Model testing and analysis of interactions between tunnels in clay
This dissertation describes a study of the interaction between closely spaced tunnels during shield tunnel construction, concentrating on the study of the short-term incremental behaviour of the liner. Carefully controlled physical model tests were carried out and the test results were complemented by a limited amount of numerical analysis. In the physical model tests described in this dissertation, two groups of tests were carried out at a laboratory scale; one set of tests studied closely spaced parallel tunnels and the other set investigated perpendicular tunnels. An important feature of the study was that a novel model tunnelling machine was designed and developed as part of the research. Thin steel tubes were used to model the tunnel linings. The experimental technique adopted in the preparation of clay samples (which is a well-established procedure) was found to produce high quality samples. Good repeatability was achieved in preparing the kaolin samples. The tunnelling machine allowed tunnel liners to be installed using similar procedures to those adopted in the construction of full scale shield tunnels using an earth pressure balance approach. The instrumentation system used in this experimental programme are shown to produce reliable data. During the model tests measurements were made of liner strains, pore water pressures and total stresses acting on the liner. Errors in the data logging system were shown to be very small (of the order of less than 2% of peak values). The mechanisms governing the structural interaction between closely spaced tunnels are highly complex. The tunnel installation was shown to modify the stresses acting on the liner of the adjacent tunnel. These stress increments led, in turn, to line deformations and induced bending moments. The nature of the interaction mechanisms depends on the geometric configuration of the tunnels, the liner properties, and overconsolidation ratio. For the parallel tunnels, the pillar width ratio (W/D) is an important parameter governing the magnitude of the interaction effects. the interaction effects increase as the pillar width ratio is reduced. Increasing the liner flexibility was found to reduce the induced bending moments but to increase the induced displacements. The interaction effects were larger in overconsolidated clay than normally consolidated clay. The worst case for interaction effects occurs when the pillar width is small, the liner is flexible and the value of OCR is large. Three-dimensional considerations suggest that interaction between parallel tunnels may be more severe than those measured in the corresponding perpendicular tunnel tests. However, the different nature of the mechanisms in the two cases appear to be more significant than this geometric effect.