Analysis of mechanized tunneling parameters to determine the overcutting characteristics

Tunneling in urban conditions requires costly measures, in order to ensure the safety of existing buildings. On average, there are up to 17–20 buildings per 1 km of Moscow Subway Lines under construction. Analysis and comparison of geotechnical monitoring data and results of geotechnical estimations for underground construction using cut-and-cover and tunneling methods in conditions of high-density urban area shows an unsatisfactory correlation between estimated and actual data. This can be described in the following way: insufcient geotechnical survey data; discrepancy between the accepted estimation model and the actual behavior of soil under load; insufcient qualication of the construction workers; and overcutting. The study was aimed at solving the urgent scientic and engineering problem of determining the characteristics of overcutting during mechanized tunnel boring. At the rst stage, the investigations were aimed at identifying the key reasons and factors which determine the quantitative parameters of overcutting in urban underground construction by tunneling. These factors include the following: mismatch between the cutting diameter and the outer lining diameter; displacement of the soil mass in front of the face; incomplete grouting of voids beyond the lining; incomplete lling of beyond-shield voids with clay mortar or slow-curing grouting mortar or no lling at all; and human factor (low qualications of personnel). The overcutting coefcient was determined on the basis of the proposed empirical dependence of its values with regard to the depth of tunneling. The experimental data allowed the depth dependence of the overcutting coefcient for different tunneling depths to be dened, as well as for tunnel diameters from 4 to 10 meters in the case of mechanized tunnel boring machine (TBM) using the earth pressure balanced tunneling method. The practical importance of the studies consists in determining the range of the empirical overcutting coefcient variation from 0.5 % (for TBMs with nominal diameter of 10 m) up to 5 % (for TBMs with nominal diameter of 4 m). The development of organizational measures and justication of process solutions, aimed at ensuring the safety of the existing buildings in conjunction with the scientic and technical support of underground construction has led to a shortening of tunneling time between the Okskaya and Nizhegorodskaya stations of Nekrasovskaya Line of Moscow Subway by about six months. It has also provided savings of about 2.5 billion rubles.