The Numerical Analysis Of Ground Surface Settlement In The Process Of Tunnel Excavation

The underground engineering develops rapidly in our country, especially the tunnel engineering. With the great development of highway and bridge basic construction, the construction of highway tunnel also continues to raise. But with the rapid development of tunnel engineering construction, more attention should be paid on the safety and reliability of underground structure. Tunnel’s numerical simulation calculation and analysis is conducted by the 3D finite difference software FLAC3D based on Dongzhi county Leading Mountain Tunnel Engineering, when through theoretical analysis, the different construction method of tunnel effects on the ground surface settlement in the construction process is studied most importantly. The main conclusions are as follows:(1) By numerical calculation, simulates tunnel excavation process under the condition of Ⅲ grade, Ⅳ grade and V grade surrounding rock by CRD method, CD method, step method and full face method, the results shows that in grade Ⅲ, Ⅳ and Ⅴ surrounding rock conditions distribution of full face method and CD method or step method and CRD method for tunnel excavation are in line with the actual situation. The numerical results show that reduction in the displacement and deformation of the surrounding rock can be very good to reduce the surface settlement, and the results also in accordance with the Peck settlement theory.(2) Under the same surrounding rock and excavation support, tunnel’s cross section shape has little effect on the surface subsidence. Steel arch’s axial force and moment is related to the tunnel excavation method. At the same time, the error is only 15% compared with the maximum surface subsidence prediction method results proposed in this paper, and the numerical simulation results basically maintaines at about 10%. So the prediction method proposed in this paper can be considered available to predict the maximum surface subsidence.(3) In the process of double tunnel excavation, the crown displacement of the tunnel has the greastest change. By analyzing monitoring chart of tunnel’s vertical displacement of arch crown, it shows that the crown displacement of the first excavation tunnel has obvious influence when the last tunnel begins excavation, and after the completion of the last tunnel excavation, the left and the right tunnels’ vertical displacement of arch crown basically stays at about 23mm. In the lateral displacement monitoring diagram of the tunnel, there is a clear inflection point, and the final lateral displacement variation is less than lmm.(4) In the process of double tunnel excavation, monitoring the displacement of surface subsidence, it shows that the displacement of surface subsidence has little difference with the four excavation methods, and the surface subsidence curves almost coincide in grade Ⅲ rock. With increasing of the distance between the left and the right tunnel, the maximum vertical displacement of ground surface settlement reduces, and the shape changes from U curve into W curve, and the ground surface settlement of the last tunnel is slightly smaller than that of the first one.(5) Stress analysis is done on the tunnel rock and the lining element. Stress concentration is inclined to come up in the crown and waist of the double tunnel model, especially the left one. So the crown and the waist of the lining element should be monitored for priority in the process of evacuation of double line tunnel. In the process, displacement and stress of the rock varied with the distance of tunnels. More smaller the distance, more bigger the stress and the displacement, or the other side.The research results show that the surface settlement curve is in accordance with the peck’s curve. Comparing surface subsidence value of four excavation methods, this paper gives a more practical method of excavation under different geological conditions, and provides a theoretical basis for actual construction.