ANALYSIS AND OPINION SMART INFRASTRUCTURE


Monitoring performance of reinforced concrete piles, by Nicky De Battista, CSIC research associate


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raditional strain gauges and extensometers used to monitor reinforced


concrete piles provide limited information as they only return measurements at discrete points, rather than along the entire length of a pile. Securing richer information about the behaviour of concrete piles supports future designs to be more accurate, enabling a reduction in the use of construction materials. CSIC has installed distributed fibre optic sensors (DFOS) in several piles because it is ideal for monitoring strain or temperature over distance, and particularly good for detecting cracks, material anomalies or embedded defects that cannot be observed with point sensors. DFOS monitoring of piles enables measurement of the concrete curing temperatures, as well as strain and displacement during pile testing, or during the operational phase of a working pile. DFOS provides a complete profile along several sides and down the entire depth of the pile, thus delivering more information than strain gauges or extensometers limited to single points.


Figure one demonstrates


the importance of obtaining a continuous measurement profile along the whole depth of a pile. Here, data from a compression test pile shows the concrete curing temperature at regular intervals. The concrete reached peak temperature after about 34 hours. Normally the temperature profile would be fairly uniform along the whole depth, but in this case there is a clear rise in peak temperature


at about 17m depth. This is indicative of an anomaly, which was confirmed through testing with cross-hole sonic logging. The pile was then load tested with a standard static load test, which showed a distinctive spike in compression strain at the location of the suspected anomaly, mainly on one side,


“CISC has developed the capability of this technology”


confirming the presence of a localised weak spot (figure two). CSIC has developed the capability of this technology, leading to rapid industry uptake in recent years. It offers a full pile testing service in partnership with spinout company Epsimon. DFOS offers a cost-effective technology for pile testing, as the equivalent cost per sensing point is very low compared with traditional point measurement sensors.


Figure one: concrete curing temperature for monitoring performance of reinforced concrete piles


Figure two: load test strain for reinforced concrete piles


Optimising the design of sprayed concrete linings, by Nicky De Battista, CSIC research associate


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SIC collaborated with industry partners Crossrail and Mott


MacDonald to use DFOS to monitor the behaviour of sprayed concrete lining (SCL), a technique used to construct underground station tunnel walls on the Crossrail project in London. CSIC embedded a DFOS system in the SCL in


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an enlargement chamber at two cross-passage junctions of Crossrail Liverpool Street Station, to monitor changes in the continuous strain profile that occurred during excavation of the cross-passages. The distributed nature of


the strain measurements made it possible to reconstruct the strain map in the tunnel lining


during every stage of the cross-passage excavations with unprecedented spatial detail. The monitoring data


indicated that the load distribution in the tunnel SCL during cross-passage excavation was localised, and significant increases in strain were limited to a relatively small distance from the cross-


passage opening. This is an important finding, indicating that the strengthening of the tunnel walls is unlikely to be necessary beyond the first 3m either side of the cross- passages, thus leading to safer construction and considerable savings in time, cost, and materials, with the resulting reduction in carbon emissions.


October 2019 Electro Optics 11


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