Thursday, 29 May 2014

Tunnel Face Stability - VB Module for Quick Estimation

The stability of the tunnel face is one of the fundamental factors in selecting the method for excavating a tunnel in soft ground and in urban areas. When using TBMs, evaluation of the face-support pressure is a critical component in both the design and the construction phases. However, specific recommendations or technical norms are not available as common guidance for the design. In current practice, different approaches are often employed, both to evaluate the stability condition of the face and to assess the required face-support pressure [6].

This blog post presents a VB module built on Microsoft Excel which is set to calculate the face support pressure in Tunnel. This post is based on Prof. Anagnostous' lecture, ITACET training seminars and related practice exercises. This code is set to calculate the Tunnel face pressure to maintain a stable face when there are no seepage forces and for closed EPB drive (with seepage forces). Although these results can not be used for detailed analysis/design but this could help in a quick check on pressure magnitudes and for rough parametric studies.

Screenshots from the VB program - Input and Output
Click here to download
[Update: One of the reader reported that the program seem to show some errors in Mac OS. I will update it soon. However, it is working perfectly fine in Windows 8 & Windows 7]

Details about the program are briefly explained below.

Part 1: Support pressure at Tunnel face (without Seepage Forces)
This part calculates face support pressure based on Horn (1961), wedge failure mechanism. As of now, this program calculates face support pressure for Cohesive Soils or for short term condition in low permeable soils. This could be further developed for all types of soils.

Part 2: Support pressure in case of Closed EPB Drive (with Seepage Forces)
The construction methods used in soft ground tunnelling beneath the water table must ensure control of the ground at the tunnel heading and additionally prevent seepage flow towards the working face. In an EPB drive, the face is stabilized by direct support of the pressurized muck and by the reduction of seepage forces. Hence, higher the head difference, the higher the effective support pressure. Higher effective support pressure will cause excessive cutter wear and will require higher torque to operate. The above program calculates effective support pressure using normalized diagrams. For detailed analysis case specific FEM coupled analysis shall be performed.

In some cases, the program shows "No pressure required". This is possible when the compensation of water pressure (along with cohesion in ground) suffices for face stability.

[1] G. Anagnostou and K. Serafeimidis, “The dimensioning of tunnel face reinforcement,” in World Tunnel Congress 2007,. May 2007.

[2] G. Anagnostou and K. Kovári, “Face stability conditions with earth-pressure-balanced shields,” Tunn. Undergr. Sp. Technol., vol. 11, no. 2, pp. 165–173, Apr. 1996.

[3] G. Anagnostou and K. Kovári, “The face stability of slurry-shield-driven tunnels,” Tunn. Undergr. Sp. Technol., vol. 9, no. 2, pp. 165–174, Apr. 1994.

[4] G. Anagnostou and K. Kovári, “Face stability in slurry and EPB shield tunnelling,” in Geotechnical Aspects of Underground Construction in Soft Ground, 1996, pp. 453–458.

[5] G. Anagnostou, “Some remarks concerning EPB and slurry shields,” in Development of Urban Areas and Geotechnical Engineering, 2008.

PS: Please let me know if I have missed any error handling scenario or any other bugs. Thank you.