Plasticity analysis and numerical modelling of tunnel collapse in cohesive soil
In this thesis, previous research works on the collapse analysis of underground excavations:
tunnels, trapdoors and plane strain headings, are reviewed. Through the discussion of the
collapse of a single circular tunnel in soil, the collapse mechanism of two parallel circular tunnels
is established under the condition of Tresca yield criterion. Employing the bound theorems of
plasticity theory, admissible velocity fields and admissible stress fields are created around the two
tunnels to obtain upper and lower bound solutions. The bound solutions for two parallel circular
tunnels are compared with the test results from a centrifuge experiment in literature.
Several new parameter definitions: field stability ratio N[, natural stability ratio Nn , external
stability ratio Ne, collapse stability ratio Nc and the stability analysis line (SAL) are introduced. A
new stability analysis plot is derived to analyse the collapse of geotechnical structures, working
with the finite element software package - CRISP. According to the stability state of tunnels in
soil, five principal procedures for searching for the collapse stability ratios are developed, based
on the stability analysis plot. Under the condition of soil with self weight, the stability ratios for
the two tunnels obtained from the finite element method agree well with the solutions obtained
using the bound theorems.
Two models of three-dimensional trapdoors, square and rectangular trapdoors, are developed to
investigate local roof collapse in twmels. Comparing several trapdoor stability solutions, it is
proposed that different trapdoor models (3D block, 3D rectangular and 3D square models) can be
used to the stability analysis for the square trapdoors located in different depths. The upper and
lower bound solutions for square and rectangular trapdoors are derived and discussed.
Block Analysis Method with a triangular element is developed to examine the stability of a
vertical cut and a plane strain heading. The interfaces between two blocks are treated as
elements, while the blocks are treated as extended nodes. The force equilibrium equations of a
soil structure and yield criterion equations are assembled into a linear programming that is solved
with the Simplex Method to obtain optimum stability ratio, via the load factor. This method
shows good potential for the collapse analysis of soil structures in geotechnical engineering
practice in the future.