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Design-build and remote PDA testing

By John C. Ryan, Ph.D., P.Eng., Ryan Structural Engineers, LLC

Sharpening the proverbial pencil
It’s a familiar saying, and in an engineering office it may sound like this: “Four per cent overstress ... maybe I can sharpen the pencil and make it work.”

Despite the fact that a given balance between load and resistance cannot be “made to work,” there is some insight that this euphemism provides. In reality, deeper understanding of a problem is being sought with the intent of removing uncertainty. As engineers, we are inherently and appropriately risk averse. If we have not personally proven or maintained control of an idea from inception to completion, we become skeptical of it. In practice, this tends result in unnecessary conservatism, particularly where often-disconnected design professionals have tangential or overlapping responsibility. Such is the case with driven piles.

Much of the excessive conservatism that persists in the driven pile industry can be reduced significantly through a design method that treats the pile foundation as a performance specified component. Utilizing bid solicitations, which include pile design criteria and subsurface reports, foundation contractors, along with a driven-pile specialty engineer, can provide design-build solutions with pricing and schedule to be evaluated for best value. Further, by incorporating remote dynamic pile testing within the scope of the design-build team, the most optimized foundation and installation schedule can be achieved. If the pile specialty engineer and contractor team are engaged from concept through design and certification, the pencil is always sharp with respect to foundation design.

Traditional design method
Consider the traditional method of foundation design by an engineer of record (EOR). The design process varies somewhat, but typically begins with engaging a geotechnical engineer for reporting on subsurface conditions. Foundation recommendations are typically included in a report that also advises on site preparation, paving, slab system requirements and seismic risk assessment. Further, the foundation recommendations are often limited to relatively few options and based on minimal information regarding the structure. This is somewhat reasonable since the geotechnical investigation often proceeds the completion of any substantive structural design, which could advise a more targeted approach. As a preliminary assessment to be used in future collaboration between engineering disciplines, this information may be adequate. However, all too often the scope of geotechnical work is complete (test pile and field monitoring not withstanding) upon issuing the initial subsurface report, leaving what would be a good initial step in developing an efficient foundation solution as final options for selection by the EOR. From the initial subsurface report, a single pile type is proportioned to provide a safe design, with the knowledge that field-testing will substantiate the geotechnical analysis. The result is often a pile foundation proposed by the geotechnical engineer, without substantive input by the structural engineer, and then specified by a structural engineer without additional geotechnical input. The expectation of this process resulting in the most economical solution is unrealistic.

The traditional model lacks attention of specialized knowledge at critical points in the process, which creates uncertainty and often leads to unnecessary conservatism from the geotechnical and structural engineers. To illustrate why, consider how the geotechnical report is used. Typically, design recommendations are taken as safe design values without consideration of soil borings or laboratory tests, which are far more useful in establishing an optimized pile design. These recommendations are the geotechnical engineer’s method of simplifying the complexity of soil-pile interaction for the sake of providing an EOR with an easily understandable design model. Adequate design is normally the result due to conservative principles used to develop a very simple soil strength model. However, the most efficient solution is rarely attained through strict adherence to these design recommendations. Optimizing the pile type and length requires depth of understanding of the geotechnical report by the EOR. In the exceptional circumstance where the EOR has a sophisticated understanding of the reported geotechnical data, uncertainty can persist with respect to installation methods, control of field-testing schedule and the ability to make adjustments to design during production pile driving. Consequently there is a high probability that the traditional method will either result in larger, longer piles than are necessary, or an expanding construction schedule necessary to coordinate the interaction of three to four separate entities.

Design-build
It seems logical that removing uncertainty inherent in the traditional design model results in substantial improvement to the efficiency of pile foundations. But how can this be done in a systematic way, such that the results are repeatable? Several years ago, Ryan Structural Engineers, LLC (RSE) and Force Pile Driving, Inc. (FPD) believed that the answer was the design-build model. At that time, RSE had become an established driven-pile specialty design firm, focused on combining efforts related to structural and geotechnical aspects of pile design. FPD was an aggressive and rapidly growing pile-driving company interested in providing Western Canada with value-added services. Recognizing a special opportunity in one of the world’s most prolific construction markets, a natural partnership formed.

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.