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    • Determination of support pressure for semi-elliptical tunnels in anisotropic and nonhomogeneous soil by lower bound limit analysis


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      This research work examines the stability of long semi-elliptical tunnels in purely cohesive and cohesionless soils. The support pressure (si) required to apply along the tunnel periphery by means of lining and anchorage system has been evaluated using lower bound limit analysis with finite elements and second-order conic programming technique (SOCP). The results are presented in terms of normalized support pressure as (i) ri/cv0 in purely cohesive soil for different combinations of gD/cn0 and ac and (ii) si/gD in cohesionless soil for different combinations of fv and af considering different values of 2h/D and H/D of tunnel where h, D and H are height of tunnel, width of tunnel and soil cover depth of the semi-elliptical tunnel, respectively; cn0, f/v and g are soil undrained shear strength of cohesive soil in vertical direction at the ground level, peak vertical friction angle of cohesionless soil and unit weight of all type of soils, respectively; ac and a/ are anisotropy factors in cohesive and cohesionless soil, respectively. The value of si/cn0 has been observed to increase with an increase in gD/cn0 and ac. The value of si/gD increases with a decrease in fv and increase in af. The increase in aspect ratio of tunnel profile (2h/D) causes an increase in normalized support pressure in both purely cohesive and cohesionless soils. The effects of increase of undrained shear strength in isotropic purely cohesive soil on the stability of tunnel have also been studied. The support pressure is observed to decrease with increase in nonhomogeneity of cohesive soil. The combined effect of soil anisotropy and linearly increasing undrained shear strength may reduce the required magnitude of si/cn0 in comparison with the value of si/cn0 in isotropic homogeneous cohesive soil, especially for H/D >1.

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