Numerical investigation of stability and settlement of tunnels in undrained clay

Numerical investigation of stability and settlement of tunnels in undrained clay

This thesis describes the development, verification and use of a numerical model for investigating circular tunnels in cohesive soils. Using this model, important problems relating to tunnel construction can be studied. In particular, it studies the stability and settlement problems that arise during the construction of single and
twin tunnels using tunnel-boring machines (TBM’s). The developed numerical model simulates the movement and relaxation of the soil around the shield and tail void that occurs due to overcutting and the time delay to lining installation and back grouting. Using this numerical model, a parametric study is conducted which covers most of the practical range. Settlement and stability data is collected for single and twin tunnels.

The settlement data is analysed using a regression of the commonly used Gaussian equation on the settlement data. This approach allows a settlement parameter (ix) to be estimated reliably and accurately for each case. The results of this study are quite positive, settlement results compare well with previous experimental and observational results. Design charts using dimensionless ratios have therefore been presented, which allow the prediction of a settlement profile based on geometry,
volume loss, and material properties.

Stability is analysed using the widely used Broms-Bennermark stability number (N). This approach allows an N to be calculated for each case, which defines the differential between surface and internal pressure. By determining the collapse stage during the relaxation method, an envelope for the critical N is developed. This
stability envelope is then compared to the rigorous upper and lower bound solutions computed by the finite-element limit analysis approach developed by the University of Newcastle Geotechnical group. The results are quite positive, with the stability results from this study remaining within 5% of the upper and lower bound solutions. Design charts using dimensionless ratios have therefore been presented. These calculated stability numbers are also considered with the settlement results, which allows some correlation between N and volume loss.