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Laboratory testing: Game changer

Accurate mimicking of soil-structure interactions is promised by new test equipment developed in the US. 

US-based Purdue University has developed a new test chamber which, it says, allows engineers to precisely simulate soil-structure interactions during the installation of piles and other structural elements. According to the university, the test chamber will be a key research tool that could improve design and construction processes of everything from “buildings and bridges to offshore wind turbines”.

“The system can be used to study many types of geotechnical structures during both their construction and service life,” says Purdue University professor of civil engineering Rodrigo Salgado. “The nice thing about the chamber is that it can be used to study many geotechnical problems for which there are neither experimental data nor theoretical solutions.”

Researchers at the university have used cone penetration testing to demonstrate the system.

“One limitation of current methods of interpreting cone penetration is that there is “no rigorous theoretical solution of the penetration problem,” Salgado says. “The problem is complicated by the fact that soil sometimes behaves as a solid – when stresses are below certain limits – and sometimes as a fluid, when those limits are exceeded.”

The system consists of a halfcircle- shaped chamber 1.2m tall and 1.6m wide with a transparent window in the side. A series of images is taken with cameras and a digital microscope as the cone penetrometer probe is pushed into the sand. The sand contains coloured particles that allow researchers to track the movement of soil particles with a technique called digital image correlation (DIC).

The researchers also developed a mechanism that precisely controls the density of the soil by uniformly “raining” the sand into the chamber through holes in a disc-shaped “pluviator.”

Experiments using the chamber will provide data for development of models and also to validate new models. Images were shown to precisely track the displacement of soil in the cone penetration experiments.

“The DIC method allows you to model it from an experimental viewpoint because you can actually see what’s happening so you can track particle groupings in images, calculate deformations, how much flow has happened, and so on,” says Salgado.

It took about five years to design and build the chamber, which was challenging because elements of the system must remain perfectly aligned while objects are forced at high pressure into the soil sample. Another challenge was integrating the transparent window, which is made of 75mm-thick Plexiglas.

“This was all done from scratch, so we had to spend a lot of time on the details,” says Purdue University professor of civil engineering Monica Prezzi.

Research findings revealed new details about how the cone penetration tip displaces soil differently at specific depths.

“Until now, nobody has been able to measure the displacement and deformation field around the cone,” Salgado says. “So this is the first time we can actually visualise that.”

“You can make the case that if you know things with a lot more accuracy and precision and you understand them on a fundamental level you will prevent failures, and you also do things more economically.”

According to Prezzi, offshore structures often are founded on carbonate sand deposits, which undergo much more severe “particle crushing” than silica sands upon loading. Properly designing pile foundations for platforms and wind turbines is essential for safe and economical energy production in onshore and offshore environments.

“The challenges posed by carbonate sands, due to their crushability and resulting different mechanical response, are well illustrated by the case of Woodside’s North Ranking A platform in Western Australia,” she says.

“Overestimation of pile capacity from designing in carbonate sand using methods developed for silica sand was a costly lesson, with AUS$340M (£165M) spent on remedial work.

“Accurate testing data might have prevented the failure and avoided expensive repairs.”

The new chamber has attracted researchers and civil engineering students from around the world. Six undergraduate students and five doctoral students are working in experimental programmes that use the chamber.

Prezzi believes that the chamber is the first such large-scale system for geotechnical research. “It is enabling the study of problems with axial symmetry, ‘or symmetry with respect to plane’ that would not otherwise be possible,” she says. 

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