Readers of this blog will know that my family goes back a long way visiting the Bahamas in general and the Abaco Islands in particular. We had some exciting times, almost sending our ship to the bottom and riding out a storm. This beautiful paradise, which looked like this when we visited: Now looks like […]

via Appeal for the Abaco Islands, and Mercy Chefs — Chet Aero Marine

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Appeal for the Abaco Islands, and Mercy Chefs — Chet Aero Marine

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Argument from authority

The Logical Place

by Tim Harding

The Argument from Authority is often misunderstood to be a fallacy in all cases, when this is not necessarily so. The argument becomes a fallacy only when used deductively, or where there is insufficient inductive strength to support the conclusion of the argument.

The most general form of the deductive fallacy is:

Premise 1: Source A says that statement p is true.
Premise 2: Source A is authoritative.
Conclusion: Therefore, statement p is true.

Even when the source is authoritative, this argument is still deductively invalid because the premises can be true, and the conclusion false (i.e. an authoritative claim can turn out to be false).[1] This fallacy is known as ‘Appeal to Authority’.

The fallacy is compounded when the source is not an authority on the relevant subject matter. This is known as Argument from false or misleading authority.

Although reliable…

View original post 475 more words

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An Important Announcement About vulcanhammer.info

After a summer of eclipses and hurricanes, we’re pleased to announce that vulcanhammer.info has finally moved to its new platform as of yesterday. Click here and check out what we have to offer. Most of the content has gone with the site; we’ve added many photographs and used the transition to correct many of the […]

via We’ve Moved At Last, But Took the Online Pile Routines Somewhere Else — vulcanhammer.info

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Some Things I Would Say if Giving the E.A.L. Smith Award Lecture for the Pile Driving Contractors Association

I found it intriguing that the Pile Driving Contractors Association has instituted the E.A.L. Smith Award with lecture following.  It looks like I’ve already made a contribution to the effort: the graphic they used for the LinkedIn announcement probably comes from my piece on E.A.L. Smith and his contribution.

I don’t anticipate actually doing this, but that’s the result of some choices I’ve made along the way.

To begin with, I allowed my technical membership in PDCA to lapse many years ago, although the organisation I work for certainly is a member.  As the musicians say, you can’t sing the blues if you don’t pay your dues, and that’s as true in deep foundations as it is in jazz music.

Beyond that, most people who have been in the driven pile industry for a while know that Vulcan Iron Works passed from the Warrington family in 1996, and that things really didn’t get better for some time thereafter.  My years in the equipment business convinced me that equipment people could not remain uninvolved in the whole business of pile dynamics, which led me to start this site twenty years ago.  Unfortunately that involvement was not accompanied by a sponsoring organisation or budget, so I had to use the emerging internet to do what I felt needed to be done: furnish information on pile dynamics and driven piles, and ultimately geotechnical and marine engineering in general, without paywall or restriction.  That effort has been successful; the award for it is in geotechnical engineers who can get their work done in a better way, students who can learn about this part of the profession, and those in countries which lack the resources to purchase materials.  If I’ve helped them, I’ve succeeded, and that’s award enough.

In any case part of that effort was my piece on E.A.L. Smith and his development of the wave equation program, and I think my piece has been just about the only one on the subject for a long time.  In looking back at the piece and the whole effort behind it, I’d like to make some observations that hopefully with shed some light on Smith’s effort.

The first is that Smith was Raymond’s Chief Mechanical Engineer.  Raymond was an organisation that defined vertical integration: they not only drove the piles, but made or modified the equipment that did the work.  Smith worked during an era when geotechnical engineering was coming into maturity as a science; Terzaghi and Peck’s Soil Mechanics in Engineering Practice was published in 1947.  Nevertheless it took a mechanical engineer to crack the forward problem in pile dynamics.  That’s because civil engineers in general and geotechnical engineers in particular don’t really like or understand things that move, but moving things (like pile driving equipment) are the centrepiece of a mechanical engineer’s work.  That simple fact invited the interdisciplinary approach to the problem that Smith took, but it also has made pile dynamics a “black box” to many of the civil engineers who work with the problem, and that in turn has guided the way the solution of the problem has been implemented.

The second is that the physics of Smith’s wave equation program is a classically mechanical engineer’s solution to a problem.  Spring-dashpot-mass systems are the core building blocks of any vibrating system; Smith basically took this, made the springs elasto-plastic, and strung them together into the system he developed.  It is a tribute to Smith’s ability to see the big picture of the system to relate the parameters of the system to the soil he was driving into, and not permit himself to get lost in the soil mechanics of his geotechnical peers.

The third is that Smith developed his numerical method at a time when numerical simulation of physical systems was itself in its infancy.  He had the feedback of the likes of W.E. Milne and the collaboration of crosstown IBM’s computers and expertise in the development.  It’s also worth noting that civil engineering, although it has pushed forward finite element modeling with people such as T.J.R. Hughes and D. Vaughn Griffiths, is still content with using “classical” methods for many of its designs, especially in the transportation field.  The singularity of Smith’s achievement needs to be seen in the context of both the situation at the time and afterwards.

The fourth is that Smith’s wave equation program was the result of an extended effort that lasted at least a decade and probably more.  That was facilitated by Raymond itself, a large organisation with considerable resources and the ability to test the model with its own work.  It was done at a time when U.S. corporations were more inclined to engage in long-term research projects and to share those results.  The government’s involvement only entered in the wake of Smith’s seminal ASCE paper, and that too was an extended effort.

So Smith’s effort is certainly worthy of celebration and commemoration, and to learn some lessons from it.  Smith’s basic model of the pile has endured to this day, finding application in both forward and inverse solutions of the wave equation for piles.  But is Smith’s model the last word on the subject?  Probably the best answer came from Smith himself.  In his ASCE paper he noted the following, in his discussion of soil mechanics:

When future investigators develop new facts, the mathematical method explained herein can be modified readily to take account of them…

It’s unreasonable to expect that Smith’s model cannot be improved on beyond tweaking the parameters.  And there are fundamental problems: Smith, wise to initially bypass much conventional soil mechanics, developed a model where the relationship between the parameters he used and the properties of the soil he was driving into is not clear.  Solving that problem might, for example, reduce the importance of the sensitivity issue of soil damping on the results of the wave equation.  Efforts have been made to solve the model-soil properties issues but they are neither as widely perfected or implemented as one would like.

That’s a special problem when he consider the inverse implementation of the wave equation for piling.  Use of Smith’s model brings with it uniqueness issues (and there are enough of those with problems involving plasticity like this one) that need to be addressed.

Numerical methods and computer power have both vastly improved since Smith’s day.  So is it possible to see another paradigm shift in the way we perform forward and inverse pile dynamics?  The answer is “yes,” but there are two main obstacles to seeing that dream become a reality.

The first is the nature of our research system.  As noted above, Smith’s achievement was done in a large organisation with considerable resources and the means to make them a reality.  It was also a long-term effort.  Today the piecemeal nature of our research grant system and the organisational disconnect among between universities, contractors and owners incentivises tweaking existing technology and techniques rather than taking bolder, riskier steps with the possible consequence of a dead-end result and a disappointed grant source.

The second is the nature of our standard, code and legal system.  Getting the wave equation accepted in the transportation building community, for example, was an extended process that took longer than developing the program in the first place.  Geotechnical engineering is a traditionally conservative branch of the profession.  Its conservatism is buttressed by our code and standard system (which is also slow-moving) and the punishment meted out by our legal system when things go wrong, even when the mistake was well-intentioned.  Getting a replacement will doubtless be a similar extended process.  And of course we should consider having been “written into the specs.”  Vulcan was certainly the beneficiary of that phenomenon, although the process was driven more by the ubiquity of the product than an effort by the company.    That last point is certainly not the case here; general acceptance would have never taken place had it been so.

However, we need to face the reality that, sooner or later, the ball will move down the field and newer techniques will be developed.  The question in front of us is whether it will be done on these shores, as was the case with Smith, or somewhere else.  As I like to say, it’s our move: we need to make it.

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STADYN Wave Equation Program 4: Eta Limiting, and More on Norm Matching

In our last post we broached the subject of different norm matching methods for the actual and computed velocity-time histories at the pile top. In this post we will go into \eta limiting, while at the same time running both norms to get a better feel for the differences in the results.

Before we begin, one clarification is in order: CAPWAP’s Match Quality and the use of the 1-norm in STADYN are similar in mathematical concept but different in execution. That’s because the Match Quality weights different part of the force-time history (in their case) differently, whereas STADYN goes for a simple minimum sum difference.

One characteristic of the inverse case both in the original study and in the modifications shown in the last post are very large absolute values of \eta . These are products of the search routine, but they are not very realistic in terms of characterising the soil around the pile. To illustrate, we bring back up one of the results from the last post, showing the optimisation track using the 2-norm and phi-based Poisson’s Ratio (which will now be the program standard):

stadyn3-2-2

Note that the #8 track (\eta for the lowest shaft layer) has a value approaching -30; this is obviously very unrealistic.

In principle, as with \xi , the absolute value of \eta should not exceed unity; however, unlike \xi there is no formal reason why this should be the case. But how much should we vary \eta ? To answer this question, and to continue our investigation of the norm issue, we will examine a matrix of cases as follows:

  1. \eta will be run for values of 1, 2, 3 and unlimited (the last has already been done.)
  2. Each of these will be run for both the 1-norm and 2-norm matching.

A summary of the results are shown below

Changed Parameter

Difference

Static Load, kN

Average Shaft \xi

Toe \xi

Toe \eta

 Norm

1

2

1

2

1

2

1

2

1

2

|\eta | < 1

0.3364

0.003690

811

1490

-0.364

-0.149

-0.62

-0.311

-0.175

0.611

|\eta | < 2

0.2381

0.002626

278

223

-0.091

-0.06

-0.588

-0.316

-0.781

-0.0385

|\eta | < 3

0.1806

0.001707

172

207

0.324

0.42

-.832

0.823

-1.01

1.45

Unrestricted \eta

0.1344

0.001456

300

218

-0.329

-0.183

-0.491

0.804

8.19

1.52

\nu = f(\xi,\eta)

0.1484

0.001495

278

187

-0.383

-0.53

0.792

0.366

3.116

1.814

To see how this actually looks, consider the runs where |\eta | < 3.  We will use the 2-norm results.

Velocity-Time Output
Impedance*Velocity Comparison, 2-norm, eta limiting = 3.
Optimization Track
Optimisation Track, 2-norm, eta limiting = 3

The results indicate the following:

  1. The average shaft values of \xi tend to be negative.  This is contrary to the cohesive nature of the soils.  The interface issue needs to be revisited.
  2. The toe values do not exhibit a consistent pattern.  This is probably due to the fact that they are compensating for changes in values along the shaft.
  3. As values of |\eta | are allowed to increase, with the 2-norm the result of the simulated static load test become fairly consistent.  This is not the case with the 1-norm.  Although limiting |\eta | to unity is too restrictive, it is possible to achieve consistent results without removing all limits on \eta .
  4. The velocity (actually impedance*velocity) history matching is similar to what we have seen before with the unlimited eta case.
  5. The optimisation track starts by exploring the limits of \eta , but then “pulls back” to values away from the limits.  This indicates that, while limiting values “within the box,” i.e., the absolute values of \eta < 1, is too restrictive, reasonable results can be obtained with some \eta limiting.

Based on these results, \eta limiting will be incorporated into the program.  The next topic to be considered are changes in the soil properties along the surface of the pile, as was discussed in the last post.