Based on my personal experience, I would say that geotechnical instrumentation is a special branch of geotechnical engineering. Some college departments offer instrumentation classes, often time as electives, in their geotechnical curriculum; most do not. If not offered, you can learn about the basis of instrumentation by enrolling in classes such as experimental stress analysis, signal processing, and other E/E classes. If you have taken these classes, you are off to a good start! Otherwise, it literally relies on the opportunity or if your organization offers such services or training. Regardless of where you pick it up, you will likely spend some time outside of your work honing your programming skills, or brushing up college physics or electrical knowledge on top of doing your routine bearing capacity or other geotechnical calculations. Having an experimental background definitely helps.
There are mainly two basic components in a geotechnical instrumentation system. First, there is the sensor or measurement component; second, the dataloggers and data acquisition systems. The sensor or measurement component relates to responses such as deformation and pressure that we want to measure. The second datalogging component refers to electronic systems that store these response data and possibly perform pre-processing of measured data (e.g. Frequency to Digits) in the background. Of course, if there is always someone there to record the data. We do not need the second component. So to speak, more accurately, this is for automated geotechnical instrumentation system. There are other components like power system to excite sensors, or if remotely automated, there are additional web hosting, cloud storage, and network systems etc. I consider all these belong to the datalogging system.
In this blog, I intend to share my experience and knowledge about geotechnical instrumentation. I disclaim that I own any of the mentioned products and I receive no incentives or compensation in return.
Sensors and Transducers
Straight out in practice, most instrumentation in geotechnical applications does not require highly precise and high-resolution measurements. This is mainly attributed to the fact that soil and geosystems response can be largely captured within seconds (s) or longer periods, unlike mechanical and electrical systems that take higher frequency measurements in milliseconds (ms) or shorter intervals. For example, a failing slope with visible scarps may still take days or months to fail. In another example, in a fully-grouted borehole, pore pressure takes about 3 days to equilibrate to its surrounding hydrostatic pressure. However, in pavement applications, the vehicular wheel load-unload period is about 1 second. This highlights the importance of "time of failure" and "rate of loading," in which most geomaterials have some time to respond rather than exhibiting an instantaneously rapid response. After all, instrumentation is meant to provide quantifiable data for use in structure or geosystem performances. Therefore, consideration must be given to the selection of the sensor type and specific applications.
In the industry, vibrating wire (VW) sensor is dubbed as "standard practice" due to its stability, reliability, and economy, while providing good enough measurements to engineers. MEMS-based sensors have become increasingly popular in the last decade or so for higher frequency and multi-directional measurements. Here, we will mostly focus on discussing VW sensors and their applications. If you are interested in other types of sensors such as electrical resistance sensors, please let me know.
Dataloggers and Data Acquisition Systems
If you have done any instrumentation in outdoor settings, chances are, you have come across Campbell Scientific, Inc. (CSI) and its measurement systems at one point in your career. In particular, you may have seen a white enclosure with a wind monitor and an antenna sticking out (like the one shown in Figure 1) in a remote area or next to the highway. This could be the CSI's weather station. Inside the enclosure, you will find at least one component to be the datalogger; like one of those shown in Figure 2. The agriculture sector, climate/weather agencies, energy sector, earth science, and many other sectors use CSI products. Not surprisingly, in civil engineering, where structural health and geotechnical monitoring are needed, CSI dataloggers are also widely used. If you are curious, check out the CSI website (www.campbellsci.com).
The one thing about using CSI dataloggers is the use of CRBasic programming language. For simple programs, CSI offers Short Cut program to generate CRBasic program for users. However, if flexibility is needed, another software, CRBasic Editor, provides that advanced coding environment to users. The language is not a high-level language, but it offers basic operators and instructions such like conditional "if...then...else" and for-loop etc. to operate the datalogger. The language itself is not difficult to learn. Nonetheless, there are some limitations, if users are aware of, would save users time to program the datalogger. Unfortunately, because there are multiple dataloggers with various capabilities (e.g. vibrating wire measurements), copy-and-paste the exact CRBasic code from a CR6 datalogger to a CR1000 datalogger does not work. For other generic measurements such as single-ended voltage, the programs are identical for two different dataloggers. Therefore, it is almost guaranteed that optimizing and maximizing the datalogger's performance require a good command of CRBasic programming and the products.
Next: CRBasic A Quickstart Guide
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