- Title
- The stability of shallow tunnels using limit analysis
- Creator
- Wilson, Daniel William
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2013
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Tunnelling is generally associated with underground mining or in the construction of rail tracks and roadways. The evolution of mass transit systems within large cities has made ever increasing use of relatively shallow tunnels to by-pass urbanised areas, provide detours around obstacles such as rivers, or simply ot preserve the environment. The engineering design of tunnels in soil must consider the stability of the tunnel or, more accurately, the stability of the soil surrounding the tunnel to ensure that it does not collapse during and after construction. In this Thesis, advanced computational and numerical techniques are used to investigate the stability of tunnels in soil. A range of tunnelling scenarios are studied to determine the influence of soil strength and tunnel geometry on tunnel stability. Rigorous upper and lower bounds on the magnitude of loads required to cause collapse of the tunnel are computed using Finite Element Limit Analysis. The failure mechanisms associated with the collapse of tunnels are also investigated using rigid-block upper bound methods. The first scenario to be investigated is the undrained stability of a tunnel through a soil profile in which the soil strength varies linearly with depth. Such conditions are encountered in geotechnical engineering as normally consolidated layers of clay often display this form of heterogeneity. This work extends the study of Assadi and Sloan (1991), and applies finite element techniques to more accurately bound the magnitude of the loads that may cause tunnel collapse. In addition, simple closed-form expressions for computing the tunnel stability have been developed for use by practitioners. The stability of pairs of parallel square and circular tunnels, positioned side-by-side, is also investigated. Stability charts are generated for a variety of tunnel depths, material properties and tunnel geometries. It is found that the stability of dual tunnels does not necessarily increase with increasing spacing between the tunnels. In fact, for tunnels which are very close together, the stability may decrease as the inter-shaft distance is increased and that the tunnel is least stable when the distance between the tunnels is approximately equal to the tunnel width. As the distance between the tunnels is increased beyond the point of minimum stability, the stability increases until there is no interaction between them. This occurs when the distance between the tunnels is six to fourteen times the tunnel size, depending upon the depth of cover. The stability of wide rectangular tunnels is also investigated. The effect of the aspect ratio on tunnel stability is found to be very small compared to the influence of the material properties and cover depth. A closed-form expression that accurately estimates the stability of rectangular tunnels is developed for used by engineers. Results from this study are used to show that the stability of parallel square tunnels, with a spacing of less than approximately one tunnel width, is approximately equal to that of a rectangular tunnel that envelops the dual square tunnels. Finally, square and circular tunnels in cohesive frictional soils are studied to examine the influence of surcharge loads applied to the ground surface on stability. This situation is important in the design of new tunnels to be constructed beneath existing urban infrastructure, or in the engineering assessment of tunnels beneath new urban developments. A range of soil parameters and tunnel depths are investigated, and the results obtained are presented in the form of dimensionless stability charts. An expression that approximates that ultimate surcharge load permitted above a tunnel is also derived.
- Subject
- tunnels; limit analysis; tunnel stability; soil parameters; surcharge load; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1043137
- Identifier
- uon:14170
- Rights
- Copyright 2013 Daniel William Wilson
- Language
- eng
- Full Text
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