The technique uses thermal imaging rather than the traditional method of checking small test panels.

The contractors believe the new method is much safer and protects site workers from the dangers of sprayed concrete lining failures.

The new technique is known as Strength Monitoring Using Thermal Imaging (SMUTI) and has been invented by Dr Benoit Jones from Cambridge University.

It uses a thermal imaging camera to track the temperature of concrete as it is sprayed to form the tunnel lining.

Knowing this temperature history enables the engineers to calculate the amount of hydration that has taken place in the concrete, and hence its strength.

Aled Davies, Costain Senior Tunnel Engineer has worked closely with Dr Jones on the trials.

He said: “SMUTI allows us to directly monitor the compressive strength development of sprayed concrete whilst remaining at a safe distance.

“This is a substantial improvement over the current method, which relies upon a small test panel being representative of the entire sprayed concrete advance to prevent personnel being at risk from sprayed concrete lining falls.

“The trials have gone very well and the workforce was very appreciative of the time and efforts being taken to improve their safety.

“The data is now being analysed but we hope to have the results by September, when we will present them to tunnelling sector clients and design partners.

“We hope to see SMUTI become the primary method of early strength monitoring on all tunnelling projects.”