Structural response of main tunnel linings during construction of connecting aisle by means of mechanized drilling

In order to study the structural safety of the linings of the main tunnel during mechanized cutting, two full-scale tests on segmental tunnel rings are carried out. The specimen each consists of seven rings. One of them is assembled by reinforced concrete segments, whereas the others by composite segments. 24 hydraulic jacks per ring provide external forces, simulating water and soil pressure. A shield machine is used to cut the segments, simulating the initial process of mechanized drilling of the connecting aisle. The experimental measurements concern the convergence displacements, relative displacements at the joints and strains of concrete and reinforcement. By analyzing the experimental data, the following conclusions are drawn: the tunnel rings together with the inner supports, prescribed before cutting of the segment, carry the additional loads resulting from the cutting process. The structure remains safe after the inner supports are removed. Redistribution of the internal forces occurs in both the longitudinal and circumferential directions. The extent of such a redistribution depends on the type of the segments, cutting thrust and stiffness of the inner supports. The most dangerous load case during the tests refers to removal of the inner supports, and the increment of the bending moment reaches 40~60 kN·m. During the cutting process, the internal forces of the cutting region are greatly changed, and the variable of bending moment reaches 50 kN·m. The most dangerous cross-sections in the longitudinal direction are located at the cutting side of the 4^th ring and the top and waist regions of the 3^rd and 5^th rings. Redistribution of the internal forces in the longitudinal direction basically occurs in a range of three to five rings nearby the 4^th ring. The inner support bears the change of the main loads during the cutting process.