Pioneering grout monitoring using thermal imaging

For the first time, thermal imaging was used to monitor grout penetration in two shafts segmentally constructed by precast concrete rings.

The shafts, located on the Thames River, were constructed as part of a gas pipeline that links Chelsea with Battersea. The project incorporates the replacement of a 29-km intermediate pressure gas pipe. It will ultimately provide a significant connection between Fulham and Battersea main. 

The 1^st shaft was built by caisson techniques while the 2^nd using the underpinning method. The project addressed difficult soil conditions (alluvium and river terrace deposits) before reaching the well-known London Clay. The cylindrical structures must be waterproof and therefore grout was pumped around the concrete rings. 

Typical grout penetration monitoring includes implementing pressure gauges attached to the concrete rings. This method has proven to have some disadvantages as the distance between the gauges creates an uncertainty concerning penetration in between. "On large shafts and tunnels the gauges could be 2m or more apart, making it difficult to be sure about grout penetration in the middle. Sometimes the pressure recorded can come from soft ground rather than grout. This could result in settlement," Barhale contracts manager Ovi Frunza stated.

Thermal imaging provides a solution to that problem. When injected, grout's temperature is increased due to a chemical reaction between cement components and water. Therefore, a thermal camera can detect areas with different temperatures where the grout has not penetrated enough. "In practice there can be as much as 5°C difference between the areas where the grout has penetrated and where it hasn't. This makes it very easy for us to identify areas where secondary grouting or increased grouting pressure is needed," Frunza added.