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One expert's test is worth a thousand unprofessional attempts

October 20, 2014

  Wernher von Braun, who with his team launched the first American satellite into space, said “One good test is worth a thousand expert opinions”.

The question is, can our down-to-earth (pun intended!) profession and industry compare themselves to and learn from the trials of the space program? Can we make a twist on Von Braun’s assertion to say one expert’s test is worth a thousand unprofessional attempts? We think yes, and in more than one way.

The Saturn V rocket of the Apollo program was about 10 m (33 ft) in diameter, weighed 3100 tons and was 111 m (360 ft) tall; GRL tested steel piles of more than 1000 tons weight and about 490 m (1600 ft) length, comparable to the height of the Empire State Building. Some drilled shafts are more than 3.5 m (12 ft) in diameter, 60 m (200 ft) in length, weighing maybe 1650 tons. So while dimensions are of the same order of magnitude, there is one major difference: each Saturn V was only used once, while our foundations have to last for decades or more.

Like the early days of space flight in the late 1950s, deep foundations construction involves many unknowns and, therefore, testing is a must. The soil conditions at each new construction site present new challenges and actually are not as predictable as the space environment near Earth! Near failure pile driving stresses, drilling in a soft collapsing soil or into hard rock make each job like designing a new rocket.Are we comparable to rocket scientists? Yes! And like those who made going to the moon possible, we need good testing - good is the key word in von Braun’s quote.

Advanced building codes have recognized the need for increased quality testing, and reward it with increased allowable design loads and therefore cost effective deep foundations. Unfortunately, there are plenty of examples of bad tests conducted with unreliable equipment, unproven methods and inexperienced personnel.Worse yet, there are anecdotal rumors of testers reporting what the client demands regardless of what good measurements and analyses reveal. Complex conditions sometimes result in a bewildering variety of test outcomes, which lead some in the industry to choose an “Ignorance is Bliss” approach rather than sort through and reconcile facts and results.

Every testing method has its specific limitations, some obvious, some not. Even a good test may therefore need an expert to interpret it properly. Here are a few examples where a good test and one that does not waste money needs input from a qualified and experienced engineer:

• A static load test has to be well designed, which means that the reaction system should be sufficient for either a proof load or a failure load, and it should not interfere with the soil condition around the pile or its results will be incomplete or wrong.

• For a dynamic load test of either driven or drilled piles, sufficient hammer energy has to be provided for an activation of the required resistance. It may be tempting to save money by employing an available low energy hammer, but the end result will disappoint. However, when a hammer is able to initially drive a pile with reasonable blow counts and then reaches refusal during a restrike, end of drive and restrike tests can be combined by an experienced analyst to expertly calculate the total long term pile capacity.

• Because soil setup adds capacity with time after pile installation, either static or dynamic testing too early after initial pile driving generally indicates low capacities (in relaxation cases high capacities). Given a good knowledge of the soil conditions, the experienced engineer can propose a testing program which would determine long term pile capacity without unnecessarily long waiting times.

• Being able to identify difficult conditions causing unusual pile behavior whether under static or dynamic loads helps avoid wrong conclusions. In situations involving, for example, fine silts or fat clays, and plugged or partially plugged open profile piles, care and caution are required for proper data analysis and interpretation.

• Erratic or inconsistent dynamic test data may cause concern regarding pile integrity; the expert tester would be able to determine both the validity of the data as well as whether or not the potentially damaged pile is capable of supporting the intended capacity.

• Judging hammers by the energy transferred to the pile may or may not be reasonable depending on pile and/or cushion behavior; experience and a rigorous wave equation analysis can help explain the condition.

• Cross Hole Sonic Logging is a reliable testing method, however, it is of limited value if the test engineer fails to recognize inspection tubes de-bonding or bleed water-caused dispersion of the ultrasonic signals in spite of sufficient concrete strength.

• Sonic pulse echo tests have been used with good success for many years in many countries, but because of its limitations it requires an experienced test engineer to look at all available design and construction information and explain limitations and potential mitigations.

So how do we become testing experts, capable of prudent judgment? Obviously, expertise can be gained through education, training, study, and other efforts. PDI and GRL both offer webinars, seminars, workshops, brown bag lunch talks and other learning opportunities; the PDA Proficiency Test (www.PDAproficiencytest.com) helps PDA testers assess their knowledge and obtain a certificate of recognition.

Let’s face it: One good test may be worth a thousand expert opinions, but one bad test can generate a thousand ignorant opinions and make a bad situation worse. As deep foundations testers, we must provide good tests and expert opinions. Now that is a worthy goal for the New Year.

Translated from the English original, published on the PDI-GRL Newsletter №74, January, 2014,
with permission from Pile Dynamics, Inc.

One_expert_s_test_is_worth_a_thousand.pdf

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