Excavations boundary walls

Fracture initiation can occur at boundary or inside the rock mass. Both the excavation boundaries (e.g. borehole wall) and the grid points in the intact rock are frequently checked to determine any possible fracture initiation. 

The boundary conditions shown in Figure 1 were determined from FEBEX in situ stress data (Pahl et al., 1989). An additional boundary condition was used to model the time-dependent stress induced by the alpine miner during excavation of the drift. A pressure value of 25 MPa was applied to the wall in successive sections of the drift when that material was "excavated" numerically in the calculation. 

Figure 6 also illustrates that the HM-induced changes in fluid pressure are temporal. For example, the HM-induced fluid-pressure increase above the drift is diminished 5 to 10 m behind the excavation front. Thus, away from the active TBM drilling, the open boundary at the tunnel wall controls the fluid pressure. 

On the whole, flow is moving from within the massif towards the laboratory tunnel (from west to east). Before excavation of the FEBEX tunnel, the equipotentials were roughly parallel to the wall of this main tunnel. A zero pressure has been assumed in the laboratory tunnel. To the north and south, the model is constrained by two major shear zones S1-1-S2, as specified in Section 2.2. Figure 3 displays the hydraulic boundary conditions adopted, in a plane view. The head and gradient adopted were deduced both from measurements conducted within the boreholes intersecting our... 

The loading of the structure is mechanically very simple since it consists in an initial isotropic stress field related to the dead weight. Concerning the mechanical boundary conditions, a zero normal stress is prescribed on the free boundaries of the pattern (wall of the drifts and well, and ground surface), and the symmetry planes are characterized by a zero normal displacement. Before excavation, the rock mass is supposed to be in a compressive stress state, and the principal minor stress 03, indicator of maximum compression, is equal to CTh (and 0i=O2=o =ayy). This stress increases (in absolute value) with depth, from -1.1 MPa at the top of the wells to -1.6 MPa at its base, and to -3.1 MPa in lower limit of the model. The excavation of drifts and wells causes a disturbance of this initial stress field (see fig.3). It is noticed that, apart... 

According to the model, the maximum induced stress 0.5 m below the tunnel floor in the pillar boundary is about 137 MPa after excavation. This stress will cause minor fracture initiation and propagation at the borehole boundaries. Fracturing along the central borehole walls occur continuously as stresses increase, Figure 5. Fractures initiates also in the intact rock. 

Stone-filled vent trenches represent a second form of passive gas control. These trenches are often used on site boundaries where gas migration has been identified, but such systems can only be effectively used when the landfill depth does not exceed 8m (DoE, 1992a), and they are seldom used at depths greater than 5m. The limit on the effective depth of trench is controlled by the depth to which an excavator may dig. To excavate deeper than this requires specialised equipment or techniques and is not cost-effective. The effectiveness of vent trenches (Figure 8) increases when they are used in conjunction with a geomembrane, placed within the trench along the wall furthest from the gas source. The vent trenches again... 

In salt rock, AE activity is detected around open cavities and at the boundaries between different rock types. Creep processes cause high AE activity due to high deviatoric stresses at the walls of the cavities in the excavation disturbed zone. This kind of AE activity is interpreted as ongoing... 

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