The Effect of Traffic Load Excavation Supported by Artificial Ground Freezing: A Case Study of Tabriz Subway

In this study, the impact of passing traffic and temperature reduction on the stability of the frozen soil wall is studied. Design geometry and mechanical properties of the frozen and unfrozen soil have been simulated according to the line 2 of Tabriz Subway. In order to numerical modeling, in all stages of the present study, the ABAQUS finite element software has been used. For modeling of unfrozen soil, mechanical properties resulted from experimental tests on the samples obtained from boreholes, then the Mohr-Coulomb constitutive model has been used. Since the frozen soil is an unconventional material, there is no reasonable constitutive model for that. Taking into account the actual site conditions (including unit weight, water content, void ratio and lateral earth pressure), more than 60 triaxial compressive tests were conducted on the frozensoil samples. Results from triaxial tests on the frozen soils showed that shear strength of frozen poorly graded sand increases with temperature reduction. Thus, by modeling the triaxial test on frozen soil in ABAQUS software, Mohr-Coulomb constitutive model was verified according to the experimental results of triaxial tests,so that it can model the strain-softening elasto-plastic behavior of frozen sand. The geometry of metro station excavation with the length of 104 m and the depth of 10 m was simulated according to the cut and cover method, consequently. Analysis results showed that lateral displacement of the frozen soil wall into the excavation due to the cut at middle section of the wall is greater than the supports section. Besides, the effect of passing traffic load on the middle section of the frozen soil wall is greater and it leads to larger lateral displacement into the excavation in thissection. The reason of this fact is the friction between unfrozen and frozen soil at the place of abutments. Therefore, in close section to abutments of frozen soil wall, thin and warmer (subzero) temperatures can be used. It is worth mentioning that the decrease in temperature has no significant effects on lateral displacement reduction of the frozen soil wallat abutments (about 2-3%), while, this value is up to 13% at the middle of the frozen soil wall. Eventually, the method of artificial ground freezing for supporting the excavations in this study (subway station of line 2, Tabriz subway) is recommended, which is adjacent to the main road and heavy vehicles traffic is possible.