Early strength of shotcrete for tunnel advances - new monitoring approach using thermal imaging

Abstract

Sprayed concrete (or ‘shotcrete’) is often used as a lining for tunnels, particularly where the low mobilisation cost and flexibility of geometry make segmental tunnel linings uncompetitive. The early strength gain of the shotcrete is a crucial aspect to ensure there is effective ground support and to ensure the safety of operatives. Currently, strength monitoring is achieved by needle penetrometer and stud-driving tests according to EN 14488-2 (2006). These tests are very local and hence may not be representative, they are time consuming, and to avoid the risk of falling fresh shotcrete, these tests are often performed in panels sprayed subsequent to spraying the lining. Strength Monitoring Using Thermal Imaging (SMUTI) is a new method which allows quick and easy monitoring of the early strength of the whole shotcrete lining from a safe remote position. A thermal imaging camera is used to build up a time-temperature history of the shotcrete, from which compressive strength may be calculated using an Arrhenius maturity function. The main advantage is the ability to scan the whole surface of the shotcrete from a safe position, in the time it would take to take a photograph. SMUTI represents a step-change in safety and quality control of shotcrete application. In this paper, a description of the method is given, along with an example calculation using real field data obtained at the Tunnel Oberau in southern Germany.