Recent trends in TBM face pressure estimation

In my previous blog post, I mentioned that the widely used method for face stability calculation in drained condition is based on Anagnostou and Kovari, 1996 [2]. In a recent paper, Perazzelli et. al 2014 [3] presented a new set of nomograms which estimates the effective face support pressure using the "method of slices" approach. This blog post attempts to compare results from above two methods and summarize the observations.

For the sake comparison, effective face pressure is estimated for 10m dia tunnel with the condition: Overburden / Diameter = 1, c’ = 0 and hydraulic head Delta h = 30m for various phi’ values and the results using both the methods are shown below.

It is observed that the effective face pressure estimated using Perazzelli et al nomograms are constantly lower than that of the Anagnostou 1996. Constant difference is maintained even with the increase of phi’ values. This suggests that the method of slices leads to lower effective support pressure values (for equilibrium condition). 

The same comparison is repeated with a constant phi’ (= 25°) but now varying the effective cohesion instead, and results are presented in figure below. Face pressure calculated using Anagnostou 1996’s nomograms are considerably lower than the one Perazzelli et al’s nomograms for higher c’ values. This is because, Anagnostou et al 1996 [2] considers only equilibrium of the prism and does not check the tensile failure.

Thus, in case of high hydraulic gradient and if the cohesion of the ground is high (which may be true for weak rocks), the necessary effective face support pressure may be much higher than the pressure required for the stability of the wedge. Because, in this case, tensile failure rather than sliding becomes the critical mode for the determination of support pressure [3]. This means that, in such situations nomograms of Anagnostou et al. 1996 [2] may underestimate the necessary support pressure and thus may be unsafe. 

This effect is further studied by comparing effective face pressure with varying hydraulic gradient for two different cohesion value (0 and 100 kPa), using both the methods and is presented in figure below. Results indicate that, as observed above, the results from Anagnostou et al 1996 [2] are underestimating the support pressure at higher cohesion. Another important observation is, as the hydraulic gradient increases, the estimate by Anagnostou et al 1996 [2] is approaching the values estimated using Perazzelli et al. i.e, the governing mechanism is changing from tensile failure back to limit equilibrium failure.

This phenomenon important to understand to prevent underestimation of face pressure in high effective cohesive soils.

References:

[1] Senthilnath, G.T (2014). Face Stability of Closed TBMs in Urban Tunnels. Politecnico di Torino, Italy.

[2] Anagnostou, G., and Kovári, K. (1996). Face stability in slurry and EPB shield tunnelling. In M. & Taylor (Ed.), Geotechnical Aspects of Underground Construction in Soft Ground (pp. 453–458).

[3] Perazzelli, P., Leone, T., and Anagnostou, G. (2014). Tunnel face stability under seepage flow conditions. Tunnelling and Underground Space Technology, 43, 459–469.

Urban Tunnel - Sink Holes & Face stability

Metro Tunnel (Source)

Lately, after a recent event (details: link 1, link 2, link 3), there has been much attention in media about the sinkholes created by underground tunnel construction in an urban setting. So, in this post I would like to discuss the theoretical basis behind the stability calculation, which is one of the engineering parameters used to avoid sink holes. 

During an urban bored tunnel drive, instability of the face is one of the prime concern for any tunnel manager. While the workers in TBM may be protected with the closed-face machine, the instability could cause over-excavation and thus excessive settlements & at the worst case, a sink hole on the surface.

Usually, based on the geology, overburden, loads, water condition etc, the type of mechanised tunnelling is chosen for the construction (more on selection of TBM is discussed here). Regardless of the type of TBM (unless its open face rock TBM), during the TBM drive, the Tunnel engineer constantly monitors the applied TBM face pressure with respect to the Target face pressure estimated for the anticipated geotechnical properties. The forces/factors contributing to stability and instability of the tunnel face are:

Factors affecting the stability

Since the cohesion of the soil depends on the pore pressure dissipation, the methods can be broadly divided into:

1. Undrained Condition (widely used method in practice - Kimura and Mair, 1981)

2. Drained Condition (widely used method in practice - Anagnostou and Kovari, 1996)

The face support could be exerted using (a) The Pore pressure in the TBM chamber and (b) The effective support pressure excerted by the TBM. Usually in EPB, the pressure is measured by load cells in the excavation chamber which measures the total stress, ie (a)+(b). The following plot clearly indicates that the total pressure required for the case with maximum delta H is always less than the case in which pressure gradient is the least. However, it is still preferred to have the pore pressure in excavation chamber that is equal to the in situ pore pressure in the ground. This is clearly explained in Dr. Benoît Jones' article in Tunnelling Journal [2]. It can also observed that, as the cohesion increases (stabilizing factor), the effective pressure required decreases (and hence the total pressure).

Comparison of Face Pressure - Above plot is prepared for a 6.6m dia Tunnel with 10m overburden and 20kPa surcharge. Ground water assumed at ground level

In Slurry TBM, the pore pressure in the TBM chamber can be increased by increasing the slurry pressure. It can be set even higher than the water pressure in the ground. Whereas in EPB, the pore pressure in the TBM chamber is maintained by soil plug (formed in the screw conveyor) and can not be set higher than the fluid pressure in the ground.

References:

[1] Anagnostou, G. & Kovári, K. (1996) Face stability conditions with earth-pressure-balanced shields. Tunnelling and underground space technology. Vol. 11, No. 2, pp. 165-173.

[2] Benoît Jones, A Bluffer's Guide to Stability (Part 1 to 3), Tunnelling Journal Magazine (Feb to Jun '14).

[3] Davis, E. H., Gunn, M. J., Mair, R. J. & Seneviratne, H. N. (1980) The stability of shallow tunnels

and underground openings in cohesive material. Géotechnique. Vol. 30, No. 4, pp. 397-416.

[4] Kimura, T. & Mair, R. J. (1981) Centrifuge testing of model tunnels in soft clay. Proceedings of the 12th Int. Conf. of Soil Mechanics and Foundation Engineering, Stockholm. Vol. 2, pp. 319-332.

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.

References:

[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.

Prof. Georgios Anagnostou's Lecture on Face Stability

50th Lecture day of the 2nd Level Specializing Masters in Tunnelling course and the Last day of  ITACET Training Seminar (28th March '14)  was presented by Prof. Georgios Anagnostou from ETH Zurich, Switzerland. Prof. Anagnostou's lecture was focused on Tunnel face instability in soil and the potential hazards related to it. Professor explained the idealized failure mechanism (wedge in front of face + prismatic body extending upto the surface based on Horm 1961) of face instability using model tests and the derivation of limit equilibrium equations for calculation of support pressure. 

Prof. Anagnostou's Lecture on Face Stability

Professor also gave examples of face stability calculation using Limit Equilibrium equations and based on monograms.

Examples of Support Face Pressure Calculation using LEM Approach

References:

[1] J. Messerli, E. Pimentel, and G. Anagnostou, “Experimental study into tunnel face collapse in sand,” Phys. Model. Geotech., vol. 1, pp. 575–580, 2010.

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

[3] R. Schuerch and G. Anagnostou, “Analysis of the stand-up time of the tunnel face,” in World Tunnel Conference 2013 Geneva, 2013, pp. 709–714.

[4] G. Anagnostou, “Urban tunnelling in water bearing ground – Common problems and soil-mechanical analysis methods,” in 2nd International Conference on Soil Structure Interacton in Urban Civil Engineering, 2002, pp. 233–240.

[5] K. Serafeimidis, M. Ramoni, and G. Anagnostou, “Analysing the stability of reinforced tunnel faces,” in 14th European Conference on Soil Mechanics and Geotechnical Engineering, 2007, pp. 1079–1084.

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

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

[8] P. Perazzelli and G. Anagnostou, “Comparing the limit equilibrium method and the numerical stress analysis method of tunnel face stability assessment,” 7th Int. Symp. „Geotechnical Asp. Undergr. Constr. Soft Gr. “. Rome, 2011.

[9] 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.

[10] 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.

[11] L. Cantieni and G. Anagnostou, “The interaction between yielding supports and squeezing ground,” Tunn. Undergr. Sp. Technol., vol. 24, no. 3, pp. 309–322, May 2009.

[12] G. Anagnostou and L. Cantieni, “Design and analysis of yielding support in squeezing ground,” in 11th ISRM Congress, 2007, p. 4.