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Hammering it home

Some foundation contractors and consultants remain sceptical of the benefits of dynamic pile testing methods, according to a recent GE survey. Suppliers of testing equipment and services say the methods have a lot to offer.

Suppliers' views

Is dynamic pile testing used to its full potential?

Manufacturers and suppliers of dynamic pile testing equipment and testing services cite a number of reasons why they believe dynamic methods are not used more fully.

'It varies from project to project, ' says Robert Miner of US firm Robert Miner Testing.

'In some cases the client or the project have very specific and limited needs, and the full scope of dynamic testing services is not desired or appropriate.'

But there are often a number of concerns that call for dynamic testing, he says.'Concerns regarding transfer energy and driveability may fade if restrike tests show that significant time dependent soil strength gains occur, and that soil resistance is adequate. In these cases dynamic testing is seldom used to its full potential.'

More commonly, techniques are not used because of the fragmented nature of construction, with different firms responsible for different aspects. 'Not all parties remain adequately involved over time, and some may not understand or accept dynamic test results, ' he says.

Goble Rausche Likins and Associates president Frank Rausche says dynamic methods'use varies across the world.

'Only a relatively small number of professionals use these methods, ' he says. 'In offshore oil construction in Australia, Brazil, the UK and the United States, high strain testing of impact-driven piles is relatively widely used although not as much as possible.

Even in countries where dynamic pile testing methods are routinely applied, a large proportion of the projects remain untested.'

Dick Stain of UK firm Testconsult comments that while dynamic testing is fairly widely used on driven piles, in the bored piling market dynamic testing has not reached its potential.

Is there full enough understanding of the techniques among foundation engineers and their clients?

A lack of understanding of the theory behind dynamic pile testing and of what the method can and cannot deliver is a key problem in promoting the use of dynamic techniques.

'There is recognition but not understanding, ' says Samuel Paikowsky of the Geotechnical Engineering Research Laboratory at the University ofMassachusetts.

'Most engineers, including foundation engineers, do not understand the fundamentals behind the methods and interpretation and as a result there is common misuse. The single most limiting factor is ignorance.'

Mark Svinkin of VibraConsult warns:

'There is a belief that hardware and software can solve piling problems. But these are only tools and cannot replace engineering judgement.'

'A fuller understanding of dynamic testing is needed, 'adds Robert Miner.'In many cases it is not necessary for engineers and clients to understand the test method, as long as they understand what questions dynamic testing can answer, how to set up a testing programme, or who to ask for help.'

Frank Rausche says: 'Although major efforts have been made by those involved in the development and application [of dynamic methods] to educate the profession, they only reach a small percentage of the geotechnical industry.' One of the problems is that the methods are not taught in universities, he explains.

Dick Stain agrees. 'In the past, theory has been complicated by the use of long and unnecessarily complex formulae which have not helped broaden the understanding of the science. A genuine understanding of the technique is rare among the engineering community.

'A common misconception is that dynamic tests are somehow able to predict the pile performance from a single small blow. Dynamic tests can only measure the performance of a pile. If ultimate capacity is required then it is necessary to load the pile to mobilise all soil reaction forces, 'he says.

Svinkin says: 'The basic argument against dynamic testing is the small time of loading compared with static tests.This is true, but also the time of loading the pile during static tests is substantially less than the time of application of design loads to pile foundations.

'Proper implementation and interpretation of dynamic pile testing is the best way to convince foundation engineers that dynamic testing is an equal partner of static techniques in determining ultimate pile capacity.'

What factors inhibit wider use of dynamic pile testing?

Cost and complexity of use.

Inability to simplify the methods to become 'idiot-proof ', ie specialist knowledge required. Lack of codes based on actual data.

Lack of understanding and knowledge of available techniques.

'Mental inertia' among engineers reluctant to try new methods.

Bad experiences of using the methods with poor or inconclusive test results.

Dynamic tests do not measure creep or consolidation (long-term) effects.

Testing of large diameter bored piles need large drop masses that are difficult to transport and use.

What are the main growth areas for dynamic pile testing in the next decade?

Software development allowing more accurate defect detection.

Less specialised, cheaper, easily updateable equipment leading to more use.

Remote load and integrity testing equipment using the telephone and the internet. Automated data interpretation software. Testing of large diameter piles in water. Bored pile testing. Quality assurance.

Results used to modify design, improving the environmental, safety and commercial aspects.

A simple system of soil anchoring so reaction forces can be generated without the use of drop masses to make testing more economical.

End-users'views Is dynamic pile testing used to its full potential?

While most piling contractors and consultants agree that the methods are not used to their full potential, some remain unconvinced of their capabilities.

'Dynamic testing techniques for bored piles offer certain advantages but also have drawbacks compared with traditional methods, ' says Alain Marcetteau of Arup Geotechnics. 'It can never replace static testing, but in many cases it may allow many more piles to be tested for the same or less cost.'

Many pile test specifiers are unaware of what is available, he says, and there are still problems with limited test capacities compared with typical bored pile capacities.

'Dynamic testing of a driven pile can also cause significant damage to the pile head and sometimes deeper in the pile, ' Marcetteau claims. Because piles that are going to be tested need extra reinforcement, they have to be chosen before construction, restricting the testing regime.

Many respondents identify the lack of correlation of dynamic with static testing as a major obstacle to its acceptance and use.

'Dynamic testing could be used more extensively if it was linked to more refined pile design, ' says Chris Heath of UK foundation contractor May Gurney. 'Suitably calibrated, it is an economical way of providing control on pile capacity, especially on driven sites.'

Piling contractor Van Elle agrees that dynamic testing could potentially be used to give more cost-effective solutions.

'It is a more powerful design tool than driving formula, as decisions regarding pile performance are made on the basis of measured data rather than assumed values, ' it says.

It adds that dynamic testing of cast insitu piles reduces costs and delays associated with traditional static testing.'Piles can be selected for testing after all of them have been installed. More piles can be tested across the site to highlight potential differential settlement.'

However, Melvyn Eng land and Ken Fleming of Kvaerner Cementation Foundations believe there are 'significant' limitations because long-term behaviour cannot be represented over the short time a dynamic test is carried out.

They say industry is eager to adopt an accurate way of refining pile installation and design but 'after several decades of being promised a technique to determine bearing capacity by stress wave analysis techniques, a general solution remains elusive'.

Their firm introduced dynamic testing commercially in the early 1990s. Client confidence grew, but fell when it attempted to rationalise results with pile designs and maintained load tests. 'Correlations were found to be consistently unsuccessful'.

The firm concluded that mechanisms controlling pile deformation under static loading were different to those mobilised by dyanmic tests, and decided to abandon stress wave techniques.

'Although some engineers have seen or produced apparently persuasive evidence that dynamic testing of piles allows either settlement behaviour under load (other than elastic shortening) or even ultimate capacity to be deduced, it remains a challenge to devise or develop a technique that is generally applicable to any pile in any type of soil conditions.'

Is there enough information in the public domain?

End users blame insufficient published information for engineers' lack of knowledge of dynamic methods. If there was more information available engineers could gain a better understanding of the benefits and the limitations of the techniques.Most literature is written for specialists rather than the wider engineering community, they say.

'Apart from a range of more in-depth technical papers, little detailed guidance is available. A CIRIA report like Report 144 on integrity testing would do much to demystify this area for non-specialist engineers, ' says Alain Marcetteau.

Van Elle says: 'It would be useful to have a definitive guide for the end user.'

However, for some there is already too much information: 'It is enough for those who are interested!' says Manfred Stocker of German foundation contractor Bauer Spezialtiefbau.

What factors inhibit the wider use of dynamic pile testing?

Many specifying bodies have not used dynamic testing and are unaware of the capabilities. Cost.

Engineer prejudice, lack of knowledge and lack of trust in techniques. Lack of published data and analysis.

Suitable testing regimes often not discussed by the engineer, piling contractor and testing engineer.

Problems comparing static tests with dynamic tests.

What are the main growth areas for dynamic pile testing in the next decade?

Bored pile testing, but only if higher capacity testing rigs become available and clearer correlation with static performance is published.

More regular use, with design and quality control. Dissemination of results.

Projects where more than one or two pile tests are required.

Cast insitu piles, if a suitable portable means of delivering the blow can be found. Remote data acquisition systems.

A user-friendly guide to educate clients and specifiers on the capabilities of dynamic testing.

Thanks to all who participated in the survey.

Some of the longer answers can be viewed in full on the knowledge bank at www.nceplus.co.uk

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