Tuesday, 30 December 2014

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 Kov├íri, 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.

1 comment:

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