For several years, the Centre for Smart Infrastructure and Construction at Cambridge University has been developing distributed fibre optic sensors, DFOS, for a range of civil infrastructure monitoring applications.
Dr Nicky de Battista, who has been researching innovative uses of DFOS, said: “After many field trials and incremental improvements, DFOS is now at a high technology readiness level and, in fact, has been adopted by a number of industry sectors as a standard monitoring tool.”
The novel aspect of this technology lies in the fact that standard optical fibre becomes the sensor and tens of kilometres of fibre can be sensed at once for continuous distributed measurement of the conditions around the optical fibre such as temperature, strain, and acoustic noise. “DFOS is ideal if you want to monitor strain or temperature over a long distance or a large area, and particularly if you want to detect the presence of an event such as cracks, material anomalies or water infiltration, but you cannot predict where these will occur,” said Dr deBattista, who was recently invited to talk about the applications and commercial viability of DFOS monitoring at the 2017 Ground Engineering Instrumentation and Monitoring conference.
Additionally, fibre optic cables use light rather than electricity – the only electrical part of the system is the analyser – which makes DFOS ideal for hazardous environments where electrical sensors are not suitable, such as in sewers or nuclear facilities.
CSIC has deployed DFOS systems in several different civil engineering applications, including:
Measuring axial shortening of a reinforced concrete high-rise building
CSIC has deployed DFOS to measure axial shortening of reinforced concrete columns and walls at Principal Tower, a residential building currently under construction in central London that will rise 163 m above ground.
The contractor and designer are keen to verify the predicted axial shortening values and adjust the column height presets in the reinforced concrete structure if necessary. CSIC embedded fibre optic sensors in two columns and two walls to measure the axial deformation of these elements.
The tower is being constructed using an automated jumpform that incorporates the whole building footprint, including the columns. This is the first building in the UK to be constructed using this type of jumpform. The fibre optic cables are housed in reels within the jumpform rig and are unwound and embedded with the concrete as the construction progresses upwards.
CSIC is continuously measuring strain and temperature from the embedded cables and this is planned to continue throughout the construction. From these data, the shortening of the instrumented columns is calculated at any point along their height. The general trend shows progressive shortening as expected, but there were also some occasions where the columns actually elongated due to thermal expansion, during periods of relatively higher ambient temperature.
Since the DFOS system is also measuring the temperature of the concrete, it is possible to isolate the effect of thermal movement to reveal the shortening due to load and concrete shrinkage and creep. From this approach, it becomes clear that thermal effects can be significant and at times they can account for up to half of the total axial displacement of the columns. The measurements recorded so far have matched very well with the engineers’ predictions which reassures both the contractor and the designers.
Monitoring of concrete piles
CSIC has been researching and improving pile monitoring techniques since 2006. During the past two years the technique has been specified more and more often by contractors to the point where it is now replacing traditional pile testing instrumentation.
DFOS can be used to monitor sacrificial test piles or working piles. CSIC has used DFOS to monitor bored piles and CFA piles, and both tension and compression piles. For this application, the fibre optic cables are installed along the whole length of the pile cage, on two or more sides, and can also be installed on central rebar bundles. The cables are attached to the cages on site, and the cages are then inserted into the pile bore and concreted. Monitoring can start from the moment the cage is inserted into the bore.
One of the advantages of DFOS over conventional monitoring is that a single instrumentation can provide profiles along the whole depth of the pile, of the concrete curing temperature, as well as continuous strain and displacement during pile testing or during the operational phase of a working pile. The continuous profiles allow identification of anomalies in the temperature or strain data, which one would be unlikely to identify from single point measurements.
Locating rainwater infiltration and blockages in a sewer network
CSIC was asked to monitor just over a kilometre of sewer in order to identify locations where rainwater was entering due to illicit household connections, and locations of recurring blockages causing flow restrictions. DFOS was used to monitor the temperature profile along the sewer, since rainwater and sewage have a different temperature. This monitoring campaign lasted a total of 80 days during which there were several storms.
The DFOS installation involved passing a pilot rope downstream from a manhole, then attaching the end of the fibre optic cable to the rope at the other side and pulling it upstream through the sewer. The cable was then connected to the analyser inside a monitoring room at the treatment plant. A rainfall gauge was used to correlate rainfall with monitoring events in the sewer. With a single fibre optic cable in the sewer, CSIC was able to observe in great detail what was happening and this monitoring can help the asset owners locate any problems with blockages and infiltration.
Additional information about DFOS
DFOS can be equally as practical as other conventional sensors, while being able to measure different physical parameters with a single system, such as temperature and strain. The cost of a fibre optic sensing cable is potentially very low compared with point measurement sensors, when considering the fact that one can obtain many measurement points per meter of cable. The optical fibre material itself is relatively inert and can be ideal for long-term monitoring when the fibre is embedded in structures. The current performance of optical fibres embedded in infrastructure is more than sufficient for most civil monitoring applications and this is expected to improve even more as capability of analysers improves. Such features can potentially provide a relatively cheap but highly effective monitoring system for both the short and long term. Most of the capital investment relates to the optical analyser, which can be connected to a number of fibre optic cables or be shared at different sites. It is expected in the near future that an increased choice of analysers from more manufacturers will bring a reduction in price. However, in the vast majority of projects it is the monitoring service provider (such as CSIC spinout company Epsimon Ltd) that owns the analyser and provides it as part of the service.
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