Fig. 1. Parameters investigated using transparent soils model
Ground movements induced by shallow tunnels affect the safety of nearby underground and aboveground structures. Therefore, the reliable prediction of these movements is important. A transparent soil model is used to investigate not only the surface settlement profile induced by shield tunneling, but also the distribution of soil deformation within the soil mass near the tunnel, as shown in Fig. 1 and 2. The measurements indicate that subsurface ground movements can be in excess of the observed surface settlement, which can adversely affect underground utilities.
Fig. 2. Displacement vectors observed in transparent soil models at 2–2:5% volume loss
The work also explored the relationship between tunnel face support pressure and associated soil movements obtained using a transparent soil model that simulates shield tunneling in medium dense saturated sand. The use of a transparent soil surrogate permits measuring the internal soil deformations within the model soil as shown in Fig. 3, below. Soil deformations associated with various face support pressures are presented for 4 cover-to-diameter (C/D) ratios. Failure is found to be sudden with sand flowing into the tunnel leading to a prismatic wedge in front of the tunnel face and a vertical chimney of soil above. A minimum support pressure was achieved with support pressures as low as 10 ± 1% of the effective vertical stress at the tunnel axis. The stability of the tunnel face was related to the coefficient of active earth pressure with C/D ratio having a small effect on the magnitude of required pressure at collapse.
Fig. 3. Contours of ground movement normalized by tunnel diameter for 4 cover to depth (c/D) ratios
Primary References
- Ahmed, M. and M. Iskander (2012). Evaluation of tunnel face stability by transparent soil models. Tunneling and Underground Space Technology, Vol. 27, No. 1, pp. 101-110, doi: 10.1016/j.tust.2011.08.001 [link]
- Ahmed, M. and Iskander (2011) “Analysis of tunneling induced ground movements using transparent soil models,” J. of Geotechnical and Geoenvironmental Engineering, Vol. 137. No. 5, pp. 525-535, doi: 10.1061/(ASCE)GT.1943-5606.0000456, ASCE [link]
