had to agree as well.  

Cheers    
        

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