Pore pressure tunnel excavation

Pore water pressure changes in the vicinity of the tunnel excavation are a direct consequence of changes in the volumetric strain of the rock. Later, pore water pressure dissipations are a consequence of the transition flow towards a new equilibrium, which now has a modified boundary condition (the tunnel surface) in the vicinity. Therefore, fully coupled hydro-mechanical analyses are required to try to capture actual measurement. In fact, one-way... 

The development and dissipation of excess pore water pressures in the vicinity of the advancing tunnel (at the time of the FEBEX tunnel excavation) was a clear example of hydromechanical interaction. It was concluded that the development of pore pressures was controlled by the initial stress field state, by the rate of excavation and by the permeability and drainage properties of the granite. However, the available information on the intensity and direction of principal stresses in the area was found inconsistent with the actual measurements. The problem posed by this discrepancy was essentially unsettled since a precise determination of the initial stress state in the vicinity of the FEBEX tunnel was not available. 

The TBM excavation process was modelled by a decrease of the effective force and pore pressure on the wall of the tunnel. This decrease depends on the time and the distance to the front of the excavation. The excavation was simulated according the excavation sequence shown in Figure 1 for an initial effective stress of Oh = Oh = 29.4 MPa (horizontal maximum and minimum compressive), and Ov = 7.14 MPa (vertical). 

Figure 4 shows the comparison carried out between the measured and simulated pressures. The general shape of the curves obtained with hydraulic simulation is quite similar to measurement results, except there is a constant difference of about 1 bar between the P4 measurements and the simulation. This point will be discussed in Section 4.5 below. Our simulation approach (a hydraulic steady flow analysis with four steady-state steps) was not able to reproduce pore pressure increase (led to a higher horizontal stress than vertical stress) as excavation neared the monitored borehole intervals and as post-tunnel face dissipation was completed. 

As a preliminary estimate, stepwise excavation is not functioned in the program temporarily. The simulation results consist of pore pressure, saturation, solubility distribution change, total gas and water well or tunnel output and portion of vicinity of well bottom (one diameter height) after 24 hour and 2 year. The following figures are some of examples. 

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