Settlement Analysis

Information

Estimate for preliminary studies; it does not replace a project-specific assessment.

Settlement trough

Gaussian trough above each tunnel j (Peck 1969; O'Reilly & New 1982), the troughs of two tunnels superposed. Greenfield, at the ground surface. Settlement S positive downwards. x is measured from the tunnel axis (1 tunnel) or from the midpoint between the axes (2 tunnels), positive to the right; the axes lie at , or at with the spacing . Only the building moves.

: axis position (0 or ±c/2), axis depth, equivalent diameter; : volume loss; : trough width factor; : distance of the inflection point from the axis; : horizontal displacement towards the axis; : horizontal strain, extension positive.

Trough width factor K

Typical values, as listed in the GEO5 online help (Fine Ltd., «Shape of subsidence trough»).

Building and damage

Building from to , width , divided into fields of equal width (structural bays). «Pointwise» means , the grid step (limit ).

: tilt; : angular distortion; : lateral strain of a field; : factor on the lateral strain; : maximum principal extension strain, the maximum over of (Cording et al. 2010). The field with the largest governs.

Damage categories

Limiting tensile strains: Franzius (2003), Tab. 2.3, after Boscardin & Cording (1989) and Burland (1995); the limit curves drawn in Cording et al. (2010), Fig. 2 deviate slightly at the upper two boundaries. Crack widths after Franzius (2003), Tab. 2.2, and Cording et al. (2010), Fig. 2, as additional information only: the category is defined by the ease of repair. A strain equal to a limit belongs to the higher category.

Screening, stage 1

Three-stage assessment after Mair et al. (1996): preliminary assessment (screening), second stage assessment, detailed evaluation. Limits of the first stage after Rankin (1988), both as given by Franzius (2003), §2.4.2: maximum slope and maximum settlement at the building ⇒ negligible risk of damage; otherwise a second stage assessment is required.

Assumptions

• Greenfield movements, no interaction between ground and building; the building follows the trough.
• Two tunnels: the troughs are superposed independently; interaction between closely spaced tunnels (narrow pillar) is not considered.
• Tilt = slope of the chord over the whole building (beam assumption, Cording et al. 2010, Fig. 5b). If the distortion concentrates in one bay (Fig. 5a), the tilt is smaller and β is underestimated; Cording et al. report β between 2Δ/L and 4Δ/L.
• Only extension counts: compressive lateral strain is set to zero.
• f_L reduces the lateral strain; f_L = 0 for floors tied to the walls or a grade beam (Cording et al. 2010, Fig. 2).
• Screening (stage 1 of the three-stage assessment after Mair et al. 1996) with the limits after Rankin (1988), see above.
• Not included: subsurface trough at foundation level, longitudinal trough, deflection ratio method.

Sources

• Boscardin, M.D., Cording, E.J. (1989): Building response to excavation-induced settlement. Journal of Geotechnical Engineering, ASCE, 115(1), 1–21.
• Burland, J.B. (1995): Assessment of risk of damage to buildings due to tunnelling and excavation. Invited special lecture. 1st Int. Conf. on Earthquake Geotechnical Engineering, IS-Tokyo '95.
• Cording, E.J., Long, J.L., Son, M., Laefer, D., Ghahreman, B. (2010): Assessment of excavation-induced building damage. In: Arduino, P., Hashash, Y., Finno, R. (eds.), Earth Retention Conference 3, Proceedings of the 2010 Earth Retention Conference, Bellevue, Washington. ASCE. doi:10.1061/41128(384)7.
• Franzius, J.N. (2003): Behaviour of buildings due to tunnel induced subsidence. PhD thesis, University of London (Imperial College of Science, Technology and Medicine).
• Mair, R.J., Taylor, R.N., Burland, J.B. (1996): Prediction of ground movements and assessment of risk of building damage due to bored tunnelling. Proc. of the International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, 713–718. Balkema, Rotterdam.
• O'Reilly, M.P., New, B.M. (1982): Settlements above tunnels in the United Kingdom – their magnitude and prediction. Tunnelling '82, The Institution of Mining and Metallurgy, London.
• Peck, R.B. (1969): Deep excavations and tunneling in soft ground. Proc. of the 7th Int. Conference on Soil Mechanics and Foundation Engineering, State of the art volume, 225–290. Sociedad Mexicana de Mecánica de Suelos.
• Rankin, W.J. (1988): Ground movements resulting from urban tunnelling: predictions and effects. Engineering geology of underground movements, 79–92. The Geological Society, London.