Jack force effect to tunnel stability

Abstract

The construction process of a bored tunnel is a complex process. During tunnel construction, loads were acting on tunnel and to support excavation, tunnel lining were installed with application of jack force (Fj). Jack force is exerted as a thrust force to ensure advancement of tunnel construction and to enhance tunnel face stability. However, the complexity of Fj to the variation of segment's thickness to ensure the overall tunnel stability is not fully studied yet. The impact of jacking force on segmental tunnel lining and surrounding soil during the tunnel construction also yet to be defined in certain. Therefore, effect of tunnel lining thickness applied with a variation of jack forces in the tunnel-boring machine (TBM) in different soil formations is investigated here in. This research presents a three dimensional (3D) numerical modelling of tunnel soil-jack force by using ABAQUS software. From the findings, the ground surface and subsurface reaction, and reaction force in segment lining were presented. It is found that, from the initial model, longitudinal and transverse surface settlement shows a similar results with previous research work thus verified the work. Next, from the simulation, loads case applied caused stress on the tunnel face which must be encountered by a certain thrust magnitude to advance the tunnel. The face pressure values changed respective to the soil formations, which recorded a different maximum values in the range of 360 MPa to 500 MPa in different soil formations. The jack force calculated from the face pressures of three different lining thicknesses (0.135 m, 0.275 m, and 0.375 m). Jack force of each lining thickness in different soil formations is ranging from 4000 kN, 2000 kN and 1000 kN for the lining thickness of 0.135 m, 0.275 m and 0.375 m, respectively. From the extended complex model, results show the subsurface soil settlement presents a heaving behaviour at the beginning of the excavation and induced a gradually increase of settlement once tunnel stabilise its excavation. The subsoil stress of the soil above the tunnel crown reaches its plastic behaviour at the end of shield contact to cause the final soil displacement. When investigate the effect of jack force to the tunnel lining reaction, a tunnel lining thickness of 0.135 m, 0.275 m and 0.375 m show a maximum reaction force in range of 20000 kN to 40000 kN, 27400 kN to 22700 kN, and 22400 kN to 27700 kN, respectively. This reaction force was varied due to the soil formations and staggered tunnel configurations. It is also found that, the lining thickness of 0.275m (t/D = 0.0458) shows the most stable uniform distribution of reaction force (RF) and thus presents none of critical segments (i.e., safe condition). The segment configuration and angle shows the favourable angle of 5 segment rings is when the staggered started at the angle 32.5°. To sum up, all factors including the geological condition, face pressure, redistribution of sub surface soil stress and jack force variation are crucial in tunnel stability, hence the tunnel lining selection should be done accordingly.