Jun 8, 2020
Thank you to all who joined our recent webinar with Geotechnical Observations, ShapeArray for Automated Deformation Monitoring on Infrastructure Projects. Over 100 geotechnical engineering professionals registered to learn how Measurand’s ShapeArray can reduce project costs, save time, increase worker safety, and provide real-time data.
Chris Gairns, Director of Sales at Measurand and Dr. Andrew Ridley, Managing Director of Geotechnical Observations took a few minutes to answer questions from attendees. We thought you might be interested in the full question and answer section to help inform your automated monitoring strategies.
Question: How many years can we use the MEMS accelerometer inside the ShapeArray system?
Chris Gairns, Director of Sales, Measurand: That’s a good question and one that comes up quite often. The theoretical tested lifespan of the sensors in the boards themselves is 38 years, which I believe is on our specification sheet now. Again, that is a theoretical lab value. It’s difficult to know exactly how things will actually work in the real world, but I can tell you that our first installations back in 2005 are still running now and unless there’s some type of physical damage to the instrument, either there’s so much movement that—I mean, we’re talking metres of movement—that the instrument either breaks or gets deformed or it’s a dug up by an excavator—there’s no real reason to expect the sensors to fail.
Having said that, these are electronic. So occasionally something does happen. Even if a sensor, for whatever reason, stops working, the whole chain does not stop. The rest of the instrument continues to work. In software, you can just tell it to ignore that segment and you can continue monitoring for all the rest of the instrument. It’s not like an old set of Christmas lights where if one goes out the entire chain is ruined. The rest of it will continue to function.
Question: Were the ShapeArray results were compared with conventional inclinometers in the Tottenham Court Road case study? If so, what was more accurate and what was the data collection frequency?
Dr. Andrew Ridley, Managing Director at Geotechnical Observations: Typically, the collection frequency on this type of work is hourly intervals. In terms of what is more accurate on Tottenham Court Road, I don’t think we did have any direct comparisons with inclinometer measurements, but all the ShapeArrays as I’ve explained were giving exactly the same measurements.
We have comparisons from other projects where we’ve ShapeArray along with an inclinometer casing or maybe we’ve had to take a ShapeArray out for some reason and we’ve been able to check the profile using an inclinometer probe afterwards and in every single case, it’s a very good comparison.
In terms of what’s more accurate, I’d say that they’re both the same, technically, the same accuracy because they effectively use most of—these days—most of the time these days they use the same sensors. So the accuracy is all to do with the accumulation of sensor accuracy along with the array and I think the important thing to remember here is that inclinometer probes are half-metre gauge length, and so are ShapeArrays at half-metre gauge length, so as a consequence, they have the same accumulation of accuracy along the string.
So, the accuracies are almost directly comparable and as are the measurements themselves. We’ve done a lot of this type of work and almost every single time—in fact, I’d probably be prepared to stake my reputation on the fact that every single time we’ve made a measurement, we’ve got a good comparison.
CG: That is a great point, Andrew. We had a project where a user did this. It was inclinometer casing and they were making manual readings and then they wanted to try a ShapeArray in this hole. So, they installed the ShapeArray permanently for a few months, noticed some movement, wanted to verify that it was what was happening on-site. So, they pulled out the ShapeArray, made another series of manual measurements, noticed it was within half a millimetre and then reinstalled the ShapeArray and continued monitoring automated. So, it was a great application for both technologies.
AR: If I could come back on that one as well because I think one of the one of the real beauties, particularly with SAAV is how we can install it using the random helical installation method. I think for monitoring in retaining structures this can be very helpful because historically, we’ve always had to install, the piling contract has to install, what’s known as a reservation tube, and then generally speaking, inside that reservation tube has gone an inclinometer casing and that has to be grouted in. The beauty of the SAAV is it could go straight inside the reservation tube and that can save an awful lot of time. On the Northern line extension project, we did a sort of time and motion study to work out how long it took from the point at which we got the access to the top of the void former to install the inclinometer casing through to the point when we actually we’re able to take some measurements. And that could be two to three weeks for various logistical reasons and you can’t do it all in one day and then the next day you come back and it’s not available to you. There’s lots of movement of traffic around the site and you’re not given access. And that in that two to three-week period quite often the contractor starts excavating because they just get frustrated, they want to get on, and they’re not going to wait for the monitoring data. So SAAV opens up a huge opportunity here for us to do away with the inclinometer casing and actually put the ShapeArrays straight into the reservation tube and literally, you’ll be monitoring from a couple of hours after you get access to that reservation tube and you won’t have to wait the two to three weeks.
Q: Can more than one ShapeArray be connected to a logger? If yes, how far apart can the ShapeArrays be?
CG: The first answer is yes, depending on the logger you are using you can obviously you can get many more ShapeArrays. We have an interface that is a 1-to-1 or we have a 5-to-1, essentially a MUX-type instrument. If you’re using a Campbell Scientific CR800 data logger, for example, you can get up to 10 ShapeArrays on a single logger. If it is the CR1000, you can get up to 20. I don’t recall off the top of my head how many communication ports there are in the CR6, but you can have multiple ShapeArrays connected to it.
A single communication cable comes out of every ShapeArray regardless of the number of sensors. You can, in theory, have thousands of ShapeArray segments on a single data logger and the length of how far these can be spaced apart depends on how much communication cable you wish to use. There are tables on our website that layout how long the communication cable can be, which is based on the length of the instrument. Typically, the amount you can use is into the hundreds of metres. If the instrument is 50 metres, you can get something like a thousand metres of communication cable to run from the ShapeArray location back to the data logger.
AR: I’m going to stick my neck out here, Chris, and I’m going to sort of say that you know, I think you know, you guys are working on radios and I think we’re going to get to a situation where you can have a lot of ShapeArrays onto a single data logger in the relatively near future. I’m not going to say more because I don’t want to get myself into hot water, but I think that’s an exciting proposition.
CG: I will mention that the industry in general since I started at Measurand 11 years ago has changed quite a bit from running these long runs of cable back to a single logger location to many more wireless and radio installations in applications. And as you mentioned Andrew, probably in the next few months, we will be launching a nice, slick, powerful wireless radio option with ShapeArray, so stay tuned for that.
Question: Is ShapeArray sensitive to temperature change and are temperature corrections needed?
AR: I can give a brief overview that the answer to that is no. It’s not sensitive to temperature change, but all instruments are temperature sensitive and generally whatever is the structure your monitoring on, or the brackets that you’re using to connect your instrument to that structure, they will all be temperature-sensitive, but the ShapeArray has onboard processing that will take out that temperature effect. Chris can probably say more about that. I think it’s an important point for people to remember that all instruments, all structures, and anything that we monitor has temperature sensitivity in some way and it’s always a good practice, to monitor, to measure the temperature at the same time as making a measurement.
CG: That’s a good point to make that the structure is sensitive, but as you mentioned the ShapeArray will actually—I will just back up a bit. Every sensor that goes inside of a ShapeArray is temperature calibrated to that range I mentioned earlier, some -35 to +60 degrees Celsius. Every segment has a temperature sensor that measures its own temperature and if the temperature varies, it will compensate for that so that you never see any, let’s say, artificial movement based solely on temperature. It will calibrate and you always maintain the instrument’s accuracy.
Q: Are there any ShapeArrays that are still working on UK projects?
AR: What I would say to that is, we have one ShapeArray that we installed, one of the first ShapeArrays we ever installed, it must be more than 10 years. It’s on a landslide. It’s still there. It’s still working. The results from it are still very comparable with all the inclinometers that we go and measure weekly. We have another in-place inclinometer (IPI) there that we’ve been using, that we installed about 10 years before the ShapeArray was installed. We’ve got a ten-year backlog of data that shows the ShapeArray is still performing well, in an outdoor environment with ongoing movements.
We’ve got some ShapeArrays that we’ve had installed on a job for about, coming on eight years, maybe nearly 10 years. It’s another landslide job. We installed them. I think we’ve had over 500 millimetres of movement along the shear plane. They were installed inside existing inclinometer casings that were already sheared too much that we couldn’t measure with a probe inclinometer anymore. So we installed the ShapeArray inside because it’ll go around the kink much easier and then carried on monitoring for the last, nearly 10 years and we’ve got about, well, last time I looked, it was certainly an excess of half a metre of movement along the ShapeArray.
CG: That’s a good point. We had some instruments that moved three metres and continued to work for another two years after that. There’s a case study on our website about that one too. That was in the USA.
Question: Is it possible to link the ShapeArray interpretation with survey point displacements for fixed ends and to get the absolute displacement of the system?
CG: That’s a really good point. That follows on the application where you showed where all of the data is relative to one end and if you maybe don’t have the bottom of the ShapeArray into stable soil, you can flip it use the top as your reference end, and then survey to it or use GPS. Andrew, do you have any applications where you have done this in projects?
AR: I do. It’s a very good point. We should have brought that out earlier, that it’s always important to do that. Even if you’re confident the bottom is moving, it’s probably good practice to measure the top, especially if you’re expecting the top not to move and then you should survey. We do have measurements of that kind. We do also have the facility within Geodaisy® to input that survey information. So, the survey information can be placed inside the software and then you can get the absolute movement of the ShapeArray. That can be done in any of the orientations, either vertically or horizontally or in the circular.
Question: What happens if the sensors in the middle of a ShapeArray are broken? Can the ShapeArray still produce data?
CG: I quickly touched on that earlier when we were talking about the theoretical lifespan, but yes. Definitely. If it’s just some type of electronic maybe malfunction or something, it’s very rare, but it does happen. The rest of the instrument is still completely working. What you would do in that situation is to ignore the data from that one sensor. or give it an average of the sensors around it. So, it doesn’t break up the rest of the chain and then you can continue monitoring with the rest of the instrument. If there’s so much movement that the entire thing physically snaps in half—we’ve seen this in rock installations where the shear zone is very kind of abrupt—even in those cases, everything above where the shear happened will still continue to work. Also, in those cases, so you can continue to monitor the site. Obviously, you miss everything below where it has been physically cut, but everything above will continue to work from thereon.
Question: Can you extract the length of a ShapeArray after using it a borehole, if you want to use it again in a deeper borehole?
CG: That’s a great question and one that comes up a lot. We build the sensorized section of a ShapeArray to the correct length. That cannot be changed in the field without having it sent back to Measurand. We can make that sensorized portion longer or shorter, if needed, and send it back. But with the SAAV specifically, we have methods where you can use the same ShapeArray in another location. So, for example, if you had a say a 50-metre ShapeArray and you wanted to use it in a 60-metre hole, you can extract it, you can buy what we call silent segments, which are essentially SAAV segments but without any sensors inside. So they can be silent or dummy segments, if you will, and they can be added to the bottom of the ShapeArray to take up some space but if you need data all the way to the bottom of the hole, of the 60-metre hole, you can install what we call extension tubes, which again, are essentially used to take up space at the top and lower the overall price, so you would install this 50-meter ShapeArray right into the 60-metre hole all the way to the bottom and then add 10 metres of extension tubes to the top. You can use the same ShapeArray in multiple holes by adding and subtracting these silent segments or these extension tubes to move the sensor zone up or down in longer holes. Hopefully, that answered that question.
Question: Do the sensors need to be recalibrated if you want to reuse them in different installations?
CG: No. Once the sensors leave our facility, they are fully calibrated and never need to be recalibrated. The sensors will not drift over time. You don’t need to recalibrate it every time you move it around. Having said that, a lot of users like to check the calibration of their instruments. They’ll remove the ShapeArray and then in our software you can connect it to a laptop and you can run some diagnostic tools and you can actually check the functionality of all the sensors and make sure it’s still within its factory calibration. So, then you have the confidence to reuse it as another project.
Elements of the transcript have been edited for clarity and concision.
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