College of Engineering

A master thesis was discussed in Department of Civil Engineering / College of Engineering at University of Mosul entitled “Effect of Support System Stiffness on the Stability of the Tunnel” submitted by (Mohammed Ali Amin), Supervised By Prof. Dr. Moataz A. Al-Obaydi on Thursday, March 31, 2022.A parametric study was carried out using the Plaxis 2D numerical finite element technique to investigate the effects of different bolt and lining parameters on tunnel stability in terms of radial and tangential stress redistribution with maximum boundary displacement of the tunnel in two soil types: soft and hard soil. Bolt’s parameters were available in five sizes 16, 22, 25, 28, and 32 mm and four lengths 3, 4, 5, 6 m were selected. The lining changeable parameter was only its thickness by taking four different thicknesses of 30, 35, 40, and 50 cm, respectively.In addition, an assessment of a section of the Haybet Sultan tunnel is conducted as a case study in this research. This tunnel is supported by preliminary support of a 28mm fully embedded bolt of 6m and the reinforced shotcrete of 25cm thickness. Then a 40cm of reinforced concrete is applied as an inner lining.During the parametric study, it was observed that in both soil types, the bolt role in the stress redistribution is minor relative to the lining effect. Its effect is more pronounced in displacement reduction. The lining effect is relatively higher than the bolt in both stresses’ redistribution and displacement reduction. The performance of the lining is best in soft soil. The combined support is best for the hard at the tunnel crown.Generally, bolts at the sidewalls performed better than at the crown. In displacement reduction, the maximum displacement in the side walls was about 166 cm in soft soils at the excavation phase. It reduced by 31.52% when using a 3m bolt with a diameter of 32 mm. In hard soil, the reduction percentage of crown displacements when using a 6-m bolt with a diameter of 32 mm, become 8.6% as the best choice of the bolt. The bolt showed a decrease in the floor displacement by 20% and 3.6% in soft and hard soils, respectively.In the soft soil, the performance of the lining was much better than the bolt effect. It’s reduced the displacement at sidewall (when using a lining of 50 cm thickness) from 165.6 cm to only 5 cm which is means a 97% reduction and also shows a 98.4% reduction for crown displacement. The combined structure of 6m length with 32mm diameter can be combined with 30cm at the tunnel sidewall but it’s not the preferred option and has the same effect as the lining at the crown. In hard soil the combined of 30cm with 6m,32mm bolt is better than the 50cm lining at the crown and sidewall.For the tunnel in the case study, it is observed that the inner lining has crucial effect on the stress ratios then the shotcrete effect as a second level. This pattern of the stress increment at the tunnel border leads to more stability. At the tunnel crown and sidewall, the 28mm, 6m rockbolt doesn’t affect these ratios effectively. Meanwhile the inner lining support amplified the (Ϭx/Ϭxo) by 4.6 times and (Ϭy/Ϭyo) by 26.74 times at crown. The (Ϭx/Ϭxo) and (Ϭy/Ϭyo) at the sidewall are amplified by 14.86 and 5.39 times, respectively. The shotcrete at the crown induced an increase by 1.06 and 13.87 times for radial and tangential ratios, respectively. The (Ϭx/Ϭxo) and (Ϭy/Ϭyo) increased by 7.91 and 1.46 times, respectively at the tunnel sidewall.