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Grouting: Magnetic attraction

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An innovative new method of grouting could save the industry both time and money

A chance conversation in 2013 between Alasdair Henderson of Bam Ritchies and Rebecca Lunn, University of Strathclyde head of the Centre for Ground Engineering and Energy Geosciences, kicked off the development of a grout that has the potential to change the industry for the better.

“I asked Ritchies what its biggest problems were,” explains Lunn, who is now the Royal Academy of Engineering and Bam Nuttall research chair in biomineral technologies for ground engineering, as well as a professor at the University of Strathclyde.

“Alasdair Henderson who was heading up Bam Ritchies at the time, said: ‘one of our biggest problems is that we just keep pumping grout into the ground and we have no idea where it has gone,” recalls Lunn.

Lunn decided this was something she could help solve and established a four-year PhD studentship through funding from the Engineering and Physical Sciences Research Council to begin research the problem and test the proof of concept.

“People have tried to solve the problem in a variety of ways,” says Lunn. “Usually by grouting the ground and trying to image it afterwards using geophysical methods. But these things depend on imaging ground properties a erwards and those properties being very different to the original ground.

“One of the difficulties in using those methods is that you need to know quite a bit about the properties before you do it, in order to interpret it. You are grouting small fractures trying to identify anything that is different in your rock and this has proved incredibly difficult because you are grouting small fractures and the rock hasn’t really changed.

“But what if whatever we inject is emitting the signal itself so that the difference is easier to identify?” asked Lunn.

Trialling a variety of materials, Lunn and the PhD student added magnetite to the cement grout at known percentage and grain size, with a known magnetic susceptibility that could be injected so the magnetic signal could be detected.

Lunn continues: “We developed a borehole sensoring system that could be used from monitoring boreholes, where can we detect the signal. Using a magnetometer we can see the magnetic anomalies that are associated with the grout and model what you would see.

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Knowing where the grout has gone is challenging with conventional methods

“If you inject a big bulb of grout, what would you expect the magnetic field some distance away from that to look like?

“The borehole is not in the grout itself, it is outside and away from it. From the magnitude of the field we detect, and its orientation, we can tell something about the shape and distance away that the grout is located. We can image the shape and distance to the grout front of the group without having to drill through it.

“You can have a set of monitoring holes that might be 6m to 10m away and you can see how far the grout has penetrated and estimate how close and in what direction they pass or approach those holes.”

Lunn believes this has been the first time that the layout for injection holes could be designed and says this could be especially useful for the installation of a grout curtain to form a hydraulic barrier.

“For example, where you might want an intact grout curtain and by seeing where you have grouted before, you need considerably less grout and reduce additional holes,” says Lunn.

To prove her concept, Lunn and the PhD student conducted field trails where they cast a number of different sized and shaped concrete discs into magnetic cement and buried them on a beach. Using a hired magnetometer they undertook tests to see if they could detect their presence.

“We found that the magnetic field was very variable, but if we did a presurvey of the monitoring holes before burying something, or in the real world inject, and then did a survey afterwards, and took the background field away and looked at the change and the magnetic field, then we can detect the magnetic cement.

“The nice thing about a magnetic signal, compared to an electrical signal, is that it doesn’t decay away. It doesn’t matter what is between you and the detection signal, it doesn’t make any difference. You do not need to know anything about the rock properties to interpret the signal.

“It should behave the same regardless of ground conditions,” adds Lunn.

Once they had established that the concept worked Lunn, in partnership with Ritchies, applied for £587,000 funding from Innovate UK in 2015, to develop the field scale technology and interpretation model to commercialise the detectable grouting system – Detectagrout was born.

“We went on to optimise the grout properties for their flow, usability and emitting a magnetic field,” says Lunn.

The system can detect individual fractures, and then tie it to the geology. 

“We have televiewer logs just of the injection holes and we know where fracture sets are in the injection hole and using the model and the monitoring holes we can see exactly where the grout has gone along which fractures and bedding plane and how far,” adds Lunn.

“Because no one knows what happens in the ground, despite the fact that we know intuitively that the grout has to travel down fractures, people still draw a picture of grouted rock as a bulb.

Lunn adds that the system does give more detailed information about individual features and how far the grout has travelled, although this is more complicated than she envisaged at the beginning of the project.

“With this, we can see is individual grout fronts that have travelled a particular distance in a particular preferential direction on individual features that can be identified in the injection holes,” she says.

A patent for Detectagrout was granted earlier this year and Lunn is keen to work with some active projects where they could demonstrate the product, in particular hydraulic containment where it would be beneficial to know exactly how it has been grouted.

“The technology is not in its final configurated state, but we are keen to build an evidence base around its use on real-life projects,” explains Henderson, who is now Bam Nuttall people and culture director. He plans to bring Detectagrout to market in the next two to five years. “We already have some projects lined up for 2019, but would like to work with more people who are interested in having it deployed on their projects.”

Lunn adds: “It is more expensive than commercial grouting, but if you are not wasting grout then there would be substantial savings.”

Henderson agrees. “Once you develop this technology it has application across all infrastructure. Being able to image what you have done, has a considerable advantage and is a significant productivity improver,” he says.

“It also gives you a confidence boost around what has been done. Where in the past grouting might have been declined as a solution because there was lack of certainty as to what was going to be achieved.

“With far less waste, you have the ability to install an insitu barrier that doesn’t require any significant removal of existing material. All of sudden grouting is an attractive alternative to diaphragm or pile seeking walls.”

 

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