New monitoring technology which combines geophysical ground imaging technology and web-based data visualisation with intelligent monitoring adds an important tool for geotechnical engineers, according to the British Geological Survey (BGS) which led development.
After five years of development, the Proactive Infrastructure Monitoring and Evaluation (Prime) system, which won the Product and Equipment Innovation Award at the GE Awards in 2016, is now ready for commercial use.
“Through deployment in a targeted way, it creates the opportunity for long term monitoring on high risk of critical geotechnical structures and the data delivered could lead to leaner design through integration on new structures,” said BGS geophysical tomography team leader Jonathan Chambers at a launch event for the technique last night at the Institution of Civil Engineers in London.
Chambers outlined the need for a new approach. He told the audience that many critical geotechnical assets were built some time ago and not to current design standards and current use and changing climatic conditions is placing great pressure on infrastructure.
“Geophysics has a role to play in understanding the issues,” he said.
Chambers compared the non-invasive and spatial data offered by geophysics to that delivered by imaging in modern medicine. “The approach is well-respected in medicine and has the same potential in geotechnics,” he explained.
The Prime system combines electrical resistivity technology with data telemetry and custom processing and uses a series of electrode pins that can be installed on a permanent or semi-permanent basis to collect and transmit motion sensing, resistivity and moisture content data.
“It delivers a low cost, low power solution to look at the internal geotechnical condition of assets remotely,” said Chambers.
The BGS worked with a series of industry clients and partners to develop and trial the technology and Chambers presented a number of case studies to demonstrate the technique.
“We have piloted the technique over the last two to three years with a focus on engineered slopes initially, but we have since looked at the use of the technique on natural slopes, water leakage issues and energy research applications,” said Chambers.
The BGS has joined forces with instrumentation and monitoring specialist ITM Monitoring to use ITM’s web-based Calyx system to present the data and make it accessible. “Going forward, ITM will be the delivery agent for the technique,” said Chambers. “With ITM’s expertise in installing geotechnical instrumentation and monitoring, they bare well placed to do this and have been involved with the project since the start.”
Chambers said the technique was not a “silver bullet” for the geotechnical sector but added a new tool to the tool box to aid geotechnical understanding of asset condition. He also added that further development of the technique is planned.
How does it work?
Central to the monitoring system is the Prim instrument, which is a standalone battery powered unit complete with GSM modem for remote communication and interrogation. Connected to this are cabled lines of electrodes which are shallow buried and deployed in lines or grids across the asset for 2D or 3D imaging respectively. Electrical current is ten passed through these electrodes into the ground and the obtained resistivity measurements are automatically processed to generate a resistivity map of the sub-surface, from which changes in groundwater can be inferred. In parallel to this, Prime can measure movements of the electrodes, potentially resolved to centrimetric accuracy, creating a truly comprehensive monitoring system.
Once the instrument is installed, Prime data is then delivered through ITM’s Calyx online monitoring software, allowing asset owners access to near real time data from any internet enabled device. The system also includes options to set response thresholds to alert stakeholders when resistivity and slope movement measurements might impact asset integrity.
“It is not a real time solution as time is needed to process and analyse the data,” said Chambers. “This limits the technique’s use for failure detection but will provide asset owners with an indication of when failure may happen.”
According to Chambers, the maximum depth that the technique can be used to deliver data is scalable. “We have undertaken survey lengths of over 1.6km and the longer the line, the deeper you can see,” he said. “As a rule of thumb, you can see to 25% of the line length but it is a trade off as there are a finite number of electrodes – 256 in total – and as you spread them out, the spatial resolution decreases.”