Investigation of the engineering geological properties of host rock masses in section third of Urmia - Tabriz freeway tunnel and suggested support system based on empirical and numerical methods

The issue of stability and maintenance of the rock mass structure is one of the inevitable cases for the construction of underground structures. The third section of the Urmia-Tabriz Freeway Project will approximately reduce the length of the path 12 km by running the two tunnels system with an approximate length of 4.4 km.

Based on field studies, two units of Trachytic (OMtr) and Gabbroic (OMgb) with Oligo-Miocene age were identified as host rocks. These two units are subdivided into smaller units called OMtr (1), OMgb (2), OMgb (2), OMtr(2) and Cr due to various engineering issues such as differences in engineering geological properties, overhead height and etc. Investigation of engineering geology and geotechnic of host rock masses usually include discontinuity identification, exploratory drilling, and rock mass engineering classification. In addition, numerical analysis is performed using PLAXIS.

Generally, 170 discontinuities were surveyed and analyzed using Dips 5.1. There are three main joints along the tunnel, critical wedges were detected with the help of Unwedge which suggested 1cm thick reinforced shotcrete for their stability.The rock masses were classified, according to the empirical methods RMR, Q, GSI and RMi based on the support system proposed. Allowable displacement in the tunnel was also determined using empirical equations (Sakurai equations). The minimum allowable displacement is 1.94 cm for unit OMgb (1) and the highest at 3.13 cm is for unit Cr.
PLAXIS which is a finite element program, was used for numerical analysis. In one-step drilling, excessive allowable displacement is obtained by empirical equations and then modeled through two-stage drilling. The results determined the thickness of the shotcrete with a compressive strength of 250 kg/cm2 in OMtr (1) unit, 5 cm, in OMgb (2), OMgb (1) and OMtr (2) units, 10 cm and in the Cr unit it was modeled 15 cm. Also, the safety factor of different units shows that the lowest is OMtr (1), which is higher than the minimum required safety factor.Phase2 was used to validate the modeling. To avoid duplication, two units of OMtr (1) and Cr were validated. The maximum displacement in unit OMtr (1) is 0.24 cm and in unit Cr it is 1.41 cm.

According to empirical classifications, rock mass quality is determined as good for OMgb (1) unit, fair for OMgb (2), OMtr (1) and OMtr (2) units and as poor for Cr unit. Due to the quality of the rock mass, the support system of 3 m long rock bolt, spaced at 1.35 to 2.5 m and 5 to 6 cm of reinforced shotcrete for OMgb (1) unit, 4 m long rock bolt, spaced at 1 to 2 m and 10 to 15 cm of reinforced shotcrete for OMgb (2), OMtr (1) and OMtr (2) units with 4 to 5 m long rock bolt, spaced at 1 to 2 m were suggested.  More than15 cm of reinforced shotcrete was suggested for Cr unit.According to the support system in PLAXIS, shotcrete with thicknesses of 5, 10 and 15 cm was suggested for OMtr (1) - OMgb (1), OMgb (2). OMtr (2) - Cr.It was observed that the maximum displacement in all units was less than the calculated value by the Sakurai method and the least safety factor is also 2.13 for OMtr (1) unit. The model was validated with Phase2 and the results were very close.