Abstract: The cracks of a complete super-long cast-in-place concrete section of an open-cut tunnel are investigated to explore its early cracking mechanism. Sensors are embedded in central lines of the sidewall and roof structure to continuously monitor the temperature, humidity, and strain. Finally, the early cracking mechanism of the tunnel concrete is analyzed using temperature stress theories and field crack investigation. The results reveal the following: (1) The temperature rise of hydration heat generated by the sidewall and roof structure of a tunnel after concrete pouring reaches over 40 °C, and the strain variation trend of the tunnel roof structure is primarily affected by temperature. (2) The tensile stress caused by the temperature shrinkage in the cooling stage under the constraint of the construction joint is the main reason for cracking. The strain difference along the length direction, the nonuniform shrinkage caused by the temperature difference between the inside and the surface, and the dry shrinkage caused by the decrease in humidity further increase the cracking risk. (3) The maximum temperature difference between the center and the outer surface of the structure is over 12 °C higher than the maximum temperature difference between the center and the inner surface of the structure, and the outer surface of the structure is more prone to cracking. (4) The middle part of the section under continuous external constraint is more prone to cracking. (5) Cracks on the surface of the structure gradually spread, causing cracks to appear in areas where the risk of cracking is low.

Key words: open-cut tunnel, cast-in-place concrete, early cracking, field test, temperature field, relative humidity, strain