Abstract: The rational determination of tunnel face support pressure is key to the normal construction of a tunnel and the safety of the surrounding environment. To explore the variation law of the upper limit value of shallow tunnel support pressure in different composite strata, an upper limit solution model of the limit support pressure of shallow tunnel face considering different proportions of mudstone is established based on threedimensional finite element limit analysis software Optum G3, and the model′s effectiveness is verified using comparative analysis of cases. Moreover, the upper limit solution and corresponding instability mode of tunnel face support pressure under different mudstone proportions are discussed, and the parameter sensitivity analysis is performed to obtain the corresponding simplified calculation formula. The results show the following. (1) The upper limit value of tunnel face support pressure increases more rapidly when the proportion of mudstone is larger than 50% than when the proportion of mudstone is less than 50%. (2) The tunnel face failure mode in composite strata significantly differes from that in a homogeneous stratum. When the proportion of mudstone is less than 50%, an instable point appears at the junction of sand and mudstone, which increases with the proportion of mudstone; when the proportion of mudstone is greater than 50%, an instable point appears at the tunnel′s center axis. (3) The upper limit solution of limit support pressure positively correlates with the ratio of tunnel buried depth C/D, soil internal friction angle φ, and cohesion value c; the larger the C/D, the greater the increase in the upper limit value of tunnel face support pressure. When the proportion of mudstone is greater than 75%, the limit support pressures under different cohesions are almost the same.

Key words: shallow tunnel, composite strata, tunnel face support pressure, finite element limit analysis, upper limit solution