Shallow Tunneling Method (STM) For Underpass Construction in Soft Ground; A Case Study: Galoobandak Underpass Project

ABSTRACT: One of the main challenges in the construction of the Galoobandak underpass project was the excavation through loose ground with low overburden. The tunnel diameter was about 6 meters and goes through clayey soil with low overburden (about 3 meters). The tunneling experiences in Tehran alluviums and also the results of numerical modeling showed that construction of an underpass without using a pre-support system or grouting was not possible. Within a specific section of the Tehran market, the potential for subway station construction using the top-down or cut-and-cover methods was not viable because of the congestion and traffic. Additionally, the forepoling technique was not employed, as the forepoling excavation machine could not maneuver and excavate forepoling holes in this project. This paper provides an innovative approach for the excavation of a shallow tunnel in shallow loose soil. The analysis showed that the maximum settlement was more than 4 cm if no pre-support or soil improvement methods were adopted. According to the type of sub-structure, the maximum allowable settlement in 150-meter length was limited to 60 mm. To reduce risk and control settlement, the NATM-STM method with suitable grouting was implemented. The results of finite element modeling showed that the maximum settlement was reduced to 32 mm and therefore, it was within the allowable range.