Real time thickness control of continuous extruded concrete lining (ECL) during production

At this moment innovative tunnel drilling techniques are developed for various types of soils. One of these techniques is based on an extruded concrete lining (ECL) production in a continuous process. This wall production technique can be economical in comparison with existing methods e.g. pre-fabricated segmented linings.

However, this extruded concrete technique needs new direct non-destructive (ND) measuring techniques to control and to guarantee that the produced tunnel walls have the required thickness. Existing ND-techniques, based on ultrasound, cannot be used during the dehydration process of in-situ injected concrete.

The aim of this feasibility study was to assay the properties of a new ND-technique that is able to measure in real time the thickness of extruded tunnel linings of at least 0.44 m during its fabrication. An advantage of process control – a direct measurement technique – is that irregularities can be recognized in an early stage so that in general corrective actions can be initiated in time. The study shows that the developed ND-technique can assess in real time with an optimum set-up of ⁶⁰Co radioactive sources in combination with 2 × 2″ NaI(Tl) detectors an ECL thickness between 0.40–0.46 m with an accuracy of ±0.011 m. Due to the fact that the ND-technique is based on the transmission of emitted gamma radiation, the technique has the advantage that the dehydration process does not disturb the collection of data. The accuracy of 0.011 m per measurement is further used for the assessment of an area resolution (minimum change in thickness of the ECL for an area leading to a significant change in collected data) of the lining thickness. For the ND-system, that consists out of 36 ⁶⁰Co sources and NaI(Tl) detectors, the area resolution is 0.013 m over an area of 2.25 m² (radial 2.8 m and axial 0.8 m). This means, that a change of the ECL greater than 0.013 m in an area of 2.25 m² or a change of 0.013 m in an area larger than 2.25 m² will be significantly detected.

Alternative ND-techniques like ground radar, ultrasound, radioactive tracers and other nuclear techniques cannot perform equally yet as the investigated ND-technique.