Rock-mass displacements

Abstract Modeling of the drift-scale heater test at the Exploratory Studies Facility at Yucca Mountain, Nevada, U.S.A. was performed. The objectives of the analysis were to investigate the (i) temperature effects on mechanical deformation surrounding the heated drift and (ii) thermal-mechanical effects on rock-mass permeability. The continuum representation of a deformation-permeability relationship based on fracture normal stress was developed to assess rock-mass permeability variations because of temperature changes. The estimated rock-mass displacements and permeability variations as a function of heating time were compared with field measurements. The estimated trend of permeability responses using a normal stress-based deformation-permeability relationship compared reasonably to that measured. 

Table 2. The statistics of surrounding rock mass displacement amount of roadway under different coal pillar width.

Catastrophic. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air. 

The constitutive model of rock masses is Mohr-Coulomb criteria and the commercial numerical software (FLAC3D) is used. The mechanical parameters of rock masses are selected through back analysis of excavation process, and compared the simulated results and monitoring data of surrounding rock mass displacement. 

Figure 4. Contours of rock masses displacement when -200 m to -240 m levels were excavated in foot-wall of FI.

Li, W.X., Mei, S.H., Zai, S.H., et al. 2004. Analysis on in-situ stress field change due to underground mining of metal ore and its influence on range of rock mass displacements. Chinese Journal of Rock Mechanics and Engineering 23(23) 4047-4051. 

The outside boundaries of the rock mass are assumed to be fixed against normal displacement, and are specified to remain at a constant temperature (12°C). The measured steady state pressure in the rock in the vicinity of the gallery varies between 0 (atmospheric) and less than I MPa. These values are very small to influence the hydraulic response of the bentonite. For the sake of simplicity, we assumed that the outside boundary of the rock mass is maintained at a constant pressure of 0 MPa. The heaters are not explicitly represented instead, the boundaries between the heaters and the bentonite are assumed to be fixed at zero normal displacement and zero fluid flux (very rigid and impermeable heaters), and an imposed power output is specified at either a constant rate or... 

Other than in the early stage of heating and regardless of the rock-mass properties or failure criteria used, the anchor displacements relative to the MPBX assembly head, located at the collar of the heated drift, were larger for the anchors located farther away from the heated drift. This displacement pattern indicated that the neighboring anchors of an MPBX continued to move away from each other as heating proceeded, suggesting an expansion of the rock mass between the two anchors. This behavior was not strictly observed in the... 

Abstract As a part of the DECOVALEX 111 project—model predictions were carried out of thermomechanical (TM) rock-mass responses at the Yucca Mountain drift scale test (DST), Nevada. This paper presents model predictions of TM-induced rock displacements at the DST carried out by two independent research teams using two different approaches and two different numerical models. Displacements predicted by the two independent analyses compare reasonably well to the measured ones, both in trends and average magnitude. The analyses indicate that the rock mass behaviour is essentially elastic and that the in situ rock mass thermal expansion coefficient is well represented a temperature-dependent thermal-expansion derived from laboratory tests on intact rock. 

The comparison is plotted in Figure 4 for 60° inclined boreholes (BH 147, 148, 154, 155, 178 and 179). The noticeable spread of the results may be attributed to the local rock mass heterogeneities, such as fractures. With the exception of the very early time, simulated displacements are within the range of the measured ones. The best agreement is obtained for Anchors 2 and 3, whereas the... 

Despite these differences, the predicted displacements along the boreholes are almost identical for LBNL and CEA for the case of elastic rock mass behaviour (Figure 7). The agreement between the two independent analyses by LBNL and CEA provide confidence that both approaches were appropriately implemented. 

The most important parameter for the prediction of TM-induced displacement is the thermal expansion coefficient. Both LBNL and CEA used a temperature-dependent rock-mass thermal expansion coefficient derived from measurements on intact rock samples (Figure 2). Judging from a good general agreement in displacement average magnitude, the thermal expansion coefficient derived from core samples appears to be appropriate in this case. 

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... 

FRACOD utilizes the Displacement Discontinuity Method (DDM) principles. It was designed to simulate fracture propagation and interaction of multiple fractures in rock masses. 

Taking account into the damage evolution of jointed rock mass, the resilient displacement increases slightly during the excavation of the ship lock. Displacement apparently increases in the region of damage evolution and the maximum is in the middle of the left slope, the left of plumb wall and middle frusta. 

The calculation model of two-dimensional model, considering the angle of coal seam, set up as shown in Figure 1 and trapezoidal model, a total of 2800 mesh, 5858 nodes. The stress boundary condition, the model surface applied uniform vertical compressive stress, the model under the surface of the vertical displacement fixed. The calculation of the model using Mohr—coulomb criterion is used as a rock mass failure criterion (Qian et al. 1991, Li et al. 2000). 

UDEC is used to simulate the discontinuous medium (such as joints and fissures in rock mass, etc.) under static load and dynamic load under the action of the response. Discontinuous medium is represented via discrete block assembly. Discontinuity handling for the block between the boundary surfaces, allow the block along the discontinuity occurs larger displacement and rotation which include fully out, and in the process of computing, automatic identification the new interface(Hao Zhiyong et al. 2007. Song Genxiang, et al. 2009). 

The near field blast loading due to main tunnel excavations, could generate displacements in the rock mass, only when the peak particle velocity exceeded the critical vibration levels (V ). After repeated exposures of vibrations due to blast rounds at MAT, CVT and TRT, plastic displacements observed even at lower levels of vibrations than the V. The extra damage due to repeated... 

The displacements observed at the V level of 98 mm/s i.e. at approximately 23% of in the Gneiss rock mass with obtuse joint orientation, after 54 numbers of occurrences of blast loading. Similarly, displacements observed atthe level... 

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